Developmental Biology Lesson 28

Developmental Biology - Lesson 28

• Metamorphosis: A dramatic, hormone-triggered transition that reactivates development, transforming the animal's form, often involving changes in habitat, food, and behavior.

Categorization of Animals Based on Development

• Direct Developers: Juveniles resemble smaller, less sexually mature versions of the adult.
• Indirect Developers: Undergo a distinct larval stage before metamorphosis to the adult form.

Types of Larvae

• Primary Larvae: Have a body plan distinct from the adult (e.g., sea urchin larva vs. adult).
• Secondary Larvae: Share the same basic body plan as the adult but modify and add structures during development (e.g., caterpillar to butterfly, tadpole to frog).

Metamorphosis in Animals

• Metamorphosis can result in adults that are morphologically, physiologically, and behaviorally distinct from larvae. Examples include Cecropia moths, whose caterpillars feed extensively while adults do not eat and live only for reproduction.
• The molecular mechanisms of metamorphosis remain largely unexplored and are understood only in a few species.

Metamorphic Changes in Anurans

• Locomotory System: Aquatic larvae with tail fins transform into terrestrial adults without tails.
• Respiratory System: Larvae have gills, skin, and lungs; adults have lungs.
• Circulatory System: Larvae have aortic arches; adults have carotid and systemic arches.
• Nutritional Adaptations: Larvae are herbivores, while adults are carnivores.
• Nervous System: Larvae have a simpler nervous system than adults.
• Excretory System: Larvae have a simpler excretory system than adults.
• Integumentary System: Larvae have a thin, bilayered epidermis; adults have a stratified squamous epidermis.

Eye Migration and Neuronal Changes During Metamorphosis (Xenopus laevis)

• Eye Position: Tadpole eyes are laterally placed, providing minimal binocular vision. During metamorphosis, eyes migrate dorsally and rostrally, creating a large binocular field in the adult frog.
• Retinal Projections: Early metamorphosis: axons project contralaterally across the midline. Late metamorphosis: Ephrin B in the optic chiasm induces the formation of neurons projecting ipsilaterally (on the same side), enabling binocular vision.

Changes in the Xenopus Skull During Metamorphosis

• Early Stage: Prominent pharyngeal arch cartilage (branchial arch), Meckel's cartilage (at the tip of the head), and ceratohyal cartilage (wide and anteriorly positioned).
• Progression: Pharyngeal arch cartilage disappears, and Meckel's cartilage elongates, forming the mandible.

Metabolism of Thyroxine (T4) and Tri-iodothyronine (T3) - Hormonal Control of Amphibian Metamorphosis

• Early Discoveries: Feeding tadpoles thyroid glands induced premature metamorphosis. Removing thyroid rudiments prevented metamorphosis, resulting in giant tadpoles.
• Thyroid Hormone Regulation: Increasing thyroid hormone levels trigger metamorphic stages: low levels for early events (e.g., limb growth) and high levels for later events (e.g., tail resorption).
• Key Hormonal Processes: T4 (thyroxine) is secreted as a precursor; T4 is activated to T3 (tri-iodothyronine) by Type II deiodinase in tissues; T3 is inactivated to T2 by Type III deiodinase; T3 binds to nuclear receptors, activating tissue-specific gene expression essential for metamorphosis.

Hormonal Control of Xenopus Metamorphosis

• T3 Levels and Repression: During early stages (embryonic and premetamorphic), T3 concentrations are low. T3 and other hormone receptor complexes can stabilize chromatin to prevent some transcription.
• Metamorphic Climax: T3 levels peak, forming a co-activator complex with TRa. This activates T3-sensitive genes, including TRβ, further amplifying the metamorphic response.
• Feedback and Decline: Post-climax, feedback inhibition lowers T3 levels, ending metamorphic processes.
• Gene Activation: Metamorphosis transitions from TR/T3-independent activation to T3-dependent activation, driving stage-specific gene expression.

Regional Specificity During Frog Metamorphosis

• T3 and TR Regulation: Regions of the tadpole body regulate T3 (tri-iodothyronine) and thyroid hormone receptors (TRs) to respond differently to thyroid hormones.
• Example: T3 stimulates limb muscle growth but induces tail muscle apoptosis.
• Tail Degeneration: Tail tissue rapidly undergoes apoptosis and is digested by macrophages using proteolytic enzymes.
• Tail-Specific Responses: Response to thyroid hormones is intrinsic to the tissue, not its location.

