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Structure and Function

Connective tissue contributes to numerous body functions, including supporting organs and cells, transporting nutrients and wastes, defending against pathogens, storing fat, and repairing damaged tissues (see Image. Transverse Section of the Ureter). Connective tissue is composed primarily of an extracellular matrix and a limited number of cells. Most connective tissues are composed of ground substances, fibers, and cells, although blood and lymph are specialized fluid connective tissues without fiber.

Connective tissue is largely organized into subcategories based on its nonliving components. While numerous subcategorizations are published in the literature, connective tissue can be organized into two main types: connective tissue proper and specialized connective tissue.

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Connective Tissue Proper

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Connective tissue proper includes both loose (or areolar) connective tissue and dense connective tissue. Loose connective tissue previously included areolar, reticular, and adipose tissues, although this system has been revised only to include areolar tissue. Loose connective tissue generally holds organs, anatomic structures, and tissues in place. The extracellular matrix is the most significant feature of loose connective tissue with large spaces between fibers. Dense connective tissue proper is composed of a higher density of fibers, which may be regular (with parallel fibers such as that of tendons and ligaments) or irregular (with multidirectional fibers such as that of the pericardium), or elastic (with significant embedded elastin such as that of arteries).

The extracellular matrix of both loose and dense connective tissue proper is composed of amorphous ground substance, extracellular fluid, and collagen and elastic fibers. The amorphous ground substance is a gelatinous material surrounding the fibers and permitting extracellular fluid to diffuse through. The collagen and elastic fibers of connective tissue proper are histologically distinguishable as three fiber types: collagen, reticular, and elastic fibers.

Collagen fibers form approximately 20-25% of the protein content of humans. They are nonelastic and have variable bundle thicknesses. They are made up of closely packed thin collagen fibrils that run a wavy course in tissues. These parallel fibrils form bundles with flexible proteoglycans to offer an essential mechanical property. They offer flexible but powerful resistance to pulling force. Specifically, collagen runs in a parallel course in loose connective tissue and then joins to form a larger bundle. They split from each other and join back together at varying locations, creating a three-dimensional meshwork. Dense connective tissue, such as ligaments and tendons, is composed mainly of densely packed collagen fibers.[1]

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Reticular fibers are generally recognized as one type of collagen fiber. They are also called argyrophilic fibers, which have a restricted abundance in the human body. They are primarily present in basement epithelial tissue, adipose cells, Schwann and muscle cells, lymphoid tissue, and endothelium of hepatic sinusoids. Under microscopy, these reticular fibers are fine, dark fibrils that are continuous with the college fibers described above. The arrangement of these fibers forms a network that underlies the basal lamina layer. A firm attachment of these fibers to the basal lamina indicates that these fibers, along with the collagen fibers, create a functional and structural unit that supports tissues. The loose arrangement of these fibers also provides space for molecular movement within the extracellular fluid.[1]

Elastic fibers have the characteristic property of elastic recoil; each fiber can be stretched to approximately 150% of its resting length. These fibers are composed of elastin surrounded by fibrillin. Typically, in loose connective tissue, elastin is a loose network. Their organization and distribution depend on the type of tissue. Concentric elastin fibers are present in the vascular wall to help maintain uniform blood pressure. Fibers are also present in distensible and contractible organs such as the lungs and urinary bladder.[1]

Specialized Connective Tissue

Specialized connective tissue includes a variety of distinct tissues with specialized cells and unique ground substances that result in wide-ranging properties. Specialized connective tissues include adipose, cartilage, bone, blood, and lymphatic tissues. These tissues contribute to diverse functions. Adipose tissue is a loose, specialized connective tissue that functions primarily in energy storage and release, temperature insulation, organ protection, and hormone secretion. Cartilage functions as a flexible but strong connective tissue that protects the bones and joints by reducing friction and working as a shock absorber. Bone is a rigid and strong connective tissue with mineralized extracellular components that contribute to numerous body functions, including support, protection of organs, enabling movement, fat and mineral storage, and hematopoiesis.[2] Blood and lymph are considered fluid connective tissues, with their cells circulating in a fluid extracellular matrix. Blood is considered a specialized connective tissue as it connects all systems of the body and transports oxygen, nutrients, and wastes. Lymph is a specialized connective tissue that connects the body systems and primarily functions to maintain fluid levels, transport substances, and participate in the immune response.

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