Intercellular junctions and other membrane specializations in developing spinal ganglia: a freeze-fracture study

Satellite Glial Cells: Morphology, functional heterogeneity, and role in pain

Neurons in the somatic, sympathetic, and parasympathetic ganglia are surrounded by envelopes consisting of satellite glial cells (SGCs). Recently, it has become clear that SGCs are highly altered after nerve injury, which influences neuronal excitability and, consequently, the development and maintenance of pain in different animal models of chronic pain. However, the exact mechanism underlying chronic pain is not fully understood yet because it is assumed that SGCs in different ganglia share many common peculiarities, making the process complex. Here, we review recent data on morphological and functional heterogeneity and changes in SGCs in various pain conditions and their role in response to injury. More research is required to decipher the role of SGCs in diseases, such as chronic pain, neuropathology, and neurodegenerative diseases.

The expression of prosaposin and its receptors, GRP37 and GPR37L1, are increased in the developing dorsal root ganglion

Prosaposin (PSAP), a highly conserved glycoprotein, is a precursor of saposins A-D. Accumulating evidence suggests that PSAP is a neurotrophic factor, as well as a regulator of lysosomal enzymes. Recently, the orphan G-protein-coupled receptors GPR37 and GPR37L1 were recognized as PSAP receptors, but their functions have not yet been clarified. In this study, we examined the distribution of PSAP and its receptors in the dorsal root ganglion (DRG) during development using specific antibodies, and showed that PSAP accumulates primarily in lysosomes and is dispersed throughout the cytoplasm of satellite cells. Later, PSAP colocalized with two receptors in satellite cells, and formed a characteristic ring shape approximately 8 weeks after birth, during a period of rapid DRG development. This ring shape, which was only observed around larger neurons, is evidence that several satellite cells are synchronously activated. We found that sortilin, a transporter of a wide variety of intracellular proteins containing PSAP, is strongly localized to the inner side of satellite cells, which contact the neuronal surface. These findings suggest that PSAP and GPR37/GPR37L1 play a role in activating both satellite and nerve cells.

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

The structure of the perineuronal sheath of satellite glial cells (SGCs) in sensory ganglia

In sensory ganglia each nerve cell body is usually enveloped by a satellite glial cell (SGC) sheath, sharply separated from sheaths encircling adjacent neurons by connective tissue. However, following axon injury SGCs may form bridges connecting previously separate perineuronal sheaths. Each sheath consists of one or several layers of cells that overlap in a more or less complex fashion; sometimes SGCs form a perineuronal myelin sheath. SGCs are flattened mononucleate cells containing the usual cell organelles. Several ion channels, receptors and adhesion molecules have been identified in these cells. SGCs of the same sheath are usually linked by adherent and gap junctions, and are functionally coupled. Following axon injury, both the number of gap junctions and the coupling of SGCs increase markedly. The apposed plasma membranes of adjacent cells are separated by 15–20 nm gaps, which form a potential pathway, usually long and tortuous, between connective tissue and neuronal surface. The boundary between neuron and SGC sheath is usually complicated, mainly by many projections arising from the neuron. The outer surface of the SGC sheath is covered by a basal lamina. The number of SGCs enveloping a nerve cell body is proportional to the cell body volume; the volume of the SGC sheath is proportional to the volume and surface area of the nerve cell body. In old animals, both the number of SGCs and the mean volume of the SGC sheaths are significantly lower than in young adults. Furthermore, extensive portions of the neuronal surface are not covered by SGCs, exposing neurons of aged animals to damage by harmful substances.

Aquaporin expression in normal and pathological skeletal muscles: a brief review with focus on AQP4

Freeze-fracture electron microscopy enabled us to observe the molecular architecture of the biological membranes. We were studying the myofiber plasma membranes of health and disease by using this technique and were interested in the special assembly called orthogonal arrays (OAs). OAs were present in normal myofiber plasma membranes and were especially numerous in fast twitch type 2 myofibers; while OAs were lost from sarcolemmal plasma membranes of severely affected muscles with dystrophinopathy and dysferlinopathy but not with caveolinopathy. In the mid nineties of the last century, the OAs turned out to be a water channel named aquaporin 4 (AQP4). Since this discovery, several groups of investigators have been studying AQP4 expression in diseased muscles. This review summarizes the papers which describe the expression of OAs, AQP4, and other AQPs at the sarcolemma of healthy and diseased muscle and discusses the possible role of AQPs, especially that of AQP4, in normal and pathological skeletal muscles.

