Chapter 3 of embyo

• Cleavage

  First phase of development of the zygote: passage from the unicellular condition to the multicellular condition after several mitotic divisions

  
• Cleavage

  • Highly conserved process among species
  • Cleavage of nucleus followed by cytoplasm
  • The zygote divides once to give 2 cells called blastomeres then 4,8,16 ....
  • The first cleavage is simultaneous in all the blastomeres, but synchronization is lost and the blastomeres divide at different times, independently of one another

  
• Blastomeres

  The cells resulting from the cleavage of the zygote

  
• The blastomeres do not increase in size before the next division begins, so with each division the resulting blastomeres are only half of the original size
• Cleavage

  1. Zygote

  2. 2 blastomeres (2n)

  3. Morula

  4. Blastula

  
• Morula

  A ball of blastomeres
• Blastula

  The embryo when the blastocoel is established, with a thin outer layer called the blastoderm enclosing the blastocoel

  
• Types of cleavage

  • Holoblastic (complete)
  • Meroblastic (incomplete)

  
• Holoblastic cleavage

  • The egg is completely divided, both animal and vegetal poles are divided into blastomeres

  
• Meroblastic cleavage

  • Division occurs only in a small area at the animal pole that contains the cytoplasm, the yolk remains uncleaved
• Types of holoblastic cleavage

  • Equal
  • Subequal
  • Unequal
• Equal cleavage

  The blastomeres are approximately of the same size
• Subequal cleavage

  The blastomeres have little difference in size
• Unequal cleavage

  Micromeres (small, at animal pole) and macromeres (large, at vegetal pole)
• Types of holoblastic cleavage by blastomere arrangement

  • Radial
  • Spiral
  • Rotational

  
• Radial cleavage

  • Blastomeres in the upper tier lie over the corresponding blastomeres of the lower tier, pattern of blastomeres is radially symmetrical

  
• Spiral cleavage

  • Blastomeres in the upper tier are shifted in the same direction in relation to blastomeres of the lower tier, the mitotic spindles are arranged in a spiral

  
• Rotational cleavage

  • One blastomere divides meridionally, the other divides equatorially

  
• Types of meroblastic cleavage

  • Discoidal
  • Superficial

  
• Discoidal cleavage

  • Only the active cytoplasm at the animal pole is divided into blastomeres, the rest of the egg remains unsegmented

  
• Superficial cleavage

  • Division of the nucleus located in the center of the egg in the first step, then the nuclei migrate to the periphery and form a syncitium, meroblastic division of the cytoplasm occurs resulting in a peripheral blastoderm

  
• Morula

  A compact mass of blastomeres
• Blastula

  The embryo when the blastomeres are arranged around a cavity called the blastocoel, with the layer of blastomeres called the blastoderm

  
• Types of blastulae

  • Regular coeloblastula
  • Irregular coeloblastula
  • Stereoblastula
  • Discoblastula
  • Periblastula
  • Blastocyst

  
• Regular coeloblastula

  Central blastocoel surrounded by a single layer of blastoderm, in eggs with poor yolk and holoblastic cleavage

  
• Irregular coeloblastula

  The blastocoel is displaced upward and surrounded by several layers of micromeres towards the animal pole and macromeres towards the vegetal pole, in heterolecithal eggs with holoblastic unequal cleavage

  
• Stereoblastula

  The blastocoel is very limited (virtual), the micromeres almost directly cover the macromeres, in annelid eggs

  
• Discoblastula

  The primary blastocoel (subgerminal cavity) is a small cavity separating the blastoderm from the uncleaved yolk at the vegetal pole, in telolecithal eggs with discoidal meroblastic cleavage

  
• Periblastula

  The peripheral blastomeres surround the uncleaved yolk, the blastocoel is very limited (virtual), in centrolecithal eggs with superficial meroblastic cleavage

  
• Blastocyst

  The blastoderm surrounds the large blastocoel, consisting of an outer trophoblast and an inner cell mass called the embryoblast, in alecithal eggs of placental mammals

  
• Gastrulation involves rapid RNA transcription and a large amount of genetic information is expressed