MAIN FEATURES AND COMPONENTS OF THE CONNECTIVE TISSUE – 2
Cells of connective tissue
Connective tissue cells are usually classified into two populations: resident cells and transient cells.
Resident cells are permanent, characteristic components of connective tissue. The most common resident cells in the so called connective tissue proper are fibroblasts, mast cells, and mesenchymal cells.
Other types and subtypes of connective tissue contain their own characteristic resident cells. For example, chondrocytes are the typical resident cells of cartilage, whereas osteoblasts and osteocytes are characteristic cells of bone.
Resident cells are generally long-lived. When connective tissue loses cells or when increased cellular activity is required, such as during inflammation or tissue repair, their numbers can increase. New cells may develop from resident mesenchymal stem or progenitor cells, or from progenitor cells derived from the hematopoietic bone marrow.
Transient cells, also called wandering cells, are migratory cells that are found in many types of connective tissue, although they are not normally present in all types, such as cartilage and bone.
These cells enter connective tissue from the blood or lymph. They perform important functions while they are there, and some eventually die or leave the tissue. For example, some lymphocytes can return from connective tissue to the blood or lymphatic system.
Most of the activities performed by transient cells in connective tissue are related to defense and immune responses. They help protect the body against foreign substances and microorganisms.
Extracellular matrix (ECM)
The extracellular matrix (ECM) is a major component of connective tissue. It surrounds the cells and provides the tissue with much of its structural organization and many of its functional properties.
The ECM consists mainly of macromolecules produced by connective tissue cells. It also contains smaller molecules that come from the blood or lymph, including water, ions, peptides, amino acids, carbohydrates, and other small molecules.
The macromolecules produced by connective tissue cells are organized into different types of fibrils and fibers, which vary in thickness, length, and organization.
Fundamental extracellular matrix
In addition to its fibrillar components, the ECM contains many free macromolecules, Together with water and other molecules, these components form a hydrated, gel-like material that fills the spaces around the cells and between the fibrils and fibers.
Many free macromolecules of the fundamental extracellular matrix, as the glycosaminoglycans and proteoglycans, constitute the semi-fluid that exist around the cells and fibrillar component of the connective tissue ECM.
Some components are not well preserved during the routine preparation of histological sections and do not stain effectively with routine histological stains. In consequence areas of the connective tissue ECM may appear under the microscope as apparently “empty spaces.” These spaces do not necessarily represent actual empty areas in the living tissue. Many of these components can be demonstrated using special stains, histochemical techniques, or immunohistochemical methods.