Early Embryonic Development

Gametogenesis & Fertilization - Spark of Life

• Gametogenesis: Formation of gametes (sperm & ova).
  • Spermatogenesis: Seminiferous tubules. Puberty onset. Spermatogonia (2n) undergo meiosis → 4 haploid (n) motile spermatozoa.
  • Oogenesis: Ovaries. Fetal life onset. Oogonia (2n) undergo meiosis → 1 haploid (n) ovum & polar bodies. Arrests: Prophase I (primary oocyte), Metaphase II (secondary oocyte, completes if fertilized).

> ⭐ Most common site of fertilization is the ampulla of the fallopian tube.

Cleavage & Blastocyst - The Great Divide

• Cleavage: Series of rapid mitotic cell divisions of the zygote. Results in ↑ cell number (blastomeres) without ↑ in overall size. Occurs during fallopian tube transit.
• Morula: 📌 (Latin 'morus' for mulberry). Solid ball of 16-32 blastomeres. Enters uterine cavity around Day 4 post-fertilization.
• Blastocyst Formation (Day 5-6):
  • Uterine fluid enters morula, forming a cavity (blastocoel).
  • Cells differentiate into:
    • Inner Cell Mass (ICM) / Embryoblast: Gives rise to the embryo proper.
    • Outer Cell Mass / Trophoblast: Forms chorion and fetal part of placenta.
  • Zona pellucida degenerates ("hatching") allowing for implantation.
• Implantation (Begins Day 6-7, completes by Day 10-12):
  • Hatched blastocyst adheres to, then invades, the uterine endometrium (typically decidua basalis).
  • ⭐ > The most common site for normal implantation is the posterior superior wall of the uterus.

Gastrulation & Germ Layers - Triple Decker

• Gastrulation: Begins Week 3. Epiblast cells invaginate via primitive streak, forming 3 primary germ layers (trilaminar disc) from bilaminar disc.
  • Primitive Streak: Transient groove on epiblast. Defines craniocaudal axis. Primitive node at cranial end.

  ⭐ The primitive streak establishes the craniocaudal axis of the embryo.

• Primary Germ Layers & Key Derivatives:

Neurulation & Folding - Tube & Twist

Neurulation: Formation of the neural tube, the primordium of the Central Nervous System (CNS).

• Neural tube closure: Cranial (rostral) neuropore by Day 25; Caudal neuropore by Day 28.
• Defects in closure lead to Neural Tube Defects (NTDs) e.g., anencephaly (cranial), spina bifida (caudal).

⭐ Folic acid (Vitamin B9) supplementation (400 µg/day) preconception and during early pregnancy significantly reduces NTD risk.

Neural Crest Cells (NCCs):

• Origin: Ectodermal cells detaching from the lateral edges of the neural folds during neurulation.
• Key Derivatives: Peripheral Nervous System (PNS) components (sensory & autonomic ganglia, Schwann cells), melanocytes, adrenal medulla, craniofacial cartilage & bone, C cells (parafollicular cells of thyroid), odontoblasts, conotruncal septum of the heart.

Embryonic Folding:

• Transforms the flat trilaminar embryonic disc into a C-shaped cylindrical embryo.
• Craniocaudal folding: Occurs in the median plane due to rapid growth of brain vesicles and caudal eminence; forms head and tail folds.
• Lateral folding: Occurs in the horizontal plane due to rapid growth of somites; sides of embryo fuse ventrally, incorporating part of yolk sac as primitive gut tube.

High‑Yield Points - ⚡ Biggest Takeaways

• Fertilization typically occurs in the ampulla of the fallopian tube; implantation around day 6-12 in the posterior superior uterine wall.
• Cleavage leads to Morula (16-cell stage, day 3-4), then Blastocyst (day 5; embryoblast & trophoblast).
• Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast (secretes hCG, basis of pregnancy tests).
• Week 2 is the "week of twos": Bilaminar germ disc (epiblast, hypoblast), amniotic cavity, and primary yolk sac form.
• Week 3: Gastrulation (forms 3 germ layers: ectoderm, mesoderm, endoderm), initiated by the primitive streak; notochord develops, inducing neurulation.