Flat embryonic disc folds to become 3-d embryo w/ internal organ systems

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  Introduction
  
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    Flat embryonic disc folds to become 3-D embryo w/ internal organ systems
    
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    All major organ systems begun
    
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    Period very sensitive to external perturbations (radiation, viruses, chemical teratogens); many congenital malformations originate during this period
    
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    Trilaminar Embryo Overview (longitudinal section):
    
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      Ectoderm/neuroectoderm dorsal to mesoderm
      
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      Mesoderm dorsal to endoderm
      
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      Oropharyngeal membrane is cranial; cloacal membrane is caudal
      
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      Central cardiogenic area located cranial to oropharyngeal membrane
      
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      Allantois forms a sm. YS diverticulum (protrusion/process) extending into connecting stalk
      
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  Folding -- like a mushroom cap folding over the stem (analogous to yolk sac/umbilical cord)
  
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    Amnion folds down over embryo & envelopes connecting stalk & yolk sac; head & tail curve ventrally into a C- shape
    
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      Fetus encased in amniotic cavity (amnion enlargement eventually obliterates chorionic cavity – see Placenta Lecture)
      
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      Results in umbilical cord ensheathed by amnion
      
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    Somatopleure (dorsal) folds around ventral portion of embryo & fuses to form body wall
    
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    Splanchnopleure (ventral) folds around ventral portion of embryo; fuses to close gut tube
    
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    Sagital Head Folding:
    
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      Neural Tube at cranial margin (forebrain) grows beyond oropharyngeal membrane; overhangs primitive heart
      
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      Heart, pericardial cavity, oropharyngeal membrane fold under & assume ventral position
      
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      Future mouth:
      
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        Depression of surface ectoderm forms stomodeum (primitive mouth); also endoderm, h/w endoderm is later obliterated
        
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        Embryonic endoderm & yolk sac form foregut which ends in oropharyngeal membrane (ruptures early in week 4)
        
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      Future diaphragm:
      
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        Septum transversum starts as a mass of mesoderm cranial to pericardial cavity
        
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        After folding it lies ventrally & caudally b/w heart and yolk sac
        
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        Eventually develops into major part of diaphragm
        
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        Most caudally displaced tissue (due to folding)
        
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    Sagital Tail Folding:
    
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      Cloacal membrane, connecting stalk, and allantois assume a ventral position as the posterior, caudal end of the embryo folds under
      
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      Embryonic endoderm & yolk sac form hindgut; hindgut ends in cloacal membrane (anus)
      
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      Connection of gut w/ yolk sac eventually reduced to yolk stalk becomes vitelline duct (connected at future “belly button”)
      
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      Allantois, connecting stalk, & yolk stalk (or vitelline duct) become closely associated, wrapped in amnion; eventually form umbilical cord
      
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    Transverse Folding:
    
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      Somatopleure (dorsal layer) folds toward midline to form ventral body wall & parietal layer; amniotic cavity now surrounds the embryo
      
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      Splanchnopleure (ventral layer) forms gut (part of yolk sac include into embryo at midgut) & visceral serous membranes; intra-embryonic coelomic cavity now surrounds the gut
      
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      Endocardial heart tubes (2) on opposite sides of embryo come together and fuse to form a single heart; due to folding, tubes are displaced medially, ventrally, & caudally
      
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      As umbilical cord forms (as result sagital folding), fusion of the lateral folds reduces communication b/w intraembryonic coelom (space b/w somatopleure & splanchnopleure) & chorionic cavity (surrounds amnionic cavity & yolk sac); this is because as a result of folding the amniotic cavity now lies between the intra-embryonic coelomic cavity (future pericardium/peritoneum) & the chorionic cavity (except at the umbilical cord)
      
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  Germ Layer Derivations (partial listing):
  
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    Ectodermal Derivatives
    
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      Surface Ectoderm
      
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        Epidermis (incl. hair & nails)
        
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        Mammary glands
        
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        Tooth enamel
        
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        Lens of eyes
        
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        Inner ear
        
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        Anterior pituitary gland (from oral ectoderm)
        
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      Neuroectoderm
      
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        Neural Tube – CNS
        
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          Brain & spinal cord
          
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          Retina
          
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          Posterior pituitary gland
          
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        Neural Crest – PNS
        
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          Pigment cells
          
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          Branchial arch cartilages
          
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          Head mesenchyme – facial morphology
          
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          Parafollicular cells of thyroid
          
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          Medulla of adrenal gland
          
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    Mesodermal Derivatives
    
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      Paraxial Mesoderm – Somites
      
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        Sclerotome – Bones & cartilage of skeleton, not skull
        
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        Myotome – Muscles of head , trunk & limbs
        
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        Dermatome – Dermis of skin
        
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        CT
        
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      Intermediate Mesoderm – Urogenital system
      
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      Lateral Mesoderm
      
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        CT & muscle of viscera
        
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        Serous membranes
        
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        Cardiovascular & lymphatic systems
        
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        Spleen
        
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        Blood & lymph cells
        
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        Adrenal cortex
        
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    Endodermal Derivatives
    
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      Epithelial lining of respiratory tract
      
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      Epithelial lining of gastrointestinal tract, liver & pancreas
      
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      Epithelium of urinary bladder & urachus (canal connecting the bladder of the fetus with the allantois; becomes part of umbilical cord)
      
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      Epithelial lining of tympanic cavity & eustacian tube
      
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      Parathyroid & thymus
      
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      Thyroid gland
      

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  Summary of Embryonic Period
  

Week 1	Week 2	Week 3	Week 4
Cleavage	Bilaminar Embryo	Gastrulation	Folding
Cavitation	Amniotic Cavity	Notochord	Neural Crest Migration
Implantation	Primitive Yolk Sac (YS)	Neuralation	Somites Forming
	2^oYS	Vasculogenisis
	Chorionic Cavity	Cardiogenisis
	1^o Villi	2^o & 3^o Villi