Development of the Nervous System
last authored:
last reviewed:
Introduction
Contributing Tissues
most of the CNS is derived from ectoderm.
Early Development
During the third week, the ectoderm thickens and clusters to form the neural plate.
Neurulation, or formation of the neural tube, is the beginning of the central nervous system and forms by day 21. Fusion first occurs cervically and extends in both directions.
By day 26, a tube forms that is closed at both ends and separate from the overlying ectoderm.
By 5 weeks, the CNS is developing from the neural tube, with the notocord acting as the beginning of the the vertebrae.
Neural Crest cells
precursors for neural and non-neural cells. These include:
- parts of cranial nerves
- some sensory neurons
- postganglionic neurons of the ANS
- Schwann cells
- pia and dura (check)
- satellite cells
- cartilage in pharyngeal arches
- olfactory epithelium
- smooth muscle in the eye
the neural tube cavity is the precursor to the ventricular system of the brain. Cell migration forms three layers:
- ependymal (ventricular)
- mantle zone contains cell bodies (gray matter)
- marginal zone contains cell fibres (white matter)
Spinal Cord Development
by 5.5 weeks, anterior and posterior differentiation of the sc is apparent. An intermediate zone is present between the dorsal (alar) plate and the ventral (basal) plate. The dorsal plate gives rise to sensory function, while the ventral plate becomes motor.
Beginnings of the dorsal root ganglion are present.
By about week 20, lower alhoa motor neurons are present
Brain Development
During the 4th week of development, there are three vesicles present: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon).
During the 5th-6th week, the five vesicle stage occurs. The prosencephalon divides into the telencephalon and the diencephalon, while the rhombencephalon divides into the metencephalon an
| primary structure | secondary structure | adult structure |
| prosencephalon | telencephalon | cerebral hemispheres |
| diencephalon | thalamus, hypothalamus | |
| mesencephalon | mesencephalon | midbrain |
| rhombencephalon | metencephalon | pons, cerebellum |
| myelencephalon | medulla |
Things that Go Wrong
Neural tube defects
- Neural tube defects can result from a number of causes, with a recognized one being folate deficiency during early pregnancy.
- spina bifida affects approximately 1:1000 babies
- can be detected by amniocentesis or maternal serum screening
- alpha fetoprotein is produced by the liver of the fetus. any break in the embryo will result in leakage of afp from the fetus into the amniotic space. maternal serum testing is sensitive but not specific, as there is a lot of overlap between normal and affected values. Amnicentesis has a much lower level of overlap, but is more invasive
spina bifida occulta
- largely benign
- strictly a skeletal problem
- common, maybe 5% of adults
- can have a tuft of hair
- if a passage between the skin and spinal cord is present it must be surgically corrected to prevent meningitis
spina bifida cystica/aperta
- can occur anywhere along the vertbral column
- meningiocele - meningeal cyst that passes through open spinal processes, but with no spinal components within the cyst
- myelomeningiocele - most common; spinal cord and roots are present within the cyst
- frequently associated with hydrocephalus
- myeloschisus - the neural tube doesn't close, leaving the spinal cord is outside. very uncommon
hydrocephalus
- results from impaired circulation and absorption of CSF; often due to congenital aqueductal stenosis
- causes dilation of ventricles and presssure on cerebral hemispheres
- usually refers to noncommunicating hydrocephalus, in which all or part of the ventricular system is enlarged
- communicating hydrocephalus results from obliteration of subarachnoid cisterns or malfunciton of arachnoid villi
- myelomeningiocele pulls spinal cord down and results in changes to brain stem anatomy
anencephaly
- lack of anterior neuropore closure results in amniotic fluid erosion of brain
- can be variable amount of cerebellum and brain stem, with little forebrain remaining
- frog-like face
- very rare to survive longer than three days
encephalocele (cranium bifidum)
- defects in formation of cranium; usually involving medial plane of calvaria
- perhaps 1:2000; higher in Asia
- cysts that form can contain nothing (cranial meningocele), brain tissue (meningioencephalocele), or brain tissue plus ventricular tissue (meningiohydroencephalocele)
Resources and References
 |