LM 1.3: Indifferent Embryos Flashcards
what makes testes develop instead of ovaries?
during the 7th week after conception, the SRY gene on the short arm of the Y chromosome produces Testis-Determining Factor
TDF increases Sox9 expression leading to testes development – without SRY, Sox9 expression is decreased and ovaries develop
whats one of the earliest genes to be unregulated in the developing ovary?
Foxl2
it suppresses genes involved in testis differentiation, including Sox9
how are the testes formed?
the primitive sex cords continue to proliferate and penetrate deep into the mesenchyme or medulla to form the testis or medullary cords
the tunica albuginea forms cutting off their connection to the surface epithelium
the outer portion of the gonadal cords become seminiferous tubules containing sertoli cells and the maturing primordial germ cells called gonocytes – the sertoli cells release anti-mullerian hormone, also known as mullerian inhibiting substance (MIS) or factor
the inner portion of the gonadal cords form a meshwork and become the straight tubules and rete testis
the mesenchyme differentiates into short-lived fetal Leydig cells during the 8th week
how are the ovaries formed?
the primitive gonadal cords and a rudimentary rete ovarii disappear and are replaced by vascular stroma that forms the ovarian medulla
unlike the male, the surface epithelium continues to proliferate and gives rise to new “cortical” cords in the 7th week
the primordial germ cells become embedded in them and begin to proliferate and differentiate into oogonia and then primary oocytes –> differentiation stops in the 22nd week; no more are made ever
at 16 wks, the cortical cords split into isolated cell clusters around one or more developing primary oocytes
the surrounding epithelial cells form follicular cells
the primary oocyte and its follicular cells constitute the Primordial Follicle
the Tunica albuginea forms, separating the ovarian epithelium from the cortex
when does meiosis in the ovaries start?
meiosis starts in primary oocytes, but it is arrested at the diplotene stage of prophase
each oocyte remains in that stage until an LH surge in adulthood causes it to ovulate
how many primordial follicles does a female have at birth?
at birth, each ovary contains about 2 million primordial follicles; however, there is a reduction in the number of primordial follicles to about 200 000 by menarche (beginning of menses)
each primordial follicle contains a primary oocyte half way through its first meiotic division –> each oocyte remains in that stage until an LH surge in adulthood causes it to ovulate
where do the mesonephric and paramesonephric ducts originate from?
MIS intermediate mesoderm
what does MIS do?
represses the development of paramesonephric (Mullerian) duct
aka makes sure ovaries don’t form
what does testosterone do?
ensures the development of the epididymis, vas deference, and seminal vesicle
once it’s converted to DHT, that leads to the formation of the urogenital sinus and external genitalia
how do the testes end up in the pelvis?
the gubernaculumis a ligament attached to the bottom of the developing gonad and it runs diagonally down the posterior wall, behind the peritoneum, to where the entrance to the inguinal canal will be, then through the abdominal wall to attach to the labialscrotal fold
the testes are pulled transabdominally to reach the inguinal region by 12 weeks gestation – they make their final descent through the inguinal canal into the scrotum in the third trimester under the control of testosterone
the gubernaculum anchors the testes in place within the scrotum
what is the processus vaginalis?
an outpouching of the peritoneum called the processus vaginalis precedes the testis as it descends
it carries with it all the layers of the abdominal wall and creates the inguinal canal around the gubernaculum
as the testis descends it pulls the vas deferens and vessels of spermatic cord with it
afterwards the inguinal canal normally contracts around the spermatic cord
the remaining portion of the processus vaginalis around the testes becomes the Tunica Vaginalis
how do the ovaries end up in the pelvis?
ovaries also descend following the path of the gubernaculum, but they only descend to posterior pelvic wall
the caudal portion of the gubernaculum becomes the round ligament of the uterus
the cranial portion becomes the ovarian ligament.
what is hypospadias and epispadias?
instead of having an opening at the normal spot of the urethra, the urethra is either higher or lower
more common in men
when the urethral folds along the penile urethral don’t meet up and close properly which leaves an opening along the bottom of the penile shape = hypospadias
epispadias: during the 6th week of gestation the genital tubercle grows in the rectal direction instead of the cranial direction which results in an opening on the top of the penis
how is the urethra formed embryologically in men ?
8th week of gestation, both men and women have a genital tubercle that grows in the cranial direction
the genital tubercle responds to dihydrotestosterone and becomes a phalice and as it grows in length the urethral plate invaginate to form a urethral groove lined with epithelial cells
in the 14th week the 2 urethral folds pinch off to form the urethra
in the 17th week the ectodermal cells of the glans penis undergo vandalization so that the urethra and penile canals connect
how is the urethra formed embryologically in men ?
estrogen stimulates external genitalia formation
genital tubercle only gets a little longer and forms the clitorus
urethral folds dont fuse and instead form labia minor and vestibule
in the 13th week the urethra forms and is situated anterior to the vagina
hypospadias results into the urethra opening into the vaginal wall while epispadius results in the urehtra developing too far anteriorly
