Embryology of Split Cord Malformations

CHAPTER 2

Ashutosh Agarwal, Shweta Kedia, and Ashok K. Mahapatra

Table of Contents

■ Introduction

■ Normal Early Human Embryo Development (Journey from 2-Layered to 3-Layered Structure)

■ Embryogenesis of Human NTDs

CHAPTER
2

Embryology of Split Cord Malformations

Ashutosh Agarwal, Shweta Kedia, and Ashok K. Mahapatra

Introduction

Split cord malformation (SCM) defines the group of disorders where the spinal cord is split into two by either a bony or fibrous septum. Based on the nature of the dividing structure, it has been classified into types I and II. However, there have been several reports in the literature where complex multilevel splits are described—some of them in association with other congenital disorders. The pathogenesis of complex SCM has always been intriguing and there is still an ongoing search for the explanation of these presentations based on embryology. Authors intend to go through the natural embryological process in this chapter and also analyze the proposed theories of embryogenesis as described in literature.

Normal Early Human Embryo Development (Journey from 2-Layered to 3-Layered Structure)

■ Gastrulation

In the postovulatory days (POD) 1 to 13, which is the first 2 weeks postfertilization, the human embryo cells divide and undergo rearrangements. It results in the formation of a blastocyst, a two-layered embryo suspended between the amniotic and yolk sacs. This is followed by formation of epiblasts on the dorsal surface of embryo and hypoblast on the ventral surface by postovulatory day 4. Prochordal plate is formed by the thickening of the cranial end of the embryo by postovulatory day 13.

This is also the time when primitive streak develops at the caudal end of the embryo and progresses cranially over the next 3 days. By day 16, it has obtained its full length. It is in the midline in the caudal half. The regression of the primitive streak begins thereafter, and it moves back toward the caudal pole of the embryo. Meanwhile, epiblasts migrate toward the primitive streak through the primitive groove running along the length of the primitive streak. Future endodermal cells ingress and displace the ventrally placed hypoblast cells laterally and form the endoderm. With the regression of the streak, future mesodermal cells ingress between the epiblast and endoderm to form the definitive mesoderm. The epiblast cells that are remaining now spread out and replace the ingressed cells to form both the neuroectoderm and surface ectoderm. It should thus be remembered that the embryonic endo-, meso-, and ectoderm are all derivatives of the epiblast.

The Hensen node needs special mention. Located at the cranial end of the primitive streak, this node acts as the organizer of the embryo. It is through this node that the future endodermal cells migrate, as the streak is elongating, and the future notochordal cells are laid down in the midline between the neuroectoderm and endoderm as the notochordal process.

Prospective neuroectoderm have been localized on the epiblast area surrounding the Hensen node toward the rostral half of the primitive streak. Neuroepithelium can be divided into areas that contribute to multiple neuraxial levels.

■ Timeline

•POD 4: Formation of epiblast and hypoblast.

•POD 13: Prochordal plate formed at the cranial end.

•POD 16: Primitive streak obtains full length at caudal end.

•POD 23-25: True notochord is formed.

•POD 24-25: Cranial neuropore closes.

•POD 25-27: Caudal neuropore closes.

■ Notochord Formation

This process starts postovulatory day 16 onward. Cord of cells arranged radially around the notochordal canal constitutes notochordal process. This notochordal canal is in continuity with the amniotic cavity dorsally through the primitive pit. It is between postovulatory days 17 and 21 that the notochordal process elongates. Fusion with the underlying endoderm happens between postovulatory days 18 to 20 and results in the formation of notochordal plate. The notochordal plate gets incorporated into the yolk sac roof and establishes the continuity of notochordal canal. On postovulatory days 17 to 19, neurenteric canal is formed (Fig. 2.1). The “true notochord” is formed by postovulatory days 23 to 25, when the plate folds dorsoventrally and separates from the endoderm. This results in obliteration of the neurenteric canal, and continuity with the amniotic and yolk sacs is closed.

Fig. 2.1 View from above onto the dorsal surface. The caudal pole of the embryo is oriented toward 11 o’clock. During gastrulation, if the primitive streak is abnormally wide, prospective notochordal cells in Hensen’s node may begin ingressing more laterally than usual. The paired notochordal streams would not integrate into a single midline notochord and would remain instead as paired paramedian notochordal processes. The caudal neuroepithelium induced by the paired notochordal processes would fail to integrate into a single midline neural plate and would form paired “hemineural plates” instead. Totipotent tissue from Hensen’s node could pass into the gap between the two notochordal processes and form a number of different caudal tissue types.

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