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Embryology, Innervation, Morphology, Structure, and Function of the Canine Intervertebral Disc
John F. Innes and James Melrose
Introduction
The intervertebral disc (IVD) is composed of a disparate collection of connective tissues of differing structure and function, and it is the dynamic interplay of these components in the composite IVD which endows it with its unique ability to withstand tensional stresses, to act as a viscoelastic hydrodynamic weight-bearing cushion, and to provide spinal flexibility [1]. While the cross-sectional area and angulation of IVDs vary with spinal level, all share common structural features. The outer region of the IVD, the annulus fibrosus (AF), is a collagen-rich tissue, while the central region of the IVD, the nucleus pulposus (NP), is rich in proteoglycans. The intervening region between the AF and NP is called the transitional zone (TZ). The areas of the IVD that interface with the adjacent vertebral bodies are called the cartilaginous end plates (CEPs); these are hyaline-like cartilaginous tissues containing cells of a rounded chondrocyte-like morphology.
Embryology of the IVD
During gastrulation, three somatic germ cell layers are initially laid down in the developing embryo: outer ectodermal, middle mesodermal, and inner endodermal layers [2–4]. A midline longitudinal rod-shaped column of the mesoderm, the notochord, subsequently develops from cell aggregates located between the ectoderm and endoderm and establishes cranial/caudal and ventral/dorsal axes in the developing embryo [2]. Ectoderm dorsal to the notochord gives rise to the neuroectoderm from which the neural tube develops. Adjacent mesodermal tissue develops into discrete tissue units, termed as the somites [5]. The somites consist of three tissue types: (1) the dermatome which gives rise to the dermis, (2) the myotome which gives rise to the axial musculature, and (3) the sclerotome from which vertebral structures arise. Cells of the sclerotome migrate medially and ventrally to form a continuous tube of mesenchymal cells (the perichordal sheath) which surround the notochord. Increased proliferation of cells at regular lengths along the perichordal tube creates areas of low and high cell density from which the vertebrae and AF, TZ, and spinal ligaments develop [5]. Formation of the vertebral bodies results in segmentation of the notochord. Each notochordal segment persists in the central region of the developing IVD to give rise to the NP [3]. Thus, during embryonic disc development, cells of the AF are derived from the sclerotome, whereas the NP originates from the notochord [3]. In nonchondrodystrophoid breeds, notochordal cells persist into adulthood, whereas in chondrodystrophoid breeds they disappear within 2 years of birth. This correlates with an earlier onset of IVD degeneration in chondrodystrophoid breeds.
Innervation of the IVD
There are major neuroanatomical differences between the human and canine spines in terms of how far the spinal cord extends along the vertebral canal. In humans, the spinal cord extends as far as the second lumbar vertebra with nerves exiting the spinal cord descending inside the remaining lumbar and sacral vertebral segments to exit through their respective foramina. The spinal cord in dogs ends at approximately L6 with nerves that serve the IVDs descending through the last lumbar, sacral and coccygeal vertebral segments. The canine cervical IVDs are served by 8 pairs of nerves, the thoracic IVDs have 13 pairs, the lumbar IVDs have 7 pairs, and the coccygeal region contains 2 nerves per IVD.
The human lumbar IVD is innervated by several nerves. The sinuvertebral nerve (meningeal rami) innervates the posterior (i.e., dorsal) aspect of the disc and the posterior (dorsal) longitudinal ligament. Branches from the rami communicantes innervate the lateral aspects of the disc and the anterior (ventral) longitudinal ligament [6]. A structure similar to the sinuvertebral nerve is not apparent in the canine thoracolumbar spine and in contrast to the human IVD, sensory nerves are sparse in the outermost annular lamellae. However, the dorsal longitudinal ligament is innervated profusely [7]. The nerves in the outer AF communicate with caudal and cranial spinal levels two positions removed from the actual site of annular innervation, which explains the referred pain reported at sites distant from damaged annular nerves.
