L3.pdf

Human Histology Lecture 3: Connective tissue. The main function of connective tissue is to link and connect other tissues. The connective tissue is mainly composed of support cells and extracellular matrix (ECM). Connective tissue...

Human Histology Lecture 3: Connective tissue. The main function of connective tissue is to link and connect other tissues. The connective tissue is mainly composed of support cells and extracellular matrix (ECM). Connective tissue Support cells ECM Fibroblast Glycosaminoglycans Chondrocytes Fibrillar proteins (collagen and elastin) Osteoblast Myofibroblast Adipocyte Diagram 1: General composition of connective tissue. 1 The support cells are important to mechanical stability of the tissue and they produce a variety of extracellular matrix materials. These support cells are of five types: Fibroblasts: secrete the extracellular matrix components in most tissues, usually collagen and elastin. Chondrocytes: secrete the extracellular matrix components of cartilage. Osteoblasts: secrete the extracellular matrix components of bone. Myofibroblasts: secrete extracellular matrix components and also have a contractile function. Adipocytes: are specially adapted lipid-storing support cells which not only act as an energy store, but also have a filling function. Important note: The addition of “blast” to the root name of a support cell indicates that the cell is actively growing while the suffix “cyte” means that the support cell is in a quiescent phase. On the other hand, the extracellular matrix (ECM) is a gel-like structure and mainly composed of fibrillar proteins surrounded by glycosaminoglycans. The glycosaminoglycans are large polysaccharides with negative charge that help to retain water and sodium ions. Example of glycosaminoglycans are hyaluronic acid and chondroitin sulfate. The fibrillar proteins of ECM are of four types: collagen, elastin, fibrillin and fibronectin. These fibrillar proteins determine the tensile 2 properties of connective tissue. For example, collagen fibers resist tensile stressor while elastin provide more tissue elasticity. Figure 1: Connective tissue. Fibro-collagenous tissue is described as: Loose when collagen fibers are thin and widely spaced. Dense when collagen fibers are broad and confluent. The degree of organization and collagen orientation varies from site to site according to local tissue stresses. Highly organized dense fibro- collagenous tissue forms tendons and ligaments. Adipocytes can store fat as an energy source and these cells exist as two types: 3  Unilocular adipose tissue (white fat) which is important in fat biosynthesis and transport. Multilocular adipose tissue (brown fat): this type of adipose tissue is found only in newborn and it is important for heat generation. Adipocytes in brown fat tissue contain high number of mitochondria and this correlates with the heat generation by using fat storage. Figure 2: Unilocular versus multilocular adipose tissues. 4