Day 13, Lecture 4: Human Development 4: Weeks 4-8 Flashcards Preview

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Flashcards in Day 13, Lecture 4: Human Development 4: Weeks 4-8 Deck (22):

Mesododerm is a germ layer not a cell type may be composed of  ____ or ___ throughout the embryonic disk except at 

  • Mesenchymal or epithelial cells 
  • throughout the embryonic disk except at the oropharyngeal and cloacal membranes


_____ results in the formation of the three germ layers

  • Gastrulation 


Mesoderm is organized into 1 midline and 3 paired regions, name them?

  • Midline
    • Notochord
  • Paired regions
    • Paraxial mesoderm
    • Intermediate mesoderm
    • Lateral plate mesoderm


What major events occur in the fourth week of development

  • Organization of mesoderm into groups of tissue
    • Notochord
    • Paraxial Mesoderm
    • Intermediate Mesoderm
    • Lateral Plate Mesoderm
  • Segmentation of paraxial mesoderm into somites
  • Body folding 
    • 3-D establishment of body plan 


Organization of Paraxial Mesoderm

  • Cranio-Caudal sequence of development initially organize into partially segmented structures termed somitomeres 
  • More caudal paraxial mesoderm completely separates into segments termed somites
    • ​​7 cephalic somitomeres
    • 42-44 somites (occipital + trunk) 


What are somites 

cuboidal blocks of mesoderm compsed of epithelial cells formed from the paraxial mesoderm


Somites disperse to form

  • sclerotome
    • forms from ventromedial side of somite
    • forms axial skeleton (vertebrae and ribs)
  • dermamyotome
    • forms from dorsolateral side of somite
    • Disperses to form two groups of cells
      • Dermatome
        • Contributes to dermis
      • Myotome 
        • Forms all skeletal muscle 



Intermediate mesoderm forms

  • Urogenital system 



cervical spinal nerves are numbered by vertebrae below with the exception of 

  • Spinal nerve C8 
  • (note the rest of spinal nerves are numbered by vertebrae above) 


Lateral Plate Mesoderm

  • Lateral plate mesoderm splits:
    • Forms embryonic coelom
    • Somatic and Splanchnic mesoderm, which flank the embryonic coelom
  • Somatopleure
    • Somatic mesoderm + ectoderm
      • body wall
  • Splanchnopleure
    • Splanchnic mesoderm + endoderm
      • Visceral wall
  • Embryonic coelom
    • Forms body cavities 
      • Pleural
      • Pericardial
      • Peritoneal 


Medial Folding

  • occurs in the 4th week of development
  • Head folding
    • Cardiogenic region and oropharyngeal membrane "swing" ventrally and caudally
  • Tail Fold
    • Cloacal membrane "swings" ventrally and cranially
  • Part of yolk sac incorporated into embryo as primitive gut tube


Embryonic Folding

  • Occurs during 4th week of development
  • Establishment of body plan
  • 3-layered disk transformed into cylindrical embryo by median and horizontal folding 
    • Median folding (head and tail folding)
    • Horizontal folding (lateral folds)
  • Purse-string closure mechanism
    • put string around periphery of embryonic disk 
    • tighten string around ventral side of embryo
    • closure region is umbilical ring


Horizontal folding (lateral folds)

  • Occurs in the 4th week of development 
  • Lateral plate mesoderm moves ventrally and medially
  • participates in forming body wall 


Lateral folding in week 4 of developement


Molecular Regulation of Somite Differentiation

  • Sclerotome formation
    • SHH produced by neural tube and notochord induce ventromedial portion of somite to become sclerotome
    • Sclerotome expresses PAX-1 initiates cartilage and bone formation 
  • Dermamyotome formation
    • WNT from dorsal neural tube and epidermis induces dorsolateral portion to become dermamyotome 
    • Medial portion expresses MYF5 and becomes epaxial musculature
    • lateral portion induced by BMP4 expresses MYOD and becomes hypaxial musculature 


Craniocaudal Patterning of embryo

  • Hox genes organized in specific spatial pattern on chromosomes
  • Hox gene expression occurs in specific spatial pattern based on gene organization
  • Spatial expression of Hox genes determines pattern of somite segmentation 
    • Clusters of genes are on separate chromosomes
    • Genes at the 3' end of the cluster are expressed cranially while genes at the 5' end are expressed caudally 
  • Hox6 normally expressed in thoracic region where ribs form 


Which hox gene expression regulates Rib formation

  • Hox6 normally expressed in thoracic region where ribs form
  • Ectopic expression of Hox6 in lumbar region promotes abnormal rib formation 



  • Insult in 3rd week of development
    • Development occuring during week after 1st missed menses 
  • Kills cells in anterior midline
  • single brain ventricle, cyclopia
  • Possible reasons: 
    • may be caused by alcohol
    • Genetic defect in SHH


Caudal Dysgenesis (Sirenomelia) 

  • Insufficient mesoderm forms in caudal-most region
  • Effects all caudal mesoderm derived structures
    • lower limbs
    • kidneys
    • genital organs
    • rectum/anus
    • bladder 


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