An excerpt from Oxytocin: The Biological Guide to Motherhood by Kerstin Uvnäs-Moberg, MD, PhD.
Also by the author: The Hormone of Closeness.
The role of oxytocin in milk ejection and giving warmth.
As described in the previous chapter, prolactin produced in the lactotrophs of the anterior pituitary is the main stimulator of milk production, even if oxytocin exerts a positive regulatory input. In contrast, oxytocin released from the posterior pituitary into the circulation is the main stimulatory factor behind milk ejection.
Oxytocin released in response to suckling induces an additional effect as it dilates the blood vessels in the skin of the mammary glands and the chest, thereby allowing “giving of warmth,” in addition to giving of milk.
Milk ejection is the process whereby milk is expressed from the alveoli and the ducts of the mammary glands to the opening of the ducts on the nipple. It is induced by suckling, but the pathways in the spinal cord and the brain, as well as the hormonal mechanisms involved, differ from those of milk production.
Milk Ejection in Breastfeeding Women
In connection with milk ejection induced by the suckling stimulus during breastfeeding, milk that has already been produced becomes available to the infant in connection with milk ejection. In lactating women, most of the milk stored in the alveolar portion of the gland is expressed into the milk ducts and ejected from the nipples during milk ejection.
The occurrence of the milk-ejection reflex during breastfeeding can be established by a sensation of tingling or pressure in the mother’s breast. Not all women sense this reflex. Alternatively, an observed flow of milk from the contra-lateral nipple or swallowing sounds from the infant can be used to establish the milk-ejection reflex. By using these types of observations, milk ejection was found to occur within 60-90 minutes after the start of breastfeeding (Widstrom et al., 1984).
Recently, an ultrasound technique has been developed which allows milk ejection to be visualized as opening of the pores on the nipple. The opening of the pores was shown to parallel ejection of milk in experiments in which milk flow was studied in the contra-lateral breast during milk pumping. The first milk ejection occurred after 120 seconds and was then followed by several others at 90-120-second intervals (Geddes, 2007; Prime et al., 2009; Prime, Geddes, Hepworth, Trengove, & Hartmann, 2011; Prime, Kent, Hepworth, Trengove, & Hartmann, 2012; Ramsay, Kent, Owens, & Hartmann, 2004; Ramsay et al., 2006).
Role of Oxytocin
A pituitary extract was found to increase milk flow in the beginning of the 20th century. Later on, this unknown substance was found to correspond to oxytocin (Ott & Scott, 1910). As described in detail in a previous chapter, oxytocin is produced in nerve cells in the hypothalamus in the SON (supraoptic) and PVN (paraventricular) nuclei. Oxytocin produced in the magnocellular neurons of the SON and PVN is transported down to the posterior pituitary by long nerve endings to be released into the blood stream, e.g., in response to suckling.
Interestingly, an oxytocin-like peptide is released into the circulation in connection with egg laying in birds, suggesting that milk ejection in mammals is functionally related to behaviors, such as egg laying in birds, and perhaps to other even more primitive animals (Burbach et al., 2006).
Oxytocin is necessary for milk letdown. Female animals lacking oxytocin fail to give milk to their young, and the offspring die of starvation (Nishimori et al., 1996; Young et al., 1996).
Oxytocin Induces Contraction of Myoepithelial Cells
During suckling, milk ejection occurs when oxytocin released into the circulation reaches the mammary glands. After binding to specific receptors, oxytocin contracts the myoepithelial cells that surround the alveoli, thereby raising the intra-alveolar pressure. Milk is consequently expelled into the smaller milk ducts. Since the myoepithelial cells lining the small milk ducts are disposed longitudinally, the ducts shorten and widen in response to oxytocin, thereby expelling the milk into the larger ducts, which lack myoepithelial cells (Ely & Petersen, 1941; Gaines, 1915; Kimura et al., 1998; Ott & Scott, 1910; Petersen & Ludwick, 1942; Prime et al., 2009; Prime et al., 2011).
Oxytocin Opens the Sphincters of the Larger Milk Ducts
Oxytocin may also widen the larger milk ducts and dilate the circular muscles surrounding the openings of the larger milk ducts on the nipple, “opening the doors” for the milk to be ejected. These effects may be mediated by oxytocin receptors on the milk ducts and in smooth muscles located in the connective tissue surrounding the ducts, as well on circular muscles surrounding the sphincters, as oxytocin receptors are not only present on the myoepithelial cells surrounding the alveoli (Kimura et al., 1998). These effects may be indirectly mediated via an oxytocin-induced release of locally produced peptides that relax smooth muscles. As will be described later in this chapter, the sensory nerves innervating the nipple and the mammary glands contain peptides, such as calcitonin gene-related peptide (CGRP), SP, and VIP, which may relax smooth muscle (Holzer, Taché, & Rosenfeldt, 1992; Pernow, 1983; Said & Mutt, 1970). In support of this assumption, oxytocin has been demonstrated to release VIP (Stock & Uvnäs-Moberg, 1985).
