Computer-assisted morphometric study on the postnatal growth of axon size and myelin thickness in the ventral and dorsal roots of the rabbit.

The morphometric analysis of the myelinated fibers is of great value in developmental, experimental, and pathological studies. The present study was performed on the ventral and dorsal spinal roots (L7) of new-born, 10-, 15-, 20-, 30-, 60-, 120- and 240-day-old rabbits. Semi-thin cross-sections were investigated by OLYMPUS Video image analyser. The average axonal diameter was measured as an average length of Feret's diameters and the specific width of the myelin sheaths as a total width of the myelin sheath. In the ventral roots, the increase of the axon size is more intensive than the increase of the myelin thickness. In adult rabbits, the mean axonal diameter for the ventral root is approximately 15% greater than the mean axonal diameter for the dorsal root, whereas the mean myelin thickness values are practically equal for both roots. For the period newborn-adult (240-day-old) rabbits, a full correlation was found in respect to the increase of the dorsal root mean axonal diameter and mean myelin thickness (380% and 380%). In the ventral roots, the mean axonal diameter increased 420% whereas the mean myelin thickness increased 350%. It is intriguing whether the greater axonal size of the ventral roots might compensate their relatively thin myelin sheaths in comparison to the dorsal roots. Our findings of essential increase (16%) of the mean myelin thickness in the last (120-240 days) period in both, ventral and dorsal roots, points to a protracted period of myelinization in the rabbit.     

Characteristic variations of relative myelin sheath thickness in 11 nerves of the rat

Summary Computer-assisted measurements of relative myelin sheath thickness (the g ratio) were made in 11 peripheral nerves of the rat. The scatter diagrams showed nerve-specific variations in the distribution of relative myelin sheath thickness. Myelinated fibers of less than 3.5 m axon diameter had relatively thin myelin sheaths, particularly in the splanchnic, vagus and glossopharyngeal nerves. The oculomoter nerve had two fiber populations clearly set apart in terms of relative myelin sheath thickness. Thickly myelinated fibers were found in facial and hypoglossal nerves. No single functional modality was evident for the thinly myelinated fibersThis study was supported by grant 609 from the Deutsche Forschungsgemeinschaft     

Myelination and remyelination in the regenerating visual system of the goldfish

Summary The remyelination of regenerated optic axons was investigated in goldfish following either optic nerve crush or ouabain retinal intoxication. Axons grown after nerve crushing acquire thinner myelin sheaths than axons originating from reconstituted ganglion cells. If axons of reconstituted ganglion cells are crushed and allowed to regenerate, the subsequent myelination is weaker than that of control axons not interrupted by crushing, but stronger than that of axons of preexisting retinal ganglion cells.The present results suggest that a neuron is capable of inducing a normally developed myelin sheath when its axon contacts an oligodendrocyte the first time, whereas a neuron whose axon contacts an oligodendrocyte the second time is not capable of forming a normal myelin sheath in the adult animal. The present results also support the notion that the oligodendrocyte requires a neuronal signal for myelin sheath formation.Supported by the Deutsche Forschungsgemeinschaft (Wo 215/5)     

A quantitative study of the facial nerve in mice prenatally exposed to ethanol

ABSTRACT  Pregnant ICR mice were given 20 % ethanol intraperitoneally twice on day 13 of gestation and allowed to give birth to offspring. The offspring were killed at 56 days of age and the motor root of their facial nerve was examined histologically and morphometrically. The cross-sectional area of the facial nerve of mice prenatally exposed to ethanol was significantly smaller than that of the control mice. There was no significant difference in the total number of myelinated axons or the mean axonal diameter between control and ethanol-exposed mice, but the mean diameter of myelinated fibers (axon + myelin sheath) and the thickness of myelin sheath were significantly decreased in the treated group. These results suggest that prenatal exposure to ethanol disturbs myelina-tion of the motor root of the facial nerve and may cause permanent neurological effects.     

大脑白质有髓神经纤维髓鞘超微结构定量研究的体视学方法

目的:运用新的体视学方法研究大鼠白质有髓神经纤维髓鞘超微结构,以期为以后各种有关髓鞘超微结构的相关研究提供可靠的方法学依据.方法:6～8月龄雌性Long-Evans大鼠.运用电镜技术及相应的体视学方法计算白质内有髓神经纤维的总长度、有髓神经纤维髓鞘总体积、有髓神经纤维及轴突直径、髓鞘断面面积、髓鞘内外周长、髓鞘平均厚度.结果:获得了正常6～8月龄雌性Long-Evans大鼠有髓神经纤维总长度及髓鞘超微结构的各项数据,两种不同方法得到的髓鞘平均厚度并没有显著性差异.结论:把新的体视学方法和电子显微镜结合起来定量研究了大鼠大脑白质有髓神经纤维髓鞘超微结构,所描述的方法为以后进行髓鞘超微结构的定量研究提供了有用的工具.     

