Cleavage PDF

Chapter 3 CLEAVAGE Cleavage (The Segmentation Process) is a series of cell divisions which, immediately after fertilization, follow upon one another in rapid or close succession. Characteristics of Cleavage. 1. The unicellular fertilized egg is transformed by consecutive mitotic di...

Chapter 3 CLEAVAGE Cleavage (The Segmentation Process) is a series of cell divisions which, immediately after fertilization, follow upon one another in rapid or close succession. Characteristics of Cleavage. 1. The unicellular fertilized egg is transformed by consecutive mitotic divisions into a multi-cellular complex. 2. No growth occurs. 3. The general shape of the embryo does not change, except for the formation of a cavity in the interior – the blastocoele. 4. Apart from transformation of cytoplasmic substances into nuclear substance, qualitative changes in the chemical composition of the egg are limited. 5. The constituent parts of the cytoplasm of the egg are not displaced to any great extent and remain on the whole in the same positions as in the egg at the beginning of cleavage. 6. The ratio of nucleus to cytoplasm, very low at the beginning of cleavage, is, at the end, brought to the level found in somatic cells. Difference of Cell Division in Cleavage and Cell Division in later Stages of Growth and Adult Organisms: In adult, cell division is intimately connected with growth. After each division, the daughter cells grow and when they are approximately doubled in size, they divide again. Thus, the cells maintain an average size in every type of tissue. In cleavage, the consecutive divisions of blastomeres are not separated by periods of growth. The blastomere does not increase in size before the next division begins. Thus, cleavage cells begin with one very large cell and ends with a great number of cells usually even smaller than most differentiated cells of an adult animal. Regions of the Egg: 1. Vegetative pole is the region of the egg in which the yolk is accumulated. 2. Animal pole is the opposite region where the nucleus and the active cytoplasm are situated.  Deutoplasm is a cytoplasm of an egg that store food material. It is non- living and inert. It plays no part in mitosis except as it exerts a local retarding effect by the mechanical impediment it offers to the process. Blastomeres are many small cells produced during cleavage. There are two types of blastomeres: 1. Micromeres are small blastomeres 2. Micromeres are large blastomeres. Process of Cleavage The cleavage of a fertilized egg is initiated by the division of the nucleus (synkaryon) followed by the division of the cytoplasm, so that the zygote divides into two daughter cells termed cleavage cells or blastomeres. The first two blastomeres divide again, thus producing four blastomeres, then eight, 16, 32 and so on. The division of blastomeres is essentially a typical mitosis, and the chromosomes have the appearance and structure of somatic chromosomes. Types of Eggs Based on Distribution of Yolk: 1. Isolecithal (Homolecithal) is an egg with small amount of yolk uniformly distributed throughout the cytoplasm. An isolecithal egg undergoes cleavage which is essentially an unmodified mitosis. The yolk is not sufficient in amount, nor sufficiently localized to alter the usual mode of cell division. Ex. Placental mammals 2. Telolecithal egg contains a considerable amount of yolk and the accumulation of the yolk at one pole has crowded the nucleus and cytoplasm toward the opposite pole. In telolecithal egg, the cytoplasm at the animal pole where there is little or no yolk divides promptly, but where the yolk mass is encountered the process is greatly retarded. Ex. bird, amphibians Types of Cleavage 1. Radial type. The pattern of the blastomere is radially symmetrical. Each of the blastomeres of the upper tier lie over the corresponding blastomeres of the lower tier. The first division is, as a rule, vertical; it passes through the animal- vegetal axis of the egg. The plane of the second division is also vertical or passes through animal-vegetal axis,  but it is not at right angles to the first plane of cleavage. The first four blastomeres all lie side by side. The plane of the third division is at right angle of the first two planes and to the animal – vegetal axis of the egg. It is therefore horizontal or parallel to the equator of the egg. Of the eight blastomeres, four on top of the other four, the first four comprising the animal hemisphere of the embryo, the second vegetal hemisphere. Ex. Sea cucumber Figure 3.1. The 8-cell stage cleavage Figure 3.2. The 16-cell stage cleavage 2. Spiral Type. The four spindles during the third cleavage