Overview
This lecture covers the embryological development of the nervous system, focusing on the formation and differentiation of the neural tube and its transformation into major adult brain structures.
Early Neural Development
- The embryonic nervous system starts as a simple straight line called the neural tube.
- Three germ layers form at 16 days: endoderm (lining tissues), mesoderm (muscle/connective tissue), and ectoderm (skin and nervous system).
- The ectoderm develops into neuroectoderm, which forms the neural plate, groove, folds, and eventually the neural tube.
- Neural crest cells migrate from the neural tube to become parts of the peripheral nervous system (PNS) and other tissues.
Neural Tube to Brain and Spinal Cord
- The anterior neural tube becomes the brain; the posterior forms the spinal cord.
- At 25 days, the neural tube starts differentiating into distinct regions.
- The neural tube also establishes dorsal-ventral orientation; dorsal regions become sensory, ventral regions become motor.
Brain Vesicle Development
- The brain forms from three primary vesicles: prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).
- Primary vesicles further divide into five secondary vesicles: telencephalon (cerebrum), diencephalon (thalamus, hypothalamus, retina), mesencephalon (midbrain), metencephalon (pons, cerebellum), and myelencephalon (medulla).
- The cerebrum arises from the telencephalon; brainstem includes midbrain, pons, and medulla; the cerebellum is separate.
Relation to Adult Structures
- The neuraxis (anterior-posterior axis) is established by the neural tube.
- The cephalic flexure creates a bend between brainstem and forebrain in humans.
- Ventricles in the adult brain originate from the hollow center of the neural tube.
Spinal Cord Development
- The spinal cord develops as a straight tube with a central canal.
- Dorsal spinal cord handles sensory input; ventral handles motor output.
Neural Tube Defects: Spina Bifida
- Spina bifida occurs if the neural tube does not close fully, especially at the posterior end.
- Types include occulta (mild, hidden), meningocele (meninges protrude), and myelomeningocele (meninges and nerves protrude).
- Surgical intervention can limit or correct some defects but neurological symptoms may persist in severe cases.
Key Terms & Definitions
- Neural tube — embryonic structure forming the brain and spinal cord.
- Neuroectoderm — specialized ectoderm that becomes nervous tissue.
- Neural crest — migrating cells from neural folds forming PNS components.
- Vesicle — embryonic sac-like brain region.
- Cephalic flexure — bend in neuraxis between brainstem and forebrain.
- Spina bifida — birth defect from incomplete neural tube closure.
Action Items / Next Steps
- Watch assigned animations and videos on brain and white matter development.
- Review Table 13.1 to link embryonic stages to adult CNS structures.