Embrology of the urogenital system. Development of the genital system.Dr Mberesero J.
Introduction.• The key to sexual dimorphism is the Y chromosome which contain the sex determining region on Y (SRY) short arm (Yp11).• Male and female morphological characteristics begin to develop by seventh week the initial period of genital development is the indifferent stage of sexual development.
Development of gonads.• The gonads (testes and ovaries ) are derived from three sources. – Mesothelium (mesoderm epithelium) lining the posterior abdominal wall. – Underlying mesenchyme ( embryonic connective tissue) – Primordial germ cells.• Indifferent gonads. – By fifth week a thickened area of mesothelium develops on the medial side of the mesonephros.
Indifferent gonads cont:• Proliferation of this epithelium and the underlying mesenchyme produce a bulge on the medial side of the mesonephros the gonadal ridge.• Finger like epithelial cords the gonadal cords grow into the underlying mesenchyme.• The indifferent gonads consist of an external cortex and an internal medulla.
Indifferent gonads cont:• In an embryo with XX the cortex of the indifferent gonads differentiate into an ovary and the medulla regresses.• In an embryo with XY the medulla differentiate into a testis and the cortex regresses.
Primordial germ cells.• These are large spherical cells visible in the 4 th week among the endodermal cells of the umbilical vesicle near the origin of allantois.• In the 6th week the primordial germ cells arise from yolk sac endoderm migrate to the gut and then through dorsal mesentery to reach the gonadal ridge.• Germ cells proliferate and migrate into cellular cords to become surrounded by supporting cells (germ cells that fail to enter a cellular cord undergo degeneration).
Sex determination.• The sex of the embryo depends on whether an X-bearing sperm or a Y-bearing sperm has fertilized the X- bearing oocyte.• Before 7th week the appearance of the two gonads is identical.• Development of the male phenotype requires a Y chromosome.• The SRY gene for a testis-determining factor (TDF) has been localized in the sex determing region in short arm of Y chromosome.
Development of the ovary.• The X chromosome bear genes for ovarian development.• Cortical cords extend from the surface epithelium of the developing ovary ito the underlying mesenchyme.• As the cortical cords increase in size the primordial germ cells are incorporated in them.• At approx. 16 weeks the cords begin to break into isolated cell clusters- Primordial follicles constisting of oogonia.• Active mitosis of oogonia occur during fetal life.
Development of the ovary cont;• No oogonia form postnataly, although many degenerate before birth, two milion or so that remain enlarge to become primary oocyte before birth.• primary oocytes commence meiosis, but remain frozen in prophase of Meiosis I until ovulation in the reproductively capable female.
Development of the genital ducts.• During 5th and 6th week there indifferent gonads.• The mesonephric ducts ( Wolffian ducts) male reproductive system.• The paramesonephric ducts ( Müllerian ducts) female reproductive system.
Female genital ducts and glands• Female sexual development does not depend on presence of ovaries or hormones.• Uterine tubes develop from unfused cranial parts of the müllerian ducts.• The caudal fused portion of these ducts form the uterus and the vagina ( superior part)• The endometrial stroma and myometrium are derived from splanchnic mesenchyme. .• Fusion of the müllerian ducts also brigs together a peritoneal fold that forms the broad ligament and two peritoneal compartments– the rectouterine pouch and vesicouterine pouch.
Uterus and vagina.• The fibromuscular wall of the vagina is derived fro the surrounding mesenchyme.• The uterovaginal primordium with the urogenital sinus form the sinus tubercle induce formation of paired endodermal growth the sinovaginal bulbs.• The sinovaginal bulbs fuse to form a vagina plate whose central cells break down later to form the lumen of the vagina.• Until late fetal life, the lumen of the vagina is separated from the cavity of the urogenital sinus by a membrane the hymen ( derived from invagination of the posterior wall of the urogenital sinus).
Development of external genitalia• The external genitalia are not fully differentiated until the 12th week.• At 4th week genital tubercle in both sexes at the cranial end of the cloaca membrane.• Labioscrotal swellings and urogenital folds develop on each side of the cloaca membrane.• The genital tubercle soon elongates to form the primordial phallus.
Female External Genitalia.• The primordial phallus in the female gradually becomes the clitoris ( relatively large at 18 weeks).• The urethral folds do not fuse, except posteriorly where they join to form the frenulum of the labia manora.• The unfused parts of the urogenital folds forms the labia minora.• Most parts of the labiaoscrotal folds form the labia mijora.