Lecture 12: Metamorphosis

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Questions and Answers

What is the primary characteristic of metamorphosis in animals?

It is a rapid change from one developmental state to another. (correct)

It is a gradual process involving minimal changes.

It includes continuous development with no distinct stages.

It primarily occurs during the embryonic stage.

Which statement accurately describes the anatomical changes during amphibian metamorphosis?

Gills are kept for terrestrial breathing.

The tail is resorbed while limbs and lungs develop for a semi-terrestrial lifestyle. (correct)

The limbs develop while the tail remains fully functional.

Tadpoles completely retain their larval organs into adulthood.

What type of change does apoptosis during metamorphosis entail?

Resorption of larval structures, such as gills and tails. (correct)

Complete replacement of adult structures by larval tissues.

Growth of new adult organs without resorption.

Differentiation of larvae into adults without tissue alterations.

Why do adult frogs require structural changes from tadpoles?

To adapt from a fully aquatic to a semi-terrestrial lifestyle. Signup and view all the answers

Which of the following is NOT a characteristic of metamorphosis?

Involves gradual growth without drastic changes. Signup and view all the answers

During which stage of amphibian development do hind limbs first begin to develop?

Tadpole stage. Signup and view all the answers

What role does tissue remodelling play in metamorphosis?

It alters retained organs significantly for adult functionality. Signup and view all the answers

What role does TH play in limb development during Xenopus metamorphosis?

TH is a critical regulator that influences nerve cell differentiation directly. Signup and view all the answers

What is the role of the Dominant Negative Thyroid Receptor (DNTR) in the experiments conducted?

DNTR blocks TH effects specifically in the cell type it is expressed. Signup and view all the answers

What was observed when DNTR was specifically expressed in nerve cells?

The tadpole had normal limbs but experienced paralysis due to disrupted nerves. Signup and view all the answers

Which statement correctly describes the impacts of TH on muscle cells?

TH directly affects muscle cell development leading to the absence of muscles. Signup and view all the answers

What conclusion can be drawn about TH's influence on limb cell types during metamorphosis?

Each major limb cell type is under the cell autonomous control of TH. Signup and view all the answers

What role does the hypothalamus play in amphibian metamorphosis?

It responds to environmental cues by releasing corticotropin-releasing hormone (CRH). Signup and view all the answers

Which hormone released by the pituitary gland is primarily responsible for promoting amphibian metamorphosis?

Thyroid-stimulating hormone (TSH) Signup and view all the answers

What is the active form of the thyroid hormone involved in amphibian metamorphosis?

T3 (triiodothyronine) Signup and view all the answers

How is T4 converted to T3 in target tissues?

By the activity of an enzyme called diodinase, which removes one iodine group. Signup and view all the answers

Which environmental cues influence the hypothalamus to regulate metamorphosis?

Temperature, light, and photo period. Signup and view all the answers

What role does prolactin play in the process of amphibian metamorphosis?

It tends to delay the metamorphic changes. Signup and view all the answers

What is the combined designation of thyroid hormones T3 and T4?

T8 Signup and view all the answers

Which statement about thyroid hormones and their feedback mechanism is accurate?

T3 and T4 provide feedback to the brain to sustain hormone release. Signup and view all the answers

What is the primary structure responsible for the production of thyroid hormones during metamorphosis?

Thyroid gland Signup and view all the answers

What triggers the pro-metamorphic changes in amphibians?

Gradual rise in thyroid hormone levels Signup and view all the answers

Which phase comes after the pro-metamorphosis phase during amphibian development?

Climax phase Signup and view all the answers

What significant event occurs during the climax phase of metamorphosis?

Gill resorption and organ remodeling Signup and view all the answers

How does the thyroid gland change during the climax phase?

It reaches its largest size Signup and view all the answers

What happens to thyroid hormone levels after the climax phase?

They drop significantly Signup and view all the answers

What is indicated by substantial radioactive iodine uptake?

Activation of the thyroid gland Signup and view all the answers

At which stage is the thyroid gland's activity at its peak?

Climax phase Signup and view all the answers

What characterizes the post-metamorphosis phase?

Final transformation into an adult frog Signup and view all the answers

What is the primary physiological change in limbs during the pro-metamorphosis phase?

Formation of limb buds Signup and view all the answers

Which of the following events occurs as thyroid hormone levels peak?

Advanced gill resorption Signup and view all the answers

What is the result of increased transcription of the TR gene?

Increased levels of TH and TR Signup and view all the answers

What role does the Dominant Negative Thyroid Receptor (DNTR) play during metamorphosis?

It binds to DNA but inhibits activation Signup and view all the answers

What happens to wild type tadpoles when given thyroid hormone (T3)?

They regress their tails and produce collagenase Signup and view all the answers

Which of the following describes the direct effect of T3 on epithelial cells?

It induces apoptosis in some epithelial cells Signup and view all the answers

How does T3 indirectly affect epithelial cells during metamorphosis?

