What can we learn from kangoroos? Lactation is induced through prolactin and oxytocin in a favourable endocrine environment and controlled via feedback loops. The oxytocin reflex also helps mother-infant bonding and can be interrupted by stress and birth trauma. During lactation, fertility is reduced.
Milk onset and flow are controlled by the two hormones prolactin and oxytocin, if the hormonal landscape is supportive. During pregnancy, prolactin concentrations steadily increase. They induce growth and development of the breast tissue in preparation for lactation. Progesterone and oestrogen prevent milk production. These hormones decrease postpartum removing the inhibition of prolactin and allow milk production (WHO, 2009).
Once the infant suckles on the mother’s breast, a signal is sent to the brain, which triggers the production of the two hormones in the pituitary gland or posterior lobe, respectively. As prolactin induces milk production in the milk-secreting cells of the alveoli, oxytocin releases milk flow to the milk ducts via contraction of myoepithelial cells of the alveoli (smooth muscle cells) (WHO, 2009, Haaksma et al., 2011). Prolactin concentrations reach their peak about 30 minutes after feeding starts. This peak is the preparation for the following feeding (WHO, 2009).
… can be stimulated in the first weeks postpartum because the longer and more often an infant suckles, the more prolactin is released (WHO, 2009, Weaver & Hernandez, 2016). Later, this association decreases until a stop of breastfeeding will reduce prolactin release and stop the lactation process altogether (WHO, 2009).
Night feedings can help establish milk flow and volume because higher prolactin concentrations are released at this time. In parallel, prolactin supports maternal rest by making the mother relaxed and sleepy (WHO, 2009).
The oxytocin reflex is also described as the "milk ejection reflex" or "let-down reflex". This reflex leads to the uncontrolled release of milk stored in the alveoli to the milk ducts in response to the mother's experiences. It happens in response to interactions with the child (touching, smelling, seeing, hearing), when she anticipates a feeding, or emotional responses like loving thoughts. The reflex is inhibited through severe pain, upset or stress. The regulation mechanism is possible because oxytocin is produced faster than prolactin. Long-lasting stress situations can severely impact milk flow. Coaching, calming and continuing with breastfeeding can help "deblock" the oxytocin reflex (WHO, 2009, Galbally et al., 2011).
Leaking can also be oxytocin independent. It occurs when the breast is not emptied sufficiently. The large milk volume then exerts pressure and pushes milk out. Emptying the breast after each feeding is one way to avoid excess leakage (Weaver & Hernandez, 2016) or initiate the inhibition of lactation through a negative feedback loop (WHO, 2009).
Oxytocin generally is involved in social interactions or bonding, learning by association, memory and exerts psychophysiological effects. It can reduce pain and acts as anti-inflammatory, anti-oxidative and anti-stress agent offering restorative effects and reducing depression (Uvnäs Moberg et al., 2019). Oxytocin is relevant for mother-infant bonding which can be interrupted through traumatic birth experiences that can be so severe as to cause post traumatic stress disorder (PTSD). Such stress easily affects maternal behaviour, bonding and lactation negatively (WHO, 2009, Witteveen et al., 2020). Maternal support, creating a calming environment, skin-to-skin contact, rooming in, and pleasant forms of tactile stimulation (touch) to induce oxytocin release and mother-infant bonding may be plausible actions (WHO, 2009). Interventions with exogenous oxytocin are also debated for trauma patients and may be worth considering for some lactating women (Witteveen et al., 2020, Donadon et al., 2018).
Women sometimes experience severe uterine pain in the first few days of breastfeeding. This is also part of oxytocin function: The hormone contracts the uterus after delivery and reduces excess bleeding (Uvnäs Moberg et al., 2019, Negishi et al., 1999).
Milk production is also regulated by several bioactive factors one of which creates a feedback-loop one of which is the "feedback inhibitor of lactation" (FIL): This polypeptide is present in breast milk. It accumulates if the breast is not emptied, for example when a child is only fed from one breast. The result can be that the breast that is not used, stops lactating. It is a protection mechanism to avoid damage to the tissue if the breast is too full and allows adaptation of milk volume to the child's demands. To avoid this inhibition mechanism, women can express their milk via squeezing or pumping if the child is unable to empty both breasts (WHO, 2009, Weaver & Hernandez, 2016).
