Cytoplasm of cleavage Cells and Its Role in Development

• Cells that produced during cleavage don’t receive the same cytoplasmic substances.

• Further elaboration of Spemann’s experiment of delayed nuclear supply to one half of Newt egg can be brought to bear on this issue.

Frontal Constriction

Medial Constriction

• The egg half without grey crescent cytoplasm will become a “belly” even if it contained the nucleus right from the beginning, remains highly abnormal; it doesn’t develop a nervous system, a notochord, or segmented muscles.

• The importance of cytoplasmic substances in the egg for the differentiation of parts of the embryo is further proved by a number of experiments carried out on the eggs of various animals.

• In the egg of mollusk Dentallium, there may be distinguished three layers of cytoplasm: a layer of clear cytoplasm at the animal pole of the egg, a broad layer of granular cytoplasm in the equatorial zone, and a second layer of clear cytoplasm at the vegetal pole of the egg.

• When the egg begins to cleave, the clear cytoplasm at the vegetal pole is pushed out in the form of polar lobe. At the end of first cleavage, the polar lobe is found to be connected to one of the daughter cells.

• If we have completely separated these two cells, only the cell that have the three zones will develop into normal trochophore, the other one abnormally developed,

• It has been concluded that the clear cytoplasm of the vegetal pole is necessary for the development of the mesoderm.

Embryo of sea urchin development

• The embryo of the sea urchin develops into a larva called a pluteus.

• If the blastomeres of the sea urchin are separated in the two-cell stage or in the four-cell stage, each of them develops into a complete pluteus.

Gastrulation

Gastrulation is a phase early in the

embryonic development of most

animals, during which the single-

layered blastula is reorganized into a

trilaminar ("three-layered") structure

known as the gastrula. These three

germ layers are known as the

ectoderm, mesoderm, and endoderm.

•

• The most prominent features of gastrulation are:

• 1. A rearrangement of cells of the embryo by means of morphogenetic movements(MM).

• 2. As a result of MM, the three germ layers are synthesized( ectoderm, mesoderm & endoderm)

• 3. The rhythm of cellular division is slowed down.

• 4. Growth is insignificant.• 5. The type of metabolism changes; oxidation is

intensified.• 6. More gene expression.• 7. Proteins of many new kinds, that were not

present in the egg, begin to synthesized.• 8. Embryo axes are determined.• 9. Blastopore formation, so we can determine if

the animal belongs to protostomia (fist mouth) or to deuterostomia(second mouth).