Animal Development

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EGG CYTOPLASM

Slide 8

The Cortical Reaction

Fusion of egg and sperm also initiates the cortical reaction:

This reaction induces a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg.

These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy.

Slide 9

Activation of the Egg

The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell.

With these rapid changes in metabolism, the egg is said to be activated.

The sperm nucleus merges with the egg nucleus and cell division begins.

Slide 10

Fertilization in Mammals

Fertilization in mammals and other terrestrial animals is internal.

In mammalian fertilization, the cortical reaction modifies the zona pellucida, the extracellular matrix of the egg, as a slow block to polyspermy.

In mammals the first cell division occurs 12–36 hours after sperm binding.

The diploid nucleus forms after this first division of the zygote.

Slide 11

Fertilization in mammals

Follicle cell

Zona pellucida

Cortical granules

Sperm nucleus

Sperm basal body

Slide 12

Cleavage = Rapid Mitosis / No Mass change

Fertilization is followed by cleavage, a period of rapid cell division without growth.

Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres.

The blastula is a ball of cells with a fluid-filled cavity called a blastocoel.

Slide 13

Cleavage in an echinoderm embryo

(a) Fertilized egg

(b) Four-cell stage

(c) Early blastula

(d) Later blastula

Slide 14

The eggs and zygotes of many animals, except mammals, have a definite polarity.

The polarity is defined by distribution of yolk (stored nutrients).

The vegetal pole has more yolk; the animal pole has less yolk.

Slide 15

The three body axes are established by the egg’s polarity and by a cortical rotation following binding of the sperm.

Cortical rotation exposes a gray crescent opposite to the point of sperm entry.

Slide 16