The Mammalian Spinal Cord

By Charles Watson, Gulgun Sengul and George Paxinos

The Mammalian Spinal Cord provides a comprehensive account of the anatomy and histology of the spinal cord. The text covers the cytoarchitecture, chemoarchitecture, motor neuron distribution, long tracts, autonomic outflow, and gene expression in the spinal cord. A feature of the book is the inclusion of segment-by-segment atlases of the spinal cords of rat, mouse, newborn mouse, marmoset, rhesus monkey, and human. This book is an essential reference for researchers studying the spinal cord.

• Includes full-color photographic images of Nissl-stained sections from every spinal cord segment in each of two rodent and three primate species, over 160 Nissl plates
• Contains comprehensively labeled diagrams to accompany each Nissl-stained section, over 160 diagrams
• Provides more than 500 photographic images of sections stained for AChE, ChAT, parvalbumin, NADPH- diaphorase, calretinin, or other markers to supplement the Nissl-stained images

• Language: English
• Publisher: Academic Press
• Release date: Dec 22, 2021
• ISBN: 9780128141472

Charles Watson

Charles Watson is a neuroscientist and public health physician. His qualifications included a medical degree (MBBS) and two research doctorates (MD and DSc). He is Professor Emeritus at Curtin University, and holds adjunct professorial research positions at the University of New South Wales, the University of Queensland, and the University of Western Australia. He has published over 100 refereed journal articles and 40 book chapters, and has co-authored over 25 books on brain and spinal cord anatomy. The Paxinos Watson rat brain atlas has been cited over 80,000 times. His current research is focused on the comparative anatomy of the hippocampus and the claustrum. He was awarded the degree of Doctor of Science by the University of Sydney in 2012 and received the Distinguished Achievement Award of the Australasian Society for Neuroscience in 2018.

Book preview

Chapter 1: Organization of the spinal cord

Charles Watson; Gulgun Sengul

Outline

The gross anatomy of the spinal cord

Segments based on spinal nerve exit patterns

Spinal nerves

Gray and white matter

White matter of the spinal cord

Gray matter of the spinal cord

The laminae of Rexed

Spinal cord nuclei

Central canal

Meninges

Blood supply

Hox gene expression

References

The spinal cord is the part of the central nervous system that controls the voluntary muscles of the limbs and trunk, and also receives sensory information from these regions. Additionally, it controls most of the viscera and blood vessels of the thorax, abdomen, and pelvis. The spinal cord is usually divided into segments according to the vertebral level of emergence of each spinal nerve. For example, the human spinal cord has 31 segments—8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. This scheme is handy for descriptive purposes, but it has significant theoretical limitations. First of all, there is not an exact correlation between vertebral regions and the six functional regions of the spinal cord (Watson and Sidhu, 2009; Watson et al., 2015; Mitchelle and Watson, 2016). Secondly, there is no external or microscopic anatomical distinction between adjacent spinal cord segments, in that they do not display anatomical boundaries or lineage restriction during development (Lim et al., 1991; Stern et al., 1991). By comparison, the segments of the hindbrain are clearly formed by lineage restriction (Fraser et al., 1990). It may be concluded that the spinal cord segments are not intrinsic; instead, they are simply territories defined by the segmental (somite defined) opportunities for exit of the spinal nerves. Rather than being an intrinsic property, the apparent segmental arrangement has been imposed on the spinal cord by the adjacent vertebrae.

The gross anatomy of the spinal cord

The spinal cord and its meninges (dura mater, arachnoid mater, and pia mater) lie within the vertebral canal. The human spinal cord is generally cylindrical, but is slightly flattened dorsoventrally in lower cervical segments. The spinal cord occupies the upper two-thirds of the vertebral canal, below which the vertebral canal contains only the spinal nerve roots and meninges. The average length of the spinal cord in humans is 45 mm in males and 42–43 cm in females. Up until the third fetal month, the spinal cord extends as far as the level of the fourth sacral vertebra, but after that the vertebral column grows faster than the spinal cord: at the beginning of the sixth month, the spinal cord extends to the base of the sacrum; at birth, it extends to the lower border of the second lumbar vertebra (Barson, 1970).

On the anterior surface of the spinal cord, there is a deep longitudinal fissure in the midline, named the anterior median fissure. It extends to nearly one-third of the anteroposterior length of the spinal cord and contains a prolongation from the pia mater and branches of the anterior spinal artery. In humans, the anterior median fissure averages 3 mm in depth and is deepest at C5 (Fountas et al., 1998). The floor of the anterior median fissure is formed by the anterior white commissure. On the posterior surface, there is a shallow groove named the posterior median sulcus. The dorsal septum, composed of pial tissue, extends from the base of this sulcus almost to the commissural gray matter and divides the dorsal column of white matter into right and left halves. On the lateral side of the spinal cord, there is an indistinct ventrolateral sulcus and a deeper dorsolateral sulcus; these correspond to the line of the origin of ventral and dorsal roots, respectively (Fig. 1.1).

Fig. 1.1

Fig. 1.1 A diagram of a transverse section of cervical spinal cord. The gray matter is divided into two ventral horns and two dorsal horns. The central canal is found at the center of the gray matter. The white matter is divided into dorsal, lateral, and ventral funiculi. The white matter is bounded in the midline by the dorsal median sulcus and the ventral median fissure. The dorsolateral suclus marks the point of entry of the dorsal roots of spinal nerves, and the ventrolateral sulcus lies at the junction between lateral and ventral funiculi.

