VIVEK DHOLE CLEAVAGE AND TYPES The process of cleavage remains one of the earliest mechanical activities in the conversion of a single celled egg into a multicellular embryo. 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. 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 equalhalves. Meridional cleavage VIVEK DHOLE Blastocoel formation 2. Vertical plane: The cleavage furrows may lie on either side of the meridional plane. The furrows pass from animal to vegetal pole. The cleaved cells may be unequal insize. 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 horizontalcleavage. 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 ofcleavage. Accordingly several cleavage patterns have been recognized. 1. Total or holoblastic cleavage - In this type the cleavage furrow bisects the entire egg. Such a cleavage may be either equal orunequal. (a) Equal holoblastic cleavage - In microlecithal and isolecithal eggs, cleavage leads to the formation of blastomeres of equal size. Eg: Amphioxus and placentalmammals. (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 sizedmacromeres. 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 twotypes. VIVEK DHOLE (a) Discoidal cleavage - Since the macrolecithal eggs contain plenty of yolk, 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. Such a cleavage is called discoidal meroblastic cleavage. Eg: birds andreptiles. (b) Superficial cleavage - In centrolecithal eggs, the cleavage is restricted to the peripheral cytoplasm of the egg. Eg:insects. Typical cleavage patterns of isolecithal, mesolecithal, telolecithal and centrolecithal eggs 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. 1. Sach’s laws - These laws were proposed by Sach in1877. i) Cells tend to divide into equal daughtercells ii) Each new division plane tends to intersect the preceding plane at rightangles. 2. Hertwig law -At the time of cleavage the formation of spindle fibres occurs in longest axis of the egg. 3. Pflugger's - During cleavage formation of spindle fibres takes place in the region of lesser resistance or less yolk. 4. Balfour’s law (Balfour 1885) - “The speed or rate of cleavage in any region of egg is inversely proportional to the amount of yolk itcontains”. VIVEK DHOLE Meroblastic discoidal cleavage Meroblastic superficial cleavage VIVEK DHOLE Cleavage in amphibians Cleavage Patterns Early cleavage patterns vary widely between different groups of animals, based largely on the orientation of the division planes. The simplest pattern is radial cleavage, in which successful division planes are at 90 degree angles relative to each other. This result in the blastomeres aligned directly over or to the side of one another. In spiral cleavage, the division planes are not at 90 degree angles, resulting in blastomeres that are not aligned directly over or beside one another. Radial Cleavage: occurs such that the resulting daughter cells are located exactly on top of one another. Radial cleavage is a characteristic of Deutrostomes, and results in indeterminant cells (Cells that can individually give rise to a complete embryo, and they don't have a determined embryological fate early on during the development of the embryo). Spiral Cleavage: occurs such that the resulting daughter cells are not located exactly on top of one another; instead, they are located at a slight angle. Spiral cleavage is a characteristic of Protostomes, and results in determinant cells (Cell that have a determined embryological fate early on during the development of the embryo). In other words, determinant cells are programmed to become a specific type of cell, early on during the process. Bilateral cleavage: Due to unequal holoblasticclevage 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. 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. VIVEK DHOLE Radial cleavage Spiral cleavage Determinate and Indeterminate Cleavage Cleavages may be classified into determinate and indeterminate types based on the potentiality of the blastomeres for the future development. 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, mollusks and ascidians which produce mosaic type of eggs exhibit determinate cleavage. 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 regulative type of eggs exhibit indeterminate cleavage. VIVEK DHOLE Types of egg The Structure of a typical Ovum (egg) Ovum is the female gamete. It stores food required for the entire process of development in the form of Yolk. It has three important functions: 1. It supplies a haploid set of chromosomes to the future embryo. 2. It contributes almost all cytoplasm to the zygote. 