Embryonic stages and brain malformations

The central nervous system (CNS) is one of the earliest to begin development in the human embryo and does not complete maturation until several years after birth when myelination is complete. The embryonic period encompasses 23 stages that are completed approximately 2 months following fertilization. These developmental stages, frequently referred to as the Carnegie stages, provide a precise morphological framework for following development and are helpful in determining the timing of many abnormalities of the brain and spinal cord in humans. The definitive account of embryonic development was published by O'Rahilly and Müller who refined and updated previous work on the Carnegie embryo collection. Additional clarification of staging and prenatal ages has also been provided. They have recently added a detailed account of the development of the human brain during the embryonic period and this reference, along with the older work of Lemire et al., provides specific information about the development of the human CNS. Table 5.1 outlines a general framework of features related to the embryonic stages and Table 5.2 provides selected information on brain development.

A useful concept in assessing the timing of human malformations is that of a “termination period,” a point in time beyond which a specific malformation cannot occur. For example, in humans the rostral neuropore closes in embryonic stage 11. Anencephaly is felt to arise in most cases when the neuropore fails to close.