Cellular inclusions

Cell inclusions, also called deutoplasm or paraplasm, are small particles or deposits containing insoluble substances or metabolites, without metabolic activity, in the cytosol, mostly with no membrane.

The defining negative is the thing to hold on to: they are not building blocks or structural constituents. They are non-living, non-moving components. An organelle does something; an inclusion is something the cell has put down and left.

They represent deposits of the main chemicals stored within the cell, and there are three kinds: glycogen granules, lipid droplets and pigment deposits.

Cell inclusions, whose synonyms are deutoplasm and paraplasm, are small particles or deposits containing insoluble substances or metabolites, without metabolic activity, in the cytosol. Mostly they have no membrane.

• They are no building block or structural constituent.
• They are non-living, non-moving components.
• They represent deposits of the main chemicals stored within the cell.

Glycogen granules are aggregates of carbohydrate polymer in which glycogen is stored.

• They are found in liver, striated muscle cells, cartilage and nerve cells.
• They are histochemically demonstrated by the PAS reaction, the Periodic Acid-Schiff, which gives pink-red clumps.

Lipid droplets are accumulations of lipid molecules, prominent in adipocytes, the fat cells.

• They are abundant in cells of the adrenal cortex.
• Under TEM they appear as small spherical structures enclosed by a single monolayer.
• They are histochemically demonstrated by Sudan III staining, giving reddish-brown, or by Sudan black staining, giving blue-black.

The single monolayer is worth noticing. Every membrane elsewhere in the cell is a bilayer; a lipid droplet is a ball of lipid, so only one leaf of phospholipid is needed to face the cytosol.

Pigment deposits are naturally coloured substances. They divide by where they were made.

Endogenous pigments are produced inside the cell. Three are named.

Lipofuscin is a yellow-brown pigment in non-dividing cells: neurons, skeletal and cardiac muscle. It is called the wear and tear pigment, because its amount increases over time, with advancing age.

Melanin consists of dark-brown pigments which affect the colour of eye, hair and skin. It serves to protect cell nuclei from damage to DNA caused by light.

Hemosiderin contains ferritin protein, which forms an iron storage complex. It is found in phagocytic cells of the liver and spleen.

Exogenous pigments are produced outside the cell:

• tattoos;
• dusts;
• carotene;
• minerals.