Honeycomb blocks composed of carbonate apatite, ß-tricalcium phosphate, and hydroxyapatite for bone regeneration: effects of composition on biological responses.

Synthetic scaffolds exhibiting bone repair ability equal to that of autogenous bone are required in the fields of orthopedics and dentistry. A suitable synthetic bone graft substitute should induce osteogenic differentiation of mesenchymal stem cells, osteogenesis, and angiogenesis. In this study, three types of honeycomb blocks (HCBs), composed of hydroxyapatite (HAp), ß-tricalcium phosphate (TCP), and carbonate apatite (CO3Ap), were fabricated, and the effects of HCB composition on bone formation and maturation were investigated. The HC structure was selected to promote cell penetration and tissue ingrowth. HAp and ß-TCP HCBs were fabricated by extrusion molding followed by sintering. The CO3Ap HCBs were fabricated by extrusion molding followed by sintering and dissolution-precipitation reactions. These HCBs had similar macroporous structures: all harbored uniformly distributed macropores (∼160 â€‹µm) that were regularly arrayed and penetrated the blocks unidirectionally. Moreover, the volumes of macropores were nearly equal (∼0.15 â€‹cm3/g). The compressive strengths of CO3Ap, HAp, and ß-TCP HCBs were 22.8 â€‹± â€‹3.5, 34.2 â€‹± â€‹3.3, and 24.4 â€‹± â€‹2.4 â€‹MPa, respectively. Owing to the honeycomb-type macroporous structure, the compressive strengths of these HCBs were higher than those of commercial scaffolds with intricate three-dimensional or unidirectional macroporous structure. Notably, bone maturation was markedly faster in CO3Ap HCB grafting than in ß-TCP and HAp HCB grafting, and the mature bone area percentages for CO3Ap HCBs at postsurgery weeks 4 and 12 were 14.3- and 4.3-fold higher and 7.5- and 1.4-fold higher than those for HAp and ß-TCP HCBs, respectively. The differences in bone maturation and formation were probably caused by the disparity in concentrations of calcium ions surrounding the HCBs, which were dictated by the inherent material resorption behavior and mechanism; generally, CO3Ap is resorbed only by osteoclastic resorption, HAp is not resorbed, and ß-TCP is rapidly dissolved even in the absence of osteoclasts. Besides the composition, the microporous structure of HC struts, inevitably generated during the formation of HCBs of various compositions, may contribute to the differences in bone maturation and formation.

Afferent and efferent fiber connections of the carp torus longitudinalis.

The efferent and afferent pathways of the carp torus longitudinalis were studied by means of degeneration and retrograde HRP methods. Efferent projections were only seen in the most superficial layer of the ipsilateral optic tectum (stratum fibrosum marginale). Afferent pathways to the torus longitudinalis were found to originate mainly in the valvula cerebelli. Degenerating fibers course in the tractus mesencephalocerebellaris posterior within the valvula, and join the tractus mesencephalocerebellaris anterior in the tegmentum. The fibers which ascend in the tract gradually invade the optic tectum through which they are distributed to the torus longitudinalis. The remaining fibers pass through the posterior commissure and terminate in the torus longitudinalis at the rostral end of the tract. Degenerating terminals were also seen in the torus longitudinalis when lesions were made in the optic tectum, tectal commissure, torus semicircularis, and in the area between the valvula and the corpus cerebelli. The possibility of projections from these areas is discussed depending upon the results of the retrograde HRP method.

Trigeminal, vagal, and spinal projections of supramedullary cells in the puffer fish, Takifugu niphobles.

The supramedullary cells (SMCs) of teleosts have been studied for nearly 100 years, but their peripheral connections have remained obscure. We examined the supramedullary cells of the puffer fish, Takifugu niphobles, using horseradish peroxidase transport. Horseradish peroxidase labeling was found bilaterally after application to the trigeminal, the posterior branch of the vagal, and the spinal nerves. No labeled neurons were found after application to the anterior or visceral branches of the vagal nerve. Thus, labeled SMCs were found only after application to the nerves containing cutaneous branches. Some rostrocaudal topographical labeling was found after selective application to each of the four branches of the trigeminal nerve. Labeled neurons were more common in the rostral than in the central or caudal part of the SMC region. Some topographical labeling was also found after application to the first, second, and third spinal nerves, but the topography was not very clear, and there was considerable overlap in the distribution of labeled cells. The sum total of labeled SMCs after unilateral horseradish peroxidase application to each peripheral nerve was more than three times the total number of ipsilateral SMCs, indicating that a single SMC projects several peripheral processes into different nerves. From these results, and taking previous studies into consideration, we propose that supramedullary neurons have a phylogenetic relationship with the spinal dorsal cells of the lamprey and with the extramedullary cells of the amphibian embryo.

