This is
characterized by having a large amount of intercellular substance. It forms
areolar tissue, the packing material of the body, the supporting tissues and
blood. Embryonic connective tissue is called mesenchyme.
Areolar tissue
The intercellular substance is semisolid and composed
of proteins and mucopolysaccharides. Three types of fibres are found: coarse collagen fibres, which are white,
flexible, inelastic and arranged in bundles, elastic fibres, which are
yellowish (in bulk), less frequent and branching; and reticular fibres, which form a very fine silver-staining
network throughout the tissues.
The cells are of five main varieties: large, slender
poorly staining fibroblasts, closely concerned with the production
of the three types of tissue fibre; tissue
macrophages, which are phagocytic and can engulf particulate matter; oval
plasma
cells with their cartwheel-like staining nucleus,
concerned with antibody production; granular basophilic mast cells, concerned
with histamine and heparin production; and the cyst-like fat-containing
cells.The relative amounts of cellular and intercellular substance vary
throughout the body. Subcutaneous tissue contains a variable amount of fat and
loose fibrous tissue. Superficially, fat is usually predominant, but more
deeply the fibrous tissue forms a well-defined superficial fascial sheet
connecting
it to the deep fascia that invests the limbs and
trunk.In other places condensations of non-elastic fibrous tissue form ligaments, tendons and aponeuroses, and retinacula.Ligaments are usually attached
to the bones on each side of
a joint, maintaining its stability; tendons join the
muscles to the bones by blending with the periosteum,aponeuroses are thin
flattened tendinous sheets through which muscles gain
wider attachments. Retinacula are usually thickenings
of the deep fascia related to joints.
SUPPORTING TISSUE
Cartilage
This is an avascular, firm tissue composed of cells in
an abundant intercellular substance. It is formed from an overlying fibrous
layer, the
perichondrium, and classified, according to its
predominant fibres, into hyaline cartilage, fibrocartilage and yellow elastic cartilage.
Hyaline cartilage contains many cells and a few
fine collagen like fibres, and is found in the rib cartilages and over most
articular surfaces. It also forms the precursor in cartilaginous ossification.
Fibrocartilage contains many dense fibrous
bundles and fewer cells, and is present in the intervertebral discs, over the
articular surface of bones that ossify in membranes, e.g. the mandible, and in
intra-articular cartilages, e.g. the menisci of the knee.
● Yellow elastic cartilage contains elastic fibres and is
foundin the auricular, epiglottic and the apices of the arytenoids cartilages
of the head.
Bone
This is a hard supporting tissue composed mainly of
inorganic calcium salts impregnating a network of collagen fibres . The basic
unit, composed of concentric layers around a central vessel, is known as a Haversian system.
The bone cells lie within spaces between the layers
and their processes pass into canaliculi in the bone. Compact bone is dense and strong and forms the
outer part of most bones. The cancellous bone within consists of a network of thin partitions
around intercommunicating spaces; the osteocytes lie within lacunae in the
trabeculae. The outer surface of a bone is covered by a thick fibrous layer,
the periosteum, many of the cells of which are
the granular, bone-forming osteoblasts. These cells, when enclosed in the hard
intercellularsubstance, become osteocytes. The blood supply of
bone is from the periosteum and muscular vessels and,
in the case of long bones, from one or two nutrient arteries that enter the
shaft.
The shape of the bones of the body, the proportion of compact
to cancellous tissue and the architecture of the trabeculae are arranged to
give maximum strength along with economy of material. Both genetic and local
factors influence the shape and size of a bone. Adjacent muscles or organs
mould the bone to some extent.Many of these factors can be investigated in the
living person by means of X-rays.
Bones are classified as long, short, flat, sesamoid or
irregular.Long
bones are present
in the limbs. The body is a cylinder of compact bone surrounding a medullary
cavity
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