3. It supplies food to the developing embryo Shape and Size Typically, the eggs are spherical or ovoid in shape. But in a few animals like insects, the eggs are elongated and cylindrical. Eggs are generally larger than the sperms and average somatic cells. The egg is covered externally by a plasma membrane or plasmalemma. Within the plasma membrane is the granular cytoplasm. Organization of Egg Cytoplasm • The cytoplasm of egg is known as ooplasm. It is granular and contains in addition to the usual cellular Organelles certain other inclusions like yolk and cortical granules. The peripheral layer of ooplasm is more Viscous and gelatinous. it is known as the egg cortex which is provided with many microvilli and cortical granules. The microvilli are formed by the outrushing of the plasmalemmaand they help in transportation of substances from the outside into the ooplasm during the development of egg. • They are membrane bound and are formed from Golgi complex. They contain homogeneous and granular polysaccharides. Cortical granules are present in the eggs of sea urchins, frogs, fishes, bivalve molluscs, several annelids and certain mammals. Yolk: • Nutritive substances are stored in the cytoplasm of egg in the form of yolk. This stored food is utilized by the embryo for its early development. The process of formation of yolk is known as vitellogenesis. The yolk is a complex material consisting of proteins, fats, carbohydrates etc. The yolk may be called "protein yolk" when it has more proteins than lipids, or “fatty yolk" when it has more fat contents than the proteins. Most animal eggs contain both kinds of yolk. Classification of Egg • 1. On the Basis of the Amount of yolk • 2. On the Basis of the distribution of yolk • 3. On the basis of potentialities of egg for further development Classification of egg on the basis of amount of yolk • Eggs are grouped into four types on the basis of the amount of yolk present in them. a. Alecithal egg b.Microlecithal egg c.Mesolecithal egg d.Macrolecithal egg VIVEK DHOLE 1) Alecithalegg: When the egg contains negligible amount of yolk, it is called alecithal egg. E.g. The eggs of Eutherian mammals 2)Microlecithal egg: When the egg contain small amount of yolk it is said to be microlecithal. Romer and Balinsky named these eggs as oligolecithal eggs. E.g. Amphioxus, Tunicates 3)Mesolecithal egg: When the amount of yolk present is moderate and is not high, these eggs are also named as mesolecithaleggs E.g. amphibian, Dipnoi, xenopus and Petromyzon 4)Macrolecithal egg: When the egg contains large amount of yolk it is said to be macrolecithal or megalecithal egg. It is also called Polylecithal egg. E.g. bony fishes amphibians, reptiles and birds. Fig On the Basis of the distribution of yolk a. Isolecithal egg b. Telolecithal egg c. Centrolecithal egg a.Isolecithal or Homolecithal Egg: inisolecithal eggs, the very little amount of yolk present is uniformly distributed throughout the ooplasm e.g. echinoderms, Amphioxus, mammals). This condition is usually observed in eggs with very little amount of yolk. b.Telolecithal Egg: In eggs containing moderate or large quantity of yolk, the distribution of yolk is not uniform. it is Concentrated more towards the vegetal pole. Such a type of egg, in which the yolk is concentrated towards one pole, is called telolecithal egg. Telolecithal eggs may further classified into three types VIVEK DHOLE 1. Slightly Telolecithal: This type of egg contains only a small quantity of yolk which is distributed unevenly. The vegetal pole has the highest concentration and the animal pole the lower (e.g. eggs of fishes). 2. Moderately Telolecithal egg:this type of egg contains a moderate quantity of yolk which is Distributed unevenly. Due to high concenteration of yolk in the vegetal hemisphere, the nucleus is shifted more towards the animal hemisphere (e.g. amphibian egg). 3. Extremely Telolecithal Egg: in this type of egg, due to the heavy deposition of yolk, the entire vegetal hemisphere and a major portion of the animal hemisphere are occupied by yolk. Due to this extremely uneven distribution of yolk, the ooplasm and nucleus are displaced towards the animal pole (e.g. reptilian and avian eggs). c.Centrolecithal egg: • Egg of many arthropods and some coelenterates are described as centrolecithal. They are relatively large and elongate and have a very great amount of yolk. The nucleus lies at the geometric centre of the yolk mass, surrounded by a small amount of cytoplasm. A thin cytoplasmic layer covers the surface of the yolk. Fine strands of cytoplasm extend from the peripheral layer to the zone occupied by the nucleus. Fig On the basis of potentialities for further development 1) Mosaic Egg: In certain eggs, every portion is predetermined with respect to its potentialities for further development if a small portion of such an egg is removed, a defective embryo is formed, this is because removal of a portion results in a permanent loss from the egg. The remaining portion of the egg cannot make compensatory development for the lost part. Such an egg, is called mosaic or determinate egg (e.g. annelids, Molluscs). 2)Regulative Egg In vertebrates and most of the invertebrates, the developmental potentialities are not predetermined in the eggs. Removal of a small portion of the egg, or even one or two early blastomeres will not affect the normal development. This type of egg in which the future developmental potentialities are not predetermined is known as regulative or indeterminate egg.
