Antisera to the sequence Arg-Phe-amide visualize neuronal centralization in hydroid polyps

Summary Antisera to the sequence Arg-Phe-amide (RF-amide) have a high affinity to the nervous system of fixed hydroid polyps. Whole-mount incubations of several Hydra species with RFamide antisera visualize the three-dimensional structure of an ectodermal nervous system in the hypostome, tentacles, gastric region and peduncle. In the hypostome of Hydra attenuata a ganglion-like structure occurs, consisting of numerous sensory cells located in a region around the mouth opening and a dense plexus of processes which project mostly radially towards the bases of the tentacles. In Hydra oligactis an ectodermal nerve ring was observed lying at the border of hypostome and tentacle bases. This nerve ring consists of a few large ganglion cells with thick processes forming a circle around the hypostome. This is the first direct demonstration of a nerve ring in a hydroid polyp.Incubation of Hydractinia echinata gastrozooids with RFamide antisera visualizes an extremly dense plexus of neuronal processes in body and head regions. A ring of sensory cells around the mouth opening is the first group of neurons to show RFamide immunoreactivity during the development of a primary polyp. In gonozooids the oocytes and spermatophores are covered with strongly immunoreactive neurons.All examples of whole-mount incubations with RF-amide antisera clearly show that hydroid polyps have by no means a diffuse nerve net, as is often believed, and that neuronal centralization and plexus formation are common in these animals. The examples also show that treatment of intact fixed animals with RFamide antisera is a useful technique to study the anatomy or development of a principal portion of the hydroid nervous system.     

Types of neurons and synaptic connections at hypostome-tentacle junctions in Hydra

Using transmission electron microscopy of thin sections we have examined neuronal concentrations at hypostome-tentacle junctions in Hydra littoralis. A total of 194 ganglion cells were counted in 587 serial thin sections of a single hypostome-tentacle junction. We found two distinct types of ganglion cells: those with and those lacking stereocilia. The majority of the neurons observed lacked stereocilia; in a single hypostome-tentacle junction only 37% of the ganglion cells possessed a kinocilium surrounded by rodlike stereocilia. Most of the ganglion cells (55%) were clustered together in the oral or upper epidermis of the hypostome-tentacle junction: Nineteen percent were in the lateral and 26% in the aboral or lower epidermis. The two types of ganglion cells did not differ significantly in their distribution. Both types of ganglion cell had synaptic contacts with other neurons and with epitheliomuscular cells. More than 85% of the neuroneuronal and 61% of the neuroepitheliomuscular cell synapses were located in the oral epidermis of a hypostome-tentacle junction. In addition, two-way chemical synapses and a gap junction between neurons were observed at hypostome-tentacle junctions. Our morphological evidence of synaptic connectivity in neuronal clusters at hypostome-tentacle junctions suggests that primitive ganglia are present in Hydra.     

Ultrastructural evidence for two-cell and three-cell neural pathways in the tentacle epidermis of the sea anemone Aiptasia pallida.

Sensory and ganglion cells in the tentacle epidermis of the sea anemone Aiptasia pallida were traced in serial transmission electron micrographs to their synaptic contacts on other cells. Sensory cell synapses were found on spirocytes, muscle cells, and ganglion cells. Ganglion cells, in turn, synapsed on sensory cells, spirocytes, muscle cells, and other neurons and formed en passant axo-axonal synapses. Axonal synapses on nematocytes and gland cells were not traced to their cells of origin, i.e., identified sensory or ganglion cells. Direct synaptic contacts of sensory cells with spirocytes and sensory cells with muscle cells suggest a local two-cell pathway for spirocyst discharge and muscle cell contraction, whereas interjection of a ganglion cell between the sensory and effector cells creates a local three-cell pathway. The network of ganglion cells and their processes allows for a through-conduction system that is interconnected by chemical synapses. Although the sea anemone nervous system is more complex than that of Hydra, it has similar two-cell and three-cell effector pathways that may function in local responses to tentacle contact with food.     

