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Bone Anatomy, Histology, & Types
general anatomy of bone
gross structure
Bone tissues
divide into two forms:
Compact bone (aka cortical bone)
- The outer dense bony layer.
Spongy bone (aka trabecular, cancellous, or medullary bone), the inner bony meshwork.
Bone regions
Diaphysis
- The shaft. Notably comprises the marrow cavity.
Metaphysis
- Comprises spongy bone.
Epiphysis
- The ends – the sites of articulation.
The epiphyseal line (the ultimate regression of the growth plate) separates the epiphysis and metaphysis.
Collaneous linings
Periosteum
- Lies along the shaft.
- Is derived from a condensation of outer connective tissue.
Endosteum
- Lies 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.
Articular cartilage.
- Derived from hyaline cartilage.
Osteoarthritis refers to when cartilage wears out and the epiphyses from adjacent bones grind on one another.
Marrow cavity
The marrow cavity is filled with:
Red marrow (hematopoetic marrow)
- Red blood cell and white blood cell precursors)
- Yellow marrow (adipose tissue).
- At birth, marrow is red, but progressively transforms to yellow marrow with age.
Key exceptions include the ribs, vertebrae, sternum, and ilia, which maintain their red marrow longer – we can remember this by the fact that the sternum is a good site for bone marrow aspiration.
Consider also that this transformation is a dynamic process and demands for red blood cells, such as chronic hypoxemia, will cause conversion of bone marrow from yellow back to red.
microscopic structure
Along the diaphysis:
The outer layer: the periosteum.
Internal to it, lies compact bone, in columns called osteons.
Centrally, within the osteon, lies a longitudinally-oriented canal, the Haversian (aka central) canal.
The osteon comprises concentric rings of lamellae – the bone connective tissue.
Osteocytes (the mature form of osteoblast (the bone-producing cells) lie within the bony matrix.
Sharpey's fibers are collagenous fibers that anchor the periosteum to the outer lamellae.
Endosteum comprises an inner circumferential lamellae and osteoprogenitor cells, internal to it.
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.
Hematopoetic marrow fills the marrow cavity.
major bone types and their functions
Long bones
- Derive their name because they are longer than they are wide. They function in support and movement.
Short bones
- The calcaneous is a short bone, which is cube-shaped (cuboidal) and supports weight.
Flat bones
- The cranial vault comprises flat bones that are thin (plate-like) and protect fragile tissue.
Irregular bones
- The vertebra of the spine is an irregular shaped bone; its shape fits its function; irregular bones have various functions (and, thus, varying shapes).
Sesamoid bones
- Sesamoid bones look like sesame seeds; they develop within tendons – such as the patellar tendons of the knee.
Wormian bones
- Wormian bones of the skull's sutures are small, space-filling bones.