Mammae in general

Mammary development

The ventrolateral ectoderm of the embryo becomes the mammary ridge and then the mammary complex. In development, mammary buds push into mesenchyme with their number equaling the number of mammae: mares 2, cows 4, ewes and does 2, sows 14, bitches 10, and queens 8. Sprouts form from each mammary bud, and the number equals the number of papillary ducts (and therefore mammary glands) per mamma: mares 2, cows, ewes, and does 1; sows 2; bitches 8 to 14; and queens 3 to 7. Each mamma has a single papilla (teat). Mammae develop in male embryos, but in domesticated species, they only regress fully in the stallion.

There is a simple ductal network at birth. At puberty, there is rapid proliferation of these simple ducts from the terminal end buds, clover leaf -shaped structures that cover the tips of the primary ducts. The primary ducts undergo secondary branching. In diestrus and during pregnancy there is subsequent tertiary branching.

As puberty approaches, the branching of ducts is mediated by oestrogen, progesterone, prolactin, growth hormone, insulin-like growth factors, and many other factors. There is an intimate interaction between the mesenchyme and epithelium in the formation of ducts and alveoli. Mammary development is maximal at the onset of lactation. Milk flows from alveoli through the lactiferous ducts to a lactiferous sinus (in large animals) and with suckling, through a papillary duct and through the papillary ostium.


There is variation between the species in the amount of regression that occurs when milking ceases. All species reduce the area of secretary epithelium and increase the relative amount of stroma of the gland. This is done by single cell death of epithelium and extracellular matrix remodelling. It is orchestrated by different cytokines. The stimulus for cessation of secretion is a buildup of milk in the gland from a lack of milk removal either by milking, cycling by offspring or obstruction to milk flow. This is followed by a second phase with remodelling of the mammary tissue. When secretion ceases completely, mammary fluid is resorbed and macrophages take up milk components and single dead cells. Bovine mammary glands do not regress as much as in other species, and they complete involution in about 2 weeks. Ewes take about 4 weeks to involute. Leukocytes, especially macrophages, increase in number in the involuting gland.

 

 

McNally S, Martin F. (2011) Molecular regulators of pubertal mammary gland development. Ann Med. 2011; 43: 212-234.

Watson CJ, Oliver CH, Khaled WT. (2011) Cytokine signalling in mammary gland development. J Reprod Immunol 2011; 88: 124-129

Mammary anatomy and histology

The mammary glands are sequestered from the outside world by the gatekeeper functions of the sphincter of the mammary papillary (teat) ostium and the papillary duct, and at least in the ruminant, its lining of keratinized squamous epithelium. The lactiferous sinus and ducts are lined by columnar epithelial cells, and the alveoli have the secretory epithelium. Secretion of milk occurs in the alveoli. Mammary epithelial cells are of two types – the luminal epithelial cells of the ducts and secretory alveoli and the basal cells and myoepithelium. The secretory epithelial cells have receptors for immunoglobulin G (IgG) that allow for transfer of antibody to form colostrum. The receptors are present for about 1 week before parturition disappear during lactation. Epithelial cells also allow transfer of IgA, which is produced locally in subepithelial plasma cells, into the alveolar lumens. The stroma of the mammary glands contains capillaries, lymphocytes and plasma cells.
Lymphoid cells in the normal glands are derived from blood, and there is homing of lymphocytes from the intestine (the enteromammary link) as part of the common mucosal immune system. Lymphocytes of the cellular immune system are present also but in low numbers.

 

 

 

 

Function

The secretion of the mammary glands provides nutrition, immunity, and some normal flora for the fetus.

Nutrition

Immunity


The structure of the placenta of animals determines how much immunoglobulin is transferred by colostrum. All of the domestic animal species except the dog and cat rely entirely on immunoglobulin transfer in colostrum for passive humoral immunity.

Chucri TM1, Monteiro JM, Lima AR, Salvadori ML, Kfoury JR Jr, Miglino MA. A review of immune transfer by the placenta. J Reprod Immunol 2010; 87: 14-20.

References