Development of the two-part pattern during regeneration of the head in hydra

The head of a hydra is composed of two parts, a domed hypostome with a mouth at the top and a ring of tentacles below. When animals are decapitated a new head regenerates. During the process of regeneration the apical tip passes through a transient stage in which it exhibits tentacle-like characteristics before becoming a hypostome. This was determined from markers which appeared before morphogenesis took place. The first was a monoclonal antibody, TS-19, that specifically binds to the ectodermal epithelial cells of the tentacles. The second was an antiserum against the peptide Arg-Phe-amide (RFamide), which in the head of hydra is specific to the sensory cells of the hypostomal apex and the ganglion cells of the lower hypostome and tentacles. The TS-19 expression and the ganglion cells with RFamide-like immunoreactivity (RLI) arose first at the apex and spread radially. Once the tentacles began evaginating in a ring, both the TS-19 antigen and RLI+ ganglion cells gradually disappeared from the presumptive hypostome area and RLI+ sensory cells appeared at the apex. By tracking tissue movements during morphogenesis it became clear that the apical cap, in which these changes took place, did not undergo tissue turnover. The implications of this tentacle-like stage for patterning the two-part head are discussed.     

Plasticity in the nervous system of adult hydra. II. Conversion of ganglion cells of the body column into epidermal sensory cells of the hypostome

Due to the tissue dynamics of hydra, every neuron is constantly changing its location within the animal. At the same time specific subsets of neurons defined by morphological or immunological criteria maintain their particular spatial distributions, suggesting that neurons switch their phenotype as they change their location. A position-dependent switch in neuropeptide expression has been demonstrated. The possibility that ganglion cells of the body column are converted into epidermal sensory cells of the head was examined using a monoclonal antibody, TS33, whose binding is restricted to a subset of epidermal sensory cells of the hypostome, the apical end of the head. When animals devoid of interstitial cells, which are the nerve cell precursors, were decapitated and allowed to regenerate, they formed TS33+ epidermal sensory cells. As this latter cell type is not found in the body column, and the interstitial cell-free animals contained only epithelial cells and ganglion cells in the part of the ectoderm that formed the head during regeneration, the TS33+ epidermal sensory cells most likely arose from the TS33- ganglion cells. The observation of epidermal sensory cells labeled with both TS33 and TS26, a monoclonal antibody that binds to ganglion cells, in regenerating and normal heads provides further support. The double-labeled cells are probably in transition from a ganglion cell to an epidermal sensory cell. These results provide a second example of position-dependent changes in neuron phenotype, and suggest that the differentiated state of a neuron in hydra is only metastable with regard to phenotype.     

Migration of neurons between ganglia in the metamorphosing insect nervous system

Migration of neurons over long distances occurs during the development of the adult central nervous system of the sphinx moth Manduca sexta, and the turnip moth Agrotis segetum. From each of the suboesophageal and three thoracic ganglia, bilaterally-paired clusters of immature neurons and associated glial cells migrate posteriorly along the interganglionic connectives, to enter the next posterior ganglion. The first sign of migration is observed at the onset of metamorphosis, when posterio-lateral cell clusters gradually separate from the cortex of neuronal cell bodies and enter the connectives. Cell clusters migrate posteriorly along the connective to reach the next ganglion over the first three days (approximately 15%) of pupal development. During migration, each cell cluster is completely enveloped by a single giant glial cell spanning the entire length of the connective between two adjacent ganglia. Intracellular cobalt staining reveals that each migrating neuron has an ovoid cell body and an extremely long leading process which extends as far as the next posterior ganglion; this is not a common morphology for migrating neurons that have been described in vertebrates. Once the cells arrive at the anterior cortex of the next ganglion, they rapidly intermingle with the surrounding neurons and so we were unable to determine the fate of the migrating neurons at their final location.     

Neuropeptide distribution in the cervico-thoracic paravertebral ganglia of the cat with particular reference to calcitonin gene-related peptide immunoreactivity

