Genital System

The chromosomal and genetic sex of an embryo is determined at the time of fertilization. Early development is the same for both sexes and is called the indifferent stage. Mesodermal epithelium that lines the posterior abdominal wall, the underlying mesenchyme, and primordial germ cells (from the yolk sac) all contribute to the gonadal formation.

Some of the mesodermal epithelium medial to the mesonephros thickens along with growth of the underlying mesenchyme. This produces a bulge called the go-nadal ridge. Epithelial cells grow into the mesenchyme, becoming, respectively, the cortex and medulla of the indifferent gonad. In males, the cortex regresses, and the medulla forms the testis. The converse occurs in females, with the cortex forming the ovary and the medulla regressing.

The primordial germ cells originate from the endo-derm of the yolk stalk, migrating along the suspending dorsal mesentery of the hindgut to reach the subperi-toneum131 (Fig. 2-71). There they are incorporated into the primary sex cords. Various hormonal influences determine the ultimate differentiation into testes or ovaries. In females, gonadal development is delayed compared with males.

The cloaca is the endoderm-lined terminal hindgut. This cavity is in continuity with the allantois (a yolk sac diverticulum).9 Urorectal folds grow in from the side wall, forming a septum that separates the ventral urogenital sinus from the more dorsal rectum (Figs. 2-65 and 2-70).

The allantois and a portion of the cloaca unite to eventually form the definitive bladder. The allantois that connects the dome of the bladder to the umbilicus eventually becomes atretic and is called the urachus. The obliterated fibrous connection can be seen in adults as the medial umbilical fold. Similar to the omphalome-senteric duct from the gastrointestinal tract, draining umbilical sinuses or a diverticulum-like outpouching of the bladder may be seen.124 A urachal sinus is the most common anomaly, with a urachal cyst next in frequency. A patent urachus is much less commonly encountered.130 The urachus may become the site of dysplasia, leading to the development of carcinoma. The distal mesonephric ducts and the metanephric diverticulum are separately incorporated into the urogenital sinus. The mesonephric ducts become the vas deferens, seminal vesicles, and ejaculatory ducts in the male and minimally contribute to the urethra, while the metanephric ducts become the ureters.124

Patent Urachus Radiology

Fig. 2-70. Sagittal schematic of 7-week embryo demonstrates the growth of the ureteric bud and development of the kidney proper from the metanephros (MET). The degenerating mesonephros (MES) and its duct are still evident. The urorectal septum has completed the compartmentalization of the cloaca into the bladder and the hindgut (rectum).

Fig. 2-70. Sagittal schematic of 7-week embryo demonstrates the growth of the ureteric bud and development of the kidney proper from the metanephros (MET). The degenerating mesonephros (MES) and its duct are still evident. The urorectal septum has completed the compartmentalization of the cloaca into the bladder and the hindgut (rectum).

Ray Genital

Fig. 2—71. Schematic of an axial section through the pelvis of a six-week embryo shows the infolding of the lateral margin of the gonadal ridge as it starts to form the paramesonephric duct (PMD). The mesonephric duct (MD) and associated tubule can be seen in the urogenital ridge. The hindgut mesentery (HGM) is the pathway for the migrating primordial germ cells. The developing adrenal cortex (AC) and medulla (AM) can be seen in the retroperitoneum.

The mesonephric ducts, which play such an important role in the development of the definitive ureter and kidney, continue to be important in the formation of the male genital tract. Under the trophic influence of androgens, the proximal end becomes part of the epididymis and the vas deferens.131 A distal outpouching becomes the seminal vesicle. Even more distally, the mesonephric duct becomes the ejaculatory duct as it joins the urethra.

An infolding occurs along the lateral aspect of the gonadal ridge, giving rise to the paramesonephric duct (Fig. 2-71). This structure is open cranially into the developing greater peritoneal cavity, and inferiorly it is incorporated into the uterovaginal primordium. It eventually becomes the Fallopian tube proximally and, with fusion of its opposite-sided partner distally, forms the uterus and upper vagina131 (Fig. 2-72). Various anomalies of the uterus and/or the vagina may result from incomplete fusion of these ducts. The anomalies range from an arcuate uterus through uterus bicornis (separate uterine horns empty via separate cervices into one vagina) to uterus didelphys (two uterine tubes, horns, and cervices open into a duplicated vagina). They respectively represent the sequelae from minimal to complete lack of fusion of the muellerian ducts.132

As the paramesonephric ducts fuse in the midline caudally, its stalk of mesoderm overlying and connecting it to the genital ridge (containing the degenerating mes-

Genital Ridges

Fig. 2—72. Schematic of an axial section through the pelvis of an 8-week female embryo shows the fusing paramesonephric ducts in the midline supported by the broad ligament (BL) that attach to the lateral pelvic wall. The developing ovary (OV) can be seen along the ligament 's posterior aspect. The developing bladder is noted anteriorly.

Fig. 2—72. Schematic of an axial section through the pelvis of an 8-week female embryo shows the fusing paramesonephric ducts in the midline supported by the broad ligament (BL) that attach to the lateral pelvic wall. The developing ovary (OV) can be seen along the ligament 's posterior aspect. The developing bladder is noted anteriorly.

onephros) is elongated and carried laterally. This connecting stalk then becomes the suspending mesentery of the uterus, the broad ligament131 (Fig. 2-72). It connects the fused paramesonephric ducts to the pelvis side wall. The mesonephric duct becomes a solid cord, the round ligament, connecting the ovary to the labia majora via the inguinal canal.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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