1. Ecological changes during metamorphosisv4

2. 1. The ecological changes are associated with the change in the aqutic to terresterial mode of life.
2.There is a change in the feeding habit the tadpole scraps off vegetable matter from submerged plants with horny teeth,stops feeding till it is able to prey upon animal food.
3.Change in respiration from gills to lungs.

3. The tadpole frequently rises to the surface to gulp air and inflate the lungs.
A good part of oxygen is also available by glandular skin. The miniature frog begins to take short excursions on the land but not far from the ancestral home the water.
The metamorphosis in anurans is rapid as compared to slamanders urodela.
The temperature required is 25 degrees.faster in warmer water than cooler water.

4. Time and causes of Metamorphosis.
Frog about 2 months for metamorphosis.
The changes are due to a hormone from Thyroid gland.
Gudernatsch 1912 fed frog tadpoles on powdered sheep thyroid and observed they promptly metamorphosed promptly into little frogs.
Allen 1918 surgically removed thyroid in tail bud stage the operated tadpoles showed normal growth but failed to metamorphose.

5. Such tadpoles when supplied with thyroid hormone either by feeding them on dried thyroid or immerssing them in water containing soluble thyroid glands they immediately started metamorphosis Allaen 1938.The thyroid glands of the same animal are not necessay.
Triiodothyronine and Tetraiodothyrynine T3 anT4 are active compounds responsible.
Iodine is important constituent of T3 and T4

6. Circulating T4 converted
to T3 in cells

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

8. Mechanism of Amphibian Metamorphosis
Etkin in 1966 showed that the thyroid hormone concentration in the blood and tissues of tadpoles increases during 2/3rd upon life cycle and drops after metamorphosis.
TRF-TSH-Thyroxine Hormone-Tadpole Tissue.-Metamorphosis. Degeneration and histolysis and growth and histogenesis.

10. Tissue Reactivity in Metamorphosis
The effects of T3 and T4 are both constructive and destructive depending on target tissues.
The tissues of tail and gills become necrotic and undergo degeneration.While the tissues of limbs increase and undergo differentiation.
The response and reactivity of larval tissue is very different to same agent.
This can be demonstrated by experiments that reactivity of tissues is tissue specific.

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

12. For example parts of tadpole if transplanted to the trunk of the tadpole undergoing metamorphosis undergoes necrosis along with the host tail shrinks and degenerates.
If an eye of a tadpole is transplanted to the tail of tadpole before metamorphosis the tail shrinks and degenerates but eye remains normal and is brought nearer to the trunk and finally comes to lie in the sacral region.
Threshold values
Molecular mechanisms.