Development I Flashcards
development
- all the changes in life cycle
- includes embryonic developmental birth/hatching, metamorphosis, etc
model organisms
species chosen for research -> easily studied
what are the consequences of fertilization?
- restores diploid number
- determines sex (sometimes)
- sperm content activates egg -> start development
external egg structure
plasma membrane with 1+ coverings
- aid in fertilization
- barrier to interspecific fertilization (especially important for species/external fertilization)
steps in fertilization
- sperm dissolves protective layers around egg
- sperm binds to egg surface receptors - ensure same species
- change to egg surface - prevent polyspermy
why are echinoderms used in research a lot?
- readily available
- easy to work with
- easy to obtain lots of gametes
- external fertilization - easy to observe
name the egg coverings of sea urchins external to internal
jelly coat -> sperm-binding receptors -> vitelline layer -> egg PM
what are the steps in sea urchin fertilization?
- Ascosomal reaction
- fast block to polyspermy
- cortical reaction - slow block to polyspermy
describe the external fertilization in sea urchins
- gametes released into water
- egg jelly coat releases chemicals -> attract sperm (chemotaxes)
acrosomal reaction (sea urchins)
- acrosome in sperm head releases hydrolytic enzymes -> break down jelly coat
- acrosomal process/filaments bind sperm-binding receptor
- recognition triggers plasmogamy - sperm and egg PMs fuse -> sperm nucleus enters egg
preventing polyspermy (sea urchins)
- more than 1 sperm nucleus enters egg (more fatal - abnormal chromosome #)
- 2 mechanisms to ensure only 1 sperm enters (fast block ~ 1-3 sec, slow block ~ 1 min)
fast block to polyspermy (sea urchins)
- seconds after membrane fusion: ion channels open in egg PM -> electric “zap” that repels other sperm on egg surface -> prevents addition sperm/egg PM fusion
- lasts ~ 1 min
cortical reaction/slow block to polyspermy (sea urchins)
- also triggered by plasmogamy (membrane fusion)
- vitelline layer separates, hardens -> fertilization envelope-protective layer -> complete block - irreversible, permanent
- requires ~ 1 minute
name the egg coverings of mammals outside to inside
PM -> zona pellucida -> follicle cells
what are the steps in mammal fertilization
- acrosomal reaction
2. cortical reaction/slow block to polyspermy
fast block to polyspermy in mammals
TRICK QUESTION! THERE IS NO FAST BLOCK TO POLYSPERMY IN MAMMALS
where are eggs fertilized in mammals?
internally - inside oviduct
acrosomal reaction (mammals)
- sperm binds receptor in zona pellucida
- acrosome bursts and release enzyme (helps sperm get through zona pellucida)
cortical reaction/slow block (mammals)
-enzymes catalyze changes in zona pellucida -> ZP hardens -> filters sperm receptors so that no sperm can bind
describe the fertilization envelope in mammals
TRICK QUESTION. THERE IS NO FERTILIZATION ENVELOPE IN MAMMALS
egg activation
- triggered by sperm binding
- series of metabolic reactions starts, accelerate (onset of embryonic development)
what are the results of egg activation?
- increase respiration
- maternal enzymes and proteins activated
- increase protein synthesis - mRNA already present
what is the 2° oocyte triggered by?
sperm binding
for humans, when does the 2° oocyte arrest?
metaphase II
what triggers continuation? what happens as a result?
fertilization. meiosis is complete
when do sea urchin eggs complete meiosis?
before release
karyogamy
- microtubules guide sperm nucleus to egg
- sea urchins ~ 20 minutes after sperm nucleus enters
- humans - several hours
- results in diploid, totipotent zygote
totipotent
potential to give rise to all cell types
end of fertilization
- when zygote undergoes first division -> embryo
- sea urchins - within 90 minutes
- mammals - 12-26 hours
compare and contrast fertilization in sea urchins and mammals
sea urchins
- echinoderms - in ocean
- external fertilization
- fast and slow blocks
- fertilization envelope
mammals
- aminotes
- internal fertilization
- slow block only
- no fertilization envelope
what is the second step in embryonic development and what is formed as a result?
cleavage. single celled zygote becomes a MC embryo
characteristics of cleavage
- rapid mitosis
- very short/no G1 or G2 Phases
- no cell growth
what are the stages of cleavage?
zygote (1 cell) -> 24 hours -> embryo (2 + cells) -> blastula: hallow (blastocoel inside = nothing)*
- at least 128 blastomeres cells of the blastula
deuterostomes
radial and indeterminate
yolk
mixture of proteins, phospholipids, fats -> food for developing embryo (like endosperm - convergent evolution)
animal pole
where embryo is
vegetal pole
where yolk is
cleavage furrow
indentation during cytokinesis
how does yolk affect the pattern of division?
amount and distribution of yolk varies between animal groups
- little yolk -> faster, equal cleavage
- lots of yolk -> slower, unequal cleavage
equal holoblastic
- cleavage furrow passes entirely through egg
- equal division of egg cytoplasm -> equal blastomeres
unequal holoblastic
- cleavage furrow passes entirely through egg
- yolk slows down cytokinesis in vegetal pore -> unequal divisions of egg cytoplasm -> unequal blastomeres (more/smaller blastomeres in vegetal pole, fewer larger blastomeres in vegetal pole)
in what species does unequal holoblastic occur?
species with medium amount of yolk - e.g. amphibians
meroblastic cleavage
cleavage furrow does NOT pass entirely through egg (ONLY through pole)
in what species does equal holoblastic occur?
occurs in species with little yolk (e.g. sea urchins, humans)
in what species does meroblastic cleavage occur?
occurs in species with a LOT of yolk - birds, other reptiles, fish
