|
TISSUE LOCALIZATION AND
FUNCTION
It appears that dimeric Inhibin is produced
by the testis in the male (B only) and ovary in the
female (A and B) together with the fetoplacental unit in
pregnancy (Inhibin A only). They act in direct negative
feedback on pituitary production of FSH in the control
of folliculogenesis and spermatogenesis.
In the testis the sertoli cells
promote spermatogenesis by raising the local
concentration of testosterone, under the stimulation of
FSH. They produce Inhibin B as a regulatory mechanism
with negative feedback on FSH at the anterior
pituitary.
The granulosa cells of the
developing follicle produce Inhibin B as part of the
exquisite regulation that leads to follicular dominance
in the menstrual cycle. Inhibin A is also produced by
the follicle prior to ovulation and subsequently by the
corpus luteum in the luteal phase.
The monomeric alpha subunit is
produced in the brain and the adrenal gland (5,6) as are
the two beta subunits which dimerise to Activin, but no
dimeric Inhibin has been shown to be produced outside
the gonads. Activin A is found in heart, brain, bone
marrow and skeletal muscle. Activin has been implicated
in embryogenesis, osteogenesis, and hematopoeisis as
well as apoptosis in hepatocyte models.
Follistatin binds Activin and to
some extent Inhibin. Its precise role is not yet fully
established but is generally seen as a regulator of
Activin, whose activity is significantly diminished when
bound to Follistatin. It may be acting as a reservoir
for Activins.
|