AIPMT-NEET Biology Aspirants, read out the next AIPMT-NEET Biology Study material/ Notes of Embryonic Cleavage. In this we will learn about patterns of embryonic cleavage and types of eggs, important for AIPMT-NEET Biology. Free online notes for AIPMT-NEET.

Process of fusion of sperm and ovum is called Fertilization. In fertilization, haploid nucleus of both sperm and ovum fuse together to form diploid Zygote. Zygote begins the production of new organism by the series of mitotic divisions termed as Cleavage. During cleavage, rapid cell divisions divide the cytoplasm of fertilized egg/zygote into numerous small nucleated cells. These cells are called as Blastomeres. Zygote first divides into half, then quarters, then into eighths and so on.

• Cleavage is the result of 2 processes in the zygote: Karyokinesis (division of cell nucleus) and Cytokinesis (division of cell cytoplasm).
• Different organisms undergo cleavage in different ways, so the pattern of cleavage in the organism depends on 2 factors: 1 ) amount and distribution of yolk within the cytoplasm, 2) angle of formation of mitotic spindle for division and the timing of its formation.
• Fertilized egg has 2 poles: Animal pole & Vegetal pole. Yolk-rich pole is vegetal pole, while animal pole has less concentration of yolk. Yolk provides nourishment to the developing embryo.
• Yolk basically hinders cleavage so the yolk free region has faster cell division than the yolk-rich region.
• To understand cleavage patterns, first have a look on type of eggs. Eggs of animals like sea urchin, mammals etc. have sparse, equally distributed yolk. These eggs are called as Isolecithal eggs.
• Amphibians egg have moderate amount of yolk in vegetal pole. These eggs are called as Mesolecithal eggs.
• Eggs of fish, birds, reptiles etc. have dense yolk throughout most of the egg and have only one small area of egg free of yolk. These eggs are called as Telolecithal eggs.
• Insects egg have yolk in the center of the egg. These eggs are called as Centrolecithal eggs.
• On the basis of type of egg, cleavage is of 2 types: Holoblastic cleavage and Meroblastic cleavage. (cells formed from cleavage are called blastomeres)
• Isolecithal and mesolecithal eggs have HOLOBLASTIC (COMPLETE) CLEAVAGE (holos = complete),i.e. in these eggs cleavage furrow extends through the entire egg.
• Telolecithal and centrolecithal eggs have MEROBLASTIC (INCOMPLETE) CLEAVAGE (meros= parts), i.e. eggs which contain large volume of yolk undergo meroblastic cleavage in which only portion of cytoplasm is cleaved. Cleavage furrow does not penetrate the yolky portion of cytoplasm.
• Holoblastic cleavage has following patterns of cleavage: Radial, Spiral, Bilateral, Rotational (all 4 in isolecithal eggs) and Displaced radial cleavage (in mesolecithal egg).
• Meroblastic cleavage has following patterns of cleavage: Bilateral, Discoidal (both in telolecithal eggs) and Superficial cleavage (in centrolecithal egg).
• As insects eggs are centrolecithal so division of cytoplasm occur only in the rim of cytoplasm around the periphery of the cell which is called as superficial cleavage. So, insects (DROSOPHILA) eggs have SUPERFICIAL MEROBLASTIC CLEAVAGE.
• Telolecithal eggs of fishs and birds have only one small area/disc free of yolk, so cell division occur only in this small disc of cytoplasm called as Discoidal cleavage.
• Sea urchin has RADIAL HOLOBLASTIC CLEAVAGE.
• Snail has SPIRAL HOLOBLASTIC CLEAVAGE.
• Frog has DISPLACED RADIAL/UNEQUAL RADIAL HOLOBLASTIC CLEAVAGE.
• Chick eggs undergo DISCOIDAL MEROBLASTIC CLEAVAGE.
• Humans have ROTATIONAL HOLOBLASTIC CLEAVAGE.

To summarize, following is the complete table of patterns of cleavage:

HOLOBLASTIC (COMPLETE) CLEAVAGE

1.Isolecithal eggs (sparse evenly distributed yolk)

Radial cleavage (Echinoderms, Amphioxus): first and second cleavage are meridional and perpendicular to each other,third is equatorial i.e. perpendicular to first two cleavages. Radial Cleavage

Spiral cleavage (Annelids, Molluscs, Flatworms): cleavage occurs at oblique angles.   Spiral Cleavage

Bilateral cleavage (Tunicates): first cleavage plane decides the future left and right sides of the organism. Bilateral holoblastic cleavage

Rotational cleavage (Mammals, Nematodes): first cleavage is meridional, while in second cleavage one blastomere divides meridionally and second blastomere divides equatorially. Rotational cleavage

2.Mesolecithal eggs (moderate yolk  in vegetal pole)

Displaced radial cleavage or unequal radial cleavage (Amphibians): first and second cleavage are meridional but second cleavage starts before the first gets complete, third cleavage is equatorial but displaced towards animal pole. Displaced radial cleavage

MEROBLASTIC (INCOMPLETE) CLEAVAGE

1. Telolecithal eggs (dense yolk throughout most of the egg)

Bilateral cleavage (Cephalopod molluscs) Bilateral meroblastic cleavage

Discoidal cleavage (Fishs, Reptiles, Birds): cleavage occurs only in the small disc of cytoplasm. Discoidal cleavage

2. Centrolecithal eggs (yolk concentrated in the center of the egg)

Superficial cleavage (Insects): division of cytoplasm occur only in the rim of cytoplasm around the periphery of the cell. Superficial cleavage

Note : 1) In human body- dorsal is back, ventral is stomach side, anterior is head, posterior is feet, laterals are left and right sides.

2) Planes of division: