What makes connective tissue durable and resilient

Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph.

Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body. All connective tissues derive from the mesodermal layer of the embryo see [link]. The first connective tissue to develop in the embryo is mesenchyme , the stem cell line from which all connective tissues are later derived. Clusters of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after a connective tissue injury.

This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body. The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix Table.

Connective tissue proper includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue —bone and cartilage—provide structure and strength to the body and protect soft tissues.

A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue , in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins. Fibroblasts are present in all connective tissue proper Figure 1. Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue.

Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body.

Figure 1: Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells. The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix.

As you might expect, a fibrocyte , a less active form of fibroblast, is the second most common cell type in connective tissue proper. Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active.

Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population. The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue.

The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers.

Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. In contrast, fixed macrophages are permanent residents of their tissues. The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin.

When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you recognize as an allergic response.

Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the immune system. Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers.

Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength.

These fibers hold connective tissues together, even during the movement of the body. Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column. Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network.

They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma the functional cells, blood vessels, and nerves of the organ. All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches.

The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance. Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues. In: Comper WD , ed. Collagen superfamily. Amsterdam, the Netherlands: Harwood Academic Publishers; : 22 — Collagen: the collagen family, structure, assembly, and organization in the extracellular matix.

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Ann Rheum Dis. The biological effect of continuous passive motion on the healing of full-thickness defects in articular cartilage: an experimental investigation in the rabbit. J Bone Joint Surg Am. Effects of movement and weightbearing on glycosaminoglycan content of sheep articular cartilage. Australian Journal of Physiotherapy. Animal models of osteoarthritis. The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage Figure 4.

Hyaline cartilage , the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints.

It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. The knee and jaw joints and the the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans.

This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.

Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily.

Without mineral crystals, bones would flex and provide little support. Osteocytes , bone cells, are located within lacunae.

The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal Figure 4. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time. Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones.

Compact bone is solid and has greater structural strength. Blood is a fluid connective tissues. Blood has two components: cells and fluid matrix Figure 4.

This connective material needs to be strong enough to withstand the constant activity required of many organ systems, yet flexible enough to bounce back from common trauma. A set of protein s, called fibulins, appear to be key components of this connective composite material, particularly because of their ability to bind calcium ions. Calcium ion bonding contributes to strong protein fibers—the reinforcement constituents of these composites.

Bacteria and fungi break down soft plant tissue and leave the tough stuff intact. Spiders turn liquid into a strong, stretchy fiber by squeezing it through a small space that helps protein molecules to connect with each other.

Birds evaluate external threats, their previous success, and the success of others in determining where and how to build their nests. The teeth of great apes help them survive times of food scarcity because they are diverse in type and material characteristics, allowing consumption of fallback foods.