DEVBIOLEC: L4 Mol and Morp Mechanisms of Cleavage

• Cleavage is the mitotic division of the fertilized egg.
• Cleavage is the first major phase of embryonic development. It creates a multicellular embryo from the zygote. It is affected by the amount and distribution of the yolk.
• Zygote is divided into smaller and smaller cells called blastomeres.
• Blastomeres are nondifferentiated and can give rise to any tissues.
• Stem cells are set aside and will continue to divide while remaining undifferentiated.
• Tissue specific cells can only give rise to only one tissue.
• Pluripotent cells can give rise to multiple differnt cell types (blastocyst - inner cell mass)
• Totipotent cells can give rise to any cell type (Morula stage)
• Classification of Egg based on amount of yolk

  1. Alecithal - negligible or no yolk
  2. Microlecithal - small amount
  3. Mesolecithal - moderate amount
  4. Mega or Polylecithal - large amount
• Classification of Egg based on distribution of yolk

  1. Isolecithal - evenly distributed
  2. Telolecithal - concentrated on one side (frog)
  3. Centrolecithal -located at the center (insects)
  4. Discoidal - majority of except a small disc shaped area
• Classification of Egg based on presence or absence of shell

  1. Cleidoic (box-like) - laid on dry land, self contained, fully laden with yolk and surrounded by albumin and waterproof shell (reptiles and birds)
  2. Non-cleidoic - eggs laid on water and are not protected by the shell (amphibians)
• Classification of Egg based on Types of Development

  1. Determinate or Mosaic - fate of each and every part of the egg is fixed before or at the time of fertilization. if a particular portion of the egg is removed, the developing embryo will be deficient in a particular organ (annelids and arthropods)
  2. Indeterminate or Regulative Eggs - no predetermination and the fate of the various parts of eggs is usually not fixed until the cleavage divisions (8 cell stage) completed.
• The ability of the blastomere to develop into a whole embryo is known as totipotency.
• Polarity

  the eggs and zygotes of many animals (not mammals) have a definite polarity that is defined by the distribution of yolk.

  • vegetal pole - most yolk, darker in color
  • animal pole - least yolk, lighter in color
• Kind of Stem Cells

  1. Totipotent - each cell can develop into a new individual (e.g. cells from early (1-3 days) embryos, each cell can form a complete organism
  2. Pluripotent - cells can form any (over 200) cell types
  3. Multipotent - can develop into a limited number of cell types in a particular lineage
• Types of Cleavage

  1. Holoblastic Cleavage
  2. Meroblastic Cleavage
  3. Superficial Cleavage
• In holoblastic cleavage, the fertilized egg divide completely.

  1. total and equal - segments are equal sizes (echinoderms, amphioxus, mammals)
  2. total and unequal - segments in animal pole are smaller in vegetal pole (fishes, amphibians)
• Holoblastic cleavage:

  Isolecithal

  1. Radial - echinoderms, amphioxus
  2. Spiral - annelids, molluscs, flatworms
  3. Bilateral - tunicates
  4. Rotational - mammals, nematodes

  Mesolecithal

  1. Displaced radial - amphibians
• In meroblastic cleavage, the partitioning of the cell is incomplete and confined to a small disc of the yolk free cytoplasm as in the egg of the birds.

  Telolecithal

  1. Bilateral - cephalopod molluscs
  2. Discoidal - fish, reptiles, birds
• In superficial cleavage, the cleavage is incomplete and only the nucleus divide. Later, daugther nuclei migrate into the peripheral cytoplasm and form a single layer of cells at the surface.

  Centrolecithal

  1. Superficial - most insects
• types of twins

  1. monozygotic twin
  2. conjoined/siamese twin
  3. dizygotic twin
  4. parasitic twin
• monozygotic or identical twin is a product of one fertilized egg that arise from subdivision or splitting of single embryo

  1. dichorionic diamniotic - divide at morula
  2. monochorionic diamniotic - divide at hatching
  3. monochorionic monoamniotic - divide at blastocyst
• Conjoined or siamese twin is physically joined at both, sometimes shared organs. Some cases separable by surgery.

  1. cephalopagus
  2. pyopagus
  3. cephalothoracophagus
  4. thoracophagus
• dizygotic or fraternal twin results from fertilization of two ovulated ovum.
• parasitic twin is conjoined twin whereas one fetus stops developing but remains attached to its twin (parasitic) while the other twin continue to develop (host twin).
• Blastulation

  • formation of blastula through cleavage stage
• blastocyst

  • cleavage in mammals continuous for 5-6 days producing a ball of cells.
  • consist of:

  1. Trophoblast - outer layer, forms the blastula, secretes enzymes to enable the blastocyst to implant in the uterine wall.
  2. Inner Cell Mass - forms the embryo, (lab - embryonic stem cell culture)
• Mammalian Blastula

  A) blastocoel

  B) inner cell mass

  C) trophoblast

  
• Bird Blastula

  A) blastodisc

  B) yolk

  C) blastocoel

  
• Cell differentiation

  • human body: more than 210 major types of differentiated cells
• Cell determination

  • commits a cell to particular development pathway
  • "seen" by experiment
  • cells moved to a different location in embryo, if they develop according to the new position, they are not determined
• Cells initiate developmental changes by using transcriptional factors to change patterns of gene expression (in cytoplasm)

  • cells become committed to follow a particular developmental pathway in one of two ways:

  1. via differential inheritance of cytoplasmic determinants
  2. via cell to cell interactions