DB Gallery and Cinema:
Images of fertilization...
Fertilization unites the parental genomes carried by the sperm and egg, reconstituting
the diploid nucleus of the "zygote," and initiates the developmental
program(s) leading to embryogenesis.
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
syngamy restores to each the element necessary to further development.
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
depicts the acrosome reaction of sea urchin sperm (used with permission
of Chris Patton; see the Sea
Urchin Embryology Web site
waves during fertilization:
many species (including sea urchins, fish, and frogs) fusion of the sperm and
egg triggers release of Ca2+
from the subcortical ER, resulting in
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
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
granules and the vitelline envelope (fertilization envelope):
The transient rise in intracellular Ca2+
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
portrays fertilization and release of the cortical granules
in a sea urchin egg. Used with permission of Chris Patton.
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
(0.7 MB) or AVI.
A shows an unfertilized
egg (just prior to fertilization). Note the sperm (arrows) bound to the egg
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.
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
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
< 800 KB). Another
sequence showing raising of the vitelline envelope in A23187-treated eggs
GIF depicting fertilization, raising of the fertilization envelope, and
first cleavage in a sea urchin egg (used by permission of Chris Patton).
pronuclear migration, and cortical rotation in Xenopus laevis:
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.
here for enlarged view of polyspermic eggs at first/second division.
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)