7. sperm-oolemma binding and fusion (lecture) Flashcards
how can the oocyte plasma membrane be divided?
into 2 major regions:
smooth surface devoid of microvilli
rich in microvillar protusions
what is the part of the membrane which has a smooth surface devoid of microvilli?
part that directly overlies the metaphase chromosomes
which part of the membrane is rich in microvilli protrusions?
the remainder of the oocyte
NOT directly overlying the metaphase chromosomes
what is the function of the part of oocyte rich in microvillar protrusions?
region of the oocyte where sperm binds and fuse
what leads to the block to polyspermy (fast block) occur?
electrical change in oocyte membrane
Na+ channels open (resting potential -75mV to +20mV)
what is the opening of Na+ channels leading to +20mV of the oocyte membrane called?
fertilisation potential
how does the fast block (block to polyspermy) come about?
wave of depolarisation starts at the site of entry of sperm and propagates across cytoplasm
what triggers the block to polysperm: slow block to occur?
Ca2+ released from ER, induces local exocytosis of cortical granules
what does local exocytosis of cortical granules in a slow block cause?
granules release enzymes to stimulate adjacent cortical granules to undergo exocytosis
what does granules stimulating adjacent granules to undergo exocytosis in a slow block lead to?
wave of exocytosis occurs around oocyte in 3 dimensions from original site of sperm entry
what is syngamy?
union of male and female pronuclei to form diploid zygote (46 chromosomes)
what are the steps leading up to syngamy?
oocyte completes meiosis II
EXPELS second polar body
male and female pronuclei migrate towards each other (23+23 chromosomes)
what is polyploidy?
embryos containing 3 or more pronuclei
what causes polyploidy?
entry of more than 1 sperm
OR
failure of extrusion of second polar body
what is cleavage?
series of rapid mitotic divisions and metabolic changes (for upcoming cell division and embryogenesis)
what does cleavage lead to?
increased number of cells (16-32 blastomeres) of decreasing size, withOUT increase in size of fertilised ovum
what is totipotency?
each cell has capacity to develop into entire individual
what leads to monozygotic twins?
in cleavage, totipotent cells become divided into 2 separate independent cell masses
(25-30% of twins follow separation after 1st cleavage)
what leads to dizygotic (non-identical) twins?
2 eggs ovulate
2 eggs are fertilised
why is cleavage important?
generation of a large number of cells that can undergo differentiation and gastrulation to form organs
increase in nuclear / cytoplasmic ratio
why is increase in nuclear / cytoplasmic ratio in cleavage important?
one nucleus cannot transcribe sufficient RNA to support the enormous cytoplasm of the zygote
which stages of cell cycle are NOT present during cleavage?
no G1 or G2 stages in cell cycle during cleavage
what is G1 stage in cell cycle?
duplication of organelles and cytosol
what is G2 stage in cell cycle?
synthesis of enzymes and proteins
what happens with each mitotic division during cleavage?
the nuclear-cytoplasmic ratio increases
how does the process of cleavage occur?
asynchronous
not all blastomeres divide at the same time
what happens during compaction?
at 8 cell stage, blastomeres undergo polarisation and form tight junctions to create ‘inner embryo environment’
what forms after compaction?
morula
what is morula?
16 blastomere (totipotent cells) stage
when does morula form?
3-4 days after fertilisation
when does the embryo pass from fallopian tube (fertilisation) into the uterus (implantation)?
as a morula (3-4 days post fertilisation)
16 blastomere
