Lecture 16: Development I Flashcards
development
all the changes that occur during a life cycle
embryonic, goes on after birth/ hatching
model organisms
species chosen for reasearch - easily studied
ex: roundworms, sea urchins, frogs , chicks
fertilization
- sperm ( usually flagellated, motile + ovum (large, nonmotile -> zygote
- haploid + diploid = diploid zygote
what are some consequences of fertilization
- restores diploid # (2n)
- determine sex of organisms
- activates egg & stimulates reactions to promoting development
what is the egg surrounded by?
plasma membrane, 1+ coverings
what is the purpose of the coverings of the egg
- aids in fertilization ( may secrete to signals to sperm cells
- barrier to interspecific fertilization, important for external fertilization
steps of fertilization
- sperm dissolves protective layers in order to reach plasma membrane
- binds to receptors of egg surface ( ensures egg and sperm are same species)
- changes to egg surface - prevents polyspermy ( prevents multiple sperms from entering)
sea urchin egg coverings
- plasma membrane - innermost
- vitelline layer - thin
- jelly coat - thick glycoprotein layer
acrosome
releases hydrolytic enzymes
-partially digest jelly coat
acrosomal process
- actin filaments
- protrude from sperm head & penetrate jelly coat proteins at tip
- bind to specific receptors on egg plasma membrane
- ( lock and key) ensures same species
polyspermy
- fertilization of egg by more than one sperm
- offspring with extra chromosome sets
- usually lethal
what happened when gametes are released into water
- jelly coat releases chemicals into H20 -> attracts sperm
what happens when the recognition between sperm and egg triggers fusions of PM’s
- sperm nucleus enters egg ( plasmogomy
- no more sperm should enter
fast block to polyspermy is triggered by
fusion of PM’s
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seq fast block to polyspermy
- unfertilized egg is polarized ( cytoplasm negatively charged relative to outside)
- sperm PM fuses and nucleus enters egg -> ion channels in egg PM open
- Na+ enter egg
- egg membrane depolarizes
- membrane depolarization prevents fusion with additional sperm
- transient effect and last only ( 1 minute)
seq slow block to polyspermy
- signal transduction pathway
- binding of sperm to egg
- release of Ca2+ from ER into cytoplasm of egg
- triggers cortical granules
cortical granules
i. vesicles in cortex
ii. releases enzymes into space between PM & vitelline layer
Cortical Reaction is triggered by
by fusing of plasma membrane between sperm and egg
what happens during cortical reaction
- Ca2+ released in egg from smooth ER
cortical granules
- cortical granules fuse with plasma membrane ( vesicles containing enzymes that are stored below the plasma membranes)
- enzymes relased ( into ZP) , trigger slow block to polyspermy
mammal egg coverings
plasma membrane
zona pellucida
follicle cells
in mammels fertilization reacts
internally
acrosomal reaction in mammals
- sperm binds to ZP3 ( glycoprotien in zona pellucida
- binding triggers acrosome to burst ( releases enzymes, digests through pellucida)
- no fast block to polyspermy
cortical reaction in mammals
- triggered by sperm binding
- ca2+ released into cytoplasm
- cortical granules release enzymes to outside of cell
- enzymes
enzymes in cortical reaction in mammals
- catalyze hardening and alter sperm receptors of zona pelllucida
- no additional sperm can bind
- no fertilization envelope formed
summary of fertilizations
- sperm has recognized egg
- sperm nucleus has entered egg
- changes to egg prevents polyspermy
egg activation
- series of metabolic reaction in egg ( onset of embryonic development)
what triggers egg activation
increased ca2+ in cytoplasm (cortical reaction)
what are the changes in egg activation
- increased cellular respiration
- maternal enzymes and proteins activated
- increased proteins synthesis (maternal mRNA in cell)
fusion of sperm and egg nuclei
- same time as egg activation
- sperm nucleus guided to egg nucleus by microtubules
egg development in sea urchins
- eggs have completed meiosis when released from female
egg development in humans
- egg ( 2° oocyte) up to this point stalled at metaphase II , fertilizations triggers completion of meiosis
- > sperm guided to egg nucleus
female and male nuclei fuse
karyogamy
when does sea urchin karyogamy
20 minutes after sperm nucleus enters egg
how long does human karyogamy take
several hours
zygote in egg development (karygomy)
- 2n
- totipotent (potential to give rise to all cell types of new individual)
end of fertilization
when zygote undergoes first division
cleavage
- series of rapid mitotic divisions with no growth,
what is a result of cleavage
cell # increases, but cells get smaller & embryo size doesn’t increase
- results in blastomere
cleavage furrow
- indentation in cell surface as cytokinesis divides
yolk
- protien, phospholipids, fats -> food developing embryo
- amount varies among animal groups -> depends on needs of embryo
stages of cleavege
- zygote ( one cell)
- embryo ( >= 2 cells )
- blastula ( >= 128 cells) - hollow ball, blastocoel
echinoderms in mammals
- very litle yolk
equal holoblastic
- uniform division across embryo
- cleavage furrow passes entirely through egg causing blastomeres of equal size
- blastocoel forms centrally
assymetric yolk distribution
- most yolk at vegetal pole of egg
- opposite end is animal pole
unequal holoblastic cleavage
- cleavage furrow passes entirely through egg, cytokinesis slowed down by yolk in vegetal hemisphere
gray crescent
- lighter color region
- opposite site of sperm entry
- gastrulation begins here
continuation of division in frogs
- yolk affects sizes of cells in two hemispheres
animal hemispheres
blastomeres are smaller and more numerous
blastocoel forms entirely in animal hemisphere
cleavge of birds
- lots of yolk, cleavege furrow cannot pass through
meroblastic cleavage
- only region without yolk undergoes cleavage
