HB 4 (Connective Tissue)

• Connective tissue is a basic type of tissue of mesodermal origin that provides structural & metabolic support for other tissues & organs, mediates the exchange of nutrients, metabolites & waste products between tissues & the circulatory system, and generally contain blood and lymphatic vessels.
• The functions of connective tissue include connecting & binding tissues, providing structural support, defense, being a medium through which nutrients & metabolic wastes are exchanged, and producing heat.
• Resident cell population normally exhibit little movement & are permanent residents of the tissue.
• Wandering cells migrated into the tissue from the blood in response to specific stimuli.
• Resident cell population in connective tissue includes fibroblasts, macrophages, adipose cells, mast cells, and undifferentiated mesenchymal cells.
• Brown adipose tissue consists of multilocular cells, is polygonal and smaller than cells of unilocular adipose tissue, and its cytoplasm is filled with numerous lipid droplets and numerous mitochondria.
• Brown adipose tissue is greatly reduced in adulthood.
• White adipose tissue has an insulating function in the skin and is richly vascularized.
• Except for the eyelids, penis, scrotum, and auricle, white adipose tissue is found in all parts of the body.
• Brown adipose tissue is the predominant type of adipose tissue in adults and its distribution and density are determined by age and sex.
• Brown adipose tissue is found in the human embryo and newborn, is important mainly in the first months of postnatal life, and produces heat and protects the newborn against cold.
• The nucleus of brown adipose tissue is eccentric but not flattened.
• Fibroblasts are the principal cell of connective tissue and are responsible for the synthesis of fibers & extracellular matrix components.
• Fibroblasts occur in either active or quiescent state.
• In H&E preparations, only the nucleus tends to be visible in fibroblasts.
• Fibroblasts are the main cell type involved in cell repair.
• Quiescent (inactive/dormant) fibroblast, also known as fibrocyte, is smaller than active fibroblast, spindle-shaped, has fewer processes, smaller, darker, elongated nucleus, and a small amount of rER.
• Active fibroblast has abundant and irregularly branched cytoplasm, is close-associated with collagen bundles, has a large and well-defined nucleolus, and is rich in rER and well-developed Golgi complex.
• Loose connective tissue includes areolar tissue, cellular connective tissue, fibers are thin and relatively sparse, ground substance is abundant, beneath the epithelia that cover the body surfaces and line the internal surfaces of the body.
• Elastic connective tissue consists of elastic fibers arranged in parallel arrays, the fibers are short and curled at their ends, cells composed of parallel rows of flattened fibroblasts which are not stained, found in ligamentum nuchae and ligamenta flava.
• Connective tissue proper, loose connective tissue, dense connective tissue, specialized connective tissue, supporting connective tissue, embryonic connective tissue.
• Adipose tissue is a specialized form of connective tissue consisting of fat-storing cells (adipocytes), two types of adipose tissue: white (unilocular) adipose tissue and brown (multilocular) adipose tissue.
• Ground substance is a viscous, clear substance that has a slippery feel, high water content, fills the space between cells and fibers, acts as a lubricant and a barrier to the penetration of invaders, formed mainly of three classes of components: glycosaminoglycans, proteoglycans, multi adhesive glycoproteins.
• Elastic fibers are thinner than collagen fibers, arranged in a branching pattern to form a 3-dimensional network, interwoven with collagen fibers to limit the distensibility of the tissue and prevent tearing from excessive stretching, stain with eosin but not well, cannot be distinguished from collagen fibers, ligamentum flava of vertebral column, vocal folds, elastic arteries.
• Dense regular connective tissue is characterized by collagen bundles arranged according to a definite pattern, examples include tendons, ligaments, aponeuroses.
• Dense irregular connective tissue is characterized by an abundance of fibers and few cells, relatively little ground substance, fibers are arranged in bundles oriented in various directions, present in the submucosa of hollow organs and reticular layer of the dermis.
• Myofibroblasts are cells that features of both fibroblast and smooth muscle, are numerous in wound healing, and have morphologic characteristics of fibroblast but contain increased amounts of actin and myosin and behave like smooth muscle cells.
• Macrophages are phagocytic cells derived from monocytes and constitute the mononuclear phagocytic system.
• Monocytes are identified by their phagocytic activity, demonstrated by ingested India ink.
• Cells of mononuclear phagocytic system include macrophages, Kupffer cells, microglia cells, Langerhans cells, dendritic cells, and osteoclasts.
• Adipose cells are narrow rim of cytoplasm, flattened and eccentric nucleus, lipid droplet (primarily as triglycerides), and appear as empty cells (lipid dissolve during tissue preparation).
• Mast cells are storage of chemical mediators of inflammatory response and substance release histamine.
• Mast cells are large, ovoid cells with spherical nucleus, cytoplasm filled with large basophilic secretory granules, and are not easily identified.
• Reticular fibers are mainly type III collagen, arranged in mesh-like pattern or network, abundant in liver, lymph node, spleen & hematopoietic organs, and support capillaries, nerves & muscle cells.
• Collagen fibers are flexible and have high tensile strength, and collagen is the most abundant protein in the human body, forming the most abundant fibers of connective tissue.
• Adipose cells are specialized for storage of neutral fats or for production of heat and are found throughout loose connective tissue as individual cells and groups of cells.
• Collagen fibers and fiber bundles in the ECM are produced by fibroblasts, appearing as bundles of variable thickness and orientation.
• Lymphocytes are spherical cells, dense staining nuclei, type of white blood cell / Natural killer cell, and are found in loose connective tissue associated with epithelial surfaces.
• Wandering cells include lymphocytes, plasma cells, neutrophils, eosinophils, basophils, monocytes, and are migrated into the tissue from the blood in response to specific stimuli.
• Plasma cells are found in loose connective tissue associated with epithelial surfaces, are large ovoid cells with eccentric nucleus, and their cytoplasm is deeply basophilic.