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ZYGOTE FORMATION, CLEAVAGE & BLASTULA FORMATION
R. Mareeswaran, (09BC011)
Second year – M.Sc., Biochemistry
EARLY EMBRYONIC DEVELOPMENT
Cleavage starts
Fertilizationof ovum
Oviduct
Secondaryoocyte
Ovulation
Ovary
Blastocyst(implanted)
Endometrium
Uterus
FERTILIZATION..... Fertilization is the process whereby two sex
cells (gametes) fuse together to create a new individual with genetic potentials derived from both parents.
Also known as Zygote formation Not a single event – occurs in a series of
steps
EVENTS …………
1. Contact and recognition 2. Regulation of sperm entry into the
egg. 3. Fusion of the genetic material of
sperm and egg. 4. Activation of egg metabolism
SPERM CAPACITATION….. Freshly ejaculated sperm are unable or
poorly able to fertilize. Must undergo a series of changes -
collectively known as Capacitation. Capacitation is associated with removal of
adherent seminal plasma proteins, reorganization of plasma membrane lipids and proteins.
Capacitation occurs while sperm reside in the female reproductive tract for a period of time, as they normally do during gamete transport.
Capacitation appears to destabilize the sperm's membrane to prepare it for the acrosome reaction
PHASES…. Phase 1 - Penetration of the corona radiata; Phase 2 - Penetration of the zona pellucida Phase 3 - Fusion of the oocyte and sperm cell
membranes.
PHASE 1: PENETRATION OF THE CORONA RADIATA
Of 200 to 300 million spermatozoa only 300 to 500 reach the site of fertilization.
Only one of these fertilizes the egg. Capacitated sperm pass freely through
corona cells
PHASE 2: PENETRATION OF THE ZONA PELLUCIDA
Zona - glycoprotein shell Facilitates and maintains sperm binding and
induces the acrosome reaction. Both binding and the acrosome reaction are
mediated by the ligand ZP3, a zona protein. Release of acrosomal enzymes (acrosin) allows
sperm to penetrate the zona, Permeability of the zona pellucida changes
when the head of the sperm comes in contact with the oocyte surface.
Results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte.
ACROSOMAL REACTION
The acrosome reaction provides the sperm with an enzymatic drill to get throught the zona pellucida.
The same zona pellucida protein that serves as a sperm receptor also stimulates a series of events that lead to many areas of fusion between the plasma membrane and outer acrosomal membrane.
Membrane fusion (actually an exocytosis) and vesiculation expose the acrosomal contents, leading to leakage of acrosomal enzymes from the sperm's head.
As the acrosome reaction progresses and the sperm passes through the zona pellucida, more and more of the plasma membrane and acrosomal contents are lost.
By the time the sperm traverses the zona pellucida, the entire anterior surface of its head, down to the inner acrosomal membrane, is denuded.
The constant propulsive force from the sperm's flagellating tail, in combination with acrosomal enzymes, allow the sperm to create a tract through the zona pellucida.
Once a sperm penetrates the zona pellucida, it binds to and fuses with the plasma membrane of the oocyte.
PHASE 3: FUSION OF THE OOCYTE ANDSPERM CELL MEMBRANES
Initial adhesion of sperm to the oocyte - by the integrins & disintegrins
After penetration egg responds in three ways:
1. Cortical and zona reactions. 2. Resumption of the second meiotic
division 3. Metabolic activation of the egg
THE ZONA REACTION
The cortical reaction refers to a massive exocytosis of cortical granules seen shortly after sperm-oocyte fusion.
Cortical granules contain a mixture of enzymes, including several proteases
Proteases alter the structure of the zona pellucida, inducing what is known as the zona reaction.
Components of cortical granules may also interact with the oocyte plasma membrane.
The critical importance of the zona reaction is that it represents the major block to polyspermy in most mammals.
This effect is the result of two measurable changes induced in the zona pellucida:
1. The zona pellucida hardens. 2. Sperm receptors in the zona pellucida are destroyed
THE ZONA REACTION ANIMATION
EGG ACTIVATION…… Prior to fertilization, the egg is in a quiescent
state, arrested in metaphase of the second meiotic division.
Upon binding of a sperm, the egg rapidly undergoes a number of metabolic and physical changes that collectively are called egg activation.
Prominent effects include a rise in the intracellular concentration of calcium, completion of the second meiotic division and the so-called cortical reaction.
FUSION OF GENETIC MATERIAL……
Pronuclear migration takes about 12 hours Male and female pronuclei are
indistinguishable
SUMMARY….
The spermapproachesthe egg
12
3
4
The sperm’sacrosomal enzymesdigest the egg’s jelly coat
Proteins on thesperm head bindto egg receptors
5 The spermnucleus enters the egg cytoplasm
The plasma membranesof sperm and egg fuse
6 Afertilizationenvelopeforms
SPERM
Nucleus
AcrosomePlasmamembrane
Spermhead
Acrosomalenzymes
Jellycoat
Vitellinelayer
Plasmamembrane
Cytoplasm
EGG CELL
Receptor proteinmolecules
Spermnucleus
Eggnucleus
7 The nucleiof spermand egg fuse
Zygotenucleus
RESULTS OF FERTILIZATION
Restoration of the diploid number of chromosomes
Determination of the sex Initiation of cleavage
BLOCKING OF POLYSPERMY
2 Types Fast block – Changing electric potential Slow block – Zona reaction
CLEAVAGE…. Mitosis and cytokinesis of the zygote produces an
increasing number of smaller cells (Blastomeres), each with an exact copy of the genome present in the zygote.
Compaction – Tight junction & Gap junction Morula – 16 cell Cleavage ends with the formation of a blastula.
CLEAVAGE PATTERN
BLASTOCYST FORMATION
Morula enters the uterine cavity Blastocele Blastocyst – Embryoblast & Tropoblast
ENDOMETRIUM
Inner cell mass
Cavity
Trophoblast
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