Embryology

Development from Zygote through Gastrulation

• Fusion of two haploid gametes forms a diploid zygote.
• Zygote undergoes cleavage via mitosis into blastomeres without size increase.
• Morula stage is reached with 16-32 blastomeres.
• Blastocoele develops within the morula, giving rise to the blastocyst.
• Inner cell mass forms a bilaminar disc of epiblast and hypoblast, establishing the dorsoventral axis.
• Epiblast cells migrate to create a primitive streak, establishing the craniocaudal axis.
• Formation of three germ layers during gastrulation: ectoderm, mesoderm, and endoderm.

Definitions

• Male Pronuclei: Male gamete post-meiosis (1n).
• Female Pronuclei: Ovum nucleus post-fertilization, before fusing with sperm.
• Cleavage: Zygote division via mitosis into blastomeres; no increase in cell size.
• Morula: Solid ball of 16-32 blastomeres.
• Blastocyst: Cavity within the morula, called blastocoele.
• Inner Cell Mass (Embryoblast): Cells that develop into the embryo.
• Trophoblast: Outer blastocyst cells that develop into placenta.
• Epiblast: Dorsal part of the bilaminar disc.
• Hypoblast: Ventral part of the bilaminar disc.
• Gastrulation: Formation of a trilaminar disc from the epiblast.
• Ectoderm: Forms skin, oral/nasal cavity linings, and nervous system.
• Mesoderm: Develops into connective tissue, muscle, mesothelium, and cardiovascular system.
• Endoderm: Forms linings and glands of digestive and respiratory systems.
• Teratogen: Any factor causing congenital anomalies during embryonic development.

Pluripotent, Totipotent, and Related Concepts

• Totipotent: Ability to form any embryonic or placental cell.
• Pluripotent: Can differentiate into any embryonic cell type.
• Multipotent: Can develop into various organ and tissue cells.
• Unipotential: Ability to differentiate into only one cell type.
• Differentiation: Gene interaction and cell signaling guiding cell specialization.

Derivatives of Germ Layers

• Ectoderm: Gives rise to epithelium and nervous tissue.
• Mesoderm: Forms connective tissue, muscle, and parts of the urogenital tract.
• Endoderm: Develops into endothelium and digestive/respiratory glands.

Mesoderm Divisions and Somite Derivatives

• Axial Part: Forms the notochord.
• Paraxial Part: Forms somites, which further differentiate.
• Intermediate Part: Develops the urogenital system.
• Lateral Plate: Forms somatic and splanchnic mesoderm.
• Somite Derivatives:
  • Dermatome: Contributes to the dermis.
  • Myotome: Develops into muscles.
  • Sclerotome: Forms vertebrae and ribs.

Common Developmental Anomalies and Teratogens

• Teratogens: Cause congenital anomalies; include genetic, radiation, chemical, and infectious agents.
• Influences of Teratogens:
  • Pre-differentiation Stage: May lead to embryo death.
  • During Organogenesis: Causes structural defects.
  • During Fetal Growth: Impacts functional maturation and minor structural defects.

Embryology of the Gonads

• Primordial germ cells migrate from yolk sac and allantois to the genital ridge -> become bipotential gonad.
• Bipotential gonad has the potential to develop into either sperm or ova.
• Default sex is female unless influenced by male-specific mechanisms.
• Sertoli cells produce Anti-Mullerian hormone, inhibiting Müllerian duct development in males.
• Male (XY): Develops from mesonephric ducts into epididymis and ductus deferens.
• Female (XX): Develops from paramesonephric ducts into uterus, uterine tubes, and cervix.

Gametogenesis

• Gametogenesis: Production of haploid gametes (sperm and ovum).
• Mitosis: Results in two identical diploid daughter cells.
• Meiosis: Produces four genetically diverse haploid cells.
• Male Gametogenesis:
  • Spermatogonia -> primary spermatocytes (mitosis).
  • Primary spermatocytes -> secondary spermatocytes (meiosis).
  • Further division leads to mature spermatozoa.
• Female Gametogenesis:
  • Fixed number of primordial oocytes at birth.
  • Primordial -> primary oocytes (mitosis), then meiosis (stops at prophase I).
  • Meiosis I completes at puberty; secondary oocyte formed.
  • Meiosis II resumes and completes post-fertilization.

Differences in Spermatogenesis and Oogenesis

• Similarities:
  • Occur in specialized organs (testes/ovaries).
  • Both involve multiplication of germ cells by mitosis and formation of gametes via meiosis.
  • Importance of genetic variability through crossing over and independent assortment.
  • Limited survival if fertilization does not occur.

Fertilization Steps

• Capacitation: Sperm undergo hyperactivation in the uterine environment.
• Acrosome Reaction: Enzymes are released to digest the zona pellucida of the oocyte.
• Cortical Reactions: Cortical granules release upon sperm contact, creating a barrier to polyspermy through zona pellucida modification.