The skeleton contains both bone and cartilage tissues.
Most cartilage later turns to bone.
Cartilage tissue: not as strong as bone, but
resists shock better. Smoother surface is better at moving joints.
No blood supply; cells nourished by diffusion only. Grows much more rapidly
than bone, until diffusion cannot meet nutritional needs; then cells die
or are replaced by bone. Types of cartilage are:
- Hyaline cartilage: simple matrix
- Fibrocartilage: collagen fibers added to matrix
- Elastic cartilage: elastic fibers added
- Calcified cartilage: calcium salts added
Bone tissue: stronger than cartilage, but less resistant to shock
and grows more slowly. Internal blood supply nourishes interior and keeps
cells alive, allowing constant restructuring, repair, and healing of
injuries.
Compact (lamellar) bone tissue: made of layers, usually organized as
Haversian systems (concentric cylinders surrounding a central Haversian
artery). As bones restructure, new Harversian systems align parallel to
greatest stress.
Spongy (cancellous) bone tissue: built of struts (trabeculae)
separated by spaces containing blood or marrow.
Bone growth: there are two types of bone growth (ossification):
- Dermal (intramembranous) ossification, within the dermis of the
skin, forming membrane bones (clavicle, bones of skull roof & palate)
- Endochondral ossification: pre-shaped or preformed as
fast-growing cartilage tissue; bone then replaces the cartilage
- Visceral bones, derived from gill arches: alisphenoid;
malleus, incus, stapes; hyoid apparatus; cartilages of the larynx and trachea
- Somatic bones, derived from embryonic somites:
- Axial (derived directly from somites): braincase (ethmoid,
sphenoid, occipital series), vertebrae, ribs, sternum
- Appendicular (derived via limb bud mesenchyme): scapula,
coracoid, humerus, radius, ulna, bones of wrist and hand; innominate
bone (pelvic girdle), femur, tibia, fibula, bones of ankle and foot
Growth of long bones:
- Cartilage first grows in all directions.
- A primary ossification (diaphysis) then forms in the middle
of the bone
- A secondary ossification (epiphysis) forms at either end.
- An epiphyseal cartilage between diaphysis and epiphysis marks the
region of fastest growth.
- The growing diaphysis gradually replaces the epiphyseal cartilage.
- Bone growth ceases when epiphyseal fusion is complete.
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