Relationship of early neural tube appearance to myeloschisis in a chick embryo model.

Myeloschisis, a form of neural tube defect involving the spinal cord, was induced in chicks by creating a window in the eggshell of the fertilized egg at 26 h after placement in an incubator. The embryos were stained and photographed through the window at 48 to 60 h after commencement of incubation and the neural tube was determined to be normal, delayed, irregular, or abnormally open for the developmental stage of the embryo. The eggs were then reincubated until 14 days of age. At that time the chicks were determined to have normal spinal cords or myeloschisis by gross examination, dissection, and histology. The appearance of the neural tube at 48 to 60 h was then correlated with the appearance of the spinal cord at 14 days of age. It was determined that in the chick embryo model of myeloschisis induced by windowing, a normal neural tube at the time of neural tube closure results in a 2.3% incidence of myeloschisis, a widely open neural tube a 47% incidence of myeloschisis, an irregular neural tube a 37% incidence of myeloschisis, and delayed closure of the neural tube a 14% incidence of myeloschisis. Thus, no appearance of an early neural tube can be said to always correlate with myeloschisis or a normal outcome. Additionally, it appears that within a single model there may be multiple mechanisms of production of myeloschisis.