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Unit 14 Cleavage and Gastrulation

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Unit 14 Cleavage and Gastrulation UNIT 14 CLEAVAGE AND GASTRULATION Structure 14.1 Introduction Objectives 14.2 Cleavage Yolk and Cleavage Planes of Cleavage Pattern of Cleavage Roducts of Cleavage (Morula and Blastula) Mechanism of Cleavage 14.3 Gastrulation Fate Maps Morphogenetic Movements Gastrulation*inSome Animals 14.4 Summary 14.5 Terminal Questions 14.6 Answers 14.1 INTRODUCTION In Unit 13 ~f this Block you have studied that spermatozoa reach the ovum by either chance or chemical attraction. Eventually, one spermatozoon fuses wi the ovum to restore the diploid genomic condition and activates all the potentials ir the fertilised egg cell or zygote to develop into a new individual of the next generation. But the zygote is one cell and the adult body in the Metazoa is constituted of many cells - from a few hundred to many billions of cells. It implies that the unicellular zygote must enter the phase of rapid divisions in quick succession to convert itself into a multicellular body. Such a series of divisions of the zygote is known as cleavage or segmentation. In this multicellular structure formed as a result of cleavage the various cells or cell groups later become tearranged as layers and sublayers during a process called gastrulation. Cleavage and gasuulation are significant phases in the ontogenetic development because cleavage transforms the unicellular zygote into a mullicellular body and gastrulation lays the foundation of primary organ rudiments so as to initiate the formation of organs according to the body plan of the particular metazoan group of animals to which the particular individual belongs. 0 bjectives After you have read this unit you should be able to: explain the various planes of cleavage furrows . list different cleavage patterns explain the purpose of gastrulation e discuss the process and mechanism of gastrulation in some animals describe the influence and the role of yok in determining the pattern and course of cleavage and gastrulation. 142 CLEAVAGE Clesvage and Castrulaliun Cleavage or segmentation is a series of cell divisions of the fertilised .egg through whic'h it is converted into .a multiccllular suucture, called blastula. The main characteristics of cleavage include: i) The unicellular fertilised egg is transformed by consecutive miroric divisions into a multicellular body. ii) hacticdly no growlh takes place during cleavage. The cell divisibns in the somatic cells is mitotic. The daughter cells or blastomeres or cleavage cells are also derived as a result of miototic divisions of the zygote. We may ask whether there are any dificrences between mitotic divisions of somatic cells and, of the zygote and the blastomeres derived from it during cleavage. From the following you will learn th3t the mitosis in the phase of cleavage has spme striking pcculiruities: a) Synchronisation of cell divisions of blastomeres: The early blastomeres divide sirnultaneuusly (synchronously) producing two blastomeres from zygote followed by 4,8,16.32 and so on, in most cases. Howevei, such synchronisation is lost, during later cleavage divisions. b) No interphase between two successive divisions, i.e. here is no growh in he amount of cytoplasm in the derived blastomeres with the result that the size of daughter blastomeres continues to decrease during successive cleavages. c) The size of the nucleus remains practically unchanged. Therefore, he nucleus: cytoplasm ratio, which is very small in he fertilized egg cell or zygote, continues to increase in the blastomeres derived from successive cleavage divisions. d) Rate of cell divisions is very rapid and very large number of cells are produced during cleavage (Fig. 14.1). This is possible due to absence of interphase. The rate. slows down later on Hours at 150C Flg. 14.1: Increase In the number of cells during early development of frog, Note the dlflerence In the rate of celi.dlvlsions during cleavage and gastrulation. 14.2.1 Yolk and Cleavage Apart from the importance of yolk as nutritive material for the deleloping embryo, yolk or deutoplasm determines he type and structure of the egg, and it also influences the rate and pattern of its cleavage. In other words, cleavage depends, to a large extent, upon the amount, distribution and orientation of yolk in the egg. Types of Eggs Depending on the amount of yolk, the eggs in various animal groups are of the following types (Fig. 14.2): i) Alecithal or yolkless-eggs as in the eutherian~mainmals.