Modeling and competition in the nervous system: clues from the sensory innervation of skin

Ceramide is responsible for the failure of compensatory nerve sprouting in apolipoprotein E knock-out mice

Apolipoprotein E (apoE) is a key transporter of the cholesterol and phospholipids required for membrane synthesis and nerve growth. We now report a virtual absence in apoE knock-out (KO) mice of normal nerve growth factor (NGF)-driven compensatory sprouting of undamaged cutaneous nociceptive nerves. In contrast, NGF-independent regeneration of crushed axons was unaffected. Essentially similar results came from aged wild-type mice. In apoE KO mice, the endogenous sprouting stimulus was suspect, because NGF administration induced normal sprouting; nevertheless, NGF increased normally in denervated skin, transported normally in the axons, and led to phosphorylation of trkA, erk1, and erk2. However, sprouting was restored in apoE KO mice (although not in aged mice) by fumonisin B1, an inhibitor of ceramide synthesis. A shotgun analysis revealed a wide array of changes in individual ceramide species in DRG neurons of apoE KO mice, and the changes for ceramide species OH_N15:0 made it a candidate inhibitor of sprouting (increased in apoE KO mice and normalized by fumonisin B1). Nevertheless, the unknown effects of individual ceramide species on sprouting, as well as the variability of their changed levels in apoE KO mice and how these were affected by fumonisin B1, support a different conclusion. We suggest that absence of apoE expression alters the balance among ceramide species to one that collectively inhibits compensatory sprouting, whereas fumonisin B1 establishes a new balance that allows sprouting. Nontoxic ceramide modulators might usefully promote sprouting and circuitry repair in neurodegenerative disorders in which ceramide species are perturbed, adding to the benefits of reducing ceramide-induced neuronal apoptosis.

Nerve growth factor accelerates seizure development, enhances mossy fiber sprouting, and attenuates seizure-induced decreases in neuronal density in the kindling model of epilepsy

Recurrent seizure activity induced during kindling has been reported to produce a functional synaptic reorganization of the mossy fibers in the hippocampus. To date, it is unclear whether this kindling-induced growth is secondary to decreases in hilar neuron density, which are presumed to reflect hilar neuronal cell loss, or whether it is related specifically to an activation-dependent plasticity. We recently demonstrated that blocking nerve growth factor (NGF) biological activity retards seizure development and inhibits the sprouting of mossy fibers. We now demonstrate that intraventricular administration of NGF itself accelerates the progression of kindling epileptogenesis, increases mossy fiber sprouting in the CA3 region and in the inner molecular layer (IML), but reduces seizure-induced decreases in hilar cell density. These findings provide support for a role of NGF in kindling and kindling-induced mossy fiber sprouting. In addition, the results dissociate this form of epileptogenesis from hilar cell loss or decreases in hilar cell density attributable to increases in hilar area, thereby supporting seizure-induced mossy fiber sprouting as being primarily attributable to the combined effects of neuronal activation and the activation-induced upregulation of growth factors.

