Neurulation is the development of the neural plate and the processes involved with its subsequent closure to form the neural tube during the early stages of embryonic development (during first 3-4 week). Neurulation involves formation on neural plate, neural folds and the neural tube.
During the 3rd week of development, the ectoderm on the dorsal surface of the embryo between the primitive knot and the buccopharyngeal membrane becomes thickened to form the neural plate. The plate develops a longitudinal neural groove which deepens so that it is bounded on either side by neural folds. The neural folds fuse, converting the neural groove into a neural tube. Fusion starts at about the midpoint along the groove and extends caudally so that in earliest stage, the cavity of the tube remains in communication with the amniotic cavity through the anterior and posterior neuropores. The anterior neuropore closes first, and 2 days later, the posterior neuropore closes. Neural tube later forms spinal cord, brain, and the bone and tissues that surround it.
Neural Tube Defects (NTD)
Neural Tube Defects are congenital defects of the central nervous system, including the spinal cord, skull and brain that occur due to the defect of neurulation process. NTDs may involve the meninges, vertebrae, muscles and skin. The two common neural tube defects are:
- Spina bifida
In spina bifida, the spines and arches of one or more vertebrae fail to develop. The condition occurs most frequently in the lower thoracic, lumbar and sacral regions. This condition is a result of failure of the mesenchyme, which grows in between the neural tube and the surface ectoderm, to form the verterbral arches in the affected region.
Spina bifida occulta:
- Defect in the vertebral arches that is covered by skin and usually does not involve uderlying neural tissue
- Marked by a patch of hair or a fatty tumor overlying the affected region
- Spinal cord and nerve roots are usually normal
Spina bifida cystica:
- Severe NTD in which neural tissue and/or meninges protrude through a defect in the vertebral arches and skin ti form cyst like sac
- Results in neurological deficits
- Spina bifida with Meningocele: Only fluid-filled meninges protrude through the defect
- Spina bifida with Meningomyelocele: The normal spinal cord or cauda equina lies within the meningeal sac, which projects through the vertebral arch defect.
- Myelocele: The neural tube fails to close in the region of the defect
- Syringomyelocele: A meningomyelocele is present and in addition, the central canal of the spinal cord at the level of the bony defect is grossly dilated.
- Hydrocephaly develops in virtually every case because the spinal cord is tethered to the vertebral column
In anencephaly, the greater part of the brain and the vault of the skull are absent, leaving the malformed brain exposed. It results due to the failure of the cephalic part of the neural tube to close. Most anencephalic infants are stillborn or die shortly after birthe.
Read more about anencephaly @ Anencephaly : A severe birth defect
Treatment and Diagnosis of NTDs
Prevention of NTDs:
- Folic acid : reduces the incidence of NTDs by as much as 70% if 4000 mg is taken daily beginning 2 months prior to conception and continuing throughout gestation
- Avoiding teratogens like valproic acid (valproate), excessive vitamin A, etc.
- Abortion in case of untreatable cases
Clinical Neuroanatomy – Richard Snell
Lanman’s Medical Embryology – Sadler