Structure and types of bones, innervation and blood supply

Functions

● Support for the body

● Motion (musculoskeletal)

● Protection of organs

● Storage of Calcium, phosphate and ions

The skeleton can be divided into two systems: - Axial skeleton: form the axis, center of the body - Appendicular skeleton: “appendages” of the axis – limbs and pelvis

Bone types

By density

● Compact (cortical) bone: dense compact bone – found on the surface of bones. in long bones the diaphysis and metaphysis are mainly composed of compact bone. *in the bones of the skull it is call internal and external laminae.

● Spongy/trabecular/cancellous bone: the bone marrow is stored between the mashes of bone. can be found within short bones, flat bones and the epiphysis and metaphysis of long bones. *In the bones of the skull it is called dipole

By shape

● Long bone: consists of 2 epiphyses and a diaphysis (shaft). In the center of the diaphysis the bone marrow is stored in the medullary cavity. The bone marrow can be yellow (fat) or red (hematopoietic)

● Flat bone: consists of two layers of compact bones with spongy bone material between them. They grow in two main directions. o Example: scapula, pelvis, sternum, ribs and some of the skull bones.

● Short bones: have spongy core surrounded by compact bone. o Example: carpals and tarsals

● Irregular bones: include all bones that don’t belong to any of the preceding groups (vertebrae…)

● Pneumatized bones: contain air filled cavities lined by mucous membrane (found in the skull) o Example: maxillary, ethmoid…

● Sesamoid bones: protect tendons from stress and wear. o Example: patella, some bone in the palm and foot.

By arrangement of fibrils

● Woven (primary) bone: structurally it is ossified CT with Irregular organization of collagen fibers. It primarily occurs only during development. In adults it is found only in the capsule of inner ear and some cranial bones.

● Lamellar (cortical) bone: more common and more important, exhibits a distinct stratification produced by layers of parallel running collagenous fibrils which are designated as Lamellae. The lamellar arrangement takes place around a vascular canal (Haversian canal) which together with osteocyte in its lacuna constitutes an Osteon (or "Harversian system"). The collagenous fibers are about 2-3 μm thick, and are arranged spirally in such a way that a right and a left spiral lamella appear alternately with one another, which forms increased stability

Types of lamellae

▪ Circumferential lamella: underline the periosteum and endosteum

▪ Interstitial lamella: fill in between the osteons

▪ Concentric lamella: surrounding the haversian canal

Parts of the bone

1. Periosteum – innervation and blood supply to the bone Covers all parts of the bone (except the joint surfaces), it is connected to the compact bone by Sharpe’s fibers (made of collagen). The periosteum contains many blood and lymph vessels, and nerves. It consists of 2 layers:
   • Outer fibrous layer: dense CT, blood and lymph vessels, nerves, collagen and fibroblast
   • Inner cambium (osteogenic) layer: contains progenitor cells – bone lining cells, osteoblasts, mesenchymal stem cells. responsible for increasing width of the bone and fracture healing
2. Endosteum – covers all trabeculae around marrow cavities. It contains bone lining cells, osteoblasts and mesenchymal stem cells
3. Blood supply
   • Periosteal vessels: blood vessels in the outer layer send small capillaries to the inner, cell-rich layer
   • Nutrient artery: blood vessels and nerves reach the bone through the nutrient foramen. They split in the medullary cavity into longitudinal branches and supply all bone parts.
   • Epiphyseal and metaphyseal vessels: support the respective parts, arise from the arteries that supply the joint
4. Innervation
   • Periosteum: rich sensory innervation, carry pain fibers 🡪 account for the intense pain in bone fractur
   • Nerves that accompany the blood vessels: most of the nerves passing through the internal cavity are vasomotor nerves that regulate the blood flow
   • Hilton’s law – the nerve supplying the muscle will also supply the bone underlying it – if a group of muscle receive nerve supply from a specific nerve, the bone over which the muscles lie will also be innervated by the same nerve.
5. Canals
   • Haversian canal (parallel to the periosteum): lined with endosteum, contain blood vessels, nerves, lymph vessels, loose CT
   • Volkmann’s canal (perpendicular-oblique to the periosteum)