Male Physiology & Fertility Flashcards
HPT axis
GnRH –> LH & FSH –>
- LH –> leydig –> testosterone
- FSH –> sertoli –> AMH + inhibin B
feedback loops in HPT axis
short: inhibin B inhibits FSH
long: testosterone inhibits GnRH
no feedback: AMH does NOT participate in feedback
what type of feedback loops are used in the male HPT axis
negative feedback loops
testosterone binds ARs on KNDy neurons –> decrease GnRH
testosterone converts to estradiol which binds to ERs –> decrease GnRH
what to Sertoli cells secrete
estrogen
AMH
inhibin B
ABP (androgen binding protein)
where are Sertoli cells located
near the basement membrane (basal compartment) of seminiferous tubules
androgen binding protein (ABP)
concentrates testosterone in the seminiferous tubules for spermatogenesis
what do Leydig cells secrete
testosterone and estrone sulphate
where are Leydig cells located
in the interstitium between seminiferous tubules
separated from developing sperm due to connection to blood vessels
primordial germ cells (PGCs)
progenitors for spermatogenesis
do NOT secrete hormones
what stimulates and initiates spermatogenesis
local testosterone production by leydig cells
how do LH and FSH support the local androgenic environment
LH stimulates leydig cells to produce testosterone
FSH stimulates Sertoli cells to produce ABP to concentrate androgens in tubules
type A spermatogonia
diploid cells that divide via mitosis along the basement membrane of the seminiferous tubules
how long are mitotic cycles of type A spermatogonia
12 days long
(a new type A cell will divide every 12 days)
type B spermatogonia
diploid primary spermatocyte (the daughter cell of type A mitosis that separates from the basement membrane and begins traveling toward adluminal compartment)
how does the type B spermatogonia divide
undergoes meiosis to produce a haploid secondary spermatocyte which crosses the Sertoli cell tight junctions to enter the luminal compartment
continues dividing via meiosis until spermatozoa is produced
how often does the tubular release of spermatozoa occur
every 50-60 days
(4-5 mitotic cycles will have occurred on the basement membrane by the time the first sperm is released)
what does the daily sperm production depend on
the number of Sertoli cells
(which is determined by proliferation during fetal/neonatal development)
what regulates spermatogenesis
pulsatile secretion of GnRH
(will stimulate LH –> testosterone)
what happens if GnRH secretion is continuous instead of pulsatile
continuous GnRH inhibits LH and testosterone secretion
ultimately decreases spermatogenesis
how does bilateral castration affect levels of testosterone, GnRH, LH, and FSH
- removal of testes –> initial decrease in testosterone
- low testosterone stimulates GnRH secretion –> increases LH/FSH
- lack of testes prevents an increase in testosterone, which results in no spermatogenesis
low testosterone
high GnRH, LH, FSH
how would injecting testosterone into a bilaterally castrated male affect GnRH, LH, and FSH concentrations
decreases GnRH, LH, and FSH
how would injecting inhibins into a bilaterally castrated male affect GnRH, LH, and FSH
only affects short feedback loop
decreases FSH
unaffected GnRH, LH (still high)
how does unilateral castration affect levels of testosterone, GnRH, LH, and FSH
- removal of a single testis –> initial decrease in testosterone
- low T stimulates GnRH, LH, and FSH
- remaining testis responds to high LH/FSH to increase testosterone production (hypertrophy)
- increased testosterone production increases spermatogenesis
causes compensatory hypertrophy of the contralateral testis
what does the degree of compensation depend on
the number of Sertoli cells (the amount of increase in sertoli cell number)
