Development of male reproductive system : lecture note

 

Development of male reproductive system

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When fertilization occurs, the genetic makeup of the male gamete (22X or 22Y) determines the sex of the developing embryo.

The process of sex differentiation is complicated and multigenic.
The most significant one is the Y chromosome gene's sex-determining region (SRY), whereas certain autosomal chromosomal genes are also involved in this process.

On the short arm of Y chromosome Yp11, the SRY gene is located. Male development is influenced by the SRY protein, whereas female development is developed in its absence.

Indifferent gonad

Up to 6 weeks of development, gonads are two longitudinal genital ridges, at the dorsal body wall of embryo which develop from intermediate mesoderm.

The genital ridge is formed by coelomic epithelium and underlying mesoderm. The coelomic epithelium forms irregular sex cords in both male and female embryos, and it is not possible to distinguish between male and female gonads at this stage, so this stage of development is known as indifferent gonads.

By the 12-week mark, the male and female structures of the external genitalia can be differentiated.

By the 20th week, the phenotypical differentiation is complete.

Source of development of gonad

1. Genital ridge: it is formed by the elevation of the intermediate mesoderm, which is covered by the coelomic epithelium. 
2. Primordial germ cells

The STY gene produces a protein known as a testis-determining factor.

TDF induces the testis to secrete FGF9, which stimulates the mesonephric duct to penetrate the gonadal ridge.

The SRY gene also produces SF1, which stimulates the differentiation of sertoli and leydig cells, which produce testosterone and Mullarrian-inhibiting factor (MIF), respectively.

In the presence of TDF, testosterone, and MIF, the indifferent gonad will produce a male phenotype.

In the absence of TDF, testosterone, and MIF, the indifferent gonad will produce a female phenotype.

Sequence of development of reproductive system

Gonads > then genital duct formation > then formation of external genitalia

Development and migration of primordial germ cells originate from the epiblast, then migrate through the primitive streak to reach the endodermal cell of the yolk sac close to allantois by the 3^rd week. During the 4^th week of development, they pass through the dorsal mesentery of the hindgut, and by the 5^th week of development, they reach near the genital ridges and start to penetrate the genital ridge by the 6^th week.

If primordial germ cells have an inductive influence on the development of the gonad, if they fail to reach the genital ridges, the gonad will not develop.

 

Development of gonads

Development of testis

1.          Intermediate mesoderm forms the urogenital ridge, a longitudinal elevation at the dorsal body wall of the fetus.
The coelomic epithelium and underlying mesoderm of the urogenital ridge protifelate to form the gonadal ridge.
The coelomic epithelium forms irregular primary sex cords and incorporative primordial germ cells, which migrate from the wall of the yolk sac.
The primary sex
The development of the testis ord extends deep into the medulla to form medullary cords, which lose their connection to the surface epithelium as the thick tunica albuginea forms in between the medullary cord and coelomic epithelium.

2.       Near the hilum medullary cord break up into network of tiny cell stands that form rete testis

3.       By the 4^th month of interuterine life medullary cord is continuous with rete testis

4.       The testis or medullary cord contain and primitive germ cells and sustentacular cell of  sertoli cell which derived from the surface epithelium of genital ridge.

5.       Sertoli cell secrete mullarian inhibiting factor (MIF) which degenerate mullarian duct

6.       Interstitial cell of leydig derive from mesenchyme of genital ridge which derived from intermediate mesoderm ,By the 8^th week of gestation leydig cell begin to produce testosterone which influence development of genital duct and external genitalia

7.       The testis or medullary cord of testis remain solid cord until puberty , then it become canalize and known seminiferous tubule, the canal of seminiferous tubules are continuous with rete testis which inturn join with efferent ductile. The efferent ductules and other excretory part of male genital system derived from mesonephric or wolffian duct

 Relative descent of the testis

The testis originally developed within the abdomen but later descent to scrotum due to disproportionate growth of upper abdomen region away from the pelvic region

The gubernaculum testis a fibrous band extend from testis to scrotum pull the testis

The gubernaculum also pull part of peritoneum with it the processus vaginalis which obliterate later except a remnat of peritoneal sac the tunica vaginalis of the testis

12^th week , testis reach inguinal region

28^th week, inguinal canal

By 33^rd week it reach the scrotum  

Genital duct in male

There are two genital ducts, mesonephric and paramesonephric ducts  are present in both sex at the initial period of development

In case male mesonephric duct is developed but paramesonephric duct mostly degenerate, while in case female para mesonephric duct is developed but mesonephric duct mostly degenerate

Gonads influence the development of genital duct and genitalia.

The Sertoli cell secrete mullarian inhibiting factor (MIF) which inhibit the development of paramesonephric or  mullarian duct

Interstitial cell of leydig derive from mesenchyme of genital ridge which derived from intermediate mesoderm ,

By the 8^th week of gestation leydig cell begin to produce testosterone which stimulates the mesonephric ducts to form genital ducts .  Structure derived from mesonephric duct : Efferent ductules, epididymis, vas deferens, seminal vesicle and ejaculatory duct

The paramesonephric duct regress due to presence of Mullarian inhibiting factor secreted from the sertoli cell

The remnant of paramesonephric duct in male: Appendix testis , appendix epididymis,

, The remnant of mesonephric duct in male: paradidymis,

 

Development of external genitalia

Indifferent stage

In the 3^rd week of development, mesenchymal cells origining in the region of the primitive streak migrate aroung the cloacal membrane to form a pair of slightly elevated cloacal folds , cranially these folds unit and form genital tubercles , caudally the folds are subdivided into urethral folds anteriorly and anal folds posteriorly .  another pair of swellings lateral to it known as genital swellings

A the end of the 6^th week of development it is impossible to distinguish between two sexes.

Development of male external genitalia

Under influence of testosterone genital tubercle is elongated known as phallus which pull urethral folds forward

Phallus forms the (glans penis, corpora cavernosa and corpus spongiosum of penis )

Urethral folds form the ventral part of the penis

The genital swelling form the scrotum