MAIN FEATURES AND COMPONENTS OF THE CONNECTIVE TISSUE – 2
Cells of the connective tissue
Connective tissue cells can be classified into two populations: resident cells and transient cells, also called wandering cells.
The resident cells are the constant, customary components of connective tissue, while the transient cells are migrating through the connective tissue, arriving mainly from the blood by the way of blood and lymphatic vessels that are always present in conective tissues.
The most common resident cells that occupy the type of typical connective tissue called connective tissue properare thefibroblasts, macrophages, mast cells and mesenchymal cells.
In other types and subtypes of connective tissue we have other resident cells, such as chondrocytes in cartilage, osteoblasts and osteocytes in bone and so on.
Most resident cells are relatively long-lived, but when necessary – for example, when they need to increase their activity during inflammatory or healing processes, or when they need to stimulate growth – their numbers can increase at the expense of mesenchymal stem cells (in the case of fibroblasts) or from progenitor cells from the hemopoietic marrow (in the case of mast cells and macrophages).
The transient type of cells of connective tissues are migratory cells. Leukocytes or white blood cells are part of this group. The function of these cells in defending the body against foreign molecules and microorganisms is carried out almost entirely within the connective tissue. The presence of these cells in the different connective tissue sites of the body is variable and often depends on chemical signals transmitted by cells (e.g. macrophages, neutrophils), which stimulate the leukocytes to leave the blood vessels and accumulate in the connective tissue.
Extracellular matrix (ECM)
The ECM is composed mainly of macromolecules, in addition to water, ions, peptides, small carbohydrates and other small molecules. Some of these molecules originate from blood plasma.
The macromolecules of the extracellular matrix of connective tissue are characteristic ans special glycoproteins, glycosaminoglycans and proteoglycans, whose molecular composition, quantity and relative proportion vary in different types of connective tissue and in different locations and organs of the organism.
An important group of proteins of the ECM is organized into long structures of varying thickness and length, forming filaments. The thicker filaments are visible under the light microscope and are called connective tissue fibers and constitute the fibrillar extracellular matrix.
Many macromolecules – mainly glycosaminoglycans and proteoglycans – do not participate in the filaments and fibers and constitute the fundamental extracellular matrix. These components of the matrix are not always well preserved in histological preparations and do not always stain very well with routine stains. Therefore, in sections they causee an apparently “empty” appearance of many parts of the exytacellular matrix of the connective tissue. Many of these components can be visualized by other stains and by histochemical or immunohistochemical techniques.