-It makes up the framework of the organs of the body.

-It is found everywhere except where the epithelial tissue, muscle cells, nerve cells, and satellite nerve cells are present.

Structure of Connective Tissue

A. Made up of 3 basic elements;

1. GROUND SUBSTANCE-an adhesive material made from carbohydrates and proteins.

2. FIBERS-connective tissue fibers combined with ground substance forms a MATRIX.

3.CELLS- connective tissue cells are embedded in the matrix.

B. Contains 3 types of fibers;

1. RETICULAR FIBERS-made of the triple-stranded coiled protein collagen. Widely found in body.

-Collagen lends high tensile strength to the tissue.(.2 to 1 micron in diameter)

-Has a coating of glycoprotein.

-Forms a network around fat cells, nerve fibers, muscle cells and blood vessels.

2. COLLAGENOUS -made of the triple-stranded coiled protein collagen.

-1 to 30 microns in diameter. Much thicker than reticular.

-Most commonly found in LIGAMENTS, TENDONS, CARTILAGE, and BONE.

-LIGAMENTS-attach bone to bone.

- TENDONS-attach muscle to bone.

-Muscle to ligament

-Muscle to muscle

-Muscle to other kinds of connective tissue.

-CARTILAGE-tough, flexible tissue. Found where both of its qualities are needed.

-BONE-supports the body.

2. ELASTIC-made of a proteinaceous substance Elastin which has only about .1 the tensile strength of collagen.

-More flexible than collagen

-Elastic fibers degenerate faster than reticular and collagenous fibers.

-This is why the skin's elasticity is lost.

-The loss of elastic fibers in the walls of blood vessels explains why the blood pressure changes as we age.

-Elastic fibers are found where tissue is distorted. They are present in large arteries, the larnyx, the outer ear, the elastic ligaments of the spinal column, and in other structures of the body.

- Fibroblasts-primitive, mesenchymal, stem cells which are present in all connective tissues. They are spindle shaped or star-shaped (stellate), with an oval nucleus. They are not free to wander, but fixed firmly in place.

-Their importance is twofold:

1. They contain the genetic information for the synthesis of ground substance and the 3 types of connective tissue fibers.

2. They have the ability to differentiate into various other kinds of specialized tissue cells, such as white blood cells.

Fibroblast cells are found in every type of connective tissue, though in some tissues they may have different names.

- Fibroblasts are called RETICULAR CELLS when they occur in the following three tissue types:

1. LYMPHOID TISSUES-thymus gland, spleen, tonsils, lymph nodes.

2. MYELOID TISSUE-bone marrow.

3. RETICULOENDOTHELIAL SYSTEM-a collection of MACROPHAGE cells (phagocytic) throughout the body especially as cells in lymphoid tissue, bone marrow, and the liver and lungs.

-Fibroblasts mature into various types of blast cells responsible for producing other connective tissues;

4. CHONDROBLASTS-(cartilage stem cells) they mature into CHONDROCYTES (mature cartilage cells).

5. OSTEOBLASTS-(bone stem cells) mature into OSTEOCYTES (mature bone cells).

Connective tissues vary greatly from one kind of organ to another, often causing confusion and difficulty in identification. Therefore, any connective tissue can be arbitrarily classified as a GENERAL CONNECTIVE TISSUE or a SPECIAL CONNECTIVE TISSUE. There are basically 2 types of general connective tissues and 7 types of special connective tissues.

GENERAL TYPES OF CONNECTIVE TISSUES

1. LOOSE CONNECTIVE TISSUE-the matrix is sparsely woven with fibers, and has a higher ratio of cells to fibers.

-Found in every soft area of the body. In the lower layer of the skin, in all of the glands, and throughout the intestinal, respiratory, excretory, and reproductive tracts.

-These layers of loose connective tissue contain ground substance, fibers, and fibroblasts are present as:

-HISTIOCYTES are MACROPHAGE cells which literally eat materials that are foreign to the body. One of their functions is to eat dead cells, internal blood clots, bacteria. and perhaps newly arising cancer cells. Another function is to eat certain kinds of substances, attach MARKERS to them, and release them (ENDOCYTOSIS) into the tissues as ANTIGENS.

