Embryology

Question 1

when does gastrulation occur?

Answer 1

week 2 (2 layers) > week 3 (3 layers)

Question 2

what happens in gastrulation?

Answer 2

formation of the trilaminar disc
invagination of the epiblast cells through the primitive streak to form the definitive endoderm and mesoderm
mesoderm is then organised into 3 main groups
- paraxial
- intermediate
- lateral plate

Question 3

what 2 types of folding occur in development?

Answer 3

lateral folding (roll up into a tube)
cranial folding (top and bottom ends of tube curve up towards middle)

Question 4

do the urinary and genital systems develop together?

Answer 4

sort of

both arise from intermediate mesoderm and excretory ducts enter a common cavity (cloaca - will become urethra)

Question 5

what are the 3 main bits of the urogenital system embryologically?

Answer 5

pronephros (degenerates quickly after forming)
mesonephros (functional)
metanephric (kidney)

Question 6

how does the indifferent gonad form?

Answer 6

primordial germ cells form in the yolk sac and migrate via dorsal mesentery to the intermediate mesoderm during week 4-6
coelomic epithelium in intermediate mesoderm proliferates and forms genital ridges and somatic support cells
somatic support cells envelop the PGCs

Question 7

what are the 2 main primitive ducts in the urogenitary system development and where do they connect?

Answer 7

mesonephric (wollfian) duct
paramesonephric (mullernian) duct
both connect to the posterior wall of urogenital sinus

Question 8

what is the ambisexual phase and when does it occur?

Answer 8

phase where male and female anatomy is the same, before sexual differences are seen
occurs up to week 7

Question 9

what stimulates the development of male anatomy?

Answer 9

SRY protein

SRY = sex determining region of Y chromosome

Question 10

what happens in the presence of SRY?

Answer 10

somatic support cells develop into sertoli cells and produce medullary cords which engulf PGCs
medullary cords connect to mesonephric duct via rete testis
cords loose connection with cortex of indifferent gonad due to loss of tunica albuginea causing cortex to degenerate while the medulla develops

Question 11

how are sertoli cells involved in male development?

Answer 11

secrete AMH (anti mullerian hormone) which causes degeneration of the paramesonephric (mullerian) duct
stimulate leydig cells which secrete testosterone which induces formation of epididymus, vas deferens, seminal vesicles
5 alpha reductase turns testosterone into Dihydrotestosterone (DHT) which induces male specific external genitals and prostate development

Question 12

what does the mesonephric duct form?

Answer 12

epididymus

vas deferens

Question 13

what is persistent mullerian duct syndrome?

Answer 13

mutations of AMH or AMH receptor genes causing failure of paramesonephric duct degeneration 
therefore female features persist
- uterus, vagina and uterine tubes
- testes in ovarian location
- male external genitalia

Question 14

name the layers through which the testes descend?

Answer 14

peritoneum
subserous fascia
transversalis fascia
transversus abdominus muscle
internal oblique
external oblique

Question 15

what is the vaginal process?

Answer 15

invagination of parietal peritoneum over testicle as it descends through layers
where an inguinal hernia protrudes through

Question 16

at what level do the testes originate?

Answer 16

T10

Question 17

what pulls the testes caudally?

Answer 17

gubernaculum

Question 18

what is the name for an undescended testicle?

Answer 18

cryptochidism

Question 19

which 3 accessory glands sprout near the junction of mesonephric duct and urethra and when does this happen?

Answer 19

prostate (endodermal invagination from urethra)
bulbourethral gland (endodermal invagnation from urethra)
seminal vesicle (from mesonephric ducts)
occurs at week 10

Question 20

how does female gonadal development differ from male?

Answer 20

no SRY so no male lineage stimulated so no sertoli cells so no AMH so the paramesonephric duct persists
germ cells differentiate into oogonia then primary oocytes
somatic support cells differentiate into granulosa cells which surround primary oocytes forming primordial follicles in the ovary
cortical cords in cortex develop and medulla degenerates

Question 21

function of primordial follicles and thecal cells in female development?

Answer 21

influence of ovarian oestrogens and from maternal sources stimulates formation of the female external genitalia and development of paramesonephric ducts

Question 22

what does the paramesonephric duct become?

Answer 22

uterine tubes
uterus
superior vagina

Question 23

what are the 3 parts of the paramesonephric duct?

Answer 23

cranial (opens into coelomic cavity)
horizontal (crosses mesonephric duct)
caudal (fuses with paramesonephric duct on opposite side)

Question 24

how do the paramesonephric ducts form the uterovaginal canal?

Answer 24

paramesonephric ducts fuse together caudally at the urogenital sinus then start to zip up
they stop at a point and the connection between the two ducts degenerates forming one big cavity
end of the ducts forms the carvix
the vaginal lumen is created via vacuolization of the paramesonephric portion portion of the vagina and the sinovagital bulbs and the fornices

Question 25

the vagina is made of what types of tissue?

Answer 25

endoderm and mesoderm

Question 26

types of problems with female development?

Answer 26

double uterus and double vagina (ducts never fuse)
double uterus (only join but dont zip up)
bicornate uterus (septum doesnt fully degenerate)
separated uterus (septum doesnt degenerate)
unicornate uterus (only 1 duct)
cervical atresia (uterus doesnt connect to vagina cavity)

Question 27

external genitalia is the same in both sexes until which week?

Answer 27

week 7

Question 28

how does external genitalia develop in males?

Answer 28

spongy urethra forms proximal to distal zipping of urethral groove
ectodermal ingrowth at tip of the gland penis which meets spongy urethra
prepuce (foreskin) is formed by circular ingrowth of ectoderm around the periphery of glans (week 12)

Question 29

what does the genital tubercle become in males and females?

Answer 29

males = glans penis
females = clitoris