Connective tissue is the most abundant of the four types of tissues present in the human body, the others being epithelial, muscle, and nervous tissue. It develops from the mesoderm, an embryonic type of tissue formed of elongated undifferentiated cells. Connective tissue is mainly responsible for providing and maintaining the general structure of organs.
Mechanically, it binds, anchors and supports both cells, and tissues. Metabolically, it forms a reservoir of factors controlling cell growth and differentiation. This type of tissue provides a suitable medium for the diffusion of nutrients and waste products between cells and their blood supply.
Structure and components
The connective tissue structure is made up of three components: cells, fibers and ground substance. Unlike the other tissue types, the major constituent of connective tissue is the extracellular matrix (ECM) rather than cells. The extracellular matrix is comprised of different combinations of protein fibers (collagen, reticular, and elastic) and ground substance.
The ground substance is a hydrophilic complex of macromolecules (glycosaminoglycans and proteoglycans) and glycoproteins (laminin, fibronectin, etc) that stabilize the extracellular matrix. Connective tissue can be divided into three types: loose, dense, and reticular. This variety reflects the differences in the composition, arrangement, number of cells, fibers and ground substance present in each.
Loose (areolar) Connective Tissue
Loose connective tissue is a common type of connective tissue that is present around structures that are normally under some form of pressure and low friction. This includes epithelial tissue, small blood vessels, lymphatic vessels and spaces between muscle, and nerve fibers. It can also be found in the papillary layer of the dermis, hypodermis, linings of peritoneal and pleural cavities, glands, and the mucous membranes supporting the epithelial cells.
Loose connective tissue has all the connective tissue constituents in roughly equal parts. Its most numerous cells are fibroblasts and macrophages. Collagen, elastic and reticular fibers are present too. Loose connective tissue has a moderate amount of ground substance, therefore having a delicate consistency. It is flexible, well-vascularized, and not very resistant to stress.
Dense Connective Tissue
Dense connective tissue has the same constituents present in the loose connective tissue, but with fewer cells and the predominance of collagen fibers over ground substance. According to the arrangement of these collagen fibers, it is subdivided into regular and irregular tissue. Dense regular connective tissue is arranged according to a definite pattern, with collagen fibers aligned with the linear orientation of the fibroblasts. Dense irregular connective tissue, on the other hand, is without any definite orientation.
Dense connective tissue offers resistance and protection. It is ,however, less flexible, poorly vascularized and has a slow damage repair rate. Examples of dense connective tissue are tendons and ligaments.
Reticular Connective Tissue
Reticular tissue is a specialized form of connective tissue that consists of reticular fibers of type III collagen. This is produced by specialized fibroblasts called reticular cells. The heavily glycosylated reticular fibers create delicate three-dimensional networks that support cells present in the reticular tissue. That provides an architectural framework that offers special microenvironments for hematopoietic organs and lymphoid organs, like bone marrow, lymph nodes, and spleen.
The reticular cells are distributed along the framework, partially covering the reticular fibers and ground substance with cytoplasmic processes. That results in a sponge-like structure, in which cell and fluids are able to move freely. Moreover, mononuclear phagocyte system cells are also dispersed along the framework. They monitor the slow flow of materials through the sinus-like spaces and remove invaders by phagocytosis.