Degeneration and regeneration of cutaneous sensory nerve formations

Auxiliary structures of the cutaneous sensory nerve formations (SNF) are dependent on sensory innervation during their critical period of development. Denervation of mature cutaneous corpuscles results in survival of the terminal Schwann cells and the capsular structures which are probably responsible for successful reinnervation of the cutaneous SNF. In addition, the basal lamina tubes of Schwann cells are connected with the terminal Schwann cells and play an important role in the guidance of regrowing axons to their original targets. Long-lasting denervation causes atrophic changes of the terminal Schwann cells and alterations of their molecular equipment. These atrophic changes in the terminal Schwann cells may be responsible for erroneous reinnervation of cutaneous SNF. A population of the cutaneous Merkel cells surviving denervation may also serve as targets for regrowing sensory axons. The basal laminae of terminal Schwann cells are produced under control of the sensory terminals during maturation of cutaneous SNF. In adult animals, the basal laminae are capable of stimulating differentiation of migrated Schwann cells to the terminal Schwann cells without the presence of the sensory terminals. Nonspecific cholinesterase (nChE) is secreted by the terminal Schwann cells and is attached to their extracellular matrix. The synthesis of these molecules in adult animals is not influenced by the sensory terminals. However, the presence of nChE molecules is associated with living terminal Schwann cells. Fetal orthotopically grafted dorsal root ganglion (DRG) neurons have the ability to reinnervate cutaneous SNF of adult hosts. When cutaneous areas are denervated, axons from adjacent sensory nerves may extend collateral branches into this area. The capacity for such extension is dependent on: (1) type of sensory nerve ending, C and A delta fibers having significantly greater capacity than sensory axons of larger caliber; (2) age of the animal, immature animals generally showing a greater capacity for collateral sprouting; (3) the state of the adjacent axons, those already in a growth mode being more capable than "resting" ones; and (4) the regional and mechanical conditions at the site of denervation, hindpaw skin being much less extensively reinnervated by collateral fibers than that of the trunk.

NGF-dependent and NGF-independent recovery of sympathetic function after chemical sympathectomy with 6-hydroxydopamine

To help clarify the distinction between the nerve growth factor (NGF)-dependent collateral sprouting of sympathetic nerves and their NGF-independent regeneration after crush, we used 6-hydroxydopamine (OHDA) to destroy the sympathetic terminals in adult rats; this leaves the axons damage-free. Would recovery occur by regeneration and/or collateral sprouting? A single 6-OHDA injection abolished the sympathetic pilomotor field revealed by electrical stimulation of a cutaneous nerve. Recovery began within 2 days, and by 20 days the field was reestablished. If the field was "isolated" by adjacent denervations at the time of 6-OHDA treatment, the recovering pilomotor field expanded extensively into the surrounding territory. In the presence of anti-NGF, however, the pilomotor field expansion ceased at about 60% of its former size; if anti-NGF treatment was discontinued, expansion recommended and extended into the surrounding skin. We suggest that the latter, NGF-dependent, growth phase corresponds to collateral sprouting, and the initial NGF-independent one to regeneration. After simple nerve crush, however, such regeneration can triple the normal sympathetic field size. This difference between crush- and 6-OHDA-induced regeneration might relate to the "cell body reaction" (CBR); the CBR is reduced with increasing distance of the lesion and is undetectable after a 6-OHDA lesion. Since the CBR and the vigor of regeneration are both increased by repeated axonal injury, we tested the effects of multiple 6-OHDA treatments; this significantly increased the initial NGF-independent expansion. We hypothesize that regeneration is regulated largely by mechanisms associated with the CBR, and that neurotrophin-dependent collateral sprouting occurs independently of these.

Allodynia-like effects in rat after ischaemic spinal cord injury photochemically induced by laser irradiation