By acting on fibroblasts to signal apoptosis Signup and view all the answers

What is a consequence of having a Dominant Negative mutation in the thyroid receptor?

Failure to regress tails despite hormone presence Signup and view all the answers

What is the effect of high levels of TH and TR on gene activation?

Activation of further gene transcription Signup and view all the answers

What does the recruitment of co-activators during metamorphosis facilitate?

Stimulation of transcription of metamorphic genes Signup and view all the answers

What characterizes the condition of both wild type and Dominant Negative Tadpoles when exposed to T3?

Only wild type regress their tails Signup and view all the answers

Which statement accurately depicts the role of thyroid hormone in intestine remodeling during metamorphosis?

It induces apoptosis in epithelial cells Signup and view all the answers

Study Notes

Metamorphosis in Animals

Metamorphosis is a rapid change in form from one developmental stage to another
It is a post-embryonic process, where a larval or immature stage transforms into an adult stage
Metamorphosis involves dramatic tissue remodeling
Larval tissues and organs are completely replaced by adult structures
Examples include amphibians (tadpole to frog), insects (butterfly metamorphosis from larval stage)

Metamorphosis in Amphibians

Amphibian metamorphosis involves distinct stages
Stages of amphibian metamorphosis are not detailed

Hormonal Control of Amphibian Metamorphosis

Metamorphosis is regulated by both hormonal and environmental factors
Environmental cues (temperature, light, photoperiod) signal the hypothalamus in the brain
Hypothalamus releases corticotropin-releasing hormone (CRH)
CRH stimulates the pituitary gland to release Thyroid-stimulating hormone (TSH)
TSH activates the thyroid gland, which then releases thyroid hormones T3 and T4
T3 and T4 initiate and promote metamorphosis
These hormones also provide feedback to the brain, sustaining the metamorphic process

Thyroid Hormones and Metamorphosis

Two main types of thyroid hormones are T3 (triiodothyronine) and T4 (thyroxine)
T3 is the active form, while T4 serves as a prohormone
Mammals convert T4 to T3 in target tissues
Iodine is crucial for proper thyroid function; deficiency can cause goiter
T3 and T4 regulate the metamorphic process in amphibians

Thyroid Hormone Receptors and Action

Thyroid hormone receptors (TRs) are DNA-binding transcription factors
TRs form a heterodimer complex with retinoid X receptors
In the absence of thyroid hormone, TRs are complexed with repressors
In the presence of thyroid hormone, T3 recruits co-activators, changing TRs from repressors to activators
Activation leads to histone acetylation, leading to transcription

Thyroid Hormone Effects During Metamorphosis

Pre-metamorphosis: low TSH, low TH, low TRs, leading to repression of TR gene activation
Pro-metamorphosis: environmental input increases TSH, TH, and TR production
Climax: high TSH, TH, and TR, further activating genes needed for metamorphosis
Dominant Negative TR Blocks Metamorphosis

Role of Thyroid Hormone in Regulating Intestinal Remodeling

Thyroid hormone induces intestinal remodeling directly and indirectly affecting epithelial cells
Thyroid hormone directly induces apoptosis in some epithelial cells and de-differentiates others into stem cells
It indirectly affects fibroblasts in connective tissue, influencing epithelial cells to undergo apoptosis or de-differentiation

Cell Autonomous Actions of Thyroid Hormone

Different body regions respond to different amounts of thyroid hormone and thyroid hormone receptors at varying times
The type of response depends on intrinsic factors within the cells
Examples include apoptosis (cell death), proliferation (cell growth), and differentiation (development of structures)

Programmed Cell Death during Metamorphosis

During metamorphosis, the thin skin of tadpoles is replaced by thicker, water-resistant adult skin via TH-regulated programmed cell death
Larval skin expresses larval keratin, while adult skin expresses adult keratin
The thin larval skin undergoes apoptosis
Epidermal cells develop into adult skin

Gut Re-Modeling During Metamorphosis

The larval intestine is convoluted and coiled with one single fold
It changes to a shorter, non-coiled structure with folds during the climax stage
These changes are orchestrated by thyroid hormones

TH Controls Limb Development during Metamorphosis

TH controls limb development in Xenopus
TH antagonist (methimazole) blocks both metamorphosis and limb development
TH effects are cell autonomous, meaning it directly affects the cell

Understanding the Concept of Gene Switching During Metamorphosis

During metamorphosis, cells switch from expressing larval genes to adult genes
This process is known as gene switching and is the reversible on/off switching of gene expression
An example is fibroblasts in the tail, initially producing collagen but switching to collagenase under TH influence

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Description

Test your knowledge on the intricate processes of amphibian metamorphosis. This quiz covers anatomical changes, the role of thyroid hormones, and key developmental stages in amphibians such as frogs. Dive into the transformative journey from tadpole to adult frog and explore the significance of tissue remodeling and apoptosis.