NORMAL SEXUAL DIFFERENTIATION
Category: Pediatric Surgery
Abstract : Normal sexual differentiation Sexual differentiation and gonadal development
is determined by the sex chromosomes (XY male, XX female). The gonads produce
hormones which influence the subsequent differentiation of internal and external
genitalia. Both sexes Gonads develop from the genital ridges (formed
by cells of the mesonephros and coelomic epithelium). At 5-6 weeks, primor
Normal sexual differentiation
Sexual differentiation and gonadal development
is determined by the sex chromosomes (XY male, XX female). The gonads produce
hormones which influence the subsequent differentiation of internal and external
genitalia.
Both sexes
Gonads develop from the genital ridges (formed
by cells of the mesonephros and coelomic epithelium).
At 5-6 weeks, primordial
germ cells migrate from the yolk sac to populate the genital ridges. Primitive
sex cords are formed, which support germ cell development.
From 4 weeks, the
mesonephric (Wolffian) ducts are incorporated into the genital system, when
renal function is taken over by the definitive kidney. At 6 weeks, coelomic
epithelium creates the paramesonephric (Müllerian) ducts, which develop
laterally and are fused to the urogenital sinus at their
bases.
Males
The testis-determining gene (SRY) is located on the Y
chromosome and stimulates medullary sex cords in the primitive testis to
differentiate into Sertoli cells, which produce Müllerian inhibiting substance
(MIS) at 7-8 weeks. This triggers regression of the paramesonephric ducts,
testosterone secretion from Leydig cells of the testis, and the initial phase of
testicular descent. During weeks 8-12, mesonephric ducts differentiate into
epididymis, vas deferens, seminal vesicles, and ejaculatory ducts. The prostate
is formed from mesenchyme (capsule) and urethral endoderm. After week 23, the
testes rapidly descend from the abdomen (via the inguinal canal during weeks
24-28) and into the scrotal sac, guided by the gubernaculum.
Testosterone and
di-hydrotestosterone (DHT) androgens are responsible for masculinization. DHT is
made from testosterone by 5-alfa-reductase enzyme in the tissues. Development of
the external genitalia occurs from week 7. Urogenital folds develop around the
opening of the urogenital sinus, and labioscrotal swellings form either side.
The penile shaft and glans are formed by elongation of the genital tubercle and
fusion of urogenital folds. The scrotum is created by fusion of labioscrotal
folds.
Females
The genital ridge forms secondary sex cords (primitive
sex cords degenerate) which surround the germ cells to create ovarian follicles
(week 15). These undergo meiotic division to become primary oocytes, which are
later activated to complete gametogenesis at puberty. Oestrogen is produced from
week 8 under the influence of the aromatase enzyme. In the absence of MIS, the
mesonephric ducts regress, and the paramesonephric ducts become the fallopian
tubes, uterus, and upper two-thirds of the vagina. The sinovaginal sinus
develops at the junction of the paramesonephric ducts and the urogenital sinus.
This forms the lower third of the vagina.
The genital tubercle forms the
clitoris; the urogenital folds become the labia minora; and the labioscrotal
swellings form the labia majora.
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