Mammary tissues from virgin, prepartum and lactating ewes were examined by in situ hybridisation with a 35S labelled cRNA probe derived from the bovine alpha-lactalbumin gene. Expression of alpha- lactalbumin was absent in the virgin udders. In the pregnant gland, alpha-lactabumin expression was found in some secretory alveoli but not in others, even though they appeared histologically similar, containing milk or colostrum and fat globules. Fully lactating ewes have several levels of alpha-lactalbumin expression. In secretory epithelium, it was high in collapsed thick walled lactating alveoli, slightly less in distended thin walled lactating alveoli with homogeneous contents and absent from alveoli containing an abundance of large fat globules. Beta-cytophasmic actin expression was the reverse of this. These observations are intriguing and suggest a number of hypotheses. Firstly; that alpha-lactalbumin gene expression is linked to the long term secretory activity of cells and drops off once cells are resting or regressing. Secondly; that alpha-lactalbumin gene expression is highest in those alveoli which are "filling up" and lowest in those which are full or have just been milked out. That is, there are cyclical variation in expression. And thirdly; that there is distinct compartmentalisation in the lactating gland, and synthesis of alpha- lactalbumin mRNA (and the protein?) occurs in cells which synthesise little fat.
Proceedings of the New Zealand Society of Animal Production, Volume 51, , 63-66, 1991
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