The widest segments of the human spinal cord are found in the cervical segments, followed in order by the lumbar, thoracic and sacral segments (Barson and Sands, 1977). The largest transverse diameter is at segment C5 (13.3 ± 2.2 mm), decreasing to segment T8 (8.3 ± 2.1 mm), and increasing slightly to 9.4 ± 1.5 mm at L3. The anteroposterior diameter shows less variation in size along the spinal cord at C5 (7.4 ± 1.6 mm), T8 (6.3 ± 2 mm), and L3 (7.5 ± 1.6 mm) (Frostell et al., 2016). In humans, the longest spinal cord segment is at T7; the segments rostral and caudal to this level become progressively shorter, except for C5 which is slightly longer than other cervical segments (Malinska et al., 1972). The anteroposterior length of segments in humans decreases gradually from upper cervical segments towards T1, remains constant in thoracic segments, and gradually increases from T12 to L3 (Ko et al., 2004). The cross-sectional area of the spinal cord increases from C1 to C6 and then decreases rapidly at C8-T2, the white matter occupying most of the cross-sectional area of the cervical enlargement. Throughout the middle and lower thoracic segments, the cross-sectional area remains constant, but then increases again from T12 to L4, and then decreases promptly below S1 (Kameyama et al., 1996).

The spinal cord gray matter is enlarged in the regions where the nerves of the limbs arise. These enlargements are called the cervical enlargement and the lumbar (or lumbosacral) enlargement. Animals with minimal limb development show little tendency to local enlargement in the cervical and lumbar regions (Ranson and Clarke, 1953).

The cross-sectional area of the cervical spinal cord in humans is 122 mm² at C4, 110 mm² at C2 and 85 mm² at C7 (Sherman et al., 1990). This enlargement is chiefly due to an increase in the transverse diameter (Fountas et al., 1998). Tomographic imaging studies have shown that the cervical enlargement is smaller in life than that observed in post-mortem studies, the cross-sectional area being about 78 mm² at C5 (Fountas et al., 1998). The sagittal diameter of the cervical enlargement was measured 7.5 mm and the transverse diameter 13 mm in cadavers (Ko et al., 2004) and 7 and 14 mm in computed tomography measurements (Fountas et al., 1998). The cross-sectional area of the lumbar enlargement measures 50 mm², the sagittal diameter about 7 mm, and the transverse diameter 9 mm in cadavers (Ko et al., 2004).

Segments based on spinal nerve exit patterns

In the rat, the spinal cord is made up of 35 segments—8 cervical (named C1 to C8), 13 thoracic (T1 to T13), 6 lumbar (L1 to L6), 5 sacral (S1 to S5), and 3 coccygeal (Co1 to Co3). Most mammals have a segmental pattern similar to the rat, but there are variations in the number of thoracic, lumbar, and sacral segments, and animals with substantial tails have many more coccygeal segments. The human spinal cord differs from that of the rat in having 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal segments, making a total of 31 segments. The cervical (brachial) enlargement in most mammals (including rodents and primates) extends from C5 to T1. The lumbosacral enlargement extends from L2 to L6 in rodents and from L2 to S2 in humans.

The caudal end of the spinal cord narrows to form the conus medullaris usually located at the lower third of L1, ranging from the middle third of T12 to upper third of L3 in different mammals (Saifuddin et al., 1998). The caudal continuation of the conus medullaris a long fibrous strand, called the filum terminale, which stretches from the end of the spinal cord at the level of the L1 vertebra to attach to the coccyx. The upper part of the filum terminale, the filum terminale internum, descends in the dural sac surrounding the cauda equina. In humans, it usually starts at the middle L1 level and fuses with the dura mater at the upper S2 level. The length of the filum terminale internum is 15 cm, the mean initial diameter being 1.5 mm, and the midpoint diameter 0.75 mm (Pinto et al., 2002). At the level of the S2 vertebra, it perforates the dura and continues as the filum terminale externum to terminate on the dorsal surface of the first coccygeal vertebra together with the coccygeal ligament. The filum terminale is composed of longitudinally arranged collagen bundles as well as elastic and elaunin fibers which give it considerable elasticity (Fontes et al., 2006). In addition to these elements, there are glial cells, nerve fibers and remnants of the ependyma-lined central canal of the developing spinal cord (Choi et al., 1992). The central canal continues in the filum terminale for 5–6 mm. In the rat, the spinal cord does not end at the conus medullaris, but its basic components (central canal, gray matter, white matter) continue in the filum terminale (Rethelyi, 1977).

Spinal nerves

Pairs of spinal nerves arise from the spinal cord and leave the vertebral column through adjacent intervertebral foramina. Because the vertebral column grows faster than the spinal cord, the spinal cord in the adult human extends to only the L1 or L2 vertebra. The result is that only the cervical segments of the spinal cord are approximately level with their corresponding vertebrae. Below cervical levels, spinal nerves run increasingly oblique downwards to their intervertebral foramina. In humans, the C8 segment is level with the C7 vertebra, T12 segment with T9-T10, and the L5 segment with T11-T12 vertebrae (Ranson and Clarke, 1953). The spinal nerves lying caudal to the conus medullaris form a bundle of nerves called the cauda equina, because it resembles a horse’s tail.

