Morphometric analysis of the effects of exenteration and enucleation on the development of third and sixth cranial nerves in the rat

Microscopic anatomy: normal structure

A peripheral nerve trunk is composed of nerve fascicles supported in a fibrous collagenous sheath and defined by concentric layers of cells (the perineurium) that separate the contents (the endoneurium) from its fibrous collagen support (the epineurium). In the endoneurium are myelinated and unmyelinated fibers that are axons combined with their supporting Schwann cells to provide physical and electrical connections with end-organs such as muscle fibers and sensory endings. Axons are tubular neuronal extensions with a cytoskeleton of neurotubules and tubulin along which organelles and proteins can travel between the neuronal cell body and the axon terminal. During development some axons enlarge and are covered by a chain of Schwann cells each associated with just one axon. As the axons grow in diameter, the Schwann cells wrap round them to produce a myelin sheath. This consists of many layers of compacted Schwann cell membrane plus some additional proteins. Adjacent myelin segments connect at highly specialized structures, the nodes of Ranvier. Myelin insulates the axon so that the nerve impulse can jump from one node to the next. The region adjacent to the node, the paranodal segment, is the site of myelin terminations on the axolemma. There are connections here between the Schwann cell and the axon via a complex chain of proteins. The Schwann cell cytoplasm in the adjacent segment, the juxtaparanode, contains most of the Schwann cell mitochondria. In addition to the node, continuity of myelin lamellae is broken at intervals along the internode by helical regions of decompaction known as Schmidt-Lanterman incisures; these are seen as paler conical segments in suitably stained microscopical preparations and provide a pathway between the adaxonal and abaxonal cytoplasm. Smaller axons without a myelin sheath conduct very much more slowly and have a more complex relationship with their supporting Schwann cells that has important implications for repair.

A semi-automated method for identifying and measuring myelinated nerve fibers in scanning electron microscope images

Diagnosing illnesses, developing and comparing treatment methods, and conducting research on the organization of the peripheral nervous system often require the analysis of peripheral nerve images to quantify the number, myelination, and size of axons in a nerve. Current methods that require manually labeling each axon can be extremely time-consuming as a single nerve can contain thousands of axons. To improve efficiency, we developed a computer-assisted axon identification and analysis method that is capable of analyzing and measuring sub-images covering the nerve cross-section, acquired using a scanning electron microscope. This algorithm performs three main procedures - it first uses cross-correlation to combine the acquired sub-images into a large image showing the entire nerve cross-section, then identifies and individually labels axons using a series of image intensity and shape criteria, and finally identifies and labels the myelin sheath of each axon using a region growing algorithm with the geometric centers of axons as seeds. To ensure accurate analysis of the image, we incorporated manual supervision to remove mislabeled axons and add missed axons. The typical user-assisted processing time for a two-megapixel image containing over 2000 axons was less than 1h. This speed was almost eight times faster than the time required to manually process the same image. Our method has proven to be well suited for identifying axons and their characteristics, and represents a significant time savings over traditional manual methods.

Morphometric nerve fiber analysis of the human inferior alveolar nerve: lateral asymmetry

We studied morphometric nerve fiber analysis and the lateral asymmetry of the inferior alveolar nerve (IAN). Human IANs were resected at the mandibular foramen. The preparation of sections involved fixation, washing, dehydration, embedding, sectioning and staining as described in our previous reports. We estimated the average total number of myelinated axons in the right IAN to be 22,808, with an average transverse area of 37.6 microm2, an average perimeter of 23.0 microm, and average circularity ratio of 0.85, with the same measurements in the left IAN being 24,289, 33.9 microm2, 21.6 microm, and 0.86, respectively. Morphological differences between the right and left side were analyzed by applying parametric tests (unpaired t-test) to all measured items. According to these results, the IAN did not demonstrate notable lateral asymmetry in any measured item. We considered that these results were caused by using subjects with the same dentulous condition in both sides.

