Udder Physiology

Physiology of the udder

The udder is to all intents a highly-developed and modified sweat gland, lined inside with a structure similar to skin. In cattle it is composed of four individual glands, known as quarters.

Diagram

The interior of each quarter of the udder is composed of a teat cistern, a gland cistern, milk ducts, and glandular tissue. The glandular tissue deep within the udder contains millions of microscopic sacs called alveoli; each alveolus is lined with milk-producing epithelial cells and is surrounded by muscle cells (myoepitheliad cells) that contract, squeezing milk from the alveolus into the milk ducts when the stimulus for milk let-down occurs during milking.

The nutrients found in milk are brought to each alveolus via the blood vessels, where epithelial cells convert them into milk. There is a large blood supply to the udder; in the order of 400 litres of blood pass through it to produce a single litre of milk. Between milkings, milk accumulates in the alveolar spaces, milk ducts, and cisterns, particularly in higher-yielding cows. During milking, this accumulation of milk is removed through the teat ducts.

There is no milk flow and a very insignificant flow of blood between the four separate glands, but due to the large volume of blood needed for milk production this is why antibiotics injected into one quarter can find their way systemically into the cow's bloodstream and be detectible in other quarters.

There is also a strong, complex structure of ligaments attaching to the pelvic bone, the abdomen and the inner thigh, to support the udder and help it to hold its shape but also to allow for expansion as milk collects between milkings and provide a degree of shock absorption.

Diagram inclusions: teat cistern, gland cistern, milk ducts, glandular tissue, alveoli, muscle cells, epithelial cells