Theories of Milk Secretion : Evidence from the Electron Microscopic Examination of Milk

Abstract

IN spite of study for over one hundred years, we still do not know how milk is secreted by the mammary cells¹. One early theory of light microscopists was that the mechanism was holocrine, involving death and release of the whole cell as in sebaceous glands. Indeed, recent biochemical studies have shown that milk contains several intracellular components (such as citrate, enzymes and microsomes), although it is uncertain whether these are derived from secretory or duct cells or from phagocytic cells (neutrophil leucocytes and macrophages), which are known to migrate into milk from the blood and connective tissue. Later, the idea that whole cells are lost was rejected because the observed rate of mitosis was too small to account for the predicted rate of cell renewal. An alternative theory was that the process was apocrine—that only the apex of the cell became detached and that milk consists essentially of cell debris. Subsequent investigators considered this theory was not proven because what seemed in histological sections to be pieces of cytoplasm recently detached from cells could have been portions of neighbouring cells, the greater part of which lay out of the plane of section².