嗅鞘细胞培养上清液促进大鼠周围神经损伤后的修复与再生

文题释义：细胞培养上清：细胞在正常生理过程中会释放一些信息物质,包括可溶性因子、细胞外囊泡、蛋白质、各种RNA等,这些物质能够在细胞间通讯,乃至在多种生理过程中发挥重要作用。在细胞培养过程中,这些物质由细胞分泌至细胞培养液中。这种含有细胞分泌的活性物质并去除了细胞碎片等杂质的培养液,称为细胞培养上清。 神经再生：损伤后的神经再生是一个复杂的生理过程,受多种因素的影响和调节。神经再生过程可归纳为3个方面：受损神经近端轴突的萌芽和伸长,再生轴突的髓鞘化,再生轴突与靶器官之间突触连接的重建。神经再生分为中枢神经再生及周围神经再生。受轴突外部再生微环境的影响,中枢神经再生较外周神经再生更为困难。 背景：既往研究发现嗅鞘细胞培养上清可以促进脊髓损伤后轴突再生及功能恢复,但应用于周围神经损伤治疗方面鲜有报道。 目的：探讨嗅鞘细胞培养上清是否有助于周围神经损伤后的神经修复。 方法：分离纯化嗅鞘细胞并鉴定,制备嗅鞘细胞培养上清。在体外环境将嗅鞘细胞培养上清作用于背根神经节组织块,观察背根神经节轴突生长情况;在体内环境将嗅鞘细胞培养上清应用于大鼠坐骨神经缺损模型,观察坐骨神经轴突再生及髓鞘化情况。 结果与结论：①嗅鞘细胞纯度高达(94.4±3.1)%;②与空白对照组和低剂量嗅鞘细胞上清组对比,高剂量嗅鞘细胞上清组背根神经节组织块的5根最长神经轴突平均长度显著增加(P < 0.05);③免疫荧光显示嗅鞘细胞上清处理组与自体神经移植组类似,再生神经贯通缺损区域,并且再生神经排列有序,神经再生情况显著优于空白对照组;④透射电子显微镜观察显示嗅鞘细胞上清处理组再生神经轴突的数量和髓鞘厚度显著高于空白对照组(P < 0.05);⑤结果表明,嗅鞘细胞培养上清能够促进周围神经损伤后轴突再生及再生轴突的髓鞘化,为周围神经损伤提供了一种新的基于嗅鞘细胞的无细胞疗法。 ORCID: 0000-0002-9558-8585(杨雨洁) 中国组织工程研究杂志出版内容重点：干细胞;骨髓干细胞;造血干细胞;脂肪干细胞;肿瘤干细胞;胚胎干细胞;脐带脐血干细胞;干细胞诱导;干细胞分化;组织工程     

Changes in axonal physiology and morphology after chronic compressive injury of the rat thoracic spinal cord

The spinal cord is rarely transected after spinal cord injury. Dysfunction of surviving axons, which traverse the site of spinal cord injury, appears to contribute to post-traumatic neurological deficits, although the underlying mechanisms remain unclear. The subpial rim frequently contains thinly myelinated axons which appear to conduct signals abnormally, although it is uncertain whether this truly reflects maladaptive alterations in conduction properties of injured axons during the chronic phase of spinal cord injury or whether this is merely the result of the selective survival of a subpopulation of axons. In the present study, we examined the changes in axonal conduction properties after chronic clip compression injury of the rat thoracic spinal cord, using the sucrose gap technique and quantitatively examined changes in the morphological and ultrastructural features of injured axonal fibers in order to clarify these issues. Chronically injured dorsal columns had a markedly reduced compound action potential amplitude (8.3% of control) and exhibited significantly reduced excitability. Other dysfunctional conduction properties of injured axons included a slower population conduction velocity, a longer refractory period and a greater degree of high-frequency conduction block at 200 Hz. Light microscopic and ultrastructural analysis showed numerous axons with abnormally thin myelin sheaths as well as unmyelinated axons in the injured spinal cord. The ventral column showed a reduced median axonal diameter and the lateral and dorsal columns showed increased median diameters, with evidence of abnormally large swollen axons. Plots of axonal diameter versus myelination ratio showed that post-injury, dorsal column axons of all diameters had thinner myelin sheaths. Noninjured dorsal column axons had a median myelination ratio (1.56) which was within the optimal range (1.43-1.67) for axonal conduction, whereas injured dorsal column axons had a median myelination ratio (1.33) below the optimal value. These data suggest that maladaptive alterations occur postinjury to myelin sheath thickness which reduce the efficiency of axonal signal transmission.In conclusion, chronically injured dorsal column axons show physiological evidence of dysfunction and morphological changes in axonal diameter and reduced myelination ratio. These maladaptive alterations to injured axons, including decrease in myelin thickness and the appearance of axonal swellings, contribute to the decreased excitability of chronically injured axons. These results further clarify the mechanisms underlying neurological dysfunction after chronic neurotrauma and have significant implications regarding approaches to augment neural repair and regeneration.     

