Lecture 1: Fertilization to Gastrulation

Lecture 1: Fertilization to Gastrulation

1. Fertilization

   • Ampulla of Uterine Tube
   • Blastocyst Formation
     • Fluid Filled Cavity, Inner Cell Mass (Embryo), Outer Cell Mass (Trophoblast)
     • Fusion of pronuclei, formation of morula by division of blastomeres, compaction of blastomeres, fluid pumped into morula
     • Pregnancy Occurs
       • Syncytiotrophoblast secretes hCG
         • Corpus luteum maintained
           • Progesterone made
     • No Fertilization
       • No hCG
         • Corpus luteum regresses
           • No progesterone
             • Menstruation leaves basal layer to regenerate endometrial glands for next cycle

2. WeekTwo

   • Week of 2's
     • 2 Layers of trophoblast
       • Syncytio-
         • Forms syncytium, responsible for invasion of blastocyst into uterine endometrium and production of hormones
       • Cyto-
         • Makes cells
     • 2 Layers embryoblast
       • Epiblast
       • Hypoblast
     • 2 Cavities
       • Amniotic
       • Yolk Sac
       • Later in the week: Chorionic cavity
     • 2 Layers Extraembryonic Mesoderm
       • Somatic
       • Splanchnic
   • What could go wrong?
     • Ectopic Pregnancy - Abnormal implantation site
       • Ampullary region of uterine tube
       • Douglas' pouch of abdominal cavity

3. Week Three

   • Week of 3's
     • Gastrulation
       • 3 Germ layers
         • Ectoderm
         • Mesoderm
         • Endoderm
         • Formed from epiblast, hypoblast disappears
     • 3 Cavities
       • Amniotic
       • Yolk Sac
       • Chorionic
   • What could go wrong?
     • Caudal dysgenesis: Mermaid syndrome
     • Situs inversus
     • Laterality sequences: incomplete situs inversus
   • Key Steps
     • AVE establishes cranial/caudal, genes for head formation
       • Head mesoderm organized by Goosecoid (secreted by node)
         • BMP-4 antagonized by Goosecoid
         • Brachycury controls formation of dorsal mesoderm caudal to head
           • Expressed by node and notochord
             • Decreased T gene: caudal dysgenesis
           • PITX2 establishes left-sidedness