Summary of Cuticle

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Summary of cuticle The cuticle consists of a relatively soft and colourless endocuticle, hardened and darkened in its outer part in some places to form a rigidexocuticle, and a complex epicuticle made up of several layers. The endocuticle consists of polyacetylglucosamine (chitin) intimately associated with a characteristic protein (arthropodin). Perhaps these are combined in the form of a mucoprotein in which the relative amounts of protein and polysaccharide vary with the type of animal and wi
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  Summary of cuticle The cuticle consists of a relatively soft and colourless endocuticle , hardened and darkened in its outer part insome places to form a rigid exocuticle , and a complex epicuticle made up of several layers.The endocuticle consists of   polyacetylglucosamine ( chitin ) intimately associated with acharacteristic  protein ( arthropodin ). Perhaps these are combined in the form of a mucoprotein in which therelative amounts of protein and polysaccharide vary with the type of animal and with the part of the body.The substance of the endocuticle separates, apparently spontaneously, into laminae of varying dimensions.In most insects the submicroscopic crystallites of chitin, and perhaps protein, tend to lie at random with thelong axis in the plane of the cuticle. They can be orientated by tension or compression. In the Coleopterathey are arranged in microscopic bundles (‘Balken’) lying parallel in a given lamina, but at an angle of 60 o insuccessive laminae. The crystallites are orientated also in the long axis of tendons and bristles.In the exocuticle the protein is tanned by quinones derived by oxidation from dihydroxy-phenols. This tanned  protein ( sclerotin ) impregnates the chitin framework to form a rigid fabric (like cellulose impregnated with aresin plastic) which is moderately impermeable to water. Hard cuticles in insects are always dark, largelybecause the quinonoid groups are chromophore, in part perhaps because during the oxidative hardeningsome true melanin is formed from tyrosine. In a few insects impregnation with lime takes the place of tanning.The cuticular substance has a tendency to crystallize in the form of multiple thin plates; these areresponsible for the iridescent colours of many insects.The epicuticle is responsible for most of the impermeability to water. It consists of a thin layer of  tanned lipoprotein ( cuticulin ), a layer of  crystalline waxes about a quarter of a micron thick, and a layer of  cement  (likewise consisting, perhaps, of tanned protein containing some lipides) protecting the wax.The wax varies in character from a soft grease to hard white crystalline materials. If the temperature israised to a critical level, some 5–10 0 C. below the melting-point of these waxes, the insect shows a suddenincrease in the rate of transpiration. If the cement and wax layer are abraded by fine dusts or removed bylipid solvents, the loss of water increases enormously. The properties of the epicuticular layers control tosome extent the entry of insecticides through the cuticle.In the deposition of the cuticle the cuticulin layer is first laid down; the lipoproteins which compose it appear to come from the oenocytes. Formation of the endocuticle takes place around cytoplasmic filaments(the  pore canals) which extend from the interior of the epidermal cells and appear to penetrate the cuticulinlayer of the epicuticle. Droplets of material rich in polyphenols are exuded from the tips of the pore canalsand fuse to form a continuous layer over the cuticulin. The wax is then secreted, during the last few hoursbefore moulting  , and covers the polyphenols. Almost immediately after moulting (in some insects beforemoulting) the cement is poured out from the dermal glands.  The inner layers of the old cuticle are digested by enzymes contained in the moulting fluid and probablysecreted by the general epidermis. The products are absorbed, together with almost all the fluid, beforemoulting occurs.Hardening and darkening take place after moulting as the result of the activities of the oxidative enzymes inthe cuticle which convert the dihydroxyphenols into quinones.The function of the pore canals is probably to enable the epidermal cells to act at a distance upon thesuperficial layers of the cuticle. In some insects they still contain cytoplasmic filaments in the fully formedcuticle; in others their contents are converted into chitin, into sclerotin, or a mixture of the two.The greater part of the endocuticle is laid down after moulting.The integument  of many insects remains alive. Removal of the wax and cement layers by gentle abrasion isrepaired by the secretion of fresh wax. And in some forms water can be actively absorbed from theatmosphere even when this is far from being saturated with moisture.METAMORPHOSIS Metamorphosis is a feature seen during the development of embryos belonging to many differentphyla. This feature entails an intermediate sexually immature feeding stage, which is commonlycalled a larva. Metamorphosis is the transition from the larva to the adult, and it is a complex andhighly regulated process. Metamorphosis has been well studied in insects and amphibians.An essential part of insect metamorphosis is molting wherein the larva sheds its external cuticleand attains a new exoskeleton. Molting is necessary because of the cuticular nature of theexoskeleton, which imposes a constraint on increase in size. Near the end of larval life, insect larvaesecrete an external case called the puparium, in which further development of the animal occurs.Pupal development entails a complex program of morphogeneis and differentiation during which acompletely new organ system different from that of the larva is laid down. The process of molting ininsects is dependent on neurosecretory circuits, which respond to a hormone called ecdysone . Theproduction of ecdysone is under the control of a peptide called prothoracotropic hormone (PTTH),which is secreted by the prothoracic gland. In response to ecdysone, epithelial cells of the bodysurface withdraw from the cuticle and produce a molting fluid containing proenzymes that, afteractivation, will digest the old cuticle. The epithelium then generates a new cuticle which isdistensible and will continue to expand as the larva grows in size, until such time that it is no longerable to expand due to a reduction in elasticity. At the end of the third-instar larval stage, a spike inecdysone levels leads to the formation of a puparium. Within the pupa, the development of the  adult organ system takes place. Most of the larval tissue undergoes apoptosis, and the tissuesdisintegrate by a process known as histolysis. The adult surface structures such as wings, halteres,legs, eyes, and antennae develop from sheets/pouches of epithelial cells which are set aside veryearly on in the embryo. These sheets/pouches of cells are called imaginal discs and undergoregulated proliferation during the larval stages and ultimately develop into the adult structures byway of extensive patterning mechanisms in the pupa. [fig 8.5 and 8.6]It is now known that ecdysone exerts its effects by influencing gene transcription. In Drosophila ,ecdysone on binding its receptor protein on the surface of a cell allows the receptor to form aheterodimer with a protein called ultraspiracle (Usp). Specific genes are then activated in responseto ecdysone receptor/Usp complex; proteins synthezised from these genes might in turn activateother genes, ultimately giving rise to the observed response.Amphibian metamorphosis involves a drastic change in physiology and habitat of the animal; theherbivore aquatic tadpole develops into a carnivorous amphibian adult. Metamorphosis in the frog isdriven by the secretion of thyroid hormones by the anterior pituitary. Absence of thyroid hormonesresults in the larvae remaining aquatic, and thus they do not metamorphose into adult frogs. Therelease of thyroid hormones is under the control of a number of hormones, such as thecorticosterone releasing hormone and the adrenocorticotropic hormone. Thyroid hormones stimulatethe pituitary to produce another hormone prolactin. Thyroid hormones along with prolactin result invarious effects such as resorption of tail tissue, and changes in the eye visual pigment from theaquatic to the terrestrial form. Furthermore, the timing of various metamorphic events is regulatedby threshold concentrations of thyroid hormones and receptors. It is now known that the thyroidhormone receptors are a downstream target of thyroid hormones, thus establishing a positivefeedback loop in the response pathway of thyroid hormones, ultimately leading to specific geneexpression.
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