Summary Paraffin sections of cervical and upper thoracic paravertebral ganglia of the cat were investigated by immunohistochemistry using antisera directed against calcitonin gene-related peptide (CGRP). The relationships of CGRP-immunoreactive structures to those exhibiting immunoreactivity to antisera against other regulatory peptides and dopamine--hydroxylase (DBH), respectively, were studied in consecutive sections. Singly scattered CGRP-immunoreactive neuronal perikarya were observed in the superior and middle cervical ganglia as well as in the stellate ganglion. These neurons also displayed immunoreactivity to vasoactive intestinal polypeptide (VIP), and some additionally exhibited faint substance-P immunoreactivity. DBH- and neuropeptide Y-immunoreactive ganglion cells were not identical with CGRP-immunoreactive neuronal cell bodies.According to the immunoreactive properties of varicosities, which abut on CGRP/VIP-immunoreactive perikarya, three types of CGRP/VIP-immunoreactive ganglion cells could be distinguished: (1) CGRP/VIP-immunoreactive neurons being surrounded by somatostatin-immunoreactive nerve fibers, (2) neurons being approached by both DBH- and met-enkephalin-immunoreactive varicosities, and (3) neurons receiving both DBH- and neurotensin-immunoreactive fibers. The stellate and upper thoracic ganglia harbored clusters of intensely VIP-immunoreactive somata, which lacked CGRP-immunoreactivity. Fine somatostatin-immunoreactive and coarse CGRP-immunoreactive fibers were distributed within these clusters, whereas patches of neurotensin-immunoreactive fibers were complementarily arranged. At all segmental levels investigated, a few postganglionic neurons were approached by both CGRP-immunoreactive and substance P-immunoreactive varicosities, but lacked a VIP-immunoreactive innervation. Therefore, CGRP/substance P-immunoreactive fiber baskets appeared rather to be of extraganglionic origin than to emerge from intraganglionic CGRP/VIP/SP neurons. CGRP-immunoreactive cell bodies or fibers were absent in clusters of small paraganglionic cells, but some of the solitary paraganglionic cells displayed CGRP-immunoreactivity. Our findings establish the presence of CGRP-immunoreactivity in a population of sympathetic neurons in the cat. A highly differentiated, segment-dependent organizational pattern of neuropeptides in cervico-thoracic paravertebral ganglia was demonstrated.Supported by Deutsche Forschungsgemeinschaft grant He 919/6-2     

Numbers, distribution, and types of neurons in the pedal disk of Hydra based on a serial reconstruction from transmission electron micrographs

The numbers, distribution, and types of neurons in a pedal disk of Hydra littoralis were determined from electron micrographs of 567 serial sections approximately 0.12 micron thick. Of 248 neurons counted, we found 234 ganglion cells in the epidermis and 14 in the gastrodermis. No sensory cells with surface projecting cilia were observed in either epithelial layer of the foot region. We found ciliary structures in 196 (84%) of the epidermal neurons: 55 had a well defined cilium-stereociliary complex, 30 had a cilium lacking stereocilia, and 111 could not be classified. In contrast, 38 epidermal neurons lacked evidence of ciliary structures; 10 of the 14 gastrodermal neurons had one or more centrioles, some with an elaborate pericentriolar rootlet system, but no cilium or stereocilia. Neuronal perikarya could be classified into those with dense heterochromatic nuclei and those with light granular nuclei; often these two nuclear variations were observed in paired or triad arrangements of epidermal neurons. In addition, 68 (29%) of the epidermal neurons were characterized by the presence of small dense granules (115-178 nm in diameter) in the cytoplasm around the periciliary space. Although 32 pairs and 5 triads of contiguous neuronal perikarya were present in the epidermis, only two paired neuronal perikarya were present in the gastrodermis. The major concentration of neurons was approximately midway between the basal surface and the region of transition of epitheliomuscular cells into glandulomuscular cells. There was no evidence of large neuronal aggregations suggestive of ganglia in the pedal disk.     

Cerebral motoneurons mediating tentacle retraction in the land slug Ariolimax columbianus

(1) Tentacle retraction in the land slug Ariolimax columbianus can be elicited by stimulation of all nerves and connectives of the ipsi- and contralateral cerebral ganglia. (2) Six neurons in the left cerebral ganglion were classified as tentacle retraction motoneurons because their action potentials are followed one-for-one with constant delay by action potentials in the left tentacle retractor nerve and their depolarization causes retraction of the ipsilateral tentacle. The motoneurons can be identified by size, pattern of pigmentation, position, and physiological characteristics. (3) Each retractor motoneuron discharges at a rather constant rate and has more than one source of excitatory input, but no IPSPs were observed. No synaptic connections between the six retractor motoneurons were found. The nerve action potentials that correspond to each motoneurons are distinguishable by waveform and size rank. (4) Each motoneuron elicits visible contractions in a particular region of the ipsilateral retractor muscle, but the motor fields of some motoneurons overlap. Some motoneurons mediate relatively rapid contractions while others cause slower responses. (5) There is one-for-one correspondence between action potentials of the largest unit recorded extracellularly in the retractor nerve and exciatory junction potentials recorded from the retractor muscle. No evidence of a peripheral neural plexus was found in serial sections of the retractor muscle.     

Cngsc, a homologue of goosecoid, participates in the patterning of the head, and is expressed in the organizer region of Hydra.

