BIO1022 Chapter Notes - Chapter Prescribed: Neural Tube, Blastocoel, Cell Migration
30 May 2018
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BIO1022 – Readings – Week 7
chapter 47
47.1
- fertilisation and cleavage initiate embryonic development
- fertilisation
• sperm dissolve or penetrate any protective layer surrounding the egg to
reach the plasma membrane e
• molecules on the sperm surface bind to receptors on the egg surface
• the acrosomal reaction
•
o sea urchins release their gametes into the water which have a jelly
coat surrounding the egg which releases soluble molecules that
attract the sperm
o when the sperm head contact the jelly coat of an effect - molecules
in the jelly coat trigger the acrosomal reaction in the sperm
o this reaction begins with the discharge of hydrolytic enzymes from
the acrisome - a specialised vesicle at the tip of the sperm
o these enzymes partially digest the jelly coat - enabling a sperm
structure called the acrosomal process to elongate and penetrate
the coat
o protein molecules on the tip of the extended acrosomal process
bind to specific receptor proteins that jut out from the plasma
membrane of the egg
o lock and key effect - recognition - important as other gametes may
be present in the water
o the recognition event between the sperm and egg triggers fusion of
their plasma membranes
o the sperm nucleus then enters the egg cytoplasm as ion channels
open into the egg’s plasma membrane
o sodium ions diffuse into the egg and cause depolarisation - a
decrease in the membrane potential - the charge difference across
the plasma membrane
- the cortical reaction
• slow block to polyspermy is established by vesicles that lie just beneath
the egg plasma membrane, in the rim of the cytoplasm known as the
cortex.
• within seconds after a sperm binds to an egg these vesicles called cortical
granules fuse with the egg plasma mebrance
• enzymes and other macro miles from the granules then trigger a cortical
reaction - which lifts the vitalise later away from the egg and hardens the
layer into a protective fertilisation envelope
• correlated with the release of calcium and the formation of the
fertilisation envelope
- egg activation
• fertilisation initiates and speeds up metabolic reactions that trigger the
onset of embryonic development
• marked rates of cellular respiration and protein synthesis in the egg
following fertilisation
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• sea urchin eggs have already completed meiosis when they are released
from the female
- fertilisation in mammals
• fertilise eggs internally
• support cells of the developing follicle surround the mammalian egg
before and after ovulation
• a sperm must travel through this layer of follicle cells before it reaches the
zone pellucid - the extracellular matrix of the egg
• there the binding of a sperm to a sperm receptor induces an acrosomal
reaction, facilitating sperm entry
• sperm binding triggers a cortical reaction - the release of enzymes from
cortical granules to outside of the cell
• these enzymes catalyse changes in the zone pellucid - which then
functions to slow block polyspermy
- cleavage
• rapid cell divisions
• cell cycle consists of primarily the S - DNA synthesis and M - mitotic
phases.
• the G1 and G2 phases are essentially skipped and little or no protein
synthesis occurs
• cleavage partitions the cytoplasm of the large fertilised egg into many
smaller cells called blastomeres
• the first five to seven cleavage divisions produce a hollow ball of cells, the
blastula - surrounding a fluid-filled cavity called the blastocoel.
• cleavage patterns in frogs
•
o cleavage is asymmetric
o
▪ reflecting the asymmetric distribution of yolk - stored
nutrients - across the egg
o yolk is often concentrated towards one pole - the vegetal pole and
away from the opposite or animal pole
o as a result - the two halves of the egg called the animal and vegetal
hemispheres differ in colour
o when an animal cell divides - an indentation called a cleavage
furrow forms in the cell surface as cytokinesis divides the cell in
half
o during these divisions the main effect of the yolk is to slow
competition of cytokinesis
• cleavage patterns in other animals
•
o yolk affects where division occurs in the eggs of frogs and other
amphibians - however the cleavage furrow still passes entirely
through the egg - said to be holoblastic
o
▪ also seen in echinoderms, mammals and annelids
o in those animals whose eggs contain a relatively little amount of
yolk - the blastocoel forms centrally and the blastomeres are often
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Document Summary
Bio1022 readings week 7 chapter 47. Regulation of cleavage initial protein development is carried out by rna and proteins deposited in the egg during oogenesis. Morphogenesis in animals involves specific changes in cell shape, position and survival. After cleavage - the rate of cell division slows considerably as the normal cell cycle is restored. The last two stages of embryonic development are responsible for morphogenesis the cellular and tissue based processes by which the animal body takes shape. During gastrulation - a set of cells at or near the surface of the blastula moves to an interior location - cell layers are established and a primitive digestive tube is formed. Further transformation occurs during organogenesis - the formation of organs. Cytoplasmic determinants and inductive signals contribute to cell fate specification. Developmental fate - where a cell resides, how it appears and what it foes.