Giant terminals in the dorsal octavolateralis nucleus of lampreys.

The dorsal octavolateralis nucleus of lampreys is a primary nucleus for electroreceptive stimuli in the medulla. In Lampetra japonica, the rostral and caudal thirds of this nucleus are exclusively occupied by giant terminals, which become evident when the primary fibers of an electrosensory nerve (recurrent branch of the anterior lateral line nerve) are labeled with horseradish peroxidase. We studied the ultrastructure of these terminals. They contain neurofilaments, mitochondria, microtubules, and tubular membranous structures. Many synapses, all of the chemical type, are located around the neck region of the terminal swellings. Many vesicular structures, which are clear, round, and uniform in size, and most of which are probably synaptic vesicles, are densely clustered in a single large mass in the neck region of the terminals. Some of the tubular structures may serve as a membrane reservoir for the large number of synaptic vesicles required in the giant terminals.

Thalamic fiber connections in a teleost (Sebastiscus marmoratus): visual somatosensory, octavolateral, and cerebellar relay region to the telencephalon.

Fiber connections of the nucleus ventromedialis thalami (VM) of Schnitzlein (J. Comp. Neurol. 118:225-267, '62) in a teleost (Sebastiscus marmoratus) were examined by means of the horseradish peroxidase (HRP) tracing method. This nucleus receives fibers from the ipsilateral telencephalon (area dorsalis pars centralis), contralateral retina, contralateral VM, ipsilateral optic tectum, ipsilateral torus semicircularis, contralateral corpus cerebelli, contralateral sensory nucleus of the trigeminal nerve, bilateral bulbospinal reticular formation, contralateral obex region, and contralateral dorsal portion of upper spinal segments. In turn, axons arising from VM terminate in the dorsal telencephalic areas (pars centralis, pars dorsalis, and pars medialis) ipsilaterally, ventral telencephalic area (pars supracommissuralis) bilaterally, nucleus prethalamicus of Meader (J. Comp. Neurol. 60:361-407, '34) bilaterally, nucleus dorsomedialis thalami bilaterally, VM contralaterally, optic tectum bilaterally, torus semicircularis bilaterally, and nucleus lateralis valvulae ipsilaterally. Based on the cytoarchitecture and fiber connections, VM is subdivided into rostral and caudal components. The caudal part of VM in Sebastiscus is considered to be a multimodal thalamic complex that contains some cells that constitute the dorsal thalamus in other vertebrate groups.

Primary sensory ganglion cells projecting to the principal trigeminal nucleus in the mallard, Anas platyrhynchos.

The trigeminal and glossopharyngeal ganglia of the adult mallard were studied following HRP injections into the principal trigeminal nucleus (PrV). The PrV consists of the principal trigeminal nucleus proper (prV) and the principal glossopharyngeal nucleus (prIX). After an injection into the prV, the labeled cells were found in the ipsilateral trigeminal ganglion. After an injection into the prIX, labeled cells were found in the ipsilateral distal glossopharyngeal ganglion, but not in the proximal ganglion of the IX and X cranial nerve (pGIX + X). In Nissl preparations, two types of ganglion cells in the trigeminal ganglion, pGIX + X, and distal ganglion of N IX could be distinguished: larger light cells and smaller dark cells. We could not determine whether the HRP-labeled cells belonged to both types or to one of them; but because all the labeled cells were over 20 microns, we concluded that the smallest cells (10-19 microns) in the trigeminal ganglion and distal ganglion of N IX did not project to the PrV. The labeling of the cells in the distal ganglion of N IX (average 34.5 microns) was uniformly moderate. In the trigeminal ganglion there were two types of labeled cells: heavily labeled cells (average 29.1 microns) and moderately labeled cells (average 35.1 l microns). These two types of labeling (moderate and heavy) may reflect two types of primary sensory neurons: cells with ascending, nonbifurcating axons, and cells with bifurcating axons. We speculate that the former are proprioceptive neurons and the latter tactile neurons. Labeled bifurcating axons in the sensory trigeminal complex gave off collaterals to all parts of the descending trigeminal nucleus except to the caudalmost laminated spinal part.

