Organ

An organ is a structure in a multicellular organism made of two or more cell-based tissues joined into a unit that performs a common function. Organs sit between tissues and organ systems in the biological hierarchy, so the heart, liver, stomach, and skin each combine several tissue types to do a specific job. The word derives from the Greek organon, meaning instrument, and organs are found in most animals and plants. Understanding organs is central to anatomy and physiology, and it underpins medicine, from diagnosing disease to replacing failed organs through transplantation.

This entry explains how organs are built and organized, how they group into organ systems, and how they are classified across animals and plants. It then covers organ development and evolution, the physiological roles organs play in maintaining homeostasis, and the clinical topics of organ failure and transplantation. It closes with the history of organ study and current research on engineered tissues and organ regeneration.

Definition and place in the biological hierarchy

In biology, an organ is a body part built from several tissues that work together to perform one specialized task. In biology, an organ is a group of tissues in a living organism that have been adapted to perform a specific function. The concept applies across multicellular life. Most multicellular organisms have one or more organs. Animals have organs such as the stomach, liver, lungs, and heart, while plants have organs such as stems and leaves.

Organs occupy a defined position in the hierarchy of biological organization. The sequence begins with the cell, the most basic level, and rises through tissues, organs, and organ systems to the complete organism, and the whole arrangement serves to maintain homeostasis. Organs and organ systems represent two levels in the structural hierarchy of living things. The hierarchy is composed of five levels of organization that interact within the body to maintain homeostasis. An organ therefore sits between tissue and organ system in that ladder. Organ systems are most clearly developed in complex animals. In the more advanced animals, there are usually 10 organ systems: integumentary, skeletal, muscular, nervous, endocrine (hormonal), digestive, respiratory, circulatory, excretory, and reproductive. These systems appear gradually in the lower animals and attain their full complexity and functional specialization in the higher animals.

The English word comes from Latin and ultimately Greek. Latin organum was taken from the Greek organon, a word that in classical usage meant an implement or tool. Middle English, partly from Old English organa, from Latin organum, from Greek organon, literally, tool, instrument. The Greek root is linked to the idea of work: from Greek organon "implement, tool for making or doing; musical instrument; organ of sense, organ of the body," literally "that with which one works". The bodily sense of the English word is comparatively late: the biological meaning "body part of a human or animal adapted to a certain function" is attested from late 14c., from a Medieval Latin sense of Latin organum. The older sense of a musical instrument, which in English eventually narrowed to the pipe organ, was never fully displaced, so the same word carries both meanings.

An organ differs from a tissue in how it is composed. A tissue is made of cells of one kind working in concert, whereas an organ combines different kinds of tissue. Tissues are formed from same type cells to act together in a function. Tissues of different types combine to form an organ which has a specific function. The distinction from the organ system is one of scale: an organ system groups organs that cooperate. Two or more organs working together in the execution of a specific body function form an organ system, also called a biological system or body system. For example, the esophagus, stomach, and liver are organs of the digestive system. The same organ can therefore be understood at two levels: as a structural unit within its own tissues, and as a component of a larger functional network.

Tissue composition and internal structure

Parenchyma and stroma

Anatomists commonly split an organ into two compartments with different roles. The parenchyma is the functional part, made of the cells that perform the organ's particular task, and it generally makes up most of the organ's bulk. The stroma is the supporting framework. Histology texts define it by exclusion: whatever is not functional parenchyma, including connective tissue, vessels, nerves and ducts, belongs to the stroma. The parenchyma of an organ consists of that tissue which conducts the specific function of the organ. An organ's parenchyma usually comprises the bulk of the organ. Stroma is everything else, and this split is useful because it lets a description of an organ avoid listing every cell type individually .

Stromal tissue is built mostly from fibroblasts, which secrete the extracellular matrix that fills the interstitial space, together with pericytes beside blood vessels and, in some organs, fat cells Stroma consists of mostly mesenchymal cells, such as fibroblasts, which produce the interstitial extracellular matrix (ECM), pericytes adjacent to blood vessels, and adipocytes. In this role, the stroma shapes the architecture of the parenchyma, for example the lobules, ducts and glands of an exocrine organ, by supplying that matrix .

The categories are applied by function rather than by tissue type, so they do not always line up with the four basic tissues. In the kidney, the parenchyma is mostly epithelial, consisting of renal tubules, while the vessels, nerves and connective tissue form the stroma The parenchyma of the kidney is epithelial tissue (mostly renal tubules). The blood vessels, nerves, and supporting connective tissue of the kidney comprise the stroma. The spleen shows the opposite pattern. Its parenchyma is connective tissue (mostly lymphocytes and other blood cells).

The four basic tissue types

Histology recognizes four basic tissue types, epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Each has a characteristic role. Epithelial tissue creates protective boundaries and is involved in the diffusion of ions and molecules. Connective tissue underlies and supports other tissue types. Muscle tissue contracts to initiate movement in the body. Nervous tissue transmits and integrates information through the central and peripheral nervous systems.

These types also follow consistent spatial relationships throughout the body. Epithelium sits on a layer of connective tissue, peripheral nerves run inside connective tissue, and muscle is surrounded by it Epithelium always rests upon (i.e., is supported by) connective tissue. Peripheral nerves travel within (surrounded by) connective tissue. Muscle tissue is enmeshed by connective tissue.

How tissues combine within an organ

Few organs consist of a single tissue type. The heart is a standard illustration: it is cardiac muscle wrapped in connective tissue, lined with epithelium, and controlled by nervous tissue.

The wall of the intestine provides a more detailed example of epithelium combined with smooth muscle. The innermost mucosa contains the epithelium, where most digestive, absorptive and secretory processes occur; the lamina propria, a layer of connective tissue, and the muscularis mucosae, a thin layer of smooth muscle. In the stomach, small intestine and colon, the epithelium is simple columnar. Its cells include goblet cells, which secrete mucus, which lubricates the passage of food along and protects the intestinal wall from digestive enzymes. Beneath the mucosa lies the submucosa, which contains nerves including the submucous plexus (also called Meissner's plexus), blood vessels and elastic fibres with collagen. The outermost muscular layer is made of two smooth muscle sheets: an inner circular and outer longitudinal layer. Connective tissue separating these sheets carries nerves (myenteric plexus of Auerbach) and blood vessels. Folds of the mucosa also raise the surface area available for absorption, and the small intestine's villi each contain a lacteal, a vessel connected to the lymph system that aids in the removal of lipids and tissue fluids.

