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Other Hormones and Factors


Other Hormones
and Factors

Piglets suckling a sow.

Estrogen

Estrogen is not directly involved in lactogenesis, but indirectly it may have an effect by increasing numbers of PRL receptors in the mammary cells. In vivo, it may also be one factor in controlling PRL secretion from the pituitary.


Growth Hormone

In vitro: There are no GH receptors on the mammary epithelial cells, therefore there should be no in vitro effect. Some reported lactogenic activities of GH (especially human GH, which has some prolactin-like activety) probably arise from the very close PRL-like structure of that GH. That is, GH in those cases is binding to the PRL receptor and acting as if it was PRL.

In vivo: The involvement of GH in lactogenesis is not known. IGF-1 (secreted from the liver in response to GH) is a mammary mitogen. Low concentrations of IGF-1 can replace the very high insuling concentratrions required for in vitro culture systems where lactogenesis is studied.

In hypophysectomized - adrenalectomized - ovariectomized rats (pretreated with E and P4), PRL, GH plus glucocorticoids gives nearly normal initiation of lactation. Again the GH effect may be acting by indirect effects mediated by IGFs.


Local Effects

Cows will secrete significant amounts of milk when milked prepartum. Thus, events of lactogenesis do not necessarily have to occur during the peripartum period of elevated hormones.

If you milk each quarter of a cow beginning at different times prepartum, you can get milk of different composition from each quarter at parturition. This suggests that some intra-mammary factor is inhibiting the final activation of milk secretion, because all quarters are exposed to the same plasma hormone concentrations.


Extracellular Matrix

Although hormones clearly are major factors in regulating cellular events and processes associated with mammary cellular functions such as lactogenesis and milk secretion, it is clear from in vitro studies that other factors are also imortant. A key factor is the interaction of the epithelial cells with their extracellular environment, specifically the interactions of the cells with the extracellular matrix, especially the basement membrane. Extracellular matrix (ECM) is broadly defined as the noncellular components of tissues or in the mammary gland the noncellular components of the stroma. Most of the ECM of most tissues is made up of collagen. There are numerous types of collagen found in different tissues. Collagen makes up a large proportion of the noncellular matrix of bone, cartilage, hair, and nails and horns, as well as the connective tissue matrix of soft tissues. The ECM is made of more than just collagen, though. Epithelial cells in all tissues are in direct contact with and are attached to a basement membrane. This association between epithelial cell and basement membrane is essential for many the cellular processes associated with the lactation function, such as maximal expression of milk protein genes, maximal protein synthesis and significant secretion of milk proteins.

Basement membrane is generally made up of several ECM proteins including collagen (type IV), laminin, and proteoglycans. These proteins complex by cross-linking among themselves and by binding to cell-surface binding proteins, thereby forming a structural mesh around the alveoli and anchoring the alveolar cells to that mesh. This anchorage allows the cells to develop the necessary polarity and cellular orientation to secrete milk components at the apical surface of the cell.


 
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