Inclusions (Cytoplasmic) are non-living substances that are not able to carry out any metabolic activity and are not bound by membranes. Inclusions are stored nutrients, secretory products, and pigment granules. Examples of inclusions are glycogen granules in the liver and muscle cells, lipid droplets in fat cells, pigment granules in certain cells of skin and hair, water-containing vacuoles, and crystals of various types.
Glycogen: Glycogen is the most common form of glucose in animals and is especially abundant in cells of muscles, and liver. It appears in electron micrograph as clusters, or a rosette of beta particles that resemble ribosomes, located near the smooth endoplasmic reticulum. Glycogen is an important energy source of the cell; therefore, it will be available on demand. The enzymes responsible for glycogenolysis degrade glycogen into individual molecules of glucose and can be utilized by multiple organs of the body.
Lipids: Lipids are triglycerides in storage form is the common form of inclusions, not only are stored in specialized cells (adipocytes) but also are located as individuals droplets in various cell type especially hepatocytes. These are fluid at body temperature and appear in living cells as refractile spherical droplets. Lipid yields more than twice as many calories per gram as does carbohydrate. On demand, they serve as a local store of energy and a potential source of short carbon chains that are used by the cell in its synthesis of membranes and other lipid containing structural components or secretory products.
Crystals: Crystalline inclusions have long been recognized as normal constituents of certain cell types such as Sertoli cells and Leydig cells of the human testis, and occasionally in macrophages. It is believed that these structures are crystalline forms of certain proteins which is located everywhere in the cell such as in nucleus, mitochondria, endoplasmic reticulum, Golgi body, and free in cytoplasmic matrix.
Pigments: The most common pigment in the body, besides hemoglobin of red blood cells is melanin, manufactured by melanocytes of the skin and hair, pigments cells of the retina and specialized nerve cells in the substantia nigra of the brain. These pigments have protective functions in skin and aid in the sense of sight in the retina but their functions in neurons is not understood completely. Furthermore, cardiac tissue and central nervous system neurons shows yellow to brown pigment called lipofuscin, some believed that they have lysosomal activity.
Secretory products: Hormones, mucus, digestives enzymes, neurotransmitters, fibrous proteins, and hydrochloric acid are all different types of secretory products found in cytoplasmic inclusions. The cytoplasmic inclusions bound for secretion in some secretory cells are sometimes not visible by microscopy; some glands secrete products that are only released when needed by the body. An example of such glands is steroid-secreting endocrine gland.
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