HISTOLOGY

REMAN A. ALINGASA, RMT

CELLS OF THE CONNECTIVE TISSUEVarious components of the connective tissue play roles in the defense or protection of the body including many of the components of the vascular and immune systems (plasma cells, lymphocytes, neutrophils, eosinophils, basophils, mast cells). The various macrophages of the body are also categorized as connective tissue cells. These all develop from monocytes and are grouped as part of the Mononuclear Phagocyte System of the body. Macrophages are important in tissue repair as well as defense against bacterial invasion. The fibroblasts of connective tissue proliferate in response to injury of organs and migrate to and deposit abundant new collagen fibers, resulting in the formation of fibrous scar

CONNECTIVE TISSUE CELLSConnective tissue cells are usually divided into two groups based on their ability to move within the connective tissue. Fibrocytes (or fibroblasts) and fat cells are fixed cells. Macrophages, monocytes, lymphocytes, plasma cells, eosinophils and mast cells are wandering cells.

FIBROCYTESFibrocytes are the most common cell type in connective tissues. They are the "true" connective tissue cells. Usually only their oval, sometimes flattened nuclei are visible in LM sections. The cytoplasm of a resting (i.e. inactive) fibrocyte does not contain many organelles. This situation changes if the fibrocytes are stimulated, for example, by damage to the surrounding tissue. In this case the fibrocyte is transformed into a fibroblast, which contains large amounts of the organelles which are necessary for the synthesis and excretion of proteins needed to repair the tissue damage. Fibrocytes do not usually leave the connective

RETICULAR CELLS

Reticular cells are usually larger than an average fibrocyte. They are the "fibrocytes" of reticular connective tissue and form a network of reticular fibres, for example, in the lymphoid organs. Their nuclei are typically large and lightly stained (H&E) and the cytoplasm may be visible amongst the cells which are housed within the network of reticular fibres.

ADIPOCYTESFat cells or adipocytes are fixed cells in loose connective tissue. Their main function is (what surprise!) the storage of lipids. If "well fed" the cytoplasm only forms a very narrow rim around a large central lipid droplet. The flattened nucleus may be found in a slightly thickened part of this cytoplasmic rim - if it is present in the section, which may not be the case since the diameter of an adipocyte (up to 100 m) is considerable larger than the thickness of typical histological sections. A "starving" adipocyte may contain multiple small lipid droplets and gradually comes to resemble a fibrocyte. Lipid storage/mobilisation is under nervous (sympathetic) and hormonal (insulin) control. Adipocytes also have an endocrine function - they secrete the protein leptin which provides brain centers which regulate appetite with

MACROPHAGESMacrophages arise from precursor cells called monocytes. Monocytes originate in the bone marrow from where they are released into the blood stream. They are actively mobile and leave the blood stream to enter connective tissues, where they differentiate into macrophages. Macrophages change their appearance depending on the demand for phagocytotic activity. Resting macrophages may be as numerous as fibrocytes. Resting macrophages are difficult to distinguish

MAST CELLSMast cells are - like macrophages, lymphocytes and eosinophils - in demand when something goes wrong in the connective tissue. Quite a few of them are present in healthy connective tissue as they stand on guard and monitor the local situation. The cytoplasm of mast cells is filled by numerous large vesicles. Mast cells discharge the contents of these vesicles if they come in contact with antigens, for example, proteins on the surface of an invading bacterium or, in allergic reactions, in response to antigens found, for example, on the surface of pollen grains. The most prominent substances contained in the vesicles are heparin and histamine. They increase blood flow in close by vessels and the permeability of the vessel walls to plasma constituents and other white

LYMPHOCYTES AND PLASMA CELLSLymphocytes are usually small cells (6 - 8 m). Their nuclei are round and stain very dark. The cytoplasm forms a narrow rim around the nucleus and may be difficult to see. There are many of them in the connective tissue underlying the epithelia of the gastrointestinal tract but usually much fewer in other connective tissues. Again, this situation may change - in this case with immunological reactions. Some lymphocytes may differentiate into plasma cells. Plasma cells are lymphocytes which produce antibodies. To accommodate the necessary organelles for this function the size of the cytoplasm increases dramatically and the cells become basophilic. Plasma cells can occasionally be spotted in the loose connective tissue present in sections.

AMORPHOUS GROUND SUBSTANCE

The intercellular ground substance is an amorphous, transparent material composed mainly of glycoproteins and proteoglycans, with a fairly high water content. The main proteoglycans consist of a core protein associated with sulfated glycosaminoglycans (GAGs). The main GAGs include : chondroitin-4-sulfate, chondroitin-6-sulfate, keratan sulfate, heparan sulfate) and the non-sulfated hyaluronic acid. All substances passing to and from cells must pass through the ground substance.

