Development

Fertilization

Chemotaxis

• Sea Urchin’s eggs have a chemotatic molecule called resact.

• This molecule is found in the outer jelly coat of the egg and attract sperm.

• Sperm have receptors on their surface that will bind to resact and trigger the sperm tail to drive in the sperm.

• Fertilization in Mammals.• Capacitation, a function of the female reproductive

system, enhances sperm function.– A capacitated

sperm migratesthrough a layerof follicle cellsbefore it reachesthe zona pellucida.

– Binding ofthe sperm cellinduces anacrosomalreaction similarto that seen in thesea urchin.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.5

Cortical reaction forms slow block

Cortical reaction forms slow block

• The Acrosomal Reaction.– Acrosomal reaction: when exposed to the jelly

coat the sperm’s acrosome discharges• Enzymes enable the acrosomal process to penetrate the

egg’s jelly coat.

• The tip of the acrosomal process adheres to the vitelline layer just external to the egg’s plasma membrane.

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– The sperm and egg plasma membranes fuse and a single sperm nucleus enter the egg’s cytoplasm.• Na+ channels in the egg’s plasma membrane opens.

– Na+ flows into the egg and the membrane depolarizes: fast block to polyspermy.

Na+

Bindin in the acrosomal process will bind to the egg’s protein recptors ensuring species recognition

• The Cortical Reaction.– Fusion of egg and sperm plasma membranes triggers

reaction• Ca2+ from the eggs ER is released into the cytosol and propagates as

a wave across the fertilized egg – cortical granules release contents

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The Cortical Reaction• Fusion of egg and sperm also initiates the

cortical reaction– Inducing a rise in Ca2+ that stimulates cortical

granules to release their contents outside the egg

Figure 47.4

A fluorescent dye that glows when it binds free Ca2+ was injected into unfertilized sea urchin eggs. After sea urchin sperm were added, researchers observed the eggs in a fluorescence microscope.

EXPERIMENT

RESULTS

The release of Ca2+ from the endoplasmic reticulum into the cytosol at the site of sperm entry triggers the release of more and more Ca2+ in a wave that spreads to the other side of the cell. The entire process takes about 30 seconds.CONCLUSION

30 sec20 sec10 sec afterfertilization

1 sec beforefertilization

Point ofspermentry

Spreading waveof calcium ions

500 m

• Activation of the Egg,– High concentrations of Ca2+ in the egg stimulates an

increase in the rates of cellular respiration and proteins synthesis.

– In sea urchins, DAG activates a protein that transports H+ out of the egg.• The reduced pH may be indirectly responsible for the

egg’s metabolic responses to fertilization.

• In the meantime, back at the sperm nucleus...• The sperm nucleus swells and merges with the egg

nucleus diploid nucleus of the zygote.– DNA synthesis begins and the first cell division occurs.

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Actin filaments

Depolarization provides Fast Block - blocks multiple fertilizations

Fusion>release of Ca2+ into cystol>IP3 production>ligand-gated Ca2+ channel opens in ER>Cortical granules fuse with plasma membrane>enzymes from granule released into perivitelline space>swelling pushes vitillene layer away from plasma membrane and along with enzymes hardening it >turns it into the fertilization membrane (Slow Block to polyspermy)

Fusion>release of Ca2+ into cystol>IP3 production>ligand-gated Ca2+ channel opens in ER>Cortical granules fuse with plasma membrane>enzymes from granule released into perivitelline space>swelling pushes vitillene layer away from plasma membrane and along with enzymes hardening it >turns it into the fertilization membrane (Slow Block to polyspermy)

• Fertilization of the egg by the sperm forms the zygote

• Cleavage follows fertilization.

• Cleavage is a type of mitosis where the cell divides rapidly with little or no growth.

– The zygote is partitioned into blastomeres.

3. Cleavage partitions the zygote into many smaller cells

Fig. 47.6Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Cleavage partitions the cytoplasm of one large cell– Into many smaller cells called blastomeres

Figure 47.7a–d

Fertilized egg. Shown here is thezygote shortly before the first cleavage division, surrounded by the fertilization envelope. The nucleus is visible in the center.

(a) Four-cell stage. Remnants of the mitotic spindle can be seen between the two cells that have just completed the second cleavage division.

(b) Morula. After further cleavage divisions, the embryo is a multicellular ball that is stillsurrounded by the fertilization envelope. The blastocoel cavityhas begun to form.

(c) Blastula. A single layer of cells surrounds a large blastocoel cavity. Although not visible here, the fertilization envelope is still present; the embryo will soon hatch from it and begin swimming.

(d)

• In both sea urchins and frogs first two cleavages are vertical.

• The third division is horizontal.

• The result is an eight-celled embryo with two tiers of four cells.

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Fig. 47.8a

• Continued cleavage produces the morula.

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Fig. 47.8b

• A blastocoel forms within the morula blastula

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Fig. 47.8d

Yolk impedes cleavage establishing vegetal pole

Yolk impedes cleavage establishing vegetal pole

• Gastrulation in a sea urchin– Produces an

embryo with a primitive gut and three germ layers

Organogenesis

• Various regions of the three embryonic germ layers– Develop into the rudiments of organs during

the process of organogenesis