The Cuticle Flashcards
function of cuticle?
provides shape and support to soft tissues
minimises loss of body fluids (aquatic + terrestrial)
- mechanical protection
- prevents desiccation
- protective barrier against bacteria, viruses, toxins
- location of colours and patterns important in behaviour
- lines:
(a) external surface of the body
(b) tracheae
(c) anterior and posterior sections of alimentary canal
(d) parts of the reproductive system - supports body + provides points for muscle attachment via connecting tonofilaments that run through epidermal cells
What do insects cuticle consist of?
living and non-living cells
living layer
row of epithelial cells resting on connective tissue
these cells secrete the non-living layers of cuticle during moulting
other cells, interspersed in epithelial cells, form sensory structures such as setae + hairs
what is the non-living layer made of?
2 main layers - thin, outer epicuticle + thicker, inner procuticle
Epicuticle
2 layers + 2 additional layers
- thicker, inner, lipoprotein layer = inner epicuticle
- very thin, outer layer = cuticulin = outer epicuticle
- wax layer - v. fragile but essential for water retention - bipolar molecules w/ hydrophilic + hydrophobic ends - hydrophobic end faces outward + rejects water
- cement layer - protects the wax layer which can be disoriented by heat or abrasion
cement layer
- absent in some, e.g. Apis mellifera
- thin layer outside wax layer
- produced by dermal glands
- consists of tanned proteins + lipids, resulting in hard shellac-like substance
- protects against abrasion
- not replaced if abraded
wax layer
varies in composition in different insects
contains: fatty acids, hydrocarbons, alcohols
lipids synthesised by oenocytes + transported via pore canals
wax = transported to surface via pore canals
changes permeability of cuticle to water
Pore canals
- cytoplasmic extensions of epidermal cells
- transport cuticular materials to surface
- occurs in large numbers e.g. >1,000,000mm-2 of cuticle
- shape probably determined by orientation of fibres making up lamellae in endocuticle
- believed to be origin of surface wax
Procuticle of the non-living component
- exocuticle - dark, outer layer
- endocuticle - pale, inner layer
Epicuticle and Procuticle
Epicuticle
- outer epicuticle (3/4 layers) 1 - 4 um
- no chitin
- outer - non-elastic lipid + protein
- inner - tanned lipoproteins
Procuticle
- inner cuticle (2 layers) <200 um
- +chitin
- first secreted as procuticle
- outer sclerotised exocuticle
- inner - undifferentiated endocuticle
- Chitin
- polymer of N-acetylglucosamine
- makes up 20-50% dry weight of procuticle
- neighbouring chains run in opposite directions
- held together by H-bonds
- form microfibrils
- embedded in protein matrix (reinforced concrete)
Complex arrangement of cuticle
- chitin macromolecules form fibrils that wrap w/ proteins + assemble into fibres
- fibres assemble into bundles
- bundles arrange themselves parallel to each other + form pseudolayers
- pseudolayers stack into helical structure w/ pseudolayers rotated around normal axis of the cuticle
Microfibril orientation
- alternate production of helicoidally arranged and uni-directional
- circadian periodicity
- daily growth layers in larval + adult Exopterygota + adult Endopterygota
- food supply may limit growth
Microfibril orientation - Lamellate cuticle
- each layer (lamella) rotates anti-clockwise by fixed angle -> helicoidal arrangement
- most insects have lamellate outer procuticle (to be exocuticle)
- found in inner procuticle (endocuticle): Apterygota - larvae and pupae of Lepidoptera, Diptera, Coleoptera
Microfibril orientation - unidirectional & pseudo-orthogonal
Lamellate alternated w/ uniformly orientated microfibrils in same direction
- Exopterygota
- Locusts and Cockroaches
Lamellate alternated w/ uniformly orientated microfibrils in in different directions
- Hemiptera
- Adult Lepidoptera, Diptera, Coleoptera
