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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

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 glands Epididymis Penis Foreskin Frenulum of penis Glans penis Prostate Scrotum Seminal vesicles Testicles	Bartholin's glands Fallopian tubes Ovaries Skene's gland Uterus Cervix Vagina Vulva Hymen Clitoris Clitoral frenulum Clitoral glans (glans clitoridis) Clitoral hood Labia Labia majora Labia minora Frenulum of labia minora

Development

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. Furthermore, as the largest sexual organ, the brain also undergoes differentiation in sex-specific neural structures, affecting, but not solely nor absolutely determining, typical sex-specific behavior, including gender identity and sexual orientation.

Intersex encompasses the development of genitalia and/or sex-specific cerebral neural structures either somewhere between typical male and female, or indeed at the opposite of the typically expected karyotype, for instance, in a CAIS individual of genetically male Y chromosome karyotype and congenitally but with a complete typically female habitus (physical outer body), typical female neural brain structures, female gender identity, and androphilic (attraction to males) sexual orientation.

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.

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.

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

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

The reproductive system of gastropods (slugs and snails) varies greatly from one group to another.

Planaria

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

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.

See also

• Andrology
• Genital modification and mutilation
• Hermaphrodite
• Human sexuality
• Hysterectomy
• Intimate part
• Obstetrics and gynaecology
• Oophorectomy
• Orchiectomy
• Emasculation

References

1. ^ Clark, Robert K. (2005). Anatomy and Physiology: Understanding the Human Body. Jones & Bartlett Learning. ISBN 9780763748166.
2. "Mosses and Ferns". Biology.clc.uc.edu. 16 March 2001. Archived from the original on 28 July 2012. Retrieved 1 August 2012.
3. "Flowering Plant Reproduction". Emc.maricopa.edu. 18 May 2010. Retrieved 1 August 2012.
4. Eberhard, W.G., 1985. Sexual Selection and Animal Genitalia. Harvard University Press
5. Marvalee H. Wake (1992). Hyman's Comparative Vertebrate Anatomy. University of Chicago Press. p. 583. ISBN 978-0-226-87013-7.
6. William Cutrer; Sandra Glahn. Sexual Intimacy in Marriage. Kregel Publications. p. 13. ISBN 978-0-8254-9819-0.
7. Brigitta Olsen (15 November 2009). Daphne's Dance: True Tales in the Evolution of Woman's Sexual Awareness. Brigitta Olsen. p. 9. ISBN 978-0-9842117-0-8.
8. Anita Allen (November 2011). Unpopular Privacy: What Must We Hide?. Oxford University Press, USA. p. 219. ISBN 978-0-19-514137-5.
9. https://science.sciencemag.org/content/253/5023/957
10. Fausto Sterling, Anne (2000). Sexing The Body. New York: New York. pp. 44–77.
11. Pūras, Dainius; Special Rapporteur on the right of everyone to the enjoyment of the highest attainable standard of physical and mental health (4 April 2016), Sport and healthy lifestyles and the right to health. Report A/HRC/32/33, United Nations, archived from the original on 15 December 2016
12. Swiss National Advisory Commission on Biomedical Ethics NEK-CNE (November 2012). On the management of differences of sex development. Ethical issues relating to "intersexuality".Opinion No. 20/2012 (PDF). 2012. Berne. Archived from the original (PDF) on 23 April 2015.{{cite book}}: CS1 maint: location missing publisher (link)
13. Involuntary or coerced sterilisation of intersex people in Australia Archived 2015-09-23 at the Wayback Machine, Australian Senate Community Affairs Committee, October 2013.
14. World Health Organization (2015). Sexual health, human rights and the law. Geneva: World Health Organization. ISBN 9789241564984.
15. Hosken, David J., and Paula Stockley. "Sexual selection and genital evolution." Archived 12 October 2017 at the Wayback Machine Trends in Ecology & Evolution 19.2 (2004): 87-93.
16. Arnqvist, Göran. "Comparative evidence for the evolution of genitalia by sexual selection." Archived 27 January 2012 at the Wayback Machine Nature 393.6687 (1998): 784.
17. Schilthuizen, M. 2014. Nature's Nether Regions: What the Sex Lives of Bugs, Birds, and Beasts Tell Us About Evolution, Biodiversity, and Ourselves. Penguin USA
18. "Male reproductive behaviour of Naja oxiana (Eichwald, 1831) in captivity, with a case of unilateral hemipenile prolapse" (PDF). 2018 – via https://www.biotaxa.org/hn/article/view/34541. {{cite journal}}: Cite journal requires |journal= (help); External link in |via= (help)

Further reading

• The Evolution of Primary Sexual Characters in Animals. Oxford: Oxford University Press. 2010. ISBN 978-0199717033. {{cite book}}: Unknown parameter |authors= ignored (help)

• Sex organs