Lactation Biology

One of the most fundamental requirements for understanding any aspect of biology is to appreciate the relationships that exist between form and function, or anatomy and physiology. In the case of lactation and the mammary gland, it is important to be able to visualize the various levels of mammary gland structure in order to understand how the mammary gland grows and functions. This module presents an overview of mammary gland anatomy and macrostructure using the dairy cow as our primary example. In addition, the tissue, cellular, and subcellular levels of mammary structure are presented. The module provides a conceptual foundation that will allow you to individually explore mammary gland anatomy and microstructure in other mammalian species.

Milk is the product of the lactation process. Such a simple statement does not come close to doing justice to the complex nature of this biological fluid. In this module, we explore the major components of milk, as well as provide a basis for understanding milk as a product that we purchase in a store. Mechanisms of synthesis of milk components are introduced. And, several videos include discussions of the nature of differences in milk composition among mammalian species, including comparisons of cow milk with human milk.

The tissue that we know of as the mammary gland (or udder or breast) may appear as relatively uncomplicated compared with some internal organs such as the heart or brain. The gland is outside the body wall, has a nipple or teat in most species, and has a series of ducts by which milk that is produced in microscopic blind tubes can be expressed from the gland. How can such a gland develop and what regulates that development? It turns out that the initial stage of development occurs extremely early in the fetal development of the animal. It then proceeds through a series of developmental phases, many of which are closely linked with the reproductive functioning of the animal, and eventually leading to the time when the gland is producing milk. Furthermore, the mammary gland is one of a few tissues that undergoes repeated cycles of development, functioning, regression, and redevelopment, again associated with the reproductive cycles of the animal. This module describes the various stages of mammary development and provides the framework for understanding how these stages relate to each other, as well as to the functionality of the gland.

The peripartum period, the time shortly before, during, and after giving birth, is a time of rapid changes in the mammary gland, the mother, and the neonate. Extensive physiological coordination occurs between the processes leading up to and giving birth, the formation of colostrum, the initiation of lactation in the mammary gland, and the subsequent removal of milk by the neonate. In this module, we explore how the mammary gland changes around the time of giving birth when the gland transitions from a non-lactating to a lactating state. Part of this transition is the production of colostrum, the first mammary secretion produced by the gland after giving birth. Another part of the transition is the changing regulation of mammary gland function from one being driven primarily by hormones associated with pregnancy and parturition to one where milk removal, by the neonate or milking machine, is the driving force in gland function after birth. In this module, we examine some basic characteristics of the neonate, how lactation is initiated (lactogenesis), and the formation and special components of colostrum (immunoglobulins) and their impact on the neonate.