Overview of Secondary Neurulation

Abstract

Secondary neurulation is a morphological process described since the second half of the 19th century; it accounts for the formation of the caudal spinal cord in mammals including humans. A similar process takes place in birds. This form of neurulation is caused by the growth of the tail bud region, the most caudal axial region of the embryo. Experimental work in different animal species leads to questioning dogmas widely disseminated in the medical literature. Thus, it is clearly established that the tail bud is not a mass of undifferentiated pluripotent cells but is made up of a juxtaposition of territories whose fate is different. The lumens of the two tubes generated by the two modes of neurulation are continuous. There seem to be multiple cavities in the human embryo, but discrepancies exist according to the authors. Finally, the tissues that generate the secondary neural tube are initially located in the most superficial layer of the embryo. These cells must undergo internalization to generate the secondary neurectoderm. A defect in internalization could lead to an open neural tube defect that contradicts the dogma that a secondary neurulation defect is closed by definition.

Keywords: Blastema; Caudal spinal cord; Cavitation; Secondary neurulation; Tail bud.

Fig. 1.

Figures from plate 5 published by Gasser [11] showing axial sections of chick embryo from rostral (A) to caudal (E). This plate presents the first historical illustration of secondary neurulation in chick embryos. Multiple lumens can be evidenced in the secondary neural tube (A and B). C : The primordium of the spinal cord is formed as a solid cellular structure (the so-called medullary cord). D and E : Dorso-ventral gradient of epithelialization.

Fig. 2.

Axial section of a sheep embryo (Maximilian Braun) [4]. The caudal neural tube (at the top of the figure) is formed by a solid rod of cells. Peripheral cells are epithelial whereas core cells are mesenchymal. This structure is now known as the medullary cord.

Fig. 3.

Axial section of the caudal region of the tail bud (chick, 25-somite-stage). Tail bud cells are mesenchymal covered by surface ectoderm (SE). Their density is higher on midline. Laterally mesenchymal cells form the paraxial mesoderm (PAM). End : endoderm, So : somatopleural mesoderm, Sp : splanchnopleural mesoderm.

Fig. 4.

Axial section involving a more rostral region of the tail bud than represented on Fig. 3 (chick, 25-somite-stage). Tail bud cells are sandwiched between surface ectoderm (SE) and endoderm (End). On the midline, dorsal cells aggregate to form an epithelium (Ep). In contrast, ventral cells are still mesenchymal. Paraxial mesoderm (PAM) is located laterally. So : somatopleural mesoderm, Sp : splanchnopleural mesoderm.

Fig. 5.

Axial section of a more rostral region than that illustrated on Fig. 4 The tail bud is still limited by surface ectoderm (SE) dorsally and endoderm (End) ventrally. The histological aspect of midline cells has dramatically changes. Neural cells aggregate to form a solid rod of cells (the so-called medullary cord). Underneath this epithelial structure, lies the chordoneural hinge (CNH) which is the region where notochord, ventral neural tube and mesenchyme of the tail bud merge. The space between CNH and ventral neural tissue is an artefact due to dehydration. Paraxial mesoderm (PAM) is located laterally. So : somatopleural mesoderm, Sp : splanchnopleural mesoderm.

Fig. 6.

Axial section from a more rostral level than represented on Fig. 5. The neural tube contains two lumens. The space between ventral neural tissue and notochord (No) is an artefact due to dehydration. SE : surface ectoderm, PAM : paraxial mesoderm, Sp : splanchnopleural mesoderm, End : endoderm, So : somatopleural mesoderm.

Fig. 7.

At more rostral level compared to Fig. 6, the two lumens fuse. SE : surface ectoderm, NT : neural tube, PAM : paraxial mesoderm, a : aorta, No : notochord, End : endoderm, So : somatopleural mesoderm, Sp : splanchnopleural mesoderm.

Fig. 8.

