Lactation - Anatomy & Physiology
Introduction
Mammary gland development is initiated prenatally in the female foetus and continues through puberty and pregnancy. The anatomy and distribution of mammary glands is diverse among mammals. Secretion of milk does not begin until shortly (hours) before parturition. Lactation provides the neonate with the opportunity to nurse and be nourished with minimal energy expenditure. It also provides immunoprotection for the neonate because initial mammary secretions (colostrum) contain antibodies that provide passive immunity. Lactation continues until the neonate is weaned. After weaning, the mammary glands undergo involution and return to a non-secretory state.
Key words
- Mammogenesis: the development of mammary tissue
- Lactogenesis: the onset of milk secretion
- Galactopoesis: the maintenance of lactation
- Milk ejection: the expulsion of milk from alveoli
- Involution: termination of milk secretion and mammary gland regression.
Involution
- As the requirement for milk by the neonate decreases, suckling is less frequent.
- Consequently, there is a buildup of pressure in the mammary gland causing secretory cells to become increasingly less functional - pressure atrophy.
- Secretory cells then remain non-functional until a subsequent pregnancy, at which time prolactin, glucocorticoids and placental lactogen restimulate alveolar milk synthesis.
- During involution, immune cells such as lymphocytes and macrophages invade the mammary tissue. These produce immunoglobulins in the subsequent lactation.
- Involution is critical to allow the replacement of senescent secretory cells which are lost by apoptosis.
- In rodents, defoliation occurs. Secretory epithelial cells are sloughed from the basement membrane at involution. Thus, more extensive regeneration is required at the onset of the subsequent lactation.