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the organs that form the sexual products (ova and spermatozoa) in animals and man.

The gonads are either male organs (testes), female organs (ovaries), or hermaphroditic (for example, in Platyhelminthes and Oligochaeta, the leeches, the majority of Gastropoda, and the barnacles). In the embryos of vertebrates the gonads are found in the sides of the dorsal mesentery in the form of sex folds (paired longitudinal folds of epithelium that line the body cavity), into which migrate the primary sex cells (which arise outside the gonads at earlier stages of the body’s development) and into which grow columns of mesenchymal cells from the truncal kidney (the mesonephros), which participate in formation of the gonadal stroma (in particular, their medullary layer). The epithelium of the gonads becomes cylindrical and its cells may be converted into germ cells (so that it is called the germinal epithelium). In fishes the sex folds arise along the entire length of the body cavity and are converted completely to gonads. In amphibians the folds are similarly situated, but only the middle part is converted to gonads. In reptiles, birds, and mammals the sex folds appear only in the region of the lateral surface of the anterior half of the mesonephros, which faces the mesentery. The germ cells multiply, grow, and mature in the gonads. The growth of the germ cells in the gonads takes place under the influence of the gonadotrophic hormones of the pituitary and by means of special nurse cells—Sertoli’s cells (in male gonads) and follicle cells, which in many animals form a follicular membrane around the ova (in female gonads). Both sexes in the process of gonadal development pass through an undifferentiated (bipotential) stage of development, after which their sexual differentiation begins. The ovaries are formed principally from the cortical layer of the gonads, and the testes from the medullary layer. The form of the ovaries differs markedly in different animals (for example, they are tubular in round-worms and insects, racemose in birds, and dense rounded bodies in mammals). The testes in all vertebrates, beginning with the frogs and toads, are constructed of seminiferous tubules, which communicate with the ductus deferens, formed from the tubules of the mesonephros. In higher animals the gonads are also endocrine glands, secreting sex hormones into the blood. The gonads, their conduction paths, and the external sex organs, constitute the genital system.


References in periodicals archive ?
Compared to male group, the female group showed significantly higher levels of the MDA in the hepatopancreas and gonad tissues, but the muscle tissues was lower (P less than 0.
This conclusion is based on three lines of evidence: 1) the presence of undeveloped bisexual gonads in small (generally <45 cm TL) fish; 2) a total lack of small mature males; and 3) the presence of a lumen, posteriorly fused gonadal lobes, and brown body remnants of yolked oocytes in the gonads of relatively large mature males (Sadovy and Shapiro, 1987; Sadovy de Mitcheson and Liu, 2008).
TABLE 1 Number of sampled gonads of Lutjanus analis for year in the Northeast of Brazil 1997 1998 1999 Total Localities M F M F M F M F Caicara do Norte (RN) 1 1 Baia Formosa (RN) 15 23 15 23 Pau Amarelo (PE) 2 2 Tamandare (PE) 1 1 Arembepe (BA) 21 30 21 30 R/V Martins Filho 17 9 17 9 F/V Reinaldo 8 8 8 8 Total 61 74 RN: Rio Grande do Norte, PE: Pernambuco, BA: Bahia.
Because the effects of in vivo Cd exposure have been described previously in mice treated prenatally with Cd (Tarn and Liu 1985), we used organ cultures of mouse gonads as our control.
By day 36, the end of the embryonic period, the gonads have differentiated into either testes or ovaries (Figure 4-4b).
The National Radiological Protection Board (NRPB) guidance notes state that gonad protection should be implemented whenever possible, (6) and ideally in all radiological procedures.
There were 5 streak gonads (the left gonad of patients 2, 6, 9, 10, and 13), which were all in an abdominal position.
For the Catarina scallop, an abnormal gonad has been observed occasionally, finding either a gonad that appeared to be only testis (completely white-cream in coloration) with no ovary area (orange-red color), or gonads with only ovary and no testis area.
The gonads and brains were fixed in Bouin's solution, embedded in paraffin, and 6 um sections were cut.
During the histological analyses of gonads, the characteristics of any trematodes found were recognized following the work of da Silva et al.
For Katla, Pb was exceeded the recommended value provided by FAO/WHO whereas Cd and Ni were within the guidelines except for Cd in the gonad.
The gonads in these individuals are asymmetric with bilateral ovarian and testicular tissue (in varying ratios), which may be separate or combined as ovo-testes.