An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. In the hierarchy of life, organs lie between tissue and organ systems. A tissue is an organizational level between cells and organs. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. 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.
A given organ’s tissues can be broadly categorized as parenchyma, the tissue peculiar to (or at least archetypal of) the organ and that does the organ’s specialized job, and stroma, the tissues with supportive, structural, connective, or ancillary functions. For example, in a gland, the tissue that makes the hormones is the parenchyma, whereas the stroma includes the nerves that innervate the parenchyma, the blood vessels that oxygenate and nourish it and carry away its metabolic wastes, and the connective tissues that provide a suitable place for it to be situated and anchored. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. Functionally related organs often cooperate to form whole organ systems. Organs exist in most multicellular organisms. In single-celled organisms such as bacteria, the functional analogue of an organ is known as an organelle. In plants, there are three main organs. A hollow organ is an internal organ that forms a hollow tube, or pouch such as the stomach, intestine, or bladder.
In the study of anatomy, the term viscus refers to an internal organ. Viscera is the plural form.
The number of organs in any organism depends on which precise definition of the term one uses. By one widely used definition, 79 organs have been identified in the human body.
Plants
See also: Plant morphology, Plant anatomy, and Plant physiologyThe flower is the angiosperm’s reproductive organ. This Hibiscus flower is hermaphroditic, and it contains stamen and pistils.Strobilus of Equisetum telmateia
The study of plant organs is covered in plant morphology. Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs include roots, stems, and leaves. The reproductive organs are variable. In flowering plants, they are represented by the flower, seed and fruit. In conifers, the organ that bears the reproductive structures is called a cone. In other divisions (phyla) of plants, the reproductive organs are called strobili, in Lycopodiophyta, or simply gametophores in mosses. Common organ system designations in plants include the differentiation of shoot and root. All parts of the plant above ground (in non-epiphytes), including the functionally distinct leaf and flower organs, may be classified together as the shoot organ system.
The vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis, while the reproductive organs are essential in reproduction. However, if there is asexual vegetative reproduction, the vegetative organs are those that create the new generation of plants (see clonal colony).
Animals
See also: List of organs of the human body and Biological systemThe liver and gallbladder of a sheep
Non-placozoan animals such as humans have a variety of organ systems. These specific systems are also widely studied in human anatomy. The functions of these organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the musculoskeletal system because of the relationship between the muscular and skeletal systems.
- Cardiovascular system: pumping and channeling blood to and from the body and lungs with heart, blood and blood vessels.
- Digestive system: digestion and processing food with salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, colon, rectum and anus.
- Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal body or pineal gland, thyroid, parathyroids and adrenals, i.e., adrenal glands.
- Excretory system: kidneys, ureters, bladder and urethra involved in fluid balance, electrolyte balance and excretion of urine.
- Lymphatic system: structures involved in the transfer of lymph between tissues and the blood stream, the lymph and the nodes and vessels that transport it including the Immune system: defending against disease-causing agents with leukocytes, tonsils, adenoids, thymus and spleen.
- Integumentary system: skin, hair and nails of mammals. Also scales of fish, reptiles, and birds, and feathers of birds.
- Muscular system: movement with muscles.
- Nervous system: collecting, transferring and processing information with brain, spinal cord and nerves.
- Reproductive system: the sex organs, such as ovaries, fallopian tubes, uterus, vulva, vagina, testes, vas deferens, seminal vesicles, prostate and penis.
- Respiratory system: the organs used for breathing, the pharynx, larynx, trachea, bronchi, lungs and diaphragm.
- Skeletal system: structural support and protection with bones, cartilage, ligaments and tendons.
Origin and evolution
Relationship of major animal lineages with indication of how long ago these animals shared a common ancestor. On the left, important organs are shown, which allows us to determine how long ago these may have evolved.
The organ level of organisation in animals can be first detected in flatworms and the more derived phyla. The less-advanced taxa (like Placozoa, Sponges and Radiata) do not show consolidation of their tissues into organs.
More complex animals are composed of different organs, which have evolved over time. For example, the liver evolved in the stem vertebrates more than 500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, and worms more than 600 million years ago.
Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently. An outstanding model for this kind of research is the placenta, which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa. Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues, the acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types.
Society and culture
Many societies have a system for organ donation, in which a living or deceased donor’s organ are transplanted into a person with a failing organ. The transplantation of larger solid organs often requires immunosuppression to prevent organ rejection or graft-versus-host disease.
There is considerable interest throughout the world in creating laboratory-grown or artificial organs.
