Secondary Neurulation

Secondary neurulation forms the caudal spinal cord distal to S2 and the filum terminale (2, 5, 6).

• Caudal to posterior neuropore: Secondary neurulation begins on POD 27 and involves tissue located within the caudal cell mass (the remnant of Hensen’s node) at the caudal end of the embryo, caudal to the posterior neuropore. It forms the spinal cord caudal to S2 as well as the future filum terminale. Mesoderm within the caudal cell mass also forms the anlagen of the sacrum, and endoderm contributes to the hindgut (colon, rectum, and anus).

Secondary neurulation in chick embryo: Small tubules form (A) from caudal cell mass (CCM) caudal to the neural tube formed from primary neurulation (1o NT), coalesce (B) to form the secondary neural tube (2o NT), and eventually fuse with the neural tube (1o NT) (C). From Dias and McLone (2)

Secondary neurulation in mouse embryo: The secondary neural tube (NT) grows by addition of cells to the primary NT from the caudal mass (CCM). NC, notochord. From Dias and McLone (2)

• Occurs beneath intact dorsal skin: Secondary neurulation occurs in the presence of an intact cutaneous ectoderm overlying the caudal cell mass; therefore defects of secondary neurulation are considered, by definition, to give rise to closed (skin-covered) malformations.
• Less organized than primary neurulation: Multiple small tubules are formed, composed of cells radially arranged about a central lumen. Smaller tubules coalesce into larger tubules independent of the neural tube formed from primary neurulation (primary neural tube). Eventually the caudal neural tube born from the coalescence of these tubules fuses with the primary neural tube in a transitional zone.
• Varies with species: Secondary neurulation is species specific. The above scheme is described in avian embryos; mouse embryos have a somewhat different scheme in which cells from the caudal cell mass are simply added on to the end of the neural tube formed by primary neurulation. It is not clear which process occurs in humans.