Colostrum Part 3: Remember the Gaps

June 7, 2021

My family had a wonderful trip to London years ago and to this day I still remember hearing the phrase - Mind the Gap - when we went to the London Underground. This was a warning to be careful about the gaps between the platform and the Tube (the subway train).

For this class we’ll stay in that spirit with - Remember the Gaps.

During pregnancy and during the early days after birth, gaps exist between the lactocytes, the cells lining the alveolar sacs in the breasts. These loose junctions allow fluid and other solutes to flow more easily between the milk space and the other parts of the mammary gland (for example: capillaries and interstitial tissue).

These gaps play a major role in making colostrum the special fluid that it is. The gaps tightening up contributes to the process of transitioning from colostrum to more mature milk.

Here is the sequence of events.

The placenta produces progesterone. Progesterone blocks the action of another hormone, prolactin, on the lactocyte. Following birth, the delivery of the placenta leads to a rapid drop in the hormone progesterone. Prolactin levels, however, remain high. Prolactin receptors in the lactocytes are then unopposed and, under the influence of prolactin, lactocytes get to work making a variety of components including lactose. The withdrawal of progesterone and the influence of glucocorticoids also cause the intercellular gaps to tighten up. Lactose made in the lactocyte is secreted into the milk compartment. The increased concentration of lactose draws water into the milk. The mother feels a sense of fullness in her breasts.

From Breastfeeding and Human Lactation, “The time at which mothers report that their milk comes in is highly variable and ranges from 38-98 hours after birth, with an average of 50-59 hours.”1

The process described above is under endocrine control (hormonal control) compared to milk production later on that is under autocrine control (local control). More on that later.2

Now it should be clear why retained placental fragments can lead to insufficient milk production. Those placental fragments continue to produce progesterone which blocks the action of prolactin on the lactocytes.

And this should help us remember that the concentration of lactose in colostrum is lower than in more mature milk.

When in London, Mind the Gap.

On the test, Remember the Gaps.


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Wambach K and Spencer B. Breastfeeding and Human Lactation, 6th edition. Jones & Bartlett Learning. 2021


Hale & Hartmann’s Textbook of Human Lactation, 1st Edition. Edited by Thomas W. Hale and Peter E. Hartmann. Springer Publishing Company. 2017