Embryology Of The Reproductive Tract
Table of contents
The female reproductive system permits the gametes (ova and sperm) to meet, and a new individual to grow and develop. The process is controlled by gonadotrophins, including FSH and LH, and sex steroid hormones, such as oestrogen.
In this article, we shall look at the embryological development of the female reproductive system: gonads, internal and external genitalia.
Development of Gonads
The indifferent gonad is derived from intermediate mesoderm and primordial germ cells (extragonadal). Beginning at the 4th embryonic week, the first phase of development is the migration of undifferentiated primordial germ cells, which arise from the endoderm lining of the yolk sac. They migrate dorsally along the yolk stalk, midgut and dorsal mesentery of the hindgut, into the retroperitoneum, until they arrive at the genital ridges. For two weeks, the primordial germ cells undergo mitosis numerously, forming many precursor gametes.
At the genital ridges, germ cells come in to contact with coelomic epithelium, which then differentiates into primitive germinal epithelium. The primitive germinal epithelium and germ cells collectively form sex cords.
Due to the lack of a Y chromosome, a genetic female XX conceptus will have no expression of the SRY gene. The products of this gene support the development of the primitive testis. Therefore, the absence of the SRY gene subsequently leads to the formation of the ovary, female internal genitalia and external genitalia.
Differentiation of the ovaries occurs two weeks later than that of the testes. The sex cords proliferate whilst connected to the supporting mesenchyme. Under the influence of placental gonadotrophins, the primitive germ cells differentiate into primordial follicles, which become enveloped by a layer of flattened epithelial cells. The sex cords are broken up by subepithelial mesenchyme, thus separating the follicles.
Development of Internal Genitalia
During early stages of development, male and female embryos have mesonephric (Wolffian) and paramesonephric (Mullerian) ducts, which make contact with the cloaca. The ureteric bud sprouts from the mesonephric duct.In males, the testes begin to secrete androgens. This supports the maintenance of the mesonephric duct. Therefore, in females, the absence of androgens consequently leads to the degeneration of the mesonephric duct. The regression of the mesonephric ducts produces a vestigial structure: Gartner’s duct. Additionally, the testes release Müllarian Inhibiting Hormone (MIH), which causes the paremesonephric duct to degenerate. The lack of MIH, in females, supports the development of the paramesonephric (Müllarian) duct.
The Müllarian duct grows caudally to be positioned medial to the Wolffian ducts. Fusion of the Müllarian ducts starts the process of development of the future fallopian tubes, uterus, cervix and upper third of the vagina. The middle septum, formed by the fusion, usually degenerates. Part of the cranial ends of the ducts remains open and differentiates into the fimbrial portion of the fallopian tubes. Cranially, mesoderm in the uterus region of the fused Müllarian ducts will form the future uterine endometrium and myometrium. The endoderm of the sinovaginal bulb canalised giving rise to the lower two-thirds of the vagina, once it has fused with the caudal most fused Müllarian ducts.
Also known as the ‘heart-shaped’ uterus, a bicornuate uterus is a uterine structural defect where there are two separate cavities cranially, but the lower part of the uterus is as normal. This occurs as a result of partial fusion of the uterus. Expectant mothers with biconuate uterus are considerd ‘high-risk’, as there is a possibility of miscarriage or premature birth.
Descent of the Ovaries
Significant growth of the abdomen results in the ovaries descending from the posterior abdominal wall into the pelvis.
A peritoneal fold, known as the gubernaculum, attaches the ovary to the uterus and the uterus to the labio-scrotal folds, and becomes the ligament of ovary proper and round ligament of uterus, respectively. The round ligament of the ovary travels through the inguinal canal before reaching the labio-scrotal folds.
Development of External Genitalia
Dihydrotestosterone (DHT) derived from the testis promotes the formation of the male external genitalia. However, in females, the absence of DHT prevents any fusion externally, thus the urethra opens in the vestibule at the anterior aspect of the urogenital sinus and vagina at the posterior aspect.
|Common Embryonic Structure||Female Development||Male Development|
|Genital Swelling||Labia Majora||Scrotum|
|Genital Folds||Labia Minora||Spongey Urethra|
|Genital Tubercle||Clitoris||Glans Penis|
Table 1: Differentiation of common embryonic precursors for external genitalia in male and females