Perikaryal surface specializations of neurons in sensory ganglia

Slender projections, similar to microvilli, are the main specialization of the perikaryal surface of sensory ganglion neurons. The extent of these projections correlates closely with the volume of the corresponding nerve cell body. It is likely that the role of perikaryal projections of sensory ganglion neurons, which lack dendrites, is to maintain the surface-to-volume ratio of the nerve cell body above some critical level for adequate metabolic exchange. Satellite cells probably have the ability to promote, or provide a permissive environment for, the outgrowth of these projections. It is not yet known whether the effect of satellite cells is mediated by molecules associated with their plasma membrane or by diffusible factors. Furthermore, receptor molecules for numerous chemical agonists are located on the nerve cell body surface, but it is not known whether certain molecules are located exclusively on perikaryal projections or are also present on the smooth surface between these projections. Further study of the nerve cell body surface and of the influence that satellite cells exert on it will improve our understanding of the interactions between sensory ganglion neurons and satellite neuroglial cells.

Differential anatomical and cellular patterns of connexin43 expression during postnatal development of rat brain

We have shown previously that connexin43 in the adult rat central nervous system (CNS) is predominantly localized at astrocytic gap junctions. Here we document immunohistochemically the emergence of connexin43-immunoreactive (connexin43-IR) structures and the regional patterns of connexin43 expression during postnatal maturation of the rat brain. On Western blots, connexin43 was detected in brain samples at postnatal day (P) 5, the earliest age studied. Immunohistochemically, most brain regions displayed a characteristic sequence of transient immunoreactive profiles that ultimately gave rise to the uneven distribution of the protein seen in adults. Generally, brains at P1-P5 exhibited long, fibrous connexin43-IR elements which were identified as radial glial cells. This fibrous immunostaining was considerably diminished at P5 and was replaced by short immunoreactive processes which predominated up to P10. These processes had a stellate appearance, emanated from partially stained astrocytic cell bodies and were heterogeneously distributed throughout the developing brain. By P15, there occurred only punctate immunolabelling similar to that seen in adult brain. Some brain regions including the amygdaloid complex, septohypothalamic nucleus, preoptic hypothalamus, zona incerta, ependyma and subfornical organ were exceptional in that they displayed adult immunostaining patterns at early postnatal ages suggesting a precocious maturation of gap junctions in these areas. We conclude that the highly heterogeneous distribution of connexin43-immunoreactivity among defined nuclear structures in adult brain does not reflect an antecedent requirement for connexin43 in early brain morphogenesis, but rather is related to the development of neuronal activity, the establishment of functional circuitry and the contribution of astrocytic gap junctions to glial metabolic coupling and potassium spatial buffering in the mature CNS.

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.

The earliest ultrastructural development of the tangential vestibular nucleus in the chick embryo

The tangential nucleus is a primary vestibular nucleus located where the vestibular fibers enter the medulla. It is composed of neurons that migrate between 5 and 8 days in the chick embryo. Although primary vestibular fibers enter the medulla at 3 days, the first synapses are formed at 5 days on the processes of neuron precursors by longitudinally coursing fibers. Since the major components, or their precursors, are present at 3 days within the presumptive nucleus, we are interested in determining what cellular interactions occur among these structures following their entry and during the time leading up to synapse formation. At 2 days, prior to the appearance of VIIth and VIIIth nerve fibers in the medulla, the tangential nucleus anlage contained processes and endfeet of primitive epithelial cells, separated from each other by enlarged extracellular spaces. Longitudinal fibers first appeared within these spaces coincident with the appearance of root fibers, including some identified VIIth motor axons, associated with the primordial VII/VIIIth ganglia. By 3 days, some vestibular and VIIth nerve fibers could be identified by their ultrastructure and relative positions within the marginal zone and nerve roots. However, it was not until 4 days that the presumptive tangential nucleus acquired its orderly, characteristic organization. Although synapses were rare from 2 to 4 days, attachment plaques and coated pits were observed commonly between structures, especially between future synaptic structures. Thus, we confirm that synapse formation begins at 5 days. This represents the first detailed ultrastructural study of cranial sensory nerve ingrowth into the medulla.

