ICL 18.1: Embryology of the Male and Female Reproductive System Flashcards
what embryological structure is the genital system derived from?
a common ridge of intermediate mesoderm that lies along the posterior abdominal wall
this is the same for the urinary system which comes from the same embryological structure!!
when do the male and female genital systems differentiate during development?
up until the end of the 6th post conceptional week, the reproductive system is indifferent, i.e., no male/female distinction
the embryo is undifferentiated until the beginning of the 7th week
gonadal differentiation (ovaries vs. testes) determines how reproductive tract and external genitalia develop
what are the structures that are part of the intermediate mesoderm during development?
urogenital ridge has 2 parts:
1. genital ridge
- nephrogenic cord
has 2 ducts that are critical for male/female reproductive organs = mesonephric duct and paramesonephric duct
where do the gonadal cells originate from?
the GI system develops from the endoderm
the gametes don’t start in the gonads! the primordial germ cells migrate from the yolk sac in the endoderm along the dorsal mesentery of the GI tract to the genital ridge of the mesoderm
where do the gonads come from?
the gonads are derived from the genital ridge (intermediate mesoderm)
primordial germ cells (precursors to the gonads) migrate from the yolk sac to the genital ridge and land in the outer epithelial layer of the genital ridge
at this time (6 weeks post conception), the gonad is undifferentiated and at this time you start to functionally differentiate into testes vs. ovaries!
at this point you have epithelial projections into the mesoderm with the developing gonads which are your primary sex cords! you primary sex cords are part of the proliferating epithelium of the genital ridge that contain the primordial germ cells
how do the gonad know to become testis?
if a Y chromosome is present, the SYR gene on this chromosome codes for testes determining factor (TDF)
TDF results in some of the primary sex cord cells being transformed the tubules of the testis and into Sertoli cells which will produce muellerian inhibiting substance (MIS)
MIS stimulates interstitial (Leydig) cells to produce testosterone
primitive sex cords contain primordial germ cells which have differentiated into spermatogonia that are supported by Sertoli and Leydig cells which product MIS and testosterone which are critical for the development of the rest of the male reproductive tract
how do the gonad know to become ovaries?
if there’s no TDF present, you develop an ovary
the primary sex cords degenerate and become an embryonic remnant and you get a secondary proliferation of the epithelial covering called your secondary sex cords which contain your primordial germ cells
as the secondary sex cords proliferate they will proliferate in follicular-like structures –> so the primordial germ cells called oogonia are surrounded by the secondary sex cells
what do the primitive sex cords become if you are destined to be a boy?
medullary cords
- seminiferous tubules (PGCs –> spermatogonia)
- straight tubules
- Rete testis
what are sertoli cell and what are they derived from?
derived from seminiferous tubules of the primitive sec cords
they surround primordial germ cells and secrete MIS
what are Leydig cells?
they’re in-between the sertoli cells
secrete testosterone
what do the primitive sex cords become if you are destined to be a girl?
primitive sex cords degenerate!
secondary sex cords develop and they become primordial follicles that contain premordial germ cells that become oogonium
which embryologic structures gives rise to a primordial follicle?
secondary sex cord
what is the origin of the internal reproductive tracts?
nephrogenic ridge:
paramesopnephric ducts –> female reproductive tract (uterus, fallopian tubes, cervix, part of vagina)
mesonephric ducts –> male reproductive tract (vas defers, seminal vesicle)
cloaca (distal part of hindgut) has the urogenital sinus which becomes the external genitalia
if you’re a girl, what happens to the mesonephric ducts?
they will degenerate
your paramesonephric ducts will persist
distally they are fused at the midline which is called the uretovaginal primordial which is critical is forming the internal reproductive tract –> it forms the uterus, cervix and upper vagina
above this fusion, the fallopian tubes form
if you’re a girl, what happens to the mesonephric ducts?
they persist and the paramesoneprhic ducts degenerate due to MIS being secreted by the sertoli cells!
the mesonephric ducts then need testosterone to differentiate into the structures of the male reproductive structures:
1. efferent ductules
- ductus epididymus
- ductus deferens
- seminal gland
- ejaculatory duct
where does the lower part of the vagina come from?
the fused part of the paramesonephric ducts (uterovaginoprimordium) connects with the urogenital sinus and becomes the sinovaginal bulb which gives rise to the lower part of the vagina
the sinovaginal bulbs fuse in the midline to form the vaginal plate –> the central cells of the vaginal plate then breakdown to form the lumen of the lower vagina
the upper part comes from the fused paramesonephric duct
when does external genitalia start to differentiate?
up until week 7, the external genitalia are similar in both sexes
differentiation of external genitalia begins at week 9
full differentiation occurs by week 12
what is the indifferent stage? what are the parts of the undifferentiated genitalia?
when the external genitalia aren’t differentiated
before week 9 you will have:
- genital tubercle which is the precursor for the phallus (penis or clitorus)
- cloacal membrane
- urogenital fold on both sides of the cloacal membrane
- labioscrotal swelling
how does external genitalia turn into male external genitalia? which hormones are responsible for which structures?
testosterone produced by Lydig cells is essential!
testosterone is converted into dihydrotesterone by 5-alpha-reductase
dihydrotesterone presence leads to development of:
- urethra
- penis
- scrotum
- prostate
testosterone leads to development of:
- epididymis
- vas deference
- seminal vesicles
phallus/genital tubercle –> penis
urogenital folds fuse along ventral surface to form the spongy (penile) urethra
labioscrotal folds fusion –> scrotum
how does external genitalia turn into female external genitalia? which hormones are responsible for which structures?
if there isn’t TDF present:
- phallus ceases to grow and becomes the clitoris
- unfused urogenital folds form the labia minoria
- most of the labioscrotal folds remain unfused and form the labia majora
how can a phenotypic female be 46, XY?
the main cause is Androgen Insensitivity Syndrome (AIS)
- this is usually caused by mutations of the gene encoding the androgen receptor AR gene located on the X chromosome or mutation in the SRY gene –> so even though you’re XY, you will undifferentiated gonads with features of both the testis and ovaries – they might have some paramesanephric duct structures and external genitalia will be female but they won’t have menstrual periods because they don’t have ovaries
- 5-alpha-reductase deficiency = you have testosterone but no dihydrotestosterone –> no male external genitalia but they have testosterone so you will have a vas deferens! and the mesonephric duct persists
rare mutations in key sex determining genes including deletion or mutations of SRY
things that could cause female XY:
1. Y chromosome that direct the gonad to become a testis but there isn’t enough testosterone from the lydig
- deletion of the SRY gene
- 5-alpha-reductase deficiency
how do gonads move from the abdomen to the pelvis in a male?
gubernaculum anchors the gonad to the labioscrotal folds
depending on whether you’re XX or XY will determine where the gonads end up
if you have testis, the gubernaculum shortens and drags the testis through the abdominal wall and the testis to end up the scrotum and then the gubernaculum anchors the testis to the scrotum –> as the testis go through the abdominal wall is the process vaginalis
how do gonads move from the abdomen to the pelvis in a female?
gubernaculum anchors the gonad to the labioscrotal folds
depending on whether you’re XX or XY will determine where the gonads end up
if you have ovarie s, the gubernaculum differentiates into the uterovarian ligament and the round ligament:
- uterovarian ligament formed by the cranial part of the gubernaculum anchors the ovary to the top of the uterus
- round ligament formed by the caudal portion anchors the lateral aspect of the uterus to the labia majora
so the ovaries dont have a true descent like the testis but rather a change in position –> ovaries move from posterior abdominal wall to the pelvic brim
