Note - Epithelia have little matrix. Connective tissues have much matrix. Matrix = fluid, ground substance, fibers, and in some sites, mineral deposits.
A. Extracellular fluid - the medium through which nutrients and electrolytes reach the cells and waste products leave. Plasma proteins are present at a concentration considerably less than in the plasma, while electrolytes and small molecules are about the same as plasma.
B. Ground substances - a variety of polysaccharides (glycosaminoglycans) bound to or associated with proteins to form proteoglycans. Much of the extracellular water and electrolytes are bound to these acidic (anionic) polymers. Hyaluronic acid is a long chain polymer to which many proteoglycans are covalently attached through small link proteins forming huge macromolecules that bind large quantities of extracellular water. Laminin and fibronectin are adhesive glycoproteins that facilitate interaction of cells with the connective tissue matrix.
C. Fibers
1. Collagen - provides the tensile strength of connective tissues. Collagen fibers seen with the light microscope (LM) are variable in width, stain lightly with eosin, and have longitudinal striations. These striations are due to aggregates of fibrils. These fibrils can only be resolved with the electron microscope. They are very uniform in diameter in any given site, and when viewed in longitudinal sections, have cross striations with a periodicity of 64 nm. The fibrils form extra-cellularly from tropocollagen, a filamentous protein composed of 3 peptide chains. Variations in the amino acid composition occur so that the collagen of cartilage is different from that of bone.
Type I skin, bone, tendon, gingiva, fibrocartilage
Type II cartilage (hyaline and elastic)
Type III fetal and some mature c.t. and reticular fibers (liver, kidney, lung and spleen)
Type IV basal lamina of epithelia, does not form fibrils but remains as procollagen
Type VII forms anchoring filaments in the basement membrane.
2. Reticular - these fibers are exceptionally small collagen fibers (type III) that, because of their glycoproteins coating, have an affinity for silver stains. This also accounts for their being stained by the PAS technique. They are the primary component of the reticular lamina of the basement membrane and reticular tissue that supports lymphoid tissues (except thymus), bone marrow, liver sinusoids, smooth muscle, adipose tissue and the Islets of langerhans.
3. Elastic fibers - at the LM level they are smooth branched fibers without longitudinal striations. They tend to be refractile and are more eosinophilic than the collagen fibers. At the EM level, they consist of a microfibrillar network (fibrillin) surrounding an amorphous matrix of elastin. The latter is a highly cross-linked (due to desmosine and isodesmosine) polymer that is the elastic component. Elastic fibers are especially stained by orcein, a reddish-brown color. Marfan's syndrome has defective fibrillin. Oxytalan and elaunin are progressive stages in the development of elastic fibers. In some sites development is arrested so that oxytalan fibers are found in the mature periodontal ligament.
II. EXTRACELLULAR MATRIX - Varies with the location and function.
A. Classification of c.t. is based on the type, amount and arrangement of matrix. The following are the described classes of c.t.:
1. Blood and lymph consist of fluid not mixed with fibers. They are found in vascular channels and transport nutrients and free cells.
a. Reticular c.t. is fluid mixed with few fibers - mostly reticular and a few collagen - and is found in marrow and lymphoid tissue. Reticular fibers are argyrophilic ("silver loving") and stain well with silver impregnation methods.
2. Loose irregular (areolar) connective tissue, consisting of fluid mixed with few fibers, compose interfascial planes. Fibers are a mixture of collagen and elastic. Permits motion. Embryonic connective tissue is called mesenchyme. Its fibrocytes have multiple processes and there is much fluid and ground substance. Mucus connective tissue (Wharton's jelly) is the connective tissue of the umbilical cord and placenta. It is more firm than mesenchyme because of its abundance of ground substance.
3. Dense irregular connective tissue, consisting of a fluid containing hyaluronic acid, mixed with many fibers. Examples are dermis and fascia. The fibers are mostly collagen and are irregularly arranged. Provides tensile strength to prevent tearing.
4. Dense regular connective tissue is composed of fluid mixed with many closely packed collagen fibers, regularly arranged (that is, parallel with one another). Tendons and ligaments are good examples. Provides great tensile strength in one direction.
5. Elastic ligaments are fluid mixed with many elastic fibers regularly arranged.
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1. Perichondrium 2. Chondrocytes in lacunae 3. Territorial matrix immediately surrounding lacunae 4. Interteritorial matrix |
6. Hyaline cartilage consists of little fluid but much viscous to gelatinous amorphous material (chondroitnin sulfate) surrounding large amounts of small collagen fibers (Type II), irregularly arranged. It provides some rigidity without loss of flexibility, and resists compression. In laboratory look for small pieces around bronchi and the immature hyalin cartilage in the larynx and trachea of the newborn. Look for evidence of territorial and inter-territorial matrix and isogenous groups of chondrocytes. Young cartilage grows both appositionally and interstitially. In appositional growth chondroblasts in the deep part of the perichondrium proliferate and lay down new matrix. When surrounded by matrix the cell is called a chondrocyte. Interstitial growth occurs when chondrocytes proliferate and make new matrix. As long as the matrix is soft the new cells get separated from one another but when the matrix is rigid the new cells remain in close association. This is then called an isogenous group. Chondronectin links chondrocytes to collagen.
7. Fibrocartilage is composed of little fluid, and much gelatinous amorphous material surrounding large amounts of large collagen fibers (Type I) that are frequently arranged in criss-crossing bundles creating a "herringbone" pattern. Chondrocytes and their lacunae are the smallest and most widely spaced of the cartilages. It has great resistance to compression and is the main component of the annular ring of intervertebral discs.
