User:Tobias Nyamoita

CLEAVAGE

Cell division during the early stages of the embryo's development is referred to as cleavage in embryology.Zygote of several species posses rapid cell cycle progression without considerable overall growth, resulting in a group of cells of identical size as the initial zygote.The diverse cells produced by cleavage are known as blastomeres,and they group toggle to form a solid mass known as morula.The development of the blastula,or the blastocyst in animals indicates the termination of cleavage. The nature of cleavage is dependent on the amount and distribution of yolk in the egg.

TYPES OF EGGS

1.Isolecithalegg:Made of up of little yolk that is evenly distributed.It is found in amphioxus, Ascidians, molluscs and mammals 2.Mesolecithal egg:Made up of moderate to dense yolk that is concentrated at the vegetal pole found in amphibians

3.Telolecithal egg:Made up of an unevenly distributed egg.It is found in birds.

4.Centrolecithal egg:Made up of more dense yolk that is concentrated at the centre of the egg.It is found in insects.

The part of the egg that has dense yolk does not cleave. There are several types of cleavage namely:

1.Complete cleavage: This is where each cut runs across the entire egg.Each cleave /division cuts across the entire cell.It is also called holoblastic cleavage.

2.Incomplete cleavage:This is where the division doesn't go throughout the cell .The denser yolk prevents the cleavage to be complete.

3: Special cleavage: The cell does not divide The nucleus is the one that divides first and then the cytoplasm follows

Species that undergo.

1.Complete cleavage

(a) Amphioxus Has isolecithal egg The first and second divisions are vertical while the third division is equatorial. The fourth division is simultaneous vertical cuts

. (b) Amphibians The first and second divisions are similar to amphioxus,the third cleavage doesn't divide the embryo into half instead it's displaced towards the animal pole which produces small cells at the animal pole and larger cells at the vegetal pole

(c)Sea urchins They undergo radial cleavage The first and second cleavage are vertical while the third is equatorial.The result of cleavage in sea urchin are cells with different sizes.

2.Incomplete cleavage (meroblastic). Occurs in sharks ,rays,bony fishes and birds. Cleavage divides the non-yolk cytoplasm but doesn't affect the yolk for example in birds.

3. Superficial cleavage The nucleus divides rapidly and the nuclei migrates towards the plasma membrane where the are then surrounded by membranes that separates them . The center of the yolk does not divide.

Reference:

Lecture notes.

The beginning of the cleavage process is marked when the zygote divides through mitosis into two cells. This mitosis continues and the first two cells divide into four cells, then into eight cells and so on. Each division takes from 12 to 24 hours. The zygote is large compared to any other cell and undergoes cleavage without any overall increase in size. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases.[5]

Initially, the dividing cells, called blastomeres (blastos Greek for sprout), are undifferentiated and aggregated into a sphere enclosed within the zona pellucida of the ovum. When eight blastomeres have formed, they start to compact.[6] They begin to develop gap junctions, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues.[7]

When the cells number around sixteen, the solid sphere of cells within the zona pellucida is referred to as a morula.[8]

Group members:

1.MWITA TOBIAS NYAMWITA

ESC/00257/022

2.MUKHWANA OBADIAH SITOKI

ESC/00072/022

3.ANITA BECKY OCHIENG

ESN/00044/022

4.OTIENO NICK

ESC/00213/022

5.JOSEPH KANJA

ESC/00100/022

6.DEBORAH CHEBET

ESC/00270/022