cleavage

• Cleavage - conversion or transformation of the fertilized egg into a multicellular body
• Blastula - a stage where conversion or transformation of the fertilized egg into a multicellular body (hollow sphere)
• Blastomeres - resulting cells during cleavage
• Morula - After a few synchronous divisions, during which the number of cells doubles with each cycle, the embryo looks like a mulberry
• Blastocoel - central cavity
• alecithal -  eggs without or with very little amount of yolk e.g., eggs of many mammals and invertebrates
• Meiolecithal-  eggs with a small amount of yolk

                                                                                  e.g., eggs of amphioxus
• Mesolecithal-  eggs with a medium yolk content

                                                                                  e.g., eggs of amphibians
• Polylecithal-  eggs with large amount of yolk

  e.g., eggs of elasmobranch fishes and birds

   
• Isolecithal/Homolecithal or Oligolecithal   -  eggs with yolk evenly distributed  throughout the cytoplasm e.g., microscopic mammalian and amphioxus eggs     
• . Telolecithal-  eggs where the yolk is more  concentrated on one side than on  another

          e.g., eggs of fishes, amphibians, reptiles and birds
• . Centrolecithal -   eggs with yolk concentrated in the center

          e.g., eggs of insects and other arthropods
• Characteristics of Cleavage -unicellular fertilized egg transformed by consecutive mitotic division into a multicellular complex
• no growth occurs
• general shape of the embryo doesn't change except the blastocoel
• qualitative change in the chemical composistion is limited
• constituent parts of the cytoplasm is not displaced
• ratio of nucleus to cytoplasm is very low at the beginning, at the end brought to the level of somatic cells
• 1. Division of the nucleus of the fertilized egg (synkaryon) 2. divison of cytoplasm into 2 daughter cells (cleavage cells or blastomeres). 3. It then divides into 4,8,16,32 and so on. 3. the first cleavage tends to be simultaneously, but sooner the synchronization is lost. 4. the blastomeres now divides in different times. It then looks like a mulberry (morula) 5. the succeeding changes in cleavage pattern is influenced by the amount of yolk.
• The chromosomes have the appearance of somatic cells. Cell divison is connected with growth (increase in size) but claevage does not increase in size. This means cleavage starts with one big cell and ends with great number of cells and remains it original size.
• Cleavage


  The division of the zygote into smaller cells
• At the end of cleavage, the cells are usually even smaller than most of the differentiated cells of an adult animal</b>
• Cellular differentiation is often accompanied by an increase in the size of an individual cell
• Blastula stage


  Embryo develops a central cavity, the blastocoel
• Beneath the unimposing and homogenous exterior of the cleaving embryo, major changes are beginning to take place
• Embryo development

  1. Synthesis of RNA's coding for specialized proteins gets underway

  2. Cells of the embryo begin to communicate with one another in new and different ways
• The cells themselves assume different, nonequivalent properties although their appearance does not show these developments
• The egg cytoplasm is not homogenous; accordingly, different cytoplasmic areas of the early embryo act on the individual cleavage nuclei in different ways thus initiating their development along different lines
• Embryo development

  1. Final polarity becomes firmly fixed

  2. Unseen developmental guidelines that keep the massive tissue displacements of gastrulation under tight control are set up
• Cleavage cell nuclei


  Considerably larger than ordinary somatic cells of the same animal
• This is mainly due to the presence of larger amounts of nuclear sap and not to more chromosomal material
• The chromosomal material in the cleavage cells is being doubled at each mitotic division, so the amount of DNA varies from diploid to tetraploid
• Cytokinesis

  1. Occurs after karyokinesis

  2. Cleavage furrow forms first in the cortex region nearest to the mitotic spindle and then moves around the cell

  3. Cleavage furrow formation is stimulated by the interaction of the cell cortex with the asters (microtubules)

  4. Calcium ions seem to be an important factor in transmitting the cleavage stimulus from the asters to the cell cortex
• Cleavage furrow


  • Contains a band of microfilaments of the actin family of contractile proteins
  • In conjunction with myosin molecules, their contractile behavior appears to be the basis for the cell constriction during cleavage
• Within 7 to 8 minutes, areas of new membranous material form along both sides of the contractile ring in the cleavage furrow
• New membrane in cleavage furrow


  Smooth and pale in color, highly permeable to ions, and seems to be deposited only in the cleavage furrow
• chemical changes during cleavage - 1. steady increase of nuclear material at the expense of the cytoplasm 2. synthesis of rna
• holoblastic (total) equal cleavage - cleavage furrow penetrates the entire yolk, equal cells ex. amphioxus
• Holoblastic (total) unequal cleavage - entire eggs cleaved but animal hemisphere has numerous blastomeres compared to the vegetal pole ex. amphibian
• meroblastic cleavage - cleavage happens only in small yolk free region