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The term cytoplasmic inclusions is used to describe foreign substances contained within a cell membrane. It pertains to nutrients, such as proteins, carbohydrates, and lipids, as well as pigment granules. Other inclusions may be secretory products stored within the cell body, and these can be expelled from the cell through exocytosis. Laboratory technicians are able to separate cytoplasmic inclusions from the cell body through ultrasonication or homogenization.
Many of these substances function as a storage location within the cell. Nutrient inclusions are used by the cell for energy as needed. Most carbohydrate inclusions are stored as polysaccharide glycogen within liver cells. Hemosiderin is a complex found within the cell membrane that is a storage location for iron. Stored secretory inclusions may consist of proteins, enzymes, acids, or crystalline substances.
Some minerals are contained within the cell as inclusions. Lead and silver are two of these. Certain dust molecules containing carbon may be stored as an exogenous substance.
Several types of pigment granules are stored as cytoplasmic inclusions. Granules that are absorbed from the environment of the cell are called exogenous pigments. They can be obtained from food, such as the carotenes in vegetables. These pigment granules are yellowish-red in color.
Another kind of pigment granule located within and formed by the cell is called an endogenous pigment. The red pigment of hemoglobin is an example of a naturally occurring pigment granule stored within the cytoplasm. Yellowish cytoplasmic inclusions may be caused by the presence of lipofuscin. This granule cannot be exported from the cell, so it accumulates within the cell membrane.
A fourth pigment stored in cytoplasm by the body is melanin. It is manufactured by the body in response to ultraviolet (UV) rays. The brownish-black color of moles and freckles is caused by the presence of melanin. Brown eyes as well as dark skin and hair are all attributed to the existence of melanin within the cytoplasm of cells.
Several different lipids, or fats, are also stored as cytoplasmic inclusions. Some visible in the Golgi apparatus are composed primarily of lipid droplets. The majority of lipid material within the body is located in adipose cells. Triglycerides (TG) and esterified (EG) cholesterol are also cytoplasmic lipid inclusions. These types of lipid inclusions are continuously undergoing hydrolysis and resynthesis, making them metabolically active.
An inclusion may contain more than one stored substance. EC and TG are formed simultaneously, and both substances may be present in the same inclusion. The lipid is able to change locations between inclusions through a carrier protein. A transfer protein can deposit the lipid into an already formed inclusion. The size of the lipid inclusion is dependent on the size of the cell and may increase in size as more lipid material is added to the lipid droplet.