I wanna join the party! I have played games in my head for a long time to envision this too. Now this is gonna sound like the old "shell game" so you'll have to read and re-read to "catch my drift". My take on it is a little bit different, but these are the fantasies in my mind's eye. (I never really let the mom in on my mind games, however;-) "Just watch the baby and feel if your breast is getting softer."

Cathy explains:<The unconsumed milk travels back up the ducts, so the milk making cells can sense how much got taken (by the pressure the milk exerts). We don't think the fat goes back into the cells. We're not sure what happens to it.>


I have often wondered about this myself. I seem to remember from an old histology reference that the term "ductule" refers to the first tiny "tubing" draining milk from a lobule where newly secreted milk from the alveoli tends to pool slightly between MER's, and that the ductule has no myoepithelial cells around it, but connects the lobules with the actual ducts.) 


Azzopardi JG, Ahmed A, Millis RR. Nomenclature of the Microanatomy of the Breast: Parts Affected in Different Diseases: Normal Structure and Involution, In: Problems in Breast Pathology In: Major Problems in Pathology, Vol. 11 in series, Bennington JL, Ed., W. B. Saunders Co. Ltd., London, 1979.



This lack of myoepithelial cells around the ductules is a contrast to the myoepithelial cells around the alveoli themselves (basketlike) and are lengthwise and spiral around all the larger and larger ducts The actual ducts, when relaxed, are anatomically longer, narrower cylinders. During the MER, contraction of the ductal spiral myoepithelial cells simultaneously shortens and widens these cylinders so the milk must move forward to adjust to the temporary new dimensions of the cylinders. 


When the MER is over, the ducts promptly start relaxing back to their narrower diameters and longer lengths, so the movement of the remaining milk looks turbulent (fractal geometry principles??) because its volume must quickly reposition itself back into the resting dimensions, with much less pressure if the breast has been effectively drained of a sufficient volume. It seems to me that the milk's "rush to reposition" itself must be what gives the visual impression on ultrasound that it is "flowing" or "traveling" back up the ducts.) It's just basic release of hydraulic pressure happening as the walls relax, which logically would mean there would be less pressure in the ducts for a while.



This lack of myoepithelial cells around the ductule made good sense to me, #1, because the non-fat components (foremilk) of the milk (heavier specific gravity and small size) could trickle down the ductule by weight and gravity continously soon after it was secreted. In contrast, the fat globule (which I seem to remember is actually secreted as larger globules) would be held back by the tiny diameer of the narrow ductule, and has to wait in the lobule for the force of the MER to reshape the fat globules ever so slightly from pressure and maybe even temporarily shorten the length and expand the diameter of the ductule (?), so fat globules could be propelled on through into the ducts themselves. 



And #2, that after the MER, though fat globules would also move back upward with the other components in the direction of the turbulence, the actual separation of the fat globules due to lighter specific gravity seems to me to be a slower more continuous gradual movement causing a slower rise of cream "to the top"**, just as it does when EBM stands in a container for a while. But since there is no reverse MER to push fat globules back upward through the 'ductule' without myoepithelial cells, this would effectively keep the fat from the "residual (unconsumed) milk" from ever again being in contact with any part of the alveoli. 



So it made me wonder if the "feedback inhibitor" of the "old milk" was being sensed by the walls of the ducts themselves? This doesn't seem likely to me simply because this is the time period when production speeds up for an hour or so. So maybe it might be solely the feedback inhibitor in the milk being newly made, plus its gradual growing  pressure, either before secretion across the membrane, or maybe still effective after secretion, while still in the alveolus and lobule between MER's that eentually begins to slow production??? I sometime tell moms "your milk has protein with two jobs: its good for your baby when inside him, and when inside your breast for long enough, starts to act like a traffic cop at rush hour and signals the cells to 'slow down' production???



And speaking of the specific gravity of cream, actually it rises toward the ceiling**! If a mom is standing or sitting, that would mean toward the tops of the ducts (just up to the ductule) and clinging to the sides of the walls. However, if the mom were lying down, say on her back, for several hours, "up toward the ceiling" would be toward the nipple end of the ducts because of the lighter specific gravity. So maybe the "foremilk" of the next feeding in this position (mom supine, baby prone) would be the fat coming out first, followed by "skimmer" hindmilk;-)



And if she were lying on her right side for a number of hours, if she used the right breast, would the cream stay in the same part of the ducts but rise to the left, or upper walls of the ducts, which would be closest to the ceiling??? So if she were to feed in this position, would the baby get milk whose content at the beginning, middle and end of that breast was about the same, MER or no??



You could drive yourself (and the mother) nuts wondering about this! I've ended up just suggesting to them to continue on the first breast till you can feel it's gotten softer, and try breast compression if your baby starts to fall asleep early and easily, and when your baby acts as if really "done", after burping, maybe tempt him with the other breast. But the milk in the other breast can safely wait till the next feeding if he's not ready for it now.



This kind of game is more rewarding to me than the Farm Game etc. on Facebook;-)


K. Jean Cotterman RNC-E, IBCLC
WIC Volunteer LC Dayton Ohio
(heading back to snowy Ohio in 48+ hours after a sunny month in AZ;-(

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