Dense regular connective tissue, found in structures such as tendons and ligaments , is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions. Special connective tissue consists of reticular connective tissue , adipose tissue , cartilage , bone , and blood. Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved.
Loose and dense irregular connective tissue , formed mainly by fibroblasts and collagen fibers , have an important role in providing a medium for oxygen and nutrients to diffuse from capillaries to cells, and carbon dioxide and waste substances to diffuse from cells back into circulation. They also allow organs to resist stretching and tearing forces.
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Dense regular connective tissue , which forms organized structures, is a major functional component of tendons , ligaments and aponeuroses , and is also found in highly specialized organs such as the cornea. In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide the stroma —or structural support—for the parenchyma —or functional part—of the organ. Mesenchyme is a type of connective tissue found in developing organs of embryos that is capable of differentiation into all types of mature connective tissue.
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Various types of specialized tissues and cells are classified under the spectrum of connective tissue, and are as diverse as brown and white adipose tissue , blood , cartilage and bone. Connective tissue Connective tissue CT is one of the four basic types of animal tissue , along with epithelial tissue , muscle tissue , and nervous tissue.
Connective tissue Section of epididymis. Connective tissue blue is seen supporting the epithelium purple. Main article: Connective tissue disease. Retrieved 26 October Dorland, W. Newman In summary, areolar tissue is tough, yet flexible, and comprises membranes. Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin.
Regular fibrous connective tissue, shown in Figure , is found in tendons which connect muscles to bones and ligaments which connect bones to bones. Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes , make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae. A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure.
The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma.
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material.
Connective tissue: macromolecular structure and evolution | UNIVERSITY OF NAIROBI LIBRARY
The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes.
Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue. Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft or diaphysis of a long bone and the surface of the flat bones, while spongy bone is found in the end or epiphysis of a long bone. Compact bone is organized into subunits called osteons , as illustrated in Figure. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae.
The wavy lines seen between the lacunae are microchannels called canaliculi ; they connect the lacunae to aid diffusion between the cells.
Spongy bone is made of tiny plates called trabeculae ; these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons. Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism.
Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs.
Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure. The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells. Blood is considered a connective tissue because it has a matrix, as shown in Figure.
The living cell types are red blood cells RBC , also called erythrocytes, and white blood cells WBC , also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma. The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7. Sheep erythrocytes are even smaller at 4.
Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues. Leukocytes are the predominant white blood cells found in the peripheral blood. Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies.
Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body.