0930 - Embryology of Reproductive Organs Flashcards
What are the major genital changes in the fourth week of the embryo?
Functional Mesonephros arises from intermediate mesoderm, connected by to cloaca by functional mesonephric (Wolffian) ducts. Contain glomerulus, Bowman’s capsule and mesonephric tubule that connects to mesonephric duct.
Outline the formation of indifferent gonads
Primordial germ cells migrate from the yolk sack to the dorsal mesentry and on to the mesonephric ridge. There, they join with the coelomic epithelium (becomes Sertoli cells) and embryonic connective tissue/mesenchymal cells (become Leydig cells). Seperately, coelomic epithelium begins to invaginate, forming the paramesonephric (Mullerian) duct. Occurs in week 5-6, and male and female genitals are identical (indifferent) until around week 7, when SRY gene switches on in males.
Describe the formation of the mesonephric (Wolffian) duct.
Forms in fourth week. Arises from the intermediate mesoderm, connecting the mesonephric (Wolffian) ducts to the cloaca. With further development, develops into the vas deferens.
In males, becomes vas deferens, in females, degenerate.
Describe the formation of the paramesonephric (Mullerian) duct.
Formed by an invagination of the coelomic epithelium. Ultimately develops into the fallopian tube in females, in males, it degenerates.
Where do the male testes, genital ducts and external genitals arise from and when?
Male reuses embryonic urinary stuff to form genitals
Differentiation begins at 7 weeks. After the SRY gene activates and produces testis-determining factor, the testis cords develop at the mesenchymal ridge. They consist of primitive germ cells surrounded by cells of Sertoli (from coelomic epithelium - secrete antimullerian hormone), and interstitial cells of Leydig (from mesenchymal cells) that secrete testosterone.
Genital ducts - mesonephric tubules become the efferent ductules of the epididymis, and mesonephric ducts become the epidydimis, vas deferens, and seminal vesicles. Mullerian ducts degenerate from around 16 weeks in response to AMH. Urethra becomes the prostate and bulbourethral glands.
Where do the female ovaries, genital ducts and external genitals arise from and when?
From 7 weeks, in the absence of a Y chromosome (with SRY and thus TDF), female development occurs. The medullary cords degenerate, and cortical cords develop at the mesonephric ridge. Germ cells become the oocytes, surrounded by epithelial follicular cells.
Mullerian (paramesonephric) ducts fuse to form uterus and upper vagina, with unfused sections becoming the Fallopian tubes. Mesonephric (Wolffian) ducts and tubules degenerate.
What are the key hormones in differentiating gonads, genital ducts and external genitals?
TDF (testis determining factor), a protein coded for by SRY gene, causes the medullary cord to develop in males. In its absence (females), a cortical cord develops.
Sertoli cells secrete AMH (anti-Mullerian Hormone) in males, causes mullerian ducts to degenerate. In females, no AMH, and Mullerian ducts fuse to form fallopian tubes, uterus, and vagina.
Leydig cells secrete testosterone in males - causes mesonephric ducts to develop into male genital ducts and formation of male external genitalia. In females, mesonephric ducts degenerate and female external genitalia forms.
What is the relationship between urinary and genital systems during development
In males, the urinary system is re-used as genitalia, including sharing the urethra between urine and semen.
In females, the urinary and genital systems are kept entirely separate - dedicated urethra and dedicated vagina.
