1 SEGMENTATION AND GASTRULATION Intro Most animals reproduce sexually, and the diploid stage usually dominates the life cycle. In the haploid stage, sperm and egg cells are produced directly by meiotic division, unlike what occurs in plants and fungi. In most animal species, a small, flagellated sperm fertilizes a larger, nonmotile egg, forming a diploid zygote. The zygote then undergoes cleavage, a succession of mitotic cell divisions without cell growth between the divisions. During the development of most animals, cleavage leads to the formation of a multicellular stage called a blastula, which in many animals takes the form of a hollow ball. Following the blastula stage is the process of gastrulation, during which the layers of embryonic tissues that will develop into adult body parts are produced. The resulting developmental stage is called a gastrula. Once egg is fertilized it started to divide in order to occur embryo’s first phase. The zygote of an animal undergoes a series of mitotic cell divisions called cleavage. Segmentation and gastrulation in sea urchin. a) Fertilized egg, b) 2 blastomere stage c) 4 blastomere stage d) 8 blastomere stage e) 32 blastomere stage f) Blastula (Appearance from above) g) Blastula (longitudinal section) h) Arising of first mesenchyme cells i) Gastrula occurrence Bld-blastoderm 1.a.g. first abdominal gap 1.m.c.first mesenchyme cells In this laboratory segmentation divisions and resultant growth stage will be examined in sea urchin (Paracentrotus lividus). 2 3 Unfertilized Egg The egg of urchin is oligolecithal type egg. Vitellus disperses homogeneously in cytoplasm in these kind eggs. These eggs are spherical. There is a big nucleus in the centre of the egg cell. Chromatin material and nucleolus exist in the nucleus which is covered by a membrane. Cell membrane, cytoplasm and nucleus can be distinguished easily in fertilized big eggs. Fertilized Egg A membrane forms covering egg once it is fertilized by sperm. This membrane is named as fertilization membrane (it is not possible to see that in the cross section). The nucleus of fertilized egg is smaller than that of unfertilized egg. Experiment 1: Examine and entitle the nucleus in both fertilized and unfertilized egg. Fertilized egg generates two cells named blastomere with longitudinal dividing at the end of the first mitosis (2-blastomere stage). Second division is meridional like first division. Second division brings about 4-blastomer stage. In longitudinal slides, blastomere shape is like orange slice at 2-balstomer and 4-blastomer stages. They seem to be shorter in cross section. Blastomeres have nucleus in their centre. At the cross section of 4-blastomer stage, segmentation gap may be seen in the middle of the 4 blastomere, which represent precursor of blastocoel. The blastula is typically a hollow ball of cells that surround a cavity called the blastocoel. Experiment 2: Please, draw and entitle two and four blastomere stages which passed from nucleus region on given slide. In most animals, cleavage produces a multicellular stage called as blastula. The blastula is typically a hollow ball of cells that surround a cavity called the blastocoel. After second division, Blastomeres divide equatorial (transverse) and occur eight equal blastomeres with occurred third division. Since the first division of the egg and then the blastomeres divided into equal-sized daughter cells, type of division of sea urchin eggs was called as total-ekval. After eight blastomers stages, divisions continue and cell numbers increase. The increased numbers of blastomeres and structure consisting of a set of cells resembling mulberry given shape was called as morula. At the morula stage, cell division continues and blastula occurs consisting of many blastomeres. Segmentation divisions are called as sequential divisions of the fertilized egg until the blastula occurring. Blastula blastomeres are drawn towards the edges and around regularly take the form of a ball with a gap in the middle part of a sequence consisting of blastomeres. Blastocoel is called as a space which is seen in the middle of blastula stage (primitive abdominal cavity) and also blastoderm is called as a single row cell layer surrounding outside of blastocoel. Blastocoele interior is filled with a gelatinous substance. In the further stage of blastula, leaving some cells of the blastoderm from the blastula vegetative pole pass to blastocoel. These cells are the primary mesenchyme cells and which constitute the skeleton of sea urchin larvae in the future. Experiment 3: Please, draw and entitle shape showing the blastula parts on given slide. 4 GASTRULATION Most animals also undergo gastrulation, a process in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer). Vegetative pole region forming the mesenchymal cells of the Blastula recess to animal pole. This recession moves into blastocoel as glove finger. This event called as Gastrulation and also resulting phase called as Gastrula. Two distinct cell layers are observed in the gastrula stage. Ectoderm is called as stage layer of cells that surrounds the outer side of Gastrula stage, and also endoderm is called inner cell layer surrounding of the recess at the Gastrula stage. Blastopore is called opening in the beginning of the recess (primitive mouth opening) at Gastrula stage, Gastrosöl (primitive gut cavity) is called the gap occurring in the end of recession. Blastocoel is gap in the outside of Gastrosöl and mesenchyme cells were located inside of that. Two layers of cells, two gaps and a mouth are available in Gastrula. Experiment 4: Please, draw and entitle shape showing all parts of gastrula stage on given slide. .