Afferent and efferent projections of the glossopharyngeal-vagal nerve in the hagfish.

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the glossopharyngeal-vagal nerve in the hagfish Eptatretus burgeri. Anterogradely labeled ganglion cells are scattered in the glossopharyngeal-vagal nerve trunk, in the saccular ganglion, and in the brainstem. Afferent fibers of the glossopharyngeal-vagal nerve terminate in both the vagal lobe and the fasciculus communis. Close observation showed no morphological differentiation between these two structures, indicating that they are not separate entities, but a single, continuous structure that is homologous with the nucleus and tractus solitarius of other vertebrates. The median part of this structure (the commissura infima) is displaced more rostrally than the same part of the solitary nucleus in many other vertebrates. Some of the afferent fibers invade the ventral portion of the trigeminal sensory nucleus, which receives the maxillo-mandibular nerve fibers, and terminate there. Our study showed that the hagfish has only one nucleus in the vagal motor system, i.e., the vagal motor nucleus, which contains both parasympathetic and branchiomotor neurons. The dendrites of the vagal motor neurons in the hagfish are more highly developed than those in other vertebrates. This suggests that the motor reflex arc of the glossopharyngeal-vagal nerve in hagfishes may be simpler than in other vertebrates.

Somatotopic organization of the primary sensory trigeminal neurons in the hagfish, Eptatretus burgeri.

Primary sensory trigeminal projections were investigated in the hagfish following application of horseradish peroxidase (HRP) to the sensory branches. In our control preparations we were able to distinguish five sensory ganglia and their respective nerves. HRP application confirmed the almost exclusive relation of each of these nerves to their respective ganglia, with very little overlap. In normal frontal sections of the medulla oblongata, five columns of fibers surrounded by neuronal cell bodies could be clearly distinguished, but the number is probably fortuitous, for there was no one-on-one relationship with the five trigeminal ganglia. From their peripheral connections, we surmised that columns 1 and 3 handle general cutaneous sensation, columns 2, 4, and 5 handle taste sensation, and column 5 handles general mucous cutaneous sensation conveyed by utricular ganglion cells. Dorsally located columns received projections from nerves with dorsal peripheral connections, and more ventrally located columns received projections from nerves with ventral peripheral connections. This relation is the reverse of that seen in other vertebrates.

Organization of sensory and motor nuclei of the trigeminal nerve in lampreys.

Anterograde and retrograde HRP transport were used to elucidate the primary central projections of the trigeminal nerve in a lamprey, Lampetra japonica, by application to the ophthalmic, apical, basilar, suborbital, and mandibular branches of the trigeminal nerve. (1) Most of the trigeminal and a few facial ganglion cells were labeled. The ganglion cells of each nerve were distributed in separate areas within their respective ganglia. (2) Some ipsilateral medullary and spinal dorsal cells were labeled after HRP application to the ophthalmic and apical nerves, but there was no contralateral labeling. (3) Most of the neurons of the trigeminal motor nucleus were labeled, and when the apical or the basilar nerve was labeled, in each case a cluster of small motor neurons was found ventrolateral to the classic motor nucleus. (4) Miscellaneous neurons were found scattered along the course of the descending trigeminal tract and nucleus in all cases except after application to the mandibular branch. The shape, size, and distribution patterns of these neurons were varied, and several characteristics indicated that they were sensory in nature. (5) In the rostral part of the medulla, sensory fibers of each nerve showed restricted localization within the descending trigeminal tract and nucleus. When compared to the distribution of the same fibers in the hagfish Eptatretus burgeri, another member of the cyclostomes, the distribution pattern in the lampreys studied was closer to the type seen in gnathostomes.

Afferent and efferent projections of the VIIIth cranial nerve in the lamprey Lampetra japonica.