In the rat, the spinal cord ends at the level of L3, and in the cat it ends at the L5 vertebra (Gelderd and Chopin, 1977; Greene, 1968; Padmanabhan and Singh, 1979; Wischnitzer, 1967). In the rat, the filum terminale is traceable into the tail beyond the third caudal nerves. The cauda equina, made up of the lumbar, sacral, and caudal nerves, conceals the extent of the cord itself (Greene, 1968; Padmanabhan and Singh, 1979).

In rodents, each segment of spinal cord gives rise to about 15 dorsal rootlets and 15 ventral rootlets on each side. Humans have only 6–8 dorsal and 6-8 ventral rootlets for each segment. The dorsal rootlets are bundled together to form the dorsal root of a spinal nerve, and the ventral rootlets form the ventral root. There is a fundamental functional difference between the ventral and dorsal roots, which was discovered by Magendie in 1822. He demonstrated that the dorsal roots contained sensory fibers, whereas the ventral roots contained motor fibers. The dorsal roots contain afferent (sensory) fibers, whereas the ventral roots contain efferent (motor) fibers.

All of the spinal nerves arising from the thoracic, lumbar, and sacral segments of spinal cord exit from the vertebral canal caudal to their respective vertebrae. For example, T5 nerve exits caudal to the T5 vertebra and the L4 nerve exits caudal to the L4 vertebra. The situation is slightly more complicated for the cervical region because there are eight cervical spinal cord segments, but only seven cervical vertebrae. The C1 to C7 nerves are therefore named after the vertebrae below their exit; the C8 nerve exits between C7 and T1.

As noted above, the spinal cord is considerably shorter than the vertebral column, and the last segment of the spinal cord is found at the level of the first or second lumbar vertebra in humans. Because of the progressive disparity between spinal segment levels and vertebral levels from rostral to caudal, the exiting spinal nerves become angled progressively downwards in their path to their intervertebral canal. The upper cervical nerves travel horizontally to their canals, but the lumbar and sacral nerves must travel almost vertically downwards to reach their exit canals resulting in the appearance of cauda equina.

Each dorsal root bears a ganglion which contains the cell bodies of primary sensory neurons. Each ganglion forms an ovoid swelling which is called a spinal ganglion or the dorsal root ganglion. Each spinal ganglion neuron gives rise to an axon which later bifurcates to two processes, one connecting to the periphery and the other one to the spinal cord.

Gray and white matter

A transverse section of the spinal cord shows that the gray matter is arranged in the form of a butterfly or the letter H, depending on the level (Fig. 1.1). The cross bar of the H (called the commissural gray matter) encloses the central canal. The dorsally projecting arms of the gray matter are called the dorsal horns, and the ventrally projecting arms are called the ventral horns. The central region, which connects the dorsal and ventral horns, is called the intermediate gray matter. The amount of gray and white matter vary significantly from one part of the spinal cord to another. The relative area of gray substance is smallest in the thoracic region, and greatest in the limb enlargements. In the thoracic spinal cord and upper lumbar segments, there is a small lateral projection of the intermediate gray matter called the lateral horn which contains the preganglionic cells of origin of the autonomic nervous system. In the sacral segments, the lateral horn contains the parasympathetic preganglionic motor neurons that constitute the sacral parasympathetic nucleus. The dorsal horn of the gray matter connects with the surface of the spinal cord at the point where the dorsal nerve rootlets enter the spinal cord.

White matter of the spinal cord

A layer of white matter surrounds the gray matter except for where the dorsal horn touches the margin of the spinal cord. The white matter consists mostly of longitudinally running axons, many of which form distinct bundles called tracts. The horns of the gray matter divide the white matter into three columns: dorsal, lateral and ventral. The two dorsal columns are located between the two dorsal horns of gray matter. The boundary between the lateral and ventral horns is not distinct. Although the two dorsal columns lie side by side with a common medial border, the two ventral columns are separated by a deep fissure named the ventral median fissure, which extends almost as far as the commissural gray matter. Blood vessels use this deep fissure as way of reaching the center of the gray matter. At the dorsal border of the ventral median fissure is a band of white matter which connects the two ventral columns.

At the region where the dorsal horn reaches the pial surface of the spinal cord, there is a prominent band of fibers called the dorsolateral fasciculus (the tract of Lissauer). This tract contains primary afferent fibers that either ascend or descend for a few segments before entering the dorsal horn. The dorsal column of the white matter is chiefly made up of the central processes of dorsal root ganglion cells. These large myelinated axons form the main pathway conveying touch sensation and position sense (proprioception) from the limbs and trunk to the brain. Because more fibers are added at each segment, the dorsal column is very small in the sacral segments, and largest in the rostral cervical spinal cord. As the fibers from one segment enter the dorsal column, they move to form a strip as close to the medial edge as possible. Fibers from the next segment move as far medially as they can and form a strip lateral to those of the more caudal segment. At the upper cervical segments, the fibers from sacral and lumbar segments form a distinct medial strip called the gracile fasciculus. The more lateral group is wedge-shaped and is called the cuneate fasciculus which primarily contains afferents from the skin of the upper limb.

