Jennifer,
Look in the archives, I posted a long discussion of this a few years
ago, but basically, milk is made by several different secretion
methods.  The proteins and carbohydrates of milk are secreted by
merocrine secretion, they exit the cell without removing any of the
cell.  The water enters the milk by osmosis, according to the
concentration gradient set up by the concentration of lactose secreted
by the aforementioned merocrine secretion.  Fat is secreted by apocrine
secretion.  It concentrates in the apex of the alveolar cells, and the
fat along with some of the cell membrane and cytoplasm is secreted into
the alveolar lumen and then pushed into the milk ducts when the milk
ejection reflex occurs and oxytocin stimulates contraction of the
myoepithelial cells that surround each cluster of alveolar cells.  The
fat is actually produced constantly, along with other milk components,
at a rate that varies with the emptiness of the breast; but it is not
available until there is a MER to dislodge it from the milk gland cells
themselves.  By that time, there might be significant quantities of the
other milk components "foremilk" already in the ducts.  The baby will
get increasing proportion of fat as the feeding progresses, as the
foremilk and mixed milk are cleared out and newly produced and secreted
milk gets to the baby.
--
Catherine Watson Genna, IBCLC  New York City  mailto:[log in to unmask]