Bone Tissue Structure, Cells, and Marrow Function

Bone Tissue: Structure and Cell Types

Bone is a vital component of body mass in vertebrates. Bone tissue is an intercellular matrix that contains abundant minerals, predominantly calcium (Ca) and phosphorus (P). Bone contains three main cell types:

Bone Cell Types

• Osteocytes: Small, mature bone cells that reside in lacunae. They are the most important cells of bone tissue and are commonly grouped, typically between 4–6 cells, within bone lacunae.
• Osteoblasts: Large, bone-forming cells located on the outer surface of bones. Their primary function is to synthesize and deposit bone matrix.
• Osteoclasts: Large, multinucleated cells located on internal bone surfaces, especially along the medullary cavity of long bones. Their function is to resorb bone to maintain proper bone proportions and remodel bone tissue.

Bone Membranes

Bones are covered externally by a connective tissue membrane called the periosteum. The inner surface of long bones is lined by another membrane called the endosteum.

Types of Bone Tissue

Spongy (Cancellous) Bone

Spongy bone is found in the heads (epiphyses) of long bones and inside flat and short bones. It has a porous, trabecular structure formed by trabeculae. The spaces between trabeculae are filled with red bone marrow, which is responsible for the production of blood cells.

Compact Bone

Compact bone forms the shaft (diaphysis) of long bones and is covered externally by the periosteum and internally by the endosteum. It also coats flat and short bones. The fundamental structural unit of compact bone is the osteon (Haversian system), which is organized as follows:

1. Haversian canals: Longitudinal channels within osteons that contain an artery, a vein, and a nerve.
2. Bone lamellae: Concentric sheets of intercellular mineralized matrix (lamellae) arranged around the Haversian canal. Between different groups of concentric lamellae are interstitial lamellae that have no apparent order.
3. Bone lacunae: Oval cavities located between the lamellae; each lacuna commonly contains between 4 and 8 osteocytes.
4. Canaliculi: Minute channels that connect lacunae to one another and to the Haversian canals, allowing intercellular communication and nutrient exchange.
5. Volkmann's (perforating) canals: Channels that run perpendicular to Haversian canals and connect different osteons and the periosteal and endosteal surfaces.

Inside the medullary cavity of long bones is yellow bone marrow, which contains a high amount of lipids (fat).