Okajimas Folia Anat. Jpn., 76(1): 17-32, May, 1999 The Mouse Vertebrae: Changes in the Morphology of Mouse Vertebrae Exhibit Specific Patterns Over Limited Numbers of Vertebral Levels By Harumichi SHINOHARA Division of Human Sciences, Faculty of Medicine, Toyama Medical and Pharmaceutical University, Sugitani 2630, Toyama 930-0194, Japan -Received for Publication, December 16, 1998- Key Words: Vertebral column, Metameric body pattern, Genes, Gross anatomy Summary: The mouse vertebrae from the cervix to the tip of the tail were characterized and anatomical features that have been lacking were added to the classical description. The vertebrae consist of six long-range and fourteen short- range substructures, with the foveal process being a newly identified substructure. The caudal transverse process, cranial hemal process and hemal ridge are substructures that are clearly defined in the mouse. Each long-range and short-range substructure has several specific morphological features such as length, width, area, shape and angle. These features exibit a crescendo, plateau or decrescendo pattern over a limited number of vertebral segments that ranges from just a few to twenty. The variety of substructural combinations and the constant changes in the morpho- logical features lead to the fact that no single vertebra has the same morphology as any other. An analysis of the patterns of changes in morphology provides some insight into the genetic plan for the metameric body axis. The vertebral column of the mouse has been morphological features of the coccygeal vertebrae studied for almost half a century (Gruneberg, 1950; are described for the first time in detail. The verte- Searle, 1954; Green, 1962). The cited early studies brae of the mouse were found to be composed were focussed on genetic variations into emphasis of a combination of substructures, such as the on changes of the number of vertebrae and in some body, arch and several additional processes. The osteological characteristics, such as the anterior tu- length, shape and other morphological features of bercles of the cervial vertebrae and the lumber rib. the substructures changed with crescendo, plateau In terms of anatomy, such observations were quite and decrescendo patterns. Some of these patterns limited. In more recent decades, systematic de- can be interpreted in the context of molecular scriptions of the vertebral column of the mouse development. were published (Cook, 1965; Hummel et al. , 1966; Cook, 1983). Furthermore, several very recent at- tempts were made to quantitate the morphological Materials and Methods changes in the vertebrae along the antero-posterior axis and some subtle but important features of the Eight C57BL/6 mice, four males and four fe- vertebral column were characterized (Johnson and males, of 15-25 weeks of age were used for the O'Higgins, 1994; O'Higgins et al., 1997). However, present study. They were euthanized with ether and such quantitation is not yet sufficient to define the skinned. The head, anterior and posterior limbs, morphology of the vertebrae from the cervix to the thoracic and abdominal viscera were removed, and tail. the trunk musculature was removed with the costae The primary purpose of the present study was as far as possible. A straight copper wire of 0.2 mm to characterize the entire vertebral column of the in diameter was inserted into the vertebral foramen mouse and to supplement previous incomplete an- of the first cervical vertebra, passed through the atomical descriptions. Moreover, in this report, the vertebral canal and pulled outfrom the interverte- Part of this work was supported by a Grant-in-Aid for Scientific Research no. 0167-0010 from the Ministry of Education, Japan. 17 18 H. Shinohara brat space between the sixth lumbar and first sacral lumbar, sacral and proximal three coccygeal verte- vertebrae. The tail was fastened to a straight cop- brae. The vertebral body, the arch and the seven per wire of 0.8 mm in diameter. These wires pre- specific processes were defined as long-range sub- vented the vertebrae from disorganized and kept structures that were present at more than 30 verte- the vertebral column straight during subsequent brae, that is to say, in more than half of the total treatments. The vertebral column was fixed in 10% vertebrae. There were also short-range substruc- formalin for several days and rinsed in tap water. tures that were present at fewer than 30 vertebral Then soft tissues were carefully and thoroughly re- levels (Table 1). The vertebrae at all levels con- moved under a dissecting microscope. The verte- sisted of combinations of these long-range and bral column was kept in distilled water and stained short-range substructures. For example, the ninth with alizarin red (several drops of a 0.1 % solution thoracic vertebra consisted of a body; an arch, of alizarin red in 100 ml of distilled water). Four which was studded with the