An ultrastructural study of the sinus epithelium in the mammary gland of the lactating ewe.

The teat and lactiferous sinus epithelium from the mammary glands of 23 lactating ewes was examined by light and electron microscopy. Most of the sinus epithelium consisted of two layers of non-secretory cells but, in the lactiferous sinus, cells with the same ultrastructural features as alveolar secretory cells were also found. Secretory cells sometimes occupied more than 50% of the total area of the sinus. Many non-secretory cells in the lactiferous sinus possessed a single cilium but they were less common in the teat sinus. 'Accessory glands', which opened directly into the lumen of the gland, were found beneath the epithelium in both the teat and the lactiferous sinuses. From their ultrastructure it was clear that these glands consisted of normal secretory alveoli and that they produced normal milk components. It is suggested that the mixed population of secretory and non-secretory cells in the lactiferous sinus provides unique material for the experimental study of many aspects of mammary gland physiology.