Bone: the organ

Compact woven bone comprises the wall of this fetal pubic bone and is surrounded by its surface coverings. Periosteum covers all external surfaces of bone, while endosteum lines all internal surfaces except for lacunae and canaliculi. A portion of the marrow cavity can be seen in the lower left. 200x

Periosteum covers all external surfaces of bone. When active (as here), it resembles an active perichondrium with a fibrous layer (black arrow) of dense connective tissue and an osteogenic layer (blue arrow). The osteogenic layer lies immediately adjacent to the bone and contains osteoblasts that lay down bone and/or osteoclasts that resorb bone.

The inner surfaces of bone, except lacunae and canaliculi, are lined by a single layer of flattened endosteal (lining) cells causing this layer to resemble a simple epithelium.

The osteocytes in this fetal, woven bone are more rounded than flattened, as they would be in adult bone. Also, they are not uniformly aligned in lamellae, but are randomly scattered throughout the extracellular matrix, as is diagnostic of woven bone. Lacunae can be seen surrounding many of the osteocytes.

Numerous blood vessels are present in bone and marrow spaces as well as the surrounding periosteum. However, blood vessels in the periosteum are not easily detected in this preparation and at this magnification.

The next image is similar to the area outlined by the rectangle.