Giving of Warmth
In addition to stimulating milk production and milk ejection, suckling also increases the temperature in the skin overlying the mammary glands and adjacent areas on the front side of the chest. In women, this phenomenon is called flushing and can be visualized by cameras sensitive to heat.
Functions of Increased Skin Temperature
The increase of maternal skin temperature helps the mother warm her offspring and prevent hypothermia, which may be lifesaving, particularly immediately after birth (Kimura & Matsuoka, 2007).
The maternal capacity to give warmth to newborns has other important functions for the offspring. Newborns are extremely sensitive to small changes in temperature. Since they are attracted by warmth, it may help them find the nipple and start suckling. Newborn piglets, for example, move towards the udder, because the temperature of its base is increased by 0.5 degrees Celsius (Algers & Uvnäs-Moberg, 2007).
The warm temperature makes the newborns feel well and calm, and their stress levels are reduced when they are close to their mothers. Together with touch and stroking, it also stimulates processes related to growth and maturation in the newborn. In this way, the mother continues to regulate metabolic and physiologic processes in the newborn after birth (Hofer, 1994). In fact, by transferring warmth to the offspring, the mother gives a more basic form of “calories or energy” than when she gives milk. All these effects will be described in more depth in the chapters describing the effects of skin-to-skin contact.
Giving of Warmth in Birds
Perhaps the ability to give warmth to mammalian offspring is related to the process by which birds warm their eggs and offspring before hatching. Some birds have a network of superficial blood vessels on their “chest,” and some species pick their feathers on the “chest” before they are going to lie on their eggs to increase the contact between their skin and the egg. In this way, the parents give warmth to the eggs when they lie on them. It is by transferring warmth to their eggs that birds make their unborn chicks grow and develop inside the egg. They also warm newborns by lying on them in the nest.
It is as if mammals have retained the capacity to give warmth to their offspring, and in this way, contribute to growth and development of the offspring.
Role of Oxytocin
The increased temperature in the skin overlying the mammary glands is due to an increased circulation in the thoracic arteries and to dilation of the blood vessels. When the blood vessels in the skin dilate, the circulation in the skin increases, which leads to increased temperature (Kimura & Matsuoka, 2007).
Circulating oxytocin plays an important role in the increase of skin temperature. In fact, the mechanisms involved are not so different from those involved in milk ejection. As described above, circulating oxytocin does not only contract the myoepithelial cells in the alveoli in connection with milk ejection, it also dilates the larger milk ducts to allow expression of milk. In parallel, blood vessels in the skin are dilated.
The role of oxytocin on skin temperature was studied by Eriksson and collaborators in a series of experiments performed on lactating rats (Eriksson, Lundeberg, & Uvnäs-Moberg, 1996). The experiments showed that:
- The temperature of the skin overlying the mammary glands increased in connection with milk letdown when the pups suckled.
- Injections of oxytocin in amounts that gave rise to the same circulating oxytocin levels as observed in response to suckling induced a similar rise of skin temperature as observed during suckling.
- Injections of CGRP (calcitonin gene-related peptide), SP (substance P), and VIP (vasoactive intestinal polypeptide) into the circulation of the mammary gland increased the temperature in the skin overlying the mammary glands to the same extent as suckling and injections of oxytocin.
Injections of NPY (Neuropeptide Y) into the circulation of the mammary glands decreased the temperature of the skin overlying the mammary glands.
These data show that oxytocin released by suckling not only induces milk ejection, it also increases the circulation and the temperature in the skin overlying the mammary glands. Peptides, such as CGRP, SP and VIP, are produced in or close to blood vessels and increase skin temperature, so the effect of oxytocin may be secondary to a release of locally produced peptides (Eriksson, Lundeberg, et al., 1996). The results do not exclude the possibility that circulating oxytocin also dilates the blood vessels in the skin by a more direct action via oxytocin receptors in the blood vessels (Kimura et al., 1998).
Stimulation of Milk Ejection and Increase of Skin Temperature in Response to Other Sensory Cues or Mental Stimuli
As will be discussed in more detail later in this book, maternal oxytocin is not only released into the circulation by suckling and skin-to-skin contact, but may be released when the mother sees and hears her newborn or simply thinks of him or her, as oxytocin release can be conditioned to other stimuli. Therefore, milk ejection may occur and the skin temperature may rise (flushing), in response to increased levels of circulating levels of oxytocin before the onset of, or even in the absence of, suckling (Tancin, Kraetzl, Schams, & Bruckmaier, 2001).
- Milk ejection is the process whereby milk is expressed from the alveoli and the ducts of the mammary glands.
- Oxytocin stimulates milk ejection by contracting the myoepithelial cells in the alveoli and smaller milk ducts, and facilitates milk ejection by dilating the larger milk ducts.
- Oxytocin increases chest and breast skin temperature by dilating the blood vessels in the skin. The relaxation of the milk ducts and the blood vessels in the skin may be induced by direct effects of oxytocin on the smooth muscles or indirectly by a local release of vasoactive peptides.
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