J. Anat. (1986), 149, pp. 157-169 157 With 7 figures Printed in Great Britain The early origin of vertebral anomalies, as illustrated by a 'butterfly vertebra' F. MULLER, R. O'RAHILLY AND D. R. BENSON Carnegie Laboratories ofEmbryology, California Primate Research Centre and Departments ofHuman Anatomy, Neurology, and Orthopaedic Surgery, University of California, Davis, California 95616, U.S.A. (Accepted 30 January 1986) INTRODUCTION Histological reports of vertebral malformations in the human embryo and early fetus are extremely rare, e.g. sagittal clefts at 17-25 mm (Orts Llorca, Ruano Gil & Jimenez Collado, 1962) and at 21 mm (Feller & Stemnberg, 1930). The present example of a 'butterfly vertebra' in an otherwise normal, early human fetus pro- vided an opportunity to devise a developmental timetable for the appearance of this and similar congenital vertebral anomalies. MATERIALS AND METHODS The anomaly, which had not been noted externally, was discovered in sections of the vertebral column in a fetus (listed as S6 in the Gardner-Gray-O'Rahilly Collection) of 63 mm in greatest length (and hence approximately 10 postovulatory weeks in age). The foot length was 11 mm and the weight was 25 g. As part of a project on vertebral development, the vertebral column had been isolated (Fig. 1), sectioned sagittally at 25 ,um and the sections stained by a modified Mallory azan technique. Graphic reconstructions of both the affected and the adjacent thoracic vertebrae were prepared from projection drawings of every second or every third section. Reconstructions similar to those of thoracic vertebrae 8 and 9 of the anomalous 63 mm fetus were made from horizontal sections of a normal 69 mm fetus (No. 4291 in the Carnegie Collection). In addition, sagittal sections of the vertebral column of another normal 69 mm fetus (S 34) were photographed and studied. In the account that follows cervical, thoracic, and lumbar vertebrae will be abbreviated to CV, TV and LV, respectively. RESULTS Normal controls Reconstructions of normal thoracic vertebrae (TV8 and 9) of the affected fetus (S6) show that the centra are equal in height both ventrally and dorsally (Fig. 2A, B). The remains of the notochord are slightly anterior to the middle of the centra and are expanded in the region of the future nucleus pulposus (Fig. 2A). Laterally, segmental arteries are seen to be symmetrical and present at each vertebral level (Fig. 2B). The reconstruction of a control fetus (No. 4291) reveals the spinal nerve roots and ganglia as well as the normal vertebral anatomy. The cartilaginous neural processes have fused dorsally, so that the neural (future vertebral) arch is complete and a short spinous process is already present (Fig. 2 C). 158 F. MULLER, R. O RAHILLY AND D. R. BENSON 0~~ O 7 o 0 0~~ 0 C~~~~~C .. MS iLZZD o OZ -0 0 ... 0>>S_-e^°~~~~~~~~~~~~~~~C)C> Fig. 1. Radiograph of isolated vertebral column of fetus (S6). In the key drawing, the square surrounds TV10-12. Only the right hand hemicentrum of TVI1I is visible. C, an ossific locus within the cartilaginous costal element of CV7, superomedial to the first thoracic rib. This is potentially but not necessarily a cervical rib (Meyer, 1978). Anomalous vertebrae On radio'graphy (Fig. 1), the ossification centres of the neural arches of TV 10-12 are found to be slightly wider than normal, and the centre for the centrum of TV I11 is small and deviated to the right hand side. In a ventral view (Fig. 3A), the centrum is seen to be cleft sagittally, without cartilaginous continuity between the right and left sides. The right portion, which is considerably larger than the left, is joined by a cartilaginous bridge to the two adjacent vertebrae (Figs. 3A, SC). Origin of vertebral anomalies 159 (A) 8 1mm {C)f Fig. 2(A-C). Graphic reconstructions of normal thoracic vertebrae. (A) Normal thoracic vertebrae 8 and 9 of fetus S6, shown in median section. Spinal ganglia are included in the intervertebral foramina. (B) The same vertebrae on left lateral view. X indicates sectioned rib. Branches of the intercostal arteries are included. (C) Thoracic vertebra 11 of a normal fetus of comparable age, reconstructed from transverse sections and seen from above. The cartilaginous neural arch is complete. The eleventh ribs are short. The spinal cord and ganglia, as well as sympathetic ganglia, are included. N, the future nucleus pulposus. The contiguous vertebrae (TV 10 and 12) appear to be adapting to the lack of vertebral material at the TV 11 level (Fig. 3 A). As seen in a dorsal view (Fig. 3B), the neural arches are closed, as would be expected at this phase of development. A medial view of the left