Modes of Insect Development

• Ametabolous Development: Direct growth through molting; insects resemble small adults after a brief pronymph stage.
• Hemimetabolous Metamorphosis: Gradual transformation; nymphs resemble immature adults, developing wings and genital organs with each molt.
• Holometabolous Metamorphosis: Complete transformation; larvae grow through molts (instars), become pupae, and emerge as adults during eclosion.

Locations and Developmental Fates of Imaginal Discs and Imaginal Tissues

• Imaginal Discs: Form adult cuticular structures (e.g., wings, legs, antennae, eyes, and genitalia). These are dormant in larvae.
• Programmed Cell Death: Most larval tissues degenerate as imaginal cells differentiate into adult organs.
• Types of Imaginal Cells: Imaginal discs, histoblasts, and organ-specific imaginal cells. These proliferate and replace degenerating larval organs, forming adult structures.

Imaginal Disc Elongation

• Limited Mitosis in Larval Cells: Most larval cells divide minimally, while imaginal discs undergo rapid and timed mitosis.
• Tubular Epithelium Formation: Imaginal discs fold into compact spirals as their cells proliferate. Metamorphic changes: imaginal disc cells proliferate, elongate, and differentiate to form adult structures.
• Metamorphic Changes: At metamorphosis, imaginal disc cells proliferate further, elongate, and differentiate to form adult structures.

Sequence of Leg Imaginal Disc Development in Drosophila

• Embryonic Stage: Leg disc type is specified.
• Larval Stage: Disc cells proliferate and are specified into different types of leg cells.
• Prepupal Stage: Disc elongates to form the leg structure.
• Pupal Stage: Leg tissues differentiate into final structures (e.g., basitarsus and tarsal segments).

Regulation of Insect Metamorphosis

• Hormones Involved: Juvenile Hormone maintains larval state, precursor to 20-hydroxyecdysone (20E), the active molting hormone; 20-Hydroxyecdysone + Juvenile Hormone triggers molting, low juvenile hormone with 20E stimulates molting to form pupa. 20E Alone triggers molting for the adult insect.
• Molting Pathway: 20E + JH induces molts to form the next larval instar; low JH + 20E triggers molting to form a pupa; 20E alone initiates imaginal disc differentiation and molting to form an adult.

Molecular Trinity Hypothesis for Holometabolous Insect Metamorphosis

• Key Transcription Factors: Chinmo promotes larval stage, Broad regulates pupal stage, and E93 activates adult stage.
• Mutual Inhibition: Each factor inhibits the others to maintain stability in larva, pupa, or adult stage.
• Stage Progression: Hormones and growth factors provide epigenetic cues to shift the regulatory network, enabling transitions between stages.

Compartmentalization and Anterior-Posterior Patterning in the Wing Imaginal Disc

• Anterior-Posterior Axis: Engrailed gene in the posterior compartment activates the hedgehog gene. Hedgehog acts as a short-range paracrine factor, activating decapentaplegic (dpp) in anterior cells.
• BMP Signaling Gradient: Dpp and Glass-bottom boat (Gbb) create a concentration gradient, measured by phosphorylated Mad (pMad).
• Threshold Activation: High Dpp + Gbb activates spalt (sal) and optomotor blind (omb); lower levels activate omb only. Defining peripheral regions.
• Wing Vein Markers: Gradients specify wing veins L2-L5.

Determining the Dorsal-Ventral Axis

• Vestigial and Apterous Proteins: Vestigial (green) marks the ventral surface, while Apterous (red) marks the dorsal surface.
• Wingless protein: Expressed at the dorsal-ventral boundary, organizes the wing disc along this axis; induces Vestigial expression in nearby cells, promoting growth and differentiation.
• Two-Layered Wing Formation: Dorsal and ventral portions telescope outward to form the double-layered wing guided by gene expression.

Explore Lesson 28 of Developmental Biology focusing on metamorphosis and the categorization of animals. Learn about direct and indirect development, types of larvae, and the significant changes that occur during metamorphosis. This lesson highlights fascinating examples and the life cycles of various species.