What is connective Tissue?

Connective tissue fills the spaces between organs and tissues, and provides structural and metabolic support for other tissues and organs.

Connective tissue is made up of cells and extracellular matrix. The extracellular matrix is made up of fibres in a protein and polysaccharide matrix, secreted and organised by cells in the extracellular matrix. Variations in the composition of the extracellular matrix, determines the properties of the connective tissue. For example, if the matrix is calcified, it can form bone or teeth. Specialised forms of extracellular matrix also makes up tendons, cartilage, and the cornea of the eye. General connective tissue is either loose, or dense, depending on the arrangment of the fibres. The cells sit in a matrix made up of glycoproteins, fibrous proteins and glycosoaminoglycans, which have been secreted by the fibroblasts, and the major component of the matrix, is in fact, water.

Classification of Connective Tissue

CONNECTIVE TISSUE PROPER (examples of these are shown below)

• - loose irregular connective tissue
• - dense irregular connective tissue

SPECIALISED CONNECTIVE TISSUES

• - dense regular connective tissue which is found in tendons and ligaments, and is shown below. The other specialised types of connective tissue are covered in other topics.
• - cartilage - (see the topic on bone and cartilage)
• - adipose tissue (see adipose cells)
• - haemopoietic tissue (bone marrow, lymphoid tissue)
• - blood (see the topic on blood)
• - bone (see the topic on bone and cartilage)

The ECM is the predominant feature in tissues with a mechanical function (ligaments, tendon & bone).

Cells are the predominant feature in tissues specialised for protection (haemopoietic tissue, blood - white blood cells) or metabolic maintenance (adipocytes, blood - red blood cells).

Connective Tissue Proper

Loose connective Tissue

This type of tissue contains many cells, a loose arrangement of fibres, and moderately viscous fluid matrix.

This is an example of loose connective tissue from a lymph gland. It contains some cells called 'plasma cells', finer elastin fibres and thicker collagen fibres. Try to identify the cells and fibres

Dense Irregular Connective Tissue

This type of tissue contains a dense woven network of collagenous (and some elastic) fibres in a viscous matrix. It is found in joint capsules, in the connective tissue that envelops muscles (muscle fascia), and it forms dermis of skin. It is impact resistant.

Look at this image and make sure you can recognise loose and dense connective tissue.

This is an example of dense irregular connective tissue. It is the layer of skin underneath the epidermis, called the dermis. It contains collagen fibres and fibroblasts. Also at the top of the picture (not labelled) you can see a small capillary - can you find it?

Specialised connective tissue

Dense Regular Connective Tissue

In this type of tissue, the collagen fibres are densely packed, and arranged in parallel. This type of tissue is found in ligaments (which link bone to bone at joints) and tendons (connections between bones or cartilage and muscle). These are powerfully resistant to axially loaded tension forces, but allow some stretch.

This is an example of dense regular connective tissue. Can you identify the cells (fibroblasts) and collagen fibres? Notice how the fibres are all aligned.