The Reproductive System; Sex Determination

Question 1

Understand when germ cells develop, and how these cells proliferate, migrate, and eventually colonize the genital ridges to form oocytes or sperm in the mature gonads

Answer 1

1. germ cell precursors (primordial germ cells) develop from embryonic ectoderm in the early embryo (post implantation)
2. then must migrate from the yolk sac to and colonize the developing gonads at the genital ridge (has the mullerian and wolffian ducts)
3. once the PGCs get to the genital ridge, the genital ridge proliferates and forms the irregularly shaped primitive sex cords (which looks similar in males and females and is therefore referred to as the indifferent gonad, which forms adjacent to the ducts)

Question 2

identify the key gene that signals sex determination in mammals

Answer 2

SRY gene; the testis determining gene in mammals, expressed on the Y chromosome and encodes a transcription factor (SOX9) that is essential to promote testis formation

Question 3

understand what key factors regulate the development of the bi-potential gonad to form either the ovary or testis

Answer 3

1. genetic (chromosomal) sex determines gonadal sex: XX = females = ovary, XY = males = testis
   -XY carries SRY gene which promotes SOX9 and testis chord formation
   -XX: absence of SRY and presence of WNT4 promotes ovary formation
2. hormones promote secondary sex characteristics: secreted by the differentiated gonads, determine phenotypic sex and internal/external genitalia

Question 4

identify the key hormones and describe how they regulate the development of the reproductive tract in males (4) and females (4)

Answer 4

males:
1. SRY/SOX9 leads to differentiation of testes, which develops sertoli cells and leydig cells
2. sertoli cells secrete anti-mullerian hormone, which causes mullerian duct regression
3. leydig cells secrete testosterone, which on its own causes the wolffian duct to differentiate and form the internal genitalia
4. testosterone, in the presence of 5alpha-reductase, is converted to 5alpha-dihydrotestosterone (5a-DHT), which leads to development of the exernal genitalia

females:
1. WNT4 (ovary-determining gene) leads to differentiation of the ovary, which develops granulosa cells and theca cells as follicles form
2. follicles secrete fetal estrogen
3. the absence of testosterone causes wolffian duct regression
4. fetal estrogen causes the mullerian duct to differentiate to form internal genitalia

Question 5

describe the fate of the mullerian and wolffian ducts in the reproductive tract formation

Answer 5

in males, the mullerian duct regresses in response to anti-mullerian hormone secreted by the sertoli cells

in females, the wolffian duct regresses in response to a lack of testosterone

Question 6

list and describe the 4 types of female repro tract

Answer 6

1. duplex uterus with 2 cervices: rodents, rabbit, marsupials
2. bicornuate uterus with highly developed uterine horns (bipartite): marine mammals and carnivores (dogs and cats)
3. bicornuate uterus with moderately developed uterine horns: bats, cows, sheep, horses
4. simplex uterus, no uterine horns: higher primates and humans