Bones are vascular living structures, being fed by a network of blood vessels from the circulatory system and nerves from the nervous system.
The main features of a long bone are:
• The diaphysis, or shaft, is the long tubular portion of long bones. It is composed of compact bone tissue.
• The epiphysis (plural, epiphyses) is the expanded end of a long bone. It is in the epiphyses where red blood cells are formed.
• The metaphysis is the area where the diaphysis meets the epiphysis.
• The medullary cavity, or marrow cavity, is the open area within the diaphysis. The adipose tissue inside the cavity stores lipids and forms the yellow marrow.
• Articular cartilage covers the epiphysis where joints occur.
• The periosteum is the membrane covering the outside of the diaphysis (and epiphyses where articular cartilage is absent).
• The endosteum is the membrane that lines the marrow cavity.
The main features of short, flat, sesamoid and irregular bones:
• A relatively thin outer layer of compact bone with spongy bone inside containing red bone marrow.
• The periosteum covers the outside layer of compact bone tissue, except the inner layer of the cranial bones where it is replaced by dura mata.
Microscopic Structure of Bone
Bone is strong and durable type of connective tissue. Its major constituent (65%) is a mixture of calcium salts, mainly calcium phosphate. The remaining third is organic material, called osteoid, which is mainly comprised of collagen. The cellular component of bone contributes to less than 2% of bone mass.
Bone cells
• Osteoblast
• Ostecyte
• Osteoclast
Osteoblasts
These bone-forming cells. They are responsible for the deposition of both inorganic salts and osteoid in bone tissue. They are found on all bone surfaces and are enlarged and active at all sites of bone growth, re-modelling and repair.
As new bone tissue is deposited around themselves, they eventually become trapped in tiny pockets (lacunae) in the growing bone, and now differentiate into osteocytes.
Osteocytes
These are mature bone cells. They monitor and maintain bone tissue and are nourished by tissue fluid in the canaliculi that radiate from the central canals.
Osteoclasts
The osteocytes break down bone, releasing calcium and phosphorus. The continuous re-modelling of bone tissue is a result of the balanced activity of the bones osteoblast and osteoclast population.
Compact bone
Compact bone, also called cortical bone, is dense bone in which the bony matrix is solidly filled with organic ground substance and inorganic salts, leaving only tiny spaces (lacunae) that contain the osteocytes, or bone cells. Compact bone makes up 80% of the human skeleton; the remainder is cancellous. Compact bone forms a shell around cancellous bone and is the primary component of the long bones of the arm and leg and other bones, where its greater strength and rigidity are needed.
Mature compact bone is lamellar, or layered, in structure. It is permeated by an elaborate system of interconnecting vascular canals, the haversian systems, which contain the blood supply for the osteocytes; the bone is arranged in concentric layers around these canals, forming structural units called osteons. Immature compact bone does not contain osteons and has a woven structure. It forms around a framework of collagen fibres and is eventually replaced by mature bone in a remodelling process of bone resorption and new bone formation that creates the osteons.
Spongy bone
Spongy bone, also called cancellous bone or trabecular bone is a light, porous bone enclosing numerous large spaces that give a honeycombed or spongy appearance. The bone matrix, or framework, is organized into a three-dimensional latticework of bony processes, called trabeculae (meaning “little beams”), which consist of few lamellae and osteocytes interconnected by canaliculi. The spaces between the trabeculae are often filled with red bone marrow.
Spongy bone makes up about 20% of the human skeleton, providing structural support and flexibility without the weight of compact bone. It is found in most areas of bone that are not subject to great mechanical stress.