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

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

3. 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

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

5. 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

6. 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

7. 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

8. 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

9. 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
February 19, 2019

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

12. Hypothalamus & Pituitary Gland

13. Circulating T4 converted
to T3 in cells

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

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