Embryology (one)

Why Study Embryology?

  • Understanding adult form requires understanding development.
  • Explains peculiarities in human anatomy (e.g., vas deferens looping over ureter).
  • Helps understand and potentially prevent developmental issues like spina bifida (preventable with folic acid).

Embryology's Links to Other Topics

  • Evolution: Homologies and phylogenetic constraints.
  • Cell Biology: Sperm and ova development, maternal/paternal contributions (e.g., maternal mitochondria).
  • Genes: Genotype decided by reproduction and embryogenesis, cell signaling, epigenetics.
  • Environment: Uterine environment (physical and chemical).
  • Phenotype: Interaction of genes and environment.

Homologies and Phylogenetic Constraints

  • Early development: All embryos start the same (e.g., reproductive/renal systems at 5-6 weeks).
  • Gonad development: Gonads descend, leading to long vas deferens.
  • External genitalia: Homologous structures (clitoris/penis).
  • Influences: Genes, cell signaling, timing, epigenetics, environment.

Lecture Objectives

  • Cellular contributions of gametes.
  • Cleavage, implantation, blastocyst formation.
  • Characteristics of embryonic development in weeks one and two.
  • Extra-embryonic structures and their functions.

Week 1 (Days 0-6)

  • Fertilization, one-layered embryo formation, implantation starts.

Week 2 (Days 7-13)

  • Two-layered embryo (bilaminar), body stalk formation.
  • Cavity formation, implantation completion, placenta development begins.

Key Point

  • Focus on weeks, not specific days, as processes are continuous.

Process of Fertilization

  • Ovulation: Secondary oocyte arrested in metaphase II.
  • Sperm: Penetrates cumulus and zona pellucida (acrosome reaction).
  • Fertilization: 23 chromosomes from each parent, forming pro nuclei.

Early Development

  • Fertilized egg: Stays single-cell for ~24 hours.
  • Embryo: After first mitotic event (two cells).
  • Fetus: Main body parts visible (after ~8 weeks).

Pronuclei Development

  • Male contribution: Moves towards female pronucleus, loses tail.
  • DNA replication: Single DNA strands become double strands for mitosis.

Maternal and Paternal Contributions

  • Ovum: DNA, cell membrane, cytoplasm, most organelles (mitochondria).
  • Sperm: 23 chromosomes, centriole (critical for pronuclei coming together).

Centriole's Role

  • Roundworm research: Centriole moves male pronucleus; mechanism for female pronucleus movement in humans not fully known.
  • Importance: Critical for nuclei meeting; dysfunction can cause infertility.

Cleavage and Blastomeres

  • Mitotic events: Two cells to four to eight to 16-32 cells.
  • Blastomeres: Cells get smaller due to large ovum cytoplasm.
  • Differentiation: Starts at eight-cell stage (inner embryoblast, outer trophoblast).

Morula and Blastocyst Formation

  • Morula (16-32 cells): Trophoblast cells transport sodium ions, water follows, forming fluid-filled cavity.
  • Blastocyst: Embryo with fluid-filled cavity.
  • Blastocyst hatching: Discards zona pellucida via enzymes and expansion/contraction.

Implantation

  • Blastocyst hatches and interacts with maternal endometrial cells.
  • Embryoblast differentiates: Epiblast (dorsal), hypoblast (ventral).
  • Trophoblast differentiates: Cytotrophoblast (single cell layer), syncytiotrophoblast (cell soup).
  • Syncytiotrophoblast: Erodes maternal uterine glands and blood vessels.

Embryo Development (Day 8 Onwards)

  • Syncytiotrophoblast: Further develops, erodes maternal structures.
  • Amniotic cavity: Forms within epiblast; amniocytes surround cavity.
  • Primary yolk sac: Cavity lined by cells.

Day 10

  • Embryo fully implanted.
  • Maternal epithelial cells cover implantation site.
  • Extraembryonic mesoderm: Forms between cytotrophoblast and developing structures; important in placental development.
  • Lacunae network: Syncytiotrophoblast invades maternal blood vessels and uterine glands.

Days 12-14

  • Openings in extraembryonic mesoderm form chorionic cavity.
  • Primary yolk sac discarded; secondary yolk sac forms.

End of Week 2

  • Embryo and cavities connected by connecting stalk (future umbilical cord).
  • Beginning of placental formation: Syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm.

Function of Chorionic Cavity

  • Provides space for embryonic growth.

Function of Yolk Sac

  • Contributes to digestive tract formation.
  • Site of first gamete and blood cell formation.

Possible Sites of Implantation

  • Optimal: Mid-uterus.
  • Suboptimal: Can lead to ectopic pregnancies (out of place).
  • Ectopic pregnancies: Occur outside the uterus (e.g., in uterine tube), are not viable and pose risks to the mother.