|MadSci Network: Anatomy|
Marc, The tissues are physiologically different in cellular structure and function. Mammals possess white, brown and yellow depot adipose tissue composed of adipocytes, fat cells. Most mammals have white adipose which serves as insulation and an energy depot. White adipocytes have very thin ring of cytoplasm surrounding a single large lipid droplet and a single, flattened nucleus, there are very few discernable organelles. The fat tissue is not profusely vascularized. Brown adipocytes are polygonal in shape with a large volume of cytoplasm and contain multiple lipid droplets of much smaller size than the white adipocyte. There are mitochondria present, full of cytochromes, which gives them their color. Their nuclei are round and usually centrally located. The brown tissue is highly vascularized, plenty of capillaries to allow for rapid in and out of lipid to the peripheral blood supply. There are reports of melanin(brown) secretions being made also giving color to the adipose tissue. While the fat droplet may be the same in each cell type, say, glyceryl tristearate… it may be mobilized and metabolized much faster by the organelles and vascularization of the brown adipose tissue. Yellow adipocytes are actually white adipocytes in the animal consuming a diet high in carotenoids and xanthophylls. The references have excellent photomicrographs of the two tissue types, they are unmistakable. Thank you for your interesting question! I hope this helps! Peter http://www.pubmedce ntral.nih.gov/botrender.fcgi? blobtype=html&artid=1202725, (Biochem J. 1964 March; 90(3): 518–521) http://physr ev.physiology.org/cgi/content/full/84/1/277 (Physiol. Rev. 84: 277-359, 2004; doi:10.1152/physrev.00015.2003) http://www.vivo.colostate.edu/hbooks/pathphys/misc_topics/brownfat.html The Linkage Relations of Yellow Fat in Rabbits, W. E. Castle,Proceedings of the National Academy of Sciences of the United States of America, Vol. 19, No. 11 (Nov. 15, 1933), pp. 947-950
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