brief echinoderm embryology

1. Echinoderm embryology

2. Radial cleavage and spiral cleavage
• Radial cleavage. : holoblastic cleavage that is typical
of deuterostomes and that is characterized by
arrangement of the blastomeres of each upper tier
directly over those of the next lower tier resulting
in radial symmetry around the pole to pole axis of the
embryo.
• Spiral cleavage is an early cleavage pattern in which
cleavage planes are not parallel or perpendicular to
the animal-vegetal pole axis of the egg. Cleavage
takes place at oblique angles, forming a “spiral”
pattern of daughter blasomeres.

3. Diagrammatic representation

4. Cleavage in sea urchin
• .Cleavage is the rapid mitotic division after fertilization.
• Sea urchin exhibit radial holoblastic cleavage
• The first and second cleavages are both meridional and are perpendicular to each other. That is to say, the
cleavage furrows pass through the animal and vegetal poles.
• The third cleavage is equatorial, perpendicular to the first two cleavage planes, and separates the animal and
vegetal hemispheres from one another .
• The fourth cleavage, however, is very different from the first three. The four cells of the animal tier divide
meridionally into eight blastomeres, each with the same volume. These cells are called mesomeres. The
vegetal tier, however, undergoes an unequal equatorial cleavage to produce four large cells, the macromeres,
and four smaller micromeres at the vegetal pole .
• As the 16-cell embryo cleaves, the eight mesomeres divide to produce two “animal” tiers, an1 and an2, one
staggered above the other.
• The macromeres divide meridionally, forming a tier of eight cells below an2. The micromeres also divide,
albeit somewhat later, producing a small cluster beneath the larger tier.
• All the cleavage furrows of the sixth division are equatorial, and the seventh division is meridional, producing
a 128-cell blastula

6. Blastulation
• The blastula stage of sea urchin
development begins at the 128-cell stage.
The cells form a hollow sphere surrounding
a central cavity, blastocoel . By this time, all
the cells are the same size, the micromeres
having slowed down their cell division.
Tight junctions unite the once loosely
connected blastomeres into a seamless
epithelial sheet that completely encircles
the blastocoel . As the cells continue to
divide, the blastula remains one cell layer
thick, thinning out as it expands.

7. • Cell cleavages last through the ninth or tenth
cell division, depending upon the species.
After that time, there is a mid-blastula
transition, when the synchrony of cell division
ends, new genes become expressed, and many
of the nondividing cells develop cilia on their
outer surfaces. The ciliated blastula begins to
rotate within the fertilization envelope. Soon
afterward, differences are seen in the cells.
The cells at the vegetal pole of the blastula
begin to thicken, forming a vegetal plate. The
cells of the animal half synthesize and secrete
a hatching enzyme that digests the fertilization
envelope . The embryo is now a free-
swimming hatched blastula.

8. Ingression and invagination
• infolding of a sheet of cells into an embryo is called invagination.
• migration of individual cells into the embryo is called ingression

9. Gastrulation in sea urchin
Sea urchin gastrulation begins with the ingression of primary mesnenchyme (also called skeletogenic cells )from the vegetal plate.
• Following ingression of Primary Mesenchyme Cells, there is a pause, and then the vegetal plate invaginates to form a stout cylinder
called the archenteron.
• After another pause, secondary mesenchyme cells appear at the tip of the archenteron, and at about this time the archenteron begins
to elongate across the blastocoel.
• Cells of the secondary mesenchyme contact the inner surface of the blastocoele wall. Once archenteron makes contact with opposite
wall, secondary mesenchyme cells disperse to eventually give rise to other mesodermal organs.
• Meanwhile, the Primary Mesenchyme Cells secrete calcium carbonate-containing skeletal rods called spicules.
• Finally, the gut, ectoderm, and the skeleton undergo further obvious differentiation to make the pluteus larva.