What are the mammary glands?
The mammary glands are a system of exocrine glands and ducts, all connecting into alveolar ducts and then into the lactiferous duct in the nipple. If you look back at Figures 5.10 & 5.11 on pages 142 - 143 of your textbook, you will see the introductory description of exocrine glands. Mammary glands are alveolar glands (although I am not sure if they are simple branched or compound). In Figure 5.11, you will see that when mammary glands secrete, they fit into the apocrine gland-type, meaning that they function by pinching off parts of their cells, rather than just secreted fluid by exocytosis. The cell bits that they pinch off break open in the ducts, so that a fluid emerges from the nipple.
Males and females both have some glandular tissue under the nipples, however, only after exposure to estrogen during puberty does this glandular tissue develop further. As the glands develop, adipose tissue also develops around them; this combination of glands and adipose tissue are the female breasts. However, just because the glands are there does not mean that they are active in secretion...
Only after giving birth to an infant are the mammary glands productive in secretion. During pregnancy, the hormones a woman experiences are different than when she is not pregnant (as you will see in the "after fertilization" web page). These hormones cause the mammary glands to develop further-- placental estrogen gets the ductile systems of the glands to develop and placental progesterone gets the glands themselves to develop. This added development of the mammary glands during pregnancy can cause the breasts to enlarge quite a bit. However, the glands do not begin to secrete until after the child is born.
Why don't the mammary glands secrete milk before the child is born, or even when we have not been pregnant at all? The hormone prolactin is required for glandular production of milk and secretion of milk. This hormone is only released in any quantity during pregnancy and after childbirth. During pregnancy, the high levels of progesterone prevent the prolactin from working on the mammary glands to cause milk production. But after pregnancy, prolactin is still available, but now it is present in the absence of progesterone. Prolactin, without interference by progesterone, can now cause milk production and secretion.
How do these glands release their contents?
Suckling of the nipple and areola around the nipple provides the sensory input that leads to ejection of milk from the breast. You see, the sensory stimulation runs back to the brain where it stimulates the release of oxytocin from the posterior pituitary. Oxytocin travels in the blood and effects epithelial cells of the ducts that have contractile capabilities, myoepithelial cells. This may sound odd, but all this requires is that certain cells in the ducts have actin and myosin arranged in an orderly fashion within them. When these cells are activated, their contractions eject milk out of the breast. This entire loop takes only about 30 seconds to get going.
This loop can continue to function as long as prolactin is available to cause glandular production of milk. That continues as long as milk is being ejected from the breast. Shortly after a woman stops breast-feeding (typically, within a couple weeks), prolactin is no longer secreted and glandular production of milk stops.
First of all, your book mentions how sometimes a woman who is breast-feeding may have months without ovulation. I have heard of quite a few cases where women have believed that offers them temporary birth control and have gotten pregnant. Many of us know of sibling who are less than a year apart in age-- probably their mothers thought they wouldn't get pregnant (or they just wanted many children). I wish to discourage this notion of birth-control by breastfeeding. It is certainly true that ovulation isn't immediately regular after giving birth and during the initial months of breastfeeding... but that doesn't mean that it never occurs.
Secondly, your book does not describe the merits of breast feeding. It does note that the initial fluid ejected from the breasts is colostrum and contains protective antibodies. But that is about all it says. Breast-feeding children receive a large helping of antibodies from their mothers through her milk, protecting them from many initial infections that may otherwise have caused some trouble. Also, a lot of research has shown the benefits to children from breastfeeding are quite far-reaching-- even an increase in IQ has been linked to it (although the reasons for this are unclear). Some newborns have digestive troubles with formulas-- and even though there are many available, it may take months to find one that a particular newborn thrives on; however, breastmilk doesn't have that side-effect. This doesn't mean that breast-feeding always comes easily. For some it does, but for others it may be more troubling. One can hire a breast-feeding consultant or get the advice of volunteers from certain organizations, like La Leche League, to help a new mother overcome difficulties with breast-feeding. Not all new mothers can breast-feed easily-- I have a friend who was told that she had inverted nipples and couldn't breastfeed.
Each new mother has to make her own decision about breast-feeding. Certainly, the benefits of breast-feeding are so great that health care workers highly recommend breast-feeding if a new mother can handle the occasional inconvenience of it. (Elliot was breast-fed until he weaned himself at only 3 months! AJ will also get the opportunity of breast-feeding).
© 2011 STCC Foundation Press