Epithelial cells are polarized. Here the apical surface, facing the lumen, is specialized for absorption. Apical surface specialized for secretion. Haversian system in adult mammalian bone. A blood vessel enters the Haversian canala at the cylindar center.
Bone is alive.
Haversion system is evidence of bone remodeling. See PJH for adult bone structure. Haversian systems in fossilized bone from a dinosaur. Slides 21 and 23-27 are from the same preparation. Cartilage always arises from mesenchymal condensations.