Day 13, Lecture 1 (Sept 8): Human Development 1: The first week of development Flashcards Preview

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Flashcards in Day 13, Lecture 1 (Sept 8): Human Development 1: The first week of development Deck (44):

Gestational Age 

  • Staging of the human embryo
  • Determine age based on time from last normal menstrual period
  • Normal time to parturition- 40 weeks 
  • Staging used clinically


Fertilization age 

  • Staging of the Human Embryo
  • Determine age based on time from fertilization
  • Normal time ot parturition 
    • 38 weeks
  • Staging used in embryology and in this class


Staging of the Human Embryo

  • Gestational age
    • Determine age based on time from last normal menstrual period
    • Normal time to parturition
      • 40 weeks
    • Staging used clinically
  • Fertilization age
    • Determine age based on time from fertilization 
    • Normal time to parturition 
      • 38 weeks 
    • Staging used in embryology in this class



The Embryonic period

  • Weeks 1-8
    • Organogenic period 
    • Time most vulnerable to teratogens


Fetal Period 

  • Weeks 9-32
    • Development of organs and organ systems 
    • Extensive growth
    • (luns developed sufficienctly at approximately 24 weeks to allow survivla of fetus with extensive critical care)


What makes the earliest an infant can survival premature birth

  • Development of the lungs
    • lungs developed sufficiently at approximately 24 weeks to allow survival of fetus with extensive critical care 


Postnatal develpment

  • Development continues after birth
    • Respiratory system 
      • Alveoli form in lungs
    • Cardiovascular system
      • Closure of fetal shunts and vessels
    • Nervous system
      • Continued neuronal development 


What are the key processes that must occur for a fertilized egg to give rise to the adult 

  1. Proliferation
  2. Growth
  3. Differentiation
  4. Pattern Formation 
  5. Morphogenesis 


What is differentiation 

  • Process by which cells or tissues become different from one another 
  • Progressive acquisition of structural and biochemical specializations leading to unique or highly developed cellular functions 


There are how many different specialized cells in adult human




  • Can differentiate into all cell types 
    • examples
      • Zygote
      • Morula 



  • Differentiate into many cell types 
    • examples
      • Inner cell mass 
      • Epiblast 



Differentiate into restricted group of cells



Determined as to differentiated cell to form 


Since all cells contain the same set of genes (genomic equivalence) how can genes direct development when same in all cells?

  • Differential Gene Activity
    • Different genes are turned on and off during development leading to specific genes being expressed in the differential cell


Does differential gene acitvity lead to irreversibly turning off of genes?

  • No
  • Think about Dolly and iPS cells 


Induced Pluripotent Stem Cells (iPS cells) 

  • Reverse differentiated cell to pluripotent state
  • Express factors found in pluripotent cells
  • Another demonstration of genomic equivalence
  • Provide possible pluripotent stem cells to be used clinically 
  • Genotype same as patient if use adult cell from patient 


Regulation of Differential Gene Activity occurs at Mutliple Levels

  • Differential gene transcription
    • Transcrption factors 
    • Histone Methylation
    • DNA Methylation
  • Selective Nuclear RNA processing 
  • Selective messenger RNA Proccessing
  • Differential Protein Modification 


Cells must acquire _______ to determine what to form and when to undergo differentiation

  • Positional Information 


Example of differntation with loss of pattern formation



Positional Information 

  • Receive cues from environment 
    • Cell-Cell interactions
    • Soluble factors including growth factors and cytokines
    • Extracellular matrix
  • Lineage
    • Restricts response to positional information 


Specification of Little Verse Great Digit 



Apoptosis is an example of 

  • Morphogenesis 


Syndactyly is a malformation due to 

  • Altered Morphogenesis
  • Lack of apoptosis in interdigital regions 



Egg at Fertilization 



  • Specialized cell division whereby daughter cells divide rapidly reducing size with each division 
  • Partition of cytoplasm of zygote into small cells (blastomeres) without an increase in cytoplasmic mass
  • Reason for cleavage
    • Zygote very large (1000 micrometers) compared to normal cell (10 micrometers) 



  • 12-16 cell stage 
  • formed on 3rd day 


Compaction of Morula

  • compaction occurs 4 days post fertilization 
    • first morphogenesis in the embryo
  • Formation of tight junctions between outer blastomeres
    • Tight junctions allow for the partitioning of inside and outside
    • Dependent on the presence of E-cadherin in outer layers of blastomeres
    • Forms two distinct lineages
      • outer and inner cells 
    • Blastomeres no longer totipotential 