Developmental alterations in membrane organization of rat subpial astrocytes

Subpial astrocytic processes were examined in developing rats, mainly with complementary replicas, to see how orthogonal arrays of particles (OAs) are formed and become numerous in membranes covered by basal lamina. Only a few (4.2%) endfeet in the membranes contacting the basal lamina (subpial membranes) had acquired OAs by the 19-day foetal stage. The number of endfeet provided with OAs increased drastically in the prenatal period, continued to increase at birth (P0), and somewhat more slowly in the early postnatal period (P0-P3), reaching 100% at P10. There were neuronal processes as well abutting on the basal lamina at the pial surface but they were easy to distinguish from astrocytic endfeet because of their larger intramembrane particles (IMPs), which are sparsely distributed and in patch-like aggregations. The distribution density of OAs in differentiated astrocytic endfeet also increased very gradually with age until P0, a little faster in the early postnatal period, and drastically from P10 to adult. Ordinary globular IMPs increased in number with age and continued to increase in the lateral membrane where OAs were still very few, though less rapidly in the subpial membrane as OAs became numerous. With maturation, larger IMPs became conspicuous in the lateral membrane but not in the subpial, suggesting that larger IMPs were predominantly used to form OAs. We have proposed the idea that relatively large IMPs line up to form single linear arrays (SLs), appearing as grooves on the E face, and that occasionally some SLs line up in multiple rows [multiple linear arrays (MLs)] and that SLs or MLs fuse with one another to become rod-like strands, then divide into squares to become OAs. SLs and MLs appeared ontogenetically earlier than OAs, and continued to appear in membranes provided with OAs. In areas where membranes were bent, transition of these three structures was observable and the proportion of OAs increased with age. Further, in such areas, alignment of OAs was different according to membrane curvature: concentric in and around protrusions, perpendicular to the edge of invaginations. This unique association of OA alignment with membrane curvature suggests that OAs contribute to some membrane stability in the area covered by the basal lamina and provide the membrane with special resistance to bending.

Development of smooth muscle: ultrastructural study of the chick embryo gizzard

The growth and differentiation of smooth muscle in the chicken gizzard were studied by electron microscopy from the 10th day in ovo to 6 months after hatching; during this period the organ grows 1000-fold in weight. At the earliest stage studied, smooth muscle cells, interstitial cells, and fibroblasts are immature but can already be clearly distinguished. The structural components of muscle cells develop in a characteristic sequence. Mitochondria are more abundant in immature muscle cells (8% in 14 days embryos and 7% in 19 days embryos) than in the adult (5%). Caveolae are virtually absent in the 11 day embryo; they become more common at the end of embryonic life, but continue to increase in relative frequency after hatching. Gap junctions appear around the 16th day in ovo as minute aggregates of connexons, which then grow in size, probably by addition of new connexons. In the earliest stages studied, myofilaments occupy 25% of the cell profile and are assembled into bundles accompanied by dense bodies and surrounded by loosely arranged intermediate filaments. By contrast, membrane-bound dense bands are scarce until the latter part of embryonic life, an observation suggesting that myofilament formation and alignment is not a process initiated near the cell membrane or directed by the cell membrane, and that only late in development bundles of myofilaments become extensively anchored to dense bands over the entire cell surface: at that time myofilaments occupy more than 75% of the cell volume. The muscle cells increase about four-fold in volume over the period studied; the 1000-fold increase in muscle volume is mainly accounted for by an increase in muscle cell number. Mitoses are found in the gizzard musculature at all embryonic ages with a peak at 17-19 days; they occur in muscle cells with a high degree of differentiation. These cells divide at a stage when they are packed with myofilaments and form junctions with neighbouring cells: the mitotic process affects the middle portion of the cell, which takes up an ovoid shape and eventually divides, whereas the remaining portions of the cell do not differ in appearance from the surrounding muscle cells. At all stages of development the population of muscle cells has a uniform appearance (apart from the cells in mitosis), and the growth and differentiation seem to proceed at the same pace in all the cells. There are no undifferentiated cells left behind in the tissue for later development.

A freeze-fracture study on the developing satellite cells of spinal ganglia in the chick embryo

A freeze-fracture analysis of the satellite cells of spinal ganglia of the chick embryo was performed in 8 successive stages of development, from the 5th incubation day to hatching. The characteristic laminar disposition of the cells were first observed on the 7th day. Tight junctions were found at the 20th incubation day. Small groups or irregular aggregates of particles, but not gap junctions, were described on the 7th and 8th days. Pinocytotic vesicles were pointed out in the different stages considered.

