18) Metamorphosis, Regeneration, and Aging Notes

Metamorphosis

  • Larva to adult transition w/ structural changes.

  • Reactivated developmental processes via hormones.

  • Prepares for new existence.

  • Example: Amphibian (tadpole to frog).

    • Urodeles: Tail fin resorption, loss of gills, skin change.

    • Anurans: Organ modification.

  • Frog Metamorphosis:

    • Regressive: Loss of teeth/gills, tail destruction.

    • Constructive: Limb/skull development.

    • Intestine shortens.

    • Gills regress, lungs enlarge.

    • Sensory changes.

  • Eye migration for binocular vision.

  • Biochemical:

    • Retinal pigment change (porphyropsin to rhodopsin).

    • Hemoglobin changes.

    • Liver enzyme changes.

    • Excretion changes (ammonotelic to ureotelic).

  • Hormonal Control: Thyroxine (T4) & triiodothyronine (T3).

    • Body regions respond differently.

    • Tail degeneration: Protein decrease, enzyme increase, cell death, macrophage digestion.

  • Tissue sensitivity coordinates.

  • Thyroid hormones activate TRa/TRb genes.

Regeneration

  • New organ creation after removal.

  • Ways:

    • Dedifferentiation (epimorphosis): Tissue dedifferentiates, forms cell mass.

    • Repatterning (morphallaxis): Tissue remodeling (Hydra).

    • Compensatory: Cells divide, maintain function (liver).

  • Epimorphic (salamander limbs):

    • Wound epidermis forms AEC.

    • Cells dedifferentiate into blastema.

    • Proliferation via nerve fibers.

  • Patterning similar to limb development.

  • Retinoic acid crucial.

Aging

  • Time-related deterioration.

  • Causes:

    • Oxidative damage: ROS; countered by enzymes/restriction. Vitamin E and C are ROS inhibitors

    • Wear and tear: Mutations, enzyme decrease.

    • Telomere shortening: Telomerase maintains length.

  • Genetic programs:

    • Progeria: Rapid aging from LMNA gene mutations.

    • Abnormal lamin A protein, nuclear instability.