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the VIIIth cranial nerve in the lamprey Lampetra japonica. Ganglion cells of the VIIIth nerve are classified into three types on the basis of their morphology. The central processes of these ganglion cells enter the medulla in two groups: the anterior group (mostly thick fibers) and the posterior group (mostly thin fibers). Afferent fibers mainly terminate within the ipsilateral ventral and octavomotor nuclei of the octavolateralis area and within the granular and molecular layer of the cerebellum. Some fibers terminate in the contralateral cerebellum, the medial and dorsal nuclei of the octavolateralis area, the descending nucleus of the trigeminal nerve, some cranial motor nuclei, and the lateral octavus nucleus, which has not been described previously. This small nucleus is located beneath the descending nucleus of the trigeminal nerve near the obex. Within the ventral nucleus, thin fibers occupy the dorsal part and thick fibers occupy the ventral part. The basic projection pattern of the primary afferents of the VIIIth nerve in the lampreys was similar to that of gnathostome fishes that have been studied to date. Cell bodies of the efferent vestibular neurons are located between the ipsilateral trigeminal motor nucleus and the facial motor nucleus. The lateral location of these cell bodies differs from that of all other fish species that have been studied.

Organization of the primary projections of the lateral line nerves in the lamprey Lampetra japonica.

The lateral line sensory system of Lampetra japonica is innervated by the anterior and posterior lateral line nerves. The anterior lateral line nerve innervates all electroreceptors throughout the body and mechanoreceptors of the head. The posterior lateral line nerve innervates trunk mechanoreceptors. The anterior lateral line nerve consists of two ganglia (anterior lateral line and intracapsular) and four major peripheral branches (superficial ophthalmic, buccal, hyomandibular, and recurrent nerves). The posterior lateral line nerve has one posterior lateral line ganglion and one peripheral branch. The location and central projection patterns of the primary sensory neurons of these branches of the lateral line nerves were studied with the aid of horseradish peroxidase labeling. The ganglion cells of the buccal nerve were found in the rostral half, and those of the hyomandibular nerve were found in the caudal half of the medial part of the anterior lateral line ganglion. The lateral part of the anterior lateral line ganglion contains ganglion cells of the recurrent nerve and the superficial ophthalmic nerve. The rostral half of the intracapsular ganglion contains ganglion cells of the recurrent, hyomandibular, and buccal nerves. The ganglion cells of the posterior lateral line nerve were found in the posterior lateral line ganglion. The buccal nerve afferents terminated mainly in the lateral part of the ipsilateral mechanoreceptive medial nucleus. The peripheral part of the electroreceptive dorsal nucleus also received several afferents. The hyomandibular afferents terminated ipsilaterally in the central part of the medial nucleus and in the dorsolateral part of the dorsal nucleus. Some afferents of the hyomandibular nerve ascended and descended in the descending nucleus of the trigeminal nerve near its dorsal margin. The ventral nucleus, the primary nucleus of the VIIIth nerve, received a few fibers of the buccal and hyomandibular nerves. In the recurrent nerve, the fibers of the lateral part of the anterior lateral line ganglion terminated throughout the entire dorsal nucleus, and the fibers of the intracapsular ganglion projected to the dorsolateral part of the nucleus. The afferents of the posterior lateral line nerve terminated in the medial part of the ipsilateral medial nucleus and in the lateral part of the contralateral medial nucleus. In the cerebellar area, afferents of the anterior lateral line nerve were located laterally to those of the posterior lateral line nerve. Several fibers terminated in some branchiomotor nuclei, the cerebellar crest, and the dorsal gray near the obex level. No efferent cell bodies were found in the place where efferent neurons of the VIIIth nerve have been previously reported.

Primary neurons of the lateral line nerves and their central projections in hagfishes.

The hagfish lateral line system was studied by horseradish peroxidase transganglionic transport. The anterior lateral line nerve innervates the group of lateral line canals situated anteriorly to the eye, and the posterior lateral line nerve innervates the group of canals situated posteriorly to the eye. Although both nerves pass through the muscle fascia at the same point, each runs a different course to the brain. The anterior lateral line nerve runs near the trigeminal nerve and its ganglion is closely attached to the trigeminal ganglion, but both systems are completely independent. The posterior lateral line nerve runs independently of any other cranial nerve and makes a peculiar U-turn at the point of entry to the brain capsule. The anterior lateral line ganglion contains both cutaneous sensory cells (small to large cells) and lateral line sensory cells (small cells); from this ganglion projections run to both the trigeminal sensory nucleus (fine and thick fibers) and medial nucleus of the area acousticolateralis (fine fibers). The posterior lateral line ganglion contains only small lateral line cells that project fine fibers to the medial nucleus of the area acousticolateralis. There are no efferent components in this lateral line system, and its only afferent terminal field is the medial nucleus of the area acousticolateralis.

