7. sperm-oolemma binding and fusion (lecture)

Question 1

how can the oocyte plasma membrane be divided?

Answer 1

into 2 major regions:
smooth surface devoid of microvilli
rich in microvillar protusions

Question 2

what is the part of the membrane which has a smooth surface devoid of microvilli?

Answer 2

part that directly overlies the metaphase chromosomes

Question 3

which part of the membrane is rich in microvilli protrusions?

Answer 3

the remainder of the oocyte

NOT directly overlying the metaphase chromosomes

Question 4

what is the function of the part of oocyte rich in microvillar protrusions?

Answer 4

region of the oocyte where sperm binds and fuse

Question 5

what leads to the block to polyspermy (fast block) occur?

Answer 5

electrical change in oocyte membrane

Na+ channels open (resting potential -75mV to +20mV)

Question 6

what is the opening of Na+ channels leading to +20mV of the oocyte membrane called?

Answer 6

fertilisation potential

Question 7

how does the fast block (block to polyspermy) come about?

Answer 7

wave of depolarisation starts at the site of entry of sperm and propagates across cytoplasm

Question 8

what triggers the block to polysperm: slow block to occur?

Answer 8

Ca2+ released from ER, induces local exocytosis of cortical granules

Question 9

what does local exocytosis of cortical granules in a slow block cause?

Answer 9

granules release enzymes to stimulate adjacent cortical granules to undergo exocytosis

Question 10

what does granules stimulating adjacent granules to undergo exocytosis in a slow block lead to?

Answer 10

wave of exocytosis occurs around oocyte in 3 dimensions from original site of sperm entry

Question 11

what is syngamy?

Answer 11

union of male and female pronuclei to form diploid zygote (46 chromosomes)

Question 12

what are the steps leading up to syngamy?

Answer 12

oocyte completes meiosis II
EXPELS second polar body
male and female pronuclei migrate towards each other (23+23 chromosomes)

Question 13

what is polyploidy?

Answer 13

embryos containing 3 or more pronuclei

Question 14

what causes polyploidy?

Answer 14

entry of more than 1 sperm
OR
failure of extrusion of second polar body

Question 15

what is cleavage?

Answer 15

series of rapid mitotic divisions and metabolic changes (for upcoming cell division and embryogenesis)

Question 16

what does cleavage lead to?

Answer 16

increased number of cells (16-32 blastomeres) of decreasing size, withOUT increase in size of fertilised ovum

Question 17

what is totipotency?

Answer 17

each cell has capacity to develop into entire individual

Question 18

what leads to monozygotic twins?

Answer 18

in cleavage, totipotent cells become divided into 2 separate independent cell masses
(25-30% of twins follow separation after 1st cleavage)

Question 19

what leads to dizygotic (non-identical) twins?

Answer 19

2 eggs ovulate

2 eggs are fertilised

Question 20

why is cleavage important?

Answer 20

generation of a large number of cells that can undergo differentiation and gastrulation to form organs
increase in nuclear / cytoplasmic ratio

Question 21

why is increase in nuclear / cytoplasmic ratio in cleavage important?

Answer 21

one nucleus cannot transcribe sufficient RNA to support the enormous cytoplasm of the zygote

Question 22

which stages of cell cycle are NOT present during cleavage?

Answer 22

no G1 or G2 stages in cell cycle during cleavage

Question 23

what is G1 stage in cell cycle?

Answer 23

duplication of organelles and cytosol

Question 24

what is G2 stage in cell cycle?

Answer 24

synthesis of enzymes and proteins

Question 25

what happens with each mitotic division during cleavage?

Answer 25

the nuclear-cytoplasmic ratio increases

Question 26

how does the process of cleavage occur?

Answer 26

asynchronous

not all blastomeres divide at the same time

Question 27

what happens during compaction?

Answer 27

at 8 cell stage, blastomeres undergo polarisation and form tight junctions to create ‘inner embryo environment’

Question 28

what forms after compaction?

Answer 28

morula

Question 29

what is morula?

Answer 29

16 blastomere (totipotent cells) stage

Question 30

when does morula form?

Answer 30

3-4 days after fertilisation

Question 31

when does the embryo pass from fallopian tube (fertilisation) into the uterus (implantation)?

Answer 31

as a morula (3-4 days post fertilisation)

16 blastomere