Holoblastic Cleavage
Radial - Sea Urchin
animal pole
vegetal pole
mesomeres
micromeres
•
•
•
•
macromeres
First cleavage is meridional.
Second cleavage is meridional, and at right angles to the first.
Third cleavage is equatorial, and at right angles to the first and second.
Fourth cleavage is meridional in the animal half, unequal equatorial in the vegetal half.
Radial - Amphibians
3
1
1
2
2
1
grey crescent
4
3
3
4
1
5
4
1
2
•
•
•
•
•
2
5
First cleavage is meridional and bisects the grey crescent.
Second cleavage is meridional, and at right angles to the first.
Third cleavage is unequal equatorial, and at right angles to the first and second.
The fourth cleavage is meridional.
The fifth cleavage is unequal equatorial.
Developmental Biology Handout 04.1
Spiral - Snail
•
•
•
•
The diagram shows the embryo looking down on the animal pole
First cleavage is oblique, but almost meridional.
Second cleavage is oblique, but almost meridional.
Third cleavage is unequal and oblique, but almost equatorial. The smaller animal half
blastomeres are displaced clockwise relative to the larger vegetal half blastomeres.
• Subsequent cleavages produce animal half cells displaced alternately anticlockwise
and clockwise.
Rotational - Rabbit
2A
1
2B
• First cleavage is meridional.
• Second cleavage is usually meridional/equatorial, and at right angles to the first.
Developmental Biology Handout 04.2
Rotational - Compaction and Blastocyst Formation
1-cell stage
2-cell stage
4-cell stage
blastocoel
8-cell stage
compaction
blastocyst
• Compare the four-cell stage with the previous diagram.
• At the eight-cell stage the blastomeres begin to adhere to each other more tightly and
the embryo undergoes compaction.
• At the sixteen cell stage one or two cells are completely internal. These form the
embryo (occasionally with an external cell. The remaining external cells form the
trophoblast, which forms the embryonic portion of the placenta.
• The trophoblast secretes fluid, leading to formation of the blastocoel. The resulting
embryonic stage is called a blastocyst.
Developmental Biology Handout 04.3
Meroblastic Cleavage
Discoidal - Chicken
cleavage furrow
blastoderm
• Cleavage is confined to a small disc of non-yolky cytoplasm on top of the yolk.
• A single layer of cells is produced which then divides producing a layer five to six
cells thick.
Discoidal - Blastocoel Formation
blastocoel
yolk
subgerminal space
epiblast
hypoblast
• Cells are lost from the center of the blastodisc.
• Ingression of cells from the center of the blastodisc and migration from the posterior
edge of the blastodisc produce the hypoblast.
Superficial - Fruit Fly
syncytial
blastoderm
•
•
•
•
pole
cells
cellular
blastoderm
The diagrams approximate embryos after 7, 9, 10, 12, and 13 rounds of mitosis.
Nuclei divide by mitosis without intervening cytokinesis.
Most nuclei migrate to the surface of the egg. This is the syncytial blastoderm stage.
Membranes form around nuclei at the posterior of the egg. The resulting pole cells will
give rise to the adult germ line.
• Membranes form around the other nuclei at the egg surface after 13 rounds of mitosis.
This is the cellular blastoderm stage.
Developmental Biology Handout 04.4