We have isolated Cngsc, a hydra homologue of goosecoid gene. The homeodomain of Cngsc is identical to the vertebrate (65-72%) and Drosophila (70%) orthologues. When injected into the ventral side of an early Xenopus embryo, Cngsc induces a partial secondary axis. During head formation, Cngsc expression appears prior to, and directly above, the zone where the tentacles will emerge, but is not observed nearby when the single apical tentacle is formed. This observation indicates that the expression of the gene is not necessary for the formation of a tentacle per se. Rather, it may be involved in defining the border between the hypostome and the tentacle zone. When Cngsc(+) tip of an early bud is grafted into the body column, it induces a secondary axis, while the adjacent Cngsc(-) region has much weaker inductive capacities. Thus, Cngsc is expressed in a tissue that acts as an organizer. Cngsc is also expressed in the sensory neurons of the tip of the hypostome and in the epithelial endodermal cells of the upper part of the body column. The plausible roles of Cngsc in organizer function, head formation and anterior neuron differentiation are similar to roles goosecoid plays in vertebrates and Drosophila. It suggests widespread evolutionary conservation of the function of the gene.     

The development of the insect nervous system. I. An analysis of postembryonic growth in the terminal ganglion of Acheta domesticus

This study describes the post-embryonic growth of the terminal ganglion in Acheta domesticus in terms of volume and cell number. All measurements were made at the beginning of each instar from hatching until the final moult on animals reared under controlled conditions. The terminal ganglion increases about 40-fold in volume from 2 × 10⁶ μ³ in the first instar to 85 × 10⁶ μ³ in the adult. A double logarithmic plot of ganglion volume against body weight shows that the ganglion volume is a function of body weight to the 0.56 power. Initially the neuropile grows at a greater rate than the cortex; in later stages they increase at the same rate. Increase in cell number was determined from serial sections. The total number of cells, based on corrected nuclear counts, increases from 3,400 to 20,000. There is little or no increase in the number of neurons. There are approximately 2,000 association neurons and 100 motor neurons in all stages. The number of glial cells increase from 1,000 to 17,000. Their multiplication rate appears to be related to the increase in neuron volume. Despite the increase in glial cell number, increase in cell volume is primarily responsible for the increase in total volume of the ganglion.     

Histology and ultrastructure of the olfactory organ of the freshwater pulmonate Helisoma trivolvis

Summary The olfactory organ of Helisoma trivolvis is located on the surface of the body at the base of the cephalic tentacles. An evagination of skin, the olfactory plica, at the base of the tentacle extends over the olfactory organ dorsally. The epithelium of the olfactory organs contains unspecialized epithelial cells, ciliated epithelial cells, basal cells, mucous secretory cells, and sensory dendrites. The surface of the epithelium has a complex brush border of thick plasmatic processes, which branch to form several terminal microvillar twigs. Long slender cytoplasmic processes form a dense spongy layer among the plasmatic processes beneath the level of the terminal twigs. Bipolar primary sensory neurons clustered beneath the epithelium of the olfactory organ send dendrites through the epithelium to the free surface. Some sensory endings have a few short cilia, but most bear only microvilli. Cilia of sensory endings and epithelial cells extend beyond the brush border of the epithelium. Small axons arise from the perikarya of the sensory neurons and enter a branch of the olfactory nerve. HRP tracing indicates that the axons pass to the cerebral ganglion without interruption. Histochemical tests indicate that the sensory neurons are neither aminergic nor cholinergic.     

Axotomy-induced apoptosis in adult rat primary sensory neurons

Neuronal death following unilateral axotomy of a sensory nerve has long been inferred from neuronal counts of dorsal root ganglion neurons, using the contralateral ganglia as a control. The counting methods used usually involved the counting of neuronal nucleoli and made assumptions about them which could conceivably be flawed. Very few studies have used direct observations of dying or degenerating neurons to address questions concerning the duration of the period of neuronal death or the mechanisms involved in this process. Here we describe a morphological, morphometric and histochemical study into the nature and duration of sensory neuron death following transection and ligation of the sciatic nerve at mid-thigh level in the adult rat. We show that at least some of this neuronal loss occurs by apoptosis as defined by morphological criteria and in situ end-labelling of damaged DNA. Absolute numbers of apoptotic neurons were counted from serial paraffin sections of ganglia and estimates of neuronal numbers obtained by disector analysis at 1, 2, 3 and 6 months after axotomy. Using this approach we show that axotomy-induced apoptosis begins at around 1 week and continues up to at least 6 months after axotomy.     

