Phyla Cnidaria and Ctenophora, Basic Animal Development ...

Phyla Cnidaria and Ctenophora, Basic Animal Development &
Introduction to Nervous Systems 5.4
Lab #5 - Biological Sciences 102 – Animal Biology
In animals such as sponges and cnidarians, cleavage is irregular and seemingly disorganized;
egg cytoplasm is partitioned randomly into daughter cells of highly variable size and shape with
no apparent relevance to future cell fates. As the metazoa evolved cleavage began to follow
precise patterns and rhythms. In virtually all animal groups above the cnidarians, cleavage is
regular; the egg cytoplasm is segregated into specific cells called blastomeres
(Gr. blastos, bud, + meros, part) occupying discrete positions and having specific developmental
fates.
Patterns of regular cleavage depend greatly on amount and distribution of yolk in the egg. In
eggs having a large amount of yolk, cleavage may be either complete (= holoblastic), as in
amphibians, or incomplete (= meroblastic), as in birds and reptiles. In birds and reptiles with
extreme telolecithal (Gr. telos, end, + lekithos, yolk) eggs, cleavage is restricted to a small disc
of cytoplasm on the animal pole; this type of cleavage is called discoidal. The eggs of most
insects follow another pattern of cleavage called superficial. In these the nuclei divide
mitotically into hundreds or thousands of "free" nuclei, which later migrate to the egg surface.
Only then do cleavage furrows form, rapidly partitioning the cytoplasm into a superficial layer
of cells.
In most invertebrates, eggs have little yolk (= isolecithal ["equal-yolk"]), and cleavage is
complete (holoblastic) and equal. Two major kinds of holoblastic cleavage exist: spiral and
radial (see text page 156, fig 8-7). The first two cleavages are the same in both kinds of eggs:
the cleavage planes are along the animal-vegetal axis, producing a quartet of cells. At the third
cleavage, however, these two patterns, spiral and radial, can be distinguished from each other
by the geometric positioning of the cells.
In radial cleavage, the third cleavage is perpendicular to the first two, yielding two quartets of
cells, with the upper quartet lying directly on top of the lower. In spiral cleavage, the third
cleavage planes are oblique to the polar axis and typically produce an upper quartet of smaller
cells that come to lie between the furrows of the lower quartet of larger cells.
There are other important differences between these two cleavage patterns. Spiral cleavage is
typically mosaic, meaning that the embryo is constructed as a mosaic, with each cell fitting
into its predetermined location in the larval body. If cells of the embryo are experimentally
separated at this early stage, each cell will develop into partial or defective larvae because the
developmental fate of each cell has already been determined. Spiral cleavage is found in
several phyla, including annelids, many molluscs, some flatworms, and ribbon worms
(nemerteans). All groups showing spiral cleavage belong to the grouping of animal phyla called
the Protostomia, in which the embryonic blastopore forms the mouth.
Early Embryonic Development - Cell Cleavages (mitosis)
Radial cleavage is characteristically regulative
because cell fate does not become fixed until after
the first few cleavages. Radial cleavage is found in
eggs of echinoderms and many chordates,
especially protochordates, amphibians, and
mammals. (As mentioned earlier, eggs of birds and
reptiles, as well as many fishes, show discoidal
cleavage.) All of these belong to the
Deuterostomia, a group of phyla in which the
mouth is formed from a secondary embryonic
opening.