We report behaviours suggesting the presence of allodynia elicited by non-noxious brushing and mechanical pressure following photochemically induced ischaemic spinal cord injury in the rat. Female rats were intravenously injected with Erythrosin B and the T10 vertebra was irradiated with a laser beam for 1, 5 or 10 min. These procedures initiated an intravascular photochemical reaction, resulting in ischaemic spinal cord injury. After irradiation a clear allodynia was observed in most rats. The animals vocalized intensely to light touch during gentle handling and were clearly agitated to light brushing of the flanks. The vocalization threshold in response to the mechanical pressure measured with von Frey hairs was markedly decreased during this period. In some animals the existence of spontaneous pain was suggested by spontaneous vocalization. The duration of the allodynia varied among animals from several hours to several days. The severity and duration of allodynia seemed not to be related to the duration of irradiation. In sham-operated rats a slight, transient allodynia was also noted around the wound within a few hours after surgery, which was effectively relieved by systemic morphine (2 mg/kg, i.p.). Morphine (2 mg/kg, i.p.) also partially relieved the allodynia in spinally injured rats 4 h after irradiation. However, morphine, even at a higher dose (5 mg/kg, i.p.), failed to alleviate the allodynia in spinally injured rats 24-48 h after the injury. Systemic injection of the GABAB agonist baclofen (0.01-0.1 mg/kg, i.p.), but not the GABAA agonist muscimol (1 mg/kg, i.p.), effectively relieved allodynia during this period. Pretreatment with guanethidine 24 h and just prior to the irradiation (20 mg/kg, s.c.) did not prevent the occurrence of allodynia in spinal cord injured rats. The present observation is the first to show that ischaemic spinal cord injury could result in cutaneous mechanical allodynia. This phenomenon is resistant to morphine and may not involve the sympathetic system. Histological examination of allodynic animals 3 days after spinal cord injury revealed considerable morphological damage in the dorsal spinal cord of a rat irradiated for 5 min. The related dorsal roots were also slightly affected in this animal, while the dorsal root ganglia were normal. However, in rats irradiated for 1 min, despite the existence of strong allodynia, no damage could be found at this time in the spinal cord, dorsal roots or dorsal root ganglia. It is suggested that functional deficits in the GABAB system in the spinal cord may be related to this allodynia-like phenomenon.(ABSTRACT TRUNCATED AT 400 WORDS)

Variation and symmetry in the lumbar and thoracic dorsal root ganglion cell populations of newly metamorphosed Xenopus laevis

The sizes of the lumbar and thoracic dorsal root ganglion cell populations in normally developing newly metamorphosed Xenopus laevis were measured in order to determine whether these neuron populations have the same characteristics as the hindlimb motoneuron population (i.e., large individual as well as sibling group differences, striking bilateral symmetry, and a rough correspondence between neuron number and body size that suggests some peripheral control of cell number during normal development (Sperry, J. Comp. Neurol. 264:250-267). Among animals from three sibling groups, the total numbers of thoracic and lumbar ganglion cells are highly variable and symmetrical, although symmetry is not uniformly present at the level of individual ganglion pairs. Significant sibling group differences in neuron number are also present. Metamorphic body size and cell number in the thoracic but not in the lumbar ganglia are significantly correlated. The motoneurons innervating the hindlimbs were also counted and measured in the same animals. While variable as well as symmetrical, motoneuron number and metamorphic body size are correlated in only two of the three sibling groups. Interestingly, the numbers of motoneurons and lumbar ganglion cells, two populations of neurons whose sizes one might predict would be significantly correlated in normally developing animals, are not correlated. The relationship between these observations and currently held views concerning how neuron numbers might be controlled during normal development is discussed.

Characterization of Merkel cells and mechanosensory axons of the rat by styryl pyridinium dyes

The epidermal Merkel cells and their sensory innervation serve tactile sensation in vertebrates. In this study the fluorescent cationic mitochondrial dye, 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4-Di-2-ASP), which has recently been used as a vital stain for motor and autonomic nerve terminals, was tested for its ability to stain Merkel cells and sensory fibers in the snout of the rat. Brightly-fluorescent structures resembling Merkel cells as well as nerve fibers and their terminations were evident in whole mounts of the vibrissal follicle. Unilateral denervation of the vibrissal follicles soon after birth resulted in a staining pattern remarkably similar to that obtained after labelling of the Merkel cells selectively with the fluorescent marker quinacrine, but all fiber staining was abolished. Likewise, in the separated epidermis of other skin regions, including the hairy and glabrous skin of the nose, the staining pattern revealed by 4-Di-2-ASP was indistinguishable from that obtained by quinacrine fluorescence. These results indicate that certain styryl pyridinium dyes may be used as vital stains for epidermal Merkel cells as well as cutaneous mechanosensory axons.