-MAST CELLS are found in loose connective tissue near blood vessels. Mast cells and BASOPHIL CELLS (WBCs) produce and secrete SEROTONIN, which causes smooth muscle in the walls of blood vessels to contract, bringing about a constriction of the vessel (VASOCONSTRICTION). The actions of HEPARIN, HISTAMINE, and SEROTONIN are important in creating the INFLAMMATION RESPONSE.

- MELANOCYTES are present in certain body regions. They are color-carrying cells that are located mainly in the skin and in the "white" of the eye. They are equally abundant in the skin of all races. They provide pigment as a dye that colors the cytoplasm in overlying epithelial cells in response to UV light. The pigment helps to protect the nucleus and cytoplasmic structures from the damaging effect of the UV.

2. DENSE CONNECTIVE TISSUE-This tissue is dense with fibers with a higher ratio of fibers to cells. It is found within or between strands or layers of loose connective tissue, as for example the wall of the uterus, or within the FASCIA (an organ that serves alone or in combination with other structures, such as LIGAMENTS and TENDONS, to hold various parts of the body together. Because of the density of the tissue it is populated mainly by fibroblasts, as there is no room for other cells.

1. ADIPOSE TISSUE composed chiefly of fat cells. Found throughout most of the body especially the lower layer (dermis) of the skin. In small amounts around the kidneys and adrenal glands, in bone marrow, in the armpit (AXILLA) at the groin and around muscle and nerve fibers.

2. BLOOD TISSUE contains neither ground substance nor fibers. With the exception of macrophages (monocytes) and lymphocytes, its cells are different from those of all other connective tissues, though they are all derived from one or more stem cells.

-BLOOD TISSUE contains about 50% fluid (PLASMA) and 50% cells. When plasma is separated from the cells and allowed to clot, it exudes a liquid, called SERUM. Serum contains ANTIBODIES which are used in reactions against antigens. Plasma contains the clotting factors.

-The cells of the blood can divided into 3 classes:

a. RED BLOOD CELLS (ERYTHROCYTES)- are used in respiration. These non-nucleated cells are produced in the marrow and extrude their nucleus before entering the peripheral blood. They contain HEMOGLOBIN and are responsible for transporting oxygen to the cells. They have a lifespan of approx. 120 days.

b. WHITE BLOOD CELLS (LEUCOCYTES)- there are 5 different WBCs found in peripheral blood. They are each derived from a common BLAST CELL and divided into two groups.

AGRANULOCYTES-LYMPHOCYTES, and MONOCYTES.

1. NEUTROPHILS

-Segmented neutrophils (segs) are the mature phagocytes that migrate through tissues to destroy microbes and respond to inflammatory stimuli. Segmented neutrophils comprise 40-75 % of the peripheral leukocytes. They are usually 9 to 16 microns in diameter. The nuclear lobes, normally numbering from 2 to 5, may be spread out so that the connecting filaments are clearly visible, or the lobes may overlap or twist.

-The chromatin pattern is coarse and clumped. The cytoplasm is abundant with a few nonspecific granules and a full complement of rose-violet specific granules. The granules are PRIMARY GRANULES containing PEROXIDASE which initiates killing whenever a bacterium or other microbe is being consumed by the cell. In addition, the cell contains LYSOSOMAL enzymes called LYSOZYME in SECONDARY GRANULES which selectively destroys vital components in the cell wall of bacteria.

2. EOSINOPHILS

-Are the mature granulocytes that respond to parasitic infections and allergic conditions. Eosinophils comprise about 1 to 4% of the peripheral leukocytes. They are usually 9 to 15 microns in diameter. Granules stain a bright reddish-orange with Wright's or Giemsa stains. The nucleus contains one to three lobes. The chromatin pattern is coarse and clumped. The cytoplasm is abundant with a full complement of bright reddish-orange specific granules.

3. BASOPHILS

-Are granulocytes that contain purple-blue granules that contain heparin and vasoactive compounds. They comprise approximately 0.5% of the total leukocyte count. Basophils participate in immediate hypersensitivity reactions, such as allergic reactions to wasp stings, and are also involved in some delayed hypersensitivity reactions. Basophils are the smallest circulating granulocytes, averaging 10 to 15 microns in diameter. The nucleus to cytoplasm ratio is about 1:1, and the nucleus is often unsegmented or bilobed, rarely with three or four lobes. The chromatin pattern is coarse and patchy, staining a deep blue to reddish-purple. The cytoplasm is a homogenous pale blue, but this is often obscured by the large dark granules.

