Early Embryology

1. Zygote

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   • Between 12 - 24 hours
2. Morula

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   • At the 16 cell stage, approximately 3 days after fertilization
   1. Inner Cell Mass

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      • Divides into the epiblast and hypoblast
      1. Epiblast

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         • Forms all tissues of embryo and lining of newly developed amniotic sac
         1. Notocord

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            • Specialized cells that migrate through the primitive node between the endo and ectoderm. They travel cranially and caudally along the midline to form tubular aggregates of cells. These provide structure to developing embryo until the vertebrae is formed and induces development of the CNS.
         2. Amnion

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            • The cavity formed above the epiblast and lined by a thin layer of epiblast cells. Fills rapidly with fluid and wraps around the embryo. It surrounds the umbilical vein as it passes from placenta to embryo
            • - To Protect against mechanical shock - Allows for symmetrical growth - Prevents adherence of fetus to amnion - Maintains consistent temperature - Important for respiratory development - Barrier to infection - Homeostasis of fluid and electrolytes 
      2. Hypoblast

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         • Will form embryonic yolk sac, which is essentially the former blastocyst cavity
         1. Yolk Sac

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            • Yolk sac is used to produce early red blood cells
   2. Outer Cell Mass
3. Blastocyst

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   • When the morula fills with fluid. This is what enters the uterine cavity.  Zona pellucida dissolves to make ready for implantation.

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• Starts to occur around 5 - 6 days after implantation. Completed by the second week. Usually implants in the anterior or posterior wall of the uterus

1. Trophoblasts

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   • Responsible for implantation.  Mutlilayered, multinucleated structures from the outer cell mass.  They erode the endometerium to implant deeper. Will form lacunae and produces hCG
   1. Placenta

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      • Developed from trophoblast cells. Their job is to secrete enzymes that digest the endometrial wall for implantation, and enlarges around the embryos. It erodes endometrial vessels to form trophoblastic lacuna to allow for early maternal-fetal exchange
      • The placenta is the chorionic villi and the associated endometrium
      • Function is placentral transfer and hormone production
      1. Placental Transfer

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         • Gases, nutrients, waste products, hormones, electrolytes, maternal antibodies (IgG), drug metabolites and some infectious agents can cross
      2. Hormone Production
         1. hCG

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            • This is the hormone measured in pregnancy tests. It is usually detectable about 1 - 2 weeks after fertilization
            • The job is to maintain the corpus luteum
         2. Progesterone

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            • Job is to maintain the endometrium
         3. Estrogen
         4. Human Placental Lactogen
         5. Chorionic Thyrotropin
         6. Corticotropin
   2. Chorionic Villi

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      • Contain embryonic vessels. They are finger-like projections that will become the site of maternal-fetal exchange beside lacunae and branches many times to increase the surface area for exchange. 
   3. Lacunae

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      • Fill with maternal blood and allow for exchange of oxygen, carbon dioxide, nutrients, waste products, hormones, maternal antibodies, electrolytes, some harmful agents
2. Ectopic Pregnancy

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   • Any pregnancy in an abnormal location.  Most common are in the uterine tube but can also occur in the abdominal cavity, cervix and ovary. The embryo will die and will result in severe bleeding and abdominal pain.
   1. Placenta Previa

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      • May result from implantation at the cervix. The placenta covers the cervical opening and is high risk for severe bleeding. Classified as either marginal, complete or low-lying.

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• The formation of the 3 germ layers from epiblast into ecto, meso and endoderm. When cells from the epiblast migrate to the midline at the caudal end of the embryo, they form a thick band known as the "Primitive Streak" which elongates cranially. The cranial tip is known as the "Primitive Node"

1. Ectoderm

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   • When cells from the primitive streak and node do not migrate. Derivatives: epidermis, brain, spinal cord, peripheral nervous system, autonomic nervous system and skull
2. Mesoderm

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   • When cells from the primitive streak and node migrate downward and fill the space between the epiblast and hypoblast. Derivatives are: Skeletal muscle, heart and blood vessels, kidneys and gonads, skeleton, most connective tissue
   1. Paraxial Mesoderm

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      • Found along the midline
      1. Somites

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         • Starting around the 3rd week. First pair develops in the occipital region and continues cranio-caudal. By end of the 5th week there are 42 - 44 pair. Divided into 3 subunits 
         1. Sclerotome

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            • Found next to the neural tube. Migrates around the neural tube and notocord to form vertebrae and ribs
         2. Myotome

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            • The part adjacent to neural tube forms epimeres. The lateral part forms hypomeres
            1. Epimeres

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               • Develops into the intrinsic muscles of the back
            2. Hypomeres

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               • Develops into muscles of the body wall and limbs, except the head and neck
         3. Dermatome

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            • Migrate to the dorsal region of the trunk to become the dermis of the back (hair follicles, glands, fatty tissue, blood vessels)
      2. Somatomeres
   2. Lateral Plate Mesoderm

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      • Extends laterally
      1. Somatic (Parietal) Layer

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         • With folding will end up along the body wall of the embryo under the ectoderm. Forms the many structures of the body wall and limbs
      2. Splanchnic (Visceral) Layer

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         • With folding will wrap around the part of the yolk sac that gets incorporated into the body cavity to form the serous layer around organs.
   3. Intermediate Mesoderm

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      • Between the paraxial and lateral mesoderms.  Forms urogenital structures and gonads
3. Endoderm

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   • When cells from the primitive streak and node migrate downward and displace the hypoblast.  Derivatives: inner lining of gut tubes, respiratory tracts, bladder, some GI organs

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• 1. Endometrial Spiral Arteries 2. Intervillous Spaces / Lacunae 3. Chorionic Villi / Fetal Capillaries 4. Umbilical Vein 5. Fetus 6. Umbilical Arteries 7. Chorionic Villi / Fetal Capillaries 8. Intervillous Space / Lacunae 9. Endometrial Veins