Human Embryology week 2

Human Embryology week 2

1. Stages of development

   • Developmental weeks of embryogenesis: from date of fertilization
   • Clinical weeks of pregnancy: start from date of last menstural period. Aprox 2 weeks before conception.
   • Prenatal: before birth Postnatal: after birth
   • First 28 days:
     • Fertilization
     • Implantation
     • Grastulation
     • Neural tube closure
     • Spinal cord differentiation begins
   • Next 28 days
     • eye formation
     • Hands and feet develop
     • Heart formation
     • Glands forming

2. Environmental causes of birth defects (teratogens)

   • Infections
   • Medications
   • Food
   • Food Contaminants
   • Recreational Drugs
   • Social Drugs
   • External agenets
   • Maternal disease

3. Fusion at first division

   • Fertilized oocyte (single cell) goes into mitosis.
   • The chromosome arterial will come together and mix up
   • Nuclear membrane reforms around all chromosomes and you then only have one nucleus
   • Cleavage
     • After fertilization zygote undergoes cleavage:
     • Mitosis of cells without cell growth
     • The volume of the cytoplasm is getting smaller as they are replicating.
     • roughly one cleavage division per day (slow!); asynchronous, Asyncronous: some cells are dividing quicker than others
   • Compaction
     • begins after 8-cell stage
     • Cells tightly adhere; Individual cells no longer distinguishable
     • Increased expression of E-cadherin: cell adhesion molecule
     • Gap and Tight Junctions important
   • Cavitation
     • Fluid-filled blastocele cavity forms within the ball of cells of the zygote = now called a blastocyst
     • It is evident there are two types of cells: trophoblast cells and those of the inner cell mass
     • Cells have made the decision to be inner cell mass or trophoblast, they have to make this decision before the blastomere cavity is formed
       • Decision is determined by position of cell within the morula

4. Cdx2

   • Cdx2 is expressed in ‘outer’ cells and trophectoderm
   • Mutant analysis: Cdx2 mutants have disruption in trophoblast cell formation - don’t maintain epithelial integrity - embryo dies before implantation
   • Cdx2 gene is a transcription factor. Cdx2 down-regulates the expression of other genes: Nanog and Oct4
   • At the morula stage, all cells express these genes [at low level], Cdx2 expression increases in outer cells but is repressed in inner cells, Cdx2 represses Oct4 and Nanog, Cells with Cdx2 become trophoblast Cells with Oct4 and Nanog/Gata6 become ICM

5. Cavitation process

   • Being on the basal surface Means that when it works its transporting potassium inside the blastocyst and moving potassium outside of the blastocyst.
   • Increased osmolarity, water follows into the blastisyct creating the blastocyst cavaty.
   • AQP3 – aquaporin channels: water pass through the membrane

6. Zona Pellucia roles

   • Immunological barrier: Lacks histocompatibility (human leukocyte) antigen
   • Structural : Holds blastomeres together
   • Signalling : Facilitates differentiation of trophoblasts
   • Preventative : Helps prevent premature implantation (ectopic pregnancy)
   • * Blastocyst must "hatch" to be able to implant: Blastocele expansion causes ZP to thin, Trophoectoderm cells release cystenine proteases to degrade ZP. Blastocyst "hatches" from ZP
     • Blastocyst attaches to uterine lining, Endometrium must be receptive
     • Blastocyst
       • Trophoblast cells (=trophectoderm, TE) – do not form embryonic structures Form the chorion – embryonic part of the placenta
       • BLUE MASS: Inner cell mass Forms the entire embryo and some extraembryonic tissues – amnion, yolk sac and allantois

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