Development of Genital System in animals
The genital organs are undifferentiated when they first appear (indifferent period), either by gross or microscopic appearance and hence during this period the term gonad is applied to them.
The gonad appears within a thickening called the genital ridge on the medial aspect the urino-genital ridge.
The ventral surface of the genital ridge shows rapid proliferation of the mesothelium which becomes thickened and many layered. The gonad consists of a superficial germinal epithelium and an internal epithelial blastemal mass.
The primordial germ cells from the yolk sac endoderm migrate by way of the dorsal mesentery of the gut and locate themselves in the epithelium of the genital ridge.
In the male, genital glands as they increase in size, shorten into more compact organs and begin to show branched and anastomosing cords of cells called the testes cords. These arise from the superficial germinal epithelium which dips into the interior of the mesenchyme forming cords of cells. These cords differentiate into the tubuli contorti and tubuli recti.
The original germ cells descend into the tubuli contorti and the mesenchyme forms the interstitial tissue and the mediastinum testis. When the migration of the germs cells is complete, the mesenchyme surrounding the gland forms the tunica albuginea and cuts off the germinal epithelium, which becomes mesothelium. Certain cells of the mesenchymal stroma transform themselves into large cells lying in the intertubular connective tissue forming the interstitial cells.
In the female, the gonad does not exhibit any distinctive ovarian features until several weeks after the gonad of the male has declared itself as a testis. Then the blastemal mass shows clusters of small indifferent cells and one or more primordial germs cells. Soon a primary cortex appears beneath the germinal epithelium and a primary medulla appears internally.
Later, the ovary enlarges by addition of a new (secondary) definitive cortex upon the original blastemal mass. In the primary medulla and cortex, the earlier ova decline, regress and are replaced by vascular fibrous stroma-the definitive medulla.
In the definite cortex, ova and the primary follicles develop. Later vesicular (Graafian) follicles develop from the primary follicles during the active sexual cycle.
It is to be remembered that while the primary medulla and primary cortex correspond to the male component of a gonad, the definitive cortex of the gonad is a distinctive female characteristic.
DEVELOPMENT OF GENITAL DUCTS
The male does not elaborate any ducts intended primarily for its own purposes. With the degeneration of the mesonephros it merely appropriates the mesonephric ducts and some to the mesonephric tubules and converts them into genital canals (mesonephric duct becomes, the duct of epididymis, ductus deferens and ureter and pelvis while the cranial group of mesonephric tubules become efferent ductules of epididymis. Seminal vesicle arises from the mesonephric duct).
In the female, the mesonephric ducts mostly degenetrate, except for ureter, pelvis and some vestigial remnants.
DEVELOPMENT OF MULLERIAN DUCTS
Both sexes develop a pair of female ducts of Muller. But its fate in the two sexes is different, as described below-
Fate of Mullerian ducts in Female
These ducts arise by formation of a groove in the thickened epithelium of the urino-genital ridge which is lateral to the mesonephros near its cranial end. This extreme cranial end of the groove remains open like trumpet while caudally the lips of the groove fuse into a tube.
In the most posterior part, the two Mullerian ducts approach each other, reach the dorsal wall of the urogenital sinus, fuse and end blindly at the Muller’s tubercle-a median projection in the dorsal wall of the cloaca, caused by the opening of the mesonephric ducts into it. This fused region is the first indication of the uterus and vagina and the unfused cranial parts serve as uterine tubes.
The fused part, by unequal growth of its walls, becomes trans formed into the uterus and vagina. The cranial end of the tube, which remains open, forms the fimbriated end of the fallopian tube. The Muller’s tubercle becomes the site of hymen.
Fate of Mullerian ducts in Male
The same primordia also develop but remain rudimentary. Degeneration of the ducts occurs in the third month and only the extreme cranial end is spared which remains as a vestige-the appendix testis.
The portion which forms the uterus and vagina persists in a rudimentary from as the uterus masculinus.
The Muller’s tubercle is represented by the colliculus seminalis.
DEVELOPMENT OF MESONEPHRIC OR WOLFFIAN DUCTS
Fate of Wolffian ducts in Male
Some of the cranial degenerating mesonephric tubules, unite with the seminiferous tubules and form the efferent ducts of the epididymis. The caudal group of degenerating mesonephric tubules persists as paradidymis. The upper end of the mesonephric duct becomes highly convoluted to form the duct of the epididymis and lower portion forms the vas deferens.
The primordia of the seminal vesicles arise from the terminal part of the mesonephric ducts and so are mesodermal in origin. The prostate and Cowper’s glands arise from the pelvic urethra which is entodermal in origin. The penile part of the urethra is ectodermal in origin.
Fate of Wolffian ducts in Feale
The cranial group of mesonephric tubules persists as the epoophoron and the caudal group as the paroophoron.
The greater part of the mesonephric duct atropies and the distal portion persists as Gartner’s canals in the wall of the vagina.
DEVELOPMENT OF EXTERNAL GENITALIA
Embryos of six weeks present a conical genital tubercle in the mid-ventral line between the umbilical cord and tail. Its caudal slope bears a shallow urethral groove, flanked on either side by slightly elevated urethral folds.
The genital tubercle elongates into a cylindrical phallus whose tip is rounded into glans. Lateral to the phallus on either side appears a rounder ridge- the labio-scrotal swelling.
Rupture of the urethral membrane in the floor of the-urethral groove provides an external opening for the urino-genital sinus. So far the development is identical in both the sexes.
In the male, the phallus elongates to become the penis. The edges of the urethral groove progressively fold together in a distal direction to transform an open urino-genital sinus into the tubular urethra within the penis. The fused edges form the raphe. The scrotal swellings shift laterally and behind and develop into the scrotum. Each half of the scrotum is separated from its mate by a septum and a superficial raphe.
In the female, the phallus lags in development and becomes the clitoris. The shorter urethral groove remains as the vestibule and the urethral folds constitute the labia minora. The primitive labio-scrotal swelling enlarge and fuse below the anus in the dorsal commissure, while the swellings enlarge to form the labia-majora.
Anomalies of developments in Genital system
Hypospadias – failure of closure of the lips of the urethral grooves, on the ventral surface of the penis.
Epispadias -urethra opening on the dorsal surface of the penis due to displacement of the cloacal membrane.