Vessels, nerves and lymphatics

Every organ depends on a supply and drainage network that runs through its stroma. Systemic tissues that include the vasculature network, immune cells, and peripheral nerves reside within the stroma. In the intestinal wall, the submucosa serves this function by providing connective tissue support and delivering blood vessels, nerves, and lymphatics. Blood vessels are themselves organs, built from all four basic tissue types and traveling in connective tissue Blood vessels (organs in their own right, built from all four basic tissues) travel within (surrounded by) connective tissue. The connection to the lymphatic-system, nervous-system and immune-system is therefore built into the structure of the organ itself, not added on afterward.

Organ systems in animals

An organ system sits one level above the organ in the hierarchy. It is a biological system made of a group of organs that work together to perform one or more bodily functions. Member organs divide the work, and each is built from several tissue types. Reference works differ on the human total: there are 11 to 12 distinct organ systems, and the count shifts depending on whether the muscular and skeletal systems are treated separately. Some anatomists join together the muscular system and the skeletal system.

The major systems of the human body

The standard eleven-system scheme covers the functions that keep the body running. The respiratory system handles gas exchange with the air and helps maintain acid–base balance. The cardiovascular system transports oxygen, nutrients and hormones throughout the body and removes cellular metabolic waste. The digestive system breaks food down mechanically and chemically so that its components can be absorbed and used as energy. The urinary system filters blood and removes unneeded compounds through urine. The nervous system initiates and regulates vital functions, sensation and body movement. The endocrine system releases hormones that regulate many processes, and the lymphatic system drains excess tissue fluid and contributes to immune defense. The reproductive system produces reproductive cells and takes part in the reproduction process. Muscular and skeletal systems supply mechanical support, posture and locomotion, and the integumentary system (skin, hair, nails and sweat glands) protects the body and helps regulate temperature.

Organs that belong to more than one system

Because organs are defined by what they do, a single organ can contribute to several systems. The nose is a standard example: it is in the respiratory system and also serves as a sensory organ in the nervous system. The gonads show the same overlap. The testes and ovaries are both part of the reproductive and endocrine systems, because they release sex hormones as well as producing gametes. The pancreas is usually assigned to two systems at once: it is part of the endocrine system, where it produces hormones such as insulin, and of the digestive system, where it secretes digestive enzymes. More generally, some organs perform activities that directly participate in the functioning of more than one system. The overlap is therefore a matter of which functions are counted, and textbooks do not always agree on the assignment.

Body systems that are not organ systems

Not every body-wide system is built from organs. The immune system is the clearest case. The immune system protects the organism from infection, but it is not an organ system since it is not composed of organs. Its components are spread across the body. Its protective mechanisms are distributed, meaning they are found in almost every anatomical location, and act on multiple levels, from genes and proteins to cells, tissues and organ systems. Immune cells circulate in blood and lymph, while others remain resident in particular tissues. One estimate puts the average adult's immune cells at approximately 1.8 trillion cells, weighing around 1.2 kg. Immune tissue is not absent from the organ framework: the thymus, spleen and lymph nodes are organs, and they are counted within the lymphatic system (see lymphatic system, thymus, spleen and lymph nodes). The immune system is thus best described as a network that crosses several organ systems, rather than one system of its own. The immune system is consequently a useful contrast case for the organ-based definition.

Complexity of organ systems across animals

Organ systems are more elaborate in some animals than in others. In simple animals, body functions are carried out by cells or by tissues that are not yet grouped into discrete organs. Sponges lack organs altogether, and cnidarians such as jellyfish have a diffuse nerve net instead of a central brain. Vertebrates, including humans, have distinct organs for each function, arranged into a full set of interlocking systems. Greater specialisation of this kind allows finer regulation of the internal environment, because separate organs can each handle one task, linked through the nervous system and hormonal signals. These links are also why the human body is usually described as a set of systems that depend on one another, with kidneys, pancreas and digestive organs working together.

Classification of animal organs

Animal organs are usually sorted along several axes, and the most basic one is location. Organs inside the trunk are collectively called viscera, the plural of viscus, a Latin word for an internal organ. In medical usage the term covers the soft organs packed into the thoracic, abdominal and pelvic cavities, including the heart, lungs, stomach, intestines, liver, spleen, kidneys, pancreas, gallbladder, bladder and reproductive organs. Bones, skeletal muscles and skin are generally excluded, because they form the body's framework and movable parts rather than the organs that work largely beyond voluntary control. External organs such as the skin sit on the surface and can be seen and touched directly. The division also has physiological consequences. Visceral structures respond poorly to cutting or burning but strongly to stretching, reduced blood supply and inflammation, which is why visceral pain is often diffuse and hard to localize.

Within the internal organs, a further subdivision separates hollow organs from solid ones. Hollow organs, such as the stomach, intestines, bladder, heart and gallbladder, enclose a cavity through which or into which material moves or is stored. Solid organs, such as the liver, spleen and kidneys, consist mainly of tissue packed with functional cells and vessels. This distinction matters for how each organ is examined, injured and treated.

Another useful grouping is by the functional role of the organ. Sense organs, principally the eyes, ears, nose and tongue, convert physical or chemical stimuli into nerve signals that are transmitted to the nervous system. The skin also contains large numbers of sensory receptors, which is one reason it is classified both as an external organ and as a sensory one. Glandular organs produce secretions. Endocrine glands, such as the thyroid, adrenal glands and pituitary, release hormones directly into the bloodstream, whereas exocrine glands such as the salivary glands release their products through ducts. The pancreas belongs to both groups, since its islet cells secrete insulin into the blood while its acinar tissue releases digestive enzymes into the intestine. These categories overlap, and lymphoid organs such as the thymus, spleen and lymph nodes are usually grouped by their role in immunity rather than by secretion or sensation.

A small subset of organs is classed as vital, meaning that their failure is fatal. Five are usually named: the brain, heart, liver, at least one kidney and at least one lung. Total loss of function in any one of these, with no replacement, causes death. Most other organs can be removed or compensated for. The body can function with one kidney or one lung, and some organs can be replaced by medical devices or transplants, as discussed in the sections on organ transplantation and dialysis. The vital list is therefore a practical classification based on survival, not a judgment about which organs are more important in ordinary physiology.

The total number of human organs is not fixed, because no official body defines what counts as one. The most frequently cited figure is 78, but some lists contain about 91 entries, and counting each bone and tooth separately raises the total to 315. Ligaments, tendons and the peripheral nerves, which number in the trillions, are counted in different ways or not at all. The underlying problem is that the definition of an organ, a collection of tissues joined in a structural unit that serves a common function, is applied with varying strictness. Whether a single tooth, an individual bone, or a whole nerve qualifies depends on the conventions of the person counting. Sex-specific organs, such as the testes or ovaries, add a further source of variation, depending on whether counts are made for one sex or for the species as a whole.

