Appearance of myoepithelial cells in developing rat mammary glands identified with the lectins Griffonia simplicifolia-1 and pokeweed mitogen.

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Abstract
The histochemical binding of peroxidase-conjugated Griffonia simplicifolia-1 (GS-1) and pokeweed mitogen (PWM) to methacarn-preserved and paraffin-embedded female rat mammary glands at different developmental stages has been undertaken with a view to investigating the ontogeny of the myoepithelial cell. Conjugated GS-1 fails to stain the outer layer of ductal cells in neonatal rats up to 3 days old, but thereafter the staining increases so that all such cells are intensely stained in rats 5 days old and in mature, pregnant, and lactating rats. Conjugated GS-1 also stains most of the inner epithelial cells that line the ducts in neonatal rats up to about 5 days after birth; thereafter no such cells are stained in mature, pregnant, and lactating rats. Conjugated PWM stains both the inner and outer cell layers of ducts in neonatal rats up to 5 days old; thereafter the fraction of strongly stained cells declines rapidly in both cell layers so that in 6-day-old rats only weak staining is visible. Staining with PWM continues to decline for the inner epithelial cells of the ducts until it ceases when the rats mature; in contrast, that for the outer cell layer of the ducts increases so that all such cells are stained intensely when the rats mature. This pattern of ductal staining with PWM is maintained for pregnant and lactating rats. In terminal end buds of mammary ducts of prepubertal rats, GS-1 binds mainly to the peripheral or cap cells, the staining intensity increasing from cap cells at the distal tip to myoepithelial cells of the subtending duct. PWM binds to many more of the cortical epithelial cells and fewer of the cap cells. At the ultrastructural level, cap cells and adjacent immature myoepithelial cells both bind GS-1 and PWM to their surfaces, but basal clear cells do not. In alveolar buds and in alveoli, both conjugated lectins GS-1 and PWM bind to myoepithelial cells but not to epithelial cells of the rat mammary gland. We suggest that the appearance of carbohydrate receptors for GS-1 and PWM marks specific stages of myoepithelial cell differentiation in developing rat mammary glands.

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