I have slides of electronmicrographs I found in the mid 1990s while preparing
a lecture.  One shows many fat globules clinging to the epithelial cells that
secrete milk  - of a mouse.  Another shows the difference in the shape of the
cells bulging into the lumen of the alveoli after the MER, and craters where
the milk fat globules were formerly clinging to the epithelial cell
membranes.   The MER helps to dislodge the HMFGs from the outside of the milk
manufacturing cells.

 Remember that the human, and, I assume from these photos, mouse milk fat
globules are packaged in a membrane and secreted via exocytosis to the
outside of the manufacturing cell.  These packaged globules cling to the cell
until the milk ejection reflex occurs which squeezes the alveoli.

If there is lots of foremilk in the ducts when the first MER occurs then
although these HMFGs will be loosed from the epithelial cells - where will
they go?  They will initially mix into the milk way back in the alveoli, not
in the larger lactiferous ducts.   And since fat floats, I would guess that
the fat globules would tend to float upward away from the nipple pores.
Only after enough foremilk has been removed from the ducts will the fat be
able to get closer to the lactiferous sinuses to be removed from the breast.
 If a baby stays on the breast long enough to elicit several MERs then I
would assume that if there are any milk fat globules sitll attached to the
epithelials cells after the first MER,  they would be dislodged and increase
the fat concentration.

Michael Woolridge told us a few weeks ago in Vermont that there is a the
gradual changeover to higher fat content milk (and that one mother had taken
over 25 minutes to have significant fat content).   Also, that when feedings
are frequent and closely spaced, the milk has a higher fat content at the
beginning of the subsequent feeds than if a long time had passed.   He
summarized that there is a negative correlation between time of feeds and the
amount of fat present, and that feeding more frequently sustains higher fat
levels.  What predicts the amount of fat present at the end of the feed?
Current volume and feed duration.  (And that current volume was the more
significant.)

If the next feeding is sooner, then there is greater residual hindmilk left
in the ducts from the last feed, so there can be two peaks in fat content.
The one at the beginning of the feed when the milk volume is higher from the
residual fat and the peak that occurs later in the feed when the hindmilk
from that feed is reached.

So, I think that if there has been a longer time in between feeds that the
balloon analogy holds.  Only when enough volume of foremillk has been removed
will the milk fat globules be in higher concentration in the milk the baby is
receiving.   There will be very little fat evident in the first milk when
there is a long interfeed interval as the volume of the milk has greatly
increased and the fat globules have receded further away from the lactiferous
sinuses.

Mardrey Swenson, DC IBCLC
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