The German histologists of the 19th Century described myoepithelial cells in a number of exocrine glands including salivary, sweat and lacrymal glands. Later classical histologists and electron microscopists workers added to the list; snake venom glands are one example.
Jim Linzell, using the silver staining techniques that revealed the mammary myoepithelium, found myoepithelial cells in the sweat glands and submaxillary salivary gland. He remarked that up to the 1950s that the greatest interest in myoepithelial cells had been shown by pathologists, with respect both to the rare carcinomas they give rise to or their in metastasis. Looking at the more recent literature that generalisation still holds. The question is, though, what do the myoepithelial cells do in glands, other than in the mammary gland where they push milk stored in the alveoli into the duct system?
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| Top (8) Myoepithelial cells lying outside cells of sweat gland in dermis. Cat. Bottom (10) Submaxillary salivary gland of cat showing smaller, less numerous myoepithelial cells. From Linzell, 1952. |
The myoepithelial cells seem smaller in these other glands than in the mammary gland so the question arises: do they have any role in emitting secretion?
One function in the mammary gland I have not mentioned in the previous two articles is in relation to the mammary ducts. As well as the stellate or ‘basket’ cells which are arranged around the secretory alveoli, there are myoepithelial cells arranged longitudinally along the ducts. These, Linzell demonstrated, when stimulated to contract by oxytocin serve to shorten and widen the ducts. Obviously, milk will flow more rapidly in a shorter, wider vessel than in a longer, thinner one and so the two types of myoepithelial cell act in concert to move milk toward the sucking young.
There is evidence that the flow of saliva can be augmented under certain circumstances and that the contractile myoepithelial cells are be responsible. The nervous control of salivary secretion is complex and there remain many unanswered questions and anomalies in experiments dating from the 19th and first half of the 20th Centuries. Although written nearly 70 years ago, the best discussion I have found on this topic is that in the Physiological Society monograph, Physiology of the Salivary Glands, written by A.S.V (now Sir Arnold) Burgen and Nils Emmelin (1914-1997) published in 1961. Later in the 1960s, Nils Emmelin provided very strong evidence that the contractile myoepithelial cells, under nervous control (in contrast to the mammary gland), are involved in augmenting the rate of salivary secretion.
The strength of mammary myoepithelial contraction can have surprising consequences. Many infants—and passing adults—have been shot in the face by a stream of milk from their mother. Such streams formed the basis of Tintoretto’s famous painting The Origin of the Milky Way which can be seen in the National Gallery in London. Greek myths are all double Dutch to me but the illustration is of Hera who was suckling Heracles; he sucked* so strongly that Hera pushed him away. Her milk shot out in all directions (remember each opening in the human nipple is from a separate gland) across the heavens and formed the Milky Way†.
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| The Origin of the Milky Way Tintoretto |
S.J. Folley (see part 1 of this series) used that painting to illustrate the depiction of milk ejection in mythology and art in his Dale Medal Lecture to the Society of Endocrinology in 1969. Folley had not played any great part in discovering or delineating the milk-ejection reflex (although he had encouraged those who did) but it remained an abiding interest. I was in the audience at that lecture and it would not have been possible to guess that Folley was virtually completely blind and that he had committed to memory the whole script complete with hand gestures that pointed to the slides‡.
In some snake venom glands, ejection of stored secretion is achieved by external muscles pressing rapidly on the gland to eject the secretion at high pressure. Whether or not myoepithelial cells are involved in these front-fanged snakes or in the low-pressure ejection system of rear-fanged snakes appears not to have been studied. Obvious questions are: are the myoepithelial cells innervated? Do they respond to mechanical stimulation such that they contract as prey is being held tightly by the jaws and thereby help squeeze venom out of the gland of rear-fanged snakes?
I hope I have illustrated that 70 years after they hit the headlines there are many basic questions that remain on the rôle of myoepithelial cells and, indeed, on how exocrine glands work, and that in the Gadarene swine-like rush into molecular biology whole areas of biological organisation have been abandoned.
*The entry in Wikipedia gets suckling wrong, as if often the case. Mothers suckle; babies suck is the accepted distinction. Mammals suckle their young. Suckle: to give suck to. Roast sucking pig not suckling pig etc etc.
†There was a famous exchange between Alec Bangham (1921-2010) and Jim Linzell (1921-1975) at Babraham. Alec Bangham then working on his later famous liposomes said to Jim who was working on lactation in goats, James, you will never get into the Royal Society on a ruminant’s back. Ah, said Jim, but there is always the Milky Way. It is an open secret that Jim was on the list for election in 1976 when he died in December 1975. Alec was elected in 1977.
#At a Gordon Conference in 1975, Jim Linzell was giving the main invited (Thursday Night) talk. Folley had recently. When Linzell came to a problem to which he didn’t have an answer, he looked towards the heavens and said, Perhaps Folley knows. Instantly, Howard Bern proclaimed loudly, He’s looking in the wrong direction!
Burgen ASV, Emmelin NG. 1961. Physiology of the Salivary Glands. London: Edward Arnold.
Folley SJ. 1969. The milk-ejection reflex: a neuroendocrine theme in biology, myth and art. Journal of Endocrinology 44, 476–90.
Weinstein SA, Smith TL, Kardong KV. 2009. Reptile Venom Glands. Form, Function, and Future. In, Handbook of Venoms and Toxins of Reptiles, edited by Stephen P. Mackessy, pages 65-91. Boca Raton: CRC Press.
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