Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Sensory endings and synaptic structures

Summary The sensory endings, neuromuscular junctions and interneuronal synapses in the crayfish muscle receptor organ have been studied by electron microscopy. The dendrites of the receptor neuron terminate as endings which are either free in the connective tissue matrix of the central region of the receptor strands, or abut on the muscle membrane forming a specialized junction with a narrow cleft of about 18 nm. Efferent nerve endings are classified into three types on the basis of their fine structural features. Type 1 endings contain mainly spherical vesicles with a diameter of about 55 nm and a few large granular vesicles with a diameter of about 100 nm, and synapse exclusively on muscle fibres. Type 2 endings have a high proportion of elongated vesicles measuring about 30 × 80 nm and a few large granular vesicles, and synapse on both sensory neurons and muscle. Type 3 endings are characterized by the high electron density of the axoplasm and numerous large granular vesicles with a diameter of about 100 nm; they synapse only on the sensory neuron of the slow receptor unit.It is suggested that Type 1 endings are excitatory, and Type 2 and 3 endings are inhibitory. Several differences in postsynaptic structure were observed between the putative excitatory and inhibitory neuromuscular junctions. Axo-axonal synapses between endings of Type 1 and Type 2, the latter being presynaptic to the former, are also found. Functional implications and possible roles of these structures are discussed.     

Ultrastructure of the surface structures and electron immunogold labeling of peptide immunoreactivity in the nervous system of Pseudothoracocotyla indica (Polyopisthocotylea: Monogenea)

 Transmission electron microscope studies of the tegument of the tropical marine fish monogenean parasite Pseudothoracocotyla indica describe surface specialisations and detail the ultrastructure of the tegument and the haptor. The tegument consists of a syncytium, numerous electron-dense granules, electron-lucent vesicles and large multivesicular bodies. The posterior tegumental syncytium is infolded to form tegumental ridges that are present on both the ventral and dorsal surfaces. A thin coat of glycocalyx is present on the tegument surface. In contrast, the tegumental syncytium of the haptor is relatively thin, containing electron-dense granules and various-sized electron-lucent vesicles. Exocytosis of the electron-dense and electron-lucent vesicles apparently occurs in the syncytium of the haptor and general body surface. Tegumental damage was observed on the dorsal surface in the mid-body region and may possibly have been due to natural mechanical forces. The haptor consists of electron-dense clamp sclerites embedded within a matrix covered by the tegumental syncytium. The sclerites are connected to each other and to the basal lamina by radially oriented muscle fibres. The haptor is richly supplied with non-myelinated nerve axons. Both uniciliated and non-ciliated presumed sensory structures are present on the body surface and haptor. Uniciliated sensory structures were found mainly around the oral sucker. Groups of neurons and nerve processes containing neurosecretory vesicles were frequently observed in the vicinity of the clamps. Electron immunogold labeling studies demonstrated that neuropeptide F [NPF (Moniezia expansa)] immunoreactivity was confined to electron-dense-cored neurosecretory vesicles in nerve fibres from the posterior and haptor regions of the fluke. Received: 8 February 1996 / Accepted: 10 April 1996     

一种鱼类复殖吸虫神经系统结构的乙酰胆碱酯酶定位观察

用硫代胆碱法定位乙酰胆碱酯酶展示寄生鱼类的巴氏叶形吸虫神经系统结构。该虫神经系统由咽后的１对脑神经节向前后各发出３对纵神经干与其间的横神经及更细神经分支构成神经网络。纵行神经干由内至外分别是背神经干、腹神经干和侧神经干。前行神经干的末梢在口吸盘内形成神经环，后腹神经干的分支进入腹吸盘形成神经环     

Morphology and ultrastructure of an identified histamine-containing neuron in the central nervous system of the pond snail,Lymnaea stagnalis L.

Summary An identifiable histamine-containing neuron is located on the anterior dorsal surface of the visceral ganglion ofLymnaea stagnalis L. After [³H]histamine was injected into the perikaryon of this neuron, labelled axonal ramifications were seen in the neuropils of all ganglia except the pedals and right cerebral, and labelled axons occurred in seven nerve trunks. Electron microscopical examination of the perikaryon of the histamine neuron revealed the presence of large aggregations of granulated vesicles (80–200 m diameter), elaborate endoplasmic reticulum, and numerous mitochondria, Golgi complexes and lysosome-like organelles.     