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.

Orthogonal arrays of particles in the gastric parietal cell of the rat: differences between superficial and basal cells in the gland and after pentagastrin or metiamide treatment

The basolateral plasma membrane of gastric parietal cells is characterized by the presence of orthogonal arrays of particles revealed by the freeze-fracture technique. These arrays were quantitatively assessed in freeze-fracture replicas of the gastric mucosa at two different levels of the gastric gland and after pentagastrin and metiamide treatment. The arrays were small and scarce in parietal cells located in the upper part of the gland, while they were markedly more abundant in parietal cells situated at the base of the gland. In both superficial and basal cells, the concentration of the arrays was significantly decreased after pentagastrin or metiamide treatment. This decrease was not due to an increase in the surface area of the basal plasma membrane. These results indicate that the concentration of the orthogonal arrays is a distinctive feature between superficial and basal parietal cells and that the arrays can be modulated by parietal cell function.

Distribution patterns of orthogonal arrays and alkaline phosphatase in plasma membranes of satellite cells in rat spinal ganglia

The plasmalemmal structure of satellite cells in the lumbar spinal ganglia of rat was examined by freeze-fracture and by a cytochemical method for the demonstration of alkaline phosphatase activity. Plasma membranes of satellite cells are the only ones in the ganglia to contain, in addition to globular intramembrane particles, orthogonal arrays of particles 6-7 nm in diameter. The arrays are most concentrated in the portions of the membranes contacting the basal lamina, or outer membranes; they decrease considerably in number in lateral membranes, and are rare in the membrane regions adjacent to the neuronal perikaryon, or inner membranes. Such gradual decrease in array density in satellite cells suggests regional differences of plasma membrane properties within the same cell. Alkaline phosphatase, which was chosen as a cytochemical marker for membrane activity because of its relation to transport function, localizes to inner and lateral membranes, and not to outer membranes of satellite cells. The absence of correlation between localization of orthogonal arrays and such enzymatic activity suggests that the membranes provided with many arrays possess some characteristics different from other membranes that may exclude transport activity. The possible significance of orthogonal arrays and their close association with the basal lamina are discussed.

Plasma membrane organization of astrocytes in elasmobranchs with special reference to the brain barrier system

The structural machinery contributing to the blood-brain barrier in elasmobranchs has been examined mainly using freeze-fracture techniques. Capillary endothelial cells, which show local aggregations of pinocytotic vesicles and infrequent fenestrations, are connected by poorly developed tight junctions. Astrocytic processes investing the capillary are linked by well-developed tight junctions between lateral membranes immediately beneath the perivascular space. The tight junctions consist of continuous strands of multiple layers coursing circumferentially around the astrocytic processes parallel to one another as well as to the perivascular space. The presence of intramembrane particles (IMPs) within E-face grooves may result in discontinuities in IMP rows on the P-face. Thus, in compensation for the capillary endothelium, perivascular astrocytes constitute the morphological site of the blood-brain barrier in elasmobranchs. Continuous strands of tight junctions are also detected between astrocytic processes forming the glia limitans at the brain surface. These may act as a barrier between meningeal connective tissue and brain parenchyma. Astrocytic membranes have numerous IMPs of 8-9 nm in diameter on their P-faces. These IMPs are uniformly distributed so that astrocytic membranes are easily distinguished from neuronal membranes even in the neuropil. Ependymal cells also have numerous IMPs in all their membrane domains. Orthogonal arrays are not detected in either astrocytic or ependymal plasma membranes.

