HISTOLOGY OF BONE
- The diaphysis is the shaft and notably comprises the marrow cavity.
- The metaphyses comprises spongy bone.
- The epiphyses, which are the ends – the sites of articulation.
The epiphyseal line (the ultimate regression of the growth plate) separates the epiphysis and metaphysis.
- Articular cartilage are derived from hyaline cartilage, at its ends.
- Periosteum, along the shaft, is derived from a condensation of outer connective tissue. It comprises an external layer of collagens fibers and vasculature and an internal layer of osteoprogenitor cells.
- Endosteum, centrally, is derived from derived from a condensation of inner connective tissue and helps separate the marrow cavity, internally, from the compact bony matrix that encapsulates it. It comprises an inner circumferential lamellae and osteoprogenitor cells.
- Filled with hematopoetic marrow (red blood cell and white blood cell precursors) and fatty marrow (adipose tissue).
We learn about hematopoetic marrow further elsewhere. It comprises stem cells, which can self-renew, committed precursor cells, and cells that are in the process of maturation.
- The medullary cavity comprises spongy bone.
- The outer layers of the diaphysis comprises compact bone.
MICROSCOPIC STRUCTURE OF BONE (ALONG THE DIAPHYSIS)
- Internal to it, lie columns of compact bone, called osteons.
- Centrally, within the osteons, run the Haversian (aka central) canals, in parallel.
- Volkman’s (aka perforating) canals run in perpendicular to them.
- These canal systems form channels for the neurovasculature.
- The osteon comprises concentric rings of lamellae – the bone connective tissue.
- Osteocytes are a mature form of osteoblast (the bone-producing cells) within the bony matrix; they are concentrically arranged in between the circles of lamellae. They lie within lacunae (cavities) that interconnect via canaliculi (which are spindly, like spider legs) and through which the osteocytic cytoplasmic cell processes connect for the transportation of nutrients and waste.
- The outer layer forms the cement line.
- Interstitial lamella lie in between the osteons, which comprises remnants of partially resorbed osteons.
- Sharpey’s fibers are collagenous fibers that anchor the periosteum to the outer lamellae.
- Lies internal to the compact bone. It comprises:
-An inner-circumferential lamellae
- Spongy bone lies internal to the endosteum and comprises a network of lamellae that do NOT form the Haversian channels and osteons found in compact bone.
- Haversian (aka central) canal
- Interstitial lamella
- Osteocyte within a lacuna
Osteon (at higher resolution)
- Concentric pattern of lamellae (the bony layer).
- External to this lamella, show a lacuna.
- Canaliculi radiate from it
- Haversian canal
- Edge of a Volkman’s canal, which perforates it perpendicularly.
- Inner circumferential lamellae of the endosteum
- Hematopoetic marrow
- Fatty marrow, which increases with age
- Vascular sinusoid