The main results of fertilization 1. Restoration of the …Classification of ova according to yolk distribution •1. Isolecithal: having the yolk small in amount and nearly uniformly

The main results of fertilization

• 1. Restoration of the diploid number of chromosomes

• 2. Determination of the sex

• 3. Initiation of cleavage

Cleavage

Definition of Cleavage

• The repeated division of a fertilized ovum, producing a cluster of cells with the same size as the original zygote

• OR

• Succession of rapid mitotic cell divisions that followed fertilization up to blastula stage.

Differences between Cleavage and Normal mitosis

• 1. Site of occurrence:

• Cleavage: In zygote or parthenogenetic egg.

• Normal mitosis: In most of somatic cells.

• 2. Interphase:

• Cleavage: shorter period.

• Normal mitosis: longer period.

• During cleavage, the cell undergo the S (DNA synthesis) and M

(mitosis) phases of the cell cycle but often virtually skip the G1

and G2 phases.

• 3. Growth:

• Cleavage: Does not occur. The total cellular volume of the early blastula is about the same size as the original zygote; just very small cells.

• Normal mitosis: Occurs during interphase

• 4. Oxygen consumption:

• Cleavage: High as is very rapid process

• Normal mitosis: Low as is slow process

• 5. Size of daughter cells:

• Cleavage: Decreases.

• Normal mitosis: Remains same after growth.

• 6. DNA synthesis:

• Cleavage: Faster.

• Normal mitosis: Slower

• 7. Nuclear-cytoplasmic ratio:

• Cleavage: Increases.

• Normal mitosis: Remains same.

Yolk in the Ovum

• Yolk defined as the region of the ovum that richer with lipoprotein and mRNA

• Yolk plays a role in cell division( it retards cell division)

• Amount of yolk determines the pattern of cleavage

Classification of ova according to yolk amount

• 1. Microlecithal: having little yolk e.g. , sea urchin, star fish and mammals eggs

• 2. Mesolecithal: an egg that has a yolk of intermediate size and strongly concentrated in one hemisphere e.g., amphibian eggs

• 3. Macrolecithal: Enormous amount of yolk is present e.g., birds, lizards and fish eggs

Classification of ova according to yolk distribution

• 1. Isolecithal: having the yolk small in amount and nearly uniformly distributed, e.g., star fish, sea urchin and mammalian eggs.

• 2. Telolecithal: Yolk becomes more abundant and tends to concentrate in one hemisphere of the egg e.g., fish, amphibian, reptiles and birds

• 3. Centrolecithal: Yolk is concentrated in the interior of the egg and the cytoplasm is distributed as a thin layer on the outside of the yolk, as in insects and many other arthropods

Patterns of Cleavage

Rotational cleavage in Mammals

• This pattern of cleavage is characterized by slowness of cell division.

• Another feature that distinguish mammalian cleavage , the first cleavage is a normal meridional(vertical); however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially (horizontally). This is called Rotational Cleavage

• As a result of that pattern of cleavage (Rotational) blastomeres

don’t divide at the same time (Asynchrony), so mammalian

embryos don’t increase exponentially, but frequently contain odd numbers of cells.

Cleavage in Human

• Once the zygote has reached the two-cell stage after 30 hours of fertilization, it undergoes a series of mitotic divisions, increasing the number of cells. These cells, which become smaller with each cleavage division, are known as Blastomeres.

• Until the eight-cell stage, they form a loosely arranged clump.

Two- cell stage

Four-cell stage

Eight-cell stage

• Each cell (blastomere) up to 8-cell stage is totipotent.

• Totipotency means, each cell able to form everything, this mean if a one cell has been separated from the others, it will be able to develop into an embryo

Compaction

• Blastomeres up to 8-cell stage are loosely arranged with plenty of space between. After the third cleavage, blastomeres undergo spectacular change in their behavior.

• Cell adhesion proteins such as Ecadherin are expressed, cells become tightly adhered(huddle) forming a compact ball of cells held together by tight junctions.

Morula( 16-32 cells )

• At morula stage and as a result of compaction. Inner cells of morula constitute the inner cell mass (ICM) small in size, and surrounding cells compose the outer cell mass (OCM) large in size.

• ICM: gives rise to tissues of the embryo proper

• OCM: forms the trophoblast, which later contributes to the placenta.

ICM

OCM

Conclusion

•Cells after the third cleavage become pluripotent (capable to form many things)

Blastula Stage of different organisms

• The blastula (from Greek βλαστός (blastos), meaning "sprout") is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals

Blastulation

• The mechanism of cavitation(blastocoel formation) in blastula stage.

• Sodium ions pumping to inside( between the inner cells in the compact ball) , as a result of that water uptake will be, cells will be displaced forming the balstoderm

Types of Blastulae

• 1. Coeloblastula:

• A. centric blastocoel in sea urchin & star fish

• B. Eccentric blastocoel: in Amphibian

• 2. Blastocyst blastula: in mammals

3. Superficial blastula: in insects

4. Stereoblastula

• It is a solid blastula because there is no blastocoel cavity,

• The type is formed due to spiral cleavage.

• E.g. Molluscs

5. Discoblastula in birds, fish and reptiles