12 Anatomy, Histology, and Cell Biology
Sertoli cells produce Müllerian inhibiting substancewhich causes the
apoptosis of paramesonephric (Müllerian) ductstructures in the male fetus.
Leydig cellsproduce testosterone and other sex hormones that reg-
ulate further male differentiation.
In the absence of testosterone, follicular cellsandoogoniadevelop.
Two pairs of genital ducts develop in both sexes. Mesonephric
(Wolffian) ductsdevelop first as part of the urinary system.
Paramesonephric (Müllerian) ductsdevelop next and are open to
the pelvic cavity at their cranial ends, and connect to each other and then
to the urogenital sinus via a sinovaginal bulb at their caudal ends. The
mesonephric system will persist in the male and the paramesonephric sys-
tem in the female. In males, the mesonephric system gives rise to the effer-
ent ductules, epididymis, ductus deferens, seminal vesicles and ejaculatory
ducts. In females the paramesonephric system gives rise to the oviduct,
uterus and upper part of the vagina.
Inmales,theurogenital sinus endodermgives rise to the epithelia of
theurethraand associated prostateandbulbourethral glands.
In the female,the endoderm of the urogenital sinusis the origin of the
epithelium of the lower vagina,the upper portion being formed by the
paramesonephric ducts.
Male differentiation of external genitalia requires androgens. Female
differentiation is the intrinsic pathway and occurs in the absence of andro-
gens and/or functioning androgen receptors.
DEVELOPMENT OF THE PLACENTA
AND FETAL MEMBRANESThe fetal portion of the placenta forms from the trophoblast.
Syncytiotrophoblastcells are in direct contact with maternal tissue,
whereas the embryo proper is separated from the cytotrophoblast by
extraembryonic mesoderm(together, the chorion).
Primary villus:syncytiotrophoblast with a cytotrophoblast core.
Secondary villus:Cytotrophoblast core invaded by extraembryonic mesoderm.
Tertiary villus:Fetal blood vessels invade the mesoderm (week 3).
The presumptive umbilical blood vesselsform in the wall of the
allantois,an endodermal outpocket of the urogenital sinus.
Theamnionic membranedevelops from epiblastand is continuous
with embryonic ectoderm. The lining of the yolk sacdevelops from
hypoblastand is continuous with embryonic endoderm.