Bone, Cartilage, Fat
There are two kinds of ossification, endochondral and membranous, implying the presence or absence of a prior cartilaginous model. In both processes bone is formed by osteoblasts, which originate from mesenchyme. Osteoblasts are spindleshaped to columnar cells with an eccentric nucleus and basophilic cytoplasm. They congregate on surfaces and lay down the organic matrix (osteoid), which then becomes mineralized. When the osteoblasts become trapped in small spaces (lacunae) within this osteoid, they are called osteocytes.