Từ điển khoa học động vật vần L | Lactation Land Mammals Species Comparisons Olav T. Oftedal Smithsonian Institution Washington . . INTRODUCTION Lactation is a diagnostic trait of all mammals involving the synthesis of a nutrient-rich secretion by specialized mammary glands. The many similarities among living mammals in mammary gland development regulation and ultrastructure as well as in secretory mechanisms and types of milk constituents indicate that lactation has a single evolutionary origin. However there are diverse lactation patterns among living mammals involving differences in the duration and intensity of lactation as well as differences in suckling frequency milk composition and milk yield. In this article aspects of milk secretion and yield that underlie species differences will be discussed and some of the variation seen among major orders of land mammals will be mentioned. ORIGIN OF LACTATION Lactation was well established before the divergence in the late Jurassic and or early Cretaceous periods of the monotremes such as echidnas and the platypus marsupials such as opossums and kangaroos and eutherians such as mice sheep and humans . 1 Lactation appears to be an ancient trait that first appeared more than 200 million years ago perhaps as a source of moisture for incubated eggs. 2 MILK SECRETION Lactation involves the production of milk by epithelial cells that line the expanded terminal ends or alveoli of an intricate system of ducts. Milk is formed as a mixture of two primary phases an aqueous phase including water electrolytes proteins and sugars released from small vesicles that migrate to the cell surface and a lipid phase that forms by the coalescence of lipid droplets and is released from the cells as membrane-bound fat globules. 3 4 Most constituents are synthesized within the epithelial cells themselves but some are transported from blood across the mammary epithelial cells or pass into milk by extracellular routes. Differences in the rates of secretion of the .
