The nerve supply of the mammary gland

Proper development and functioning of the mammary gland depends not only on hormones, but also on the nervous system. The aim of this paper was to summarize the existing knowledge on the innervation of the mammary gland in various mammalian species, including domestic animals. In general, the pattern of innervation of this gland is similar in all studied species, i.e. large farm animals, rodents, dogs and beavers, as well as humans. Using the pan-neuronal marker of neural structures PGP-9.5, it was found that the mammary gland is well supplied with nerve fibres. Very rich innervation was discovered in the nipple, and less numerous nerve endings were found in the parenchyma. Nerve fibres supply the skin, especially the nipple, and many nerve terminals are associated with blood vessels and smooth muscles throughout the organ. Immunohistochemical investigations have made it possible to distinguish two major subpopulations of nerve fibres supplying the mammary gland. One of them consists of putative sensory fibres expressing CGRP, SP, GAL and/or PACAP, and the other is presumably composed of adrenergic axons containing TH, DBH, SOM and/or VIP. Studies on the sources of innervation of the mammary gland have confirmed assumptions about the nature of the nerve fibres and have revealed that they derive from the spinal ganglia (DRG) and sympathetic trunk ganglia (SChG). In the rat, mammary gland-projecting (MGP) neurons were also found in the nodose ganglion. So far, the rat and the pig have been tested most comprehensively for the sources of the mammary gland innervation. The MGP sensory neurons were observed ipsilaterally in several consecutive DRG, suggesting that the same neurons supply several successive glands. In the case of the SChG, it was found that MGP neurons are located, among others, in the L1-L4 ganglia, which constitute a specific „mammary gland nerve centre”. Previous studies on the sources of nerve supply to female reproductive organs have revealed that the SChG L1-L4 ganglia also contain nerve cells that project to the ovary, fallopian tube and uterus. Some clinical implications of the mammary gland innervation are also discussed.