Intraganglionic portal sinus located between small intensely fluorescent (SIF) cells and principal ganglionic neurons in the inferior mesenteric ganglion of the guinea pig

Summary The vascular system in the inferior mesenteric ganglion of the guinea pig was studied to clarify the transport pathway of transmitters released by the small intensely fluorescent (SIF) cells to the principal ganglionic neurons. Reconstruction of about 1500 1-m-thick serial sections of the ganglion demonstrated its portal system. SIF cells were tightly packed and formed two or three clusters under the capsule of the ganglion. Branches from the inferior mesenteric artery ran directly toward these clusters and broke up into a number of coiled and looped sinusoid capillaries among the SIF cells. They then drained into a large sinus surrounding the clusters in the ganglion. Capillaries were derived from this sinus and ramified among the principal ganglionic neurons. After supplying the neurons, these vessels drained into veins surrounding the ganglion. Therefore, as we observed two distinct groups of capillaries, we call this sinus the intraganglionic portal sinus. All the transmitters secreted from the SIF cells are collected into this intraganglionic portal sinus and are then conveyed through the capillaries to the principal ganglionic neurons.     

Larval apical sensory organ in a neritimorph gastropod,an ancient gastropod lineage with feeding larvae

The Neritimorpha is an ancient clade of gastropods that may have acquired larval planktotrophy independently of the evolution of this developmental mode in other gastropods (caenogastropods and heterobranchs). Neritimorphs are therefore centrally important to questions about larval evolution within the Gastropoda, but there is very little information about developmental morphology through metamorphosis for this group. We used immunolabeling (antibodies binding to acetylated α-tubulin and serotonin) and serial ultrathin sections for transmission electron microscopy to characterize the apical sensory organ in planktotrophic larvae of a marine neritimorph. The apical sensory organ of gastropod larvae is a highly conserved multicellular sensory structure that includes an apical ganglion and often an associated ciliary structure. Surprisingly, the apical ganglion of Nerita melanotragus (Smith, 1884) does not have typical ampullary neurons, a type of sensory neuron consisting of a cilia filled inpocketing that has been described in all other major gastropod groups. N. melanotragus has cilia-filled pockets embedded within the apical ganglion, but these so-called “sensory cups” are cassettes of multiple cells: one supporting cell and up to three multiciliated sensory cells. We suggest that an internalized pocket that is filled with cilia and open to the exterior via a narrow pore may be essential architectural features for whatever sensory cues are detected by ampullary neurons and sensory cups; however, morphogenesis of these features at the cellular level has undergone evolutionary change. We also note a correlation between the number of sensory elements consisting of cilia-filled pockets within the larval apical sensory organ of gastropods and morphological complexity of the velum or length of the trochal ciliary bands.     

A 3-D model of membrane specializations between human auditory spiral ganglion cells

A three-dimensional model of the cell membrane contact area was made between two large spiral ganglion cells (type 1 cells) from a cell cluster in a normal human cochlea. The freshly fixed cochlea had been removed during skull base surgery, processed, and sectioned for ultrastructural analysis. 400 consecutive serial thin sections were prepared from the apical portion where the nerve cell density is high and cell clusters are numerous. A cell cluster is defined as a conglomerate of two or more nerve cell bodies, surrounded by common Schwann cells. Direct physical contact between ganglion cell membranes (ephapse) was possible, in places where adjacent cells lacked a separating Schwann cell layer (gaps). One such gap was selected, and observed in 57 of 90 consecutive sections. Membrane specializations, observed in 36 sections, were found to be of principally three different types namely: (1) symmetrical, (2) asymmetrical, and (3) asymmetrical subplasmalemmal. The functional properties of these membrane specializations are still unknown. Asymmetrical densities were seen on one or other of the two cell membranes. A graphic model based on serial thin sections was made to illustrate the gap area. Superficially membrane specializations were seen to form small disk-like areas varying in size, the largest measuring 3 × 2 μm. It is speculated whether these unique formations between human spiral ganglion cells, which have not been observed in other species, may constitute interactive electrotonic or ephaptic transmission pathways. These may be in the low-frequency region and may increase plasticity and signal acuity related to the coding of speech.     

