The spectrum of congenital brain malformations is complex and classification schemes are imperfect. Neuroimaging evaluations provide extensive information concerning abnormal brain morphology and cellular organization. Correlation of this information with well-established concepts of normal embryological and fetal neural development provides a conceptual framework for a greater understanding of brain anomalies. The most frequently utilized neuroimaging classification scheme for congenital malformations of the brain recognizes 4 major types that correlate to stages of development: (1) dorsal induction (primary neurulation), (2) ventral induction (telencephalization), (3) neuronal proliferation, differentiation, and histogenesis, and (4) neuronal migration. It is important to recognize, however, that this system is an oversimplification of a heterogeneous and complex group of disorders.
At about 16 days' gestational age, the embryo consists of a 3-layered disc. Cells from the primitive node migrate between the endoderm and ectoderm cranially and form the notochordal process, the structure around which the vertebral column forms. The notochordal process fuses with endoderm and reaches the oropharyngeal membrane by day 18. There is concomitant thickening of ectodermal cells that overlie the notochord, thereby forming the neural plate. On day 18, neural plate cells along the dorsal aspect of the notochordal process invaginate, forming the neural groove. Cells lateral to the groove thicken and give rise to the neural folds. The neural folds move medially and close over the neural groove, forming the neural tube. Closure of the neural tube begins in the cervical region and proceeds toward both ends of the embryo. The brain ventricular system and the spinal cord central canal develop from the lumen of this tube. The walls of the neural tube eventually thicken to form the brain and the spinal cord. Dorsal induction refers to the sequence of embryological events in the dorsal aspect of the embryo that culminate in formation of the neural tube.
Neurulation is the process of neural tube closure. Primary neurulation encompasses initial closure of the neural tube from the anterior neuropore to the upper lumbar segments. Secondary neurulation refers to the events that produce closure of the more caudal segments. Various congenital anomalies result from failure of appropriate neural tube closure during the third and fourth weeks of development. Deficient neurulation encompasses a spectrum of malformations that includes anencephaly, exencephaly, encephalocele, and Chiari II malformation. Craniorachischisis is total failure of neurulation: central nervous system (CNS) tissue exists only as a neural plate that is devoid of dorsal osseous or dermal covering.
Ventral induction refers to the series of events after neurulation that lead to the formation of 2 separate cerebral hemispheres (i.e., diverticulation). Luminal epithelial cells secrete a watery liquid into the closed neural tube. The cephalic end becomes somewhat distended with fluid and forms the primary brain vesicles: the prosencephalon, mesencephalon, and rhombencephalon. The prosencephalon (forebrain) consists of 2 secondary vesicles: the telencephalon and the diencephalon. The telencephalon gives rise to ...