Fat Bodies Definition
Fat bodies are tissues that serve as reservoirs for energy reserve. In entomology, the term “fat body” particularly refers to the insect fat tissues that are involved in many metabolic functions, e.g. lipid and carbohydrate metabolism, protein synthesis, and amino acid and nitrogen metabolism.
While vertebrates have blood that transports nutrients and removes metabolic wastes, insects have hemolymph. Hemolymph is the circulating fluid in insects that performs functions similar to vertebrate blood. In essence, it brings nutrients to the different parts of the insect body and removes the metabolic waste products from the organs to be excreted via the malpighian tubules. (Ref. 1) One of the tissues it bathes is the fat body. While vertebrates have adipose tissues (for storage) and liver (for its metabolic functions), insects have fat bodies that carry out similar functions. These dynamic tissues in insects store fat, glycogen, and protein for utilization in many biological processes, such as growth and reproduction. (Ref. 2)
Fat Bodies Function
Fat bodies consist largely of storage cells called trophocytes (or adipocytes, as in other references). (Ref. 2, 3) They are distributed throughout the hemocoel (insect’s body cavity). Within the adipocytes, lipid reserves are stored in the form of triglycerides and glycogen cytoplasmic droplets. Insect fat bodies also serve as the site for the biosynthesis of many hemolymph proteins and circulating metabolites. (Ref. 2) Fat bodies release their reserves for use in diverse metabolic activities, e.g. in response to the energy demands of the insect during the insect’s metamorphosis. (Ref. 1, 2) The insect fat body is also key to the synthesis and secretion of vitellogenins, which are precursor proteins essential for egg maturation in adult female insects. (Ref. 2)
Apart from serving as a reservoir for fat, glycogen, and proteins, the other functions of the insect fat body are as follows:
- Serves as an endocrine organ (Ref. 4)
- Immune function, by producing antimicrobial peptides (Ref. 5)
- Involved in the detoxification of nitrogen metabolism (Ref. 6)
The insect fat body is of mesodermal origin based on the studies in D. melanogaster larva. (Ref. 3) The cell clusters arising from the embryonic mesoderm develop into fat cells. The predominant cell type in the insect fat body is the trophocytes (adipocytes) that contain mostly lipid droplets. Other cell types are urocytes (store urate) and mycetocyte (containing microbial symbionts). (Ref. 2)
Fat bodies are not exclusive to insects. They are also found in other arthropods, such as crustaceans. (Ref. 7)
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