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor for sensory neurons: Comparison with the effects of the neurotrophins

We compared the effects of glial cell line-derived neurotrophic factor (GDNF) on dorsal root ganglion (DRG) sensory neurons to that of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3). All of these factors were retrogradely transported to sub-populations of sensory neuron cell bodies in the L4/L5 DRG of neonatal rats. The size distribution of ¹²⁵I-GDNF-labeled neurons was variable and consisted of both small and large DRG neurons (mean of 506.60 μm²). ¹²⁵I-NGF was preferentially taken up by small neurons with a mean cross-sectional area of 383.03 μm². Iodinated BDNF and NT-3 were transported by medium to large neurons with mean sizes of 501.48 and 529.27 μm², respectively. A neonatal, sciatic nerve axotomy-induced cell death model was used to determine whether any of these factors could influence DRG neuron survival in vivo. GDNF and NGF rescued nearly 100% of the sensory neurons. BDNF and NT-3 did not promote any detectable level of neuronal survival despite the fact that they underwent retrograde transport. We examined the in vitro survival-promoting ability of these factors on neonatal DRG neuronal cultures derived from neonatal rats. GDNF, NGF, and NT-3 were effective in vitro, while BDNF was not. The range of effects seen in the models described here underscores the importance of testing neuronal responsiveness in more than one model. The biological responsiveness of DRG neurons to GDNF in multiple models suggests that this factor may play a role in the development and maintenance of sensory neurons. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 22–32, 1997.     

Transient expression of somatostatin peptide is a widespread feature of developing sensory and sympathetic neurons in the embryonic rat.

Previous studies from this and other laboratories demonstrated that many embryonic sensory ganglion cells in the rat transiently express the catecholamine synthesizing enzyme tyrosine hydroxylase (TH), a trait not expressed by most mature sensory neurons. We, therefore, sought to determine whether transient expression was uniquely associated with catecholaminergic traits, or, alternatively, whether embryonic ganglion cells transiently expressed peptidergic properties as well. Of the four peptides examined (somatostatin [somatotropin release inhibiting factor] (SRIF), galanin (Gal), calcitonin gene-related peptide (CGRP), and substance P (SP)), only SRIF was found to be transiently expressed during early stages of sensory gangliogenesis. Surprisingly, SRIF immunoreactivity was observed in virtually all cranial and spinal sensory ganglion cells on embryonic day (E) 12.5. In addition to perikaryal labeling, intense SRIF immunoreactivity was also observed in the central and peripheral processes of E12.5 sensory neurons, suggesting the peptide may be released from nerve endings. The time course of SRIF appearance in cranial ganglion cells paralleled that previously described for TH, and double-labeling studies revealed extensive co-localization of these two phenotypes. By E16.5, however, the number of neurons expressing SRIF had diminished markedly, indicating that SRIF is only transiently expressed by most sensory neurons during early stages of ganglion development. An unexpected finding was that transient expression of SRIF is also a prominent feature of sympathetic ganglion cells; however, the temporal pattern of staining in the sympathetic and sensory ganglia differed substantially. Whereas virtually no SRIF staining was observed in E12.5 sympathetics, the vast majority of cells in the E16.5 superior cervical ganglion (SCG) were labeled. This contrasted sharply with the adult SCG, in which only low levels of SRIF expression were found. These findings demonstrate that SRIF peptide is transiently expressed at high levels in peripheral sensory and sympathetic neurons during embryogenesis. The time course and widespread distribution of SRIF expression indicates that the peptide may play a role in early stages of ganglion cell growth and development. Moreover, these data, in conjunction with previous studies demonstrating SRIF immunoreactivity in developing central neurons, suggest that transient expression of this peptide is a common property of diverse neuronal cell types.     

Profile of adult rat sensory neuron loss, apoptosis and replacement after sciatic nerve crush

Following permanent transection of the adult rat sciatic nerve, sensory neuron apoptosis in the contributing L4 and L5 dorsal root ganglia can be observed for at least 6 months afterwards. To establish the profile of any sensory neuron apoptosis and loss over time when axonal regeneration is allowed, serial sections of L4 and L5 ganglia were examined and the neurons counted using a stereological technique 1, 2 and 3 months after crushing the right sciatic nerve at mid-thigh level. Our results show that an identical degree of sensory neuron loss and apoptosis occurs 1 month after crush as at 1 month after permanent transection. However, at 3 months no neurons undergoing apoptosis could be observed and no significant loss could be detected in the ipsilateral ganglia when compared to unoperated controls. One explanation was a neuronal replacement mechanism, which was investigated by administering bromodeoxyuridine to rats for 1 month after sciatic nerve transection or crush, prior to detection using immunohistochemistry on sections of their ganglia after 2 months. The presence of bromodeoxyuridine in the nuclei of occasional cells that would be counted as neurons on the basis of size and morphology indicates that a process of apparent neurogenesis may underlie the profile of sensory neuron loss after axotomy.