METAMORPHOSIS IN AMPHIBIA CONTENTS INTRODUCTION OCCURRENCE OF METAMORPHOSIS

METAMORPHOSIS IN AMPHIBIA

CONTENTS • • • INTRODUCTION OCCURRENCE OF METAMORPHOSIS TYPES OF METAMORPHOSIS IN AMPHIBIA TIME OF METAMORPHOSIS HORMONAL CONTROL OF FROG METAMORPHOSIS MECHANISM OF HORMONE ACTION TISSUE REACTIVITY IN AMPHIBIAN METAMORPHOSIS NEOTONY IN URODELA

• Metamorphosis is an important phenomenon during Development in a number of animals which involves dramatic changes in habit , habitat , morphology , physiology and behaviour of larva so that it is transformed into an adult having entirely different habit and structure. • The radial changes which converts a larva into an adult is called metamorphosis. ( Metamorphoun - to transform ).

• Metamorphosis occur in phyla of invertebrates from sponges to echinoderms and in several chordates including amphibians . • Based on presence or absence of metamorphosis , two types of embryonic development is seen • Direct Development • Indirect Development

• DIRECT DEVELOPMENT • In many animals the hatched organism resembles an adult , except it is small in sizeand there is no larval stage. • Eg: planaria , ascaris , pheretima , reptiles , birds , mammals.

INDIRECT DEVELOPMENT • In many animals the egg develops into a different organism from the adult • . It leads to independent free life Known as larva then gradually changes into adult. • The larval stage is free living and feeds voraciously to store material for further development. Eg : Amphioxus , frog , sea urchin and insects.

TYPES OF METAMORPHOSIS RETROGRESSIVE METAMORPHOSIS • Metamorphosis is a change from the juvenile to adult stage in which larval stage is quite different from the adult stage. • In retrogressive metamorphosis the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters.

2. PROGRESSIVE METAMORPHOSIS • In progressive metamorphosis simple larval organization changes into more complex organization of the adult. • Eg: insects and amphibians.

METAMORPHOSIS IN AMPHIBIA • In annuran amphibians , the egg hatches into a larva called tadpole. • To become adult it undergoes metamorphosis . The changes undergone by tadpole can be divided into 3 categories. They are • 1. Ecological changes • 2. Morphological changes • 3. Physiological and biochemical changes

ECOLOGICAL CHANGES • These include changes in habitat and feeding habit • * The tadpole is an aquatic larva , the adult is a terrestrial being. so during metamorphosis there is a transtition from aquatic to terrestrial life • * Changes from aquatic mode of respiration ( by gills ) to terrestrial ( by lungs) mode of respiration . • * The tadpole is herbivorous but the adult is carnivorous.

MORPHOLOGICAL CHANGES • The changes that take place on the structural organization of the tadpole during metamorphosis are grouped into 3 categories • Regressive changes • Progressive changes • Remodelling

• REGRESSIVE METAMORPHIC CHANGES : Certain structures of the tadpole are not essential for the adult. Hence they degenerate and disappear. The following are the regressive changes in tadpoles 1. The tail and the fin folds are completely resorbed. 2. The gills are resorbed and Gill clefts are closed

4. The mouth changes it's shape and the horny teeth of the perioal disc disappear 5. The length of the cloacal tube is reduced 6. The lateral line sense organ and ventral suckers disappears

PROGRESSIVE METAMORPHIC CHANGES : Certain characters develop and become functional only during and after metamorphosis. These constitute progressive changes. 1. The limbs develop progressively and undergo differentiation 2. Head becomes board and triangular. 3. The middle ear , tympanic membrane and tympanic cartilage develops 4. The eyes protrude out and they develop eyelids and nictitating membrane

• 5. . The tongue develops from the floor of buccal cavity. • 6. The pronephros of the tadpole is replaced by a mesonephros. • 7. The Lungs develop for aerial breathing.

• REMODELLING OF EXISTING LARVAL STRUCTURES 1. The heart becomes three chambered and maintains isolation of oxygenated and deoxygenated blood. 2. In tadpole the blood vascular system is like that of fish. During metamorphosis it is modified to supply oxygen 3. In herbivore tadpole , the intestine is very long and spirally coiled like a watch spring. As it changes the intestine becomes shortened and straight as the tadpole changes into carnivorous adult.

• The tadpoles skin is thin and delicate and is quickly dehydrated on expore to air. During metamorphosis skin thickens and a protein called keratin deveops in the outer layer of skin and becomes less sensitive to evaporation and mechanical injury.

