L38 Embryology of the Gonads Flashcards
What determines sex?
For a period of time the external genitalia is the same in the male and the female. Genetic determination of sex however occurs at fertilisation - it is dependent on the sperm.
SRY region of the chromone determines the sex of the baby. Sex-determining region of the Y chromosome gene initiates the production of the Testis-Determining Factor (TDF). If there is a Y chromosome there is an SRY region. SRY makes the protein TDF. This kicks off a cascade that makes testes. Without the chromosome, there is not TDF and so female development occurs.
What is the reproductive system derived from?
Intermediate mesoderm - from the urogenital ridge
What is the indifferent gonad?
The epithelium of the genital ridge starts to proliferate and thickens. It then penetrates the underlying mesenchyme to form primitive sex cords (the projections). Male and female are indistinguishable.
The indifferent gonad can be split it into a an:
- Outer cortex - where there is the developing epithelium that is projecting into the inner medulla of the gonad
Inner medulla
What happens in week 6?
Towards the end of week 6 there is migration of primordial germ cells - cells from the yolk sac migrate to the genital ridge. The primitive germ cells get to the posterior wall through the dorsal mesentery. Primordial germ cells migrating to this area is very important, if they do not, no gonads will develop.
How do the gonads differentiate?
Within the primitive sex cords, there are sematic support cells. The primordial germ cells migrate to the genital ridge and become invested within the primitive sex cords.
The primordial germ cells have their genetic information within them. XX/XY getting to the indifferent gonad determines there we make an ovary or gonad. In 44+XY we have the SRY region which makes the TDF. This protein means no cortical cords develop but stimulates the medullary cords (primitive sex cords projecting into the medulla) to develop further. It also encourages an outer layer called the Tunica Albuginea to develop. In the female, without the SRY region, we do not make TDF we develops cortical cords and the medullary cords regress and the tunica albuginea regresses.
How do the ovaries develop?
In a female, the medullary cords degenerate without the TDF. Secondary sex cords develop called cortical cords. This is a thickening of the epithelium, they develop in the cortex of the developing gonad. The cortical cords invest the primordial germ cells and separate from the epithelium but they stay within the cortex region. The sematic support cells around the area invest the primordial germ cells and form follicular cells. The primordial germ cells are then surrounded by follicle cells. The follicle cells signal back to the primordial germ cells so they undergo the first stage of meiosis - Primary oocyte.
At what week do the gonads become distinguishable?
Week 7
Describe the descent of the testes?
The gubernaculum is an tendinous cord that attaches to the inferior pole of the gonad. The ovaries go from the posterior wall to the pelvis. The testis move from the posterior wall to the pelvis where it stays for a while before being pulled to the scrotum through the inguinal canal. The gubernaculum extends out and attaches to the pelvis. It is thought the gubernaculum shortens pulling the testes towards the anterior abdominal wall (week 7-12). The testes arrive at the deep inguinal ring. It stays there from month 3 to 7. The gubernaculum grows again into the scrotal swellings. There is an extra-abdominal portion of the gubernaculum attaching to the scrotal swelling. Females also have this scrotal swelling - there is an indifferent stage of the external genitalia. As the gubernaculum shortens, now attached to the scrotum, it pulls the testes through the anterior abdominal wall. This is also aided by the abdominal contents due to the increased intraabdominal pressure.
How do the testes become covered by layers of fascia?
The fascial that sits just on top of the muscle, goes through with the testes as it descends.
The gubernaculum shortness and the testes descends to the level of the deep ring. The parietal peritoneum has formed the processes vaginalis. At week 12 this has gotten even bigger. The testes follows the invagination. As it goes, it takes the layer of fascia with it. There are then layers covering the testes - around the testes there are called internal spermatic fascia. They are derived from facia from the anterior abdominal wall.
The processus vaginalis closes off, forming a tunica vagnialis. The next layer that goes through is the transveralis fascia which forms the internal spermatic fascia. The fascia from the internal oblique forms the cremasteric fascia. As it goes through the fascia, it takes some of the muscle fibres with it and so the muscle can contract. The external oblique muscle forms the external spermatic fascia.
What is a patent processes vaginalis?
There is a slight invagination of the parietal peritoneum - the processus vaginalis. This makes the road by which the testes can get pulled through. Within the first year after birth this invagination should have closed off. If it doesn’t close off, the abdominal contents can follow through leading to a hernia. When it closes off, it flattens and becomes a layer sat on the testes - the tunica vaginalis. It is another coat on the testes and a remnant of the parietal peritoneum.
If it doesn’t close off, it can form a fluid filled cysts called a hydrocele. If it remains patient completely, abdominal contest can protrude into the through the inguinal canal - indriect inguinal hernia.
What is a hydrocele?
A hydrocele is a collection of fluid in the scrotum. Most develop for no apparent reason and are harmless. Hydroceles are normally painless. It is difficult to tell whether it is a tumour or a hydrocele. To test this, pass a light through it, a hydrocyle will transmit the light but a tumour will not. A tumour is treated by closing of off the opening. It is not uncommon for someone to present later.
What is cryporchidism?
Testes can get stuck within its path - most get stuck in the high scrotum. It can be surgically corrected, pull the testes through - there may be tension of the testicular artery which lengthens during descension. Or may ether the testes to the scrotum. Cryptorchidism has an increased risk of testicular cancer.
Describe the descent of the ovaries.
The gubernaculum shortens and pulls the ovaries into the pelvis. The gubernaculum passes through the inguinal canal and inserts onto the labia majora. Hormones play a role in how far some of the structures descent - it shortens less in a female - they descend to the pelvic brim. The gubernaculum persist in the adult as the ovarian ligament proper and the round ligament of the uterus (from the uterus through the inguinal canal connecting to the labia majora).
Describe the development of the testes.
Development of the testes
Under the influence of TDF, the gonad develops into a testes. The primitive sex cords (medullary cords) invest primordial germ cells. The TDF causes the medullary cords to continue to develop. Further into the medulla - towards the hilum - the cords break up and are called Rete testis. The testis cords develop to from C shaped tubules called seminiferous tubules. The rete testis join up with the seminiferous tubules. Under the influence of TDF, the medullary cords develop into seminiferous tubules. An outer tunica albuginea develops along the outer edge of the testes (a thickening). The seminiferous tubules have separated away from the outer cortex. When a male is born up until puberty, the seminiferous cords are solid. At puberty, the tubules canalise. Once they are solid tubules, the male can make mature sperm.
What is the cremasteric reflex?
The cremasteric reflex is where the muscle contracts and is controlled by the genital femoral nerve. The femoral branch supplies some of the area of the thigh. Touching the inner aspect of the thigh elicits the cremasteric reflex - the cremaster muscle will contract and pull the testes towards the body.
