Editor In Chief
During the first 4 weeks after fertilization, the CNS of the embryo undergoes a series of morphological and structural changes that result in the formation of the neural tube (brain and spinal cord). These can be divided into four phases: 1) blastogenesis, 2) gastrulation, 3) primary neurulation, and 4) secondary neurulation. Once neural tube formation is completed, a series of additional morphological changes occurs within the brain and spinal cord, divided into four additional phases: 1) ascent of the conus medullaris, 2) development of the cranial vesicles and flexures, 3) formation of the basal and alar plates and their derivatives, and 4) migration of neuronal and glial cells. Each of these processes will be summarized in this chapter. The term “postovulatory day” (POD) will be used to describe the phase or gestational age of the embryo; this designation is used in standard embryology studies to describe human embryonic development.
- Gastrulation – conversion of two-layer to three-layer embryo: Gastrulation results in conversion of a two-layered embryo, containing epiblast and hypoblast, to a three-layered embryo, containing ectoderm, mesoderm, and endoderm.
- Primary neurulation – CNS formation to S2: Primary neurulation results in formation of the brain and spinal cord as far caudal as the second sacral segment (S2) and separation (dysjunction) of the cutaneous ectoderm from the neuroectoderm.
- Secondary neurulation – cord formation distal to S2: Secondary neurulation forms the spinal cord caudal to S2 and the filum terminale from a different embryonic mechanism.
- Ascent of conus to L1-L2: Ascent of the conus medullaris results in the tip of the conus lying most commonly opposite the L1-L2 disk space; a conus below the mid-body of L2 is tethered radiographically.
- Vesicle and flexure formation shapes brain: The formation of the primary and secondary vesicles and three flexures contributes to the ultimate shape and structure of the brain.
- Neuronal migration region specific: Neuronal migration occurs in a specified pattern from basal and alar plate neurons, a process that is modified somewhat within each regional level of the neuraxis.