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WHAT HAPPENS DURING FERTILIZATION?

NOVA

1. Recognition

Sperm secretes protein that binds to receptors on vitelline layer

Vitelline layer:

- Glycoprotein layer surrounding plasma membrane of egg- Called the zona pellucida in humans

- Ensures same species fertilization

2. Penetration

Plasma membranes of sperm and egg fuse

Sperm nucleus enters egg

3. Formation of Fertilization Membrane

Penetration causes electrochemical change in Vitelline layer

Vitelline layer becomes a sperm-blocking Fertilization Membrane

4. Fusion of nuclei

Sperm and ovum nuclei fuse

Zygote forms with 23 chromosome pairs (human)

Zygote formation PBS

WHAT HAPPENS AFTER THE ZYGOTE FORMS?

Rapid cell divisions of zygote(without cell growth) form blastomeres

Blastomeres have much less cytoplasm than original zygote

Early cleavage characteristics:

1. Embryo polarity develops (except in mammals)

Animal pole

- Upper pole, contains less yolk

- Usually becomes anterior end of embryo

Vegetal pole

- Lower pole, contains more yolk (stored food)

- Heavier, so settles to bottom pole

2. Polar and equatorial cleavage

Polar (vertical) cleavage -divide egg pole to pole

Equatorial (horizontal) cleavageparallel with equator

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WHAT HAPPENS AFTER THE ZYGOTE FORMS (continued)?

3. Radial and spiral cleavages

Radial Cleavage

- At 8 cell stage, 4 upper cells directly over 4 lower cells

- Characteristic of deuterostomes (ex. echinoderms, chordates)

Spiral Cleavage

- At 8 cell stage, 4 upper cells sit in grooves between 4 lower cells

- Characteristic of protostomes (ex. mollusks, annelids, arthropods)

4. Indeterminate and determinate cleavages

Indeterminate

- Isolated early cleavage cells can develop into complete embryo

- Characteristic of radial cleavage of deuterostomes

Ex. Identical twins of humans

Determinate

- Isolated early cleavage cells form inviable embryo lacking parts

- Characteristic of spiral cleavage protostomes

Continued cleavage characteristics (beginning at 8 cell stage):Morula

Solid ball of blastomeres that results from cleavages

Blastula (in mammals, called Blastocyst)

Hollowed out morula made of single cell layer

Blastocoel fluid filled cavity of morula

Inner Cell Mass (embryonic disc)

Group of cells in blastocyst that becomes embryo

(source of stem cell lines)

Trophoblast Outer layer of blastocyst

Embeds into endometrium of uterus

Produces HCG (human chorionic gonadotropin)

- Maintains progesterone production of corpus luteum

WHAT HAPPENS AFTER THE ZYGOTE FORMS?

Cat ovary

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Some cells at or near surface of blastula invaginate (move inward)

Forms a three-layered embryo called gastrula

Three primary germ layers of gastrula1. Ectoderm

Outer layer; forms skin and nervous system in vertebrates

2. Mesoderm

Middle layer; forms skeleton, muscles, circulatory system in vertebrates

3. Endoderm

Inner layer; forms lining of digestive tract & respiratory tract in humans

Archenteron

Center cavity surrounded by endoderm cells; forms primitive gut

BlastoporeOpening in archenteron; mouth in protostomes; anus in deuterostomes

WHAT IS GASTRULATION?

Gastrulation PBS

Development of organs after cell differentiationFirst organs to take shape in chordates is notochord and neural tube

Notochord

Stiff dorsal skeletal rod formed from mesoderm Becomes vertebrae in higher animals

Neural Plate

Forms from ectoderm above notochord Folds inward, rolling into neural tube which forms brain and Spinal cord Neural tube becomes brain and spinal cord (CNS)

WHAT IS ORGANOGENESIS?

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Figure 20.3

Zygote formation PBS

Figure 20.4