Vascular supply

Perichondrium and most cartilage is enveloped by a layer of dense connective tissue, the perichondrium, which contains the vascular supply and fibroblast-like stem cells from which additional chondrocytes may arise. Few blood vessels are found within cartilage, thus the composition of the ground substance is crucial to the percolation of nutrients and oxygen to chondrocytes from the surrounding vessels.

INTERVERTEBRAL DISKS

The intervertebral disks act as cushions between the vertebrae, allowing limited movement of the vertebral column. They are bound to the vertebrae by ligaments. Each disk has 2 parts:

1. Annulus fibrosus. This outer ring is composed mainly of fibrocartilage and is covered on its outer surface by the dense connective tissue of associated ligaments. The fibrocartilage is arranged in concentric layers, with the collagen bundles of each layer oriented at right angles to those in the next. This organization may appear as a "herringbone" pattern when seen through a light microscope at low power.

2. Nucleus pulposus. This structure forms the center of the disk and derives from the embryonic notochord. It is composed of mucous connective tissue, with a few fibers and rounded cells embedded in syrupy, hyaluronic acid-rich ground substance. The nucleus pulposus smaller in adults than in children, because it is partially replaced by fibrocartilage.

BONE

Bone is the main constituent of the adult skeletal system. Like cartilage, it is a skeletal connective tissue specialized for support and protection.

All mature bone tissue has cells (osteocytes, osteoblasts, and osteoclasts), fibers (type I collagen), and ground substance. It differs from other connective tissues primarily in having large quantities of inorganic salts in its matrix, accounting for its hardness.

Functions. Bone is second only to cartilage in its ability to withstand compression and second only to enamel in hardness. It supports and protects the more fragile tissues and organs, harbours hematopoietic tissue and forms a system of levers and pulleys that multiply and focus the contractile forces of muscle. The constant turnover of bone tissue results from a balance between the activities of the bone-forming osteoblasts and the bone-resorbing osteoclasts and allows bone matrix to function as an important storage site for calcium and other essential minerals.

Types of bone tissue. Bone tissue is classified according to its architecture as spongy or compact and according to its fine structure as primary(woven) or secondary(lamellar). All bone tissue begins as primary bone, but after all is eventually replaced by secondary bone. The term "bone" refers both to bone tissue and to an individual named element of the adult skeleton - a bone. A bone is an organ composed largely of bone tissue but also containing other connective tissues, as well as bone marrow, blood vessels, and nerves.

Bones are classified by their shape (eg, long bones, flat bones) and the process by which they form (endochondral bones, membrane bones). Most exhibit protuberances that serve as attachment sites for muscles, tendons, and ligaments. A double-layered coat of connective tissue, the periosteum, covers the outer surfaces of bones. The outer or fibrous layer of the periosteum is dense connective tissue; the inner or osteogenic layer is a looser tissue containing bone cell precursors. Sharpey's fibers are periosteal collagen fibers penetrate bone matrix and anchor the periosteum to the bone. The internal surfaces of bones are covered by a thinner, condensed reticular connective tissue called endosteumthat contains bone and blood cell precursors. The endosteum lines the marrow cavity and sends extensions into the Haversian canals. Most bones of the arms and legs (eg, the femur) are termed long bones, and knowledge of their parts is important to the study of regional differences in bone histology. The diaphysis is the shaft of a long bone, and the epiphysis is its bulbous end. In adults, the diaphysis is cylindric with walls of compact bone and a central marrow lined with endosteum. Each of the 2 epiphyses contains mostly spongy bone. Where bones contact other bones to form movable joints, their surfaces are covered by articular cartilage.

Bone is a connective tissue composed of cells, fibers, and ground substance. Bone matrix, containing abundant mineral salts, is the predominant tissue component The hardness of bone makes it difficult to section. Special techniques for obtaining thin sections include grinding bone slices until they become translucent or demineralizing fixed bone by immersion in solutions of dilute acid or calcium-chelating agents (eg, EDTA). Demineralized bone can be sectioned and stained by standard histological methods.