sexual organ

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.

Moss

es,

fern

s, and some similar plants have gametangia for reproductive organs, which are part of the gametophyte. The

flower

s of

flowering plant

s 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".

Evolution of sex organs

Evolution of gonads

It is hard to find a common origin for gonads but, gonads most likely evolved independently several times. At first

testes

and

ovaries

evolved due to natural selection.

Evolution of genitalia

A consensus has emerged that sexual selection represents a primary factor for genital evolution. Male genitalia show traits of divergent evolution that are driven by sexual selection.

Animals

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:

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 the differentiation. The absence of it allows the gonads to continue to develop into ovaries.

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 female-like: a pair of urogenital folds with a small protuberance in the middle, and the

urethra

behind the protuberance. If the fetus has testes and 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.

The process of sexual differentiation 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.

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

In most plant species an individual has both male and female sex organs.

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 plant

s have small gametophytes that develop inside the pollen grains (male) and the

ovule

(female).

Flowering plants

In flowering plants the

flowers

contain the sex organs.

Sexual reproduction in

flowering plant

s 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.

Fungi

The sex organs in

fungi

are called gametangia. In some fungi the sex organs are indistinguishable from each other but in other cases male and female sex organs are clearly different.

Similar gametangia that are similar are called isogametangia. While male and female gametangia are called heterogametangia, which occurs in the majority of fungi.

See also

* Andrology
* Emasculation
* Genital modification and mutilation
*

Hermaphrodite

* Human sexuality
*

Hysterectomy

* Intimate part
* Obstetrics and gynaecology
* Oophorectomy
* Orchiectomy

References

Further reading

*

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