Mammary Gland - Physiology

Structural Aspects of the Udder

The mammary gland in the domestic species can be likened to an inverted bunch of grapes i.e. a cluster of secretory acini connected by a duct system to the teats. In the cow and ewe the major lactiferous ducts empty into a single teat cistern which drains through one teat orifice (the streak canal). The solitary nature and integrity of the lining of the streak canal are very important in relation to the genesis of infective mastitis in the cow and ewe. In the mare, sow, bitch and queen several (2-20) ducts open through each teat.

Microanatomy
Major ducts are double-lined (low cuboidal/columnar epithelium). Arcuate myoepithelial cells surround the ducts together with elastic fibres.

• Minor ducts (leading to acini) are also double lined (low cuboidal epithelium). Arcuate myoepithelial cells surround the ducts.
• Acini (secretory alveoli) are lined by a single layer of cuboidal secretory epithelial cells, stellate myoepithelial cells surround acini.

Vascularity and lymphatic drainage
Production species, cow ewe and sow, have a well developed vascular network. This is significantly involved in:

Inflammation

Vascular shock Syndromes

Trauma

The lympahtic system is important in the production species in the spread of infectious disease or inflammation. Note the important role of the lymphatic system in mammary neoplasia in the dog and cat relative to metastasis.

Physiological aspects of the udder

This glandular organ is subject to a cyclic hormonal control.

• Before puberty: growth of simple ducts only in pads of fat
• After puberty: growth of ducts only at a greater rate, waxing and waning under cyclic hormonal influence when non-pregnant.
• In pregnancy: secretory acini are 'budded on' to the enlarged duct system under the influence of corpus luteum of pregnancy or placenta. During pregnancy, acini mature and milk precursors are accumulated = lactogenesis.
• After partutition: maintenance of milk formation and secretion is controlled by the anterior pituitary = galactopoiesis. The release of milk at suckling or mechanical milking is mediated via the release of oxytocin from the posterior pituitary.

Milk in produced in acinus by:

1. Synthesis (fat, casein, lactose) by acinar cells
2. Selective permeability of acinar cells to blood constituents i.e. calcium, potassium, phosphates

Damage to the acini therefore produces altered milk with abnormal levels of some constituents. During involution, the acini stop secretion and shrink with desquamation of cells, hence why there is an increase in cell number in the milk at the end of lactation.

• Dry period: the udder comproses ducts, contracted inactive acini and fat, the latter depends on the age of the animal. Damage during lactation tends to produce ductal epithelia hyperplasia with acinar necrosis or degeneration i.e. only ducts regenerate. The organ as a whole can undergo regeneration but only under hormonal control at the next pregnancy.

Information by permission of Professor RW Else