Prolactin has an atomic mass of 23 kDa and comprises 198 amino acids. Lactotroph cells in the brain’s pituitary gland are responsible for its production (Canul-Medina & Fernandez-Mejia, 2019). Prolactin plays a key role in lactogenesis: It induces mammogenesis, which is the growth and development of mammary glands, alveologenesis, which is the development of alveoli  from mesenchymal and epithelial progenitor cells, synthesis of milk proteins, and galactopoiesis, which is the maintenance of milk secretion (Canul-Medina & Fernandez-Mejia, 2019).

Prolactin has its own prolactin receptor (PrlR) and mediates its effects via different signalling pathways activated by Janus kinase 2 (JAK2)-Stat5 or PI3K-Akt (Canul-Medina & Fernandez-Mejia, 2019). Jak2-Stat5 is responsible for activation of milk protein gene transcription, thus for synthesis of the milk proteins β-lactoglobulin, β-casein and whey acidic protein. PI3k-Akt controls transport of glucose, lactose synthesis and lipid synthesis in the mammary gland (Canul-Medina & Fernandez-Mejia, 2019).

Prolactin also regulates mammary gland secretion of the parathyroid hormone-related protein (PTHrP). In turn, this signalling molecule releases calcium from maternal bone to serve in milk production (Canul-Medina & Fernandez-Mejia, 2019).