Milk Synthesis and Secretion

Milk synthesis and secretion is the work of the differentiated alveolar epithelium: each secretory cell assembles the proteins, lipids, sugars, and ions of milk and exports them through several distinct cellular pathways. This topic covers how copious secretion is switched on after birth and how the individual milk components are made and released.

Definition

Milk synthesis and secretion is the production by alveolar epithelial cells of the constituents of milk and their export into the alveolar lumen through several parallel secretory pathways, switched to copious output after delivery by secretory activation.

Scope

The entry covers the onset of copious secretion (secretory activation, or lactogenesis II), the cellular routes of milk-component secretion (exocytosis, the milk-fat-globule pathway, transcytosis, membrane transport, and the paracellular pathway), and the hormonal control of milk synthesis. It treats milk composition only insofar as it reflects these mechanisms, and is reference physiology rather than infant-feeding guidance.

Core questions

• What hormonal change switches on copious milk secretion after birth?
• By what cellular pathways are proteins, lipids, lactose, and ions secreted into milk?
• How is milk-fat secreted as membrane-bound globules?
• How does the closure of tight junctions change milk composition after secretory activation?

Key concepts

• Secretory differentiation (lactogenesis I)
• Secretory activation (lactogenesis II)
• Progesterone withdrawal as trigger
• Exocytotic (Golgi) secretion of proteins and lactose
• Milk-fat-globule secretion
• Transcytosis and membrane transport
• Paracellular (tight-junction) pathway
• Prolactin and galactopoiesis

Mechanisms

After secretory differentiation in pregnancy, the gland is poised to make milk but is held in check by high progesterone. Delivery of the placenta removes the progesterone brake and, together with sustained prolactin and permissive cortisol and insulin, triggers secretory activation, the onset of copious milk secretion within roughly two to three days. The differentiated alveolar cell then exports milk components by several parallel routes: aqueous-phase proteins and lactose are packaged in the Golgi and released by exocytosis (lactose synthesis osmotically drawing water into milk); triacylglycerols bud from the apical membrane as membrane-enveloped milk-fat globules; immunoglobulins and some other molecules cross by transcytosis; ions and small molecules move through membrane transporters; and, before tight junctions close at secretory activation, a paracellular pathway allows exchange between plasma and milk. Established secretion is maintained (galactopoiesis) by recurrent suckling and prolactin.

Clinical relevance

The dependence of secretory activation on progesterone withdrawal and prolactin explains the physiology behind milk coming in after birth and informs how delayed or incomplete onset is understood mechanistically. This entry is a reference for secretory physiology and does not provide individualised advice on milk supply or feeding.

Evidence & guidelines

The secretory pathways and the hormonal trigger of activation are established in physiology reviews; many molecular details of lipid and protein secretion derive from animal-model and cell studies, as the cited reviews note.

History

The recognition that progesterone withdrawal triggers copious milk secretion, and the dissection of the distinct cellular pathways by which milk components are exported, were developed across the late twentieth and early twenty-first centuries, building on earlier endocrine studies of lactation.

Key figures

• James McManaman
• Margaret Neville
• Peter Hartmann

Seminal works

• mcmanaman-neville-2003
• pang-hartmann-2007
• neville-2001

Frequently asked questions

Why does milk come in a couple of days after delivery rather than immediately?

Copious secretion (secretory activation) is triggered mainly by the fall in progesterone after the placenta is delivered, acting with sustained prolactin; this switch typically takes about two to three days to take full effect.

How is the fat in milk secreted?

Triacylglycerols accumulate as droplets in the secretory cell and bud from the apical surface enveloped by plasma membrane, forming membrane-bound milk-fat globules — a pathway distinct from the exocytosis used for proteins and lactose.