Figure 5.

Evolution of breastfeeding. (A) Simplified phylogenic tree of mammals and their evolutionary forerunners. Relative time scale of the acquisition of homeothermy and the advent of lactation are shown. Placentalia have a core body temperature of 37 °C, and develop white adipose tissue (WAT) and thermogenic adipose tissue: brown adipose tissue (BAT) and beige- or induced brown adipose tissue (BeAT). Marsupialia have a lower core body temperature (35–36 °C) than the mammals, such as the Monotremata (around 32 °C). Marsupialia and Monontremata develop WAT, while BeAT only upon induction [160,161]. Developing BeAT depots is a form of adaptive thermogenesis. (B) Histology of the mammalian skin glands: sebaceous gland (sb) and sweat gland (sw) [162]. The sebaceous gland cells are filled with lipid droplets and undergo apoptosis as a mechanism of holocrine lipid secretion. The sweat glands have an apocrine secretion mechanism, and the image shows large secretory droplets in the apical regions of the gland cells. Hematoxylin and eosin staining, scale 25 μm. Both glands are associated with the hair follicle. (C) Lipid secretion mechanism of the lacteal gland (or breast gland) [12]. Note the apocrine release of the lipid droplet (lp), enveloped in the cell membrane (cm). er: endoplasmic reticulum. Transmission electron microscopy images of gland cells in the breast of lactating mouse and cattle. nc: nucleus, lp: lipid droplet, pc: plasma cell. Scale bar 2 μm.