Organs in plants and other organisms

Plant organs and their division of labor

The body plan of a vascular plant rests on two organ systems. The stem, leaf, flower, and fruit make up the shoot system, whereas the roots and their parts make up the root system. The shoot is the aerial part, and the root system lies underground. Because the two systems share work, their organs are often described in terms of function. Roots anchor the plant and absorb water and mineral nutrients. Stems have two roles: they hold leaves, flowers, and fruits in position, and they move water and plant-made materials through the plant. Leaves are the main light-capturing organs, using light, water, and carbon dioxide to make sugars. Flowers, fruits, and seeds relate mainly to reproduction. Flowers often take part in pollination, and fruits protect seeds and can help them disperse. Botanists also sort these into vegetative organs (roots, stems, and leaves) and reproductive organs (flowers, fruits, and seeds).

These organs are not functionally isolated. Roots, stems, and leaves all contain vascular tissue and function as one connected system. Water taken up by roots is conducted upward to the leaves, and the sugars made in leaves are distributed back to the rest of the plant.

Plant tissue systems and organ structure

Plant tissues are grouped into three systems. The dermal system is the protective outer layer. In non-woody species it is the epidermis, often covered by a waxy cuticle, while in woody plants, the periderm replaces the epidermis during secondary growth. The vascular system contains xylem and phloem. Xylem conveys water and minerals up from the roots, while phloem distributes food produced in the leaves. Because the vascular system contains more than one cell type, it is classed as a complex tissue, whereas simple tissues consist of a single cell type. The ground tissue fills most of the plant interior. It serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars. Its cells include parenchyma, collenchyma, and sclerenchyma. Parenchyma is the most common type and supports metabolism and wound repair.

Growth follows from the organ's layout. Growth in length happens at the tips, where apical meristems divide. Apical meristems are located at the tips of stems and roots and enable a plant to extend in length, while lateral meristems facilitate growth in thickness or girth. Because organs are built from these tissue systems, the plant's overall form emerges from organ-level arrangement rather than from individual cells alone.

Animal groups without organs

Sponges illustrate the opposite arrangement. Although they have specialized cells for particular functions, they lack true tissues in which specialized cells are organized into functional groups. Sponge cells are instead loosely associated, and they can differentiate into several cell types. Choanocytes, the collar cells, use flagella to draw water through the body, while pinacocytes form the outer skin. Feeding and gas exchange therefore rely on cell-level processes. Digestion takes place inside the cells, which limits food to particles small enough to be taken in by a single cell. Gas exchange, circulation, and excretion occur by diffusion between cells and the water. Cnidarians come next in complexity. They are the simplest animal group that displays true tissues, although they possess only two tissue layers. Organs are still absent from the group.

Simple and complex organs

An organ can be very simple or highly intricate. A simple example is the nervous system of a flatworm, which concentrates neurons into a pair of anterior ganglia, or a small brain, linked to longitudinal nerve cords. This arrangement shows a few tissue types working together for one function. The human liver is far more complex. It contains hepatocytes arranged in lobules, a network of bile ducts, and a dense blood supply from both the hepatic artery and the portal vein, all enclosed in a connective-tissue capsule. Complexity thus grows with the number of tissue types, the degree of vascular supply, and the number of distinct cell populations that must be coordinated.

Organization across the plant and animal kingdoms therefore ranges from cellular to tissue-based to organ-based designs. The sponge-to-liver sequence shows how these levels of organization accumulate over the course of evolutionary history, a theme discussed in more detail in the section on organ evolution. Plants and animals reached organ-level structure independently, and the functional demands of each lineage shaped the structures that emerged.

Evolution of organs

The gradual appearance of organs

Organs emerged in stages rather than all at once. Sponges arose roughly 500 million years ago, and they are classified into the subkingdom Parazoa, distinguishing them from Eumetazoa, which includes all animals with true tissues. Sponge cells are loosely aggregated within the mesohyl, a gelatinous matrix, so their bodies lack the coordinated tissue structure that organs require. The phylum Cnidaria contains what are considered the first eumetazoans, meaning that they possess true tissues. Tissue-level organization therefore preceded organ-level organization, and the later elaboration of organs in bilaterian animals built on it. The evolution of the mesoderm is an important step towards the establishment of these organ systems.

Some animals without organs nonetheless carry the molecular parts that later organs use. Sponges have no neurons, yet these animals still have the genetic components of synapses. Researchers at the University of California, Santa Barbara interpret this as exaptation, in which evolution co-opts an existing structure for another purpose. On this view, an ancestral toolkit is repurposed when a complex structure such as a nervous system arises.

Homologous and analogous organs

Comparative anatomy distinguishes two patterns. Homologous organs are structurally similar to each other but perform different functions. Vertebrate forelimbs are the standard example: they share a common internal bone plan but serve different roles. Because the shared structure is inherited, homology is interpreted as evidence of common ancestry, and the resulting divergence is termed divergent evolution.

Analogous organs show the opposite pattern. If any two structures or organs perform similar functions but are structurally different, they are analogous structures or analogous organs. Their resemblance arises through a form of evolution where other species evolve independently to achieve a similar type of function. Standard examples include wings of insects and wings of birds, pectoral fins of sharks and flippers of dolphins, and the eye of octopus and eye of a cat. In cladistics this pattern is called homoplasy. Telling the two apart requires examining developmental origin and ancestry, since surface function alone can mislead.

Developmental genes and the origin of new organs

Changes in regulatory genes are a central mechanism for new organs and body plans. Hox genes are homeodomain transcription factors that are involved in the control of a number of key processes in metazoan development, including segmentation and appendage formation. Their conserved function is striking: a Hox gene from a chicken can do its job just as well in a fruit fly, even though the two are separated by hundreds of millions of years of evolution. Their ancestral function in axial patterning at the dawn of Bilateria, together with their lineage-specific functions, most likely contributed to the emergence of morphological novelties in numerous clades.

Gene-regulatory changes can also alter whether a structure forms at all. In flies, the haltere fate is imposed by the Hox gene Ubx, which is expressed in T3. In T2, where Ubx is absent, functional wings develop instead. Studies of pigmentation in Drosophila santomea found that a difference in pigmentation along the fruit fly body plan involved changes in Hox expression as well as in other genes, so that the accumulation of small-effect changes in individual Hox gene targets drives body plan change. Hox-regulated networks also appear older than bilaterians: a Hox–TALE interaction is an ancient regulatory module common to all Eumetazoa which was later co-opted for anterior-posterior patterning in bilaterians.

