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INSTITUTE OF ZOOLOGY AND FISHERY SCIENCE INFRONT OF M S COLLEGE MAIN GATE CHANDMARI, MOTIHARI, EC, BH-845401 Mobile- +91 78709 61145 [email protected] www.izfs.net GASTRULATION INTRODUCTION The ontogenic development requires six processes: 1. Gametogenesis 2. Fertilization 3. Cleavage 4. Gastrulation 5. Organogenesis 6. Growth and Differentiation As the fertilized egg is formed by the fusion of female, ovum with the sperm of the same species the development begins. The cleavage begins forming blastula. Now the embryo enters into the so called Gastrulation, by which the origin of three germinal layers take place. It will later on enter into the organogenesis and the various organs are formed. DEFINITION According to BALINSKY, 1970, it is one of the displacements of parts of the early embryo so that the endodermal and mesodermal organ rudiments are removed from the surface of the embryo. The single layer of cells, blastoderm gives rise to three germinal layers, the ectoderm, mesoderm and endoderm. PROMINENT FEATURE 1. A rearrangement of cells of the embryo by means of morphogenetic movements. 2. The rhythm of cellular division is slowed down. 3. Growth is insignificant. Following types of morphogenetic movements are involved in Gastrulation of vertebrates: 1. Epiboly 2. Emboly 3. Invagination 4. Involution 5. Convergence 6. Divergence 7. Infiltration 8. Delamination 9. Concrescence etc. 1. 2. 3. 4. 5. 6. GASTRULATION IN AMPHIOXUS The egg is microlecithal. Cleavage is holoblastic. First cleavage is meridional or vertical. Second is also vertical but at right angle to the first. The third cleavage is horizontal but it occurs above the equatorial plate giving rise to four smaller cells called micromeres and four larger cells called macromeres. 4th, 5th and 6th divisions are latitudinal and a ball of cells, morula is formed. A cavity called blastocoel is formed. The roof of this cavity is occupied by micromeres and the vegetal poles by the macromeres. During Gastrulation the main processes involved are Invagination, involution and Epiboly. As Gastrulation begins, increase in mitotic activity occurs in the presumptive ectoderm (both neural and epidermal), notochord and mesoderm. The onset of Gastrulation is marked by flattening of prospective endoderm or macromeres of vegetal pole. It gradually invaginates inwardly into the blastocoel. Thus, the blastocoel is reduced 1 and a new cavity, the archenteron is formed. Its opening is called blastopore and its rim is bounded by four lips. 7. A crescent of cells, the chordal cells lying along the mid dorsal edge of the roof of archenteron. 8. The cells of the mesodermal crescent pass up dorsally to a position on either side of the chordal cells and forming the part of the roof of archenteron. 9. While the cells of the lips are rolling into their position by involution the ectodermal cells of the animal half are spreading out by Epiboly. 10. The blastopore is very broad in early stage, but its lips begin to contract and it reduces. It also shifts from the antero-dorsal to the posterior. 11. The lateral horns of the mesodermal crescent converge towards dorsal side and come to lie on both sides of presumptive notochord. 12. The remainder of the lateral, ventral and anterior part of the gastrula consists of endodermal cells. The presumptive material of nervous system comes over the notochordal material. In this way, the embryo becomes of three layers. GASTRULATION IN AMPHIBIA Eggs are telolecithal. Cleavage is holoblastic. By some divisions the morula is formed. At the animal pole there is micromere and at the vegetal pole there are yolk laden megameres. The cells are arranged in a layer surrounding a cavity called blastocoel. The layer of cells at the vegetal pole is very much thicker than the animal pole, so the blastocoel becomes eccentric. The first trace of Gastrulation is the formation of a depression on the dorsal side of the embryo (invagination). So a cavity is soon formed. This cavity is lined on all sides by invaginated cells and represents the archenteron. Its opening is called blastopore. FATE MAP According to VOGT, 1925, a fate map has been prepared: 1. Animal pole area will develop into ectoderm to form nervous system and skin. 2. Immediate grey crescent or marginal zone material for notochord and mesoderm. 