Particle assemblies in astrocytic plasma membranes are rearranged by various agents in vitro and cold injury in vivo

Distinct aggregates of small intramembranous particles and assemblies characterize the P-face of freeze-fractured astrocytic membranes. To test the lability of the assemblies, astrocytes were treated in vitro with different chemical agents and in vivo by cold injury. The assemblies appeared either to contain or be associated with protein because exposure to medium containing cycloheximide, an inhibitor of protein synthesis, led to a sharp decrease in assemblies, down to 1% of the control levels within three hours. To ascertain whether the assemblies were tethered to the cytoskeleton, the cells were treated in vitro with disruptors of microtubules (colchicine) or microfilaments (cytochalasins); the assemblies became consistently rearranged. Protein denaturants, urea and guanidine HCl, brought about a selective aggregation of assembly with assembly. The lectin, concanavalin A, did not alter the distribution of the assemblies within the plane of the membrane fracture. Surface replicas of in vitro, non-fractured, astrocytes revealed surface particles which did not resemble assemblies. In vivo, the plasma membranes of astrocytes were altered within minutes of cold injury to the brain surface. In the centre of the lesions, damaged astrocytes had assemblies that were clumped like those of in vitro astrocytes exposed to denaturants. In the periphery of the lesions, however, the assemblies did not aggregate but increased in number. These results provide indirect evidence that assemblies may consist of protein, that the recognizable particle constituent of the assembly is confined to the interior of the membrane and is not present on the uncleaved cell surface, and that assemblies are connected with the cytoskeleton. Therefore, certain changes in the environment of the astrocyte caused by injury in vivo or addition of chemical agents in vitro alter the distribution of assemblies in the astrocytic plasma membrane either by a direct effect on the assemblies or indirectly by an alteration of the cytoplasmic proteins.

Tight junction contact events and temporary gap junctions in the sciatic nerve fibres of the chicken during Wallerian degeneration and subsequent regeneration

Tight and gap junctions are described on the basis of freeze-fractures in normal chicken sciatic nerves as well as during Wallerian degeneration and subsequent regeneration. 1. Small calibre nerve fibres display a fairly continuous tight junction contact zone in the membranes of the mesaxons, paranodal loops and Schmidt-Lanterman incisures. Large fibres with more than 40 lamellae have only focal tight junction contacts in the mesaxonal membranes. 2. With the onset of Wallerian degeneration (days 2-4 post-crush, distal stump) myelinic tight junctions become arranged as maculae composed of one circular or several polygonally oriented strands that are criss-crossed by other tight junctional strands. These maculae are subsequently found in the membranes of cytoplasmic vacuoles of the Schwann cells, indicating an endocytotic mode of uptake. Tight junctions are not found between the 5th and 6th day after crush. 3. During the proliferation phase of the Schwann cells and the arrangement of these cells into Büngner cell bands (2 to 8 days post-crush) gap junctions appear between the Schwann cells of the bands. These junctions then disappear with the onset of remyelination (8 days post-crush). 4. With the onset of remyelination (from the 8th day onwards) short focal tight junctions appear in the membranes of the outer mesaxons. Shortly thereafter, when the sheaths possess 4 to 8 lamellae, tight junctions also appear in the membranes of the inner mesaxons, the paranodal loops and the cytoplasmic inclusions. The characteristic differences of tight junction elaboration in small versus large nerve fibres are re-established after three months of regeneration. The elaborated tight junctions in small and early remyelinating fibres point to a specific function; in small fibres (versus large fibres) the tight junctions might effect a separation of the intramyelinic extracellular space as a single compartment. The tight junction contacts in early remyelinating fibres support the hypothesis that myelin growth occurs within the myelin spiral and not by a free rotation and elongation of the Schwann cell tongues. It is assumed that the gap junctions between the Schwann cells contribute to the co-ordination of the Schwann cell band formation, which is involved in the guidance of sprouting axons.

