Gonads

Although the sex of the embryo is determined genetically at the time of fertilization, the gonads do not acquire male or female morphological characteristics until the seventh week of development.

Gonads appear initially as a pair of longitudinal ridges, the genital or gonadal ridges (Fig. 14.17). They are formed by proliferation of the epithelium and a condensation of underlying mesenchyme. Germ cells do not appear in the genital ridges until the sixth week of development.

Primordial germ cells first appear at an early stage of development among endoderm cells in the wall of the yolk sac close to the allantois (Fig. 14.18A). They migrate by ameboid movement along the dorsal mesentery of the hindgut (Fig. 14.18, B and C), arriving at the primitive gonads at the beginning of the fifth week and invading the genital ridges in the sixth week. If they fail to reach the ridges, the gonads do not develop. Hence the primordial germ cells have an inductive influence on development of the gonad into ovary or testis.

Shortly before and during arrival of primordial germ cells, the epithelium of the genital ridge proliferates, and epithelial cells penetrate the underlying mesenchyme. Here they form a number of irregularly shaped cords, the primitive sex cords (Fig. 14.19). In both male and female embryos, these cords are connected to surface epithelium, and it is impossible to differentiate between the male and female gonad. Hence, the gonad is known as the indifferent gonad.

Testis

If the embryo is genetically male, the primordial germ cells carry an XY sex chromosome complex. Under influence of the SRY gene on the Y chromosome, which encodes the testis-determining factor, the primitive sex cords continue to proliferate and penetrate deep into the medulla to form the testis or medullary cords (Figs. 14.20 A, Fig. 14.21). Toward the hilum of the gland the cords break up into a network of tiny cell strands that later give rise to tubules of the rete testis (Fig. 14.20, A and B). During further development, a dense layer

Mesonephros

Mesonephric duct

Excretory tube Gl°merulus \ Mesonephric duct

Mesonephros

Mesonephric duct

Allantois Genital Ridge

Figure 14.17 A. Relation of the genital ridge and the mesonephros showing location of the mesonephric duct. B. Transverse section through the mesonephros and genital ridge at the level indicated in A. C. Scanning electron micrograph of a mouse embryo showing the genital ridge (arrows). D. High magnification of the genital ridge showing the mesonephric duct (arrow) and the developing gonad (arrowheads).

Figure 14.17 A. Relation of the genital ridge and the mesonephros showing location of the mesonephric duct. B. Transverse section through the mesonephros and genital ridge at the level indicated in A. C. Scanning electron micrograph of a mouse embryo showing the genital ridge (arrows). D. High magnification of the genital ridge showing the mesonephric duct (arrow) and the developing gonad (arrowheads).

of fibrous connective tissue, the tunica albuginea, separates the testis cords from the surface epithelium (Fig. 14.20).

In the fourth month, the testis cords become horseshoe shaped, and their extremities are continuous with those of the rete testis (Fig. 14.20B). Testis cords are now composed of primitive germ cells and sustentacular cells of Sertoli derived from the surface epithelium of the gland.

Allantois Genital Ridge
Figure 14.18 A. A 3-week-old embryo showing the primordial germ cells in the wall of theyolk sac close to the attachment of the allantois. B. Migrational path ofthe primordial germ cells along the wall ofthe hindgut and the dorsal mesentery into the genital ridge.

Aorta

Mesonephric duct

Mesonephric duct

Genital Ridge

Primordial germ cells

^ ■ Proliferating K body epithelium

Primitive sex cords

Paramesonephric duct

Figure 14.19 Transverse section through the lumbar region of a 6-week embryo showing the indifferent gonad with the primitive sex cords. Some of the primordial germ cells are surrounded by cells ofthe primitive sex cords.

Primordial germ cells

^ ■ Proliferating K body epithelium

Primitive sex cords

Paramesonephric duct

Figure 14.19 Transverse section through the lumbar region of a 6-week embryo showing the indifferent gonad with the primitive sex cords. Some of the primordial germ cells are surrounded by cells ofthe primitive sex cords.

Interstitial cells of Leydig, derived from the original mesenchyme of the gonadal ridge, lie between the testis cords. They begin development shortly after onset of differentiation of these cords. By the eighth week of gestation, Leydig cells begin production of testosterone, and the testis is able to influence sexual differentiation of the genital ducts and external genitalia.

Testis cords remain solid until puberty, when they acquire a lumen, thus forming the seminiferous tubules. Once the seminiferous tubules are canalized, they join the rete testis tubules, which in turn enter the ductuli effer-entes. These efferent ductules are the remaining parts of the excretory tubules of the mesonephric system. They link the rete testis and the mesonephric or wolffian duct, which becomes the ductus deferens (Fig. 14.20B).

Wolffian Ducts

Figure 14.20 A. Transverse section through the testis in the eighth week, showing the tunica albuginea, testis cords, rete testis, and primordial germ cells. The glomerulus and Bowman's capsule of the mesonephric excretory tubule are degenerating. B. Testis and genital duct in the fourth month. The horseshoe-shaped testis cords are continuous with the rete testis cords. Note the ductuli efferentes (excretory mesonephric tubules), which enter the mesonephric duct.

Figure 14.20 A. Transverse section through the testis in the eighth week, showing the tunica albuginea, testis cords, rete testis, and primordial germ cells. The glomerulus and Bowman's capsule of the mesonephric excretory tubule are degenerating. B. Testis and genital duct in the fourth month. The horseshoe-shaped testis cords are continuous with the rete testis cords. Note the ductuli efferentes (excretory mesonephric tubules), which enter the mesonephric duct.

Primitive Gonads
Figure 14.21 Influence of primordial germ cells on indifferent gonad.

Ovary

In female embryos with an XX sex chromosome complement and no Y chromosome, primitive sex cords dissociate into irregular cell clusters (Figs. 14.21 and 14.22 A). These clusters, containing groups of primitive germ cells, occupy the medullary part of the ovary. Later they disappear and are replaced by a vascular stroma that forms the ovarian medulla (Fig. 14.22).

The surface epithelium of the female gonad, unlike that of the male, continues to proliferate. In the seventh week, it gives rise to a second generation of cords, cortical cords, which penetrate the underlying mesenchyme but remain

Paramesonepnric duct

Degenerating mesonephric tubule

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Responses

  • freyja
    Why germ cells do not appear in genital redge until six week?
    2 years ago

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