BIOL 346 Developmental Biology Lesson 28 PDF

BIOL 346 Developmental Biology Lesson 28 Chapter 23: Metamorphosis: The Hormonal Reactivation of Development  Metamorphosis Metamorphosis is a dramatic, hormone-triggered transition that reactivates development, transforming the ani...

BIOL 346 Developmental Biology Lesson 28 Chapter 23: Metamorphosis: The Hormonal Reactivation of Development  Metamorphosis Metamorphosis is a dramatic, hormone-triggered transition that reactivates development, transforming the animal’s form and often involving changes in habitat, food, and behavior. Animals are categorized as: 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 with no resemblance). 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 can result in adults that are morphologically, physiologically, and behaviorally distinct from larvae (e.g., 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. Table 23.1 Some metamorphic changes in anurans System Larva Adult Locomotory Aquatic; tail fins Terrestrial; tailless tetrapod Respiratory Gills, skin, lungs; larval hemoglobins Skin, lungs; adult hemoglobins Circulatory Aortic arches; aorta; anterior, posterior, and Carotid arch; systemic arch; cardinal veins common jugular veins Nutritional Herbivorous; long spiral gut; intestinal Carnivorous; short gut; proteases, large mouth with symbionts; small mouth, horny jaws, labial long tongue teeth Nervous Lack of nictitating membrane; porphyropsin, Development of ocular muscles, nictitating lateral line system, Mauthner neurons membrane, tympanic membrane; rhodopsin; lateral line system lost, Mauthner neurons degenerate Excretory Largely ammonia, some urea (ammonotelic) Largely urea; high activity of enzymes of ornithine- urea cycle (ureotelic) Integument Thin, bilayered epidermis with thin dermis; Stratified squamous epidermis with adult keratins; no mucous or granular glands well-developed dermis contains mucous and granular glands secreting antimicrobial peptides : Data from C. Turner and J. T. Bagnara. 1976. General Endocrinology. Saunders: Philadelphia; D. S. Source Reilly et al. 1994. Dev Biol 162: 123–133. Fig. 23.1: Eye migration and associated neuronal changes during metamorphosis of a Xenopus laevis tadpole 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. Fig. 23.2: Changes in the Xenopus skull during metamorphosis Early Stage: Prominent pharyngeal arch cartilage (branchial arch, open arrowheads). Meckel’s cartilage (arrows) is at the tip of the head. Ceratohyal cartilage (arrowheads) is wide and anteriorly positioned. Progression: Pharyngeal arch cartilage disappears. Meckel’s cartilage elongates, and the mandible forms around it. Fig. 23.3: 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: T3 binds to nuclear receptors, activating tissue-specific gene expression essential for metamorphosis. Fig. 23.4: Hormonal control of Xenopus metamorphosis T3 Levels and Repression: During early stages (embryonic and premetamorphic), T3 concentrations are low. Thyroid hormone receptor (TRα) forms a co-repressor complex, stabilizing chromatin and preventing transcription. Metamorphic Climax: T3 levels peak, forming a co-activator complex with TRα. This activates T3- sensitive genes, including TRβ, further amplifying the metamorphic response. _Feedback and Decliine______________________: 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. Fig. 23.5: 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 (e.g., collagenases). Tail-Specific Responses: Response to thyroid hormones is intrinsic to the tissue, not its location. Example: Tail tips regress even when transplanted to the trunk, but eye cups remain intact in the degenerating tail. Fig. 23.6: Modes of insect development. Molts are represented as arrows Metamorphosis in Insects Ametabolous Development: Direct growth through molting; insects resemble small adults after a brief pronymph stage (e.g., silverfish). Hemimetabolous Metamorphosis: Gradual transformation; nymphs resemble immature adults, developing wings and genital organs with each molt (e.g., grasshoppers). Holometabolous Metamorphosis: Complete transformation; larvae grow through molts (instars), become pupae, and emerge as adults during eclosion (e.g., butterflies). Purpose: Holometabolous insects switch from foraging (larva) to reproduction (adult) during metamorphosis. Fig. 23.7: Locations and developmental fates of imaginal discs and imaginal tissues in the third instar larva (left) of Drosophila melanogaster Imaginal Discs In holometabolous insects, larval cells perform juvenile functions, while imaginal cells lie dormant, awaiting signals to form adult structures. Programmed Cell Death: Most larval tissues degenerate as imaginal cells differentiate into adult organs. Types of Imaginal Cells: Imaginal Discs: Form adult cuticular structures (e.g., wings, legs, antennae, eyes, and genitalia). Histoblasts: Develop into the adult abdomen. Organ-specific imaginal cells: Proliferate and replace degenerating larval organs. In Drosophila, imaginal discs originate as epidermal thickenings, with 19 discs forming the adult head, thorax, limbs, and genitalia. Fig. 23.8: Imaginal disc elongation Imaginal Disc Proliferation and 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: At metamorphosis, imaginal disc cells proliferate further, elongate, and differentiate to form adult structures. Fig. 23.9: 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). Fig. 23.11: Regulation of insect metamorphosis Hormones Involved: Juvenile Hormone Maintains larval state. Ecdysone: Precursor to 20- hydroxyecdysone (20E), the active molting hormone. 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 (imago). Fig. 23.12: The “molecular trinity” hypothesis for holometabolous insect metamorphosis Key Transcription Factors: Chinmo (yellow): Promotes larval stage. Broad (purple): Regulates pupal stage. E93 (blue): Activates adult stage. Mutual Inhibition: Each factor inhibits the others to maintain stability in the larva, pupa, or adult stage. Stage Progression: Hormones and growth factors provide epigenetic cues to shift the regulatory network, enabling transitions between stages. Unconfirmed Mechanisms: Some relationships between factors remain hypothetical. Fig. 23.13: 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 near the boundary. _BMP Signaling Gradient: Dpp and Glass-bottom boat (Gbb) create a concentration gradient measured by phosphorylated Mad (pMad). High Dpp + Gbb activates spalt (sal) and optomotor blind (omb); lower levels activate omb only. _Threshold Activation: When Dpp + Gbb drop below the threshold, brinker (brk) is no longer repressed, defining peripheral regions. Wing Vein Markers: Gradients specify wing veins L2–L5, with L2 being the most anterior. Fig. 23.14: Determining the dorsal-ventral axis Vestigial and Apterous Proteins: Vestigial (green) marks the ventral surface, while Apterous (red) marks the dorsal surface. Yellow region indicates overlap at the dorsal-ventral boundary. Wingless protein: Wingless (purple), expressed at the dorsal- ventral boundary, organizes the wing disc along this axis. Wingless 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 distinct gene expression patterns.