portion of TV 11 shows a ventral deficiency and wedging (Figs. 4 C, 5 B). The adjacent vertebrae are larger, thereby compensating partially for the lack of vertebral mass. A medial view of the right portion shows compression in the middle, accompanied by inadequate expansion ventrally and dorsally. The adjacent vertebrae have expanded into the deficient area. Lateral views indicate that the left side of TV11 is almost normal in height dorsally but is defective ventrally (Fig. 4D), whereas the larger, right hand side appears more nearly normal, although it is slightly wedged ventrally (Figs. 4A, 5A). Projections in a transverse plane reveal clearly the distribution of vertebral material between the right and left portions of the centrum (Fig. 3 C, D). The cartilaginous bridges to TV 10 (Figs 3 C, 5 C) and TV 12 (Figs 3D, 5 C) extend for a considerable distance dorsoventrally. A slight partition in the median plane is visible in TV 12. Notochord The notochord (Fig. 3 A) is very slightly deviated to the right hand side from TV9 to LV 1 and passes through the right hand component of TV 11 (Fig. 5 C). No signs 160 F. METLLER, R. O'RAHILLY AND D. R. BENSON (D) 1 mm Fig. 3(A-D). Graphic reconstructions of thoracic vertebrae 1-12 offetus S6. (A) Ventral view. The butterfly vertebra (TV 1 1) is seen to consist of two parts, the larger of which is connected by cartilaginous bars with the suprajacent and subjacent centra. The right twelfth rib is continuous with the transverse process of TV 12. The pattern of the arteries is abnormal on the left side. (B) Dorsal view. All three neural arches are complete. (C) Inferior aspect of TV 11 projected onto TV10. The black area represents a section through the bar from TV 12. N, notochord. (D) Superior aspect of TV 11 projected onto TV 12. The black area represents a section through the bar from TV10. of notochordal duplication are present. The forerunners of the nuclei pulposi are small between TV11 and the adjacent vertebrae, and that between TV 12 and LV1 is branched (Fig. 3A). Intervertebral discs The discs (Fig. 5 D) between the left portion of TV 11 and the adjacent vertebrae appear to be transversely placed rings and the upper and lower rings seem to be fused by a mass of fibrous tissue towards the median plane. Intervertebral discs in relation to the right portion of TV 11 are present only at the periphery and are separated by the centrum. In brief, TV 11 is in contact with the adjacent vertebrae without the intervention of disc material in the central areas of both the right and left portions of the upper and lower discs. (This deficiency of disc material might favour bony fusion of TV 10-12 at a later period, resulting in a block vertebra.) Intervertebral foramina The openings are larger on the right side (Figs. 4A, B, SD) than on the left (Fig. 4 C, D) and, on both sides of the malformation, they are larger than in the Origin of vertebral anomalies 161 1 mm V Fig. 4 (A-D). Graphic reconstructions of thoracic vertebrae 10-12 of fetus S6. (A) Right lateral view. X indicates a sectioned rib. (B) Right portion of sectioned column, medial aspect. The notochord is outlined. Two vascular canals on the dorsal aspect of TV 10 are noticeable. (C) Left portion of sectioned column, medial aspect. (D) Left lateral view. X indicates a sectioned rib. normal spine at the level of TV 8/9 (Figs. 2A, B, 6F). The spinal ganglia lie close to the inferior vertebral notches. Ribs The ribs of TV 1 1 and 12 are abnormal. Rib 11 on the left side is short, articulates with the centrum of TV 11 only and fails to articulate with the transverse process. Rib 11 on the right side is shorter and articulates with centra 10 and 11 (Fig. 4A). Joint cavities have not yet appeared. The twelfth ribs are short and are not visible radiographically. That on the right side is fused with the centrum of TV 12 (Fig. 3A). Arteries The segmental (intercostal) arteries to the vertebrae show a normal pattern on the right hand side (Figs. 3 A, 4A). On the left side, however, intercostal arteries 10 and 11 share a common stem, whereas the subcostal artery is very narrow and extends no 162 F. MULLER, R. O'RAHILLY AND D. R. BENSON ;- .... .. .- 4- ..... .. .: r: st ... ': - .. ss ..''. -.. \:..........' ....:..::,;," :x2, w.t i. * ^ tL:;_-.z...:E.'.;.::*:.:3.}.*'. twli. i. I. *, 3R m m Origin of vertebral anomalies 163 further than rib 12 (Fig. 3A). In spite of these differences the number of branches (2 dorsally and 4 ventrally) entering the malformed centrum is the same as that for centra 10 and 12. Ossification Areas of calcification are present in the centra of TV 10 and 12 and are beginning to be vascularised.