Myelin-axon relationships in the rat phrenic nerve: longitudinal variation and lateral asymmetry

It is known that the myelin sheath thickness-axon perimeter relationship varies between peripheral nerves. This study examines the possibility that that relationship may vary between levels along a given nerve or between corresponding levels of the right and left examples of the same nerve. The relationship is examined for large and small fibre classes at well separated upper and lower intrathoracic levels in the rat phrenic nerve. The study shows that the myelin-axon relationship differs between levels along the same nerve bundle in the same (intrathoracic) environment. Thus, for a given increase in the perimeter of large axons, sheath thickness increases significantly more at lower than at upper levels. In addition, myelin sheath thickness shows a statistically significant lateral asymmetry in favour of the left side for the large fibre class at the upper thoracic level. The setting of the myelin sheath thickness-axon perimeter relationship also differs between the large and small fibre classes at each level examined. Large fibres have proportionately thicker sheaths than small fibres and this difference is reflected in the significantly smaller g-ratio of the former. Systematic differences in the setting of the myelin sheath thickness-axon perimeter relationship between large and small fibre classes may be a widely occurring phenomenon. It may be concluded that the myelin-axon relationship varies significantly both within and between nerves and also between fibre classes. Accordingly, morphometric studies of normal or pathological nerves should take into account possible consistent longitudinal variation or lateral asymmetry in fibre parameters and myelin-axon relationships within a given nerve bundle or fibre class, in order to avoid introducing systematic bias and to minimize variance between samples.

Axon-myelin relationships in rat cranial nerves III, IV, and VI: a morphometric study of large- and small-fibre classes

The primary objectives of this study were to determine (1) if quantitative axon-myelin relationships are similar for large- and for small-fibre classes within individual nerves and (2) if the same axon-myelin relationships hold for equivalent fibre classes in closely similar nerves. The oculomotor, trochlear, and abducent nerves of the rat were examined since they each contain distinct large- and small-fibre classes and are similar in a wide range of anatomical and developmental respects. Accordingly, morphometric analyses of axon-myelin relationships were performed separately on large and small fibres of each of the three nerves. Within each nerve, the setting of the relationship between the two parameters was found to be different for the two fibre classes: Scatterplots relating sheath thickness to axon perimeter for large fibres were shifted upwards relative to those for small fibres. These differences were also reflected in the positions of the regression lines fitted to the plots and in the g-ratios. Significant differences were found between nerves in relation to their large fibres: Those of the abducent nerve had significantly thicker sheaths, those of the oculomotor nerve had significantly smaller axon perimeters, and the myelin sheath-axon perimeter relationship of the abducent nerve differed significantly from that of the other two. This study therefore shows that morphometric axon-myelin relationships may differ significantly between equivalent fibre classes of nerves that are closely similar in respect of morphological class, central origin, peripheral distribution, developmental environment, and function.

Extraocular muscles in the microphthalmic rat

The extraocular muscles in a mutant microphthalmic strain of rat were studied. The eyeball of this strain of rat is reduced to about a third in diameter of that of the normal rat. Nevertheless, in the orbit of the mutant rat, every one of the extraocular muscles was identified; their origins and courses were the same as in the normal rat, but differences existed in the insertions. These insertions could be classified into three groups: Group A (retractor bulbi): like normal insertion into the eyeball. Group B (superior rectus and superior oblique): attachment of tendonlike insertions to each other; these muscles come from opposite directions and form a loop. Group C (lateral, medial, and inferior rectus and inferior oblique): insertion into connective tissue surrounding the reduced eyeball. The volume of each muscle of the mutant rat was smaller than that of the normal rat; moreover, significant differences existed in the degree of reduction in the volume of each muscle group classified according to the change of insertion. In the group A muscle the volume was only 33% of the normal volume, whereas group B was 74% and group C was about half of normal.

Anterograde transneuronal degeneration in the ectomamillary nucleus and ventral lateral geniculate nucleus of the chick

The effects of anterograde transneuronal atrophy were studied in two visual nuclei of the chick--the ectomamillary nucleus (EMN), which shows marked degenerative changes following enucleation, and the ventral lateral geniculate nucleus (GLv), which shows less severe changes following enucleation. The chicks were enucleated on the day of hatching and killed between 2 and 81 days later. Reconstructions of the EMN and GLv revealed that enucleation retarded the growth of these two nuclei. The volume of the control EMN and GLv, ipsilateral to the removed eye, continued to increase after eye removal. The experimental EMN did not increase in volume during this time while the experimental GLv increased in volume but at a slower rate than the control GLv. The volume of the experimental GLv remained smaller than the control volume. In order to determine whether the volumetric changes were due to arrest of cellular growth or to atrophy of the neurons, a morphometric study was carried out in the two nuclei. Measurements of the cross-sectional area of EMN neurons revealed a 20% decrease in soma area in the experimental EMN in comparison with those in the control EMN. Since neurons in the control EMN did not increase in area after hatching, it was concluded that the changes were due to atrophy rather than arrest of neuron growth. Furthermore, there was a 35% neuron loss in the EMN. The GLv, which is composed of two laminae, consistently showed a greater decrease in soma cross-sectional area and neuron loss in its neuropil lamina (comparable to the transneuronal effects in the EMN) than in its lamina interna. Thus, in both nuclei, eye removal led to neuron loss and a decrease in soma cross-sectional area when compared with the contralateral (control) nucleus.

