Overview
This lecture introduces basic embryology, focusing on the stages of prenatal development, structures involved in fertilization, and key embryological events leading to tissue and organ formation.
Overview of Embryology and Development
- Embryology explains how cellular and tissue structures form during prenatal development.
- Prenatal development is divided into germinal, embryonic, and fetal periods.
- Early developmental errors have significant effects, sometimes appearing at birth or much later.
Gamete Anatomy and Fertilization
- Testes (in males) produce spermatozoa (sperm cells) under the influence of testosterone.
- Epididymis is where sperm mature before ejaculation.
- Ovaries (in females) produce ova (eggs), released during ovulation.
- Fertilization usually occurs in the uterine (fallopian) tube, forming a zygote.
Stages of Prenatal Development
- Germinal period: 0–2 weeks, includes fertilization, cell cleavage, blastulation, and implantation.
- Embryonic period: 3–8 weeks, major organ systems begin forming (organogenesis).
- Fetal period: 9–38 weeks, organs mature and rapid growth occurs.
Germinal Period Events
- Fertilization: Fusion of sperm and ovum creates a diploid zygote (46 chromosomes).
- Cell cleavage: Zygote divides without increasing in size, forming the morula (16–32 cells).
- Blastulation: Fluid forms a blastocyst with embryoblast (inner cell mass), blastocoel, and trophoblast layers.
- Implantation: Blastocyst embeds in the uterine endometrium, which develops special layers (decidua basalis, capsularis, parietalis).
- Placentation: Trophoblast develops into chorion and chorionic villi, forming the placenta for nutrient and waste exchange.
Hormonal Regulation of Early Development
- Placenta and chorion secrete human chorionic gonadotropin (hCG) to maintain pregnancy.
- hCG stimulates ovaries to produce estrogen and progesterone, which sustain the placenta until it becomes self-sufficient.
Embryonic Period Events
- Embryogenesis involves germ cell formation and differentiation of the inner cell mass into epiblast (future ectoderm) and hypoblast (future endoderm).
- Gastrulation: Formation of three germ layers—ectoderm (outer), mesoderm (middle), endoderm (inner).
- Mesoderm differentiates into mesenchyme (connective tissue) and somites (precursors to bone, muscle, skin).
- Neurulation: Ectoderm forms neural tube (CNS) and neural crest cells (various structures like nerves, melanocytes).
- Organogenesis: Major organ systems begin around week 3.
Fetal Period Events
- Begins at week 9 and continues until birth.
- Organs and systems mature, with rapid increase in size and weight.
- Typical full-term fetus: ~50 cm long, ~3500 g weight.
Key Terms & Definitions
- Zygote — fertilized cell with full (diploid) chromosome set.
- Morula — solid ball of cells (16–32 cells) after cleavage.
- Blastocyst — fluid-filled structure with cell layers forming embryo and placenta.
- Trophoblast — outer blastocyst layer, forms placenta.
- Embryoblast/Inner Cell Mass — cluster forming embryo.
- Gastrulation — formation of three germ layers.
- Somites — paired blocks of mesoderm forming bone, muscle, skin.
- Neurulation — creation of the neural tube and neural crest cells.
- hCG (Human Chorionic Gonadotropin) — hormone supporting early pregnancy.
Action Items / Next Steps
- Download and review the embryology handout with figures.
- Study the three prenatal development periods and their key events.
- Prepare for next lecture on tissues and their embryological origins.