are arranged in a sort of spiral direction. All the blastomeres in the upper tier are shifted in the same direction in relation to the blastomeres of the lower tier, so that they come to lie not over the corresponding vegetal blastomeres, but over the junction between each two of the vegetal blastomeres. The mitotic spindles are seen in an oblique position. a. Dextral, the spiral turn of blastomeres seen from above is in clockwise direction. b. Sinistral, the spiral turn of blastomeres seen from above is in counterclockwise direction. Ex. Mollusk During the subsequent cleavage the spindles continue to be oblique, but the direction of spiraling changes in each subsequent division. Dextral spiraling alternates with sinistral, so that the spindle of each subsequent cleavage is approximately at right angles to the previous one. 3. Bilateral Type. In some animals which otherwise have an approximately radial type of cleavage, two of the first four blastomere may be larger than the other two, thus establishing a plane of bilateral symmetry in the developing embryo. 4. Discoidal Cleavage. The process of segmentation is limited to the small disc of protoplasm lying on the surface of the yolk at the animal pole. 5. Determinate (Mosaic) Cleavage. Definite blastomeres give rise to specific parts of the embryo. Thus, blastomere A, B, and C give rise to the skin of the animal, blastomere give rise to the skin of the animal, blastomere E gives rise to the endoderm of the alimentary tract,  blastomere MSt give rise to the mesoderm and the stomodeum, and blastomere P3 eventually produces the reproductive cells. Ex. Nematodes. Patterns of Cleavage. The pattern of cleavage depends upon the amount of yolk present in the zygote. 1. Holoblastic (Complete) Cleavage. Eggs, which are completely divided into blastomeres. Ex. Placental mammals. 2. Meroblastic (Partial) Cleavage. Those with incomplete cleavage. (meros=partial) The cleavage furrows never reach the vegetal pole, remains uncleaved resulting to incomplete cleavage. The egg is divided into a number of separate blastomeres and a residue, which is an undivided mass of cytoplasm with numerous nuclei scattered in it. Ex. Birds. Sometimes the terms holoblastic cleavage and meroblastic are applied to the eggs having a particular type of cleavage; thus one find in the literature the term “haloblastic eggs,” meaning eggs having holoblastic cleavage, and also the term “meroblastic eggs” for eggs having meroblastic cleavage. Period of Cleavage from Fertilization in some animal species: Cow – 12 days Ewe – 10 days Sow – 6 days THE MORULA STAGE Since the zona pellucia persists intact throughout the cleavage period, the blastomeres are forced to dispose themselves within its spheroidal cavity. When the embryo is viewed as a solid ball of cells suggestive of a mulberry or a blackberry, the condition is known to be in the morula (translated as little mulberry) stage. The morula, contains cells too numerous to accurately count. Compaction is a condition when the blastomeres of the morula lose their spheroidal appearance and become tightly apposed to each other. Stages of Cleavage in Domestic Animals 1 2 3 4  2 cells 4 cells 8cells 16 cells … 8 7 6 5 Blastocoel Compaction Morula Shedding of formation of Stage corona radiata blastomeres 9 10 11 12 Blastula Elongation of Rupture of Embryo breaks Stage the zona free Blastocyst pellucida 14 13 Larger cells Embryonic disk/ Inner cell Mass/ (will form) Blastodisk (in which the embryo develops) Trophoblast Smaller cells Cells (will form) In the isolecithal zygote, the planes of the 2 nd and 3rd cleavages are at right angles to the preceding division plane. Likewise, after the four-cell stage, cleavage is often asynchronous and it is common to find five-, six- and seven- cell embryos. Cleavage results in the formation of a solid cluster of cells termed the morula. Prior to this stage, the cells of the corona radiata are shed, but the embryo remains surrounded by the zona pellucida. The blastomeres of the morula lose their spherical appearance and become tightly apposed to each other termed compaction. The blastomeres secretes fluid which collects within the morula to form a cavity called the blastocoel. The embryo at this stage is called blastocyst or blastula. During this stage, the zona pellucida ruptures and the embryo breaks free. All the cells in the blastocyst are not identical. Those larger cells constitute the embryonic disk (inner cell mass, blastodisc), from which the embryo will develop. The cells on the periphery of the blastodisc are the trophoblast cells. Trophoblast cells facilitate absorption of nutrients in early development. They participate in the formation of extraembryonic membranes which contributes to the formation of placenta. (trophe=nourishment) Stages of Cleavage in Birds 1 2 3  Zygote Formation of Four or Five Embryonic disc Cell Divisions A large single cell in the yolk A tiny white spot which Meroblastic type with observed upon breaking the indicates the site of the complete karyokinesis and egg nucleus and other incomplete cytokinesis. cytoplasmic organelles. 