Membrane ultrastructure of the giant synapse of the squidLoligo pealei

The ultrastructure of the ‘giant synapse’ of the stellate ganglion of the squid was studied with freeze-fracture and thin-sectioning techniques. A sheath of glial cells separates the pre- and post-synaptic axons. At intervals, round-topped processes of the postsynaptic axon pierce the sheath to contact the presynaptic axon. This area of synaptic contact is marked by a widened intercellular cleft containing electron-dense material and by a cluster of synaptic vesicles within the presynaptic cytoplasm. The number of synaptic vesicles in such clusters was greatly reduced by electrical stimulation of the synapse during fixation. Freeze-fracture reveals a roughly circular patch (0.3 μm diameter) of 10 nm particles on the cytoplasmic leaflet of the presynaptic membrane. A similar patch of particles lies on the external leaflet of the apposed postsynaptic membrane.The squid giant synapse thus consists of multiple small pre- and postsynaptic active zones where neurotransmitter is released from the presynaptic terminal and sensed by postsynaptic receptors. Comparison of the structure of these postsynaptic active zones with those at synapses where the transmitter or transmitter action is known suggests that the excitatory transmitter at this synapse is an amino acid.Presumptive gap junctions, marked by particles in the cytoplasmic leaflet, are found between small-diameter axons in the stellate ganglion but not at the giant synapse. Glial-cell membranes contain aggregates of particles and pits suggestive of gap junctions. The aggregates of pits are embedded within linear arrays of particles which somewhat resemble tight junctions.     

The lack of a structured blood-brain barrier in the onychophoran Peripatus acacioi

Summary Onychophorans are ‘living fossils’ frequently purported to have evolved from the same ancestor as the arthropods and annelids. In the CNS ofPeripatus acacioi, beneath an outer acellular neural lamella, glial cells ensheath the cerebral ganglion and the nerve cords. These glial cells are, however, attenuated and rather few in number and, although they interdigitate with one another, they seem to lack intercellular junctions. Exogenous tracers penetrate between them and into the underlying neuropile, suggesting that there is no structural blood-brain barrier. Throughout the nervous tissue, extracellular spaces occur which contain banded collagen fibrils embedded in a matrix material. Thin glial cell processes, characterized by dense filaments, surround these regions and frequently form hemi-desmosomes with the extracellular matrix. The peripheral nerve cell bodies have a range of diameters; some have the characteristics of neurosecretory neurons. Granules in such neurons are produced by the Golgi saccules and associated fenestrated membranes which also possess many coated vesicles. Comparable granules are also found in axonal tracts, but no distinct peripheral neurohaemal areas have been found. Lysosomes are common in the nerve cell bodies and are frequently in the form of multivesicular bodies or large phagocytic vacuoles. Beneath the outer nerve cells lie many tracheae, arranged as a ring around the central neuropile which consists of glial processes, extracellular matrix, axons and nerve terminals. These nerve terminals occur throughout the central neuropile and are characterized by dense pyramidal presynaptic specializations and postsynaptic subsurface cisternae. The nervous system ofPeripatus is relatively simple in its organization, in the lack of glial intercellular junctions and in the ready accessibility of substances from the external milieu.     

Topography and ultrastructure of the tegument of Deropristis inflata Molin, 1859 (Digenea: Deropristidae), a parasite of the European eel Anguilla anguilla (Osteichthyes: Anguillidae)

The tegumental ultrastructure of the intestine fluke Deropristis inflata was studied using scanning and transmission electron microscopy. The surface of the tegument was covered by transverse cytoplasmic ridges from which protrude numerous thorn-like spines showing crenelated tips on the posterior part. Spines were arranged in staggered rows. Cobblestone-like units of the tegument were observed on a semicircle-shaped formation over the oral sucker. A tegumental excrescence was observed in the dorsal anterior side of the fluke. Ultrastructural study revealed that the tegument of D. inflata had a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons. Cytoplasmic bridges allowed transit of secretory vesicles and granules packed in gland cells. Two types of sensory structures were examined. Type 1 sensory receptor was a button-like uniciliated papilla mounted on a folded tegumental base and surrounded by cytoplasmic ridges. This receptor consisted of a nerve bulb and a cilium that extended from a centriole. Type 2 sensory receptor was a smooth bulb-like non-ciliated papilla. It was only recovered on the ventral sucker. This receptor consisted of a nerve bulb enclosing an ovoid electron-dense structure. For both receptors, the nerve bulbs contained numerous mitochondria, nerve fibers, and electron-lucent material. Particular distributions of the sensory receptors were observed with a concentration on the anterior third of the body around the oral and ventral suckers. Diagrams were made to help in understanding the nature of these structures.     

