Connective tissues

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General Features

Tissue is a part of an organism consisting of a large number of cells having a similar structure and function. Connective tissue is one group of the four tissues in the body. It maintains the form of the body and its organs and provides cohesion and internal support.

Origin

All connective tissue types derive from Mesenchyme, an embryonic form of connective tissue.

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Function

Connective Tissue provides mechanical support to tissues and organs allowing them to resist tension and compression. Furthermore it provides metabolic support in the form of growth factors, hormones, and high energy lipids through blood vessels. It contains also different cells that generate immune responses to foreign cells.

Main components

Connective tissue is composed of two main components

Cells

Depending of the type of connective tissue, different cells can be found.

Generally connective tissue cells can be divided into fixed and wandering cells.

Connective tissue and its main components

Fixed Cells: Mesenchymal Cells, Fibroblasts, Reticular Cells, Adipose Cells

Wandering Cells: Mast cell, Macrophage, Plasma Cell, White blood cells

Extracellular Matrix (Fibers + Ground Substance)

Usually the extracellular matrix consists of large protein fibers and nonfibrous areas of ground substance rich in various glycosaminoglycans and water.

Fibers

There are two types of fibers, Collagen and Elastic.

Grundsubstance

Ground substance is a complex of anionic, hydrophilic proteoglycans, glycosaminoglycans (GAGs), and multiadhesive glycoproteins.

Tissue must not only accommodate tensile or pulling forces but they must also resist compression. The primary component in connective tissue that responds to compression are glycosaminoglycans. GAGs resist compression by occupying a large volume and retaining water. The base component of GAGs is a disaccharide of two different sugars. These disaccharides are joined into polymers that can contain 1000s of disaccharides. The sugars that make up the disaccharides differ between GAGs, but the main feature is that they are negatively charged. The largest GAG is Hyaluronic Acid, a long polymer of the dissacheride glucosamine-glucuronate. It forms a dense, viscous network of polymers, which binds a high amount of water, giving it an important role in allowing diffusion of molecules in connective tissue.

Proteoglycans contain a core of protein with one or many side chains of sulphated GAGs as well as branched oligosacchardies.

Types of connective tissue

Depending of the proportion of the cells, fibers and intercellular substance, there are different types of connective tissue. A strong connective tissue for example needs a greater proportion of the collagen fibers and fewer cells, like dense connective tissue in tendons. Connective tissue composed of mostly cells isnt very strong, like adipose connective tissue.

Connective tissue proper

Loose connective tissue

This type has many cells, Fibroblasts, a loose arrangement of fibers, and moderately viscous fluid matrix.

Dense irregular connective tissue

It has randomly distributed bundles of Collagen I and is for example in joint capsules or the layer of skin underneath the epidermis.

Dense regular connective tissue

Here the collagen fibres are densely packed, and arranged in a parallel way. It is typically found in ligaments or tendon. This are resistant to loaded tension forces, but allow some stretch.

Adipose Tissue

White Adipose Tissue

Found in many organs throughout the body. Contains Adipocytes, with one large lipid droplet, which store fat in form of triglycerides, which can be broken down for energy by the organism.

Brown Adipose Tissue

This tissue type is to find mainly in newborn. Adipocytes contain many small lipid droplets. The main function of the brown adipocyte is to produce heat ba nonshivering thermogenesis.

Cartilage Tissue

Hyaline Cartilage

Is the most common type of Cartillage tissue. Its cells, Chondrocyten are mostly arranged in isogenous groups. It is located in the outer surface of movable joints for example.

Elastic Cartillage

It generally resembles hyaline cartilage in its chondrocytes and major ECM components, but its matrix includes abundant elastic fibers, which increase tissues flexibility.

Fibrocartilage

Contains varying combinations of Hyaline cartilage in small amount f dense connective tissue. It provides very tough, strong support at tendon insertions and in intervertebral discs.

Bone tissue

Bone is a type of connective tissue with a calcified extracellular matrix, specialized to support the body, protect many internal organs, and act as the bodys CA 2+ reservoir. Bone tissue contains three types of cells: osteobalsts, which deposit bone, osteocytes, which maintain the bone, and osteocalsts which resorb bone.

References

• Junqueiras Basic Histology Text & Atlas, Anthony L. Mescher 13th Edition ISBN 978-1-259-07232-1
• Histology & Cell Biology, Douglas F. Paulsen 4th edition ISBN978-0-8385-0593-9
• http://en.wikipedia.org/wiki/Connective_tissue
• http://www.britannica.com/EBchecked/topic/132995/connective-tissue
• http://www.histology.leeds.ac.uk/tissue_types/connective/connective_tissue_types.php

Picture: http://commons.wikimedia.org/wiki/File:408_Connective_Tissue.jpg?uselang=ang