Killifish Embryos
These three panels are images of killifish embryos (Fundulus heteroclitus).
Since fish embryos have a lot of yolk, development begins with cells forming
on top of the embryo. The cells then spread to cover the sphere in the
process of epiboly. If you look closely at the first image, you should
see that the top half is covered with cells (looks a little fuzzier), while
the bottom half of the sphere is much smoother (the yolk has not yet been
covered by cells). In later-stage killifish embryos, the embryonic axis
is visible as a curved line along one side of the embryo (middle image).
In even older embryos, a close-up of a small region of the yolk sac shows
black and yellow pigment cells, and blood vessels with streaming blood
cells.
Embryonic Killifish Epithelial Cells
These two panels are of embryonic killifish epithelial cells, stained
with a fluorescent lipid. This bright orange molecule is incorporated
into the plasma membrane of the cells. In killifish (but not most other
fish) the epithelial cells have special membrane properties such that
you can actually watch membrane turnover using these fluorescent markers.
With time, the fluorescent lipid is cleared from the membrane (internalized
by a process called endocytosis)--first at the "corners" where
neighboring
cells come in contact (first panel), and then spreading to the entire
margin of each cell. This turnover is visible as black dots/lines, because
the fluorescent orange lipid is taken inside the cell, and no longer
visible in this surface view.
Urchin Embryos
These four panels are of sea urchin embryos. The first shows a newly fertilized egg. The large "dot" you can see near the top of the spherical zygote is the egg nucleus, and the smaller dot near 5 o'clock is the nucleus delivered by the successful sperm. The embryo is surrounded by a clear fertilization envelope, which prevents any other sperm from fusing with the egg. In the second panel, you are looking at a sixteen-cell-stage urchin embryo. At this stage, the embryo is made of four small cells (foreground), four larger cells, and 8 middle-sized cells (out of focus in the background). The third panel shows a gastrula-stage embryo, and the large tube in the middle is the archenteron, or embryonic gut. In the fourth panel, you can see a mature, swimming, feeding urchin larva called a pluteus. The bright lines you can see are the larval skeleton, made of calcium carbonate. To see a video of such an embryo, go to the Biology 305 class video page.
Tiny Rear Fin of a Larval Squid
This is the tiny rear fin of a larval squid. The embryo was soaked in
a fluorescent lipid, which was taken up by the peripheral nerve cells.
You can see the brightly-staining orange nerve cell bodies along the
periphery of the fin, and the long filamentous axonal processes extending
inward.