Chapter 15 - Metamorphosis

metamorphosis

the hormonal reactivation of development that gives the animal a new form

- the remodeling of existing larval tissues into adult structures

- aquatic larva = becomes terrestrial adult frogs

- tails = become legs

- gills = become lungs

what occurs in amphibian metamorphosis?

1. decrease in protein synthesis

2. increase in digestive enzyme : proteases, RNases, DNases, and collagenases

3. cell death by apoptosis when these enzymes are released to the cytoplasm in the cells of the tails for tail regression

what are the 3 effects of thyroid hormones on the tail of an amphibian during metamorphosis?

lysosomal protease

apoptotic gene product in tail of amphibian that causes tail regression through apoptosis

- the more of it, the more the tail regresses

1 limb growth before tail regression (locomotion)

2. adult lung function before gill regression (breathing)

- otherwise, the terrestrial animal will have no way of moving or breathing

what is the specific sequence of the events of metamorphosis and why?

differential responses to hormone concentrations thresholds ensures that the correct sequence of changes occur

what ensures the correct specific sequence of events of metaphorosis?

Threshold Hypothesis

states that tissues that change earlier can respond to lower concentrations of thyroid hormone , while tissues that change later require higher concentrations of the hormone

- early response : limb growth, requires less thyroid hormone

- late response: tail regression, requires more thyroid hormone

what is considered an early response vs late response for thyroid hormone concentration for tails?

1. Duration of metamorphosis (larva to adult)

2. Timing of appearance of changes

- some amphibians spend their entire lives as aquatic larvae, and a few amphibians skip the aquatic stage

what 2 developmental events in amphibian life cycles vary due to heterochrony?

neoteny

the retention of juvenile features in the adult animal

- no terrestrial stage

- observed in salamanders

1. No metamorphosis is observed in specimens in the wild

2. They retain gills, larval skin, and tails but grows legs

3. Sexual maturity occurs while larval features are retained

4. the thyroid gland does not produce thyrotropin, a thyroid hormone necessary for metamorphosis

what are the 4 features of neoteny in the Mexican axolotl?

the gills regress and lungs grow + changes in the skin

what happens if you feed a Mexican axolotl thyrotropin?

direct development

feature in which amphibian does not have aquatic larval stage

- extremely rare in amphibians

- seen in Eleutherodactylus coquí, the common Puerto Rican

coquí

1. No aquatic tadpole stage

2. The limb buds appear shortly after the neural tube closes

3. The hatching frogs are fully terrestrial individuals

what are 3 features of direct development?

Oviviparous

Type of reproduction in which eggs stay in mother's body after internal fertilization and they give birth to fully terrestrial juveniles

1. ametabolous - direct development, ex: silverfish

2. hemimetabolous - gradual changes ex: grasshoppers and roaches

3. holometabolous - complete metamorphosis ex: butterflies, drosophila

what are the 3 types of insect metamorphosis?

1. There is no larval stage

2. The embryo hatches into a pronymph (a miniature version of the adult)

3. The animal grows with each molt until it reaches the adult size

what are 3 features of ametabolous insect metamorphosis?

1. the embryo hatches into a pronymph, then molts into a nymph (immature adult)

2. the rudiments of the adult organs are present in the nymph such as wings and genital organs

3. the animal grows and the structures mature with each molt until it reaches the adult size : the imago

what are the 3 features of hemimetabolous insect metamorphosis?

1. there is no pronymph stage

2. a feeding larva hatches from the egg

3. the larva becomes progressively larger with each molt

what are the 2 features of holometabolous insect metamorphosis?

1. metamorphic molt

2. imaginal molt

what are the 2 types of molts in holometabolous insect metamorphosis?

metamorphic molt

the larva transforms into a pupa (embryo inside cocoon), which does not feed while metamorphosis is occuring, the larval tissues are destroyed and replaced with adult tissues

imaginal molt

the adult (imago) ecloses from the cocoon

Regeneration

the restoration of severed tissues/body parts that involves the reactivation of development

1. Stem cell mediated - occurs in most animals

2. Epimorphosis - occurs in salamanders

3. Morphallaxis - occurs in hydra

4. Compensatory Regeneration - occurs in most mammals

what are the types of regeneration?

Stem-cell mediated regeneration

populations of pluripotent cells are present in animals which allow for the regrowth of lost tissues or severed body parts

1. adult cells dedifferentiate at the site of the wound to become neoblasts

2. the neoblasts form a mass of undifferentiated cells called the regeneration blastema

3. cell division occurs

4. the blastema cells become re specified to form the missing structures

what are the 4 steps of epimorphosis?

stem cell mediated regeneration

what type of regeneration is this?

epimorphosis

what type of regeneration is this?