猫肠系膜下神经节神经元与肠壁内传入神经元的突触联系

采用神经纤维和突触溃变的光镜和电镜结合方法，探讨肠系膜下神经节与来自肠神经丛内的传入神经元间的突触联系。切断肠系膜下神经节发出的分支，光镜下见节内存在溃变神经纤维；电镜下见有突触溃变，溃变轴突终末的大小多为１μｍ左右，主要含有球形透明囊泡，线粒体多少不等，与肠系膜下神经节神经元的树突或胞体形成轴－树型或轴－体型突触；溃变突触见有三种类型：电子致密型，电子透明型和神经丝型。确切证明了来自肠壁内的传入     

Histofluorescent and ultrastructural studies on the nervous system ofDiplostomum pseudospathaceum Niewiadomska, 1984 cercariae (Digenea)

The aminergic nervous system in cercariae ofD. pseudospathaceum was studied by fluorescence histochemistry. Green fluorescence, showing the presence of catecholamines, was present in ganglia, ventral and dorsal (the short part only) nerve trunks and also in the tail stem and furcae. The ultrastructure of various parts of the nervous system (brain, nerve trunks of body and tail stem) was also studied. Two different types of profiles were found in brain neuronal processes; six different types of vesicles were identified in nerve cells, as well as the single and shared synaptic junctions. The possible function of these structures is discussed.This work was supported by research grant No PB 1832/4/91 from the State Committee for Scientific Research     

A histochemical and ultrastructural study of serotonin-containing nerves in cerebral blood vessels of the lamprey

Lamprey, Entosphenus japonicus, cerebral blood vessel autonomic nerve supply was studied with fluorescence and cholinesterase histochemistry and electron microscopy. Nerve fibers emitting a yellow fluorescence characteristic of serotonin (Exc./Em. max.; 380/530 nm) were found on the major cerebral and pial arteries, but not acetylcholinesterase (AChE)-positive ones. Single ganglion cells also emitting a strong yellow fluorescence were seen in the artery adventitia. On rare occasions these cells were observed in pairs. Terminal varicosities of central catecholamine-containing nerves (Exc./Em. max.; 410/475 nm) were observed on parenchymal capillaries, but not central AChE-positive nerve terminals. In ganglion cells, dense cored vesicles (ca. 130 nm in average diameter; DCV) were abundant in the Golgi area, suggesting their formation at this site. Two types of DCV were observed; one with a homogeneous dense core and the other with a granular core. DCV were numerous in axons as well, axons in which many small clear vesicles (40–60 nm in diameter) as well as DCV were occasionally observed. The question of whether the small clear vesicles or the DCV contained serotonin could not be resolved.     

Nerve endings in human sympathetic ganglia

Forty-eight human sympathetic ganglia from 22 sympathectomies were examined ultrastructurally after one of three different fixations: (1) glutaraldehyde + osmium tetroxide, (2) glutaraldehyde + potassium dichromate + osmium tetroxide, or (3) potassium permanganate. Three different kinds of synapsing nerve ending could be identified after all fixation schedules. Type 1: “Cholinergic,” containing small, agranular vesicles 40–60 nm in diameter (75% of all vesicles) and some large granular vesicles (100 nm in diameter). The number of type 1 profiles decreased with increasing age. Type 2: “Adrenergic,” containing small granular vesicles 40–70 nm in diameter (over 90% of the vesicles). Type 3: “Nonadrenergic, noncholinergic,” characterized by large opaque vesicles 80–160 nm in diameter (over 50% of all vesicles). The frequencies of the types were counted after potassium permanganate fixation: type 1–71%, type 2–23%, and type 3–6%. In addition, at least two types of nerve profiles were observed which did not form synapses: (1) profiles entirely filled with mitochondria (mitochondrial accumulation), and (2) large nerve profiles full of different kinds of vesicles, myelin figures, and mitochondria (axonal dilatation). It is concluded that the sympathetic ganglion cells receive cholinergic and adrenergic innervation and also a third type of innervation, the possible peptidergic nature of which is discussed.     

