Insect fat bodies role in their metabolism

1. Role of fat body in insect
metabolism
Presented by:
K. HARIKARAN
M.Sc., (Agri) Entomology
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2. What is Fat body ?
Fat body is a relatively a large organ distributed throughout
the insect body, preferentially underneath the integument that
surrounding the gut and reproductive organ.
It is a loose tissue, where the other organs are solid
structures.
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4. Fat body
 Fat body plays a major role in insect’s life.
 Involved in multiple metabolic function.
 One of the essential function is to store and release energy in response
to the energy demand.
 Stores energy in the form of GLYCOGEN and TRIGLYCERIDES.
 Fat body is the centre of metabolism.
 It is the main tissue responsible for innate and acquired humoral
immunity.
 It is important in early stage of an insects due to the production of
vitellogenin (yolk protein) needed for the development of oocytes.
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5. Insect fat body cells
 Fat body is made up of five main types of cells, which vary in composition,
size, and function during different growth stages of insects.
 They are,
 TROPHOCYTES
 OENOCYTES
 MYCETOCYTES
 CHROMATOCYTES
 UROCYTES
 At initial stages, growth of this tissue is observed in insects and the largest
cells of the fat body are found in the oldest larvae.
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6. Trophocytes
a) Structure of trophocytes
b) TEM of the plasma membrane
of trophocytes.
c) Gap junction between two
trophocytes
d) Desmosomes joining two
trophocytes.
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7. Contd….
 Trophocytes are polymorphic cells of mesodermal origin.
 Responsible for storage, secretion & detoxification of the organic
substances present in the insect body.
 The central cell consists of irregular nucleus surround by a
cytoplasmic ring, storage structures and larger vacuoles.
 Four types of major vacuoles are there in fat body:
 Digestive vacuoles – process and digest and release the spare
substances into the body during diapause.
 Storage vacuoles – store the spare substance in the form of Tyrosine,
urates, proteins, lipids drop.
 Condensation vacuoles – related to golgi apparatus and lysosomes
and often contains proteins.
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8.  Superficial vacuoles – formed by fusion of vesicles.
 These cells can vary in size due to accumulation of nutrients and
the swelling of the vacuoles therein.
 The no. of. Trophocytes is lower in male insects than in females,
with a higher production of vitellogenin.
 Higher no. of cells also present during moulting.
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9. Oenocytes
 These are circular or oval cells associated with the epidermal layer
of the cuticle.
 Oenocytes have centrally placed nucleus, mitochondria, smooth
endoplasmic reticulum & vacuoles.
 Distributed throughout the body or occur in small groups (around
the spiracles).
 Often darker in colour than trophocytes (brown/yellow/amber).
 In some species (especially Diptera insects, Eg. Chironomus
thummi), oenocytes also contain haemoglobin.
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11. Mycetocytes
 These are the cells contain mainly of symbiotic prokaryotic micro –
organisms.
 Especially seen in Periplaneta americana.
 As like other cells, granules of fat and glycogen are observed in
mycetocytes with a reduced amount of cytoplasm.
 Mycetocytes are found in organisms that eat poor quality,
unbalanced food, synthesizing some nutrients (amino acids or
vitamin B).
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12. Structure of fat bodies and the lobes of P. americana under fed (A and B) and
starved (C lobes having many large vacuoles and some mycetocytes (arrows).
C: mycetocytes (arrows) and unclear small vacuoles.
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13. Chromatocytes
Chromatocytes are flat cells.
Central nucleus and transparent cuticle.
They accumulate fats that the insects uses during
metamorphosis.
These cells are present in some aquatic insects.
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14. Urocytes
 These cells are characterized by the presence of a reduced
endoplasmic reticulum.
 Vacuoles are accumulated with Urate granules.
 Urate comes from metabolism of nucleic acids or protein
degradation.
 Main purpose – store the urate granules.
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15. Metabolic functions
 Fat body participates in various metabolic activities.
 Absorption from hemolymph and buildup of intracellular storage
nutrients in the form of lipid droplets, glycogen and protein granules
during immature stages are aimed at accumulating reserves for the
later stages.
 The cells can produce various blood proteins (lipoproteins,
glycolipoproteins) which include JH carrier protein, diapause proteins.
 Trehalose, produced by the fat body, constitute the major diglycerides
of insect blood.
 Glycogen is synthesized by (glycogen synthase) and hydrolyzed by
(glycogen phosphorylase) enzymes in fat body. The hydrolyzed
products are mobilized at moulting to serves as precursors required
for chitin synthesis and new cuticle formation.
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16. Endocrine control of fat body
 Neuroendocrine secretions from brain and ganglia, moulting
hormones, JHs, and the corpora cardiaca neurosecretion affect the
metabolic state of the trophocytes.
 The blood level of trehalose is regulated by a corpora cardiaca
neurohormone.
 Synthesis and release of vitellogenins by the female fat body cells
usually are under the control of JH.
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17. Fat body during metamorphosis
 During the period of metamorphosis the fat body tissue undergoes
extensive morphological, histological, biochemical and
organizational changes.
 These actions are triggered by the MH on the presence of low
levels of the JH.
 Such changes are studied in Dipterans and Lepidopterans.
 Two major strategies for transforming the larval fat body into an
adult tissue exist:
1. HISTOLYTIC pathway
2. REMODELING pathway
 In certain holometabolous insects species, combination of both
process takes places.
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18. Dynamic exchange of nutrients between fat body and
hemolymph are evident through this cycle.
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19. Fat bodies during Chilling
 Temperate, polar and alpine insects generally do not feed over winter and hence must manage
their energy stores to fuel their metabolism over winter and to meet the energetic demands of
development and reproduction in the spring. In this Review, we give an overview of the
accumulation, use and conservation of fat reserves in overwintering insects and discuss the
ways insects modify fats to facilitate their selective consumption or conservation. Many insects
are in diapause and have depressed metabolic rates over winter; together with low
temperatures, this means that lipid stores are likely to be consumed predominantly in the
autumn and spring, when temperatures are higher but insects remain dormant. Although there is
ample evidence for a shift towards less-saturated lipids in overwintering insects, switches
between the use of carbohydrate and lipid stores during winter have not been well-explored.
Insects usually accumulate cryoprotectants over winter, and the resulting increase in
haemolymph viscosity is likely to reduce lipid transport. For freeze-tolerant insects (which
withstand internal ice), we speculate that impaired oxygen delivery limits lipid oxidation when
frozen. Acetylated triacylglycerols remain liquid at low temperatures and interact with water
molecules, providing intriguing possibilities for a role in cryoprotection. Similarly, antifreeze
glycolipids may play an important role in structuring water and ice during overwintering. We also
touch on the uncertain role of non-esterified fatty acids in insect overwintering.
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20. Conclusion
 Fat body tissue is important due to its variuos no .of functions it
performs.
 The metabolism occurs in fat body allows for proper growth and
development.
 Fat body tissue integrates many signals from many pathways
that receive and transmit to the hemolymph, which affect the
functioning of entire body.
 It allows the insects to survive in harsh conditions.
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21. THANK YOU
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