- The process of neurulation involves the formation of the neural plate and the folding of the neural plate into the neural tube.
- The notochord induces the overlying ectoderm to develop into the neural plate.
- The neural plate folds into the neural tube and as it closes, the neural crests are pinched off.
- The neural tube derives the central nervous system (the brain and spinal cord).
- The neural crest cells derive the peripheral nervous system (eg, ganglion cells and Schwann cells) and also select other cell types (eg, melaoncytes).
THE DEVELOPING EMBRYO
Trilaminar germ disc
Three layers of the trilaminar germ disc.
- Ectoderm (and amniotic cavity)
- Endoderm (and yolk sac)
The prochordal knot
- A strand of cells that extends toward the cranial end of the prochordal knot.
- The prochordal knot lies within the mesoderm (in between the ectoderm and endoderm).
ASSOCIATED EMBRYONIC STRUCTURES
Key associated embryonic structures:
- The primitive streak exists within the ectodermal layer of the germ disc; it dimples along the embryonic disc.
- The primitive node (aka primitive knot, Hensen’s node) lies at the cranial end of the primitive streak.
- The prochordal knot lies farther cranially.
- The notochord develops cranially, (towards the head of the embryo) and because it is blocked at the prochordal plate, it also develops caudally (towards the tail of the embryo) as the primitive streak regresses. There are multiple stages of notochord development, which we omit, here, for simplicity.
Key notochord actions:
- Forms the embryonic central axis,
- Induces neural plate formation,
- Establishes the central column of the spine and then degenerates to become the nucleus pulposus of the intervertebral discs.
DAY 17 OF EMBRYOGENESIS
- Early regression of the primitive streak.
- Development of the neural plate.
- The notochord lies within the mesoderm (it induces neural plate formation).
DAY 18 OF EMBRYOGENESIS
- The primitive streak has regressed.
- The neural plate invaginates to form the neural groove (the dip, centrally) and the neural folds (the peaks, laterally). The neural crests lie at the tips of the neural folds.
- Within the mesoderm, somites develop.
- Sclerotome (which derives bone and cartilage),
- Dermatome (which derives dermis),
- Myotome (which derives skeletal muscle).
DAY 21 OF EMBRYOGENESIS.
- The primitive streak has nearly completely regressed and the neural groove starts to fully fold to form the neural tube, which enters the mesoderm.
- It closes off in the center first, with the cranial and caudal ends still open at this point, and resides within the mesoderm.
- The neural crest cells have pinched off and reside in the ectoderm layer.
DAYS 23 – 26 OF EMBRYOGENESIS
- The anterior (cranial) neuropore closes at approximately Day 24.
- The posterior (caudal) neuropore closes at approximately Day 26.
- The somites form ridges underneath the ectoderm.
- The neural crests migrate to within the mesoderm.
CONGENITAL NEURO EMBRYOLOGICAL DISORDERS
- Chiari Malformation
- Dandy Walker Malformation
- Septo-Optic Dysplasia
- Zellweger Syndrome