… that the tammar wallaby (Macropus eugenii) is able to provide milk for several joeys when they are at different stages in development. Adjacent mammary glands secrete a different milk stage. This is called "asynchronous lactation". This way, an older joey outside the pouch can be fed at the same time than a joey still in the pouch. The tammar wallaby can produce three distinct milk stages. A joey suckles for about 300 days (Weaver & Hernandez, 2016, Sharp et al., 2017). That one mammary gland produces milk for a joey in the pouch and the mammary gland next to it, milk suitable for a much older joey demonstrates tight local control of the mammary tissue and the lactation process. To learn more about these mechanism – that may teach us more about human lactation as well - tammar wallaby lactation is actively investigated (Sharp et al., 2017).
Donadon MF, Martin-Santos R, Osório FdL. The Associations Between Oxytocin and Trauma in Humans: A Systematic Review. Frontiers in Pharmacology 2018; 9:154. at: pubmed.ncbi.nlm.nih.gov/29545749
Galbally M, Lewis AJ, van Ijzendoorn M, Permezel M. The role of oxytocin in mother-infant relations: a systematic review of human studies. Harvard review of psychiatry 2011; 19(1):1–14. at: pubmed.ncbi.nlm.nih.gov/21250892
Haaksma CJ, Schwartz RJ, Tomasek JJ. Myoepithelial Cell Contraction and Milk Ejection Are Impaired in Mammary Glands of Mice Lacking Smooth Muscle Alpha-Actin1. Biology of reproduction 2011; 85(1):13–21. at: pubmed.ncbi.nlm.nih.gov/21368298
Negishi H, Kishida T, Yamada H, Hirayama E, Mikuni M, Fujimoto S. Changes in uterine size after vaginal delivery and cesarean section determined by vaginal sonography in the puerperium. Archives of gynecology and obstetrics 1999; 263(1-2):13–6. at: pubmed.ncbi.nlm.nih.gov/10728621
Sharp JA, Wanyonyi S, Modepalli V, Watt A, Kuruppath S, Hinds LA, Kumar A, Abud HE, Lefevre C, Nicholas KR. The tammar wallaby: A marsupial model to examine the timed delivery and role of bioactives in milk. General and comparative endocrinology 2017; 244:164–77. at: pubmed.ncbi.nlm.nih.gov/27528357
Uvnäs Moberg K, Handlin L, Kendall-Tackett K, Petersson M. Oxytocin is a principal hormone that exerts part of its effects by active fragments. Medical hypotheses 2019; 133:109394. at: pubmed.ncbi.nlm.nih.gov/31525634/
Weaver SR, Hernandez LL. Autocrine-paracrine regulation of the mammary gland. Journal of dairy science 2016; 99(1):842–53. at: pubmed.ncbi.nlm.nih.gov/26299162
WHO. Infant and Young Child Feeding. Model Chapter for textbooks for medical students and allied health professionals. Geneva, Switzerland: World Health Organization (WHO) press; 2009. at: www.who.int/maternal_child_adolescent/documents/9789241597494/en/
WHO Task Force on Methods for the Natural Regulation of Fertility. The World Health Organization multinational study of breast-feeding and lactational amenorrhea. III. Pregnancy during breast-feeding. World Health Organization Task Force on Methods for the Natural Regulation of Fertility. Fertility and sterility 1999; 72(3):431–40. at: www.ncbi.nlm.nih.gov/pubmed/10519613
Witteveen AB, Stramrood CAI, Henrichs J, Flanagan JC, van Pampus MG, Olff M. The oxytocinergic system in PTSD following traumatic childbirth: endogenous and exogenous oxytocin in the peripartum period. Arch Womens Ment Health 2020; 23(3):317–29. at: pubmed.ncbi.nlm.nih.gov/31385103back