EMBRYOLOGY
Lec.5
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Cleavage
refers to the stereotyped pattern of early mitotic divisions that
divides up the large volume egg cytoplasm. The early zygote is
unique in being so large. Most cells undergo a period of growth
between cycles of mitosis, but this is not true for early cleavage
stage blastomeres. With each division the cells get smaller.
PATTERNS OF EMBRYONIC CLEAVAGE
Pattern of embryonic cleavage is determined both by the
position of the mitotic spindles and by the amount and
distribution of yolk.
The types of eggs based on yolk characteristics are described as:
Isolecithal: sparse evenly distributed yolk, eg., sea urchin,
mouse
Mesolecthal: moderate amount of yolk, often unevenly
distributed,
eg.,
frog
Telolecithal: dense yolk concentrated at one end, eg., bird,
reptile
Centrolecithal:yolk concentrated at the middle of the egg,eg.fly
There are several types of cleavage symmetry seen in nature:
radial (echinoderms, amphibians), spiral (mollusks, annelids),
Bilateral (ascidians,tunicates), Rotational (mammals). The two
figures below show examples of holoblastic and meroblastic
cleavage symmetries.
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2
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Sea urchin cleavage
Sea urchins also have radial holoblastic cleavage, but with some
interesting differences. First cleavage is meridional. Second
cleavage is meridional. Third cleavage is equatorial Fourth
cleavage is meridional, but while the four animal pole cells split
equally to give rise to eight equal sized animal blastomeres
termed MESOMERES, the vegetal cells divide asymmetrically
along the equatorial plane to give 4 large MACROMERES and
4 much smaller MICROMERES at the vegetal pole. Fifth
division the MESOMERES divide equatorially to give two tiers
of eight MESOMERES an1 and an2 , the MACROMERES
divide meridionally forming a tier of eight cells below an2, the
MICROMERES divide to give a cluster of cells below the veg1
layer. The sixth divisions are all equatorial, giving a veg2 layer.
The seventh divisions are all meridional giving a 128 cell
blastula.
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AMPHIBIAN CLEAVAGE
Cleavage in many amphibians is holoblastic with radial
symmetry, however the large volume of yolk (its mesolecithal)
interferes with cleavage. At the animal pole first cleavage
proceeds
proceeds
cleavage
egg the
at about 1mm/min, while through the vegetal pole is
50-100 times slower (.02mm/min). While the first
is still incomplete in the yolky vegetal region of the
second meridional cleavage begins to take place.
WHAT FUNCTION DOES THE BLASTOCOEL SERVE?
The blastocel spatially separates cells so they do not touch one
another. Cells at the roof of the blastocoel normally become
ectoderm.
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MAMMALIAN CLEAVAGE
The mammalian egg is released from the ovary into the oviduct
where it is fertilized. First cleavage begins about a day after
fertilization within the oviduct. In sharp contrast to most
animals, cleavage in mammals can be very slow---1/day.
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Additionally, the cleavage planes are somewhat different from
other animals. First cleavage is meridional just like sea urchin
and frog. However, the second cleavage division sees one of the
blastomeres dividing meridionally and the other equatorially!
This type of cleavage is called ROTATIONAL HOLOBLASTIC
CLEAVAGE.
MEROBLASTIC CLEAVAGE
In telolecithal and centrolecithal eggs the large dense yolk
prevents cleavage. Telolecithal eggs are characteristic of birds,
fishes, and reptiles while centrolecithal eggs are characteristic of
insects. Telolecithal eggs result in meroblastic discoidal
cleavage. Cleavage is restricted to the blastodisc at the animal
pole of the egg. At early cleavages, because cleavage cannot
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proceed through the yolk, the blastomeres are continuous at their
vegetal margins.
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