The oculomotor nucleus and extraocular muscles in a mutant anophthalmic mouse

The object of this study was to determine whether the oculomotor nucleus and the extraocular muscles are present in the adult of a mutant mouse strain in which eye formation fails early in development (Chase and Chase, '41). Neuronal counts for the oculomotor (IIIrd) nucleus were obtained from serial paraffin sections made through the midbrains of seven mice of the anophthalmic mutant group (ZRDCT-An) and seven mice of the same strain in which eyes were present (ZRDCT-N). The orbital structures of the anophthalmic and control groups were reconstructed from 1-micrometer plastic serial sections. There was in the anophthalmic mouse, in the same location as in the normal, an oculomotor nucleus whose mean neuronal cell number was 206 (+/- 40). The normal mouse had a mean value of 262 (+/- 63) oculomotor neurons. On the basis of these results, this difference between the two groups was not statistically significant (P = 0.073). The neuronal number per cubic millimeter was similar in the two groups (P = 0.81). In the orbit of the anophthalmic mouse, several bundles of striated muscle occupied a location comparable to that of extraocular muscle in the normal mouse. Neuromuscular junctions were present on fibers of this orbital muscle. It is concluded that the early failure of eye formation does not prevent the development of extraocular muscles and the oculomotor nucleus, and their retention in the adult.

Quantitative study of the non-circularity of myelinated peripheral nerve fibres in the cat

1. One hind limb of each of four cats was either chronically de-efferentated, or chronically de-afferentated, and perfused with buffered glutaraldehyde fixative. Up to three different muscle nerves were dissected from each limb, post-fixed in osmium tetroxide and embedded in Epon. Ultrathin transverse sections were mounted on Formvar-coated single-hole specimen grids so that all the fibres in each nerve could be examined individually by electron microscopy.2. Non-circularity was expressed as the ratio (ø): [Formula: see text] The degree of non-circularity of all the afferent axons, or all the efferent axons, in each muscle nerve was determined. The proportion of fibres cut through the paranodal region, or through the Schwann cell nucleus, was as expected for group I afferent and for alpha and gamma efferent fibres, but hardly any typical paranodal sections of group II or III afferent fibres were encountered which suggests that their paranodal arrangement differs from that of other groups. In a quantitative comparison of noncircularity in different functional groups, fibres cut through paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were rejected.3. All the gamma efferent fibres in one nerve were studied in a series of sections cut at 25 mum intervals. The degree of non-circularity was found to be relatively constant along the internode of most fibres when the values at paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were ignored.4. The value of ø varied widely from 1.0 (circular) to 0.5 or less from fibre to fibre within every functional group. However, the mean value of ø was less for gamma axons (0.68) than for alpha axons (0.78), and less for group III axons (0.79) than for axons in groups I and II (both 0.84). When the results for all the nerves were aggregated, these differences were statistically very highly significant, as was the difference in ø between group I and alpha fibres. If values of ø < 0.5 were rejected, the difference between the mean ø for group III and group II was then of doubtful significance whereas that between alpha and gamma fibres was still very highly significant.5. The external perimeter (S) of a non-circular fibre differs from pi times the diameter of a circle just enclosing the fibre (D). It is shown that S = 0.95 pi D for group I and II fibres, S = 0.90 piD for alpha and group III fibres, and S = 0.85 piD for gamma fibres.6. The myelin period, or interperiod repeat distance, varied from 14.1 to 15.6 nm in different cats, implying radial shrinkage of the myelin sheath from 15 to 23%. The myelin period in a particular cat was the same for several nerves, and the same for fibres in different functional groups.7. The possibility that repetitive firing of axons during fixation contributed to the varying degree of non-circularity is considered but rejected as unlikely.8. It is deduced that about 10% radial shrinkage of the myelin sheath, but little or no osmotic shrinkage of the axon, occurred during fixation and rinsing. Further radial shrinkage of about 8% in all components of the fibre probably occurred as a result of subsequent histological processing. It is concluded that the non-circularity of all axons, and the greater non-circularity of small axons, is unlikely to have been due to histological processing.9. It is concluded that axons are non-circular in vivo. The hypothesis that non-circularity allows axons to accommodate swelling during repetitive activity is discussed. Suggestions are made as to why gamma axons may be more non-circular than alpha or group III axons in an anaesthetized cat immediately prior to fixation, and why alpha axons may be more non-circular than axons in groups I and II.

