Connective Tissue Part 1 PDF

CONNECTIVE TISSUE Part 1 | Embryonic and Adult Connective Tissue Histology course for Veterinary Medicine students DVM, Lect. Arnoldas Pautienius  Connective tissue Connective tissue lies under the epithelia of all tissues and organs. It provides both s...

CONNECTIVE TISSUE Part 1 | Embryonic and Adult Connective Tissue Histology course for Veterinary Medicine students DVM, Lect. Arnoldas Pautienius  Connective tissue Connective tissue lies under the epithelia of all tissues and organs. It provides both structural and metabolic support for the surrounding tissue, as it contains the blood vessels, and can also contain adipocytes. The extracellular matrix also regulates cell proliferation, migration, and differentiation.  Connective tissue serves many functions including: providing support and form to the body and organs, aiding in defense and protection, serving as a medium of exchange of nutrients between tissues, storage of fat and thermoregulation  In general, connective tissue consists of cells and an extracellular matrix (ECM). ECM includes protein fibers (collagen, elastic, and reticular) and an amorphous component containing specialized molecules (proteoglycans, multiadhesive glycoproteins, and glycosaminoglycans) that constitute the ground substance.  Connective Tissue Fibers There are three principal types of connective tissue fibers: collagen, reticular, and elastic fibers. Collagen fibers are the most abundant structural components of the connective tissue. They are flexible, have a high tensile strength, and are formed from collagen fibrils Collagen fiber formation involves events that occur both within the fibroblasts (production of procollagen molecules) and outside the fibroblasts in the ECM (polymerization of collagen molecules into fibril, which are assembled into larger collagen fibers).  Connective Tissue Fibers Elastic fibers are produced by fibroblasts and smooth muscle cells. They allow tissues to respond to stretch and distension. Elastic fibers are composed of a central core of elastin associated with a network of fibrillin microfibrils, which are made of fibrillin and emilin.  Connective Tissue Fibers Reticular fibers are composed of type III collagen and provide a supporting framework for cells in various tissues and organs (abundant in lymphatic tissues). In the lymphatic and hemopoietic tissues, reticular fibers are produced by specialized reticular cells. In most other tissues, reticular fibers are produced by fibroblasts.  Ground Substance In addition to protein fibers, the ECM contains ground substance that is rich in proteoglycans, hydrated glycosaminoglycans (GAGs), and multiadhesive glycoproteins. By means of special link proteins, proteoglycans indirectly bind to hyaluronan, forming giant macromolecules called proteoglycan aggregates. The binding of water and other molecules to proteoglycan aggregates regulates movement and migration of macromolecules, microorganisms, or metastatic cancer cells in the ECM.  Connective Tissue Cells Connective tissue cells are classified as part of the resident cell population (relatively stable, nonmigratory) or the wandering (or transient) cell population (primarily cells that have migrated from blood vessels). Resident cells include fibroblasts (and myofibroblasts), macrophages, adipocytes, mast cells, and adult stem cells. Wandering (transient) cells include lymphocytes, plasma cells, neutrophils, eosinophils, basophils, and monocytes.  Fibroblasts are the principal cells of connective tissue. They are responsible for the synthesis of collagen and other components of the ECM. Macrophages are phagocytic cells that contain an abundant number of lysosomes and play an important role in immune response reactions. Other immune system cells (lymphocytes, plasma cells, neutrophils, eosinophils, basophils and etc.) Adipocytes are specialized connective tissue cells that store neutral fat and produce a variety of hormones Mast cells develop in bone marrow and differentiate in connective tissue. They contain basophilic granules that store mediators of inflammation. Adult stem cells reside in specific locations (called niches) in various tissues and organs. They are difficult to distinguish from other cells of connective tissue.  Connective tissue classification Different types of connective tissue are responsible for a variety of functions. 1. Embryonic Connective Tissue 2. Adult Connective Tissue 2.1. Connective Tissue Proper 2.2. Specialized Connective Tissue  1. Embryonic Connective Tissue Mesenchyme is primarily found in the embryo. It contains small, spindle- shaped cells of relatively uniform appearance. Processes extend from these cells and contact similar processes of neighboring cells, forming a three-dimensional cellular network. The extracellular space is occupied by a viscous ground substance. Mesenchymal stem cells differentiates into adult connective tissue cells  Mucous connective tissue is present in the umbilical cord. It consists of a specialized, almost gelatin-like ECM. Its ground substance is frequently referred to as Wharton's jelly. The spindle-shaped cells are widely separated and appear much like fibroblasts in the near-term umbilical cord. Some of the cells isolated from Whartons jelly express significant amounts of mesenchymal stem cell and have the ability to differentiate under adequate condition into osteocytes, chondrocytes, adipocytes, and neural- like cells. These cells are called Wharton's jelly mesenchymal Stem cells.  2. Adult Connective Tissue 2.1. Connective Tissue Proper Connective tissues that belong to this category are divided into two general subtypes: Loose connective tissue, sometimes called areolar tissue Dense connective tissue, which can be further subcategorized into two basic types based on the organization of its collagen fibers: dense irregular connective tissue and dense regular connective tissue.  