Dorsal root ganglion neurons induce transdifferentiation of mesenchymal stem cells along a Schwann cell lineage

It has been reported that mesenchymal stem cells (MSCs) can transdifferentiate into Schwann cell-like cells by a series of treatments with a reducing agent, retinoic acid and a combination of trophic factors in vitro, and can transdifferentiate into myelin-forming cells to repair the demyelinated rat spinal cord in vivo. We now report that when co-cultured with dorsal root ganglion (DRG) neurons, MSCs were induced to transdifferentiate into Schwann cell-like cells that had ensheathed DRG axons. Following differentiation, MSCs underwent morphological changes similar to those of cultured Schwann cells and express GFAP and S100, the marker of Schwann cells. Moreover, 6 weeks later, MSCs wrapped their membrane around DRG axons. Further, initiation of myelination was observed in the co-cultured DRG neurons, which was determined by signals to MBP and this initiation of axon myelination by MSCs is similar to that of Schwann cells. However, electron micrographs show that no compact myelin was present in the MSCs co-cultures, whereas the Schwann cells co-cultures had formed a multilammelar myelin sheath around the axon. These indicate that the release of cytokine by DRG neurons may promote the transdifferentiation of MSCs, but is not sufficient to elicit compact myelination by transdifferentiated MSCs. These results improve our understanding in the mechanism of MSC transdifferentiation, and the mechanism underlying ensheathment and myelination by transdifferentiated MSCs.     

Structure of the satellite cell sheath around the cell body, axon hillock, and initial segment of frog dorsal root ganglion cells

The structure of the satellite cell sheath of frog dorsal root ganglion cells was studied in thin sections and freeze-fracture replicas. The sheath around the cell body is composed of thin satellite cell lamellae closely applied to the neuronal plasma membrane. At the axon hillock the sheath divides into outer and inner components separated by a broad space containing a distinctive extracellular matrix and occasional flattened satellite cell processes. The sheath around the initial segment is usually multilayered but less compact than that around the cell body, and in some places it exhibits node-like interruptions. Apart from occasional particle groupings characteristic of tight junctions and gap junctions, the satellite cells display homogeneously distributed intramembranous particles in both fracture faces in all regions of the sheath.     

Experimental diabetic neuropathy. Morphometric studies on the rat N. suralis in short-term streptozotocin-induced diabetes

Morphometric studies of sural nerves were performed in diabetic rats 35 or 44 d, respectively, after the administration of 60 mg/kg b.w. streptozotocin. Morphometry of photographed semithin sections was performed after whole-body glutaraldehyde perfusion both with the semiautomatic MOP Videoplan and the MOP AM 02 (Kontron, Munich, F.R.G.). The following parameters were registered: Area of nerves and fibers, perimeter of fibers, diameter of axons, thickness of myelin sheaths, form factor. No decrease of the total nerve area or of the myelinated area were found. Parameters area of fibers, thickness of myelin sheath and form factor decreased in diabetic animals. Axon diameter, ratio axon diameter-myelin sheath thickness and perimeter of fibers increased in the diabetic nerves. It is suggested that primary Schwann cell lesion is responsible for the observed myelin reduction.     

