Nature and chemical composition
A critical characteristic of cell inclusions is their transitory nature. When examining the entire cytoplasmic contents, one can distinguish membrane-bound organelles, non-membrane-bound organelles, and permanent structural components. Inclusions do not belong to any of these categories, as they represent exclusively non-permanent structural components.
The chemical nature of these structures is highly diverse and depends on the tissue type and the current needs of the organism. In the cytoplasm, they may appear as:
- Lipid droplets;
- Starch granules;
- Glycogen granules;
- Essential oils;
- Salts of various acids.
The primary biological function of these components is that they are actively accumulated within the cell or, conversely, removed from it as metabolic processes take place.
Trophic (storage) inclusions
This group serves as an intracellular reserve of nutrients that can be utilized by the organism during periods of increased energy demand.
Key examples of trophic inclusions:
- Glycogen — a complex carbohydrate that abundantly accumulates in the form of granules within hepatocytes (specialized liver cells).
- Lipid droplets — accumulations of fats that are a characteristic feature of fat cells, also known in histology as adipocytes.
Pigment inclusions
These non-permanent structures give cells a specific coloration and participate in performing highly specialized tasks.
Depending on the tissue type, pigments are represented by the following substances:
- Hemoglobin — a vital pigment found in vast quantities in erythrocytes (red blood cells).
- Lipofuscin and various carotenoids — these pigment inclusions are predominantly localized in muscle fibers as well as in nerve cells.
Excretory and secretory granules
Cellular metabolism is inevitably accompanied by the formation of products that must be isolated and eliminated. For this purpose, excretory granules exist. They contain metabolic waste products that are destined for removal from the intracellular space.
The mechanism for eliminating the contents of excretory granules is called exocytosis. During exocytosis, the granule fuses with the plasma membrane, expelling its contents outside the cell.
Most frequently, such granules (along with related secretory inclusions) are localized in tissues performing barrier, excretory, or digestive functions. The primary sites where they are found include:
- Renal tissues;
- Liver;
- Intestinal epithelium.
Briefly: answers to topical questions
What chemical substances can form cell inclusions?
Cell inclusions have various chemical natures. They include fat droplets, starch, glycogen granules, essential oils, acid salts, and other substances.
What are some examples of trophic inclusions?
Examples of trophic (storage) inclusions:
- Glycogen — in hepatocytes (liver cells).
- Lipid granules — in fat cells (adipocytes).
- Starch and glycogen — reserve intracellular inclusions in bacteria.
- Yolk — protein-lipid inclusions in the cytoplasm of egg cells; performs a trophic function, providing nutrition for the embryo.
What types of pigment inclusions are found in the human body?
The following types of pigment inclusions are found in the human body:
- Hemoglobin — contained in erythrocytes.
- Lipofuscin and carotenoids — present in muscle and nerve cells.
- Melanin — melanin granules that provide light absorption and protection against light radiation in specialized cells.
- Bilirubin.
For quick memorization of the classification, use the mnemonic «SPET»: Secretory, Pigment, Excretory, Trophic.
Class and exam questions
Are cell inclusions organelles?
No, unlike membranous and non-membranous organelles, which are permanent structures, inclusions are non-permanent components of the cytoplasm.
Where in the body is glycogen stored?
In the form of trophic inclusions, glycogen is localized predominantly in liver cells — hepatocytes.
How does a cell get rid of metabolic waste products?
Metabolic products are packaged into excretory granules, which are then removed from the cytoplasm via exocytosis.
Which cells are characterized by the presence of lipofuscin?
Pigment inclusions of lipofuscin, as well as carotenoids, are characteristic of muscle and nerve cells.
What else to study on this topic
- Mechanism of exocytosis during the removal of excretory granules
- Structure and functions of hepatocytes and adipocytes
- Chemical properties and functions of hemoglobin in erythrocytes
- Origin of lipofuscin in nervous and muscular tissue
- Functions of secretory inclusions in the intestinal epithelium
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