HISTOPHYSIOLOGY AND CYTOPHYSIOLOGY OF THE LACTATING MAMMARY-GLAND OF THE AFRICAN ELEPHANT (LOXODONTA-AFRICANA)

The lactating mammary gland of the African elephant (Loxodonta african a) has been studied with a panel of morphological techniques focusing on (1) the functional changes during the secretory process, (2) prolif erative process [by application of proliferating cell nuclear antigen (PCNA) immunohistochemistry] and apoptotic phenomena [by use of the TU NEL technique] in the individual lobules, and (3) components of milk a nd milk-fat-globule membrane. In the lactating gland, the lobules are variably differentiated; within a lobule, however, the alveoli are usu ally similarly differentiated. The morphology of their alveoli suggest s a classification of the lobules into types 1-3. Lobules of type 1 ar e composed of immature tubular alveoli with mitotic figures and numero us PCNA-positive nuclei; advanced type 1 alveoli contain abundant glyc ogen and specific secretory granules. Lobules of type 2 are further su bdivided. In type 2a lobules, the epithelial cells of the alveoli form tall apical protrusions, which in part are occupied by small lipid dr oplets and which are pinched off in an apocrine fashion. The number of lysosomes varies considerably. Type 2b is the most common type, with striking basal membrane foldings, abundant rough endoplasmic reticulum cisterns, large Golgi apparatus, numerous mitochondria, lipid droplet s, and protein vesicles with 30- to 90-nm-wide casein micelles. The li pid droplets are pinched off with minimal amounts of cytoplasm. Type 2 c is composed of alveoli with a cuboidal epithelium and few signs of s ecretory activity. Increasing expression of peanut-agglutinin-binding sites parallels the maturation and differentiation of the glandular ce lls. Type 3 lobules are marked by numerous TUNEL-positive nuclei and l arge lipid droplets and are apparently degenerating structures. Cytoke ratin (CK) 14 is usually present in the myoepithelial cells; CK 19 and CK 7 mark ductal and immature alveolar epithelia. Milk protein conten t varies between 2.6% and 6.3%, and casein micelles range from 35 to 9 0 nm in diameter. The diameter of intra-alveolar milk fat globules ran ges from 5 to 25 mu m and the membranes bear a filamentous surface coa t composed of membrane-anchored mucins; gel-electrophoretic analysis o f these mucins from different individuals demonstrates the presence of mucin MUC 1, which is expressed with considerable genetic heterogenei ty.