In rodents and many other mammals (but not primates or carnivores), the dorsal corticospinal tract occupies the ventralmost part of the dorsal column. The fibers of this tract are smaller in diameter compared to that of the dorsal root ganglion cell axons that form the gracile and cuneate fasciculi. Because the dorsal corticospinal tract contributes fibers to each segment as it descends, it diminishes in size from rostral to caudal levels. The dorsal corticospinal fibers terminate in the dorsal horn, intermediate gray matter, and, to a lesser extent, on the interneuron pools of the ventral horn. In primates, carnivores, and some other mammals, the corticospinal fibers form a prominent tract in the lateral column. The lateral corticospinal tract of primates is unusual in that some of its fibers make direct synaptic connections with motor neurons in lamina 9. In humans, about 20% of the lateral corticospinal fibers make direct motor neuronal connections—by far the largest percentage in any primate.

The lateral and anterior columns contain a variety of ascending and descending fiber groups. The ascending tracts include the spinothalamic and spinocerebellar tracts, and the descending tracts include the corticospinal, vestibulospinal, rubrospinal and reticulospinal tracts. In addition to these long ascending and descending tracts, there are many fibers in the white columns that connect one spinal cord segment with another. These are called propriospinal pathways (or fasciculi proprii), because they often lie very close to the gray matter. The largest propriospinal pathways connect the brachial and lumbosacral enlargements to coordinate upper and lower limb movements.

Gray matter of the spinal cord

The spinal cord gray matter is made up of neuronal cell bodies and neuropil. The neurons are mostly multipolar, but vary greatly in size. The gray matter can be macroscopically divided into dorsal and ventral horns and intermediate region between them. Microscopic analysis of the spinal gray matter reveals a complex structure, characterized by successive layers of cells from dorsal to ventral. The landmark description of these layers by Rexed (1952, 1954) has formed the basis of the majority of detailed anatomical and physiological studies of the spinal cord in recent times.

The laminae of Rexed

Rexed divided the spinal gray matter into 10 regions on the basis of cytoarchitecture as seen in transverse sections. The first nine laminae are arranged from dorsal to ventral. The tenth (which is called an area rather than a lamina) is merely a circle of cells surrounding the central canal. Laminae 1–4 are the main cutaneous receptive regions; lamina 5 receives afferents from the viscera, skin and muscles. Most importantly, lamina 5 contains the limb motor pattern generators that are controlled by the corticospinal tract (Levine et al., 2014). Lamina 6 receives mainly proprioceptive and some cutaneous afferents. Lamina 7 contains the interneuron pools that project to motor neurons. Lamina 8 contains propriospinal interneurons which project to motor neurons on the same and opposite side, and lamina 9 motor neurons that supply limb and trunk muscles. A detailed description of the Rexed laminae can be found in Chapter 6.

Lamina 1

This very thin layer was previously known as the marginal nucleus of spinal cord or posteromarginal nucleus. Lamina 1 has a reticular appearance and consists mostly of fusiform neurons.

Lamina 2

This is the substantia gelatinosa of the dorsal horn. The cell density is greater than in lamina 1. It can be subdivided into two sublayers, lamina 2 outer (2o) and lamina 2 inner (2i) (Rexed, 1954; Molander et al., 1984, 1989).

Lamina 3

Rexed’s laminae 3, 4, and 5 are collectively called the nucleus proprius. Lamina 3 constitutes the superficial part of the nucleus proprius comprising many myelinated fibers. The neurons are less densely packed and larger than those in lamina 2. Laminae 2 and 3 are functionally related and consist mostly of Golgi Type 2 neurons.

Lamina 4

Formerly known as the base of the nucleus proprius, lamina 4 is about twice as thick as lamina 3. Its medial end curves ventrally along the margin of the dorsal horn to reach lamina (area) 10; at lumbar and sacral segments, it makes contact at the midline with its counterpart of the opposite side. In thoracic and upper lumbar segments, the medial extent of lamina 4 is limited by the dorsal nucleus (of Clarke).

Lamina 5

Formerly called the neck of the dorsal horn, this is the thickest layer of the dorsal horn. Its lateral part has a characteristic reticular appearance on account of fiber bundles running through it. From T2 to L2, the medial end of lamina 5 touches the dorsal nucleus (see Lamina 7 below). In rodents, lamina 5 is the main area of termination of the corticospinal tract which was previously an apparent contradiction, since this lamina was thought to be merely a sensory region. However, Levine et al. (2014) have shown that lamina 5 contains the pattern generators that initiate upper and lower limb movements.

Lamina 6

This is the most ventral layer of the dorsal horn. It is best seen in sections through the cervical and lumbosacral enlargements. Lamina 6 does not seem to exist in upper cervical levels or in thoracic levels below T1. It forms a thin layer between the thicker laminae 5 and 7 and the boundaries with these laminae are often indistinct.

Lamina 7

The dorsal part of this lamina was formerly described as the intermediate zone of spinal gray matter. The ventral part forms the dorsal region of the ventral horn. Lamina 7 cells are less densely stained than those in the adjacent laminae 8 and 9. The cells in the large central part of lamina 7 are the interneurons that connect to motor neuron pools. Lamina 7 neurons are involved in the regulation of posture and movement. Many descending motor pathways control motor neurons by means of connection with interneurons in lamina 7, rather than direct connections to motor neurons, which are rare in mammals other than primates. The functional arrangement of lamina 7 interneurons is matched to the topographical arrangement of motor neuron columns—the interneurons supplying trunk musculature are more medially placed, and the interneurons supplying limb musculature are more laterally placed.