The development of sensory projection patterns in embryonic chick hind limb

1. The distribution within individual dorsal root ganglia (d.r.g.s) of sensory neurones projecting to different targets in the embryonic chick hind limb was determined using the retrograde transport of horseradish peroxidase (HRP). The segmental pattern of sensory neurone projections was also defined, using retrograde and orthograde HRP labelling and electrophysiological techniques, from the onset of axonal outgrowth into the limb until after the period of sensory cell death. 2. At stage (St.) 29-30, shortly after initial axonal outgrowth into the limb, the large lateroventral neurones in the d.r.g.s projected both to skin and to muscle. At St. 36-37, after cell death in the d.r.g.s, cells from both the lateroventral and mediodorsal populations projected both to skin and to muscle. Thus these two cell populations do not correspond to cutaneous and proprioceptive afferents, respectively. 3. The cells projecting along individual cutaneous nerves or to individual muscles were always widely distributed throughout the ganglia. Thus, sensory neurones cannot be specified to project to particular peripheral targets as a result of their position in the d.r.g. Nevertheless, small clusters of cells were frequently found to project along the same peripheral nerve. 4. Since there is a correlation between position in the d.r.g. and time of origin, neurones projecting to each target have a wide range of birthdates, and, therefore, sensory neurones cannot be specified as a result of their birthdate. 5. At St. 36-38, after cell death, afferents to a given muscle or cutaneous nerve arise primarily from two or three adjacent segments out of the eight lumbosacral segments. For muscles these are the same segments that supply the motoneurones to that muscle. Each d.r.g. sends a characteristic proportion of axons down each of several peripheral nerves in a consistent and orderly pattern. 6. During initial outgrowth, the segmental projection pattern is similar to the pattern found in mature embryos. Thus, extensive projection 'errors' are not made and neither cell death nor retraction of axons is necessary for establishing the appropriate connectivity pattern. The majority of neurones do not send branches down more than one peripheral nerve. 7. Axons projecting to the same target are initially dispersed in the spinal nerves, and gradually segregate out in the plexus region, ultimately to form a separate nerve trunk. Axons projecting to different targets cross each other. Cutaneous and muscle nerves first form at the same stages. Therefore, the particular pathways axons take do not depend in any simple way on either axonal position in the plexus or time of arrival at the base of the limb. Simple timed outgrowth mechanisms and models in which axons maintain constant topographical relationships with each other therefore cannot generate the observed projection pattern. 8...

Electrotonic synapses are formed by fetal rat sympathetic neurons maintained in a chemically-defined culture medium

Principal neurons from the superior cervical ganglia of rat fetuses were maintained for up to 101 days in dissociated cell cultures in a serum-free, chemically-defined medium; non-neuronal cells were killed by the periodic addition of fluorodeoxyuridine to the medium. Intracellular recordings, obtained at various times between 16th and 98th day in vitro, showed that these neurons could generate substantial (up to 90 mV) action potentials in response to depolarizing current injections; these responses were dependent on tetrodotoxin-sensitive Na+ channels, cobalt-sensitive Ca++ channels, and tetraethylammonium-sensitive K+ channels. Action potentials were often followed by prominent, long hyperpolarizing after-potentials (10-15 mV, greater than 150 ms); the duration of these after-potentials was reduced by the addition of Co++ (2-5 mM) to the perfusate. Acetylcholine depolarized these neurons by a hexamethonium-sensitive mechanism. To determine whether sympathetic neurons formed synapses in a defined medium, intracellular recordings were obtained from pairs of neighboring neurons. Synaptic interactions were frequently observed at all times in vitro (up to 60% of all pairs tested). At many synapses, both hyperpolarizing and depolarizing DC potential changes spread from one neuron to another. At other synapses, the spread of DC potential changes could not be directly demonstrated; however, interactions at such synapses were not inhibited by antagonists of several neurotransmitters, by elevation of the Mg++/Ca++ ratio, or by the addition of Co++. Thus most, if not all, of the synaptic interactions among sympathetic neuron were electronic; such electrical synapses were not observed among dorsal root ganglion neurons maintained in the same medium. These data indicate that, when maintained in a chemically-defined culture medium, sympathetic neurons of rat fetuses express many of the basic membrane properties observed in neurons of superior cervical ganglia recently removed from adult rats. However, fetal sympathetic neurons maintained in this defined medium also differ from their counterparts in vivo; they adopt a mode of synaptic transmission (electrical) that has not been observed in the sympathetic ganglia of the adult rat. Thus, as late as the 21st embryonic day, not only the choice of neurotransmitter, but also the mode of transmission has not been irrevocably determined in sympathetic neurons.

Changes in number and distribution of orthogonal arrays during postnatal muscle development

