Embryology   Biology 441   Vertebrate Embryology, Spring 2016   Albert Harris



Lecture Notes for April 13, 2016



Tadpole ----> Frogs & toads

1* Self-destruction of tail - Apoptosis

Experiment in which an eye was grafted onto the tail
The eye remains intact, and shifts to the location where the tail had been
NOT analogous to cutting off blood supply

2* Formation of legs
also: loss of ability to regenerate legs (?analogy to salamanders?)

3* Change in retina: different light-detecting pigment
Also formation of new optic nerve fibers

4* Movement of eyeballs to more anterior location
binocular vision 3-D predator

5* Formation of lungs

7* Degeneration of gills

8* Changed hemoglobin; different cells blood-producing stem cells for breathing air

9* Kidney changes - adult uses urea instead of ammonia

10* loss of lateral line system

11* Digestive tract herbivorous--> carnivorous (shorter in length)

12* Skin thicker epidermis and dermis


Important that these changes occur in the right sequence: "Don't destroy your tail until you have formed your new legs"

Threshold theory
steady increase in thyroxine concentration

the lower the threshold, the sooner the stage will occur
(sudden, high concentration will cause everything to happen at once.

Gilbert regards as an over-simplification


Stimulation of amphibian metamorphosis by thyroxine :
two tyrosines, attached end to end, with 3 or 4 Iodine atoms
(We use this same chemical as a stimulant for metabolism.)

The thyroid gland evolved from a mucus-secreting groove, which then became separated from the floor of the throat

Thyroxine is made from mucus inside epithelial spheres; covalent binding of iodine to tyrosines + binding of pairs of tyrosine.

This polymerization occurs in the mucus inside the epithelial spheres.

? related to evolution of thyroid from the mucus-secreting gland used for filter-feeding by sea squirts and larval lampreys


Salamanders also have larvae, with legs, that undergo metamorphosis, in a sub-group of salamanders called newts (common in this area: more in New England; very poisonous skin)

Larva -> Eft --> Newt


Neoteny   -   Axolotl

Reach sexual maturity and breed when anatomically still larvae.


Also there are species of frogs and salamanders with "direct development"


Invertebrate examples of metamorphosis

II) Ammocoetes larva ---> Lamprey

III) Sea Squirt "tadpole" ----> Sea Squirt

IV) Pluteus ----> Sea urchin (and star-fish, etc.)

V) Sponges have a swimming larval form, whose cells then rearrange to form a sponge (in which cells keep on rearranging forever)

VI) Trochophore larvae, Pilidium larva, and several other planktonic larvae

VII) Larvae of barnacles look like shrimp, etc.


VIII) Caterpillar ----> ----> ----> Butterfly, or moth

Maggot ----> ----> ----> fly

grasshopper -> grasshopper -> grasshopper -> grasshopper -> grasshopper

silverfish -> silverfish -> silverfish-> -> silverfish -> silverfish


Metamorphosis by molting (shedding outer layer of skin)

All arthropods molt. Some insects have a metamorphic molt.

The new body surface is made by differentiation of imaginal discs.

Hormonal control of arthropod molting

    Ecdysone (a steroid) stimulates molting

    Juvenile hormone (a lipid) inhibits metamorphosis

If you keep treating caterpillars with juvenile hormone, they will molt into bigger & bigger caterpillars.

Precor - chemical name methoprene - analog of juvenile hormone

Eliminates fleas by preventing them from going through metamorphosis to become breeding adults





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