In thoracic and upper lumbar segments, lamina 7 contains two notable cell groupings—the preganglionic column (PGC) and the dorsal nucleus. The PGC can be identified at the dorsolateral tip of lamina 7 in levels T1 to L2. These preganglionic motor neurons project to the ganglia of the sympathetic chain. The axons of preganglionic neurons leave the spinal cord in the ventral roots and cross to the sympathetic chain via short connecting nerves called gray rami communicantes. Within the sympathetic chain, they travel to reach sympathetic ganglion cells on which they synapse. Because the preganglionic sympathetic neurons are restricted to the T1 to L2 region, those serving the rostral and caudal ends of the body must travel some distance in the sympathetic chain before they reach a ganglion close to their target organ or blood vessels. The ganglion cells that receive the preganglionic fibers send their postganglionic axons directly back to the closest spinal nerve for further distribution. The short nerves connecting the ganglion to the spinal nerve are called white rami communicantes. Parasympathetic preganglionic neurons are found in the intermediolateral horn in lamina 7 at levels L6 to S2 in the rat, and at levels S2 to S5 in humans.

The dorsal nucleus receives primary afferents from the dorsal roots of the lower spinal cord and its axons form the dorsal spinocerebellar tract in the lateral funiculus. The axons do not cross the midline and end in the ipsilateral cerebellum. The dorsal nucleus extends from upper thoracic levels to L3; in some species, it may extend rostrally as far as C8 and caudally as far as L4. The dorsal nucleus primarily receives cerebellar afferents from the lower limb. The equivalent cell groups serving the upper limb are found in the external cuneate nucleus of the caudal hindbrain.

In upper cervical levels (C1-C3), the central cervical nucleus is seen in the medial part of lamina 7, immediately dorsolateral to the intermediomedial nucleus. It receives input from neck muscle spindles and projects to the contralateral cerebellum and vestibular nuclei. It also receives a substantial input from the contralateral vestibular nuclei and plays an important role in the control of head and neck movements (Donevan et al., 1990; Thomson et al., 1996).

Lamina 8

Lamina 8 is found in the ventromedial or ventral region of the ventral horn. The cells of this layer are propriospinal interneurons, which are heterogeneous in size and more densely stained than lamina 7. The large cells in this lamina project to motor neurons on the same and opposite side.

Lamina 9

The most prominent feature of lamina 9 is clusters of large multipolar neurons. These are the alpha motor neurons which supply the extrafusal fibers of the striated muscles of the axial skeleton and the limbs, and gamma motor neurons innervating the intrafusal fibers in muscle spindles. Lamina 9 motor neurons are located in three distinct columns in the ventral horn—the medial motor column (MMC), the hypaxial motor column (HMC), and the lateral motor column (LMC). The MMC and HMC are found at all spinal cord levels and supply the back muscles and body wall muscles, respectively. The LMCs are restricted to the upper and lower limb regions (C5 to T1 and L2 to S2 in humans) (Fig. 1.2). Within the LMCs of the upper and lower limbs, the motor neurons innervating proximal limb muscles are more ventromedially placed in the ventral horn and the neurons innervating distal limb muscles are more dorsolaterally placed. In addition to the dorsolateral/ventromedial subdivision, there is also a rostrocaudal arrangement of motor neuron columns in each of the limb enlargements, with more proximal limb muscles represented rostrally and more caudal muscles represented more caudally. For example, the deltoid muscle is supplied by motor neurons at the C5 level whereas the muscles of the manus (hand) are found at the T1 level. The detailed arrangement of motor neurons supplying the limbs is described in Chapter 7.

Fig. 1.2

Fig. 1.2 The six spinal cord regions as defined by motor neuron distribution and Hox gene expression. The brachial and crural regions possess a prominent lateral motor column (LMC), whereas the postbrachial and postcrural regions possess a preganglionic column (PGC). The prebrachial and caudal regions lack both lateral motor and preganglionic columns. The column labeled Histology contains spinal cord sections from each of the six regions, stained to show the presence of choline acetyl transferase.

In addition to the supply of limb and trunk muscles, a special group in the HMC of the cervical spinal cord supplies the diaphragm, which is the principal muscle of breathing. The supply to the diaphragm has some unusual features. Although the diaphragm sits between the abdomen and thorax, its embryological origin is from the prevertebral muscles of middle of the neck. The phrenic nerve, which supplies the diaphragm, arises from HMC neurons at C2, C3, and C4 and travels down through the neck and thorax to reach the diaphragm. This explains why individuals who have their spinal cord severed at C5 or C6 can continue breathing, even though their thoracic and abdominal muscles are paralyzed.

As well as the large alpha motor neurons referred to above, area 9 contains a population of smaller gamma motor neurons, which supply muscle spindles.

Area (lamina) 10

This is the area surrounding the central canal. Neurons in this region are nociceptive and project to the midbrain, thalamus and hypothalamus.

