Embryology of Occult Spinal Dysraphisms

Writing a chapter on the embryology of occult spinal dysraphic malformations (OSDM) is a little like writing a historical fiction novel. We begin with a substantial knowledge about the cellular events, molecular biology, and biomechanics underlying normal avian and mammalian neural development - knowledge derived from a host of embryonic experimental manipulations and genetic mutations. From this we propose, largely by extension, a paradigm for normal human neural development that comports with our clinical observations and theorize about the embryogenesis of human neural tube defects (NTD) of myelomeningocele and exencephaly/anencephaly, for which we have reasonable experimental models. We are finally left to essentially fantasize about the embryogenesis of OSDM, for which we have virtually no animal models or experimental data. Many of the theories we will discuss in this chapter were created de novo decades ago from clinical observations and a basic understanding of morphogenesis, unmodified by subsequent advances in cellular and molecular biology. It is our hope that the future will bring greater clarity to our understanding about the embryogenesis of these malformations. Until then, a work of fiction will have to suffice. We begin this chapter with an update about what is known about normal human embryogenesis and our present understanding about NTDs and then review present theories about the embryogenesis of various OSDM.