CONNECTIVE TISSUE1. LOOSE CONNECTIVE TISSUE 2. DENSE CONNECTIVE TISSUE

LOOSE CONNECTIVE TISSUE

Loose connective tissue (areolar tissue) is the more common type. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, is present in the serosal lining membranes (of the peritoneal, pleural and cardiac cavities), in the papillary layer of the dermis and in the lamina propria of the intestinal and respiratory tracts etc.

Loose or areolar connective tissue. Thick pink bands are the protein collagen, while the thin dark threads are the protein elastin.

DENSE CONNECTIVE TISSUEDense connective tissue is divided into two sub-categories:A) Dense irregular connective tissue B) Dense regular connective tissue Dense connective tissue contains relatively few cells with much greater numbers of collagen fibers.

DENSE IRREGULAR CONNECTIVE TISSUE

Dense irregular connective tissue has bundles of collagen fibers that appear to be fairly randomly orientated (as in the dermis). Found in joint capsules, the dermis of the skin and in hollow organs on the digestive tract. It is dense with collagen fibers that run in many different directions. This design allows the tissue to resist tension pulling from numerous directions.

DENSE REGULAR CONNECTIVE TISSUE

Dense regular connective tissue has closelypacked densely-arranged fiber bundles with clear orientation (such as in tendons) and relatively few cells. Found in tendons and ligaments and it is dense with collagen fibers that run parallel to one another (thus the name). This design allows the tissue to resist tension pulling in one

Tendon, Regular Connective Tissue

Dense regular connective tissue contains densely packed collagen fibers that run in the same direction.

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SPECIALIZED CONNECTIVE TISSUECARTILAGE

BONE

CARTILAGECartilage is a form of connective tissue that has an extensive matrix surrounding the cartilage producing cells (chondroblasts or chondrocytes). These cells are trapped inside of small spaces called lacunae (little lakes). The major difference in identifying the different cartilage types is found in the visible components of the matrix. Hyaline cartilage lacks any visible fibers in the matrix, where elastic cartilage looks similar to hyaline cartilage, but it has elastic fibers in the matrix. Fibrocartilage is very dense with thick collagen fibers throughout the matrix. Cartilage cells (chondrocytes) secrete the fibers and ground substance that make up the cartilage matrix. The space in the matrix occupied by

HYALINE CARTILAGE

Hyaline cartilage, shown at right, has fine, closelypacked collagen fibers. In microscope slides these fibers are not distinguishable from the rest of the matrix, thus the matrix has a smooth appearance.

ELASTIC CARTILAGE

Similar to hyaline cartilage, but elastin is the predominant fiber, giving the tissue great elasticity. It is prominent in cartilage which give the external ear and the eustachian tubes their structures.

FIBROCARTILAGE

Fibrocartilage contains large bundles of collagen which are visible in microscope slides. Elastic cartilage contains many elastic fibers which are visible in microscope slides.

ADIPOSE TISSUEThe cells of adipose (fat) tissue are characterized by a large internal fat droplet, which distends the cell so that the cytoplasm is reduced to a thin layer and the nucleus is displaced to the edge of the cell. These cells may appear singly but are more often present in groups (Figure 11). When they accumulate in large numbers, they become the predominant cell type and form adipose (fat) tissue. Adipose tissue, in addition to serving as a storage site for fats (lipids), also pads and protects certain organs and regions of the body. As well, it forms an insulating layer under the skin

Adipose tissue is seen here in the layer between the skin and the muscles. Adipose tissue is basically areolar connective tissue that has a high concentration of adipocytes. Two adipocytes are indicated by ad. Most of the volume of an adipocyte is taken up by a large droplet of stored fat. The typical method of making tissue slides involves chemicals that dissolve fat; a white space

FUNCTIONS OF BONE TISSUE

The skeleton is built of bone tissue. Bone provides the internal support of the body and provides sites of attachment of tendons and muscles, essential for locomotion. Bone provides protection for the vital organs of the body: the skull protects the brain; the ribs protect the heart and lungs. The hematopoietic bone marrow is protected by the surrounding bony tissue. The main store of calcium and phosphate is in bone.

MACROSCOPIC STRUCTURE OF BONE

There are two main categories of bone : Spongy bone (trabecular bone, cancellous bone) Compact bone (cortical bone) Spongy bone Spongy bone is composed of a lattice or network of branching bone spicules or trabeculae. The spaces between the bone spicules contain bone marrow. Compact bone Compact bone appears as a mass of bony tissue lacking spaces visible

ANATOMICAL CLASSIFICATION OF BONES Bones are characterized anatomically as:long bones (e.g. humerus, femur) flat bones (membrane bones) irregular bones (such as the vertebrae) All these bone types, regardless of their anatomical form, are composed of both spongy and compact bone.

MONONUCLEAR PHAGOCYTE SYSTEM