Mammary homeostasis in the mother-offspring dyad

Homeostasis during lactation is a special case in which the unit for regulation is a dyad comprising the mother and her currently nursing offspring (the mother–offspring dyad). This arrangement is not a trivial. A litter of mice can have a mass greater than the mother and nutrient demands that far exceed her. Homeostasis for milk secretion, appetite, and calcium metabolism must come under integrated control, responding seamlessly to the needs of the mother and the offspring. Serotonin (5-HT) is a primary local regulator of mammary homeostasis. 5-HT synthesis in the mammary epithelium is high during lactation and increases during milk stasis. Two important functions are attributed to the 5-HT system. Firstly, when alveolar spaces are filled with milk 5-HT inhibits milk secretion and opens tight junctions. This feedback induces early phases of involution. Secondly, 5-HT induces synthesis and secretion of parathyroid hormone-related peptide (PTHrP). •Homeostasis during lactation is governed by the mother and the offspring, unlike homeostasis in a single individual.•Prolactin is the main systemic regulator of lactation, and serotonin is the main local (intramammary) regulator of lactation.•During sustained milk stasis serotonin inhibits lactation and initiates involution acting via 5-HT7 receptors.•Durin lactation serotonin acts via 5-HT2B receptors to stimulate PTHrP secretion, thereby controlling calcium homeostasis.