1. LYMPHOCYTES

-Are 25% to 30% of the total WBC and about 80% of the agranulocytes. They are important in the immunological system. There are B LYMPHOCYTES (bone-derived) that differentiate into PLASMA CELLS and produce antibodies against various antigens. As T CELLS (thymus derived) they react directly with other kinds of antigenic material and initiate its destruction. Lymphocytes in the peripheral blood have been described on the basis of size and cytoplasmic granularity.

-Small lymphocytes are the most common, ranging in size from 6 to 10 microns. The nucleus is usually round or slightly oval, occasionally showing a small indentation due to the adjacent centrosome. Except in the smallest cells, the nucleus is about 7 microns in diameter, a size that has been convenient for estimating the size of the surrounding erythrocytes. Nuclear chromatin stains a dark reddish-purple to blue with large dark patches of condensed chromatin. The nuclear cytoplasm ratio is 5:1 to 3:1, and the cytoplasm is often seen only as a peripheral ring around part of the nucleus.

2. MONOCYTES

-Are large mononuclear phagocytes of the peripheral blood. They are the immature stage of the macrophage. Monocytes vary considerably, ranging in size from 10 to 30 microns in diameter. The nucleus to cytoplasm ratio ranges from 2:1 to 1:1. The nucleus is often band shaped (horseshoe), or reniform (lima bean). It may fold over on top of itself, thus showing brainlike convolutions. No nucleoli are visible. The chromatin pattern is fine, and arranged in skeinlike strands. The cytoplasm is abundant and blue gray with many fine azurophilic granules, giving a ground glass appearance. Vacuoles may be present. They are out numbered by the lymphocytes 4 to 1.

3. PLATELETS

-Are known as THROMBOCYTES. Platelets are the cytoplasmic fragments of megakaryocytes, circulating as small discs in the peripheral blood. They are responsible for HEMOSTASIS (the stoppage of bleeding) and maintaining the endothelial lining of the blood vessels. During hemostasis, platelets clump together and adhere to the injured vessel in this area to form a plug and further inhibit bleeding. Platelets average 1 to 4 microns in diameter. The cytoplasm stains light blue to purple, and is very granular. There is no nucleus present. Normal blood concentrations range from 130,000 to 450,000/microliter.

-Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins.

-Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved.

-During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.

-Three types of muscular tissue are recognized: skeletal, smooth, and cardiac.

Skeletal, Striated, or Voluntary Muscle

n This type of muscle is composed of long fibers surrounded by a membranous sheath, the sarcolemma.

-Skeletal muscle is supplied with nerves from the central nervous system, and because it is partly under conscious control, it is also called voluntary muscle. Most skeletal muscle is attached to portions of the skeleton by connective-tissue attachments called tendons. Contractions of skeletal muscle serve to move the various bones and cartilages of the skeleton. Skeletal muscle forms most of the underlying flesh of vertebrates. There are 600 skeletal muscles in the human body. 300 on each symmetrical side.

-The fibers are elongated, sausage-shaped cells containing many nuclei and clearly display longitudinal and cross striations.

-Smooth, visceral, or involuntary muscle is composed of spindle-shaped cells, each having a central nucleus. The cells have no cross striations, although they do exhibit faint longitudinal striations. Stimuli for the contractions of smooth muscles are mediated by the autonomic nervous system. Smooth muscle is found in the skin, internal organs, reproductive system, major blood vessels, and excretory system.

-Cardiac Muscle-This muscle tissue composes most of the vertebrate heart. The cells, which show both longitudinal and imperfect cross striations, differ from skeletal muscle primarily in having centrally placed nuclei and in the branching and interconnecting of fibers (intercalated discs). Cardiac muscle is not under voluntary control. The tissue is considered myogenic because it is autoregulated. It is supplied with nerves from the autonomic nervous system, but autonomic impulses merely speed or slow its action and are not responsible for the continuous rhythmic contraction characteristic of living cardiac muscle.