Reticular connective tissue

Reticular connective tissue

Reticular connective tissue consists of reticular cells and the network of reticular fibres formed by them. Most connective tissues contain reticular fibres, but only in reticular connective tissue are they the dominant fibre type. In a number of tissues and organs, reticular connective tissue forms the structural framework in which the cells of the organ are suspended. The open meshwork of fine fibres is particularly useful in tissues and organs in which diffusion and / or cell movements are functionally important, for example, in the liver, lymph nodes and the spleen.

Adipose tissue

Adipose tissue is essentially loose connective tissue containing large numbers of adipocytes. There are two types of adipose tissue, which derive their names from the colour of the tissue (white or brown) and the number of lipid droplets found in the adipocytes.

• Adipocytes of white, unilocular adipose tissue contain one large lipid droplet.
• Adipocytes of brown, multilocular adipose tissue contain many lipid droplets.

White adipose tissue does not only function in the storage of lipids. For example, in the palms of the hands, on the plantar surface (sole) of the feet and in the gluteal region (buttocks) it has a structural, cushioning function. In these regions, accumulations of adipocytes are surrounded by strong connective tissue fibres. Also, the distribution of white adipose tissue is different in males and females and is part of the secondary sexual characteristics. The storage and mobilisation of lipids does require quite some metabolic activity of the tissue. Consequently, adipose tissue has a rich supply of capillaries.
Brown adipose tissue occurs mainly during development and may account for 2 - 5 % of the body weight in a newborn. In adult individuals most of the brown fat has further differentiated into white fat. Adipocytes in brown fat contain plenty of mitochondria. A very rich capillary supply and the cytochromes found in the mitochondria give the tissue its characteristic colour. A protein (UCP-1 or thermogenin) found in these mitochondria decouples the oxidation of fatty acids from the generation of ATP. Instead, these cells generate heat.

The location of the brown fat reflects its heat-generating function. It is located in the axilla (armpits), between the shoulder blades, in the region of the neck and along large blood vessels. The heat generated by the brown fat warms the blood which supplies nearby organs or which re-enters the trunk from the limbs.

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Suitable Slides

white adipose tissue: sections of skin - H&E, trichrome, van Gieson
Section are rarely prepared to show just white adipose tissue. The hypodermis, i.e. lightest and loosest appearing layer, of skin sections will typically contain large areas of adipose tissue. Other good candidates are bone sections which contain yellow bone marrow or sections of lymph nodes which are often embedded in adipose tissue. Small spots of adipose tissue should be present in many other sections.
brown adipose tissue: sections of brown adipose tissue or kidney - trichrome, H&E
Brown adipose tissue is often located in the connective tissue close to the renal hilus or in the renal sinus of sections which contain the entire kidney of small laboratory animals. Look for an indentation in the outline of the kidney, which corresponds to its hilus.

Thick Skin - H&E In well-preserved thin sections, the adipocytes form a mosaic of rounded or slightly angular white tiles, which correspond to the locations of the lipid droplets, separated from each other by darker seams, which correspond to the cytoplasm of the adipocytes and the sparse connective tissue components between them. Because of the large size of the adipocytes you will only rarely see it "typical" signet ring-like appearance of the cells. Although the tissue my be a little distorted, thicker sections give a good three dimensional feel of the adipose tissue. The cytoplasmic rims of the adipocytes form thin veils which enclose the open spaces which were occupied by the lipid droplets. Oval adipocyte nuclei are often located close to the corners at which the adipocytes meet. Draw two or three adipocytes at high magnification and a survey image which illustrates the appearance of adipose tissue at low magnification.

Kidney - trichrome In the renal sinus, islands of brown adipose tissue are often surrounded by white adipose tissue, which emphasises the different appearances of the two tissue types. In brown adipose tissue, the nuclei of adipocytes are round and located more or less centrally in a cytoplasm which, after the extraction of lipids during tissue preparation, looks very frothy. Cell borders can be difficult to identify. Capillaries are very frequent. Sketch the appearance of the brown adipose tissue. Contrast the characteristic features of white and brown adipose if both types are present side by side.