Coated glass and vicryl microfibers as artificial axons

The complex interactions that occur between oligodendrocytes and axons during the process of central nervous system myelination and remyelination remain unclear. Elucidation of the cell-biological and -biochemical mechanisms supporting myelin production and elaboration requires a robust in vitro system that recapitulates the relationship between axons and oligodendrocytes in a manner that is open to molecular dissection. We provide evidence for an artificial axon culture system in which we observed oligodendrocytes extending large plasma membrane projections that frequently completely ensheathed fibers coated with a variety of extracellular matrix molecules. These membrane projections varied in extent and thickness depending upon the substrate and upon the diameter of the coated fiber. Matrigel-coated glass microfibers were found to support the development of thick membrane sheaths that extended for hundreds of microns and exhibited many features suggestive of the potential for true myelin deposition. Likewise, Matrigel-coated Vicryl fibers supported plasma membrane extensions that covered extremely large surface areas and occasionally wrapped the coated Vicryl fibers in more than one membrane layer. Our findings suggest that the deposition of molecular cues onto glass or polymer fibers either via adsorption or chemical modification may be a useful tool for the discovery or validation of axonal factors critical for myelination and remyelination. Herein, we provide evidence that polyglactin 910 and glass microfibers coated with adhesion factors may provide a reasonable system for the in vitro analysis of myelination, and may eventually serve a role in engineering artificial systems for neural repair.     

Axo-glial relations in the retina-optic nerve junction of the adult rat: electron-microscopic observations

Summary The retina-optic nerve junction (ROJ) was examined by electron microscopy in adult rats, with particular emphasis on the unmyelinated-myelinated nerve fibre transition. Both single sections and serial sections were used. The non-retinal part of the ROJ is covered by an extensively folded glia limitans, facing the choroidea, sciera and pia mater. The blood vessels within the ROJ follow a transverse course and are surrounded by unusually wide perivascular spaces with a glia limitans-like outer delimitation. The endothelial cells exhibit numerous pinocytotic vesicles on their abluminal aspect. In the unmyelinated part of the ROJ the axons are embedded in an extensive meshwork of fibrous astrocytic processes. Some unmyelinated axons exhibit patches of axolemmal undercoating with externally associated astrocytic processes. Typical oligodendrocytes are not found, but a few small dark glial cells of unknown identity can be observed. Atypical ensheathment and myelination of axons at this level by ectopic Schwann cells occurred in one case. In the transition segment of the ROJ a pattern similar to that along dysmyelinated axons is observed, including aberrant axo-glial contacts, unusually thin and short myelin sheaths, intercalated unmyelinated segments, distorted myelin termination regions, bizarre paranodes and myelin termination regions without associated nodally differentiated axolemma. Neither sheath length nor number of myelin lamellae is related to axon diameter in the transition region. Axon diameter tends to be somewhat larger at myelinated than unmyelinated levels of the same axon. We suggest that the unusual axo-glial relations in this region are due to a deficient proliferation and differentiation of oligodendroglial cells, and that the pattern of glial ensheathment in the ROJ might be a consequence of the locally deficient blood-brain barrier.     

Chromatolysis of A-cells of dorsal root ganglia is a primary structural event in acute acrylamide intoxication

To test the hypothesis that a somatofugal wave of atrophy moving distally in the axon of primary sensory neurons leads to loss of myelinated nerve fibers in acrylamide neuropathy, rats (N = 18) were intoxicated with an initial dose of 75 mg acrylamide per kg body weight followed by daily treatment with 30 mg/kg for three, six and 12 days. Ten age matched saline treated rats served as controls. Numbers and mean volumes of A- and B-cell perikarya of the L5 dorsal root ganglion, numbers of myelinated axons and the mean cross sectional myelinated axon area 3 and 18 mm from the ganglion in the dorsal root and in the sural nerve were estimated using stereological techniques. After three days no changes in the number or size of primary sensory perikarya or myelinated axons were observed. However, after six days 11% of the A-cell perikarya showed signs of chromatolysis (P < 0.001). After 12 days the rats showed signs of ataxia and 23% (P < 0.001) of A-cell perikarya were chromatolytic. There was a tendency for atrophy of the mean perikaryal volume of A-cells (2P = 0.059). The size-frequency distributions of axonal area of myelinated fibers in the dorsal root 3 mm from the ganglion were displaced to the left towards smaller sizes (25–50% quartile: 2P < 0.005 and 75–100% quartile: 2P < 0.05). In conclusion, the primary structural event in acute acrylamide intoxication is chromatolysis of A-cells of the dorsal root ganglion without the occurrence of somatofugal axonal atrophy.     