Reinnervation of sweat glands in the mouse: axonal regeneration versus collateral sprouting

The capacity of sudomotor axons to reinnervate sweat glands by regeneration after freezing was compared with their capacity to reinnervate by the mechanism of collateral sprouting. Two groups of mice were prepared in the same manner, with a single exception. All nerves to the hind paw were cut and tied to prevent regrowth. The exception, the lateral plantar nerve, was left intact in one group and frozen at the ankle in the other group. Reinnervation of sweat glands in the paw was charted frequently for 10 weeks using a silastic mold method. We found that sudomotor axons regenerating from the frozen nerve functionally reinnervated sweat glands of the paw sooner, beginning at 15 days postoperation, and at a higher rate than reinnervation from the collateral branching intact nerve, that began at about 25 days. Full recovery occurred by 41 days in most animals of both groups. We also found that stimulation of sweating by pilocarpine activated more sweat glands during the reinnervation period than stimulation by heating the mice, suggesting that the newly regenerating sudomotor axons have a high threshold.

Axonal domains within shared touch domes in the rat: a comparison of their fate during conditions favoring collateral sprouting and following axonal regeneration

Low-threshold mechanosensory nerves in the adult rat differ both from their counterparts in lower vertebrates and from high-threshold nociceptive nerves in mammals in that they appear not to undergo collateral sprouting into adjacent denervated skin, although they will clearly regenerate into it after they are damaged. We have now studied the growth capabilities of the low-threshold nerves supplying touch domes, the visible mechanosensory structures scattered throughout the hairy skin. Touch domes in the rat are often multiply innervated. A serendipitous observation on such domes allowed us to investigate the possibility that a functional collateral sprouting of their nerves can indeed occur, but only to a spatially very restricted extent, e.g., within the confines of a partially denervated dome. We used a "prodder" with a tip diameter of 16 micron to examine the mechanosensory profile across single domes that were preselected as being supplied by only two axons, one running in each of two adjacent dorsal cutaneous nerves (DCNs). Simultaneous recordings were made of the afferent discharges evoked in these nerves when the prodder was applied at about 17 or more locations on a selected dome; the spatial resolution was better than 55 micron. We found that within such a shared dome, one axon can supply a discrete territory (its "domain"), which may or may not overlap with the corresponding domain of the other axon. In a preliminary electron microscopic study, we found no evidence for a sharing of single Merkel cells, which are the specialized sensory cells in touch domes, even in the regions of a shared dome where two domains overlapped; each innervated Merkel cell appeared to be contacted by a single nerve ending, implying that in a shared dome each axon probably supplies an exclusive subpopulation of the Merkel cells. We tested for functional collateral sprouting by eliminating one nerve to a shared dome, and at a selected time thereafter mapping the domain of the remaining axon to see whether it had enlarged. The result was the same whether the two domains initially had a region of overlap or not; no expansion of the surviving domain occurred over postoperative periods up to 4 months (an expansion of the domain by 55 micron would have been detected). Thus functional collateral sprouting had failed to occur.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of Merkel cell populations with contrasting sensitivities to neonatal deafferentation in the rat whisker pad

In this study, we used the quinacrine fluorescence technique to investigate the embryonic and early postnatal development of two distinct populations of Merkel cells in the rat whisker pad and the consequences of neonatal deafferentation on their subsequent development. Annular clusters of Merkel cells first appear in the epidermis near the caudal margin of the mystacial region between embryonic days E14 and E15 at dome sites located on horizontal ridges where the primordial vibrissal follicles develop. The development of these cells progresses in a caudorostral sequence across the whisker pad as does the development of the vibrissal follicles. Each cluster eventually forms a conical ridge or collar of about 130 Merkel cells that surrounds the vibrissal hair shaft as it penetrates the overlying pad epidermis. In the vibrissae, which develop as downgrowths from the horizontal ridges at the dome sites, Merkel cells first appear (caudally) between E16 and E17 and form a cylindrical cuff within the outer root sheath; cells are added progressively until about the end of the first postnatal week when a plateau level of about 750-800 cells is reached. Following unilateral transection of the infraorbital nerve at 24-36 hr after birth, these vibrissal Merkel cells continued to develop along a time course that was indistinguishable from normal, at least over the first 2 weeks of postnatal life. In contrast, all or most of the Merkel cells that normally develop within collars or annular clusters in the pad epidermis (around both the vibrissal and intervibrissal or pelage hairs) either disappeared within a few days or failed to develop. Other light and electron microscopic procedures supported the main findings and confirmed that the denervation was successful. Thus, the vibrissal Merkel cells, like those in the glabrous hindpaw, behaved as a distinct class which develops postnatally and is maintained (at least over a 2-week period) without the presence of sensory nerves. Since both the mystacial vibrissae and glabrous hindpaw have specialized cortical representations, a possible relationship between these findings and the organization of the somatosensory cortex during development is discussed.

