Prolactin is the hormone that regulates mammary gland development in man. However, in animals it is the main functional regulator to transmit the physiological reaction to the seasons, its expression is dependent on the day length, which is measured in calendar cells close to the hypophyseal stalk, and which activate prolactin expression when the day time increases, and vice versa.

How prolactin could in turn influence different functions such as increase in mating behaviour, coat colour changes or molt, the song in birds, e.g., has been an open question. A minireview in Molecular Endocrinology by Sackmann-Sala and colleagues from the  Institut Necker in Paris, France, may shed light on this issue. They show that in humans, mice, and rats prolactin acts on stem cells in a tissue specific way. The tissues in question are reproductive tissues, but apart from that also special regions of the brain, and peripheral tissues.

If each of the functions as mediated from the progeny of individual stem cells, then a stimulating role of the pleiotropic prolactin activities is easily understood. The paper does not address this question, but it opens a new way of thinking.

For this reason, do not miss it if you are concerned with circannual regulation.