Regional variation in morphology of vertebral centra and intervertebral joints in striped bass, Morone saxatilis

Abstract

The vertebral column of fishes has traditionally been divided into just two distinct regions, abdominal and caudal. Recently, however, developmental, morphological, and mechanical investigations have brought this traditional regionalization scheme into question. Alternative regionalization schema advocate the division of the abdominal vertebrae into cervical, abdominal, and in some cases, transitional regions. Here, we investigate regional variation at the level of the vertebrae and intervertebral joint (IVJ) tissues in the striped bass, Morone saxatilis. We use gross dissection, histology, and polarized light imaging to quantify vertebral height, width, length, IVJ length, IVJ tissue volume and cross-sectional area, and vertical septum fiber populations, and angles of insertion. Our results reveal regional differences between the first four (most rostral) abdominal vertebrae and IVJs and the next six abdominal vertebrae and IVJs, supporting the recognition of a distinct cervical region. We found significant variation in vertebral length, width, and height from cranial to caudal. In addition, we see a significant decline in the volume of notochordal cells and the cross-sectional area of the fibrous sheath from cranial to caudal. Further, polarized light imaging revealed four distinct fiber populations within the vertical septum in the cervical and abdominal regions in contrast with just one fiber population found in the caudal region. Measurement of the insertion angles of these fiber populations revealed significant differences between the cervical and abdominal regions. Differences in vertebral, IVJ, and vertical septum morphology all predict greater range of motion and decreased stiffness in the caudal region of the fish compared with the cervical and abdominal regions.

Regional variation in the mechanical properties of the vertebral column during lateral bending inMorone saxatilis

“…In contrast to these findings of constant or increasing IVJ stiffness from cranial to caudal, a recent morphological study predicted decreasing flexural stiffness and increasing ROM from cranial to caudal in striped bass, M. saxatilis [19]. The study found a longitudinal variation in the soft tissues of the IVJ; the volume of the notochordal cell mass and the cross-sectional area of the helically wound fibrous sheath both decreased from cranial to caudal, and regional variation was found in the number of fibre populations and the insertion angle of those populations in the vertical septum.…”

“…Estimates of the second moment of area for the IVJs along the length of the fish predicted a lower stiffness in the most rostral and caudal regions, relative to the abdominal (middle) joints. Thus, on the basis of the soft-tissue morphology and geometry of vertebrae and IVJs, this study predicted an increase in ROM from cranial to caudal and a lower stiffness in the caudal than in the abdominal IVJs [19].…”

“…This morphological study of M. saxatilis also proposed, based on vertebral and IVJ morphology, the division of the striped bass vertebral column into three regions: a cervical region containing vertebrae 1 -4, an abdominal region containing vertebrae 5 -12 and a caudal region containing vertebrae 12-24 [19]. Vertebrae were shown to vary in size, overall shape and position of bony projections between the cervical and abdominal regions and between the abdominal and caudal regions.…”

“…These soft tissues of the IVJs in fishes include fluidfilled extracellular lacunae, a mass of notochordal cells with varied degrees of vacuolization and a complex of three encapsulating tissues that bind the rims of the centra adjacent to one another (figure 1d) [9,10,18,19]. A sheet of connective tissue, the vertical septum, spans the space between adjacent neural and haemal spines (figure 1a,c).…”

“…A sheet of connective tissue, the vertical septum, spans the space between adjacent neural and haemal spines (figure 1a,c). This combination of fluid-filled structures and tensile elements has led some researchers to hypothesize that the amphicoelous IVJ acts as a hydrostatic hinge, resisting compressive loads but allowing the lateral bending associated with swimming in fishes [10,19].…”