6 5 4 Formation of Proliferation of Formation of Subgerminal Cavity Blastomeres Blastomeres Space created by the Result to formation Occurs through growth of proliferating mass of of… membranous furrows blastomeres that separates between nuclei from the underlying yolk. 7 8 Central Marginal Blastomeres Blastomeres Lie directy over the Periherally located cells that subgerminal cavity that will will form some of the form the body extraembryonic membranes of the embryo. The avian zygote is the yolk that one observes upon breaking an egg open. On the upper side of this large single cell is a tiny white spot, the embryonic disk which indicates the location of the nucleus and other cytoplasmic organelles of the zygote. Meroblastic cleavage commences at this site. Four or five cell divisions occur with complete karyokinesis (nuclear division) but incomplete cytokinesis (cytoplasmic division). Cell membranes do not fully surround the new cells and separate them from each other. Thus, the early embryo is a syncytium, a term used to any multinucleated cell. Formation of blastomeres occurs through the growth of membranous furrows between nuclei. Soon the proliferating mass of blastomeres separates from the underlying yolk creating a space called the subgerminal cavity. Cells directly over the subgerminal cavity are the central blastomeres and, like the embryonic disk of mammals, they will form the body of the embryo. The more peripherally located cells are called marginal blastomeres, and these will form some of the extraembryonic membranes. Marginal blastomeres continue to divide rapidly and the population expands peripherally over the surface of the yolk. Only those located near the outer perimeter remain syncytial. Table 3. Comparative Outline of the Different Types of Yolk and its Effects on Cleavage  SIZE OF YOLK DISTRIBUTION CLEAVAGE ANIMAL OF YOLK Macrolecithal Telolecithal Meroblastic Bird (at one end) (at the embryonic disc) Mesolecithal Telolecithal Holoblastic (because Amphibians (increasing gradient cleavage furrows pass from animal to through the entire egg) vegetal pole) Microlecithal Isolecithal/ Holoblastic Placental Homolecithal mammals (equal) Some anatomists study embryonic development to compare different organisms and help determine the evolutionary relationships between them. Sea urchins, frogs, humans, and many other animals are remarkably similar in their early development. Figure 3.3. Comparative Embryology All begin with a single cell that divides into two cells, the first step in the process of cleavage (1a, 2a, 3a). During cleavage, cell divisions occur so rapidly that the cells do not have time to grow between divisions, and the result is smaller and smaller cells. Cleavage produces a solid ball of cells called a morula (1b, 2b, 3b). Within the morula, a fluid-filled cavity called the blastocoel develops, converting a morula into a blastula (1c, 2c, 3c).  In a process called gastrulation, certain cells of the blastula migrate to different regions of the blastula to create the gastrula, a structure with three cell layers (1d, 2d, 3d). o The outer cell layer of the gastrula, called the ectoderm (shown in blue), forms the outer covering of all animals, and in the frog, human, and other higher animals, it also forms the nervous system. o The inner layer of the gastrula, known as the endoderm (shown in yellow), gives rise to the gut in all animals, and in higher animals, other organs including the stomach, pancreas, liver, and lungs. o The mesoderm, which forms between the ectoderm and endoderm, produces the simple excretory system of the sea urchin and frogs and the kidneys of humans. In higher animals, the mesoderm also gives rise to blood, bone, muscle, and other structures. Cell specialization is followed by the development of primitive organs, organogenesis, which marks the larval form of sea urchins and frogs, and the embryo stage of human development (1e, 2e, 3e). Size and time of development vary widely among species. The sea urchin larva, for example, forms in 12 to 76 hours and measures 0.1 to 0.3 mm (0.004 to 0.01 in), while the human embryo takes eight weeks to fully form, and measures about 30 mm (about 1.2 in) from crown to rump.