Spinal cord nuclei

Several discrete nuclei have been identified in the spinal cord. In the white matter, there are two significant neuronal groups located lateral to the dorsal horn in the lateral column: the lateral cervical nucleus in upper cervical segments and the lateral spinal nucleus throughout the spinal cord. In the gray matter, the internal basilar nucleus is found in upper cervical segments, the intermediomedial nucleus in cervical, thoracic and upper lumbar segments, the lumbar dorsal commissural nucleus in upper lumbar and the sacral dorsal commissural nucleus in sacral spinal cord segments. The sympathetic intermediolateral nucleus is located in thoracic an upper lumbar segments, and the parasympathetic sacral parasympathetic nucleus throughout the sacral spinal cord.

The precerebellar nuclei of the spinal cord comprise the central cervical nucleus (located in upper cervical segments), the dorsal nucleus (located in thoracic and upper lumbar segments), and the lumbar and sacral precerebellar nuclei.

Central canal

The central canal of the spinal cord is a remnant of the embryological ventricular system and is present throughout the whole length of the spinal cord. It is continuous with the fourth ventricle rostrally and the terminal ventricle of the conus medullaris caudally. The central canal is lined with columnar ependymal cells, may be blocked with debris in older adults.

Meninges

The spinal cord is enclosed by a tube formed by the spinal meninges. The meninges are the pia mater, arachnoid mater, and dura mater, separated from each other by the subdural and subarachnoid spaces. The subarachnoid space is filled with the cerebrospinal fluid. Between the dura and the bony vertebral canal is the epidural space, filled with fat and lymphatic tissue, small arteries, and a venous plexus. On each side of the spinal cord, the pia mater is tethered to the arachnoid and dura between the ventral and dorsal spinal roots at each vertebral level by a saw-toothed fibrous extension of the pia called the denticulate ligament.

Blood supply

The spinal cord is supplied with a single anterior spinal artery and two posterior spinal arteries. The anterior spinal artery originates from the vertebral artery and descends within the anterior median fissure of the spinal cord. The posterior spinal arteries originate either from the vertebral artery or its inferior posterior cerebellar branch, and descend in the posterolateral sulcus of the spinal cord. Segmental branches from the vertebral, deep cervical, intercostal and lumbar arteries anastomose with the anterior and posterior spinal arteries. The veins of the spinal cord form a surface plexus, which drain rostrally into the cerebellar veins and cranial sinuses, and through the intervertebral veins and external venous plexuses to the azygous system.

Hox gene expression

Hox genes play an important role in spinal cord development, both in regional specification and in motor neuron column differentiation. As diagrammed in Fig. 1.2, Hox genes specify the presence of LMC motor neurons in the areas that supply the upper limb muscles (mainly Hox6 paralogs) and lower limb muscles (Hox 10) paralogs. Between the two areas as single Hox paralog, Hoxc9, specifies the presence of the PGC motor neurons (Philippidou and Dasen, 2013).

The motor neuron expression patterns created by Hox gene specification support a functional subdivision of spinal cord into six regions that was suggested by Watson and Sidhu (2009). Based on histological studies, the six region organization appears to be common to amphibians, reptiles, birds, and mammals; the six regions can be described as prebrachial (C1 to C4 in humans), brachial (C5 to T1), postbrachial (T2 to L1), crural (L2 to S1), postcrural (S2 to S4), and caudal (S5 to Co) (see Fig. 1.3). The brachial and crural regions contain LMC motor neurons, the postbrachial and postcrural regions contain PGC motor neurons, and the prebrachial and caudal regions contain neither LMC or PGC motor neurons. Because of co-repression of Hox paralogs at the boundaries between regions, the boundaries are quite sharp, with minimal overlap of motor neuron types.

Fig. 1.3

Fig. 1.3 The segmental position of the motor neuron columns of the spinal cord in the mouse. The arrangement of columns of motor neurons in the spinal cord indicates that the spinal cord can be divided into six functional regions, prebrachial, brachial, postbrachial, crural, postcrural, and caudal. The upper part of this figure shows the extent of each of the columns along the length of the spinal cord. The medial motor column (MMC, red ) supplies the postvertebral muscles, and extends the whole length of the cord. The hypaxial motor column (HMC, blue ) supplies the prevertebral muscles (including the muscles of the diaphragm), and also extends the whole length of the cord. The lateral motor column (LMC, green ) supplies the muscles of the upper and lower limbs, and is therefore restricted to segments C5 to T1 and segments L2 to L6 in the mouse. The preganglionic column (PGC, yellow ) is restricted to segments T2 to L2 and S1 to S3 in the mouse. The branchial motor column (BMC, orange ), which supplies the sternomastoid and trapezius muscles, is restricted to segments C2 to C4 in the mouse. The lower part of the diagram shows the position of each of the motor neuron columns in transverse sections of the mouse spinal cord at C3, C6, T12, L3, S2, and S4 ( Ad9 , adductors; Bi9 , biceps; De9 , deltoid; Ph9 , phrenic nucleus; Q9 , quadriceps; Si9 , supraspinatus and infraspinatus; Tail9 , tail muscles; ThAb9 , thoracoabdominal wall muscles; TzSM9 , trapezius and sternomastoid). Modified from Mitchelle A, Watson C (2016) The organization of spinal motor neurons in a monotreme is consistent with a six-region schema of the mammalian spinal cord. J Anat 229, 394–405.