Increasing specialization and evidence

Organ complexity increased over the course of animal evolution. In bilaterian lineages, the anterior concentration of nervous tissue, sensory organs and the appearance of dedicated feeding structures surrounding the mouth reflects a progressive process known as cephalization. Sensory organs appear in fairly early lineages as well: eye-like sensory organs can already be detected in box jellyfish.

Evidence for organ evolution comes from several sources. Comparative anatomy supplies homologous and analogous structures, while embryological and palaeontological evidence provide independent lines of support. The fossil record is limited for early soft-bodied animals; for example, recent research suggests the earliest sponges were soft and skeleton-free, explaining why their fossils don't appear until much later. Such gaps mean that the sequence of organ origins is reconstructed largely from comparative and genetic data, and those data are central to modern studies of evolution in species and their genetics.

Development and organogenesis

From germ layers to organs

Organs are built during embryonic life. Organogenesis begins at the end of gastrulation and continues until birth. In humans, the main work of organ formation falls in the embryonic period, from weeks 3 through 8, and by week 8, organogenesis is complete.

Each organ traces back to one of three germ layers. The endoderm gives rise to the epithelial linings of the gastrointestinal and respiratory tracts and organs such as the liver, lungs, and pancreas. The mesoderm contributes the blood, heart, kidney, muscles, and connective tissues. The ectoderm forms the epidermis of the skin, the brain, and the nervous system. The germ layer assignments are not always obvious from adult anatomy. For example, the thymus, parathyroid glands, bladder, and urethra also derive from endoderm, which is why the thymus can be discussed alongside the lymphatic system while its origin lies in the gut-forming layer.

Differentiation and the physical shaping of organs

Organs form as cells become progressively more specialized. Each cell type expresses specific sets of genes during differentiation that determine its ultimate cell type. This process depends on cell signaling cascades that act on large populations of cells over time. The shape of an organ also depends on how tissues move. According to a common textbook summary, the germ layers form organs by three processes: folds, splits, and condensation. In vertebrates, for example, the coelom forms from a split of the mesoderm along the somite axis. Accordingly, organogenesis is characterized by rapid and precise movements of the cells within the embryo. The cells that form an organ are often cells from different germ layers that must physically meet to build a single structure.

Inductive signaling and the control of position

Where an organ forms depends on signals passed between neighboring tissues. A key family of regulatory genes is the Hox genes, which are transcription factors. Their main task is structuring the embryo along the body's longitudinal axis. They are activated in a fixed sequence, so that the 3′ most genes are expressed earlier and their expression domain reaches a more anterior limit than those occupying 5′ positions. Because of this arrangement, specific combinations of HOX proteins provide a unique address to defined regions. In the pharyngeal region, the thyroid, thymus, parathyroids, and ultimobranchial bodies derive from the pharyngeal endoderm, and each of the distinct segmented regions has a unique pattern of Hox expression, which conveys crucial positional information.

Signals also pass directly between tissue layers. In the frog Xenopus, zygotic Hox gene expression is first initiated in the non-organizer mesoderm, and this expression then spreads during gastrulation to the overlying prospective neurectoderm. In the chick hindgut, regional tissue mechanics are specified by upstream HOXD13 expression, which patterns the tissue through transforming growth factor β (TGF-β) signaling. Positional identity persists after development as well: expression of HOX is known to persist in many tissues in the postnatal period.

Growth, patterning, and coordination

As the embryo grows, organs develop in parallel and must be placed relative to one another. Organogenesis is important for the concurrent development of multiple organs and organ systems. The arrangement of Hox genes along chromosomes is linked to the order in which they act. Hox genes are sequentially activated in time and space, in a way that reflects their organisation into clusters in the genome. Although this co-linearity of expression of the Hox genes has been conserved during evolution, it is a phenomenon that is still not understood at the molecular level.

Regeneration

Some animals can replace organs or body parts after injury, while mammals regenerate poorly. "Regeneration" is therefore a spectrum, not one universal superpower. The axolotl is among the best-studied examples. It can regenerate missing limbs, tails, and parts of their brain, heart, and lower jaw, and after damage, a scarless regeneration of tissues and organs is observed. Its repair begins when once they're injured, they grow a stub called the blastema from nearby undifferentiated cells. Regeneration often reuses developmental programs: the animals reactivate the same genetic circuits that guided the formation of those structures during embryonic development. The axolotl also has a constraint, since the axolotl can rebuild only if the main body axis is intact.

Planarian flatworms rely on a different cellular source. Their bodies contain large populations of pluripotent stem cells called neoblasts. After a suitable cut, these cells can produce missing tissues, including organs and major body structures. Organ-specific regeneration can also be studied in isolation. In one assay, researchers found that RNAi of the forkhead transcription factor FoxA, which is expressed in a subpopulation of stem cells, specifically inhibited regrowth of the pharynx.

Physiological roles and homeostasis

An organ's job within the body is seldom self-contained. Each organ contributes to homeostasis, the maintenance of a stable internal environment, and it does so through the combined activity of many tissues. Homeostasis refers to the body's ability to physiologically regulate its inner environment to ensure its stability in response to fluctuations in external or internal conditions. The organs most often named in this role are the kidneys, lungs, liver, and pancreas, together with cellular mechanisms such as the regulation of gene expression, and the work is driven by the nervous system and endocrine signalling.

Integrated control loops

Organs are linked into control loops in which a sensing centre, a signal, and a responding organ work in sequence. In textbook models, an effector is any organ or tissue that receives information from the integrating centre and acts to restore balance. Nervous signals act quickly, while hormones act more slowly. The endocrine system coordinates slower, longer-lasting homeostatic responses through hormone secretion. Hormones travel through the bloodstream to modulate the activity of target organs across the body. The pancreas illustrates this pattern: when blood sugar levels become too high, insulin is released from the pancreas.

A well-studied loop links the kidneys, liver, lungs, and adrenal glands. When blood pressure or volume falls, juxtaglomerular cells in the kidneys release renin. Renin converts a protein made by the liver into angiotensin I, and angiotensin-converting enzyme (ACE), primarily in the lungs, converts angiotensin I into angiotensin II. Angiotensin II acts directly on the vessels and stimulates aldosterone release from the adrenal cortex. Aldosterone then promotes Na+ and water reabsorption in the kidneys, increasing blood volume and pressure. The sequence shows how one organ's output becomes another organ's input, and how the loop closes on the original stimulus.

