a-short-review-of-histology

Breast in Pregnancy:

As pregnancy progresses, and under the high levels of progesterone and oestrogen secreted by the corpus luteum and placenta, there is an expansion of the duct system and some "sprouting" of secretory cells from the tips of the ducts
In late pregnancy, the expansion (proliferation) of secretory cells increases so that by the end of gestation the breast is largely occupied by glandular tissue
Around the outside of the secretory epithelium, but within the basement membrane of the secretory epithelium, myoepithelial cells differentiate
These are cells of epithelial origin but contain actin and myosin and are contractile
When stimulated, they squeeze the secretory parts of the gland and push the secretion, which has already been exocytosed from the secretory cells, into the ducts
As the end of gestation approaches, the secretory cells have begun to produce the first milk, known as the colostrum

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Breast - Active:

In late pregnancy there is an expansion of glandular tissue as shown in this micrograph
The adipose tissue that was present in the inactive breast has been replaced by glandular tissue