Collateral sprouting of sensory axons in the hairy skin of the trunk: a morphological study in adult rats

The capacity of sensory axons in the hairy skin of adult rats to extend branches into adjacent denervated skin has been studied by anterograde tracing with wheat germ agglutinin-horseradish peroxidase conjugate. In one series of experiments the Th10 intercostal nerve area was isolated and the distribution of sensory axons from the lateral cutaneous nerve was examined in control experiments and at various times after the denervation. In a second series of experiments, the entire Th10 was isolated, and the caudal extension of sensory axons from the Th10 spinal nerve was examined in control experiments and at various times after the denervation. The findings indicate that thin as well as coarse cutaneous sensory nerve axons can extend collateral sprouts within their 'own' dermatome as well as outside their normal segmental territory. Thus the dermatomal border does not seem to be a limit for collateral sprouting of coarse sensory nerve axons.

Collateral reinnervation of skin by C-fibres following nerve injury in the rat

Evidence for collateral reinnervation of skin by saphenous nerve C-fibres, following either crush or section of the ipsilateral sciatic nerve, was obtained in the rat using a dye-labelled plasma extravasation technique. During the period of regeneration of the crushed nerve the spread of collateral reinnervation was halted and may even have been reversed.

Collateral sprouting of sensory axons in the glabrous skin of the hindpaw after chronic sciatic nerve lesion in adult and neonatal rats: a morphological study

The anterograde transport of wheat germ agglutinin-horseradish peroxidase conjugate was used to study the normal distribution of sensory nerve axons in the plantar skin of the rat hindlimb and at various times after chronic sciatic nerve injury in adult and neonatal rats. In adults, thin saphenous nerve axons were found in a small area laterally to the normal saphenous nerve territory 2-24 weeks after sciatic nerve lesion. In neonatal rats, at 6 and 10 weeks after sciatic nerve injury thin saphenous nerve axons were found almost or all over the sole of the foot, respectively, and in all 5 toes. At longer survival times, the area innervated by saphenous nerve axons became smaller. However, this area was now occupied by thin, as well as coarse axons. When adult animals were subjected to saphenous nerve crush simultaneously with the sciatic nerve lesion, thin, as well as coarse, nerve axons were found laterally to the normal saphenous nerve territory. The findings indicate that thin cutaneous sensory axons of adult mammals can extend collateral sprouts in glabrous skin for a short distance. This capacity appears to be greater in neonatally lesioned animals, where it is present for coarse cutaneous sensory axons as well. However, after neonatal nerve injury collateral sprouts seem to disappear from the initially most distally reinnervated area. Regenerating sensory axons in adult rats seem to have a greater capacity for collateral sprouting than intact axons. Coarse and thin cutaneous sensory axons could be found in this instance. In all instances a great part of the plantar skin remained denervated, suggesting that there is an upper limit for the territory which can be maintained by cutaneous sensory neurons reinnervating glabrous skin.

