Metamorphosis in Amphibians Instructor: Dr. Shahzad A. Mufti Advisor Department of Biosciences

Metamorphosis in Amphibians In many animals, embryonic development…………. larval stage, Important for growth and dispersal, Morphological, physiological and biochemical chnages, February 19, 2019

Metamorphosis in Amphibians Regressive: teeth, gills, tail(caspace-9 imp. In apoptosis). Progressive: limbs, bony skull, tongue muscles. Modifications: intestine, lungs, eyes & ear, new neural development in eyes etc, Haemoglobin February 19, 2019

Metamorphosis in Amphibians Biochemical Changes: retinal photopigment changes from Porphyropsin to Rhodopsin, hemoglobin, Liver enzymes: ammonotelic to ureotelic excretion. February 19, 2019

Metamorphosis in Amphibians Hormonal Control: Thyroid hormone (Gudernatsch, 1912). Allen(1916) experiments, different concentrations of Thyroid hormones….Systemic changes; Threshold Model. Thyroxin(T4), Tri-iodothyronine(T3). T4 into T3 & T2 by type II & III de-iodinase. February 19, 2019

Metamorphosis in Amphibians T3 binds to nuclear Thyroid Receptors (TRs) Two types of TRs, as TR & TR. Premetamorphosis, Corticotropin Releasing Hormone (CRH) acts on pituitary release of Thyroid Stimulating Hormone (TSH). Limbs first to react, by converting T4 to T3, through TR. February 19, 2019

Metamorphosis in Amphibians Prometamorphosis and Metamorphosis Climax: TR distributed widely, so tail reacts, and so on. Then with brain, so negative feedback through pituitary…..less and less of hormones. February 19, 2019

Metamorphosis in Amphibians So gradual, regionally specific responses, involving proliferation, apoptosis and migration of cells etc. Organ specific responses shown through transplantation experiments, of tail tip and eye cup. February 19, 2019

Table 18.1. Summary of some metamorphic changes in anurans System Larva Adult Locomotory Aquatic; tail fins Terrestrial; tailless tetrapod Respiratory Gills, skin, lungs; larval hemoglobins Skin, lungs; adult hemoglobins Circulatory Aortic arches; aorta; anterior, posterior, and common jugular veins Carotid arch; systemic arch; cardinal veins Nutritional Herbivorous: long spiral gut; intestinal symbionts; small mouth, horny jaws, labial teeth Carnivorous: Short gut; proteases; large mouth with long tongue Nervous Lack of nictitating membrane; porphyropsin, lateral line system, Mauthner's neurons Development of ocular muscles, nictitating membrane, rhodopsin; loss of lateral line system, degeneration of Mauthner's neurons; tympanic membrane Excretory Largely ammonia, some urea (ammonotelic) Largely urea; high activity of enzymes of ornithine-urea cycle (ureotelic) Integumental Thin, bilayered epidermis with thin dermis;no mucous glands or granular glands Stratified squamous epidermis with adult keratins; well-developed dermis contains mucous glands and granular glands secreting antimicrobial peptides Source: Data from Turner and Bagnara 1976 and Reilly et al. 1994. February 19, 2019

Formulae of thyroxine (T4) and triiodothyronine (T3). February 19, 2019

Hypothalamus & Pituitary Gland

Circulating T4 converted to T3 in cells

X T3/T4 Dopamine -- [TRbeta] peaks -- Metamorphic Climax PO ME TRH TSH PRL X [TRbeta] peaks -- T3/T4 Metamorphic Climax

Differential & Temporal Tissue Sensitivity (Schwind 1933) Differential & Temporal Tissue Sensitivity Fig. 18.5, pg. 561