Loose connective tissue Loose connective tissue is a cellular connective tissue with thin and relatively sparse collagen fibers. The ground substance is abundant and it occupies more volume than the fibers do. It has a viscous to gel-like consistency and plays an important role in the diffusion of oxygen and nutrients from the small vessels that course through this connective tissue as well as in the diffusion of carbon dioxide and metabolic wastes back to the vessels.  Loose connective tissue Loose connective tissue is primarily located beneath the epithelia that cover the body surfaces and line the internal surfaces of the body. It is also associated with the epithelium of glands and surrounds the smallest blood vessels. This tissue is thus the initial site where pathogenic agents such as bacteria that have breached an epithelial surface are challenged and destroyed by cells of the immune system. Most cell types in loose connective tissue are transient wandering cells that migrate from local blood vessels in response to specific stimuli.  Dense irregular connective tissue Dense irregular connective tissue is characterized by abundant fibers and few cells. Dense irregular connective tissue contains mostly collagen fibers. Cells are sparse and are typically of a single type, the fibroblast. This tissue also contains relatively little ground substance. Because of its high proportion of collagen fibers, dense irregular connective tissue provides significant strength.  Dense irregular connective tissue Typically, the fibers are arranged in bundles oriented in various directions that can withstand stresses on organs or structures. Skin contains a relatively thick layer of dense irregular connective tissue called the reticular layer (or deep layer) of the dermis. The reticular layer provides resistance to tearing as a consequence of stretching forces from different directions. Similarly, hollow organs (e.g., the intestinal tract) possess a distinct layer of dense irregular connective tissue called the submucosa in which the fiber bundles course in varying planes. This arrangement allows the organ to resist excessive stretching and distension.  Dense regular connective tissue Dense regular connective tissue is characterized by ordered and densely packed arrays of fibers and cells. Dense regular connective tissue is the main functional component of tendons, ligaments, and aponeuroses. As in dense irregular connective tissue, the fibers of dense regular connective tissue are the prominent feature, and there is little ECM.  Dense regular connective tissue Tendons are cord-like structures that attach muscle to bone. They consist of parallel bundles of collagen fibers. Situated between these bundles are rows of fibroblasts called tendinocytes. Tendinocytes are surrounded by a specialized ECM that separates them from the loadbearing collagen fibrils.  Dense regular connective tissue The substance of the tendon is surrounded by a thin connective tissue capsule, the epitendineum, in which the collagen fibers are not nearly as orderly. Typically, the tendon is subdivided into fascicles by endotendineum, a connective tissue extension of the epitendineum. It contains the small blood vessels and nerves of the tendon.  Dense regular connective tissue Ligaments, like tendons, consist of fibers and fibroblasts arranged in parallel. The fibers of ligaments, however, are less regularly arranged than those of tendons. Ligaments join bone to bone, which in some locations, such as in the spinal column, requires some elasticity.  Dense regular connective tissue Aponeuroses resemble broad, flattened tendons. Instead of fibers lying in parallel arrays, the fibers of aponeuroses are arranged in multiple layers. The bundles of collagen fibers in one layer tend to be arranged at a 90° angle to those in the neighboring layers. The fibers within each of the layers are arranged in regular arrays; thus, aponeurosis is a dense regular connective tissue.  2. Adult Connective Tissue 2.2. Specialized Connective Tissue Adipose tissue Reticular Connective Tissue Cartilage Bone Blood  Adipose tissue Adipose tissue is a specialized connective tissue that plays an important role in energy homeostasis (stores energy in lipid droplets in the form of triglycerides) and hormone production (adipokines). There are two types ofadipose tissue: white (unilocular) and brown (multilocular).  White Adipose tissue White adipose tissue with supporting collagen and reticular fibers forms the subcutaneous fascia, is concentrated in the mammary fat pads, and surrounds several internal organs. White adipocytes are very large cells (100 micrometers or more in diameter) with a single, large lipid droplet White adipose tissue secretes a variety of adipokines, which include hormones (e.g., leptin), growth factors, and cytokines (immune system molecules).  Brown adipose tissue Brown adipose tissue is abundant in newborns (5% of total body mass) but is markedly reduced in adults. Brown adipocytes are smaller than white adipocytes, contain many lipid droplets. Metabolism of lipids in brown adipose tissue generates heat (thermogenesis) by uncoupling the oxidation of fatty acids in the mitochondria from ATP production. The metabolic activity of brown adipose tissue is regulated by norepinephrine released from sympathetic nerves and is related to ambient outdoor temperature (cold weather increases the amount of brown adipose tissue).  Trans-differentiation of adipose tissue Adipocytes are able to undergo white-to-brown and brown-to-white transformation (trans-differentiation) in response to the thermogenic needs of the body. Cold exposure and physical activity induce white-to-brown trans- differentiation.  Reticular Connective Tissue The stroma of all lymphatic organs (spleen, lymph node, hemal node, tonsils), diffuse lymphatic tissue, solitary lymphatic nodules, and bone marrow is made of reticular connective tissue. This tissue is composed of stellate reticular cells and a complex three dimensional network of reticular fibers.