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Mammary Development
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Mammary Gland Development

Overview of
Mammary Gland

Cross section of a beef heifer's udder.

Mammogenesis is the term used for development of the mammary gland and refers to the development of mammary gland structures. This is most often assumed to occur during pregnancy, however mammary development begins when the animal is an early fetus and proceeds beyond initiation of lactation. The mammary gland is one of a few tissues in mammals which can repeatedly undergo cycles of growth, functional differentiation, and regression. This resource covers topics related to the growth of the tissue. Functional differentiation is covered in the Lesson on Mother & Neonate: The Peripartum Period, and the Lactation Lesson.

In the lactating mammary gland, the parenchymal tissue is composed of epithelial structures, such as alveoli and ducts, and the associated stromal connective tissue. The stroma of a lactating gland primarily is composed of cellular and non-cellular components of the connective tissue surrounding the epithelial structures. Cellular components of the stroma include fibroblasts, endothelial cells associated with blood vessels, and leukocytes localized in the tissue, while non-cellular components include collagen and other connective tissue proteins. In contrast to a lactating gland, considerable white adipose tissue exists in a gland from the early phases of fetal development and extending through much of pregnancy. This fat pad often is included as part of the stroma of the developing gland, but is considered as extraparenchymal tissue.

This resource includes sections on mammary gland development. Structural development of the mammary gland is generally broken into five phases. These are development during : Fetal Period, Prepubertal Period, Postpubertal Period, Pregnancy, and Lactation. Also included are sections on mammary gland involution, endocrine abnormalities, and endocrine modulators.

As a point of reference, the images below will give you an idea of how much mammary development may occur normally in non-pregnant, cycling heifers and in pregnant heifers.

12 mos old heifer

Heifers, ~6 mos pregnant

Primigravid heifer
1-2 weeks prepartum

Older cow
1-2 weeks prepartum

Mammary Development
Mammary Development Resources