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| Gonad | |
|---|---|
 A pair of ovaries ofCyprinus carpio (common carp) placed in dissecting dish | |
| Identifiers | |
| MeSH | D006066 |
| FMA | 18250 |
| Anatomical terminology | |
Agonad,sex gland, orreproductive gland[1] is amixed gland andsex organ that produces thegametes andsex hormones of an organism.Female reproductive cells areegg cells, andmale reproductive cells aresperm.[2] The male gonad, thetesticle, produces sperm in the form ofspermatozoa. The female gonad, theovary, produces egg cells. Both of these gametes arehaploid cells. Somehermaphroditic animals (and some humans— seeOvotesticular syndrome) have a type of gonad called anovotestis.
It is hard to find a common origin for gonads, but gonads most likelyevolved independently several times.[3]
The gonads are controlled byluteinizing hormone (LH) andfollicle-stimulating hormone (FSH), produced and secreted bygonadotropes or gonadotrophins in theanterior pituitary gland.[4] This secretion is regulated bygonadotropin-releasing hormone (GnRH) produced in thehypothalamus.[5][6]
The gonads develop from three sources; the mesothelium, underlying mesenchyme and the primordial germ cells. Gonads start developing as a common primordium (an organ in the earliest stage of development), in the form ofgenital ridges,[7] at the sixth week, which are only laterdifferentiated to male or femalesex organs (except when they are not differentiated). The presence of theSRYgene,[8] located on the short arm of theY chromosome and encoding thetestis determining factor, usually determines male sexual differentiation. In the absence of the SRY gene from the Y chromosome, usually the female sex (ovaries instead of testes) will develop. Thedevelopment of the gonads is a part of thedevelopment of the urinary and reproductive organs.[citation needed]
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The gonads are subject to many diseases, such ashypergonadism,hypogonadism,agonadism, tumors, and cancer, among others.[citation needed]
A delay in having children is common in the developed world and this delay is often associated with ovarianfemale infertility and subfertility. Ovarian aging is characterized by progressive decline of the quality and number ofoocytes.[9] This decline is likely due, in part, to reduced expression of genes that encode proteins necessary forDNA repair andmeiosis.[10][11] Such reduced expression can lead to increasedDNA damage and errors in meioticrecombination.[9]
The testes of older men often havesperm abnormalities that can ultimately lead tomale infertility.[12] These abnormalities include accumulation of DNA damage and decreased DNA repair ability.[12] During spermatogenesis in the testis, spontaneous new mutations arise and tend to accumulate with age.[13]
The diversity of modes in which gonads are formed makes it hard to substantiate a common origin of gonads. It appears to be more likely that gonads evolved independently several times.
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