Kidneys: water, salts, and pH

The kidneys hold a central place in this network. The primary function of the kidneys is to maintain water and electrolyte balance for the body by controlling the production and concentration of urine. They also regulate plasma ion concentrations, such as sodium, potassium, chloride, and calcium, by controlling the quantities lost in the urine and the synthesis of calcitriol. Acid–base control is achieved in a similar way: stabilizing blood pH by controlling loss of hydrogen and bicarbonate ions in the urine is among the kidneys' regulatory tasks. The kidneys also secrete hormones that act elsewhere. The kidney hormone EPO stimulates erythrocyte development and promotes adequate O2 transport. Because of this blood flow demand, the kidneys receive about 20% of total cardiac output, which is roughly 1200 mL of blood per minute in normal adults.

Liver: metabolic regulation

The liver's homeostatic role is primarily metabolic. The liver is responsible for metabolizing toxic substances and with signaling from the pancreas maintains carbohydrate metabolism. The liver also helps to regulate lipid metabolism and is the primary site of cholesterol production. Research on steroid hormones describes the liver as a central metabolic organ that plays a crucial role in steroid hormones homeostasis and in the elimination of toxic metabolites. Its output also feeds the systemic loops described above, since the angiotensinogen that renin acts upon circulates from the liver.

Lymphoid organs and immune function

Lymphoid organs contribute to a different kind of homeostasis: maintaining the body's defences. Their structure is usually divided into primary and secondary groups. Primary lymphoid organs are where lymphocytes form and mature. The principal function of the thymus gland is to educate T lymphocytes to differentiate between self and nonself antigens. The thymus is most active early in life, since the thymus reaches maturity before puberty and slowly loses function thereafter. Immature cells arrive from the bone marrow and migrate from bone marrow to the thymus gland where they further mature.

Secondary lymphoid organs are where mature lymphocytes encounter antigens and respond. Secondary lymphoid organs include lymph nodes and spleen, which filter lymph and blood, respectively, and where naïve B and T cells are introduced to antigens. The spleen's filtering role also has a non-immune side: its job is to process old and damaged blood cells and microorganisms. Lymph nodes act as checkpoints along the lymphatic network. They serve as filter stations for the lymph of a certain body region and contain special cells of the immune system, the lymphocytes, which fight infections attacking the body. Hence, the lymph nodes clean the lymph and free it from pathogens and infectious bodies.

Classification is not entirely uniform. Some educational sources place the spleen among the primary organs, with the primary lymphoid organs being the bone marrow, spleen, and thymus gland. Most specialist sources instead treat the spleen as secondary, and the division is itself debated: one review notes that lymphoid organs are generally classified in a hierarchy with primary lymphoid organs such as the thymus and bone marrow and that the boundaries between groups are not always clear-cut.

Organ failure, transplantation, and clinical practice

When an organ loses the ability to perform its function, clinicians first establish the diagnosis. Imaging such as ultrasonography, computed tomography, and magnetic resonance imaging shows structural change in the organ, while blood and urine tests measure its output. In the kidney, for example, serum creatinine and estimated glomerular filtration rate indicate how well filtration is working, and liver enzymes and bilirubin reflect hepatic injury. Tissue sampling by biopsy often provides the definitive answer, because histological examination can distinguish inflammation, fibrosis, and neoplasia that imaging alone cannot separate. Many of these findings are interpreted within the framework of pathology.

When function cannot be restored by medication or surgery, replacement by transplantation becomes the main option. Transplant practice uses several categories of graft source, defined by the genetic relationship between donor and recipient. An autograft is taken from the patient's own body. A syngeneic graft comes from a genetically identical individual, such as an identical twin. An allograft, or allogeneic transplant, comes from a genetically non-identical donor of the same species, and it is the usual form of solid-organ transplantation. A xenograft comes from another species, a category that remains experimental for whole organs. Allotransplantation is widely regarded as the standard treatment for end-stage disease of the kidney, liver, lung, and heart, and it is generally regarded as a last resort for chronic organ failure when other treatments no longer work.

The central obstacle after transplantation is rejection. Donor tissue carries alloantigens, mainly human leukocyte antigen (HLA) molecules, that the recipient's immune system recognizes as foreign. Recipient T cells can detect donor HLA molecules directly on the graft's dendritic cells, or indirectly through recipient antigen-presenting cells that process donor HLA fragments. Clinically, rejection is grouped into several forms. Hyperacute rejection occurs within minutes to hours when preformed antibodies, often anti-ABO antibodies, attack the graft vasculature. Acute rejection, which is largely driven by T cells and can also involve antibodies, typically appears within weeks of surgery. Chronic rejection develops over months to years and is marked by fibrosis. Matching donor and recipient HLA types reduces the risk, but because other genetic differences can still provoke strong responses, tissue matching has contributed less to outcomes than advances in drug therapy have.

For this reason, nearly all recipients of solid organs other than identical-twin grafts must take immunosuppressive drugs for life. Calcineurin inhibitors, antiproliferative agents, and corticosteroids are the core of these regimens, supplemented by antibodies that target T cells or cytokines. Immunosuppression lowers the risk of rejection but exposes patients to infection, malignancy, and drug toxicity, especially nephrotoxicity. Adherence is critical: inadequate or interrupted dosing is a recognized cause of late rejection, and monitoring of drug levels and graft function is therefore continuous.

Not every organ failure leads to transplantation, and the period before a donor organ becomes available can be long. Substitute therapies fill this gap. dialysis replaces the excretory and fluid-regulating functions of the kidney through hemodialysis, in which blood is cleared across an artificial membrane, or peritoneal dialysis, which uses the abdominal lining as the exchange surface. It is the standard treatment for patients with advanced chronic-kidney-disease who are not transplant candidates or who are awaiting a graft. Other substitutes include extracorporeal membrane oxygenation for temporary cardiopulmonary support and ventricular assist devices for heart failure. Liver support systems that remove toxins from the blood are used as bridges to transplantation or to recovery, though none fully replaces hepatic metabolism.

History of organ study

Galen and the ancient anatomical tradition

Galen, a physician working in the Roman Empire, shaped the study of organs for more than a millennium. The voluminous writings of his school drew on the advances of the classical Greeks, including Hippocrates and Aristotle. The school's influence on medicine and physiology lasted some 1,300 years, up to the European Renaissance. Anatomy was central to his system: he used dissection to argue for the structure and purposes of the body, and he tied those arguments to diagnosis, prognosis, and treatment (He made anatomy central to medical reasoning and used dissection, especially of animals, to argue for the structure and purposes of the body.).