History
The English word “organ” dates back to the twelfth century and refers to any musical instrument. By the late 14th century, the musical term’s meaning had narrowed to refer specifically to the keyboard-based instrument. At the same time, a second meaning arose, in reference to a “body part adapted to a certain function”
Plant organs are made from tissue composed of different types of tissue. The three tissue types are ground, vascular, and dermal. When three or more organs are present, it is called an organ system.
The adjective visceral, also splanchnic, is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Roman pagan priests like the haruspices or the augurs in order to divine the future by their shape, dimensions or other factors. This practice remains an important ritual in some remote, tribal societies.
The term “visceral” is contrasted with the term “parietal“, meaning “of or relating to the wall of a body part, organ or cavity” The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides.
Antiquity
Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal), and to describe more abstract “parts” of an interconnected whole (e.g. his logical works, taken as a whole, are referred to as the “organon“).
Some alchemists (e.g. Paracelsus) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows:
| Planet | Organ |
| Sun | Heart |
| Moon | Brain |
| Mercury | Lungs |
| Venus | Kidneys |
| Mars | Gall bladder |
| Jupiter | Liver |
| Saturn | Spleen |
Modern times
The variations in natural language definitions of what constitutes an organ, their degree of precision, and the variations in how they map to ontologies and taxonomies in information science (for example, to count how many organs exist in a typical human body) are topics explored by writer Carl Engelking of Discover magazine in 2017 as he analyzed the science journalism coverage of the evolving scientific understanding of the mesentery. He explored a challenge now faced by anatomists: as human understanding of ontology generally (that is, how things are defined, and how the relationship of one thing to another is defined) meets applied ontology and ontology engineering, unification of varying views is in higher demand. However, such unification always faces epistemologic frontiers, as humans can only declare computer ontologies with certainty and finality to the extent that their own cognitive taxonomy (that is, science’s understanding of the universe) is certain and final. For example, the fact that the tissues of the mesentery are continuous was something that was simply not known for sure until it was demonstrated with microscopy. Because humans cannot predict all future scientific discoveries, they cannot build a unified ontology that is totally certain and will never again change. However, one of the points made by an anatomist interviewed by Engelking is that, finality aside, much more could be done even now to represent existing human knowledge more clearly for computing purposes.
Organ Procedures
Beginning in the 20th century transplants began to occur as scientists knew more about the anatomy of organs. These came later in time as procedures were often dangerous and difficult. Both the source and method of obtaining the organ to transplant are major ethical issues to consider, and because organs as resources for transplant are always more limited than demand for them, various notions of justice, including distributive justice, are developed in the ethical analysis. This situation continues as long as transplantation relies upon organ donors rather than technological innovation, testing, and industrial manufacturing. https://en.wikipedia.org/wiki/Organ_(biology)
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Sex organs
A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, are called the primary sex organs. All others are called secondary sex organs, divided between the external sex organs—the genitals or genitalia, visible at birth in both sexes—and the internal sex organs.
Mosses, ferns, and some similar plants have gametangia for reproductive organs, which are part of the gametophyte. The flowers of flowering plants produce pollen and egg cells, but the sex organs themselves are inside the gametophytes within the pollen and the ovule. Coniferous plants likewise produce their sexually reproductive structures within the gametophytes contained within the cones and pollen. The cones and pollen are not themselves sexual organs.
Terminology
The primary sex organs are the gonads, a pair of sex organs, which diverge into testes following male development or into ovaries following female development. As primary sex organs, gonads generate reproductive gametes containing inheritable DNA. They also produce most of the primary hormones that affect sexual development, and regulate other sexual organs and sexually differentiated behaviors.
Secondary sex organs are the rest of the reproductive system, whether internal or external. The Latin term genitalia, sometimes anglicized as genitals, is used to describe the externally visible sex organs: in male mammals, the penis and scrotum; and in female mammals, the vulva and its organs.
In general zoology, given the great variety in organs, physiologies, and behaviors involved in copulation, male genitalia are more strictly defined as “all male structures that are inserted in the female or that hold her near her gonopore during sperm transfer”; female genitalia are defined as “those parts of the female reproductive tract that make direct contact with male genitalia or male products (sperm, spermatophores) during or immediately after copulation”
Mammals
External and internal organs
Further information: Mammalian reproductive system and Human reproductive system
The visible portion of the mammalian genitals for males consists of the scrotum and penis; for females, it consists of the vulva (labia, clitoris, etc.) and vagina.
