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 migrates through a layer of follicle cells before it reaches the zona pellucida. –Binding of the sperm cell induces an acrosomal reaction similar to that seen in the sea urchin. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

–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 Ca 2+ from the eggs ER is released into the cytosol and propagates as a wave across the fertilized egg – cortical granules release contents Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The Cortical Reaction Fusion of egg and sperm also initiates the cortical reaction –Inducing a rise in Ca 2+ that stimulates cortical granules to release their contents outside the egg Figure 47.4 A fluorescent dye that glows when it binds free Ca 2+ 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 Ca 2+ from the endoplasmic reticulum into the cytosol at the site of sperm entry triggers the release of more and more Ca 2+ in a wave that spreads to the other side of the cell. The entire process takes about 30 seconds. CONCLUSION 30 sec 20 sec 10 sec after fertilization 1 sec before fertilization Point of sperm entry Spreading wave of calcium ions 500  m

Activation of the Egg, –High concentrations of Ca 2+ 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Actin filaments Depolarization provides Fast Block - blocks multiple fertilizations Fusion>release of Ca 2+ into cystol>IP 3 production>ligand-gated Ca 2+ 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)

Enzymes from the acrosome enable the sperm cell to penetrate the zona pellucida and fuse with the egg’s plasma membrane. –The entire sperm enters the egg. –The egg membrane depolarizes: functions as a fast block to polyspermy. –A cortical reaction occurs. Enzymes from cortical granules catalyze alterations to the zona pellucida: functions as a slow block to polyspermy. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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 Copyright © 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 the zygote 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 still surrounded by the fertilization envelope. The blastocoel cavity has 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.8a

Continued cleavage produces the morula. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.8b

A blastocoel forms within the morula  blastula Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.8d

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