Connective Tissue Cells

Connective Tissue Cells

Connective 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.

Fibrocytes

Fibrocytes 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 tissue. They are, however, able to perform amoeboid movement.

The terms fibrocyte and fibroblast refer here to the inactive and active cells respectively - at times you will see the two terms used as synonyms without regard for the state of activity of the cell.

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.

Adipocytes

Fat cells or adipocytesare 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 µm)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 feedback about the bodies fat reserves.
Leptin deficiency in experimental animals results in obesity.

Adipocytes are very long-lived cells. Their number is determined by the number of preadipocytes (or lipoblast) generated during foetal and early postnatal development.

Macrophages Macrophages 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 from fibrocytes in H&E stained sections.

Suitable Slides

sections of liver or lymph nodes - carbon injected animal - trichrome or H&E
 
sections of lung - H&E
In the lung, macrophages "patrol" the respiratory surfaces and ingest airborne particles which settle there. They "retire" to the connective tissue of the lung were they can be identified by the accumulations of fine particles in their cytoplasm.
 
sections containing irregular connective connective tissues - Alcian blue & van Gieson
In Alcian blue & van Gieson stained sections macrophages may appear as intensely green, roundish cells with a dark nucleus.

Liver, rabbit - ink injected, trichrome Macrophages are usually difficult to distinguish from other cell types in connective tissues. One way to visualize them is to inject an experimental animal with very fine carbon particles. Macrophages which come into contact with the circulating particles will phagocytose some of them. In sections the particles will be visible as dark, black-brown accumulations in the cytoplasm of the macrophages. Draw a few macrophages in situ. Macrophages found in the liver are also called Kupffer cells. They adhere to the epithelial lining of the liver sinusoids, i.e. blood filled spaces between the liver cells. Blood will not be visible in some types of preparations and the sinusoids appear empty. Once you have identified macrophages, go hunting for some good collagen - in this trichrome stains the collagen fibres will appear green(ish). Typically you will see them only in the connective tissue surrounding larger blood vessels. Improve your knowledge on epithelia and look out for ducts lined by a simple cuboidal or columnar epithelium.