Embryology and Development
I. Divisions
Prosencephalon
Telencephalon—cortex, amygdala, caudate, putamen
Diencephalon—thalamus, hypothalamus, globus pallidus
Mesencephalon—midbrain
Rhombencephalon
Metencephalon—pons, cerebellum
Myelencephalon—medulla
II. Basal and Alar Plates
Basal Plate (ventral)
Anterior horn cells
Motor nuclei of cranial nerves
Alar Plate (dorsal)
Dorsal sensory horn
Sensory nuclei in brainstem
Cerebellum (from rhombic lips)
Inferior olive
Quadrigeminal plate (superior and inferior colliculi)
Red nucleus
Sulcus Limitans
Boundary between basal and alar plates
III. Neural Tube Formation
Fusion begins on day 22 in region of the lower medulla.
Fusion begins at the fourth somite.
Anterior neuropore closes on day 25.
Posterior neuropore closes on day 27.
Neural crest cells give rise to:
IV. Caudal Neural Tube Formation (Secondary Neurulation)
Forms on days 28 to 32
Forms sacral/coccygeal segments, filum terminale, ventriculus terminalis
V. Disorders of Primary Neurulation
Anencephaly
Failure of anterior neuropore closure (<24 days)
Effects forebrain and variable portions of brainstem
Holoacrania—to foramen magnum
Meroacrania—slightly higher than foramen magnum
75% are stillborn
Most common in whites, Irish, females, children born to either very old or very young mothers
Risk in subsequent pregnancies is 5% to 7%
Myeloschisis
Failure of posterior neuropore closure
Associated with iniencephaly—malformation of skull base
Most are stillborn
Encephalocele
Restricted defect in anterior neural tube closure
75% occipital
50% with hydrocephalus
Myelomeningocele
Restricted defect in posterior neural tube closure
80% lumbar
90% have hydrocephalus if lumbar region is involved
60% have hydrocephalus if another region is involved
Symptoms include motor, sensory, and sphincter dysfunction
Arnold-Chiari Malformation
Type I
Isolated displacement of the cerebellar tonsils into the cervical canal
Kinked cervical cord
Type II
Displacement of the cerebellum into cervical canal
Displacement of the medulla and fourth ventricle into the cervical canal
Long, thin medulla and pons, tectum deformity
Skull base and upper cervical spine defects
Hydrocephalus resulting from fourth ventricle obstruction or aqueductal stenosis
100% with myelomeningocele
96% with cortex malformations (heterotopias, polymicrogyria)
76% with brainstem malformations (hypoplasia of cranial nerves, pons)
72% with cerebellar dysplasia
30% with thoracolumbar kyphoscoliosis
30% with diastematomyelia (bifid cord)
Syringomyelia
Type III
All in types I and II, plus encephalocele (occipital)
Meckel Syndrome
Associated with maternal hyperthermia or fever on days 20 to 26
Includes encephalocele, microcephaly, microphthalmia, cleft lip, polydactyly, polycystic kidneys, ambiguous genitalia
Neural Tube Defects
Causes
Chromosomal abnormalities (trisomy 13, 18)
Teratogens (thalidomide, valproate, phenytoin)
Single mutant gene (Meckel syndrome)
Multifactorial
Diagnosis
Increased α-fetoprotein
Increased acetylcholinesterase
Ultrasound
Prevention: Folate
Recurrence rate: 2% to 3%
VI. Disorders of Secondary Neurulation—Occult Dysraphic States
General Information
Intact dermal layer over lesion
80% have an overlying dermal lesion (dimple, hairy tuft, lipoma, hemangioma)
Frequent tethered spinal cord
100% have abnormal conus and filum
90% have vertebral defects
4% have siblings with a disorder of primary neurulation
Caudal Regression Syndrome
20% are infants of diabetic mothers
Characterized by dysraphic sacrum and coccyx with atrophic muscle and bone
Symptoms include delayed sphincter control and walking, back and leg pain, scoliosis, pes cavus, leg asymmetry
Myelocystocele—cystic central canal
Diastematomyelia—bifid cordRelated posts:
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