The limits of his evidence were a constant problem. Human dissection was forbidden in ancient Rome, so his work relied on other species. His anatomical reports were based mainly on the dissection of Barbary apes. Later anatomists found that his descriptions matched the bodies of those animals more closely than human ones, and this mismatch became one of the main sources of error that persisted in medical teaching.

Vesalius and the anatomical atlas of 1543

The challenge to Galen came from the anatomy classroom at Padua. Andreas Vesalius lectured on Galen's authority as was customary, but in his dissections he kept noticing discrepancies (In dissections he performed, however, he began to notice discrepancies between what he observed and what Galen had described.). From 1539 he gained access to the bodies of executed criminals, and repeated dissections convinced him that Galen had largely described the anatomy of apes, pigs, and dogs (Beginning in 1539, corpses of executed criminals were made available to him. Repeated dissections made it increasingly apparent to Vesalius that Galen's descriptions were erroneous and that Galen had based his descriptions on the anatomy of animals, primarily apes, pigs, and dogs.)

His atlas, De humani corporis fabrica, was printed at Basel in 1543. In De fabrica, Vesalius corrected more than two hundred errors of Galenic anatomy and described certain features that either were previously unknown or had been described only partially. Its detailed illustrations, which were a key part of its impact, were not free of error themselves. The 1543 plates of the placenta followed Galenic models, and the illustrations are corrected in the 1555 edition. Vesalius also kept some of Galen's ideas, such as the belief that veins and arteries carried different kinds of blood (Even with his improvements, however, Vesalius clung to some of Galen's errors, such as the idea that veins and arteries carried different respective types of blood.). Historians often divide anatomy into a pre-Vesalian and a post-Vesalian period (The history of anatomy is traditionally divided into two periods: pre-Vesalian and post-Vesalian.).

Bichat and tissues as a level of organization

Before the nineteenth century, anatomy was mainly organized around whole organs. Marie-François-Xavier Bichat (1771–1802) changed this. His major contribution to medical science was the identification and systematic classification of 21 distinct tissue types, a revolutionary departure from the traditional organ-based view of human anatomy. He argued that tissues, not organs, were the basic units of biological organization, each with its own properties, functions, and vulnerabilities to disease (Bichat proposed that tissues—not organs—constituted the true fundamental units of biological organization, each with its own characteristic properties, physiological functions, and pathological susceptibilities. This conceptual shift redefined the way scientists understood disease, emphasizing that illnesses could originate not in entire organs but in specific tissue types.). His Anatomie générale of 1801 became the most widely cited statement of this view (The Anatomie générale appliquée à la physiologie et à la médecine (1801) is the most well-known work of Bichat.).

Bichat's tissue-level approach is often credited with founding histology, although he worked without the microscope. His approach demonstrated that a rational and systematic understanding of the body required an intermediate scale of analysis—tissue-level anatomy—which would eventually lead to the microscope-based histology of the 19th century. His scheme was not without critics. Contemporaries such as Magendie and Comte challenged the fit between his anatomical classification of tissues and the physiological functions he assigned to them.

From tissues to cells and molecules

The next step moved the focus below the tissue. Cell theory, developed in the late 1830s, and the microscope-based work that followed made the cell the basic structural unit. Rudolf Virchow's cellular pathology, set out in the 1850s, applied this to disease, holding that disease processes arise in the cells of an organ, and that new cells arise only from existing cells. The next generations move from histology (study of tissues) to cytology (study of cells).

In the twentieth century, research on organs passed further, to molecular biology. Organs came to be understood through their component cell types, the signalling pathways that connect those cells, and the regulation of gene activity during development. Study of how organs form and how their cells specialize, through genetics and gene expression, replaced purely descriptive anatomy as the main means of explaining organ structure and disease. Clinical practice still draws on this older anatomical framework: pathology and surgery continue to describe disease by organ and tissue, while basic research now analyses those same organs at the level of the cell and the molecule.

Tissue engineering and organ regeneration research

Organoids and engineered tissues

Organoids are small, self-organizing three-dimensional cultures built from stem cells. Because such aggregates can give rise to several organ-specific cell types and keep cells in contact with one another and with their surrounding matrix, they reproduce some key functions and structures of real human organs in vitro. Two cell sources are used: pluripotent stem cells and organ-specific adult stem cells (Organoids can be derived from two types of stem cells). The most immediate application is preclinical drug testing. Liver, kidney, and heart organoids are used to model toxic responses, and personalized, disease-specific organoids are used to improve the prediction of drug efficacy (Our study explores current methods used to model toxic responses in different organs-such as the liver, kidney, and heart). Patient-derived tumor organoids can be used in high-throughput drug screening (Patient-derived organoids (PDOs) can be obtained by the 3D culture of a patient's tumor tissue in vitro. PDOs highly summarize the characteristics of the tumor source). In cystic fibrosis, intestinal organoids from patients have been used to test CFTR modulators in a mutation-specific way. Regulators' growing reluctance to rely on animal testing has encouraged this shift, and organoids are being considered as tools that could reduce or replace animal models. Neural organoids remain constrained by biological variability, the absence of universal standards, and ethical questions (Complex neural organoids show great promise but continue to face significant challenges, including biological variability, a lack of universal standards, and ethical concerns.).

Decellularized organ scaffolds

A second strategy starts from a donor organ rather than from stem cells. Removing all cells from an organ leaves an extracellular matrix framework that preserves the three-dimensional architecture and the vascular channels (Decellularization is a process by which organs are denuded of cells, leaving behind the entire blueprint in form of the architectural scaffold). Detergents, enzymes, chemicals, and mechanical forces, used alone or in combination, have been applied to lung, liver, kidney, heart, and pancreas tissue. Rat kidney scaffolds that had been cryopreserved kept their extracellular matrix, and mesenchymal stem cells repopulated them as readily as scaffolds from fresh kidneys (The light microscopy exploiting different microscopic stains as well as immuno-histochemistry and Scanning electron microscopy (SEM) demonstrated that ECM framework is well retained following kidney cryopreservation.). Recellularization is the harder step. Whole-organ repopulation is difficult because the scaffold is complex, cells must reach every region, and different cell types are required (During the recellularization process, whole organ repopulation is difficult due to the complexity of the scaffold). Using a bioreactor with low fluid shear instead of static culture increased the number of cells in decellularized mouse lungs and reduced cell death. Autologous cells are preferred because they avoid immune rejection, although stem cells at different stages of differentiation are also used (The use of autologous cells can avoid the problem of immune system rejection.). A review has noted that no fully functional engineered organ has yet been reported after transplantation in vivo, and that graft survival has not lasted beyond a few weeks in the experiments to date.

