CONNECTIVE TISSUE
Author: Dr. R. Menaka
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The various types of adult connective tissues in contrast to epithelium have relatively a few cells and a large amount of intercellular substance. But the proportion of these two elements shows variations.
In some type, especially loose connective tissue cells are quite numerous. In other types, they are few in number and tissue is composed almost entirely of closely packed fibers.
Two categories of connective tissue are recognized.
1. Embyronal connective tissue and
2. Adult connective tissue
1. Embryonal connective tissue: Embryonal connective tissue are two varieties
a. Mesenchyma and
b. Mucous
Mesenchyma consists of a network of branching cells, in the meshes of which is a homogenous intercellular fluid. The processes of a cell appear to anastomose with those of other cells. As development progresses, wavy primitive fibers appear between branching cells and the fluid matrix becomes viscid due to formation of mucoproteins. This developmentally more mature tissue is mucous connective tissue which is widely distributed in the embryo. The primitive fibers are later become collagenous. The elastic fibers appear later. e.g. umbilical cord and the term wharton's jelly is applied to it in that location.
Adult connective tissue
1. Connective tissue 2. Cartilage 3. Bone
The type and the nature of the substance in which they are embedded (ground substance) furnish the basis for the subdivision of adult connective tissue in to two main groups of connective tissue proper.
Adult connective tissue proper
1. General 2. Special
i. Loose connective tissue (areolar) i. Reticular connective tissue
ii. Dense connective tissue ii. Adipose connective tissue
i. Loose connective tissue
Loose or areolar connective tissue is very widely distributed in the body. It forms the superficial and of the deep fascia, forms part of the frame work (stroma) of most of the organs, of surrounds blood vessels and nerved and fills in any otherwise unoccupied spaces. Areolar tissue contains cells fibers and ground substance.
Cells: Cells constantly present are : Out of these cells, fibroblast and histiocytes are the most numerous.
1. Fibroblasts
2. Macrophages or histiocytes
3. Mast cells
4. Plasma cells
5. Wandering cells
6. Pigment cells and
7. Fat cells
Fibroblast:
Fibroblast are large r, flat branching cells with extensive processes which may join the processes of other fibroblasts. The cell membrane is delicate and usually not seen. The nucleus is spherical or oval and in ordinary preparations is lightly stained.
Macrophages or Histiocytes:
Histiocytes are irregular cells with short processes. The nucleus is smaller and darkly staining than fibroblasts. Cytoplasm coarsely granular and shows vacuoles. The are present in all fasciae and in the stroma of organs. They also occur in sinusoids of liver in lymphoid organs and in bone marrow. They show amoeboid movement and phagocytosis in pathological conditions. When activated , the histiocytes become larger and more rounded and the cytoplasm will be filled with granules of ingested material. They from part of the macrophage system or reticuloendothelial system.
Mast cells:
Mast cells occurs mostly in loose connective tissue especially long the course of blood vessels. They are large oval or round cells with pale-staining nucleus and coarse cytoplasmic granules which stain with certain basic dyes. They resemble basophils of blood and contain an anticoagulant heparin, histamine and serotonin.
Plasma cells:
Plasma cells are comparatively rare in loose connective tissue, but are numerous in the alimentary mucous membrane and the great omentum also in pathological conditions. They are smaller than macrophages of round or irregular shape, with basic staining homogenous cytoplasm. The nucleus is small and eccentrically placed. Large granules of chromatin radially arranged in a regular manner give a cart wheel appearance to the nucleus. There is acharacteristic unstained or lightly stained area in the cytoplasm at the side of the nucleus, where the cytoplasm is more abundant. They are derived from lymphocyte like cells and are active in forming antibodies.
Wandering cells:
Wandering cells are lymphocytes, eosinophils and neutrophils. These are migrated from blood.
Pigment cells:
The pigment cells occurs in the choroid and iris of the eye and in the corium (dermis) of dark skinned animals. The cells cytoplasm is filled to a varying degree with brown or black pigment which is usually melanin. They are specialized cells are called as Melanocytes, irregular in outline with cytoplasmic processes.
Fat cells:
Fat cells are regarded as arising from specific cells i.e. known as steatoblasts. The cells are found isolated or in groups in an areolar connective tissue. The largest deposits of fat are found in subcutaneous connective tissue (panniculus adiposus) in the kidney region, in the mesenteries and mediastinum and in the cervical axillary and inguinal region.
Fat cells may occur singly or in groups any where in the connective tissue. In severe starvation the fat in the cells is reduced to small droplets.
Dense connective tissue:
Based on the nature of arrangement of fibers, they are classified into two groups
1. Irregularly arranged
2. Regularly arranged
Dense irregularly arranged connective tissue occurs in the form of sheets. Main component a coarse collagenous tissue but elastic and reticular are also present. The fibers interlase to form tough network. Fibroblasts and macrophages are present.
e.g. dermis of the skin, periosteum, perichondrium and capsules of some organs.
Dense regularly arranged connective tissue occurs in the form of cordlike or band like structures. Fibers are arranged parallel to one other.
e.g. Tendons, ligaments and aponeurosis.
Additional notes:
1. Inactive fibroblasts are often called fibrocytes.
2. The fibroblasts associated with reticular fibers. They are usually called reticular cells.
3. Fat cells are called adipocytes or lipocytes.
4. Aggregation of fat cells constitute adipose tissue.
5. macrophage cells of connective tissue are also called histiocytes or clasmatocytes.
6. Sometimes macrophages may fuse together to form multinucleated giant cells.
7. Mast cells are also called mastocytes or histaminocytes
8. Mast cells are considered to release the most important substance known as histamine.
9. Apart from histamine mast cells may contain various enzymes and factors that attract eosinophils or neutrophils.
10. Very few plasma cells can be seen in normal connective tissue. The number of plasma cells increases due to various types of inflammation.
11. The plasma cells produce antibodies which may be discharged locally, may be stored within the cell itself in the form of inclusions called Russell's bodies.
12. It is believed that plasma cells represent B-lymphocytes that have matured and have lost their power of further division.
2. Special type of Connective tissue
i. Reticular Tissue
It is composed of cells and reticular fiber network. The cells have stellate shape with processes which extended in all directions and joins those of wrapped by reticular fibers.
The reticular tissue forms the frame work of organs like lymphatic glands, spleen, red marrow of bones. The reticular cells line the lymph sinuses in lymphatic glands or blood sinusoids in spleen, liver red marrow etc. These cells are actively phgocytic and form part of the reticulo-endothelial system.
ii. Adipose Tissue
In many situations fat performs a mechanical function. The fat around the kidneys keeps them in position. If there is a sudden depletion of this fat the kidney may become mobile. i.e. known as floating kidney.
Brown fat tissue is different from the common or white fat tissue. In rats and other rodents, it is highly developed and forms yellowish lobated masses in certain parts of the body between the scapulae, on the neck, in the mediastinum, in the inguinal region eleswhere.