Different stages of intramembranous bone formation are seen in the skull cap (calvarium) when moving from the center to the right.
Condensation - mesenchymal cells migrate and aggregate into areas where bone will form (a bone blastema).
Osteoprogenitor Cells - the condensation of mesenchymal cells initiates the differentiation of osteoprogenitor cells.
Osteoblasts - the synthesis of components of the bone matrix indicates the further differentiation of osteoprogenitor cells into osteoblasts. Their basophilic cytoplasm is from the synthesis of proteins in osteoid (mostly type I collagen).
Osteoid - unmineralized, organic portion (type I collagen and ground substance) of bone matrix. Narrow, light-pink region between osteoblasts and the bone spicule.
Osteocytes - osteoblasts trapped inside lacunae of the bone.
Bone spicules - composed of women immature (or primary) bone with mineralized osteoid that is more basophilic than unmineralized osteoid.
Trabeculae - form from bone spicules that fuse with each other to provide the general shape of a developing bone.
Haversian System - developing osteons (#1, #2 and #3) can be recognized by blood vessels at the center of an area surrounded by osteoblasts.