(Fig. 14.2 A). ii) Microlecithal or oligolecithal eggs have liule yolk in the form of granules, e.g., echinoderms, Amphioms, rnohscs (except cephalopods), annelids, flativorrns (Fig. 14.2 B). iii) Mesolecihal eggs wih moderabe mount of yolk, e.g. tunicares and amphibians (Fig. 14.2 C). iv) Megahitha1 or macrolecithal or heavily yolked eggs, e.g. cephalopod mo!!.;xs, bony fshes, reptiles, buds and egg laying mammals. The yolk occupies almost the . whole of the interior of the egg with a small disc-shaped clear area of cytoplasm near the animal pole where the germinal vesicle (or nucleus) lies. Most of such eggs are large sized (Fig. 14.2 E). ., Based on the placement and orientation of yolk, the egg may be: ai Isolecithal with more or less evenly distributed yolk e.g., (echinoderms, Amphioxus, molluscs (except cephalopods), annelids. (Fig. 14.2 B). I b) Telolecirhal with yolk granules or yolk mass occupying the vegetal hemisphere. In highly telolacithal eggs the yolk fills up almost the entire interior of the egg leaving only a small disc of clear cytoplasm containing the germinal vesicle (nucleus) near the animal pole of the egg. (Figs. 14.2 C, D, E). c) Centrolecithal - In insects rhe yolk granules are concentrated in the interior of the egg wheteas the cytoplasm is distributed as a thin peripheral layer around the yolk. mere is also island of cytoplasm in the centre of egg. This island surrounded by yolk on all sides contains the nucleus of the egg cell (Fig. 14.2 E). Flg. 14.2: Different types of eggs A4uman egg surrounded by follicle cells; B-microlecithal egg of Amphloxus; C-mesolecithal egg of fra; D-4elolecithal egg of the mollusc Apalysia limacina; E--macrolecithal egg of hen inside the ovarian follicle and F-centrolecithal egg of insects. Influence of Yolk on Cleavage Cleavage and G.strul.Ua Though biological significance of yolk is to provide nourishment to the developing embryo, it is not part of the active cytoplasm. Yolk is dead and inert component not participating in the cellular activities, but, it influences cleavage in ,the following ways: i) With gradual increase in the amount of stored yolk, the total amount of the active cytoplasm tends to decrease. ii) Cell division is the activity of only the nucleus and cytoplasm. With increase in the yolk amount the formation of spindles, cell membranes and cleavage furrows takes place in the active cytoplasm which is restricted to relatively smaller areas of the zygote and its daughter blastomeres. iii) The speed of cleavage is inversely proportional to the amount of yolk present. In the telolecithal eggs, blastomeres nearer to the animal pole divide at rr faster rate than the blastomeres located towards the vegetal pole because the passive behaviour L of the inert yolk in the yolky parts of the zygote and its daughter blastomeres obsnucts the formation of cleavage furrows. Therefore, the nature of various metabolic activities of the egg and the blastomeres derived from it depends upon the amount and placement of yolk mass. Principles Governing Cleavage a) The nucleus and mitotic achromatic figure tend to occupy the centre of active cytoplasmic density of the dividing cells, e.g., in isolecithal eggs or microlecithal eggs, the spindle is formed centrally in the cell while in the telolecithal eggs, it is formed nearer the animal pole. b) Each new cleavage furrow tends to intersect the plane of the preceeding cleavage furrow at right angle. c) The cells or blastomeres tend to divide into two equal sized daughter cells unless yolk is unevenly distributed. d) Free sides of the blastomeres tend to become rounded. SAQ 1 Fill in the blanks with appropriate words: i) During cleavage zygote and tlastomeres divide by ............................. ii) There is no ........................... or ...........................between two consecutive divisions of blastomeres during cleavage. iii) The egg of frog isdescribed as ...........................and ...........................: iv) Large size of hen's egg is due to ........................... amount of yolk. v) Achromatic figure or ........................... tends to be formed in the centre ,of cytoplasmic ........................... 14.2.2 Planes.of Cleavage The'ava of most of the animal groups (except some specific cases like insects) are spherical or nearly spherical having their own actual centre comparable to earth shape. Similar to north and south poles on earth, the egg has animal and vegetal poles. The yolk platelets have more density than active cytoplasm and are concentrated more towards vegetal hemisphere. Therefore, when the egg lies in any fluid medium (the fundamental feature of most of the eggs even in the app~ntlyterrestrial eggs like those of birds etc.), the vegetal pole tends to face the centre of gravity and animal pole away from it. With this picture in mind, we can now define the planes of cleavage 'of zygote or b!astomeres, keeping in mind the imaginary lines (latitudes and longitudes) drawn on the earth surface (Fig. 14.3). Anlmd Development I Fig. 143: A-Meridians (longitudes), B-Latitudes (imaginary lines on the earth surf'ace which are comparable to the cleavage planes d a spherical egg). The basic planes along which the egg and its daughter blastomeres are divided during early cleavage are: i) Meridional Plane - the cleavage furrow passes from the animal pole to the vegetal pole through the centre of the spherical egg or the blastomere so as to divide the egg into two equal halves, e.g., first cleavage furrow in the chick and first as well as second cleavage furrows in the frog's egg (Fig.
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