Quantitative freeze-fracture electron microscopy was utilized to study the changes in number and distribution of orthogonal arrays (aggregates of 6-7 nm particles) of the sarcolemmas of the fast twitch extensor digitorum longus (EDL) and slow twitch soleus muscles during the first few weeks of postnatal development in the rat. In the adult rat, orthogonal arrays are present in high densities in the fast twitch type II fibers but only in low densities in slow twitch type I fibers. In this study, the changing histochemical profiles of fiber types in the EDL and soleus muscles were also determined for the first month of postnatal development and correlated with the changing number and distribution of orthogonal arrays during the same time frame. At day 3 postpartum, EDL and soleus fibers possessed few orthogonal arrays. The developing EDL fibers rapidly acquired additional orthogonal arrays until the approximate adult number and distribution were attained at postpartum day 25. In contrast, the slow twitch soleus fibers rapidly acquired orthogonal arrays and type IIA fibers until day 35 when both were in excess of adult values. Subsequently, the number of arrays and type IIA fibers declined to normal adult ranges. We suggest that the patterns of development of orthogonal arrays and fiber types are different in the EDL and soleus because the types of innervating motor units are different in the two muscles. The EDL is innervated almost entirely by fast motor units throughout early development and maturity. The soleus, however, is initially innervated by a more heterogeneous population of motor neurons. Thus, during the period of polyneuronal innervation which occurs normally during the first weeks of postnatal development, many individual soleus fibers may possess simultaneous innervation by axons from different motor neuron types. These dual influences may be responsible for the irregular pattern of development of orthogonal arrays and type IIA fibers in developing soleus fibers. Later, as the adult pattern of monosynaptic innervation is developed, expected adult values of orthogonal arrays and fiber types are attained.

Effects of colchicine on gap junction formation during retinal neurogenesis

Colchicine, injected into the amniotic cavity of 3 to 10 day-old chick embryos, has the following effects on the developing retina: 1. Cells in arrested metaphase accumulate in the ventricular portion of the matrix region. If colchicine is applied at the end of the first week of incubation, the kinetic migration of the cell nuclei is inhibited. 2. The cells lose their typical slender, bipolar shape to become ovoid to spherical. These morphological changes are more obvious when colchicine acts during the first week of incubation. 3. The formation of temporary gap junctions between the matrix cells of the retina occurs during the first week of development. When colchicine is administered around the 3rd day of incubation, the assembly of intramembranous particles (IMPs) during gap junctions formation is incomplete. These results suggest that an intact microtubular system within the submembranous cytoskeleton is essential for the assembly of these intercellular contacts.

Freeze fracture studies of muscle plasma membrane in human muscular dystrophy

Freeze fracture analysis of intramembranous particle density in skeletal muscle plasma dystrophy from 7 patients with Duchenne muscular dystrophy (DMD), 5 patients with facioscapulohumeral muscular dystrophy (FSH) and 5 patients with myotonic dystrophy (MyD) were carried out. Marked depletion of intramembranous particles including orthogonal arrays was noted in DMD while only orthogonal arrays were significantly decreased in FSH. No abnormalities were noted in MyD.

Synaptogenesis in the vestibular sensory epithelium of the chick embryo

The formation of synapses between sensory cells and the terminals of afferent axons has been examined in the embryonic chick labyrinth. Neurites initially cross the otocyst basal lamina and ramify among the undifferentiated epithelial cells by stage 25 Hamburger and Hamilton. At the same time granular vesicles, with diameters averaging 130nm, appear in the basal cytoplasm of a few of the epithelial cells. These vesicles often exist in groups at sites contact with; neurites. By stages 27-28, non membrane-bound densities are frequently found in association with groups of granular vesicles at the plasma membrane. Smaller, clear synaptic vesicles are also a prominent component of these arrangements in presumptive hair cells. Presynaptic ribbons opposite postsynaptic densites are identifiable at about stage 28, and their number increases during subsequent embryonic stages. Specialized appositions, including adherent, postsynaptic and possibly gap junctional contacts, join epithelial cells and nerve terminals throughout this period. The distribution of these junctions is variable, and is not necessarily correlated with the sites of formation of presynaptic ribbons. By stage 32, well-developed chemical synapses consisting of presynaptic ribbons witah vesicle halos and postsynaptic densities are common features of hair cell-afferent nerve terminal contact regions. In addition, possible sites of gap junctional contact between adjacent intra-epithelial nerve endings found at stage 32 presage those found in the cristae and maculae of pre-hatch (stage 45) embryos and adults.

