EMBRYOLOGY NOTES-Different types of cleavage in animals

The process of cleavage reamains one of the earliest mechanical activity in the conversion of a single celled egg into a multicellular embryo. It is initiated by the sperm during fertilization. However in parthenogenetic eggscleavage 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. The entire process of cleavage in frog’s egg was studied by Prevost and Dumas in 1824. With the development of microscopes cleavages and further stages were observed in the eggs of sea urchin, star fishes, amphioxus and hen’s eggs.

From all these studies it has become clear that all divisions in cleavage are mitotic. The mitotic process is very rapid. In the eggs of sea urchin division of the blastomeres can be observed every 30 minutes.

 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.

The 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.

1. Meridional plane: The plane of cleavage lies on 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 cleavage furrows may lie on either side of the

meridional plane. The furrows pass from animal to vegital pole. The cleaved

cells may be unequal in size.

3. Equatorial plane: This cleavage plane bisects the egg at right angles

to the main axis. It lies on the equatorial plane. It 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. However the fertilized egg has to undergo all stages of development and result in a suitable ‘young form’ initiating next generation. Somehow with all the influences of yolk the developmental procedures are so adapted and modified that a well formed embryo will result. The initial influence of yolk is felt during the process of cleavage.The amount of the yolk and its distribution affect the process of cleavage.

Accordingly several cleavage patterns have been recognised.

1. Total or holoblastic cleavage - In this type the cleavage furrow bisects

the entire egg. Such a 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 andplacental mammals.

(b) Unequal holoblastic cleavage - In mesolecithal and telolocithal 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.

2. 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 yolk, the cytoplasm is restricted to the narrow region in the animal pole. Hencecleavage furrows can be formed only in the disc-like animal pole region. Sucha cleavage is called discoidal meroblastic cleavage. Eg: birds and reptiles.

(b) Superficial cleavage - In centrolecithal eggs, the cleavage is restricted to

the peripheral cytoplasm of the egg. Eg: insects.

PATTERN OF CLEAVAGE                                                                                                                        On the basis of arrangement of blastomere cleavage is of four types-

         (1)   Radial cleavage - In this regular clevage divisions are at right angle to the earlier. So the     cells are placed just one above the other. So the upper four blastomere are arranged just above the lower four blastomeres. e.g. Mostly Deuterostomia animals. Echinodermata, Chordata. 
        (2)   Spiral cleavage - In this cleavage planes are oblique. In this cleavage four blastomere of     lower plane rotate clock wise or anticlock wise. If the blastomere rotates clock wise than cleavage is called as Dextral spiral cleavage e.g. Mollusca. And if the blastomere rotates anticlock wise than cleavage is called as Sinistral Spiral Cleavage 
                e.g. Helminthes    (Aschelminthes and platyhelminthes), Annelids. 
         (3)   Bilateral cleavage- Due to unequal holoblastic clevage bilateral symmetry is established. Blastomere of one lateral side are small in size and another four lateral blasotmere are large in size. e.g. Amphibia, Tunicata (Urochordata), Cephalochordata animals. 
          (4)  Biradial cleavage - First two cleavage are meridional and third cleavage is vertical so    eight blastomeres are formed. In which four central blastomere are large and four blastomere are small e.g. Ctenophora. 

Classification of cleavageon the basis of fate of blastomere It is of 2 types —

       (a) Determinate             (b) Indeterminate

       (a) Determinate clevage - (Mosaic development)

                   The fate of the blastomere is fixed. In this cleavage a specific blastomere forms a                specific part of embryo. In this clevage if any blastomere of embryo is removed or destroyed than the related part of embryo will not form. e.g. Annelida, Mollosca, Platyhelminthes, Nematoda. 

          (b)Indeterminate cleavage - (Regulative development) 

The fate of blastomeres is not fixed. Each blastomere has capacity to form the        complete embryo. There will be no effect on embryo formation if any blastomere or part is removed from embryo e.g. Deuterostomia animals (Echinoderms, Chordates). Due to totipotent nature of blastomere they can form indentical twins, when these cells are separated. 

Laws of cleavage

Apparently there are several cleavage patterns. However, all cleavages follow a common procedure. The cleavages are governed by certain basic principles or laws.

Sach's law - Cleavage divisions occurs repeatedly. Each successive division is at right angle to the earlier.
 Hertwig law -At the time of cleavage the formation of spindle fibres occurs in longest axis of the egg.
 Pflugger's - During cleavage formation of spindle fibres takes place in the region of lesser resistance or less yolk. 
Balfour's law - The rate of cleavage is inversaly proportional to the amount of yolk.