Blastocyste formation 

  • 4 day post fertilization 
  • Following compaction 
  • Forms fluid filled cavity- blastocyst cavity 
  • First sorting of embryonic cells into lineages
    • Inner cell mass cells will form embryo
      • Pluripotenital
    • Trophoblast
      • will form protective membranes 
      • Extraembryonic membrane (chorion)


Hatching of Blastocyst

  • Weakening of Zona Pellucida
  • Blastocyste squeezes out of zona pellucida
    • Formation of floating blastocyst (6th day of development) 




  • end of the first week of development
  • Floating blastocyste makes contact with the endometrial lining of the uterus 
  • Trophoblast will form two distinct layers of cells 
    • Cytotrophoblast 
      • Actively dividing 
      • gives rise to Syncytiotrophoblast 
    • Syncytiotrophoblast
      • nuclei don't divide
      • highly envasive into endometrium allowing for implantation 
        • Produces matrix metalloproteinases (MMPs) to promote invasion 
          • (note: the most invasive carcinoma known is choriocarcinoma similar process of invasion used during metastasis)
        • Produces protein and steroid hormones
          • ex. hCG, which stops the menstrual cycle 


Reorganization of the inner cell mass 

  • occurs at the end the first week of development
  • Delaminates into:
    • Hypoblast layer- forms extraembryonic structures
    • Epiblast- Forms embryo proper 


  • Review of 1st week of Development


Spontaneous Abortion

  • Spontaneous abortion thought to be as high as 60%
    • almost all are early in pregnancy
      • only about 5% are after 8 weeks of development 
  • Many times women don't realize they are pregnant
  • Found out due to careful monitoring of early pregancy that is now possible
  • about 50% is due to chromosomal abnormalities 


Abnormal Implantation sites 

  • any place place other than the normal (superior part of the body of the uterus) 
    • most frequent normal location is in the posterior part of the superior body of the uterus 
  • Placentia Previa 
    • implantation in the lower part of the uterus
    • placenta is covering the internal os of uterus 
    • may result in early placential 
    • fetus must be delivered by cesarean section
  • Tubal Implantation
    • is the most common ectopic site (about 98%) 
      • Result of delayed transport along uterine tube
      • In approximately 50% of tubal pregnancies if the embryo remains will result in rupture 6-8 weeks post-ferilization. leading to potential death of mother due to internal bleeding  
  • Ovarian and abdominal implantation
    • Relatively rare
    • Risk of internal bleeding high



  • Abdominal pregnancy 
  • results in fetus becoming calcified if it remains in abdomen 


Multiple pregnancy

  • Natural cycle:
    • Twins
      • 1:90
    • Triplets
      • 1:902
    • Quadruplets
      • 1:903
  • ​2/3 of twins dizygotic 
  • 1/3 monozygotic 


Monozygotic Twinning 

  • Formation of Identical Twins from splitting of Blastomeres
    • Twinning occurs during cleavage
    • two morulas are formed forming two embryos demonstrates
      • blastomeres are totipotential 
      • Blastomeres can regulate
  • Formation of Identical Twins from splitting of inner cell Mass
    • Twinning occurs at blastocyst stage
    • inner cell mass divides forming two embryos demonstrates
      • inner cell mass cells are pluripotential 
      • inner cell mass cells can regulate 


In Vitro Fertilization

  • Can be used as an effective treatment for infertility
  • Indications for IVF
    • Blocked fallopian tubes or pelvic adhesions
    • Male factor infertility
    • Failed 2-4 cycles of ovarian stimulation with intrauterine insemination
    • Unexplained infertility
    • Vasectomy or tubal ligation
    • Preimplantation genetic diagnosis 
  • In Vitro Fertilization Protocol
    • Usually have 10-15 blastocysts per cycle
    • Implant no more than 3 blastocysts 
    • Freeze remaining blastocysts for subsequent use 


Preimplantation Genetic Diagnosis

  • Screening of embryos for genetic disorders
  • Perform genetic diagnosis on embryos produced by IVF
  • Implant embryos not carrying genetic disorde
  • Can be screened for monogenic disorders, chromosomal structural aberrations, or X-linked diseases
  • Frequently screened disorders
    • Cystic Fibrosis
    • Beta-thalassemia
    • Fragile X syndrome
    • Duchenne muscular dystrophy
  • Procedure
    • Remove one blastomere from morula 
      • possible because
        • blastomeres are totipotent 
        • morula can regenerate
    • Perform genetic analysis 
    • Only implant blastocysts not carrying genetic disorder 

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