A gonad, sex gland, or reproductive gland is a
mixed gland that produces the
gametes and
sex hormones of an organism.
Female reproductive cells are
egg cells, and
male reproductive cells are
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
. The male gonad, the
testicle, produces sperm in the form of
spermatozoa. The female gonad, the
ovary
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
, produces egg cells. Both of these gametes are
haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
cells. Some
hermaphroditic animals have a type of gonad called an
ovotestis.
Evolution
It is hard to find a common origin for gonads, but gonads most likely
evolved independently
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
several times.
Regulation
The gonads are controlled by
luteinizing hormone and
follicle-stimulating hormone
Follicle-stimulating hormone (FSH) is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the gonadotropic cells of the anterior pituitary gland and regulates the development, growth, pubertal maturation, an ...
, produced and secreted by
gonadotropes or gonadotrophins in the
anterior pituitary gland. This secretion is regulated by
gonadotropin-releasing hormone produced in the
hypothalamus.
Development
Gonads start developing as a common primordium (an organ in the earliest stage of development), in the form of
genital ridges,
which are only later
differentiated to male or female
sex organs. The presence of the
SRY gene, located on the
Y chromosome and encoding the
testis determining factor, determines male sexual differentiation. In the absence of the SRY gene from the Y chromosome, the female sex (ovaries instead of testes) will develop. The
development of the gonads is a part of the
development of the urinary and reproductive organs.
Disease
The gonads are subject to many diseases, such as
hypergonadism,
hypogonadism,
agonadism, tumors, and cancer, among others.
Aging
Ovarian aging
A delay in having children is common in the developed world and this delay is often associated with ovarian
infertility
Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...
and subfertility. Ovarian aging is characterized by progressive decline of the quality and number of
oocytes
[Park SU, Walsh L, Berkowitz KM. Mechanisms of ovarian aging. Reproduction. 2021 Jul 14;162(2):R19-R33. doi: 10.1530/REP-21-0022. PMID: 33999842; PMCID: PMC9354567]. This decline is likely due, in part, to reduced expression of genes that encode proteins necessary for
DNA repair and
meiosis. Such reduced expression can lead to increased
DNA damage and errors in meiotic
recombination[.
]
Testicular aging
The testes of older men often have sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
abnormalities that can ultimately lead to infertility
Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...
[. These abnormalities include accumulation of DNA damage and decreased DNA repair ability][Dong S, Chen C, Zhang J, Gao Y, Zeng X, Zhang X. Testicular aging, male fertility and beyond. Front Endocrinol (Lausanne). 2022 Oct 13;13:1012119. doi: 10.3389/fendo.2022.1012119. PMID: 36313743; PMCID: PMC9606211]. During spermatogenesis in the testis, spontaneous new mutations arise and tend to accumulate with age[Cioppi F, Casamonti E, Krausz C. Age-Dependent De Novo Mutations During Spermatogenesis and Their Consequences. Adv Exp Med Biol. 2019;1166:29-46. doi: 10.1007/978-3-030-21664-1_2. PMID: 31301044].
See also
* Gonadosomatic Index
* Hypothalamic–pituitary–gonadal axis
* Sex organ
References
{{Authority control
Glands
Endocrine system
Human reproductive system