Bone is the hard, calcified connective tissue that forms all the anatomical pieces that make up the skeletons of vertebrates. Like cartilage tissue, it contains living cells, called osteocytes, which are embedded in an extracellular matrix that is reinforced by collagen fibrils. Bone matrix, however, is heavily calcified, making bone harder and less supple than cartilage. Like chondrocytes, osteocytes occupy spaces called lacunae. Osteocytes are mature osteoblasts, which in turn derive from the mesenchyme. The mesenchyme is the embryonic connective tissue in chordate and vertebral animals, which, of course, include human beings.
As the individual grow, osteoblasts produce and form the hard organic bone matrix, where they fully develop into osteocytes as they become enclosed in tiny spaces called lacunae. Shut off in the lacunae, osteocytes are fed through tiny canals, called canaliculi, which link up the lacunae with one another, also connecting them to bone surfaces bathed by tissue fluid. Within each canaliculus, a slender osteocyte process is also bathed in tissue fluid. Thus, the canaliculi are extremely thin lifelines that bring tissue fluid, oxygen, and nutrients to all the osteocytes. The bone tissue also contains another type of cell, osteoclast, which takes part in bone resorption. Bone resorption is the breaking down of osseous tissue, releasing the minerals it the bone contains.
The bone consists of the hard matrix, which gives shape to individual bone pieces, the marrow, lymphatic and blood vessels, nerves, and the periosteum, which is the connective tissue that invests bones. According to their shapes, bones are classified as long (or tubular), such as the femur and humerus; flat, such as the bones of the skull; and short, like the spinal vertebrae. The middle section of a long bone is called the diaphysis, while each one of the two ends is called the epiphysis. The articulations between bones are either immobile (synarthroses), such as the skull sutures, or mobile (dyarthroses/joints), which are the articulations of the limbs.
Bone is highly vascular tissue. In contrast to cartilage, it is permeated with blood capillaries that become incorporated during its development. As a consequence, all the osteocytes are located within a 0.2 mm radius of a capillary, and the tissue fluid that this capillary produces reaches the surrounding osteocyte population by way of the canaliculi.
Bone development is known as ossification or osteogenesis. All bones are derived from mesenchyme but by two different processes, depending on which bone they are. The flat bones that form the cranium and the face develop directly in areas of vascularized mesenchyme by the process known as intramembranous ossification. Long bones, on the other hand, develop indirectly from mesenchyme through an elaborate process, which is known as endochondral ossification. In endochondral ossification, each long bone is preceded by a cartilage model; then bone tissue replaces most of the cartilage in the model during fetal life.
Below, images showing the bone tissue, with the osteocytes in their lacunae and the canaliculi.
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