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If we talk through the endocrine response that allows for the milk to be produced as well as let
down to get to the right spot, we have four different classifications of endocrine hormones we
need to discuss. The first are mammogenic. These promote alveolar and ductal cells so that you
can produce more milk. Things that are involved in this manner are estrogens, growth
hormones, cortisol, progestins, prolactin, and even relaxin. All of those help this proliferation
of breast tissue in response to pregnancy and lactation. Then you have lactogenic-specific
hormones and these help by initiating milk production. So if you initiate production, this is
getting the process started of getting those secretions across so that you have milk collecting
in those open areas of those alveolar cells. Prolactin is a great example of a hormone that does
this. We also have hCS. Now hCS is not hCG that you measured to determine if someone is
pregnant, this is a different hormone. We'll get to it in just a second. Cortisol, insulin,
insulin-like growth factor, all are lactogenic and they promote milk production. Thyroid
hormones can also have a small role as well. Interestingly if you start to change hormone
levels, this can also stimulate this process. So if you start to remove estrogens and progestins,
this will also allow for the initiation of milk production. There are galactokinetic hormones and
this particular thing will be to help to get the milk out, which is milk ejection. This is contracting
of mild epithelial cells to be able to get the milk to the right place, so when the infant
suckles, it will be able to drink the milk. Oxytocin is the most important hormone at this regard
and this is released from the posterior pituitary. Then you also have galactopoietic hormones
and these help maintain milk production. So you have to initialize milk production but then you
have to keep it going and to keep it going you need things like prolactin, cortisol as well as
some other metabolic hormones but prolactin and cortisol are the most important. So you can
see this is a very tightly regulated system with a lot of hormones involved. Each have a
different step. Having different steps is good because you can transmit the different types of
signals differently but that also means there's a lot of places for this to go wrong and
sometimes it is difficult for one of these different steps to happen and the mother is not able
to lactate her particular infant. So let's re-visit this hCS a little bit more. hCS is secreted by those
specialized cells that synced up from that blastocyte and that was called a triple-blast layer.
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Those particular cells formed the pre-component of the placenta. This is very similar to growth
hormone and this will increase in proportion to the size of the placenta. So as the placenta
gets larger, you'll get more hCS being released. If it worked similar to growth hormone, it is
going to stimulate growth both for the infant as well as for the mom and you remember it is
also helpful for the development of the breast tissue. The suckling reflex is very important for
delivering certain hormones at the right time. How this reflex responses is that first you need
an event and that event is the infant suckling on the breast. This sense sensory afferents
that then provide information back to the brain to change the release of two hormones. Those
two hormones are prolactin and oxytocin. Prolactin is a hypothalamic anterior-released hormone
and oxytocin is released from the posterior pituitary. Both of these are involved in the
regulation of milk production as well as maintaining that myoepithelial tissue so that you can
have milk let down. The decrease in GnRH also does one other thing. It served to suppress the
menstrual cycle and in certain lactating females can cause amenorrhea. So the suckling reflex
becomes very important in releasing two prominent hormones, prolactin and oxytocin.
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Therefore, if you don't have a suckling infant, there these two hormones will be lower.