Ultrastructural dimensions of myelinated peripheral nerve fibres in the cat and their relation to conduction velocity

1. The ultrastructure of all the afferent fibres, or all the efferent fibres, was studied in selected nerves from chronically de-afferentated or de-efferentated cat hind limbs perfusion-fixed with glutaraldehyde.2. The following parameters were measured: number of lamellae in the myelin sheath (n), axon perimeter (s), external fibre perimeter (S), axon cross-sectional area (A). Fibres were allocated to afferent groups I, II, III or efferent groups alpha and gamma according to the number of lamellae in the myelin sheath.3. The thickness of the myelin sheath (m) was linearly related to axon perimeter within the range s = 4 mum to s = 20 mum (groups II, III and gamma). The relation m = 0.103 s - 0.26 provided a good fit for all afferent and efferent axons in this range in several different anatomical muscle nerves in three cats. The myelin sheaths were thinner in a fourth, presumably younger, cat.4. The myelin sheaths were relatively thinner for large fibres in groups I and alpha (s = 20-50 mum). The results are interpreted in one of three ways. Either m tends to a limit of about 2.2 mum, or m is linearly related to s such that for large fibres m = 0.032 s + 1.11.5. Alternatively, m may be considered to be proportional to log(10)s for all sizes of axon so that m = 2.58 log(10) S - 1.73. The interpretation that there are two separate linear relations for large and small fibres is favoured.6. The ratio of axon to external fibre perimeter (g) falls from about 0.70 for group III and small gamma fibres in the cat to about 0.62 for group II and large gamma fibres and then rises again to 0.70, or even 0.75 for group I and alpha axons.7. The above relations between m and s are combined with the observations of Boyd & Kalu (1979) that Theta = 5.7 D for groups I and alpha and Theta = 4.6 D for groups II, III and gamma. It is shown that Theta = 2.5 s approximately for all sizes of axon (s from material fixed for electron microscopy) in rat, cat and man. The accuracy of this equation may be improved by deducting 3 m/sec in the case of small fibres. This conclusion is compatible with experimental observations of the relation between l and D (Hursh, 1939; Lubinska, 1960; Coppin, 1973) and between l and Theta (Coppin & Jack, 1972).8. From the theoretical analyses of Rushton (1951) and others Theta should be proportional to the external dimensions of the fibre rather than to axon size. It is shown that the thinning of the myelin sheath ought to affect Theta substantially. Thus some other factors must compensate for the thinning of the sheath.9. Small fibres are significantly more non-circular than large fibres. From the quantitative data of Arbuthnott et al. (1980) it is concluded that non-circularity may contribute to the fact that Theta proportional, variant s rather than Theta proportional, variant S, but cannot wholly account for it. Other possibilities considered are that axoplasmic resistivity or specific nodal conductance may differ for large and small fibres.10. It is suggested that myelinated peripheral nerve fibres may fall into two distinct classes with different properties, one comprising groups I and alpha and the other groups II, III and gamma. The conclusion predicted from theory may apply to each of these classes separately so that Theta = 2.0 S for the large-fibre class and Theta = 1.6 S for the small-fibre class.

A contribution to the electron microscopic morphometric analysis of peripheral nerve

Several aspects of data collection and analyses of peripheral nerve experiments employing light and electron microscopic morphometric techniques have not been adequately discussed in the literature. From statistical tests performed on nerve data, it was found that light compared with electron microscopic morphometry underestimates the number of small fibers. An optimum sampling strategy must take into account a potential bias toward small fibers introduced by measuring fibers from electronmicrographs. It must also take into account a potential bias introduced by the non-random distribution of nerve fibers of different sizes in nerves. These biases are offset by sampling a large enough number of fibers from large enough area electron micrographs. A method is presented for analysing periopheral nerve data using the nested analysis of variance. This requires first dividing the usual bimodal nerve fiber distribution into component normally distributed parts. The number of fibers in the two portions of a bimodal distribution must be considered in data analysis. Knowledge of the variances of parameters to be studied in any particular nerve is necessary for optimum sampling strategies.