11.12.1 Animal Development: A Close-up Look at Fertilization Events

Question 1

Animal Development: A Close-up Look at Fertilization Events

Answer 1

• Fertilization, the union of sperm and egg creating a diploid zygote, is not a single event but a complex series of processes.
• Polyspermy, the fertilization of an egg by more than one sperm, in animals will result in an incorrect number of chromosomes. To prevent this, two major cellular events occur after one sperm succeeds in penetrating the egg.
• The acrosomal reaction is characterized by a depolarization event preventing polyspermy during the first minute after fertilization.
• The cortical reaction converts the vitelline layer into the fertilization membrane while creating a water barrier in the perivitelline space. The events provide a more long-term prevention of polyspermy.

Question 2

note

Answer 2

• The phylogenetic tree of animals shown to the left reflects the fact that echinoderms and chordates share a recent common ancestry. Because the groups share ancestry, they also share genes that control development.
• Much of our knowledge of fertilization is based on research on sea urchins (Phylum Echinodermata) because they share a lot of evolutionary history with us, yet are much simpler organisms.
• Sea urchins use external fertilization and the gametes have adaptations to accomplish the required species recognition. A gelatinous coat surrounds the plasma membrane of the sea urchin egg providing a physical barrier from foreign sperm. A proteinaceous vitelline layer lies interior to the plasma membrane and contains species-specific receptors. Cortical granules play a role in preventing polyspermy.
• Each sperm is equipped with a flagellum to travel and find the egg. A single mitochondrion is wound around the tail of the sperm and fuels the energy required for sperm motility. The sperm head is capped by a vesicle called the acrosome, which contains digestive enzymes. Release of these enzymes enables the sperm to create a passage through the gelatinous coat and gain access to the vitelline layer.
• The illustration on the left shows a sperm penetrating the gelatinous layer of an egg. The acrosomal membrane breaks down and the acrosome releases digestive enzymes. A protrusion called the acrosomal process extends from the sperm head. The process extends through the jelly coat and attaches to the vitelline layer. The sperm fuses with the plasma membrane of the egg. The sperm releases its nucleus
  into the egg’s cytoplasm.

Question 3

Polyspermy Prevention:

Answer 3

• The acrosomal reactionis called the fast block to polyspermy because it occurs one to three seconds after the sperm contacts the vitelline layer. As soon as the sperm fuses with the plasma membrane of the egg, a sodium ion depolarization occurs. Sodium ion channels in the egg open allowing sodium to rush into the egg. Membrane characteristics of the egg change
  preventing any additional sperm from entering.
• During depolarization, an electrical charge occurs inside of the cell resulting in a net negative charge relative to the outside of the cell.
• The cortical reaction provides long-term prevention of
  polyspermy. The endoplasmic reticulum (shown in the upper right corner of the illustration) of the egg releases calcium ions which cause the cortical granules to fuse with the plasma membrane. Upon fusing, the cortical granules release enzymes and mucopolysaccharides into the perivitelline space. The enzymes change the character of the vitelline layer, while the mucopolysaccharides cause water to osmotically enter the perivitelline space. The resulting barrier is called the fertilization membrane, through which no sperm can enter.

Question 4

Why would it be important to prevent polyspermy?

Answer 4

• Polyspermy results in an incorrect chromosome number.

Question 5

In sea urchin eggs, the cortical reaction

Answer 5

• results in the formation of the fertilization membrane.

Question 6

The fusion of sperm and egg cell membranes triggers the release of __________ from the egg’s endoplasmic reticulum into the cytosol.

Answer 6

• calcium ions

Question 7

Place the following three events that take place during fertilization in order from earliest to latest.

1. membrane depolarization
2. release of enzymes and mucopolysaccharides into perivitelline space
3. increase in intracellular calcium

Answer 7

• 1, 3, 2

Question 8

What is released from the acrosome of the sperm?

Answer 8

• hydrolytic enzymes

Question 9

Why can fertilization events of sea urchins be used to learn about fertilization in humans?

Answer 9

• Echinoderms share ancestry with the chordates, and many of their fertilization events are similar.

Question 10

The acrosome of a sperm

Answer 10

• carries enzymes that are necessary for fertilization.

Question 11

Which statement below is the best description of polyspermy?

Answer 11

• More than one sperm releasing its nucleus into the egg