Genesis of the notochord

media/multuse/i4_vueac3D.gif — **Commentary**

Around the 19th day cells that invade into the primitive node region and migrate along the median line cranially form the chordal process (stage 7) (also known as the axial process). One can compare this phenomenon with the sliding of a finger into a glove. Thanks to the transparency of the ectoblast this cell migration can be observed in the embryos of experimental animals.

The chordal process grows longer through proliferation of the primitive node cells at its front end up to the prechordal plate (stage 6). At the same time the primitive streak recedes back into the caudal region.

media/module7/h2a3_ligneprim.gif — **Fig. 5 -** Primitive streak seen dorsally

Legend

Fig. 5

The red arrow shows schematically the migration direction of the epiblast cells coming from the primitive node to form the notochordal process and later the notochord.

media/module7/h2ia1_astad7_19.gif — **Fig. 5a -** Chordal process
19th day (Stage 7)

media/module7/h2ia2_cstad7_19.gif — **Fig. 5b -** Chordal process
19th day (Stage 7)

Legend

Fig. 5a

Schematic diagram showing the genesis of the chordal process at the 19th day through the invagination of epiblast cells coming from the primitive.

Fig. 5b

Cut along C.

media/module7/h2ia3_astad7_21.gif — **Fig. 5c -** Chordal process
21rst day (Stage 7)

media/module7/h2ia4_cstad7_21.gif — **Fig. 5d -** Chordal process
21rst day (Stage 7)

Legend

Fig. 5c

Schematic diagram showing the genesis of the axial canal through invagination of epiblast cells that migrate into the region of the primitive node (on roughly the 21rst day).

Fig. 5d

Cut along C

On the 23th day the chordal process consists of chordal mesoderm and a central axial canal (stage 7). At this point in time, the chordal process fuses with the endoderm lying below it. While the chordal process and endoderm are merged, for a short time (approximately one day), the amniotic cavity communicates with the umbilical vesicle, via the so-called neurenteric canal (stage 8). The chordal tissue that thereby goes over into the endoderm becomes thus the chordal plate (stage 8-9).

media/module7/h2ib1_astad8_23.gif — **Fig. 6a -** Chordal process
23rd day (Stage 8)

media/module7/h2ib2_cstad8_23.gif — **Fig. 6b -** Chordal process
23rd day (Stage 8)

Legend

Fig. 6a

The notochord has separated itself from the endoblast. At the caudal end of the notochord, in the region of the primitive node, thanks to the neurenteric canal, a temporary communication takes place between the umbilical vesicle and the amniotic cavity (arrow).

Fig. 6b

Cut along C

On the 25th day (stage 9) the chordal plate cuts itself off from the entoderm, which then fuses again, and forms a complete cord: the notochord. This is in the middle of the mesoderm, between the ectodern and endoderm, and plays a role in the induction of the neuroectoblast that lies over it. Moreover, the notochord plays a role in the genesis of the vertebral body and becomes the nucleus pulposus in the intervertebral disk.

media/module7/h2ic1_astad9_25.gif — **Fig. 7a -** Chordal process
25th day (Stage 9)

media/module7/h2ic2_cstad9_25.gif — **Fig. 7b -** Chordal process
25th day (Stage 9)

Legend

Fig. 7a

At around the 20th day the chordal tissue bonds with the endoderm and thus forms the chordal plate.
Between the 22nd and 24th day it cuts itself off from the endoderm (the endoderm fuses again) in order to form a complete cord: the notochord. This finds itself in the middle of the mesoderm, between the ectoderm and endoderm.

Fig. 7b

Cut along C

media/module7/h2ic3_astad10_28b.gif — **Fig. 7c -** Chordal process
28th day (Stage 10)

media/module7/h2ic4_cstad10_28b.gif — **Fig. 7d -** Chordal process
28th day (Stage 10)

Legend

Fig. 7c

The endoderm, under the notochord, comes together again. At this point the neural groove has partly fused into becoming a neural tube.

Fig. 7d

Cut along C

Summary, the notochord determines the longitudinal axis of the embryo. It defines the future situation of the vertebral body and induces the ectoblast in its differentiation to become the neural plate.