At last, for more rostral levels, the neural tube (NT) formed by cavitation is strictly similar to a neural tube produced by primary neurulation. SE : surface ectoderm, PAM : paraxial mesoderm, a : aorta, No : notochord, End : endoderm, So : somatopleural mesoderm, Sp : splanchnopleural mesoderm.

Fig. 9.

Sagittal section of a chick embryo (24-somite-stage). The rostral pole is located at the right of the figure. The neural tube (NT) and underlying notochord (No) merge with tail bud (TB) mesenchyme forming the so-called chordoneural hinge (CNH).

Fig. 10.

A : Axial sections of a chick embryo showing progression of secondary neurulation. B : These sections were used to perform 3D reconstruction. Following this reconstruction, we see that secondary lumens freely communicate with the primary lumen of the neural tube. Note that section 5 is located more caudally and consequently is not included on Fig. 10B. Modified from Schumacher [37]. Ect : ectoderm, Mes : mesoderm, End : endoderm, No : notochord, ENT : epithelium of the neural tube, Do : dorsal, Ve : ventral.

Fig. 11.

3D reconstruction of a caudal neural tube from a human embryo (8 mm-long, probably Carnegie stage 15. The multiple lumens (grey) are communicating with the rostal neural tube. Modified from Schumacher [37]. d : dorsal, v : ventral.

Fig. 12.

The three regions that have been transplanted in the avian tail bud. Region 1 corresponds to the most caudal part of the neural tube and notochord. Furthermore, the chordoneural hinge is also involved in this experiment. Region 2 corresponds to 2/3 rostral of the tail bud whereas region 3 to its caudal 1/3.

Fig. 13.

Axial section of a chimera for which a quail region 1 was transplanted into a chick host. Quail cells can be recognised by their condensed heterochromatin (arrowheads). Caudally to the level of sacral nerves 3 to 6, the graft yields the caudal notochord (arrowheads) and the floor plate (double arrow). All the rest of the neural tube is derived from the host (Feulgen-Rossenbeck staining).

Fig. 14.

Axial section of a chimera for which a quail region 2 was transplanted into a chick host. The spinal cord, located caudally to the level of sacral nerves 6, derived from the graft (arrowheads pointing to cells with a condensed heterochromatin) except for its floor plate (arrow) that is produced by the host (Feulgen-Rossenbeck staining). Comparing Fig. 13 and 14 shows the complementary pattern observed after grafting.

Fig. 15.

Section of a chimera in which a quail region 2 was placed at the level of region 3 in a chick host. QCPN antibody marks specifically the quail nucleus and not the chick one. Grafted cells contribute to neural tube (NT) (arrowheads) and somitic derivatives (arrows). This heterotopic transplantation shows that the fate of the grafted region is not modified by its new environment (Immunohistochemistry using QCPN antibody). No : notochord.

Fig. 16.

Section of a chimera in which a quail region 3 was placed at the level of region 2 in a chick host. QCPN antibody marks specifically the quail nucleus and not the chick one. Grafted cells contribute to somitic derivatives (arrows). There is no contribution of grafted cells to the neural tube (NT). This heterotopic transplantation shows that the fate of the grafted region is not modified by its new environment (Immunohistochemistry using QCPN antibody).

Fig. 17.

Dorsal view of a chick embr yo at 6 somite (So)-stage. Neurectoderm is still largely open forming a neural plate (arrowheads). At the caudal end of notochord (N), lies a depression corresponding to regressing Hensen’s node (arrow). PS : primitive streak.

Fig. 18.

A graft containing the latero-caudal part of the neural plate from a 6-somite quail embryo is placed at the same level in a chick host. After inculation, grafted cells (recognized by QCPN antibody) develop and give riose to the dorsal part of the medullary cord (Med Cord) (arrows). Modified from Catala et al. [6].

Fig. 19.

A and B : The superficial neural plate of a 6 somite chick embryo is stained by CFDA-SE. After incubation, marked cells (in grey) generate superficial ectoderm (e) and the secondary neural tube (nt). The paraxial mesoderm (pm) is not stained. Modified from Dady et al. [9].