Embryology Flashcards
What happens in weeks 1 and 2 of gestation?
Blastocyst forming and implanting in the uterus, starts to form 3-layered embryo
What happens in week 3 of gestation?
Gastrulation
Neurulation
Somite formation
What happens in weeks 4-8 of gestation?
Organogenetic period
What happens from week 9 onwards of gestation?
Maturation of organ systems
What is the timeline of reproductive system organogenesis?
Reproductive system organogenesis occurs beyond 8 weeks and beyond the post-natal period, it is not complete when the child is born
What are the 3 initial sets of kidneys that develop in gestation?
Pronephros - first rudimentary kidney
Mesonephros - briefly functional kidney with ducts and primitive urethra
Metanephros - functional, in pelvic region
What does the cloaca divide to form?
Rectum and urogenital sinus
What does the mesonephric duct drain into?
Urogenital sinus
What does the urogenital sinus form later in development?
Urinary bladder, caudal end forms urethra
The epithelium of the mesonephros invaginates to form
a tube which then becomes the paramesonephric duct
What does the paramesonephric duct develop lateral to in development?
Developing gonads and mesonephric duct
Where does the cranial end of the paramesonephric duct open?
Into the peritoneal cavity
What happens to the mesonephros at week 6 of development?
Forms a long ovoid structure in association with the thoracic and upper lumbar vertebrae
The medial part of the mesonephric ridge thickens to form
a genital/gonadal ridge
At week 6, germ cells migrate from the yolk sax via the
dorsal mesentery, and embed into the primitive sex cords
What are the primitive sex cords formed from?
Mesenchyme of genital ridges
What is the indifferent gonad?
A gonad that has not developed into a definitive ovary or testis, there is no migration of germ cells at this stage and so no true gonads
What primitive structures are needed to form the reproductive system?
Indifferent gonad Mesonephric duct (disappears in females) Paramesonephric duct (disappears in males)
What happens to the indifferent gonad, mesonephric duct, paramesonephric duct and urogenital sinus in males?
Gonad forms testis
Mesonephric duct develops into the epididymis, vas deferens and seminal vesicle
Paramesonephric duct degenerates
Urogenital sinus forms bladder, urethra and prostate
In the early stages of development, what is the only difference between the male and female embryo?
XY compared to XX
What makes the indifferent gonad develop into a testis?
SRY on the sex-determining region of the Y chromosome encodes a protein called TDF (testis determining factor) which makes indifferent gonad develop into a testis
What cells express the SRY gene?
Primordial sex cells within he sex cords of the indifferent gonad
What do Sertoli cells secrete?
Mullerian Inhibiting Substance (MIS)
What does MIS cause?
Paramesonephric duct to degenerate so only mesonephric duct remains
Also transorms mesenchymal cells between sex cords into Leydig cells which produce testosterone
At what week of gestation does TDF stimulate the primary sex cords to extend into the medulla?
Week 7
What gives rise to the interstitial cells of Leydig?
Mesenchyme between sex cords
Male external genitalia is formed under the influence of what?
Testosterone
In females, what happens to the indifferent gonad, mesonephric duct, paramesonephric duct and urogenital sinus?
Gonad forms ovary
Mesonephric duct degenerates
Paramesonephric duct forms Fallopian tube, uterus and cervix
Urogenital sinus forms bladder and lower part of vagina
Why do females not express TDF?
Two X chromosomes so no SRY region (on Y chromosome) and no TDF
How do the primordial follicles form?
Primordial germ cells bud off with a single surrounding layer of cortical cord cells
How are additional follicles formed, once primordial follicles are formed?
Active mitosis of the primordial follicles
By what month of development have all follicles formed?
7
In what phase of meiosis are oocytes arrested in?
Prophase (first meiotic division)
How many primordial follicles are present at birth?
2 million
After puberty, how many follicles per cycle are developed?
12-15 follicles per 28 day cycle
What causes the degeneration of the mesonephros and mesonephric duct?
Absence of testosterone
What do the cranial and caudal ends of the paramesonephric duct become?
Cranial end becomes associated with the ovary
Caudal end becomes uterus and superior part of vagina
In what region are gonads initially formed? What does this mean as development takes place?
Initially form in lumbar region
Positional changes occur as development takes place
Where do the gonads descend to in the male and female?
Descend out of the abdomen through inguinal canal to the scrotum in males
Descend to pelvis in females
What is the level of the testis at week 7 and week 12 of development?
Week 7 - level of T10
Week 12 - level of deep inguinal ring
What is the level of the ovary at week 7 and week 12 of development?
Week 7 - level of T10
Week 12 - broad ligament
What aspects of development are linked to the descent of the gonads?
Male - formation of the larger inguinal canal
Female - presence of round ligament and its passage through the inguinal canal
What is cryptorchidism?
Absence of one or both of the testes from the scrotum
What are the complications of cryptorchidism?
Infertility
Malignant transformation
Testicular torsion
What is the treatment of cryptorchidism?
Orchiopexy
What are the complications of remnants of the mesonephric duct in females?
Remnants can collect fluid which can compress the ovary (causing infertility) or interfere with implantation or childbirth