On nerves and teeth in the lower jaw of the cichlid Tilapia mariae

The anatomy of the teeth and tooth-related nerves in the lower jaw was examined in the cichlid Tilapia mariae. This was done in order to establish a basis for studies on dental neuroplasticity in a polyphyodont vertebrate. The region of interest was explored in specimens fixed by glutaraldehyde perfusion, and by using X-ray photography, maceration, scanning electron microscopy, gross dissection, and light microscopic examination of serial sections. The results show that the lower jaw carries some 60-65 functional teeth. In addition, numerous replacement teeth and tooth germs in various stages of development are located in a cavity in the dentary bone. Numerous nerve bundles are present in immediate relation to the dental follicles of tooth germs. Unerupted teeth do not contain light-microscopically discernible pulpal axons, but the pulps of functional teeth contain myelinated axons. Both perifollicular and pulpal nerve bundles derive from a nerve plexus, which is formed by branches from r. mandibularis trigemini. This nerve is easily accessible to experimental manipulation, where it courses through the adductor mandibulae muscular complex. Thus, the lower jaw of T. mariae seems to represent a suitable system for the study of tooth-nerve interactions in a polyphyodont species.

Temporal and spatial constraints on the collateral sprouting of low-threshold mechanosensory nerves in the skin of rats

We have studied the collateral sprouting of intact low-threshold mechanosensory nerves into adjacent denervated skin in rats. An "isolated" field was produced by extensive denervations of the surrounding skin; sprouting of the remaining cutaneous nerve supplying the field was looked for in the form of field expansion into the surrounding denervated territory at various postoperative intervals. Such isolated fields failed to expand in the adult rat for periods up to at least 85 days. "Nonfunctional" sprouting is unlikely to explain this failure. However, similar experiments done in very young animals gave a different result. In rat pups aged less than 20 days, isolated fields did expand, but this ceased at about 20 days, and attempts to evoke it after this time were unsuccessful. There seems to be a critical period for sprouting of these touch-sensitive nerves into denervated skin, and our evidence suggests that it may not begin until about 15 days of age. Within this developmental window the sprouting that occurs is spatially constrained, an isolated field expanding preferentially into denervated skin of the parent dermatome; if only skin of neighboring dermatomes is available there is no expansion. In contrast, low-threshold nerves regenerated readily after a crush at all ages studied, and the mechanosensory fields established by regenerating nerves expanded progressively into denervated skin without apparent constraints at dermatomal boundaries. The temporal and spatial constraints found for the sprouting of intact low-threshold axons are in marked contrast to their absence for the well-described sprouting of high-threshold (nociceptive) nerves.

The effects of neural crest deletions on the development of sensory innervation patterns in embryonic chick hind limb

Selected lumbosacral dorsal root ganglia (d.r.g.s) were eliminated from chick embryos by removing the parent neural crest, and the dermatomes and axonal projection patterns established in the hind limb by the remaining intact d.r.g.s were studied physiologically and/or anatomically. Dermatomes of intact d.r.g.s expanded into denervated skin regions, partially but never completely replacing the lost innervation; some skin regions consistently remained without apparent innervation. Dermatome expansion was detected in young embryos, soon after skin innervation was established, suggesting that skin sensory axons in operated embryos ignored their usual dermatome borders. The axonal pathways (nerve trunks) normally supplied by axons from the deleted d.r.g.s were missing, and axons from intact d.r.g.s were only rarely found in aberrant pathways. In contrast, the relative distribution of axons from intact d.r.g.s within their usual complement of pathways was altered, with axonal projections shifting toward, but not into the deleted pathways. Shifts in axonal projection patterns were observed in embryos prior to the onset of sensory cell death. Thus, d.r.g. deletions appeared to affect the initial growth of axons into the limb. Together these results suggest that during normal development competitive interactions among axons from neighbouring d.r.g.s play an important role in establishing the borders between dermatomes and in determining the distribution of axons within projection pathways in the limb.