Differential distribution of nerve terminals immunoreactive for substance P and cholecystokinin in the sympathetic preganglionic cell column of the filefish Stephanolepis cirrhifer.

Immunoreactivity for substance P and cholecystokinin-8 was examined in the nerve fibers in the central autonomic nucleus, a cell column for sympathetic preganglionic neurons, in the filefish Stephanolepis cirrhifer. Substance P-immunoreactive fibers were distributed throughout the entire rostrocaudal extent, but were more abundant in the caudal part of the column, where substance P-immunoreactive varicosities sometimes made contacts with the sympathetic preganglionic neurons. Cholecystokinin-8-immunoreactive fibers were found almost entirely in the rostral part of the column, where a dense network of varicosities was in close apposition to a considerable number of the sympathetic preganglionic neurons. Double labeling immunohistochemistry showed that substance P fibers and cholecystokin-8 fibers were entirely different, and distinct from serotonin-immunoreactive fibers. By using immunoelectron microscopy, synaptic specialization was sometimes observed between the dendrites of preganglionic neurons and varicosities immunoreactive for substance P and cholecystokinin-8. Substance P- and cholecystokinin-8 fibers were seen from the descending trigeminal tract, through the dorsolateral funiculus and the ventral portion of the dorsal horn, to the central autonomic nucleus. After colchicine treatment, substance P-immunoreactive perikarya were found in the cranial and spinal sensory ganglia. These results suggest that the sympathetic preganglionic neurons of the filefish receive innervation by substance P fibers and cholecystokinin fibers, and that the former might be of primary sensory origin. Topographical distribution of cholecystokinin-8-immunoreactive terminals in the central autonomic nucleus along the rostrocaudal extent might underlie the differential regulation of sympathetic activity via a distinct population of sympathetic preganglionic neurons.

Giant lateral-line afferent terminals in the electroreceptive dorsal nucleus of lampreys.

In HRP studies of the lateral line nerve in lampreys, the dorsal nucleus of the area octavolateralis received projections mainly from the recurrent branch of the anterior lateral line nerve. Furthermore, the recurrent branch projected exclusively to the dorsal nucleus. Besides the common type (1-3 micron) of nerve terminals, a hitherto unreported type of giant (10-30 micron) nerve terminal was found aggregated at the rostral and caudal ends of the nucleus. Since the dorsal nucleus mediates electroreception in lampreys, we conclude that the giant terminals are very probably the terminals of the electroreceptive primary fibers.

Substance P immunoreactivity in the vagal nerve of mice.

After horseradish peroxidase was applied to the main trunk of the mouse vagal nerve, anterogradely labeled cells in the vagal ganglia and fibers in the solitary complex, and retrogradely labeled cells in the dorsal motor nucleus and the ambiguous nucleus were observed. Most of the cells in the nodose ganglion were labeled, but only a few cells in the jugular ganglion were labeled. Heavily labeled nerve terminals and fibers were found in 3 areas in the solitary nucleus: i.e., the lateral half of the medial nucleus, the ventrolateral nucleus, and the commissural nucleus. There was only weak labeling in the dorsolateral nucleus, ventral nucleus, and intermediate nucleus. Substance P immunoreactive neurons in the vagal ganglia were found in the jugular ganglion and the dorsal part of the nodose ganglion, but not in the ventral part of the nodose ganglion. Substance P immunoreactivity in the solitary nucleus was moderate in the commissural nucleus and the intermediate nucleus, but was lacking or very weak in the lateral half of the medial nucleus, ventral nucleus, dorsolateral nucleus, and ventrolateral nucleus. We conclude that most substance P containing fibers in the main trunk of the vagal nerve project centrally to the commissural nucleus and peripherally to some of the thoracic viscera.

Somatosensory and visual correlation in the optic tectum of a python, Python regius: a horseradish peroxidase and Golgi study.