8. Elastic cartilage has little fluid but much gelatinous amorphous material surrounding large amounts of elastic and some type II collagen fibers, that are irregularly arranged. Lacunae are large and close together. It is especially flexible and supports the ear and epiglottis.
NOTE - Items 6-8 are three different kinds of cartilage. Blood vessels, capillaries and nerve endings are not found in cartilage. Nor are the cells of the immune system found in cartilage. It is nourished from the capillaries found in the perichondrium, a condensation of fibrillar c.t. surrounding the cartilage. Articular cartilage, both hyaline and fibrocartilage, are nourished from the nutrient vessels in the cancelous bone supporting it.
9. Bone contains little fluid, but much crystalline material (hydroxyapatite, a form of calcium phosphate) is found in and surrounding large amounts of regularly arranged collagen fibers. It is only slightly flexible and has great resistance to compression.
10. Adipose tissue is a unique c.t. in that it has little intercellular matrix (a few reticular fibers and capillaries) but many fat cells. It serves to pad the organs and, in the hypodermis insulate from heat loss. It consists mostly of cells with relatively little intercellular matrix that is rich in nutrient capillaries and nerve endings. (Norpinephrine stimulates fatty acid mobilization.) There are two kinds: unilocular = adipose tissue (white or yellow fat); multilocular = brown fat. In the adult most fat cells are of the unilocular variety; a single large fat droplet is surrounded by a thin rim of cytoplasm. These cells produce the hormone leptin that down regulates the appetite and the amount of fat deposited. Cytoplasmic fat droplets are surrounded by a single leaflet of phospholipids and proteins (a half membrane). This "half membrane" facilitates the movements of lipids in and out of storage. Multilocular (brown) fat is widely distributed in the fetus and infant but has limited distribution in the adult. In these cells numerous fat droplets are surrounded by numerous mitochondria that have thermogenin in their inner membrane. This protein uncouples the back flow of protons from ATP production resulting in the liberation of heat.
III. CELLS
1. Fibroblasts - fibrocytes. Produce fibers and ground substance. In response to a wound, a population of fibroblasts differentiate that produce types III and V collagen. Fibrocytes are elongate cells whose shape is maintained by intermediate filaments consisting of vimentin.
a. Collagen fibers are formed extra-cellularly from tropocollagen (The "Collagen" molecule). Its precursor, procollagen, is produced and released from fibrocytes. Matrix proteases remove peptides from both ends of procollagen to form the the tropocollagen that then self assembles to form fibrills. Maturation of these fibrils occurs when lysyl oxidase promotes cross linkage within the fibrils. Vitamin C is required for the synthesis of hydroxyproline that is required for normal tropocollagen assembly. Evidence of vitamin C deficiency is first seen in sites of high collagen turnover especially the periodontal ligament and the gingiva.
b. Elastic fibers
c. Ground substance adds viscosity or even a gelatinous character to extracellular fluid. Hyaluronic acid, chondroitin sulfate, are the main components.
2. Reticular cells (fibrocytes) are stellate epithelioid cells that produce and maintain reticular fibers in lymphoid organs and bone marrow.
3. Chondroblast produces a matrix especially rich in ground substance. It is called a chondrocyte in a lacunae when surrounded by its products. The matrix immediately surrounding the lacunae is called the territorial matrix, more distant is the inter-territorial matrix. The chondrocyte may continue to divide, giving rise to isogenous groups with a minimum of matrix between the cells. Different ratios of ground substance and fibers gives different kinds of cartilage.
4. Osteoblasts produce the organic matrix of bone consisting of regularly arranged collagen fibers surrounded by ground substance, and then direct the orderly deposition of hydroxyapatite crystals. When completely surrounded by bone, it is an osteocyte in a lacunae.
5. Osteoclasts are large multinucleate cells that break down bone by dissolving mineral and digesting the organic matrix. They are derived from blood the monocyte family of cells.
6. Adipose cells - unilocular and multilocular
7. Defense team:
a. macrophages (histiocytes) - nearly as numerous as fibrocytes they remove and digest tissue debris; they are derived from blood monocytes.
b. plasma cells - They have extensive well ordered RER and a highly developed Golgi apparatus. They secrete antibodies constitutively and therefore have few secretion granules.
c. neutrophils - blood leukocytes that migrate out of the blood to engulf and destroy small foreign particles, particularly bacteria.
d. lymphocytes - migrate out of blood to detect and react in very characteristic ways to foreign substances.
e. mast cells - particularly found near blood vessels. They react to injury (several kinds) by releasing the basophilic and metachromatic (anionic polysaccharide) secretion granules that contain histamine and heparin. These along with factors synthesized when the cell is activated initiate inflammation. Mast cells are found in all irregular connective tissues but are more numerous in sites subject to exposure to foreign invasion such as the lamina propria of the oral and respiratory tracts.
f. eosinophils come from blood. These are numerous in the lamina propria of the intestines along with lymphocytes and plasma cells.
8. Undifferentiated stem cells. These cells are the source of many different kinds of cells during repair of injured tissues.
IV. CONNECTIVE TISSUE GENERAL FUNCTIONS. In addition to providing physical support, they provide metabolic and integrative hormonal and neural support.
A. Metabolic - vascular and capillary networks of blood and lymph vessels provide nutritional and hormonal support for the tissues surrounded by c.t.
B. Nerve plexuses - follow connective tissue planes to reach tissues controlled.
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Last Revised: Tue, Nov 14, 2006
