6.5: Lactation
- Page ID
- 62161
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We’ll now say more about one of the defining features of mammals - milk production. This feature isn't talked about in any detail in LoM but it's such a remarkable and unique mammalian process that it warrants attention here. The production of milk (technically called lactation) makes sense only if we look inside mammals to find out how this life-sustaining substance is produced.
Milk is a very rich form of food. You've probably already heard about some of the major constituents of milk - proteins, fats and carbohydrates. These large molecules have to be built up (synthesized) from the simpler chemicals that the mother obtains from her diet or from her body reserves. By looking at the structure of a typical mammary gland we'll see how this biological 'production line' is put together.
The term 'gland' is used for specialized structures that produce (or more technically secrete) one or more chemical products, and many glands have the type of structure that Figure \(\PageIndex{1}\) shows. (Glands are usually made up of different types of cells - a group of cells that have similar structure and function is often called a tissue.
Figure \(\PageIndex{1}\) a shows that each mammary gland consists of a central teat or nipple, into which feed a number of channels (or ducts) that convey and temporarily store the milk, following its production by the great mass of cells that make up the bulk of the gland. We get a better sense of the fine-detailed structure of the mammary gland by magnifying just one part of what's shown in (a).
Lactation in marsupials has a particular importance; for example, the newborn red kangaroo weighs less than a gram - or, in the more familiar language of the TV commentary, 'less than a lump of sugar'. On complete emergence from the pouch, some eight months later, it weighs about four to five kilograms. It may then often double in weight before becoming fully independent of the mother's milk (i.e. becoming weaned), which happens between four and eight months after leaving the pouch.