In snakes with infrared receptors the optic tectum receives infrared input in addition to visual and general somatosensory inputs. In order to observe their tectal termination patterns in ball pythons, Python regius, we injected horseradish peroxidase (HRP) into the nucleus of the lateral descending trigeminal tract (LTTD) which mediates infrared information, the optic nerve, and the nucleus of the trigeminal descending tract (TTD) which relays general somatosensory information. Fibers from LTTD were found in layers 5-13 of the contralateral optic tectum, and were especially dense in layers 7a-8. Optic nerve fibers terminated in layers 7a-13 of the contralateral tectum, and mainly in layers 12-13. TTD fibers were few, and could be seen in only the rostral half of the contralateral tectum. These fibers were found in layers 5-7b, but mainly in layers 6-7a. Among various types of neurons stained by the Golgi-Cox method, we focused on six types of neurons whose dendritic arborization overlapped with the distribution of the terminals of these sensory afferents described above. It is possible that these different sensory modalities converge on a single neuron of the various types.

A new tectal afferent nucleus of the infrared sensory system in the medulla oblongata of Crotaline snakes.

The existence of an infrared sensory neuron group with ascending fibers which directly reach the optic tectum in Crotaline snakes was confirmed with three methods. (1) With the retrograde horseradish peroxidase (HRP) method, labeled neurons were not found within the nucleus descendens lateralis nervi trigemini (DLV), but in an unnamed cell group located immediately ventral to the DLV of the contralateral side at the transitional portion between the nucleus oralis (DVo) and the nucleus interpolaris (DVi). This unnamed cell group, which was seen only in the Crotalinae, was provisionally called the 'new nucleus'. (2) Normal brain series of 15 species were stained by the methods of Bodian-Otsuka, Klüver-Barrera and Nissl staining to compare the cytoarchitecture of the medulla oblongata. The 'new nucleus' was found only in species belonging to the Crotalinae. This nucleus was situated in fiber tracts which appeared to correspond to the lemniscus spinalis and tractus spino-cerebellaris of the reptilian medulla oblongata, and contained medium-sized multipolar or fusiform neurons. (3) In an electrophysiological study 16 single units responding unimodally to an infrared stimulus were recorded. Three of these recording sites were determined with Pontamine sky blue marking to be near or within the 'new nucleus'.

Visual and infrared input to the same dendrite in the tectum opticum of the python, Python regius: electron-microscopic evidence.

In snakes with infrared receptors, the optic tectum receives input from both the visual and the infrared senses. We investigated the infrared and optic fiber terminations in the tectum with a combination of horseradish peroxidase and degeneration labeling. In addition to synapses by visual and infrared fibers onto individual neurons, we were able to observe for the first time visual and infrared synapses on one and the same dendrite.

Primary vagal nerve projections to the lateral descending trigeminal nucleus in boidae (Python molurus and Boa constrictor).

The primary vagal axons and terminals within the lateral descending trigeminal tract (dlv) and nucleus (DLV) of two species of Boidae are studied following HRP injections of the vagal nerve. Labeled fibers and terminals are found in the tail portion of the dlv and DLV, partly forming a neuropil at its margin. The labeled thin fibers and neuropil seem to correspond to the C-fibers and marginal neuropil of Crotalinae.

Infrared sensory neurons in the trigeminal ganglia of crotaline snakes: transganglionic HRP transport.

Trigeminal neurons were labeled by inserting HRP into holes cut in the pit receptor membranes of a crotaline snake, Agkistrodon blomhoffi brevicaudus. Neurons were labeled in the ophthalmic ganglion and the maxillary division of the maxillo-mandibular ganglion, and the HRP was further transported across the ganglia and through the lateral descending trigeminal tract (dlv) to label axon terminals exclusively in the dlv nucleus (DLV). In 6 successful preparations, 7.1-19.3% of totals of 5568-5986 cells in the maxillary division of the ganglion were labeled, but none at all were labeled in the mandibular division. Only a few or none at all were labeled in the ophthalmic ganglion. Cells in the two ganglia ranged in size from 10 to 55 micrometers, but large cells (greater than or equal to 40 micrometers) were scarce (4.9% of the total population). All HRP-labeled neurons fell in the median range of 20-39 micrometers. We concluded that these ganglion cells were infrared neurons, and were therefore the origin of the A delta fibers in the pit membrane. There were no HRP-labeled neurons above or below this range, in spite of the fact that smaller cells (less than or equal to 19 micrometers) made up 35.8% of the total population. In normal Nissl preparations we found both light- and dark-staining cells, but the size range of neither corresponded to the size range of infrared neurons.