Fertilization

The ripe egg possesses all of the elements necessary for development
save an active division center. The sperm... ....possesses such a center
but lacks the protoplasmic substratum in which to operate. ...their union in
syngamy restores to each the element necessary to further development.

The acrosome reaction in sea urchins: Components of the egg jelly trigger the acrosome reaction in sea urchin sperm. The the acrosomal membrane fuses with the plasma membrane of the sperm, releasing the acrosome contents (including proteases and glycosidases for penetrating the egg jelly). Rapid polymerization of actin in the periacrosomal cap extends the acrosomal process, which can be visualized with fluorescent phalloidin (figure at left; U of U). This animation (animated GIF) depicts the acrosome reaction of sea urchin sperm (used with permission of Chris Patton; see the Sea Urchin Embryology Web site).

Ca²⁺ waves during fertilization: In many species (including sea urchins, fish, and frogs) fusion of the sperm and egg triggers release of Ca²⁺ from the subcortical ER, resulting in a Ca²⁺ wave that begins at the site of sperm penetration and passes throughout the egg.

Fertilization and Ca²⁺ wave in sea urchins: (used with permission of Mark Terasaki ). In this sequence, a sea urchin egg was viewed by phase contrast and fluorescence microscopy, using the indicator “Ca²⁺ green” to measure free intracellular Ca²+. The sperm enters from upper right (arrow), triggering the Ca²⁺ wave and elevation of the fertilization envelope (vitelline envelope). QT (1.5 MB) or AVI (1.5 MB).

This sea urchin egg was activated by injecting IP₃ (right). Intracellular Ca²⁺ was monitered by fluorescence microscopy. Used with permission of Richard Nuccitelli. QT (~85 kB). For time-lapse images of the Ca2+ wave in medaka (fish) eggs, see the the Developmental Biology Society's DB cinema!

Cortical granules and the vitelline envelope (fertilization envelope): The transient rise in intracellular Ca²⁺ initiates exocytosis of the cortical granules, elevating the vitelline envelope to provide a “slow” block to polyspermy (a “fast” or “electrical” block to polyspermy results from the transient depolarization of the egg membrane). This animated GIF portrays fertilization and release of the cortical granules in a sea urchin egg. Used with permission of Chris Patton.

In this time-lapse sequence, a lipophillic fluorescent dye (FM1-43) was used to visualize fusion of the cortical granules with the plasma membrane of a sea urchin egg. Used with permission of Mark Terasaki. QT (0.7 MB) or AVI.

Figure A shows an unfertilized egg (just prior to fertilization). Note the sperm (arrows) bound to the egg surface.

Figure B shows a fertilized egg in the process of raising the vitelline envelope (VE). Note the many sperm bound to the VE surface as it elevates.

Figure C shows a fertilized egg after complete elevation of the vitelline envelope, now referred to as the “fertilization envelope (FE).” Note the lack of sperm bound to the FE surface.

A time lapse sequence depicting raising of the vitelline envelope during the fertilization of sea urchin eggs, taken by students in the DB lab (QT or AVI; < 800 KB). Another sequence showing raising of the vitelline envelope in A23187-treated eggs (AVI). An animated GIF depicting fertilization, raising of the fertilization envelope, and first cleavage in a sea urchin egg (used by permission of Chris Patton).

Fertilization, pronuclear migration, and cortical rotation in Xenopus laevis: Xenopus eggs are arrested in second meiotic metaphase (for more detail on oogenesis in Xenopus). Upon fertilization, cell cycle arrest is released and meiosis is completed, forming the second polar body. The sperm centrioles recruit maternal proteins to become an active microtubule organizing center, and nucleating an extensive microtubule "sperm aster." The female pronucleus migrates along these MTs, meeting the male pronucleus near the center of the animal hemisphere. The nuclear envelopes breakdown as the egg enters the first mitotic division.

When MTs of the sperm aster reach the vegetal cortex at about 0.35 NT*, they form a complex mesh in the subcortical cytoplasm. These “cortical” MTs serve as tracks for motorproteins in the cortex, powering a 30 degree rotation of the egg cortex relative to the inner yolky cytoplasm. Cortical rotation begins at about 0.5 NT, and is completed just prior to initiation of the first cleavage division (1.0 NT). The cortical rotation signals the side opposite the sperm entry point to adopt a dorsal fate. * NT: normalized time, where fertilization =0 NT and first cleavage=1.0 NT.

Fertilization in low-sodium seawater disrupts the fast electrical block to polyspermy, resulting in fertilization by multiple sperm. The presence of extra centrosomes results in abberant cleavage at first and subsequent divisions.

Click here for enlarged view of polyspermic eggs at first/second division.

To ensure that gravity does not interfere with specification of the dorsal-ventral axis, raising of the vitelline envelope 10-20 minutes after fertilization (0.12-0.25 NT) allows the fertilized egg to rotate so that the A-V axis is aligned with the gravity vector (the animal pole is up and the "heavier" yolk-filled vegetal hemisphere is down). The "activation waves" occuring within minutes after sperm entry are also apparent in this video sequence (AVI: 1.5 MB; ~3-33 min post-fertilization)