Two major classes of hormones act together to control amphibian metamorphosis: the thyroxin from thyroid gland and prolactin from pituitary gland. Thyroid hormones function somewhat like the molting hormones of insects, in that an increase of their concentration relative to prolactin leads to metamorphosis of the larva into the adult. Prolactin functions somewhat like the juvenile hormones of insects, in that it tempers the action of the thyroid hormones. In most species, thyroid hormones increase dramatically in concentration during metamorphosis and this stimulates resorption of certain larval organs and differentiation of new adult organs.
The molting process is initiated in the brain, where neurosecretory cells release prothoracicotropic hormone (PTTH) in response to neural, hormonal or environmental factors. PTTH is a family of peptide hormones with a molecular weight of approximately 40,000, and it stimulates the production of ecdysone by the prothoracic glands. Ecdysone, however, is not an active hormone, but a prohormone that must be converted into an active form. This conversion is accomplished by a heme-containing oxidase in the mitochondria and microsomes of peripheral tissues such as the fat body. Here the ecdysone is changed to the active hormone 20-hydroxyecdysone.
The second major effector hormone in insect development is juvenile hormone(JH). JH is secreted by the corpora allata. This hormone is responsible for preventing metamorphosis. As long as JH is present, the hydroxyecdysone-stimulated molts result in a new larval instar. In the last larval instar, the medial nerve from the brain to the corpora allata inhibits the gland from producing juvenile hormone, and there is a simultaneous increase in the body’s ability to degrade existing JH (Safranek and Williams 1989).
Both these mechanisms cause JH levels to drop below a critical threshold value. This triggers the release of PTTH from the brain. PTTH, in turn, stimulates the prothoracic glands to secrete a small amount of ecdysone. The resulting hydroxyecdysone, in the absence of JH, commits the cells to pupal development. Larval-specific mRNAs are not replaced, and new mRNAs are synthesized whose protein products inhibit the transcription of the larval messages.
After the second ecysone pulse, new pupal-specific gene products are synthesized and the subsequent molt shifts the organism from larva to pupa. It appears, then, that the first ecdysone pulse during the last larval instar triggers the processes that inactivate the larva-specific genes and prepare the pupa-specific genes to be transcribed. The second ecdysone pulse transcribes the pupa-specific genes and initiates the molt.
Majority of metamorphoses are progressive in which larvae possess primitive characters and as they attain adulthood they develop advanced characters, as seen in amphibians and majority of insects. But sometimes larvae possess advanced characters such as locomotory organs and sense organs but as they metamorphose into adult, these advanced features degenerate and adult develops primitive features. This type of metamorphosis is called retrogressive and is generally found in sedentary and parasitic animals, which do not require advanced features for their mode of life.
Herdmania is a urochordate sedentary animal whose larva is tadpole-like, with a notochord in tail, a dorsal hollow nervous system, pharyngeal gill slits, sense organs in the form of statocyst and ocellus and muscle bundles, which are all advanced chordate characters. As the larva attaches to a rock and undergoes metamorphosis, all chordate characters are lost except for the pharyngeal gill slits. However, the adult attains advancement in digestive system, circulatory system, reproductive and excretory systems, which are useful in adult life.
Parasitic animals such as helminthes, crustancean parasites such as Sacculina, and sedentary animals such as scale insects and lac insects, barnacles have an active larval stage that is necessary for dispersal of the species, but as they metamorphose into adult, advanced characters are lost as they are not needed in sedentary way of life.