Myoepithelial cells (MECs) of the mammary glands of rats, at 10 and 18 days of pregnancy and at 10 days of lactation, were examined under scanning electron microscopy. Generally, MEC presents an elongated cell body and a few cytoplasmic processes forming a basket-like network around the acinar endpiece. Single acinar endpieces are surrounded by 4 to 5 MECs during both pregnancy and lactation. As pregnancy advances, cell belies and processes of MECs become extended and slender. Regarding the

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... esses, the number of primary processes remains unchanged, but the number of secondary and tertiary processes increases moderately. The network of MECs around the acinar endpieces becomes denser and finer at the end state of pregnancy. MECs of lactating mammary glands show slightly bulged cell bodies and processes toward the surrounding connective tissues. The relative surface area covered by MECs per acinar endpiece does not vary greatly, ranging f rom25 to 29 % during states of pregnancy and lactation.