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

Thank You !