Xenotransplantation and bioprinting

Xenotransplantation addresses the same organ shortage by using organs from animals, mainly genetically modified pigs. Organs from pigs are engineered to remove antigens that humans naturally have antibodies against and to express human proteins that regulate complement and coagulation (genetic manipulation of the organ-source pigs has largely been directed to two major aims). The number of edits differs between developers: one trial product comes from a source pig with 10 gene edits, while another uses a pig with 69 edits (The United Therapeutics pig kidney, which the company is calling UKidney™, comes from a 10 gene-edited source pig while eGenesis uses a 69 gene-edited source pig.). Conventional immunosuppressive drugs do not block the adaptive immune response to pig cells, whereas blockade of the CD40–CD154 costimulation pathway has been effective in experiments (Conventional (FDA-approved) immunosuppressive therapy is unsuccessful in preventing an adaptive immune response to pig cells). The FDA cleared the first U.S. clinical trials of pig kidneys in early 2025. In one first-in-human case reported in 2026, a patient lived 271 days with a pig kidney without dialysis before receiving a human kidney (Andrews, who was 66 years old at the time of xenotransplantation, successfully lived with the genetically edited kidney for 271 days-during which he did not need dialysis-before receiving a human kidney from a donor.). 3D bioprinting, which deposits cells and biomaterials layer by layer to form tissue shapes, is a parallel approach. It remains largely confined to simple or thin structures and has not yet produced transplantable solid organs.

Vascularization as the central obstacle

The most persistent problem is blood supply. Whole-organ constructs such as kidney, heart, lung, and liver, however require immediate vascular supply and thus represent a challenge for regeneration. Cells deeper than roughly 100–200 micrometers from a capillary cannot obtain enough oxygen and nutrients by diffusion alone, which is why thick engineered tissues die from the inside out. Researchers have tried to address this by perfusing endothelial cells through the native vascular tree of a decellularized organ before implantation (The decellularized organ can be pre-treated prior to perfusion of cultured endothelial cells in order to enhance the attachment of cultured cell populations to the decellularized organ.). Another approach joins the scaffold's vessels to the recipient's circulation at two or more points, so that the engineered organ can mature in place (establishing at least two vascular connections of the scaffold vasculature and the subject vasculature). Building functional, hierarchical networks of capillaries that match native organ microvasculature, and sustaining them after transplantation, remains the central open challenge in organ engineering.

