Formation of the Neural Floor Plate
The formation of the floor plate is not so straightforward as had been thought. Previously, it had been assumed that only the midline cells of the neural plate formed the floor plate of the neural tube. That is, when the neural plate closed up to form a tube, the most centrally located cells of the neural plate would end up at the bottom of the tube. The most peripheral regions, the neural folds, would become the dorsalmost portion of the tube. This probably is how the head region forms. However, recent evidence suggests that the floor plate of the trunk neural tube has a separate origin—that it arises in part from Hensen’s node and is "inserted" into the center of the neural plate.
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| Figure 1 Two chick-quail chimeras and a control chick 4d after hatching. In the chimeras, a piece of the neural tube and neural crest from an early quail embryo was placed into a similar position in a 12-somite chick embryo. |
The chick and quail cells differ, however, in two critical ways (Figure 1.10 on page 13 of the textbook). First, the quail heterochromatin in the nucleus is concentrated around the nucleoli. This creates a large, deeply staining mass that is easily distinguishable from the diffuse heterochromatin of chick cells. Second, there are some antigens that are quail-specific and are not seen on chicken cells. Both of these phenomena allow one to readily distinguish individual quail cells, even when the majority of the cell population is chick.
These investigators removed Hensen’s node and the caudal end of the elongating notochord from 6-somite (1.5-day) chick embryos and replaced them with their quail counterparts. From that level down to the tail, both the notochord and the floor plate were made of quail cells. The walls of the neural tube were made from the chick neural plate (Figure 7.6). Interestingly (as predicted by the regression of the node, discussed in Chapter 6), the floor plate and notochord cells associated with neural plate located more caudally than the node itself. Thus, Hensen’s node contains the cells needed to form the caudal floor plate and the notochord.
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| Figure 2 The floor plate of the trunk neural tube of the chick is derived from Hensen’s node. (A) Schema of the operation whereby the node of a 6-somite chick embryo is replaced by its quail counterpart. (B) Analysis of the chimeric axis (stained for the quail-specific antigen), showing quail cells in the notochord (arrow) and floor plate (arrowheads). (B from Catala et al., 1996; photograph courtesy of N. M. Le Douarin.) |
The floor plate cells become inserted into the center of the dorsal ectoderm. Only later does the notochord separate from the floor plate by the formation of a basement membrane between them (Figure 3). The neural tube, then, has two distinct sources–one ectodermal and one from Hensen’s node.
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| Figure 3 Hensen's node contributes to both the notochord and neural floor plate. Section through Hensen's node at the 6-somite stage, showing that it contributes to the upper layer of embryonic cells. (From Catala et al., 1996; photograph courtesy of N. M. LeDouarin.) |
While this idea that the notochord and floor plate are derived from the same population of cells has just recently been appreciated, this phenomenon had been documented in one of the famous books of embryology. Hans Spemann’s 1938 Embryonic Induction and Development has an illustration of the famous Spemann and Mangold grafting experiment. On pages 144 and 146 of that book (and reproduced in the textbook as Figure 10.20), the graft of dorsal blastpore lip is shown as giving rise to dorsal mesoderm (notochord and somites) and the floor plate of the neural tube.
Literature Cited
Catala, M., Teillet, M.-A., De Robertis, E. M. and Le Douarin, N. M. 1996. A spinal cord fate map in the avian embryo: while regressing, Hensen’s node lays down the notochord and floor plate thus joining the spinal cord lateral walls. Development 122: 2599-2610.
Le Douarin, N. M. 1969. Particularités du noyau interphasique chez la Caille japonaise (Coturnix coturnix japonica). Utilisation de ces particularités comme "marquage biologique" dans les recherches sur les interactions tissulaires et les migrations cellulaires au cours de l’ontogenèse. Bull. Biol. Fr. Belg. 103: 435-452.
Le Douarin, N. M. and Teillet, M.-A. 1973. The migration of neural crest cells to the wall of the digestive tract in avian embryo. J. Embryol. Exp. Morphol. 30: 31-48.