The number of frog sciatic axons increases continually during body growth

Summary Frog sciatic nerves show a continuing addition of new myelinated and nonmyelinated fibers with body growth. Along with increases in fiber number there are also marked changes in axon calibers and in the relative thickness of the myelin sheaths. Our data show that frog nerves, as opposed to mammalian nerves, are composed of fibers of different ages, stages of growth and stages of myelination.This study was supported by grant 609 from the Deutsche Forschungsgemeinschaft     

A strong myelin thickness-axon size correlation emerges in developing nerves despite independent growth of both parameters

The axon determines whether or not it is myelinated by the Schwann cell. At maturity there is a positive correlation between sheath thickness and axon calibre. This correlation is initially very low or absent, but gradually strengthens during development. This increase could come about because the axon continuously controls Schwann cell myelinating activity, so that a given axon calibre is associated with a particular myelin sheath thickness, an interaction which would entail the Schwann cell continuously monitoring and responding to axon size. This seems unnecessarily complex. This theoretical study shows that the strong correlation between the 2 parameters within a given myelinated fibre population may come about in a much simpler way than outlined above. This is demonstrated by modelling the growth and myelination of a hypothetical population, utilising data from earlier studies on cervical ventral motoneuron axon development. The hypothesis tested shows that the only instructive interactions by the axon on the Schwann cell necessary for the strong correlation between the 2 parameters to emerge are for the initiation of myelination, its continuation and its termination. These could result from a single stimulus being switched on, persisting for a time and being switched off. Under this influence, the Schwann cell is assumed to proceed to form the myelin sheath at a constant rate which it itself inherently determines, in the absence of any quantitative influence exerted by the axon. This continues until the stimulus for myelination ceases to emanate from the axon. The validity of the hypothesis is demonstrated, because the resulting myelin-axon relationships correspond closely to those observed during development.     

Double myelination of axons in the sympathetic nervous system

Summary Relationships between axons and Schwann cells in myelinated fibres of the superior cervical (sympathetic) ganglion have been examined in normal adult rats. In cross-sections through the ganglion up to 4 % of myelinated fibres were focally encircled by an additional myelinating Schwann cell, forming regions termed double myelination. In these regions and elsewhere in the ganglion, the structure of the inner fibre (axon and myelinating Schwann cell) conformed to the relationships expected on the basis of numerous previous investigations on normal peripheral nerve. However, the outer Schwann cell and myelin sheath, which formed an annulus around the inner fibre, was remarkable in that it apparently made no direct contact either with the centrally enclosed axon or with any neighbouring axon, yet appeared largely if not completely intact. In addition, the increasing frequency of double myelination in older animals and the rarity of myelin degeneration in the same ganglia indicate that the outer Schwann cell, and in particular its myelin sheath, persist for some period in an isolated form. Double myelination was not located in non-sympathetic peripheral nerve samples from the same animals. Double myelination may result from the displacement of one myelin internode by the interposition of another Schwann cell rendering the original Schwann cell redundant. There was no involvement of haematogenous cells as occurs in some demyelinating conditions. While some parallels may be found with previous studies, this would appear to be the first report of apparent survival of myelin in a Schwann cell not making, as far as could be determined in the present study, at least partial direct axonal contact. These observations on sympathetic nerve may provide a new perspective on axon-Schwann cell signalling.     

The Rabbit Brachial Plexus as a Model for Nerve Repair Surgery—Histomorphometric Analysis

One of the most devastating injuries to the upper limb is trauma caused by the avulsion. The anatomical structure of the rabbit's brachial plexus is similar to the human brachial plexus. The aim of our study was to analyze the microanatomy and provide a detailed investigation of the rabbit's brachial plexus. The purpose of our research project was to evaluate the possibility of utilizing rabbit's plexus as a research model in studying brachial plexus injury. Studies included histomorphometric analysis of sampled ventral branches of spinal nerves C5, C6, C7, C8, and Th1, the cranial trunk, the medial part of the caudal trunk, the lateral part of the caudal trunk and peripheral nerve. Horizontal and vertical analysis was done considering following features: the axon diameter, fiber diameter and myelin sheath. The number of axons, nerve area, myelin fiber density and minimal diameter of myelin fiber, minimal axon diameter and myelin area was marked for each element. The changes between ventral branches of spinal nerves C5–Th1, trunks and peripheral nerve in which the myelin sheath, axon diameter and fiber diameter was assessed were statistically significant. It was found that the g‐ratio has close value in the brachial plexus as in the peripheral nerve. The peak of these parameters was found in nerve trunks, and then decreased coherently with the nerves travelling peripherally. Anat Rec, 298:444–454, 2015. © 2014 Wiley Periodicals, Inc.     