PHYSIOLOGICAL AND BIOCHEMICAL CHANGES • The ammonotelic larva are changes to ureotelic adult and as such the larval kidneys are replaced by mesonephric kidneys. • The islets of langerhans of pancreas start functioning to produce insulin

• As metamorphosis proceeds , the larval haemoglobin is replaced by adult haemoglobin and there is a steady increase in the amount of serum albumin in the blood. • The uptake of oxygen increases • The larval visual pigment , porphyropsin ( retinene -2 ) is replaced by rhodopsin or retinene -1 • Changes in the metabolism of carbohydrates, lipids and nitrogen occur. • Autolysis is the disappearance of larval gills , tails and other structures.

According to Grant (1978) the sequence of events during anuran metamorphosis is divided into 3 distinct periods 1. Pre metamorphic period - this is first phase which is marked by tadpole growth with little morphogenic change except for hindlimb growth 2. Post metamorphic period- during this phase the body grows at reduced rate as hindlimbs grow. The ratio of hindlimb length to body length increases rapidly

• Metamorphic climax - this is the third and most dramatic phase and is marked by cessation of growth accompanied by rapid changes in limbs , tail , head and internal organs. TIME OF METAMORPHOSIS • Frogs metamorphose after various periods of growth according to the species. The ameriy bullfrog tadpole metamorphose at the end of the third summer growth season in the north but at the end of the second summer growth season in the south.

• It has also been known that addition of iodine to the water or feeding with thyroid gland tissue causes metamorphosis to occur earlier.

HORMONAL CONTROL OF FROG METAMORPHOSIS • Metamorphosis in amphibians is initiated by thyroid hormone , thyroxine and tri- iodothyronine • Gudernatsch(1912) when he fed some frog tadpoles on dried and powdered sheep thyroid gland found that they metamorphosed. Allen (1918) removed thyroid rudiment in frog tadpole at tail bud stage. The thyriodless tadpole showed normal growth and attained much larger size but failed to metamorphose.

• When these thyroid less tadpole were immersed in water containing soluble extracts from thyroid gland they underwent metamorphosis. This proves that metamorphosis is initiated only with thyroid gland.

• The amphibian metamorphosis is under neuroendocrine control, involving neurosecretory cells in the brain (the hypothalamus) and two endocrine glands, the pituitary (anterior pituitary) and the thyroid. • The trigger to metamorphosis may be an environmental signal affecting the larval brain through the nervous system, or there may be an endogenous ‘clock’ in the hypothalamus.

• Hypothalamus integrates the information received from body with the environmental information. • Neurosecretory cells in the hypothalamus are stimulated to produce TRF or thyroid-releasing factor which stimulates the anterior pituitary gland to secrete a TSH or thyroid-stimulating hormone which causes orderly increase of thyroid secretion. • Increase in thyroid hormone then trips the orderly sequence of tissue changes that transforms the tadpole larva into the frog.

• Another pituitary hormone, called prolactin is also found to be involved as an inhibitor. • The overall control of metamorphosis is the interaction of TSH and prolactin. • During premetamorphosis period TSF increases by the action of hypothalamus. • TRF induces secrection of thyroxine • prolactin acts as an antagonist to TSH. The metamorphosis is a result of dynamic balance of these two hormones.

TISSUE REACTIVITY IN AMPHIBIAN METAMORPHOSIS • The entire process of metamorphosis is stimulated by the thyroid hormone. • In the presence of the hormone , the tissue respond in different ways. While some tissues. (tail , gills, etc. ) degenerate and disappear , the remaining tissues ( limbs, liver, erc. ) grow and differentiate under the influence of the same hormone

• When a small pellet of thyroxine is implanted in the tail of a tadpole it causes tissue- resorption. • When the Pellet is embedded in a hindlimb, it induces growth and not dissolution. • Parts of a tadpole’s tail transplanted to the trunk undergo metamorphosis together with the host’s tail and become absorbed. • On the other hand, an eye transplanted to the tail of a tadpole before metamorphosis remains healthy, while all the surrounding and underlying tissues undergo necrosis.

• As the tail shrinks, the eye is brought nearer to the trunk and eventually becomes fused to the body in the sacral region after the entire tail has disappeared. NEOTENY • In the Amblystoma (urodele) the animal sometimes becomes sexually mature in the larval condition and does not metamorphose. • This phenomenon of the retention of larval characters in the sexually mature state has been called neoteny or paedogenesis • Inneoteny the terrestrial phase is eliminated, the aquatic larva becomes sexually mature. The larval stage is often considered as a survival of the ancestral fish type.