Post Embryonic Development

• METAMORPHOSIS


  Hormonal reactivation of development
• Types of developers


  • Direct developers
  • Indirect developers
• Direct developers


  • Young are essentially smaller, less sexually mature versions of the adult
• Indirect developers


  • Life cycle includes larval and adult stages
• Types of larvae


  • Primary larvae
  • Secondary larvae
• Primary larvae


  Represent dramatically different body plans and are morphologically distinct from the adult form
• Secondary larvae


  Larvae and adults possess the same basic body plan
• AMPHIBIAN METAMORPHOSIS


  Morphological changes prepare an aquatic organism for a primarily terrestrial existence
• Amphibian


  Greek "amphi" (double) and "bios" (life)
• Amphibian metamorphosis


  1. Initiated by hormones, thyroxine (T4) and tri-iodothyronine (T3)

  2. T3 induces certain adult-specific organs to form

  3. T3 induces the proliferation and differentiation of new neurons to serve these organs

  4. Blocking activity prevents neurons from forming thus causing paralysis of the limbs

  5. Eyes move from the original lateral position to the front of the head, creating a large binocular field of vision for the adult frog

  6. T3 causes the degeneration of the paddlelike-tail and oxygen-procuring gills

  7. Tadpole hemoglobin is replaced by adult hemoglobin, which binds oxygen more slowly and releases it more rapidly

  8. Larval RBCs are digested by macrophages in the liver and spleen

  9. Larval intestine with numerous coils is converted into a shorter intestine

  10. Nervous system is remodeled as neurons grow and innervate new targets
• Remodeling


  Undergoes enormous restructuring as some neurons die, others are born, and others change specificity
• Respecification


  • T3 induces a new set of proteins in existing cells
  • Tadpoles are ammonotelic while many adult frogs are ureotelic
  • Liver begins to synthesize enzymes necessary to create urea
  • T3 activates adult hepatic genes while repressing larval hepatic genes in the same cell
• Types of insect metamorphosis

  • Ametabolous
  • Hemimetabolous
  • Holometabolous
• Ametabolous


  Has no larval stage, undergoes direct development
• Hemimetabolous


  Undergoes gradual metamorphosis, brief period as pronymph, nymph stage like an immature adult, imago is winged and sexually mature adult
• Holometabolous


  No pronymph stage, juvenile form is called larva, metamorphic molt to pupal stage, imaginal molt turns pupa into adult
• Locations and developmental fates of imaginal discs and imaginal tissues in the third instar larva (left) of Drosophila melanogaster
• REGENERATION

  Reactivation of development in post-embryonic life to restore missing or damaged tissues
• Epimorphic regeneration

  Adult structures undergo dedifferentiation to form a relatively undifferentiated mass of cells that then re-differentiates to form the new structure
• Key stages of salamander limb regeneration

  1. Skin and muscle retracted from the tip of the humerus

  2. Blastema cells are seen beneath the thickened epidermis, where the AEC forms

  3. Large population of mitotically active blastema cells lies distal to humerus

  4. Blastema elongates by mitosis, dedifferentiation occurs

  5. Early redifferentiation can be seen

  6. Precartilaginous condensations for the carpal bones and the first 2 digits can be seen
• Limb will regenerate only those distal tissues removed
• Compensatory regeneration


  Differentiated cells divide but maintain their differentiated functions, each cell produces cells similar to itself, no mass of undifferentiated tissue forms
• Remaining lobes enlarge to compensate for the loss of the missing tissue
• Removal or injury of the liver is sensed through the bloodstream
• Amount of liver regenerated is equivalent to the amount of liver removed
• s for the carpal bones and the first 2 digits can be seen
• Limb regeneration


  • Regenerates only those distal tissues removed
  • Key positional information provides instructions for cell fate patterning during regeneration
  • Involves complex interactions between various signaling pathways, cells, and tissues
• Compensatory regeneration in liver

  • Differentiated cells divide but maintain their differentiated functions
  • Each cell produces cells similar to itself; no mass of undifferentiated tissue forms
  • To recover the structure and function of an injured organ
• Compensatory regeneration in liver

  1. Remaining lobes enlarge to compensate for the loss of the missing tissue

  2. Removal or injury of the liver is sensed through the bloodstream

  3. Amount of liver regenerated is equivalent to the amount of liver removed
• Compensatory regeneration in liver

  • Normal, mature, adult hepatocytes are instructed to rejoin the cell cycle and proliferate until they have compensated for the missing part
  • Hepatocytes, duct cells, fat-storing (Ito) cells, endothelial cells, and Kupffer macrophages each retain their cellular identity, and the liver retains its hepatic functions even as it regenerates itself
• Hepatic progenitor cells


  • Population activated when the injury is severe due to senescence, alcohol abuse, or disease
  • Oval cells - can produce hepatocytes and bile duct cells; kept in reserve
  • Result is to downregulate the genes involved in the differentiated functions of liver cells while activating those committing the cell to mitosis
• Morphallactic regeneration


  • In hydras, through the repatterning of existing tissues (trans-differentiation)
  • There is little new growth; no proliferation
• Hydra


  • Genus of freshwater cnidarians
  • Most are tiny - about 0.5 cm long
  • "Head" - found at the distal end; consists of a conical hypostome region containing the mouth and a ring of tentacles beneath
  • "Foot" or basal disc - located at the proximal end; enables the animal to stick to rocks
• Hydra


  • Diploblastic animals
  • Contains secretory cells, gametes, stinging cells (nematocytes), and neurons
  • Can reproduce sexually only under adverse conditions
  • Usually multiply asexually by budding which forms about 2/3 down the body axis
• Cell replacement in hydra

  • Generated from three cell types
  • Endodermal and ectodermal cells - unipotent, divide continuously, produce more epithelia
  • Interstitial stem cell - multipotent, generates neurons, secretory cells, nematocytes, and gametes
• Tremblay (1741) - "there are reborn as many complete animals similar to the first"
• Polarity in hydra


  • Every portion can form both a head and a foot
  • Coordinated by a series of morphogenetic gradients
  • Head (↑ hypostome) and foot (↑ basal disc) activation gradient
  • Permit the head to form only at one place and the basal disc to form only at another
• Aging


  Time-related deterioration of the physiological functions necessary for survival and fertility
• Senescence


  Physiological deterioration of an organism
• Aging and senescence have both genetic and environmental components