In placental mammals, females have two genital orifices, the vagina and urethra, while males have only one, the urethra. Male and female genitals have many nerve endings, resulting in pleasurable and highly sensitive touch. In most human societies, particularly in conservative ones, exposure of the genitals is considered a public indecency.
In humans, sex organs include:
| Male | Female |
|---|---|
| Bulbourethral glandsEpididymisPenisForeskinFrenulum of penisGlans penisProstateScrotumSeminal vesiclesTesticlesAn image of human male external sex organs (shaved pubic hair) | Bartholin’s glandsFallopian tubesOvariesSkene’s glandUterusCervixVaginaVulvaHymenClitorisClitoral frenulumClitoral glans (glans clitoridis)Clitoral hoodLabiaLabia majoraLabia minoraFrenulum of labia minoraAn image of human female external sex organs (shaved pubic hair) |
Development
Main article: Development of the reproductive system
In typical prenatal development, sex organs originate from a common primordium during early gestation and differentiate into male or female sexes. The SRY gene, usually located on the Y chromosome and encoding the testis determining factor, determines the direction of this differentiation. The absence of it allows the gonads to continue to develop into ovaries.
Thereafter, the development of the internal, and external reproductive organs is determined by hormones produced by certain fetal gonads (ovaries or testes) and the cells’ response to them. The initial appearance of the fetal genitalia looks basically feminine: a pair of “urogenital folds” with a small protuberance in the middle, and the urethra behind the protuberance. If the fetus has testes, and if the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings grow, wrap around the penis, and fuse in the midline to form the penile urethra.
Each sex organ in one sex has a homologous counterpart in the other one. See a list of homologues of the human reproductive system. In a larger perspective, the whole process of sexual differentiation also includes development of secondary sexual characteristics such as patterns of pubic and facial hair and female breasts that emerge at puberty.
Because of the strong sexual selection affecting the structure and function of genitalia, they form an organ system that evolves rapidly. A great variety of genital form and function may therefore be found among animals.
Intersex encompasses the development of genitalia and/or sex-specific cerebral neural structures either somewhere between typical male and female, or at the opposite of the typically expected karyotype, for instance, in a complete androgen insensitivity syndrome (CAIS) individual of genetically male Y chromosome karyotype but with a congenitally complete female habitus (physical outer body) and typical female neural brain structures.
In human societies, once a child is born, parents may be given the authority to decide whether or not to modify the child’s genitalia, and if so what sex to assign the child. Some scenarios have medical staff choose, either by the parents’ or their own authority. If it is decided that the genitalia should be modified, and the result is incongruent with the child’s future gender identity, the child may begin to show symptoms of gender dysphoria, which can lead them to a life of discomfort until they are able to remedy the issue. Modifying the genitalia of intersex children is broadly considered to be a violation of the child’s human rights.
Other animals
In many other animals a single posterior orifice, called the cloaca, serves as the only opening for the reproductive, digestive, and urinary tracts (if present). All amphibians, birds, reptiles, some fish, and a few mammals (monotremes, tenrecs, golden moles, and marsupial moles) have this orifice, from which they excrete both urine and feces in addition to serving reproductive functions. Excretory systems with analogous purpose in certain invertebrates are also sometimes referred to as cloacae.
Insects
Main article: Insect reproductive systemThe female genitalia of Lepidoptera
The organs concerned with insect mating and the deposition of eggs are known collectively as the external genitalia, although they may be largely internal; their components are very diverse in form.
Slugs and snails
Main article: Reproductive system of gastropods
The reproductive system of gastropods (slugs and snails) varies greatly from one group to another.
Planaria
Main article: Reproductive system of planarians
Planaria are flat worms widely used in biological research. There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Each planarian transports its excretion to the other planarian, giving and receiving sperm.
Plants
Main articles: Alternation of generations and Plant reproductive morphology
The life cycle of land plants involves alternation of generations between a sporophyte and a haploid gametophyte. The gametophyte produces sperm or egg cells by mitosis. The sporophyte produces spores by meiosis which in turn develop into gametophytes. Any sex organs that are produced by the plant will develop on the gametophyte. The seed plants, which include conifers and flowering plants have small gametophytes that develop inside the pollen grains (male) and the ovule (female).
Flowering plants
Sexual reproduction in flowering plants involves the union of the male and female germ cells, sperm and egg cells respectively. Pollen is produced in stamens, and is carried to the pistil, which has the ovary at its base where fertilization can take place. Within each pollen grain is a male gametophyte which consists of only three cells. In most flowering plants the female gametophyte within the ovule consists of only seven cells. Thus there are no sex organs as such.