CLEAVAGE- PLANES AND PATTERNS
The series of cell divisions that transform a single cell- the zygote into a multicellular embryonic stage – blastula is called cleavage or segmentation.
The mitotic divisions during this phase are called cleavage divisions and the resulting daughter cells are known as blastomeres.
• It is initiated by the sperm during fertilization.
• However, in parthenogenetic eggs cleavage can commence without the influence of fertilization.
• The process of cleavage or cellulation happens through repeated mitotic divisions.
•These divisions result in cells called blastomeres.
• In later stages of development the blastomeres occupy different regions and differentiate into several types of body cells.
• The first cleavage of frog’s egg was observed by Swammerdam in 1738.
CHARACTERISTICS OF CLEAVAGE
• All divisions in cleavage are rapid mitotic divisions.
• As the cleavage progresses the resultant daughter cells, namely the blastomeres get reduced in size.
• During cleavage, there is no growth in the blastomeres.
• The total size and volume of the embryo remains the same.
• The cleavages result in a compact mass of blastomeres called morula.
• It gets transformed into blastula. While the wall of the blastula is called the blastoderm, the central cavity is called the blastocoel.
Planes of cleavage
• An egg can be divided from different planes during cleavage.
• Depending on the position of the cleavage furrow the planes of cleavage are named.
- Meridional plane: • The plane of cleavage furrow passes through the center of the animal-vegetal axis.
• It bisects both the poles of the egg.
• Thus the egg is divided into two equal halves.
2. Vertical plane: •The vertical plane of cleavage is a furrow which tends to pass in a direction from animal pole towards the vegetal pole.
• But like a meridional plane, it does not pass through the median axis of the egg, but courses to one side of this axis.
3. Equatorial plane: •This cleavage plane bisects the egg at right angles to the main axis.
• It lies on the equatorial plane and divides the egg into two halves.
4. Latitudinal plane: •It is similar to the equatorial plane, but it lies on either side of the equator.
• It is also called as transverse or horizontal cleavage.
Influence of yolk on cleavage •Yolk is needed for embryonic development. The initial influence of yolk is felt during the process of cleavage.
• The amount of yolk and its distribution affect the process of cleavage.
• Accordingly several cleavage patterns have been recognized.
1. Total or holoblastic cleavage – • In this type, the cleavage furrow bisects the entire egg.
• Such cleavage may be either equal or unequal.
a)Equal holoblastic cleavage – In microlecithal and isolecithal eggs, cleavage leads to the formation of blastomeres of equal size.
Eg: Amphioxus and placental mammals.
b)Unequal holoblastic cleavage – In mesolecithal and telolecithal eggs, cleavage leads to the formation of blastomeres of unequal size.
• Among the blastomeres, there are many small sized micromeres and a few large sized macromeres.
Meroblastic cleavage • In this type the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (macrolecithal egg) or superficially surrounding the egg (centrolecithal egg).
• Meroblastic cleavage may be of two types:
a)Discoidal cleavage – Since the macrolecithal eggs contain plenty of yolks, the cytoplasm is restricted to the narrow region in the animal pole.
• Hence cleavage furrows can be formed only in the disc-like animal pole region. Discoidal cleavage – S
• Such cleavage is called discoidal meroblastic cleavage.
Eg: birds and reptiles.
a) Superficial cleavage – In centrolecithal eggs, the cleavage is restricted to the peripheral cytoplasm of the egg.
Laws of cleavage
1.Sach’s laws – These laws were proposed by Sach in 1877.
i) Cells tend to divide into equal daughter cells
ii) Each new division plane tends to intersect the preceding plane at right angles.
2. Balfour’s law (Balfour 1885) • The speed or rate of cleavage in any region of the egg is inversely proportional to the amount of yolk it contains
• Early cleavage patterns vary widely between different groups of animals, based largely on the orientation of the division planes.
1.Radial Cleavage: • The simplest pattern is radial cleavage, in which successful division planes are at 90-degree angles relative to each other.
• Occurs such that the resulting daughter cells are located exactly on top of one another.
• Resultant blastomeres become symmetrically disposed around the polar axis.
• When such an egg is viewed from either pole, the blastomeres are found to be arranged radially symmetrical form.
• Radial cleavage is a characteristic of Deuterostomes and results in indeterminant cells.
SPIRAL CLEAVAGE- It is found in those forms in which there is rotational movement of cell parts around the animal- vegetal axis of the egg leading to displacement or inclination of the mitotic spindles.
Here the cleavage planes are neither vertical nor horizontal but are slanting in relation to this axis. Moreover each blastomere divides to form one bigger cell (macromere) and a smaller cell (micromere)
In such a cleavage the blastomeres of the upper tier (micromeres) sits over the junction between each two of the vegetal blastomeres (macromeres).
This is due to the oblique position of the mitotic spindles, hence also called oblique cleavage.
In successive cleavages the mitotic spindles are arranged in a sort of spiral
Examples- Turbellaria, nematoda, rotifera, annelids and molluscs.
BILATERAL CLEAVAGE- In this type, the blastula can cut vertically only along one plane to get two identical halves, the right and the left. Cleavage activity on one side is mirrored by the activity on the other side.
In most cases, the plane of bilateral symmetry is established by the plane of first cleavage furrow.
Examples- tunicates, Amphioxus, amphibians and higher mammals.
Determinate and Indeterminate Cleavage
• Cleavages may be classified into determinate and indeterminate types based on the potentiality of the blastomeres for the future development.
1. Determinate: the developmental fate of each embryonic cell is established very early. If a cell is isolated from the 4-cell stage the embryo will not fully develop.
• This is because the fate of each blastomere is predetermined in the early embryonic stage itself.
• Annelids, molluscs, and ascidians which produce the mosaic type of eggs exhibit determinate cleavage.
2.Indeterminate: early embryonic cells retain capacity to develop into a complete embryo if isolated from other cells.
• Cleavage produces blastomeres which are qualitatively equipotential or totipotent.
• When they are isolated, they develop into complete embryos.
• This is because the fates of blastomeres are not predetermined in the early embryonic period.
• Vertebrates and certain invertebrates such as echinoderms which produce the regulative type of eggs exhibit indeterminate cleavage.
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