Fertilization - fusion of 2 gametesensure encounter of sperm & oocyte/egg

External fertilization # of gametesstimulus for release

Internal fertilizationcopulatory organs

receptors PH-30 humans

activation of metabolic dormant oocyte/egg - different stages of oogenesis

fusion of 2 haploid nuclei - syngamy to produce diploid embryo

The Acrosomal Reaction.Acrosomal reaction: when exposed to the jelly coat the sperm’s acrosome discharges it contents by exocytosis.

Hydrolytic enzymes enable the acrosomal process to penetrate the egg’s jelly coat..

B locks to Polyspermy1. Fast Block - short-lived

Na+ channels in the egg’s plasma membrane opens.Na+ flows into the egg and the membrane depolarizes

2. The Cortical Reaction - slow, long-lastingCa2+ from the eggs ER is released into the cytosol and stimulates cortical granules to fuse with the plasma membrane and release their contents into the perivitelline space.

The vitelline layer separates from the plasma membrane.An osmotic gradient draws water into the perivitelline space, swelling it and pushing it away from the plasma membrane.The vitelline layer hardens into the fertilization envelope: a component of the slow block to polyspermy.The plasma membrane returns to normal and the fast block to polyspermy no longer functions.

Activation of the Egg,High concentrations of Ca2+ in the egg stimulates an increase in the rates of cellular respiration and

proteins synthesis.

The sperm nucleus swells and merges with the egg nucleus diploid nucleus of the zygote.

DNA synthesis begins and the first cell division occurs.

In amphibians a rearrangement of the egg cytoplasm occurs at the time of fertilization.The plasma membraneand cortex rotatetoward the pointof sperm entry.

The gray crescentis exposed and marksthe dorsal surfaceof the embryo.

Fertilization in Mammals.Capacitation, a function of the female reproductive system removes inhibitory enzymes on the sperm head which allows the acrosomal reaction to occur.

A capacitatedsperm migratesthrough a layerof follicle cellsbefore it reachesthe zona pellucida.Binding ofthe sperm cellinduces anacrosomalreaction.

Voltage Clamps and Fast Block to Polyspermy

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Cleavage follows fertilization.

Functions of cleavage:Multicellular for differentiation

The zygote is partitioned into blastomeres.Each blastomere contains different regions of the undivided cytoplasm and thus different cytoplasmic determinants.

Restores Somatic Nuclear to Cytoplasmic Ratio1:500 -> ->-> 1:6

sea urchin fertilized egg at end of cleavage - somatic cell

Controls on # of cleavage divisions?2n = 6 divisions4n = ?n = ?

Holoblastic unequal where thereis more yolk at the vegetal pole.Cleavage occurs morerapidly in the animalpole than in thevegetal pole.

first two cleavages are vertical.The third division is horizontal.The result is an eight-celled embryo with two tiers of four cells Yolk is most concentrated at the vegetal pole and least concentrated at the animal pole.

In animals with less yolk there is complete division of the egg: holoblastic cleavage.

Holoblastic equal

A blastocoel forms within the morula blastula

In birds the yolk is so plentiful that it restricts cleavage to the animal pole: meroblastic cleavage.

• Superficial cleavage in insects

• Holoblastic equal in mammals

What Controls Cleavage?Information in Cytoplasm – Source?

Spiral Cleavage in SnailsRt = Dextral (dominant)Left= recessive

Female dd X male DDF1 genotype vs. phenotype

F1 crossdD X dDPrediction? Genotype

Phenotype

Gastrulation rearranges the embryo into a triploblastic gastrula.– The embryonic germ layers are the ectoderm,

mesoderm, and endoderm.

Gastrulation rearranges the blastula to form a three-layered

embryo with a primitive gut

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fate map – vital stain

Invagination

InvolutionEpiboly

Avian Development.Cleavage is meroblastic, or incomplete.Cell division is restricted to a small cap of cytoplasm at the animal pole.Produces a blastodisc, which becomes arranged into the epiblast andhypoblast thatbound theblastocoel, theavian versionof a blastula.

During gastrulation some cells of the epiblast migrate (arrows) towards the interior of the embryo through the primitive streak

Ingression(immigration)

Once again, the embryonic membranes – homologous with those of shelled eggs.

Chorion: completely surrounds the embryo and other embryonic membranes.Amnion: encloses the embryo in a fluid-filled amniotic cavity.Yolk sac: found below the developing embryo.

Develops from the hypoblast.Site of early formation of blood cells which later migrate to the embryo.

Allantois: develops as an outpocketing of the embryo’s rudimentary gut.Incorporated into the umbilical cord, where it forms blood vessels.

Activation of embryonic genome

Mexican axolotl o-mutant strain The “o” gene is a recessive gene “O” gene is the normal, dominant gene

In embryos obtained from female axolotls homozygous for gene “o”, development is always arrested during gastrulation.

WHY? “O” protein is necessary to activate the embryonic genome.

PROOF?

The eggs can be rescued by injecting eggs of o/o females with cytoplasm from normal eggs.

Where is the "corrective component" (O protein) produced?Rescue Experiments Cytoplasm from Egg

vs.Primary Oocytes Cytoplasm

vs. Primary Oocyte fluid from germinal vesicle

What is the “O” protein doing?

activation of the embryonic genome.

How can we prove it?

When is mammalian embryonic genome activated?

Two-cell stage determined by radioactive labeling experiments

Imprinting of mammalian gamete’s nucleus

Experiments combining:

Female pronucleus combined with Female pronucleus

Male pronucleus combined with Male pronucleus