References

1. Organbritannica.com
2. organ - Studentskids.britannica.com
3. ORGAN Definition & Meaning - Merriam-Webstermerriam-webster.com
4. Organ - Etymology, Origin & Meaningetymonline.com
5. Organ (biology)en.wikipedia.org
6. Organ - Definition, Types and Examplesbiologydictionary.net
7. organum - Wiktionary, the free dictionaryen.wiktionary.org
8. Basic Tissue Types - Histology at SIUhistology.siu.edu
9. Stromalink.springer.com
10. Histology at SIU, Introductionhistology.siu.edu
11. Types of tissue: Structure and functionkenhub.com
12. What Are the Four Types of Tissues in the Body? - ScienceInsightsscienceinsights.org
13. Gastrointestinal wallen.wikipedia.org
14. Histology at SIU, gastrointestinal systemhistology.siu.edu
15. Gastrointestinal System Ihistologylab.ctl.columbia.edu
16. Submucosa - an overviewsciencedirect.com
17. Organ systemen.wikipedia.org
18. List of systems of the human bodyen.wikipedia.org
19. Human body systems: Overview, anatomy, functionskenhub.com
20. [FREE] Which of the following organs belong to more than one organ system? Select all that apply. - Male - brainly.combrainly.com
21. 5.1: Organs and Systems of the Human Organism - Medicine LibreTextsmed.libretexts.org
22. Immune System - an overviewsciencedirect.com
23. The total mass, number, and distribution of immune cells in the human bodypnas.org
24. How many organs are in the human body?livescience.com
25. How Many Organs Are There in the Body?medicinenet.com
26. List of organs of the human bodyen.wikipedia.org
27. Organs in the Bodystudy.com
28. Medical Definition of Viscerarxlist.com
29. Visceral painen.wikipedia.org
30. What Is Viscera? Your Body’s Internal Organs Explained - Biology Insightsbiologyinsights.com
31. Splanchnologyen.wikipedia.org
32. What are the 78 Organs In the Human Body? - GeeksforGeeksgeeksforgeeks.org
33. Plant Organs - an overviewsciencedirect.com
34. What are the basic organs of a plant? Roots, stems, leaves, flowers, fruits, and seedsplant-note.com
35. How Roots, Stems, and Leaves Work Togetherplant-note.com
36. General Biology Study Guide: Vascular Plant Structure & Growthpearson.com
37. Plant Tissues and Organscourses.lumenlearning.com
38. 30.1 The Plant Bodypressbooks.online.ucf.edu
39. 20.2: Sponges and Cnidarians - Biology LibreTextsbio.libretexts.org
40. 30.1: The Plant Body - Biology LibreTextsbio.libretexts.org
41. 10.2 The Plant Bodylouis.pressbooks.pub
42. Organization of Tissue Systems in Plant Organsblogs.ubc.ca
43. 7.1: Plant Anatomy - Biology LibreTextsbio.libretexts.org
44. Phylum Porifera - Advancedshapeoflife.org
45. Phylum Poriferacourses.lumenlearning.com
46. Sponges - Advancedflexbooks.ck12.org
47. Do Sponges Have Organs? Explaining Their Unique Structure - Biology Insightsbiologyinsights.com
48. Invertebrate Evolution - Advancedflexbooks.ck12.org
49. Simple sponges provide clues to origin of nervous systemnationalgeographic.com
50. Notes on Analogous Organs and homologous organunacademy.com
51. 90% found this document useful (10 votes)scribd.com
52. Giving examples of homologous and analogous organs class 11 biology CBSEvedantu.com
53. Careers360 Logomedicine.careers360.com
54. Hox, homology, and parsimony: An organismal perspective - ScienceDirectsciencedirect.com
55. The Evolution of Body Plans: A TALE of two proteinsnature.com
56. Evolution: Remodelling Animal Body Plans, Gene by Gene - ScienceDirectsciencedirect.com
57. Body plan evolution not as simple as once believedsciencedaily.com
58. Frontiersfrontiersin.org
59. No Sponge In Human Family Tree: Sponges Descended From Unique Ancestorsciencedaily.com
60. Homologous organs and analogous organs..slideshare.net
61. Homologous vs Analogous organs - ScienceQuerysciencequery.com
62. Organogenesisen.wikipedia.org
63. Embryology, Weeks 6-8 - StatPearls - NCBI Bookshelf - NIHncbi.nlm.nih.gov
64. Organogenesis: Definition, Stages & Embryology Explainedvedantu.com
65. Organogenesisbionity.com
66. 43.7: Organogenesis and Vertebrate Formation - Biology LibreTextsbio.libretexts.org
67. Frontiersfrontiersin.org
68. YY1 Acts as a Transcriptional Activator of Hoxa5 Gene Expression in Mouse Organogenesisncbi.nlm.nih.gov
69. Developmental Signals - Homeobox - Embryologyembryology.med.unsw.edu.au
70. Vertical Signalling Involves Transmission of Hox Information from Gastrula Mesoderm to Neurectodermncbi.nlm.nih.gov
71. Hox gene activity directs physical forces to differentially shape chick small and large intestinal epithelia: Developmental Cellcell.com
72. Organ- and Site-Specific HOX Gene Expression in Stromal Cellsintechopen.com
73. Developmental regulation of the Hox genes during axial morphogenesis in the mousejournals.biologists.com
74. 9 Animals That Can Regrow Body Parts—Some Rebuild Whole Bodiesanimalso.com
75. Pictures: 5 Animals That Regrow Body Partsnationalgeographic.com
76. Axolotl: A resourceful vertebrate model for regeneration and beyond - Bölük - 2022 - Developmental Dynamics - Wiley Online Libraryanatomypubs.onlinelibrary.wiley.com
77. How These 4 Animals Can Regenerate and Why Humans Can’tdiscovermagazine.com
78. From stealing genes to regrowing limbs, how life finds a way to survive and thrivescience.org
79. Selective amputation of the pharynx identifies a FoxA-dependent regeneration program in planariancbi.nlm.nih.gov
80. Organogenesis - Laboratory Noteslaboratorynotes.com
81. 43.6A: Organogenesis - Biology LibreTextsbio.libretexts.org
82. Research Organism Superheroes: Axolotls - MDI Biological Laboratorymdibl.org
83. CH103 - Chapter 8: Homeostasis and Cellular Function - Chemistrywou.edu
84. Homeostasisen.wikipedia.org
85. Homeostatic Regulation Across Organ Systemsfiveable.me
86. RF electrodes on multiple flexible wires for renal nerve ablationimage-ppubs.uspto.gov
87. The Urinary System and Homeostasiscourses.lumenlearning.com
88. Liver and Steroid Hormones—Can a Touch of p53 Make a Difference?ncbi.nlm.nih.gov
89. Lymphoid Organ - an overviewsciencedirect.com
90. Anatomy of the Lymphatic and Immune Systemscourses.lumenlearning.com
91. Lymphoid Organ - an overviewsciencedirect.com
92. Structure and function of the lymphatic systemgpoh.de
93. Plasticity and heterogeneity of lymphoid organs: What are the criteria to call a lymphoid organ primary, secondary or tertiary? - ScienceDirectsciencedirect.com
94. Editorial: Systemic regulation of organ homeostasis and implications of hormones and immunity, Volume IIncbi.nlm.nih.gov
95. Tissues - Immune System - MCAT Contentjackwestin.com
96. Transplant rejection: T-helper cell paradigmimmunology.org
97. Responses to alloantigens and transplant rejection - Immunobiology - NCBI Bookshelfncbi.nlm.nih.gov
98. The Biology and Molecular Basis of Organ Transplant Rejection - PubMedpubmed.ncbi.nlm.nih.gov
99. Allotransplantation - an overviewsciencedirect.com
100. Galen and the beginnings of Western physiologyjournals.physiology.org
101. Galen in Medical Historyhistoriamedica.org
102. Galenen.wikipedia.org
103. Andreas Vesaliusebsco.com
104. De Humani Corporis Fabrica Libri Septemen.wikipedia.org
105. De humani corporis fabrica libri septem.collections.library.utoronto.ca
106. (PDF) Xavier Bichat (1771-1802): Architect of Modern Histologyresearchgate.net
107. Cancer's conceptions of Marie François Xavier Bichat ( ...jbuon.com
108. The Physician is Only Nature’s Assistantmcgill.ca
109. Evaluating the Efficacy of Neoadjuvant Chemotherapy Drugs Based on Organoid Technologyclinicaltrials.gov
110. Organoids Explained: How They’re Grown and Used in Disease Research and Drug Testingnews.cuanschutz.edu
111. Stem Cell-Derived Organoids as a Next-Generation Platform for Drug Toxicity and Efficacy Testing: Toward Replacing Animal Models in Regulatory Science - PubMedpubmed.ncbi.nlm.nih.gov
112. Novel organoid model in drug screening: Past, present, and future - ScienceDirectsciencedirect.com
113. Preparation of Organ Scaffolds by Decellularization of Pancreas and Re-functionalizationbiorxiv.org
114. Decellularized scaffold of cryopreserved rat kidney retains its recellularization potentialncbi.nlm.nih.gov
115. Frontiersfrontiersin.org
116. Easy-to-Assembly System for Decellularization and Recellularization of Liver Grafts in a Bioreactorncbi.nlm.nih.gov
117. Pig kidney xenotransplantation: Progress toward clinical trialspubmed.ncbi.nlm.nih.gov
118. FDA greenlights first clinical trials for genetically modified pig kidney transplants in humanskidneyfund.org
119. Pig kidney xenotransplantation offers a bridge for patients awaiting human donor kidneysnews-medical.net
120. Methods and compositions for organ decellularizationimage-ppubs.uspto.gov
121. Organ/tissue decellularization, framework maintenance and recellularizationimage-ppubs.uspto.gov
122. Human organoids: Fit for drug discovery?: Stem Cell Reportscell.com
123. Stem Cells and Organoids: A Paradigm Shift in Preclinical Models Toward Personalized Medicine - PMCpmc.ncbi.nlm.nih.gov
124. Recellularization of Decellularized Lung Scaffolds Is Enhanced by Dynamic Suspension Culturencbi.nlm.nih.gov
125. Xenotransplant Clinical Trial Update for Donate Life Monthkidney.org
126. Xenotransplantation Milestones in the U.S.kidney.org
127. First Gene-Edited Pig Kidney Transplant Clinical Trial Begins at NYU Langone Healthnyulangone.org