Neurotrophic and neuronotrophic effects in the regenerating newt limb bud after electrical stimulation of brachiospinal nerves

An earlier work demonstrated that electrical stimulation of newt brachiospinal nerves produces a 20% increase in protein synthesis in the regenerating limb bud at 6 h post-stimulation. The present study shows that if stimulation of nerve cell bodies is prevented by placing procaine between the cell bodies and the stimulating electrode, there is no increase in limb bud protein synthesis compared to the non-stimulated, contralateral control limb bud. Similarly, if colchicine is applied to the brachiospinal nerves at the site of and prior to stimulation, there is no increase in limb bud protein synthesis after stimulation. Colchicine applied to brachiospinal nerves in the absence of stimulation results in a reduction of limb bud protein synthesis that is of the same magnitude as the increase seen with stimulation. The results suggest that the neurotrophic increase in limb bud protein synthesis after stimulation is under the control of the cell body and that this control is mediated by changes in fast axonal transport. A neuronotrophic increase in axonal density in the stimulated side limb bud is seen at the same time as the increase in protein synthesis after stimulation.

A quantitative study of the time course of the reduction in Merkel cell number within denervated rat touch domes

By using the fluorescent dye quinacrine as a marker for the Merkel cells in rat touch domes we have shown that denervation results in a progressive reduction in the number of these cells to a level that remains relatively constant at about 40% of that present at the time of denervation. The time-course of quinacrine fluorescent cell changes after denervation could be described by assuming that (i) there are two populations of quinacrine fluorescent cells, one labile and the other stable, and (ii) the labile population is the one most sensitive to denervation and disappears exponentially with a half-time of ca 10 days. It appeared that this time-course of decay of the labile quinacrine fluorescent cells was the same whether the denervation was performed during the period of postnatal development studied (at 7 and 14 days), when normally Merkel cells are rapidly added to the dome, or later (at 35 and 60 days) when the adult population is virtually established. Correlative electron microscopic studies confirmed that quinacrine fluorescent cell counts reflect fairly accurately the Merkel cell population in denervated domes. These quantitative findings based on the use of quinacrine to visualize the entire Merkel cell population of touch domes show that the normal development and maintenance of these cells are trophically dependent on sensory nerves, although a subpopulation may persist even in long-term denervated domes. In addition, the similarity of the first order rate constant for the decay of quinacrine fluorescent cells after denervation and for the normal generation of quinacrine fluorescent cells suggests that the labile Merkel cell population is one that turns over continuously in the normally innervated touch dome.

Myosin transitions in developing fast and slow muscles of the rat hindlimb

Myosin isozymes from the slow soleus and fast EDL muscles of the rat hindlimb were analyzed by pyrophosphate gel electrophoresis, by peptide mapping of heavy chains, and by antibody staining. At the earliest stage examined, 20 days gestation, distinctions between the developing fast and slow muscles were seen by all these criteria; all fibers in the distal hindlimb reacted strongly with antibody to adult fast myosin. Some fibers also reacted with antibody to adult slow myosin; these fibers had a precise, axial distribution in the hindlimb. This pattern of staining which includes the entire soleus, foreshadows the adult distribution of slow fibers and may indicate that the specific pattern of innervation of the limb is already determined. In the early developing soleus there are four fetal and neonatal isozymes plus two isozymes present in equal proportions in the 'slow' area of the pyrophosphate gel. The mobility of these two slow isozymes decreases with maturity and the slowest moving isozyme gradually becomes the dominant species. Thus early diversity between the soleus and EDL is expressed by myosins which are distinct from the mature isozymes. The relative proportion of slow isozymes significantly increases with development and as this occurs the fetal and neonatal isozymes are progressively eliminated. Transiently at least one mature fast isozyme appears in the soleus. This is present at 15 days postpartum and probably correlates with the population of fast, type II fibers, which comprise 50% of this muscle cell population at 15 days. The EDL contained three fetal and neonatal isozymes and only one slow isozyme which does not change in mobility with age.(ABSTRACT TRUNCATED AT 250 WORDS)