Embryology 5 - sex determination

Question 1

sex in the initial development of embryos

Answer 1

same for both males and female, foundations for both sexes are created

Question 2

where are the gonads developed

Answer 2

in the trunk of the body, around halfway between shoulder and pelvis

Question 3

where does the germ line come from

Answer 3

the epiblast cells that were removed from the body around the time of gastrulation

Question 4

where does the germ line end up

Answer 4

outside the body in the yolk sac - problem because germ line needs to be in the gonads

Question 5

what is the mesentery

Answer 5

the sheet like connection between the gut and the rest of the body

Question 6

how do germ cells invade the body

Answer 6

by use of the connection to the yolk sac, and the gut and its mesentery

Question 7

what happens when the germ cells reach the level of the developing gonads

Answer 7

they move up the mesentery and then move across to enter the gonads themselves

Question 8

chromosome constitution of females

Answer 8

XX

Question 9

chromosome constitution of males

Answer 9

XY

Question 10

what determines sex

Answer 10

a gene on the Y chromosome (SRY)

Question 11

how does SRY act

Answer 11

primitive gonads consist of somatic cells and germ line cells
somatic cells express SRY if it is present
SRY forces somatic cells to develop into testis cells - otherwise they form ovary cells

Question 12

what does the Y chromosome affect

Answer 12

only the production of testis. everything else stems from cues from the testis - communication by excreting androgenic hormones

Question 13

key somatic differentials in females - Y2

Answer 13

mullerian ducts make oviducts, uterus, cervix uteri and upper vagina
wolffian ducts disappear
cloaca makes lower vagina
phallus develops into clitoris
labioscrotal folds remain separate and develop into labia
gonads remain internal

Question 14

key somatic differentials in males - Y2

Answer 14

mullerian ducts disappear due to AMH - anti-mullerian hormone
wolffian ducts become vas deferens
cloaca does not become vagina
phallus develops into penis
labioscrotal folds fuse to become scrotum
gonads descend into scrotum

Question 15

sexual dimorphism - external obvious differences between males and females

Answer 15

average height and mass
body shape
development of external genitalia
development of mammary glands
body hair pattern - extent varies with race

Question 16

sexual dimorphism - differences between the male and female brain

Answer 16

example - gender specific sexual behaviours

Question 17

androgen insensitivity

Answer 17

where chromosomes dont match external features

Question 18

cause of androgen insensitivity

Answer 18

testosterone is a weak androgen meaning it only weakly stimulates androgen receptors
5⍺-reductase converted by tissues into 5⍺-dihydrotestosterone which stimulates androgen receptors strongly

Question 19

what do XY children with deficient 5⍺-reductase produce

Answer 19

female presenting bodies

Question 20

when do testosterone levels rise and to what level

Answer 20

beginning of puberty
high enough to act as an androgen even in absence of 5⍺-reductase

Question 21

vast range of intersex phenotypes

Answer 21

generally children born with them undergo ‘corrections’ to conform with societal expectations of belonging clearly to one binary sex or another, even if they are at no clinical risk because of their phenotype