Connective tissue is one of the four major tissue types that make up the human body. It is structurally very diverse and includes bone, blood, cartilage, loose connective tissues, and dense connective tissues. Connective tissue plays many vital roles in the body by holding organs together, transporting substances, storing energy, and supporting soft structures.
Connective tissue is the most diverse of the four major tissue types found in the human body. All connective tissues share one common feature: they contain cells and an extracellular matrix surrounding the cells. The structure and components of the extracellular matrix determine the characteristics of each specific type of connective tissue.
The extracellular matrix can be further divided into two major components: protein fibers and ground substance. Protein fibers are long strands of collagen, elastic, or reticular proteins produced by fibroblast cells.
Collagen fibers are thick fibers made of collagen, the strongest protein in nature.
Elastic fibers contain several proteins, including elastin, that can stretch or flex and return to their original shape.
Reticular fibers contain thinner collagen fibers that form a mesh-like network.
Ground substance is the medium that fills the space between the protein fibers and cells. It helps to transport substances and determines the consistency of the tissue.
Liquid ground substance, or extracellular fluid, is the most usual form of ground substance. It contains mostly water and proteins to maintain the osmotic balance of the tissue. Many solutes, including oxygen, carbon dioxide, and glucose, are dissolved in extracellular fluid.
Gel ground substance is found in cartilage tissue. It is a flexible gelatinous material made of chondroitin sulfate polymers and collagen protein.
Solid ground substance consists of mineral salts, primarily calcium and phosphate. These salts form several types of solid crystals, the most common of which is hydroxyapatite. Solid ground substance is hard and strong but less flexible than liquid or gel matrix.
Connective tissue cells come in many diverse types. One group of cells plays an important role in producing and maintaining the extracellular matrix of various tissue types. The most common cells of this group, the fibroblasts, produce the collagen, elastin, and reticular protein fibers found in the matrix of many tissues.
Osteocytes and osteoblasts are found in osseous tissue and maintain and produce a solid bone matrix, while osteoclasts digest the bone matrix to release minerals. Chondroblasts produce gelatinous cartilage matrix while chondrocytes maintain and repair cartilage.
Another group of connective tissue cells is found in the blood and lymphatic tissues. These cells include white blood cells – lymphocytes, macrophages, and granular leukocytes – that fight pathogens, produce antibodies, clean up dead or damaged cells, and produce inflammation during allergic reactions and infections. Erythrocytes, or red blood cells, transport oxygen and carbon dioxide around the body. Finally, platelets perform the vital role of hemostasis, or blood clotting, to prevent blood loss.
Connective tissue comes in many types but can be organized into six major groups: embryonic, loose, dense, cartilage, osseous, and blood.
Embryonic connective tissue is found in developing embryos and consists of two tissue types: mesenchymal tissue and mucous connective tissue. Mesenchymal tissue differentiates to form many of the tissues present in the adult body, including bones, cartilage, fat, and muscles. Mucous connective tissue is found in the umbilical cord and provides a strong yet flexible material to support blood vessels.
Loose connective tissue has loosely connected protein fibers with significant amounts of liquid ground substance. Areolar connective tissue contains collagen, elastic, and reticular protein fibers and is found in the subcutaneous layer and membranes. It loosely anchors and reinforces surrounding tissues and supplies blood to neighboring tissues. Reticular tissue contains many reticular fibers that form a net-like mesh. It forms the filter portion of the spleen and lymph nodes and acts as a soft skeleton for soft organs. Adipose tissue, or fatty tissue, contains very few fibers and many adipocytes that store energy in the form of triglycerides. Adipose is found in the subcutaneous layer and around organs, where it insulates the body from cold and protects the body from mechanical stresses.
Dense connective tissue has many tightly packed protein fibers with fibroblasts to maintain the fibers. Dense regular connective tissue features collagen fibers arranged in a regular pattern of rows, which makes it ideal for withstanding stress from one direction. Ligaments and tendons are made almost exclusively of dense regular connective tissue. Dense irregular connective tissue also contains many collagen fibers, but these are irregularly arranged and run in all directions. It is ideal for withstanding stress from all directions and is found in the dermis, periosteum, joint capsules, and serous membranes. Dense elastic connective tissue is like dense irregular connective tissue but contains more elastin protein. It can withstand stress from all directions and can also stretch in all directions. Dense elastic tissue is found in the lungs, walls of elastic arteries, and respiratory airways.
Cartilage is unique among tissues because it has a tough and flexible gel matrix. Hyaline cartilage contains gel matrix and chondrocytes and forms the articular cartilage, costal cartilage, and support for the tip of the nose. Fibrocartilage has collagen protein fibers reinforcing its gel matrix and is found where greater stress is placed on the body. It is specifically found in the intervertebral disks, the pubic symphysis, and the menisci of the knees. Elastic cartilage has elastin proteins distributed in its gel matrix and is found where flexible support and greater elasticity are needed. It is found in the auricles of the ear and the epiglottis.
Osseous tissue, or bone, makes up most of the skeleton. It is a unique tissue of solid mineral matrix with collagen fibers for support and osteocyte cells trapped within. Each osteocyte remains in tiny holes, or lacunae, connected to one another by canaliculi, or tiny canals. Osseous tissue is the hardest and strongest of the body’s tissues.
Blood tissue, including lymph, is the only connective tissue with a completely liquid matrix. The liquid part of blood, known as plasma in the blood vessels and lymph in the lymphatic vessels, is a mixture of water, proteins, gasses, nutrients, hormones, and other solutes that transports materials around the body. Blood cells, including erythrocytes, leukocytes, and thrombocytes, transport gasses, fight off infections and prevent blood loss through clotting.
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