Assemblies of particles in the cell membranes of developing, mature and reactive astrocytes

Orthogonal arrays of small intramembranous particles characterize freeze-fractured astrocytic plasma membranes. The normal variation of assemblies in plasma membranes of subpial astrocytic processes in mature and developing rats was established and compared with assemblies in plasma membranes of reactive astrocytes. In mature rats, subpial astrocytic processes had the greatest number of assemblies. As the parenchyma was approached, this number decreased in each successive layer of astrocytic processes. The advent of assemblies within the plasma membrane is a new criterion of astrocytic differentiation. The foot processes of subpial astrocytes in foetal rats began to acquire assemblies between day 19 and 20 and continued to mature postnatally by a constant addition and rearrangement of assemblies. In contrast to the paucity of assemblies in deeper laminae of the normal brain, reactive astrocytes comprising the lower lamellae in glial scars had an increased number of assemblies while the most striking feature of the superficial, astrocytic processes within the scar was a rearrangement of assemblies. Although the function of these intramembranous particles in still unknown, it appears from our measurements that they are localized primarily in the outermost astrocytic foot process and to a progressively lesser degree in the underlying astrocytic lamellae.

Intercellular junctions and the development of the blood-brain barrier in Manduca sexta

In early embryonic development of the tobacco horn moth no blood-brain barrier is present, as shown by the unimpeded entry of exogenous tracers into the nervous system. However, later on, just before hatching, lanthanum and horseradish peroxidase (HRP) are unable to move inwardly beyond the level of the perineurium, which is the morphological site of the blood--brain barrier in the adult moth, as well as in other insects. Freeze-fracture studies indicate that in the early embryo, 10 nm particles are scattered about in the perineurial membrane PF, either as separate entities or as short linear arrays. By hatching or just before, however, the 10 nm particles have become aligned into lengthy linear aggregates as PF ridges with EF grooves. These would appear to be the simple, arthropod-form of tight junction, and are presumed to be the basis of the perineurial blood-brain barrier. At about the same time, gap junctional elements appear both between adjacent perineurial cells and between glial cells. In both cell types, the gap junctions form from free 13 nm EF particles which gradually become aligned or clumped into strands and aggregates which ultimately coalesce to form first irregular masses and then the macular plaques typical of mature gap junctions. Many of the latter stages are coincident with the hatching of a motile larvae, so that the perineurial and glial cells are by this stage coupled via the channels of the gap junctional particles. They are therefore able to undergo both ionic and metabolic exchange and cooperation during larval life, in addition to being able to respond to hormonal substances in an integrated way. During the 5 larval instars more gap junctions form as the perineurial layer grows thicker. These junctions become more regular in outline and their particles more tightly packed; these larval structures are compared with junctions found in the adult which tend to be more extensive but otherwise similar. Since no septate junctions are apparent during Manduca embryonic or larval life when the blood-brain barrier forms, nor in adults, the results of this report support the contention that it is the tight junctions rather than septate ones which form the basis of permeability barriers in this, and probably other, arthropod systems.

Communicating junctions of the human sensory retina. A freeze-fracture study

Using the freeze-fracture technique communicating junctions could be identified between the cells of the adult human sensory retina. Communicating junctions with particles arranged in linear or circular rows as well as in small aggregates were found between adjacent photoreceptor endings. Communicating junctions with particles forming plaques were localized in the inner plexiform layer. They were isolated between unidentified cell processes or lay close to active sites of chemical synapses, thus suggesting the occurrence of "mixed" (chemical and electrical) synapses in this layer. Furthermore, communicating junctions were detected between portions of Müller's cells.

The development of a zonula occludens in peripheral myelin of the chick embryo. A freeze-fracture study

Sciatic nerves of chick embryos, 12 to 18 days incubation, were examined in freeze-fracture replicas with special emphasis placed on the development of tight junctional contacts in the myelin sheaths. In stages of beginning myelination short isolated particulate chains (focal tight junctions) appear in fracture faces of the adjacent membranes in the outer myelin lamellae, i.e., the outer mesaxon. In stages of progressing myelination these tight junctional elements elongate and become more numerous. They can also be found in the membranes of the inner mesaxons, the paranodal loops and the intramyelinic cytoplasmic inclusions. In fibers of advanced myelinogenesis a fusion of these isolated tight junctions--either end-to-end or at an angle--gives rise to continuous zonulae occludentes. This contact zone extends in the mesaxonal membranes along the direction of the fiber, whereas in paranodal myelin it acquires a helical course joining the membranes of the paranodal loops. It is proposed that this zonula occludens, which seals the cytoplasmic border of the Schwann cell, separates an intramyelinic from an extramyelinic, extracellular space already during the developmental stages of myelinogenesis.