Bipotential Gonads

The development of the gonads begins with the primordial germ cells. The primordial germ cells may be first identifiable in the inner cell mass of the blastocyst in the first postfertilization week [1,14]. There is good evidence that they are in the amniotic ectoderm at the junction of the amnion with the epiblast at the caudal end of the bilaminar disc early in the second week [15]. During the dissolution of the primary yolk sac and the formation of the definitive yolk sac, they migrate by ameboid action through the extraembryonic mesenchyme of the body stalk and reach the endoderm of the caudal yolk sac near its junction with the allantois in the third week. During the fourth week they continue to migrate through the wall and mesentery of the proximal hindgut and toward the adjacent ventromedial side of the mesonephric bodies [16].

In the middle of the fifth week, shortly after the appearance of the ureteric bud and crista ingui-nalis, the primordial germ cells enter the ventromedial border of the mesonephric bodies. At the same time, the mesonephric mesoderm proliferates, differentiates into gonadal mesoderm, and induces the coelomic epithelium to proliferate and form a thickened layer. The thick coelomic epithelium and the enlarging underlying nest of differentiating mesoderm form the early gonadal ridge on the ventromedial surface of the mesonephric body [17].

The subsequent events are somewhat controversial. One view proposes that primordial germ cells and coelomic epithelium migrate into the differentiating mesoderm, and that these three components form the gonadal blastema [15]. The coelomic epithelium serves as a scaffolding around which the other components of the gonadal blastema are organized. The mesodermal tissue forms a network of poorly defined, irregular gonadal cords in the deep portion of the gonad near its border with the mesonephric body. The coelomic epithelium, primordial germ cells, and local mesenchyme lie between the cords.

A recent study utilizing serial semithin sections of Epon-embedded gonads from human embryos and fetuses proposes that gonadal cords arise from mesonephric glomerular and tubular epithelium. These cords grow though the mesorchium into the hilum of the gonad and, hence, into the cortex. These primitive mesonephric cords never intermingle with coelomic epithelium and give rise to the rete and definitive sex cords [18].

The gonadal blastema, whatever its origin, rapidly enlarges and forms a long oval gonad with a broad attachment to the ventromedial surface of the mesonephric body [11]. It does not reach either the upper or lower pole of the mesonephric body (see Fig. iV-4). At the junction between the gonad and the mesonephric body, the gonadal blastema pushes deeply into the mesonephros.

By the middle of the sixth week, the bipotential period of gonadal development ends. The mesonephros extends from the midcervical to the midlumbar regions while the gonad extends from approximately T1 to T10 (see Fig. IV-4). A groove forms along the length of the junction between the gonad and mesonephros, and the two begin to separate. This separation continues during the following period of testicular and ovarian differentiation.

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