Home to connective tissues!
Connective tissue proper, the second broad category of connective tissues, contains a variety of types. All possess visible protein fibers embedded in a fluid ground substance. Hydrophilic muco-polysacharrides and glycoproteins of this ground substance attract water to induce variation in its viscosity; it can be almost gel-like. Regardless, the ground substances of all connective tissue proper types flows around and makes intimate contact with cells. Therefore, these cells will not be seen occupying distinct spaces or lacunae within the matrix. Lacunae as we will see, are key features for identifying the supportive connective tissues, cartilage and bone, where ground substances are semisolid gels or crystals.
As you explore the specific types of connective tissue proper you will see these contain three types of protein fibers and in some cases, a variety of cell types. Type, abundance, and orientation of protein fibers determine the kind of connective tissue proper. The forming and mature cells of connective tissue proper are called fibroblasts or fibrocytes respectively. A clear distinction between these two types is difficult. Try to remember the fibroblast as the more active cell that forms fibers and ground substance. Therefore, in a young forming tissue these cells prevail. Fibrocytes on the other hand, are more abundant mature cells in a fully-formed tissue such as a tendon or ligament. However, tendons and ligaments can be strengthened or repaired. Fibroblasts, as resident or derived cells(differentiated from mesenchyme cells) within the tendon or ligament form the fibers and ground substance required for this process.
If fibers are smaller and arranged in loose, random arrays as in the diagram shown above, you have a connective tissue proper classified as a loose connective tissue type. There are three loose types!
loose connective or areolar tissue
fat or adipose tissue
reticular connective tissue
If fibers are large and occupy most of the extracellular spaces between cells you have a dense connective tissue type.
The protein fibers of dense types can be arranged randomly within the matrix as irregular c.t. or in a common direction as regular c.t.. In dense regular types, fibers can be so abundant that fibrocytes and fibroblasts are squeezed into flattened rows between the fibers. Three types of dense connective tissue are recognized:
Here is an example of a dense irregular type! Notice how the protein fibers run in all directions?
Compare the dense irregular type to a dense regular type! See how protein fibers are parallel in orientation?
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