Jay,
There are three kinds of secretion possible in glands.  The human breast
uses two of them.  Merocrine secretion is when products (proteins, etc)
are released from the cell through the little pores in the cell
membrane.  The proteins, lactose, and many other components of milk are
secreted this way from the cells of the alveoli into the alveolar lumen
(empty space in the center of the ball of cells).  This occurs at a
variable rate depending on the prolactin stimulation of the alveolar
cells, the lack of feedback inhibition from stored milk, and the lack of
cell "squashing" (geometric distortion) from milk in the lumen.
        At the same time protein and lactose are formed (as an offshoot of the
same metabolic pathway, all catalyzed directly or indirectly by
prolactin), the fats are formed.  These fats work their way to the top
(apex) of the cell, and are budded off with a bit of cell membrane
surrounding them, with a loss of some of the cell.  This is called
apocrine secretion.  We think the apocrine secretion happens faster when
the myoepithelial cells are contracting under the influence of oxytocin
and squeezing the alveolar cells so their "tops pop off", or that the
fat globules are sticky after they are secreted, and need this push to
work their way down the ducts into the lactiferous sinuses under the
areola, pushing the less fatty milk in front of them.  (I am unsure what
evidence there is as to which of these views is more correct.)
        Babies can (within certain parameters) control milk transfer by varying
their sucking.  Strong, rapid "call down" sucking elicits an MER.  Less
rapid sucking allows the baby to snack rather than feast.  I think it's
not a matter of the mom "not having MER's" (though this can - rarely -
occur), but rather is the baby engineering the situation.  As for
whether mom not being able to pump is an MER deficiency, I would like to
see how she does with a different pump, with manual expression, and with
the same pump after some manual nipple stimulation or olfactory
stimulation (such as smelling the baby's head).  Not every gizmo, no
matter how good, works for every mom.  In the same vein, it is difficult
to assess whether the mom, the baby, or both are responsible for low
milk supply without observing them and taking a good history.  The
answer will be different in each dyad.  That's one of the appeals of our
work for me, no easy answers, gotta work the noodle!
        What does everyone else think?
--
Catherine Watson Genna, IBCLC  NYC  mailto:[log in to unmask]