MALE GENITAL SYSTEM Dr Mujahid Khan Development of

MALE GENITAL SYSTEM Dr. Mujahid Khan

Development of Gonads The gonads are derived from 3 sources: Ø The mesothelium (mesodermal epithelium) lining the posterior abdominal wall Ø The underlying mesenchyme (embryonic connective tissue) Ø The primordial germ cells

Indifferent Gonads Ø The initial stages of gonadal development occur during the fifth week Ø A thickened area of mesothelium develops on the medial side of the mesonephros Ø Proliferation of this epithelium and the underlying mesenchyme produces a bulge on the medial side of the mesonephros called Gonadal ridge

Indifferent Gonads Ø Finger like epithelial cords or Gonadal cords soon grow into the underlying mesenchyme Ø The indifferent gonad now consists of an external cortex and an internal medulla Ø In embryos with an XX sex chromosome complex, the cortex differentiates into an ovary and the medulla regresses Ø In embryos with an XY sex chromosome complex, the medulla differentiates into a testis and the cortex regresses

Primordial Germ Cells Ø These large, spherical cells are visible early in the fourth week among the endodermal cells of the yolk sac near the allantois Ø During folding of the embryo, the dorsal part of the yolk sac is incorporated into the embryo Ø With this the primordial germ cells migrate along the dorsal mesentery of the hindgut to the gonadal ridges Ø During the sixth week the primordial germ cells enter the underlying mesenchyme and are incorporated in the gonadal cords

Sex Determination Ø Chromosomal and genetic sex is determined at fertilization Ø It depends upon whether an X-bearing sperm or a Y-bearing sperm fertilizes the X -bearing ovum Ø The type of gonads develop is determined by the sex chromosome complex of the embryo (XX or XY)

Sex Determination Ø Before the seventh week, the gonads of the two sexes are identical in appearance called indifferent gonads Ø Development of the male phenotype requires a Y chromosome Ø The SRY gene for a testes-determining factor (TDF) has been localized in the sex-determining region of the Y chromosome Ø Two X chromosomes are required for the development of the female phenotype

Sex Determination Ø The Y chromosome has a testes-determining effect on the medulla of the indifferent gonad Ø The absence of a Y chromosome results in the formation of an ovary Ø Testosterone, produced by the fetal testes, determines the maleness Ø Primary female sexual differentiation in the fetus does not depend on hormones Ø It occurs even if the ovaries are absent

Development of Testes Ø Embryos with a Y chromosome usually develop testes Ø The SRY gene for TDF on the short arm of the Y chromosome acts as the switch that directs development of indifferent gonad into testes Ø TDF induces the gonadal cords to condense and extend into the medulla of indifferent gonad, where they form rete testes

Development of Testes Ø The connection of gonadal cords or seminiferous cords with the surface epithelium is lost as tunica albuginea develops Ø The development of a dense tunica albuginea is the characteristic feature of testicular development in a fetus

Development of Testes Ø The enlarging testis separates from the degenerating mesonephros and becomes suspended by its own mesentery called mesorchium Ø The seminiferous cords develop into the seminiferous tubules, tubuli recti, and rete testis Ø The seminiferous tubules are separated by mesenchyme that gives rise to the interstitial cell of Leydig

Development of Testes Ø By the eighth week, these cells begin to secrete testosterone androstenedione Ø These hormones induce masculine differentiation of the mesonephric ducts and external genitalia Ø Testosterone production is stimulated by HCG Ø Fetal testes also produces a glycoprotein called antimullerian hormone (AMH) or mullerian inhibiting substance (MIS)

Development of Testes Ø AMH is produced by sustentacular cells of Sertoli Ø AMH suppresses development of the paramesonephric ducts Ø Seminiferous tubules remain solid until puberty

Development of Testes Ø The walls of seminiferous tubules are composed of two kinds of cells Ø Sertoli cells, supporting cells derived from the surface epithelium on the testis Ø Spermatogonia, primordial sperm cells derived from the primordial germ cells

Development of Testes Ø The rete testis becomes continuous with 15 to 20 mesonephric tubules that become efferent ductules Ø These ductules are connected with the mesonephric duct Ø It becomes the duct of the epididymis

Development of Genital Ducts Ø Both male and female embryos have two pairs of genital ducts Ø The mesonephric ducts (wolffian ducts) play an important role in the development of the male reproductive system Ø The paramesonephric ducts (mullerian ducts) have a leading role in the development of the female reproductive system Ø Till the end of sixth week, the genital system is in an indifferent state, when both pairs of genital ducts are present

