General Embryology Lecture Notes

Key Topics Covered

• Duration of Pregnancy
• General Timeline: Pre-embryonic, Embryonic, and Fetal
• Primordial Germ Cells (PGCs)
• Clinical Correlations: Teratomas
• Mitosis and Meiosis
• Nondisjunction and Related Disorders: Down Syndrome and Turner Syndrome

Embryology

• Definition: Study of the formation, development, and maturation of embryos.

Duration of Pregnancy

• Timeline: 9 calendar months and 7 days, or 280 days.
• Gestational Age: Calculated from the first day of the last menstrual period.

Pregnancy Periods

• Pre-embryonic: 0-2 weeks
• Embryonic: 3rd to 8th week
• Fetal: 9th week to birth

Primordial Germ Cells (PGCs)

• Also Known As: Primitive sex cells
• Originate From: Epiblast, which is the primitive ectoderm.
• Migration Path:
  • 4th Week: Migrate to the wall of the yolk sac.
  • 5th Week: Migrate to genital ridge/gonadal ridge — source of spermatogonia and oogonia.

Clinical Correlations

Teratomas

• Craniofacial Teratoma: Result from abnormal PGC migration to the neck region.
• Sacrococcygeal Teratoma: PGCs migrate between the sacrum and coccyx.

Mitosis and Meiosis

Mitosis

• Chromosome number remains the same in daughter cells (e.g., 46 to 46 chromosomes).

Meiosis

• Types: Meiosis I (Reductional Division) and Meiosis II (Equational Division)
  • Meiosis I: Chromosome number reduced by half (e.g., 46 to 23 chromosomes).
  • Meiosis II: Similar to mitosis, maintains chromosome number.
• Stages: Prophase, Metaphase, Anaphase, Telophase

Nondisjunction

• Definition: Failure of homologous chromosomes to separate during meiosis I.
• Consequences:
  • Trisomy (e.g., Down Syndrome): Extra chromosome (47 total).
  • Monosomy (e.g., Turner Syndrome): One chromosome less (45 total).

Down Syndrome (Trisomy 21)

• Cause: Nondisjunction during meiosis I
• Association: Increased maternal age
• Common Features:
  • Congenital cardiac defects, hypotonia, intellectual disability
  • Clinical mnemonic: CHILD Has PROBLEM
  • Child: Cardiac defects, Hypotonia, Increased gap between toes, Leukemia, Dudodnal atresia
  • Has: Hirschsprung disease, Alzheimers, Simian crease
  • Problem: Protruding tongue, Rolling eyes, Flat face, Brushfield spots, Low nasal bridge, Epicanthal folds, Mongolian slant
• Screening Tests:
  • Triple Test: Low AFP, high hCG, low estradiol
  • Quadruple Test: Includes inhibin A (increased)

Turner Syndrome

• Karyotype: 45, XO
• Gonads: Streak ovaries (underdeveloped, not malignant)
• Prevalence: 1 in 2500 live female births
• Clinical Features:
  • Cardiac defects (bicuspid aortic valve, coarctation of aorta)
  • Lymphedema of hands and feet, widely spaced nipples, short stature
  • Webbed neck, cubitus valgus, shield-shaped chest
• Management:
  • Estrogen alone for breast development; followed by estrogen and progesterone.
  • Growth hormone therapy before age 8.
  • Possibility of conception via IVF.

Conclusion

• Covered important topics including embryonic development timelines, germ cell migration, genetic disorders related to nondisjunction, and clinical implications.