BIO-Connective Tissue

Classification of Connective Tissues 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. 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.

Embryonic Connective Tissue All connective tissues derive from the mesodermal layer of the embryo. 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. A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton's jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.

Dense Connective Tissue (a) Dense regular connective tissue consists of collagenous fibers packed into parallel bundles. (b)

Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like network. From top, LM × 1000, LM × 200.

A mesenchymal cell is next to one of the adipocytes. The cell is rectangular and has four projections stemming from each corner of the cell. The projections appear to attach to the nearby collagen fibers. A fibroblast is located at the center of the diagram. The fibroblast appears similar to the mesenchymal cell, except that it is larger and has more projections.

Finally, a white macrophage is in the lower right of the diagram. The macrophage is a white, oval shaped disc with a prominent nucleus.

The right diagram is a micrograph of connective tissue. The tissue is mostly stained pink, however, the thick collagen fibers crisscrossing the tissue are white.

Five adipocytes also appear white, except for their cell membrane and nucleus, which stained dark. A mesenchymal cell occupies the space between two adipocytes. It stains a very deep purple, but its shape is unclear in the micrograph. A fibrocyte is also visible as an oval shaped cell with a deep purple nucleus.

Image A shows a collection of yellow adipocytes that do not have a consistent shape or size, however, most have the general appearance of a kernel of corn with a wide end that tapers to a point. Each adipocyte has a nucleus occupying a small area on one side of the cell. Nothing else is visible within the cells.

Image B shows a micrograph of adipose tissue. Here, the adipocytes are stained purple. However, only their edges and their nuclei stain, giving the adipose tissue a honeycomb appearance. The adipocytes in the micrograph are large and round, but still show a diversity of shapes and sizes. The nucleus appears as a dark staining area very close to the cell membrane.

Part A shows a diagram of regular dense connective tissue alongside a micrograph. The tissue is composed of parallel, thread-like collagen fibers running vertically through the diagram. Between the vertical fibers, several dark, oval shaped fibroblast nuclei are visible. In the micrograph, the whitish collagen strands run horizontally. Several dark purple fibroblast nuclei are embedded in the lightly stained matrix. Part B shows a diagram of irregular dense connective tissue on the left and a micrograph on the right.

In the diagram, the collagen fibers are arranged in bundles that curve and loop throughout the tissue. The fibers within a bundle run parallel to each other, but separate bundles crisscross throughout the tissue. Because of this, the irregular dense connective tissue appears less organized than the regular dense connective tissue. This is also evident in the micrograph, where the white collagen bundles radiate throughout the micrograph in all directions. The fibroblasts are visible as red stained cells with dark purple nuclei.

Adipose Tissue This is a loose connective tissue that consists of fat cells with little extracellular matrix.

It stores fat for energy and provides insulation. LM × 800.

Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations.

Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic.

Fibroblasts are present in all connective tissue proper. 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.

Functions of Connective Tissues Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body.

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.

Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver.

Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means "little net."

Connective tissue proper

Supportive connective tissue Fluid connective tissue

As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix.

The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibers.

The left image shows a diagram of connective tissue. As a whole, the connective tissue appears somewhat disorganized, with fibers and cells mixed together heterogeneously. There are many open spaces between the embedded elements, suggesting that the connective tissue is somewhat loosely packed.

The thickest fibers are collagen fibers; the thinner fibers are elastic fibers. Both the collagen fibers and the elastic fibers crisscross randomly throughout the tissue. In addition, a net of reticular fibers appear in the upper part of the diagram. Two yellow and oval shaped adipocytes are embedded below the reticular fiber net, with a small dark nucleus squeezed into one corner of the cell.

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.

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.

Adipose tissue consists mostly of fat storage cells, with little extracellular matrix. A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food.

White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term "baby fat."

In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by

alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching.

This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large

amounts of amorphous ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells; for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris.

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.

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.

Connective Tissue Proper Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells

fibrocytes, adipocytes, and mesenchymal cells. LM × 400.

Connective Tissue Fibers and Ground Substance 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.

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).

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.

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. The macrophage cells are an essential component of the immune system, which is the body's defense against potential pathogens and degraded host cells. 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.

This figure shows reticular tissue alongside a micrograph. The diagram shows a series of small, oval cells embedded in a yellowish matrix. Thin reticular fibers spread and crisscross throughout the matrix. In the micrograph, the reticular fibers are thin, dark, and seem to travel between the

many deeply stained cells.

In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at

metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.

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.

Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph

vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.

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.

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.

Cell Types 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.