Amphibian Metamorphosis | Zoology Optional Notes for UPSC PDF Download

Introduction

Amphibian metamorphosis is a fascinating process marked by significant morphological changes as these creatures transition from an aquatic to a terrestrial existence. Let's delve into the intricacies of this remarkable journey:

Urodeles (Newts and Salamanders)

• Resorption of the tail fin.
• Destruction of external gills.
• Alteration in skin structure.

Anurans (Frogs and Toads)

• More intricate changes involving every organ.
• Larval structures respond to thyroid hormones T4 and T3 in four distinct ways: growth, death, remodeling, and respecification.

Structural Developments

• Emergence of limbs, nictitating membranes, and eyelids.
• T3 induces the proliferation and differentiation of new neurons, crucial for the development of these organs.
• Eyes shift from a lateral to a frontal position.
• Ipsilateral pathways emerge, facilitating input from both eyes to reach the same brain area.

Cell Death during Metamorphosis

• T3-induced degeneration of the paddle-like tail and oxygen-procuring gills.
• Tail resorption initiated by apoptosis, with the last remnants being eliminated.
• Later in metamorphosis, tail muscles are destroyed by phagocytosis.
• Specific digestion of larval red blood cells by macrophages in the liver and spleen.

Remodeling during Metamorphosis

• Larval intestine transforms into a shorter form suitable for a carnivorous diet.
• Nervous system undergoes extensive remodeling as neurons grow and innervate new targets.
• Degeneration of the tadpole's lateral line system.
• Further differentiation of the ears, including the development of the middle ear and the characteristic tympanic membrane.

Biochemical Respecification

• Transition from ammonotelic to ureotelic processes.

Hormonal Control

• Metamorphic changes are attributed to:
  1. Secretion of thyroxine (T4).
  2. Conversion of T4 into the more active tri-iodothyronine (T3).
  3. Degradation of T3 in target tissues.

Role of Thyroid Hormones (T3 and T4)

• T3 binds to nuclear thyroid hormone receptors (TRs), essential in each tissue.
• Concentration of T3 depends on T4 and two crucial enzymes: Type II deiodinase and Type III deiodinase.

Receptor Types

• Two types: TRα and TRβ.
• TRα is widely distributed and present before the organism has a thyroid gland.
• TRβ is activated by hormones and plays a key role in gene transcription.

Phases of Metamorphosis

• Premetamorphosis:
  • Thyroid gland begins to mature, secreting low levels of T4.
  • Initiation of T4 secretion may be induced by corticotropin-releasing hormone (CRH).
  • Tissues responding earliest express high levels of deiodinase II.
• Prometamorphosis:
  • Rising levels of thyroid hormones induce higher levels of TRβ.
• Metamorphic Climax:
  • Deiodinase II is expressed, initiating tail resorption after functional limb development.
  • Frog brain undergoes changes to downregulate metamorphosis once the climax is reached.

The journey of amphibian metamorphosis is a complex and meticulously orchestrated transformation that showcases the wonders of biological adaptation and development.