Dasen et al. (2008) showed that the gene encoding the transcription factor Foxp1 acts as an important target effector on behalf of Hox proteins. In the spinal cord regions associated with the upper and lower limbs, Hox proteins generate high levels of Foxp1, which in turn specifies LMC motor neuron fates. In the region between the limb outputs (mainly thoracic), the level of Foxp1 generated is low, and PGC motor neurons are specified (Dasen et al., 2008). Moreover, experimentally raised levels of Foxp1 in the thoracic cord forces prospective HMC and PGC motor neurons to assume an LMC identity.

Jung et al. (2014) speculate that the emergence of Hoxc9 expression may have played a vital role in the appearance of tetrapod limbs. They point out that in early finless fish, the capacity to form an LMC extends the whole length of the spinal cord. It appears that the evolutionary emergence of Hoxc9 expression in the middle of the spinal cord suppressed Foxp1 expression, which led to suppression of LMC motor neurons at thoracic levels. These led to the separation of rostral and caudal LMC populations, which was a precursor to tetrapod limb function.

In addition to specifying regional subdivision of the spinal cord, Hox proteins act on transcription factors to specify separate functional motor columns. In a landmark study of forelimb LMN pools in the chick (Dasen et al., 2005), using retrograde tracer labeling showed that a Runx1, Pea3, and Scip transcription factors were separately responsible for specification of particular muscles: Runx1  + motor neurons were selectively labeled after tracer injection into the scapulohumeralis posterior muscle; Pea3  +, Isl1  + motor neurons were labeled after tracer injection into the pectoralis muscle; Pea3  +, Lim1  + motor neurons were labeled after HRP injection into the anterior latissimus dorsi; Scip  + motor neurons were labeled after tracer injection into the flexor carpi ulnaris muscle.

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Chapter 2: Development of the spinal cord

Ken W.S. Ashwell

Outline

From epiblast to neural tube

Neural crest development

Alar and basal plates and their derivatives

Segmentation of the developing spinal cord

Motor neuron development and cell death

Development of spinal cord afferents and dorsal horn interneurons

Development of glia in the spinal cord

Development of major ascending and descending tracts

Myelination of spinal cord pathways

Relative growth of the spinal cord and vertebral column

References

Further reading

From epiblast to neural tube

Fate mapping studies of embryos at the end of primary gastrulation have found that a relatively small region of epiblast is mapped to become spinal cord compared to the three primary brain vesicles (Steventon and Arias, 2017). Recent perspectives on the very early development of the spinal cord suggest that much of the mammalian cord beyond cervical levels is derived from expansion of a neuromesodermal progenitor pool (NMp) (Steventon and Arias, 2017). These NMp cells emerge within the epiblast at about 7.5 pc (days postconception) and have the capacity to pursue neural (i.e. spinal cord) or mesodermal fates.

The central nervous system develops by two distinct processes—primary and secondary neurulation (Massarwa et al., 2014). During primary neurulation, the neural plate, a tadpole-shaped thickening of the ectoderm, is converted to a neural tube (Fig. 2.1A). This can be seen at approximately 18 to 19 days pc in the human (Carnegie stages 6 to 7, see Table 2.1 for comparison with mouse and rat) (Kaufman, 1992). Induction of the neural plate appears to be due to an inhibition of epidermis formation due to signals released from the primitive node at the cranial end of the primitive streak (Sadler, 2005). In other words, the default option for the ectoderm in this region is to produce epidermis rather than neurectoderm, and the signal for neurulation involves suppression of bone morphogenetic protein (BMP) and Wnt signaling pathways (Sadler, 2005). In all vertebrates that have been studied, the notochord underlying the future floor plate and the floor plate itself excretes the molecule Sonic hedgehog (SHH), which is the signal which induces floor plate formation of the neural groove and tube and effectively ventralizes the neural tube (Massarwa et al., 2014).

Fig. 2.1

Fig. 2.1 Neural plate and tube of a human embryo at (A) 19 days, (B) 20 days, and (C) 22 days pc showing folding of the neural groove to produce the neural tube. The first point of fusion between the neural folds is at the hindbrain/spinal cord junction.

Table 2.1

a See text for references and comments.

b Carnegie stage.

c Theiler stage.

d Wistchi stage.

Within a day of the appearance of the neural plate in the human, the edges of the neural plate elevate to form the neural folds and a neural groove emerges in the midline (Fig. 2.1B). The first site of closure (closure point I) occurs in the future cervical spinal cord region at 6 to 7 somites and closure continues caudally toward the tail and rostrally toward the head (Massarwa et al., 2014). The initial step in elevation of the neural folds depends on proliferation of the underlying mesoderm and production of hyaluronic acid (Solursh and Morriss, 1977), but later stages involve furrowing and folding at three regions of neurectoderm (one median and two lateral hinge points, see Fig. 2.2 and Massarwa et al., 2014 for review). The median hinge point is stimulated by signals from the notochord (SHH), whereas the lateral hinge points are believed to be dependent on a balance between the bone morphogenetic protein antagonist Noggin and signals from the notochord and surface ectoderm. Shaping of the neural folds through folding requires apical concentrations of microfilaments (Cearns et al., 2016) and lengthening of the cell cycle at the hinge points. The latter ensures that nuclei of dividing cells remain at the base of the neurectoderm for longer periods of time, thereby widening the bases and narrowing the apices of neural plate cells at these regions (Fig. 2.2). Some cell death contributes to the shaping of the tube, but is not obligatory (Massarwa et al., 2014).