Development of Male Genital Ducts Ø Distal to the epididymis, the mesonephric duct acquires a thick investment of smooth muscle and becomes the ductus deferens Ø A lateral outgrowth from the caudal end of each mesonephric duct gives rise to the seminal gland or vesicle Ø The secretion from this pair of glands nourishes sperms Ø The mesonephric duct between the duct of this gland the urethra becomes the ejaculatory duct

Prostate Ø Multiple endodermal outgrowths arise from the prostatic part of the urethra Ø Grow into surrounding mesenchyme Ø The glandular epithelium of the prostate differentiates from these endodermal cells Ø The associated mesenchyme differentiates into the dense stroma and smooth muscle of the prostate

Development of External Genitalia Ø Up to the seventh week of development the external genitalia are similar in both sexes Ø Distinguishing sexual characteristics begin to appear during the ninth week Ø External genitalia are not fully differentiated until the twelfth week

Development of External Genitalia Ø Early in the fourth week, proliferating mesenchyme produces a genital tubercle in both sexes at the cranial end of the cloacal membrane Ø Labioscrotal swelling and urogenital folds soon develop on each side of the cloacal membrane Ø The genital tubercle soon elongates to form a primordial phallus

Development of External Genitalia Ø When the urorectal septum fuses with the cloacal membrane, it divides it into a dorsal anal membrane and a ventral urogenital membrane Ø The urogenital membrane lies in the floor of a median cleft, the urogenital groove, which is bounded by urogenital folds

Development of Male External Genitalia Ø Masculization of the indifferent external genitalia is induced by testosterone Ø The phallus enlarges and elongates to become the penis Ø The urogenital folds form the lateral walls of the urethral groove on the ventral surface of the penis to form the spongy urethra

Development of Male External Genitalia Ø The surface ectoderm fuses in the median plane of the penis, forming a penile raphe and enclosing the spongy urethra within the penis Ø At the tip of the glans of the penis, an ectodermal ingrowth forms a cellular ectodermal cord Ø It grows towards the root of the penis to meet the spongy urethra

Development of Male External Genitalia Ø This cord canalizes and joins the previously formed spongy urethra Ø This completes the terminal part of the urethra and moves the external urethral orifice to the tip of the glans of the penis Ø During the twelfth week, a circular ingrowth of ectoderm occurs at the periphery of the glans penis

Development of Male External Genitalia Ø When this ingrowth breaks down, it forms the prepuce (foreskin) Ø It is adherent to the glans for some time Ø Usually not easy to retract at birth Ø Corpora cavernosa and corpus spongiosum of the penis develop from mesenchyme in the phallus

Development of Male External Genitalia Ø The labioscrotal swellings grow towards each other and fuse to form the scrotum Ø The line of fusion of these folds is clearly visible as the scrotal raphe Ø Agenesis of scrotum is an extremely rare anomally

Development of Inguinal Canals Ø Inguinal canals develop in both the sexes Ø The gubernaculum passes obliquely through the developing anterior abdominal wall at the site of future inguinal canal Ø The gubernaculum attaches caudally to the internal surface of the labioscrotal swellings

Development of Inguinal Canals Ø The processus vaginalis develops ventral to the gubernaculum and herniates through the abdominal wall along the path formed by the gubernaculum Ø The vaginal process carries extensions of the layers of the abdominal wall before it, which form the walls of the inguinal canal

Development of Inguinal Canals Ø In males, these layers also form the coverings of the spermatic cord and testis Ø The opening in the transversalis fascia produced by the vaginal process becomes the deep inguinal ring Ø The opening created in the external oblique aponeurosis forms the superficial inguinal ring

Descent of Testes Testicular descent is associated with: Ø Enlargement of the testes and atrophy of the mesonephroi, allow caudal movement of the testes Ø Atrophy of paramesonephric ducts enables testes to move transabdominally to the deep inguinal rings Ø Enlargement of processus vaginalis guides the testes through the inguinal canal into the scrotum

Descent of Testes Ø By 26 weeks the testes have descended retroperitoneally to the deep inguinal rings Ø This change in position occurs as the fetal pelvis enlarges and the trunk of the embryo elongates Ø Little is known about cause of testicular descent Ø The process is controlled by androgens

Descent of Testes Ø Passage of testis through the inguinal canal may also be aided by the increase in intra-abdominal pressure resulting from growth of abdominal viscera Ø Descent of testes through the inguinal canals into the scrotum usually begins during 26 th week Ø It takes 2 to 3 days

Descent of Testes Ø More than 97% of full-term newborn males have both testes in the scrotum Ø During the first 3 months after birth, most undescended testes descend into the scrotum Ø Spontaneous testicular descent does not occur after the age of one year Ø When the testis descends, it carries its ductus deferens and vessels with it

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