Embryology

Embryology

1. Week 1: FERTILISATION TO IMPLANTATION

   • Clinical age (last period) Postovulatory age (from ovulation) - reason why due date off
     • PRENATAL PERIOD: GERMINAL period (d1-14) germ layers EMBRYONIC period (d14-56) organogenesis FOETAL period (d56-birth) growth
   • FERTILISATION
     • Gonads contain primordial germ cells. GAMETOGENESIS (meiosis) > gametes.
       • 200-300million sperm enter, 2-7hrs later 300-500 reach ampulla. Oocyte degenerates 24h if unfertilised.
   • DAY 1-4
     • Mitosis Day 1
       • Totipotent (before Day 5) = can form ANY placenta/embryo tissue
         • Day 4: MORULA reaches ISTHMUS of uterine tube
   • Day 5: BLASTOCYST Formation
     • Day 6: BLASTOCYST Formation - sheds Zona Pellucida to prep for implantation
     • Pluripotent = can form neural, cardiac, blood cells on germ line
     • Outer cell mass OCM > placenta/supporting tissues
     • Inner cell mass ICM > embryoblast
   • Day 7: IMPLANTATION
     • Blastocyst arrives at Fundus. Uterine endometrium (secretory phase = multiple glands, coiled arteries). Trophoblast cells INVADE wall.
       • PROBLEMS?
         • Fails to implant?
         • Abnormal blastocyst = Embryoblast absent? Trophoblast > hydatidiform mole?
         • Abnormal implantation site = Ectopic (most die in 2nd month, can be fatal
   • [IVF] Fertilised egg > implanted at 8 cell stage [ICSI] Intracytoplasmic sperm injection - inject sperm into oocyte

2. Week 2: WEEK OF TWOS/DIFFERENTIATION

   • Day 8-9
     • Trophoblast (OCM) into 2
       • CYTOtrophoblast
         • Lining of embryoblast. Chorionic villi.
       • SYNCYTIOtrophoblast
         • INVADES endomentrium wall. Multinuc. No cell boundaries. Non-antigenic (don't cause immune response).
     • Embryoblast (ICM) into 2
       • HYPOblast
         • 1° YOLK SAC outside
       • EPIblast (in direction of implantation. Hypoblast outside)
         • Contains AMNIOTIC CAVITY - Sample for prenatal diagnosis.
       • "BILAMINAR GERM DISC"
   • Day 10-12: 1° Yolk sac membrane into 2
     • INNER side: 2° yolk sac OUTER side: Extraembryonic mesoderm/mesochyme
   • Day 13-14: Extraembryonic mesoderm cavitates > CHORIONIC CAVITY
   • Day 9 *Placental tissues form* SEE BELOW

3. Week 3: GASTRULATION

   • Formation of trilaminar germ disc
     • Bilaminar disc becomes oval. Proliferating cells migrate to CAUDAL end - forms primitive streak
   • PROBLEMS?
     • Migration of cells disrupted - genetic abnormalities/toxic insults
     • Disorders affecting anterior midline
       • Affects face/brain development
       • Assoc w. ETOH
     • "Situs inversus"
       • transposed thoracic/abdo viscera (whole body flips)
         • NB Dextrocardia = just heart flips side
       • 20% have 'Kartagener Syndrome'

4. Week 4: NEURULATION & FOLDING

   • Prenotochordal cells - ORAL & CLOACAL MEMBRANE
     • Notochord (basis axial skeleton) - Prenotochordal cells detach from endoderm > NOTOCHORD. Cell signalling tube - Tells cells what to do next.
       • NEURAL TUBE
         • PROBLEMS?
           • Abnormal neurulation
             • > Spina bifida / Anencephaly
           • Multifactorial
             • Genetic/Freq varies between pop's/Freq higher with affected sibling/Environmental (teratogens)
           • Screening
             • High AFP in pregnancy
           • Prevention
             • 0.4mg/day Folic acid
   • FOLDING: Rapid growth > CEPHALOCAUDAL (head-to-toe) & LATERAL folding
     • > Body cavities & connecting stalk
       • AMNIOTIC FLUID
         • Shock absorption. Prevents foetus adhesion to amnion. Allows foetal mvmt. Foetus swallows & urinates into > helps kidneys/lungs form.
         • PROBLEMS?
           • Too much = "POLYhydramnios" > oesophageal dysgenesis
           • Too little = "OLIGOhydramnios" > underdeveloped kidneys/lungs
           • Tears > amniotic bands (indent/amputate limb)
           • Rupture > pre-term labour

5. IN UTERO

   • Chorionic Villi & Placental tissues
     • Villi function: ^SA exhange of gases, nutrients, waste. NO MIXING of blood.
       • Mixing > Haemolytic (rhesus) disease of newborn
         • WHEN RhD-mother & RhD+ baby. Women treated with immunoglobulin anti-D.

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