3. Area on and around the vegetal pole endodermal lining of the alimentary canal. A. ECTODERM: - By the further Invagination the archenteron increases and the blastocoel decreases. The upper margin of blastopore is called dorsal lip and the lower edge is called ventral lip. Inward moving cells form a border beneath the outer cell by a process called involution. When the inward movement of the cells is in the progress through the dorsal lip another type of movement occurs in outside called Epiboly. The pigmented cells of animal pole start to enclose the micromere of vegetal pole. It covers the outer part of embryo. Small mass of macromeres remain uncovered for a while and acts as a yolk plug. At this stage, the embryo becomes two layered. Each layer having many layers of cells. The outer layer is ectoderm having the material for the epidermis and nervous system. B. MESODERM AND NOTOCHORD: - The notochord cells rolls over the dorsal lip of blastopore in the interior. The notochordal material becomes concentrated on the dorsal side. The prechordal plate which in the blastula lies just below the presumptive notochord is the first mesoderm to invaginate and becomes a part of archenteron roof in front of the notochordal material. Most of the mesoderm invaginates into the interior by the rolling over the lateral and ventral lips of the blastopore and comes to lie between the ectoderm on the outside, on the endoderm on the inside. The notochordal and mesodermal materials in this stage are in the form are one continuous epithelial sheet called chorda mesodermal mantle. 2 C. ENDODERM: - Finally the two lips of blastopore become fused and two mesodermal sheets unite to form a continuous mesoderm. The endoderm which until now was on the ventral side starts to move upwards along the two borders. When the two ends meet dorsally it forms the third innermost strata. The cavity is between the endoderm is known as gut cavity. In this way, the embryo becomes three layered and layer formed are ectoderm, mesoderm and endoderm. The special feature in amphibian development is the formation of mesoderm first and then the endoderm. GASTRULATION IN BIRDS The egg is telolecithal. The cleavage is meroblastic. The segmentation is restricted only at the blastodisc. By early divisions a disc is formed having smaller cells. A cleft appears which separates the disc from the underlying yolk. The new cavity in between is called sub-germinal space. Disc is then called as blastoderm, the cells of which continue to divide. It has two types of cells: 1. The peripheral part which lies in contact with yolk is called area opaca and 2. Inner layer is called area pellucid. Later on, the blastoderm becomes double layered. Upper layer is called epiblast and the lower layer is called hypoblast. The cavity in between these two is called blastocoel. The space between the hypoblast and yolk is called sub- germinal cavity. It is the primordial archenteron. The epiblast contains presumptive ectodermal and ventral areas at the anterior portion while the posterior half comprises of presumptive notochordal and mesodermal cells. The hypoblast transforms into the endoderm and the epiblast is converted into ectoderm and mesoderm. ORIGIN OF ECTODERM, MESODERM AND ENDODERM: - The Gastrulation begins with the movement of cell called immigration. The hypoblast cells from posterior end start migration towards the anterior end along the median line. The cells of epiblast move downward the hypoblast. These involuted cells occupy a position in between the epiblast and hypoblast, and migrate from there the lateral and anterior ends between the epiblast and hypoblast. This movement has been studied by SPRATT, 1946. The stripe of the blastoderm becomes thicken at the median line called primitive streak. It has a narrow furrow called primitive groove. At this anterior end there is a Hensen’s node. The entire primitive streak is a mass of moving cells. As the cells of epiblast migrate into an anterior, whose areas of the blastoderm disappear from the surface. They are replaced by the adjoining areas moving towards the mid-line and taking their place in the primitive streak. Thus the primitive streak persists, although the cells do not stay in the same place but are constantly replaced. The first areas to start Invagination are presumptive endoderm, the notochord and the presumptive head process. The presumptive notochordal cells become concentrated in the primitive streak in the deeper parts in the Hensen’s node. It may be called as head process or notochordal process. The narrow canal penetrates into the notochordal process and its cavity may be recognized as a part of archenteron. The endoderm starts invaginating at an early stage, the cells of presumptive endoderm penetrates into the hypoblast. The endoderm lying in the posterior part of the primitive streak after Invagination moves laterally replacing the original hypoblast. As the notochordal and endodermal material disc appears from the surface the presumptive somites areas converge medially and enter the primitive streak at its inner end. After passing into the interior the cells of the somites migrate outward and forward and distributed in stripes, on each side of the notochordal process. The elongation of the primitive streak is only temporary. As the cells destined to become notochord, endoderm and mesoderm migrate into anterior. The primitive streak begins to shrink. The gut of embryo is derived from epiblast and not by the hypoblast. The neural plate appear in brain region while the Gastrulation in movement are in full swing as the Hensen’s node read the parts of the neural plate becomes differentiated. The neural tube is formed. In this way the embryo becomes three layered. ------------------****------------------ 3