Insulin is required in vitro to cause a lactogenic-like response in mammary tissue. Insulin causes the nonsecretory epithelia to undergo one cell division. This cell division seems to be necessary for lactogenesis to occur.
The role of insulin in lactogenesis in vivo is unknown. However, it is known that mammary cells in vivo undergo a large burst of cell division in late pregnancy.
In vivo, IGF-1 may be the primary mitogen involved in this cell division leading up to lactogenesis, with insulin playing a minor role in this function.
Both insulin and the IGFs may be involved in glucose uptake by the mammary cells. This glucose uptake is of critical importance for lactose synthesis. Insulin also may be directly involved in expression of milk protein genes.