Chapter Summary

Metamorphosis is a feature seen during the development of embryos belonging to many different phyla. This feature entails an intermediate sexually immature feeding stage, which is commonly called a larva. Metamorphosis is the transition from the larva to the adult, and it is a complex and highly 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 cuticle and attains a new exoskeleton. Molting is necessary because of the cuticular nature of the exoskeleton, which imposes a constraint on increase in size. Near the end of larval life, insect larvae secrete 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 a completely new organ system different from that of the larva is laid down. The process of molting in insects is dependent on neurosecretory circuits, which respond to a hormone called ecdysone. The production 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 body surface withdraw from the cuticle and produce a molting fluid containing proenzymes that, after activation, will digest the old cuticle. The epithelium then generates a new cuticle which is distensible and will continue to expand as the larva grows in size, until such time that it is no longer able to expand due to a reduction in elasticity. At the end of the third-instar larval stage, a spike in ecdysone 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 tissues disintegrate 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 very early on in the embryo. These sheets/pouches of cells are called imaginal discs and undergo regulated proliferation during the larval stages and ultimately develop into the adult structures by way 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 a heterodimer with a protein called ultraspiracle (Usp). Specific genes are then activated in response to ecdysone receptor/Usp complex; proteins synthezised from these genes might in turn activate other genes, ultimately giving rise to the observed response.

Amphibian metamorphosis involves a drastic change in physiology and habitat of the animal; the herbivore aquatic tadpole develops into a carnivorous amphibian adult. Metamorphosis in the frog is driven by the secretion of thyroid hormones by the anterior pituitary. Absence of thyroid hormones results in the larvae remaining aquatic, and thus they do not metamorphose into adult frogs. The release of thyroid hormones is under the control of a number of hormones, such as the corticosterone releasing hormone and the adrenocorticotropic hormone. Thyroid hormones stimulate the pituitary to produce another hormone prolactin. Thyroid hormones along with prolactin result in various effects such as resorption of tail tissue, and changes in the eye visual pigment from the aquatic to the terrestrial form. Furthermore, the timing of various metamorphic events is regulated by threshold concentrations of thyroid hormones and receptors. It is now known that the thyroid hormone receptors are a downstream target of thyroid hormones, thus establishing a positive feedback loop in the response pathway of thyroid hormones, ultimately leading to specific gene expression.

Chapter Summary

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