seven processes; and a specimens were dehydrated in a graded ethanol se- pair of short-range substructures, namely, the fo- ries, cleared in xylene and examined for continuous veal processes. The morphology of the individual changes in vertebral substructures and in their substructures was, however, unstable and there morphological features (Figs. 1, 2 and 3). The ap- were both gradual and sudden changes. For exam- proximate sizes of intervertebral foramina were ple, visual estimation of the craniocaudal length of measured with a micrometer under a dissecting mi- the coccygeal body (Fig. 1) revealed an increase in croscope. In the case of the remaining four speci- the proximal part of the tail and a gradual decrease mens, the individual vertebrae were separated, in the distal part (see also Fig. 5). This change in cleaned and air-dried. The craniocaudal length of length can be described as a crescendo-decrescendo each vertebra was measured with a micrometer pattern. Meticulous observations of the vertebral under a dissecting microscope. The cranial surface column revealed that the vertebrae were composed of each vertebra was photographed (Fig. 4) and the of long-range and short-range substructures whose area of the vertebral foramen was calculated with a morphological features changed from one segment computerized image analyzer (CIA 102; Olympus, to the next, with combinations of crescendo, Japan). plateau and decrescendo patterns. Thus, no single vertebra was morphologically identical to any other. Results General features Table 1. The long-rangeand short-rangesubstructures that The vertebral column (Figures 1, 2 and 3) con- formthe vertebraeof the mouse sisted of 59 to 61 vertebrae. There were seven cer- vical vertebrae (C1—C7),thirteen thoracic verte- brae (T1—T13),six lumbar vertebrae (L1—L6),four sacral vertebrae (S1—S4)and from 29 to 31 coccy- geal vertebrae (Col—Co31). The vertebral column had two curvatures; lordosis at the cervico-thoracic border and kyphosis at the thoraco-lumbar border. For convenience, the number of coccygeal verte- brae was taken as 29 because five out of eight mice (more than 60%) had 29 such vertebrae. The actual number of coccygeal vertebrae seemed to depend on the extent of ossification of a few segments at the tip of the tail. Thus, older mice tended to have a larger number of coccygeal vertebrae. However, no analysis of differences between young and old or male and female mice was performed in the present study. Each vertebra consisted of a body and an arch (Fig. 4). Seven processes, namely, a pair of trans- verse processes, a pair of cranial articular proc- esses, a pair of caudal articular processes and an unpaired spinous process, extended from the body and arch and were found in the cervical, thoracic, Morphology of Mouse Vertebrae 19 Fig. 5. The pattern of changes in craniocaudal length of the vertebral body. A crescendo-decrescendopattern with a clear maximum (arrow) is present for the cervical, lumbar, sacral and coccygeal regions, respectively.The thoracic region exhibits a crescendo- plateau-crescendo pattern. The lengths at Cl and C2 are exceptional due to the absence of the vertebral body at Cl and the presence of the dens axis at C2. N: Number of specimensexamined. Long-range substructures midsagittal line to form the vertebral foramen. The 1. The vertebral body area of the vertebral foramen (Fig. 6) exhibited a The vertebral body was one of the most consis- decrescendo pattern from Cl to T6, with small tent substructures. It was found in all vertebrae fluctuations at C5, C6 and C7. Then it exhibited a with the exception of C1 (atlas). The morphological crescendo-decrescendo pattern from 17 to Co4, features of the vertebral body included the cranio- with a peak at Ll. There was no vertebral foramen caudal length, the shapes and the depth. The mean at Co4 and Co5 (Figs. 1 and 4) as a result of the craniocaudal lengths of the vertebral bodies are abscence of curved laminae (spina bifida). Spina shown in Figure 5. The length exhibited a crescendo- bifida occurs normally at Co4 or CoS; spina bifida is decrescendo pattern in the cervical, lumbar, sacral anomalous only if it occurs at additional cranial and coccygeal regions, respectively (arrows in levels. Fig. 5). The thoracic region exhibited a cresendo- plateau-crescendo pattern. The shape of the cranial 3. The transverse process surface of the body also changed with vertebral The representative bone extension to the lateral levels: it was oval from C3 to T2, bell-shaped from side was designated the transverse process. In a T3 to T10, trapezoid from T12 to L2 and triangular series of thoracic vertebrae, the transverse process from L6 to S3 (Fig. 4). The depth of the vertebral was replaced by three bony extensions. These ex- body was reported by O'Higgins et al. (1997). tensions, namely, the foveal process, cranial tuber- cle and caudal tubercle, were independent sub- 2.