Connective tissue

• 1. Introduction

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  Connective tissue is the biological material that holds the rest of the body together. It sits between other tissues, linking them, cushioning them, and carrying vital signals from one part of the body to another. Yet despite being one of the four primary types of animal tissue, it rarely gets the attention given to muscle or nerve.

  The term was only formally introduced in 1830, by the German anatomist Johannes Peter Müller, who called it Bindegewebe. Müller was naming something that had already been recognized as distinct for more than a century, but he gave it a label that stuck. That single act of naming opened a door to a field far stranger and more varied than anyone expected.

  What exactly is connective tissue? The short answer is that it comes in dozens of forms. Some of those forms are soft and gel-like. Some are rigid enough to bear the weight of a human body. One of them flows through veins. Understanding how so many different structures share a common origin is the central puzzle this documentary will work through.

• 2. Three Components

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  Most connective tissues are built from three ingredients working in combination. The first two are fibers: collagen fibers, which bind structures together, and elastic fibers, which allow tissues to stretch and then spring back. The third is ground substance, a clear, colorless, viscous fluid that fills the spaces between fibers and cells.

  Ground substance contains glycosaminoglycans and proteoglycans. Together with the proteins in the fiber network, they create the matrix, the scaffolding in which everything else is embedded. Cells of many kinds live within this matrix: fibroblasts, adipocytes, macrophages, mast cells, and leukocytes, each with its own role.

  Collagen fibers deserve particular attention because of their sheer scale in the body. Type I collagen alone makes up roughly twenty-five percent of the total protein content in a mammalian body. That single statistic reveals why so many disease processes, from scurvy to inherited syndromes, center on what happens when collagen is disrupted.

• 3. Fiber by Fiber

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  The three fiber types found in the extracellular matrix each handle a different mechanical job. Collagen fibers bind bones and tissues to each other. They appear in tendons, ligaments, skin, the cornea, cartilage, bone, blood vessels, the gut, and intervertebral discs.

  Elastic fibers, built from elastic microfibril, elastin, and fibrillin, let organs recoil and resist stretch forces. They appear in the walls of large blood vessels and in certain ligaments, including the ligamenta flava, the yellow ligaments that connect adjacent vertebrae in the spine.

  Reticular fibers are made of Type III collagen and serve as a fine scaffolding for other cells. They form the stroma, the supportive framework, for organs such as the liver, bone marrow, and lymphatic organs. Without reticular fibers, the soft tissue of those organs would have nothing to hold its shape against the pressures of daily life.

• 4. Loose, Dense, and in Between

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  Dense regular connective tissue, found in tendons and ligaments, arranges its collagen fibers in parallel lines. That arrangement gives it powerful tensile strength in one direction, exactly what a tendon needs when a muscle pulls hard against bone.

  Dense irregular connective tissue takes the same dense bundles but scatters them in all directions, producing strength that works against forces coming from any angle. The skin relies heavily on this arrangement.

  Loose connective tissue holds much more ground substance and proportionally fewer fibers. That ratio makes it flexible and porous in a useful way. Loose and dense irregular connective tissue together form a medium through which oxygen and nutrients can diffuse outward from capillaries to reach cells, while carbon dioxide and waste substances travel the reverse route back into circulation. Adipose tissue, which is classified as a type of loose connective tissue, provides mechanical cushioning. Even without a dense collagen network, groups of fat cells are bound by collagen fibers and collagen sheets, which keep compressed fat tissue in place in locations such as the sole of the foot.

• 5. Special Forms

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  Special connective tissue includes bone, cartilage, blood, and lymph. Blood and lymph are classified as fluid connective tissues. They contain no fiber at all, which sets them apart from nearly every other connective tissue type, yet they share the same broad category because of how they develop and what they do.

  Blood and lymph are also described as liquid fascia, a classification that places them within the wider family of fascial tissues that envelop and support structures throughout the body. The meninges, the three membranes that envelop the brain and spinal cord, are made of connective tissue as well. So are the synovial membranes that line the cavities inside joints.

  Wound healing produces yet another form. When the body repairs a cut or an injury, it generates granulation tissue, a new vascularised connective tissue that fills the wound site. Granulation tissue is a temporary structure, a working scaffold the body raises while permanent repair proceeds beneath it.

• 6. Origins in the Embryo

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  Connective tissue develops mostly from the mesenchyme, which itself derives from the mesoderm, the middle of the three embryonic germ layers. Mesenchyme is remarkable because it retains the capacity to differentiate into all mature connective tissue types. It is, in that sense, a material of pure potential.

  A second embryonic connective tissue is Wharton's jelly, the mucous connective tissue found inside the umbilical cord. Wharton's jelly is relatively undifferentiated and disappears after birth. What remains are scattered mesenchymal cells distributed throughout the body, traces of the embryonic origin that produced so many different structures.

  Immune system cells, including macrophages, mast cells, plasma cells, and eosinophils, are found scattered in loose connective tissue in the adult body. Their presence there is not incidental. Loose connective tissue provides the environment in which inflammatory and immune responses begin when antigens are detected.

• 7. When It Fails

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  Marfan syndrome and Ehlers-Danlos Syndrome are both congenital diseases of connective tissue, present from birth. They arise from inherited defects that alter the structure or quantity of collagen and related proteins.

  Scurvy reaches the same endpoint through a different route. Vitamin C is essential for the synthesis of collagen, and a deficiency of it causes connective tissue to break down. The disease was observed and recorded long before anyone understood collagen, but its effects, bleeding gums, joint pain, and failing skin, are the visible signs of connective tissue losing its integrity.

  Autoimmune diseases also target this system. Systemic lupus erythematosus is a major autoimmune disease of connective tissue. Mixed connective tissue disease, and its close relative undifferentiated connective tissue disease, represent cases where the immune system turns against the very matrix that holds the body together. Fibromuscular dysplasia, a disease of the blood vessels, causes abnormal growth in arterial walls, a reminder that even fluid connective tissues and the structures that carry them are not immune to disruption. Connective tissue neoplasms, including sarcomas such as hemangiopericytoma and malignant peripheral nerve sheath tumor, round out a clinical picture that spans nearly every organ system in the body.