Fig. 2.2

Fig. 2.2 This figure illustrates the mechanisms involved in the folding of the neural plate to form a neural tube. Most folding occurs at paired lateral and median hinge points where cell division is delayed and nuclei spend more time at the base of the neuroepithelium, thereby narrowing the apical processes of the neuroepithelial cells. Note the aggregation of nuclei at the periphery in these regions and the abundant mitotic figures at nonhinge regions. Glycoprotein and cellular processes on the surface of the adjacent neural folds facilitate adhesion when these points are brought into contact.

Fusion of the paired neural folds to form a neural tube first occurs at the junction of the hindbrain and spinal cord (level of the 5th somite) at approximately 20 pc in the human (Carnegie stage 9) and 8 days pc in the mouse and rat (Table 2.1), and depends on a combination of glue-like coatings of glycoprotein on the opposing surfaces and the extension of cellular processes across the gap (Sadler, 1978; see review in Massarwa et al., 2014). Fusion of the neural tube extends rostrally and caudally over the next few days (O’Rahilly and Muller, 2002) to effect the complete closure of the neural tube (Fig. 2.1C). After initial closure, the remaining open ends of the neural tube are known as the neuropores. In humans, the rostral or anterior neuropore closes at about 25 pc, while the caudal or posterior neuropore seals at 27 to 28 pc. After closure of the neuropores, the neural tube expands rostrally to form the brain vesicles, while the caudal tube begins to differentiate into the primitive spinal cord.

Secondary neurulation generates the neural tube at caudal levels (S4 and beyond) and involves the transformation of mesenchymal cells into an epithelial rod that develops a central cavity in continuity with the central canal formed by primary neurulation (Massarwa et al., 2014; Henrique et al., 2015; Cearns et al., 2016).

The central canal of the spinal canal usually closes up during postnatal life, but cyst formation and forking of the caudal end (i.e. within the filum terminale) has been seen in human fetuses. Communication between the central canal of the filum terminale and the pial surface of the cord may provide a clinically significant fluid connection between the canal and the subarachnoid space (Saker et al., 2016).

Neural crest development

During the elevation of the neural plate, cells appear along the edge (or crest) of the neural folds. These neural crest cells are found along the entire length of the neural tube and initially lie between the neural tube and the overlying ectoderm. Vagal, truncal and caudal groups are identified contributing to discrete components of the peripheral nervous system: the vagal group gives rise to the parasympathetic supply of heart and neurons of the enteric nervous system; the truncal group gives rise to sensory and autonomic ganglia; and the caudal group gives rise to glia of enteric nervous system (Butler and Bronner, 2015). Neural crest cells subsequently migrate along two pathways to give rise to a variety of mature cell groups. The dorsolateral pathway differentiates into pigment cells and the ventrolateral pathway gives rise to neural elements (autonomic ganglia, Schwann cells, adrenal medulla) (Kunisada et al., 2014). The cells of the truncal neural crest are transformed into sensory (or dorsal root) ganglia. Young neurons of the sensory ganglia develop central processes which invade the dorsal horn (see afferent development below) and a peripheral process, which innervates somatic or visceral structures. The dermamyotome, notochord and ventral spinal cord exert chemorepulsive effects through semaphorins and other molecules on the growing peripheral processes of developing dorsal root ganglia (Masuda and Shiga, 2005).

Alar and basal plates and their derivatives

The nuclei of the neuroepithelial cells migrate between the neural tube lumen and the outer limiting membrane in a process known as interkinetic nuclear migration. When the nuclei reach the luminal surface of the neural tube they undergo mitotic division, thereby producing either further neuroepithelial cells (during early stages) or primitive nerve cells (which have been inappropriately named neuroblasts) during later stages. Progressive accumulation of postmitotic differentiating neuroblasts beneath the external limiting membrane of the neural tube leads to the formation of a mantle layer (future spinal cord gray matter) around the neuroepithelium. The mantle layer on each side of the primitive spinal cord shows dorsal and ventral thickenings, which are known as the alar and basal plates, respectively (Fig. 2.3). The paired alar plates will give rise to sensory areas of the spinal cord, while the basal plates contribute to the motor areas of the cord. The neuroepithelium of the early spinal cord also shows a roof plate dorsally and a floor plate ventrally. The ultimate fate of the tissue external to the roof and floor plates is to serve as sites of dorsal and ventral white commissures for crossing axons in the postnatal spinal cord. The region external to the mantle layer is known as the marginal layer, and it contains nerve fibers emerging from the immature neurons of the mantle layer. The marginal layer will ultimately become the white matter of the fetal and postnatal spinal cord.

Fig. 2.3

Fig. 2.3 This figure shows a cross section through a typical developing therian spinal cord at the caudal cervical level ( Trichosurus vulpecula at 13.5 mm body length, from the Hill collection held at the Museum für Naturkunde, Berlin). Ventricular germinal zone (vz), roof plate (rp), floor plate (fp), sulcus limitans (sl), alar plate (ap) and basal plate (bp) of the mantle layer are labeled on the left side. On the right side, dorsal and ventral progenitor zones within the vz (pd1 to pd6 and pdIL; p01 to p03 and pMN, respectively) are labeled. Mature derivatives of the mantle zone are also shown emerging on the right side (dorsal horn—DH; lateral motor column—LMC; medial motor column—MMC). dr—dorsal roots; mz—marginal zone; vr—ventral