Week 3 - Fertilisation, cleavage and mitosis

• What aspects of gamete production of pictures and spermatozoa are the same?

  Meiosis, extensive morphological differentiation, cannot survive for very long if fertilisation does not occur.
• At what developmental stage do oogonia commence meiosis? How about spermatogonia?

  Oogonia: Embryogenesis Spermatogonia: Puberty
• At what point does the sperm complete meiosis? How about the oocyte?

  Sperm: After fertilization And does not stop Oocyte: After fertilization.
• How many gametes formed at the end of oogenesis and spermatogenesis?

  Oogenesis: 1 gamete (and 3 polar bodies) Spermatogenesis: 4 gametes
• At what point does sperm undergo capacitation?

  Female reproductive tract.
• What changes in the sperm with capacitation?

  Hyperactivation
• Where are the enzymes for the acrosomal reaction stored?

  Golgi apparatus
• Which comes first, fast or slow block?

  Fast block
• Steps after the sperm reaches the egg?

  Binding of sperm to zona pellucida

  acrosome reaction ( release digestive enzymes to break through cumulus cell layer)

  Immediate fast block to block polyspermy (membrane massive depolarisation through Na+ influx)

  penetration through zona pellucida

  fusion of plasma membrane

  slow block (cortical reaction: Ca2+ release into egg. Cortical granules under egg surface fuse and release contents into space between cell membrane and zona pellucida causing Hardening of zona pellucida)

  sperm nucleus enters egg cytoplasm
• Does the cell size and number change during cleavage?

  No only number changes
• Why is an embryo described as a morula?

  Because it is a solid ball of cells.
• What are two things the sperm Bring to the zygote? What is the role of each?

  Genetic material: carries paternal DNA. Centrioles: forms mitotic spindle to help with cell division.
• Cleavage stages?

  Zygote, 2 cell blastomere, 4 blastomeres, 8 blastomeres or morula stage, blastocyst (cavitation of embryo Occurs)
• blastocyst description

  Trophoblast (forms placenta) and inner cell mass
• Cell cycle phases?

  G1, S, G2, M
• cyclin function?

  Cyclin drives the cell cycle by interacting with cyclin dependent kinases which phosphorylate and activate proteins
• steps of mitosis

  Interphase (loose), early (chromatin begin to condense to form chromosomes) then late prophase, metaphase (most condensed state) 4N, anaphase, telophase ( two cells separate in cytokinesis with help of actin filaments)
• functions of the second polar body

  Marks the anterior and axis of first cleavage
• What major change has to occur to allow blastocyst to interact with uterine wall?

  Implantation
• What basic cellular structure is required to move the embryo?

  Cilia
• Layers of bilaminar disc?

  Epiblast and hypoblast
• When does gastrulation occur?

  14-16 days after fertilisation after implantation
• first Sign of gastrulation?

  Formation of primitive stresk
• What is the source of all germ layers

  Epiblast
• Two types of cell movement required in gastrulation?

  Ingression and invagination.
• What is nodal flow?

  leftward movement of fluid at the central node, a central process in symmetry breaking on the left right axis. it is generated by cilia
• Steps of cell cycle?

  Interphase: normal function, replicate DNA then prepare for division

  cell division

  
• Identify and name anatomical structures of embryos undergoing cleavage, delamination, implantation and gastrulation

  Cleavage: two cells,

  four cells,

  eight cells, sixteen cells: morula, blastocyst: inner cell mass and trophoblast and zona pellucida

  During implantation (day 6-8): delamination: inner cell mass become epiblast (dorsal: upper layer) and hypoblast (ventral): extra-embryonic endoderm that lines the yolk sac and extra-embryonic splanchnic mesoderm

  
  

  gastrulation: primitive streak forms: epiblast becomes ectoderm, mesoderm and endoderm, ingression and vagination. epiblast cells
• the origin and fate of 3 germ layers and their derivatives 


  Epiblast: ectoderm: nervous system, skin mesoderm: muscles, blood, endothelium, reproductive tract endoderm: digestive system and liver and pancreas, pharynx
• Anterior-posterior (cranial-caudal) axis

  Second polar body marks the anterior (and axis of first cleavage)
• Dorsal-ventral axis


  Forms at right angle of anterior-posterior axis
• Left-right axis

  Determined by first two axes in gastrulation
• Axis formation

  1. Anterior-posterior axis forms

  2. Dorsal-ventral axis forms

  3. Left-right axis determined
• Primitive streak

  Makes it easy to identify the anterior-posterior and right-left axes
• Gastrulation


  The primitive streak forms
• Node

  Takes over as the organiser of the body axis after axes formation
• Left-right patterning/asymmetry

  1. Nodal flow: first symmetry breaking events occurs at the ventral surface of the primitive node

  2. Hundreds monocilia on the node rotate clockwise to cause leftward flow of extraembyonic fluid

  3. Controlled by FGF and Shh secreted by the node

  4. Asymmetric diffusion of them controls asymmetrical structures like heart, liver, intestines, spleen

  5. Left-right dynein: a protein involved in node monocular is conserved among species
• Aspects of fertilisation

  Acrosomal reaction: acrosome has digestive enzyme hyaluronidase and acroson to break through cumulus cells layer

  fast and slow block to polyspermy:

  fast: membrane electrical potential for egg massive depolarisation from Na+ influx

  slow: cortical reaction release Ca2+ cause cortical granules fuse and release contents between zone pellucid and cell membrane then hardening

  unification: sperm centriole duplicate and form mitosis spindle for first cleavage.
• How is the G1 or S phase of the cell cycle controlled?

  Cyclin D binds to CDK4 allowing expression of Cyclin E. Cyclin E-CDK complexes bind and activate prepare for S
• Mitosis steps?

  Interphase, prophase, metaphase,

  Anaphase, telophase