Morphallaxis (transdifferentiation)

redifferentiation and re-patterning of existing tissues into the missing structures (direct conversion)

- very little cell division occurs

compensatory regeneration

cells do not re differentiate but divide to become a mass or tissue (scar tissue) that is equivalent, but not identical to the mass of tissue lost, because there is no memory of the exact pattern of the lost tissue

Morphallaxis (transdifferentiation)

what type of regeneration is this?

compensatory regeneration

what type of regeneration is this?

Hematopoiesis

stem cell mediated regeneration of blood cell type

clonogenic neoblasts which are always present

- they migrate and accumulate at the site of the wound to form regeneration blastema

what does stem cell mediated regeneration in planarians depend on?

BMPs (the dorsal signals) and their inhibitors such as Noggin

what 2 things does dorsal-ventral polarity depend on in regards to Morphogen gradients and positional information for regeneration in planarians?

Wnts (the posterior signals) and their inhibitors

what 2 things does anterior-posterior polarity depend on in regards to Morphogen gradients and positional information for regeneration in planarians?

if tail is cut, instead of regenerating another tail, a head is regenerated

what happens if Wnt1 is blocked by RNAi in the posterior morphogen gradient for regeneration of planarians?

distal = amputation close to the wrist in which regeneration produces only a wrist and a foot

proximal = amputation behind elbow in which regeneration produces an elbow, forearm, wrist, and foot

what is the difference between distal vs proximal regeneration in a salamander?

in stem cell mediated proliferation in planarians but not in epimorphic regeneration in salamanders

when are neoblasts clonogenic (always present)?

the presence of nerves and the newt anterior gradient protein (nAG, a nerve growth factor)

what does the proliferation of salamander blastema cells depend on in the regeneration of salamander limbs (epimorphic regeneration)?

1. denervate the right limb

2. amputate both limbs seven days later

3. electroporate newt anterior gradient protein (nAG) into the right limb blastema five days later

what are the 3 steps of the experiment on regeneration of salamander limbs with newt anterior gradient protein (nAG)?

1. no growth occurs after the amputation of the denervated limb if no nAG is administered (control)

2. if nAG is administered, regeneration occurs even if the limb was denervated before amputation

what are the 2 results of the experiment on regeneration of salamander limbs with newt anterior gradient protein (nAG)?

a proximal to distal gradient of retinoic acid which tells the neoblasts at the blastema what to form

what is the positional information for the epimorphic regeneration of the salamander limb defined by?

wound epidermis

produces retinoic acid after amputation in the regeneration of salamander

the position of the wound along the proximal-distal axis of the limb

what does the amount of retinoic acid produced in the regeneration of salamander limbs depend on?

regeneration following a distal amputation with a high concentration of RA will result in elbow, wrist and foot instead of only wrist and foot

what occurs in the experiment to test what happens if retinoic acid concentration is altered?

hydra

Freshwater cnidarian that can reproduce by budding

budding

Asexual reproduction in which a part of the parent organism pinches off and forms a new organism

morphallactic regeneration (transdifferentiation)

- if an animal is cut in half, the head half regenerates a basal disc and the basal disc half regenerates a head

what type of regeneration does budding resemble?

1. Cut a section of the hypostome (head) or the basal disc (foot) from a donor animal

2. Place the section in the trunk region of a host animal

results = a secondary axis is formed in the host and a new head or foot grows by the trunk (body)

what are the 2 steps and results of the experiment in regeneration of Hydra grafts?

no graft growth on trunk happens unless the head of the host is removed

In the experiment in regeneration of Hydra grafts, what happens if the donor head graft is placed near the head in the host?

head inhibitor

originates in the head and forms a gradient near the head along the anterior posterior axis in order to prevent the formation of a second head

- in the young = the sources of the 2 inhibitors are still too close so no budding can occur

- in the adult = the sources of the 2 inhibitors are farther apart so there are no inhibitors in the mid trunk section and budding can occur

- inhibitors ensure that budding occurs only in adult animals

how is budding affected by head/foot inhibitor in the young vs the old?

shinguard

foot inhibitor

-inhibits budding but also activates foot formation

1. Liver epithelial cells are arrested in G0 but can return to cell division cycle to regenerate lost portions of the liver

2. Fully differentiated cells proliferate to become a mass of tissue that is equivalent but not identical to the lost mass of tissue (scar tissue)

what are the 2 steps of compensatory regeneration in the mammalian liver?