东方对虾有髓神经纤维的个体发生的电镜观察

用电镜技术观察了东方对虾（Ｐｅｎａｅｕｓｏｒｉｅｎｔａｌｉｓ）有髓神经纤维的髓鞘化，以及位于其髓鞘与轴突间被命名为髓鞘下间隙和微管鞘这两个侍有结构的个体发生过程。结果如下：（１）在无节幼体（ｎａｕｐｌｉｕｓ）已有由神经历细胞和神经胶质母细胞构成的神经成分出现；（２）在状幼体（ｚｏａｅａ）神经细胞发生裸露的突起，其直径不等，走向不一；（３）在糠虾幼体阶段（ｍｙｓｉｓｓｔａｇｅ）已形成了与成体虾相似的腹神经索，其中的一对内侧巨大纤维已出现，并开始髓鞘化。在此发育阶段后期这两根巨大纤维的髓鞘开始脱离轴突，形成间隙，并在轴突周围开始出现由微管束构成的鞘。在此发育阶段虽已可辨认出一对外侧巨大纤维，但通常尚未髓鞘化；（４）在仔虾（ｐｏｓｔ－ｌａｒｖａ）阶段神经索中许多较粗神经纤维开始髓鞘化，也是由新生的髓鞘直接包绕轴突，然后与之脱离形成髓鞘下间隙和形成直接复盖轴突的微管鞘；（５）髓鞘的形成不是如在脊椎动物髓鞘通常所见到的那样，由一许旺细胞的浆膜片以螺旋方式包绕轴突而形成的，对虾髓鞘的形成则是由许旺氏细胞向两侧伸出许多浆膜片以同心方式包绕轴突，当两侧的浆膜片在某处相遇时，均形成内含２至数根微管的终扣（ｔｅｒｍｉｎａｌｌｏ?     

Interactions between Schwann cells and CNS axons following a delay in the normal formation of central myelin

Summary Irradiation of the rat spinal cord during the first postnatal week results in a profound reduction of oligodendrocyte myelin formation in the dorsal funiculi (DF). Despite this absence of myelin, however, axons in the irradiated region in the DF increase in diameter and approximate the size distribution seen in the control spinal cord. By 25 days of age Schwann cells are present in the irradiated DF where they undergo cell division and myelinate the axons. During the early stages of this myelin formation, these intraspinal Schwann cells exhibit a relationship to axons that is somewhat different from that seen in the normal developing peripheral nervous system (PNS). For example, within a given region, intraspinal Schwann cells myelinate axons of large diameter prior to ensheathing bundles of small diameter axons. Additionally, during myelination a Schwann cell will surround a single axon with multiple processes which appear to compete for contact with the axolemma. On axons of larger diameter, the elaboration of these processes is so excessive that it is often difficult to trace them back to the parent Schwann cell. Later, when a single process establishes several spirals about an axon, additional processes are no longer elaborated, and the extra processes disappear as myelin formation advances to the stage of compact lamellae. Thereafter, the myelin sheath continues to form in a normal manner. Excess processes have been observed during myelinogenesis in the normal developing PNS, but their frequency in that environment is much less than in the irradiated cord. These observations support the hypothesis that the signal(s) to initiate myelin formation are expressed on the axolemmal surface and are controlled by the neuron. In addition, these observations suggest that the delay in myelination results in an affinity or tropism between axons and Schwann cells which exceeds the level existing at the normal time of myelin formation.     

Immunocytochemical study of the appearance of P2 in developing rat peripheral nerve: comparison with other myelin components

Summary Using indirect immunofluorescence on both dissociated cell cultures and frozen sections from rat sciatic nerve, dorsal root ganglion and superior cervical ganglion, we have examined the development and distribution of the peripheral myelin protein P₂. It first appears in development in sciatic nerve and dorsal root ganglion on the first day after birth, at about the same time as P₀ and P₁. Like galactocerebroside, P₀ and P₁, P₂ disappears gradually from dissociated Schwann cells in culture. In adult sciatic nerve, and dorsal and ventral roots, it shows an uneven distribution and is absent from some myelinated axons. In electron micrographs the onset of myelination in the sciatic nerve occurs between the day of birth (day 0) and the first day after birth (day 1). Immunofluorescence studies on freshly dissociated cell suspensions, frozen sections and dissociated cell cultures at these early time points indicate that the myelin glycolipids galactocerebroside and sulphatide are present on the surface of many Schwann cells at least one day before myelination starts while the myelin proteins P₀, P₁ and P₂ are not detected until myelination begins. This suggests that the early appearance of galactocerebroside and sulphatide is an important step preceding the formation of compact myelin.