Metabolic aspects of the synthesis and intra-axonal transport of noradrenaline storage vesicles

1. Constricted cat hypogastric nerve/inferior mesenteric ganglion preparations have been maintained for 24 hr in vitro, in either test-tubes or a two compartment box, and used to study the roles of energy metabolism, protein synthesis and calcium ions in the synthesis and transport of noradrenaline storage vesicles in sympathetic neurones.2. When preparations were incubated so that the nerve trunks but not the ganglia were made anoxic, dense-cored vesicles lost their store of noradrenaline and did not accumulate above the ligature. This was accompanied by profound ultrastructural damage to the nerves.3. When in contact with the ligated nerve trunks, sodium fluoride inhibited the intra-axonal movement of dense-cord vesicles but did not deplete them of their stored noradrenaline. When sodium fluoride and pyruvate were present in the medium bathing the nerve trunks the movement of noradrenaline storage vesicles was unaffected.4. Whilst a reduction of the calcium ion concentration in the incubation medium and the inhibition of protein synthesis by cycloheximide prevented the synthesis of dense-cored vesicles in the neuronal perikaryon they had no effect on the movement of preformed dense-cored vesicles along non-myelinated axons.     

Histological and electron microscopic milestones in the development of the retina of a marsupial wallaby, Macropus eugenii

Summary Retinal differentiation in the pouch young of the wallaby Macropus eugenii was characterized microscopically and morphometrically. Mitosis occurs until early in the second month in the central retina, and until early in the fourth month, peripherally. Separation of the neuroblast layer by the outer plexiform layer did not immediately halt cell division. The retinal surface continued to expand well past the time of cessation of proliferation. Cell death in the ganglion cell layer continued through the fourth month centrally and to nearly five months in the periphery. The major period of cell death was coincident with the segregation of retinal afferents and the refinement of topography in the superior colliculus and dorsal lateral geniculate necleus. Beginning in the third month retinal thickness, measured between the outer limiting membrane and nerve fiber layer declined equally in peripheral and central regions. At all stages the combined thicknesses of the outer and inner nuclear layer in the retinal periphery was greater than that in the center. Together with a late thickening of the inner plexiform layer, the data are consistent with the suggestion that expansion of peripheral non-ganglion cell elements may play a role in development of center to periphery differences in ganglion cell distributions.Retinal differentiation of the wallaby follows the pattern of most mammals. The onset of development of key milestones for the acquisition of retinal function occurred in the sequence: conventional synapse formation prior to ribbon synapse formation in the inner plexiform layer, and photoreceptor outer segment differentiation prior to terminal triad synapse formation.     

Anatomical study of the pudendal nerve adjacent to the sacrospinous ligament

The pudendal nerve (S3-S5) is a major branch of the sacral plexus. After branching from the sacral plexus, the pudendal nerve travels through three main regions: the gluteal region, the pudendal canal, and the perineum. In the gluteal region, the pudendal nerve lies posterior to the sacrospinous ligament. The relationship of the pudendal nerve to the sacrospinous ligament has important clinical ramifications, but there is a lack of literature examining the variations in pudendal nerve anatomy in the gluteal region. This study investigates the pudendal nerve trunking in relation to the sacrospinous ligament in 37 cadavers (73 sides of pelves) of 21 males and 16 females, ranging from 18-83 years of age. Pudendal nerve trunking could be grouped into five types: Type I is defined as one-trunked (41/73; 56.2%), Type II is two-trunked (8/73; 11%), Type III is two-trunked with one trunk as an inferior rectal nerve piercing through the sacrospinous ligament (8/73; 11%), Type IV is two-trunked with one as an inferior rectal nerve not piercing through the sacrospinous ligament (7/73; 9.5%), and Type V is three-trunked (9/73; 12.3%). In summary, 56.2% of pudendal nerves adjacent to the sacrospinous ligament were one-trunked, 31.5% were two-trunked and 12.3% were three-trunked. Fifteen inferior rectal nerves originated independently from the S4 root and never joined the main pudendal nerve. Eight of fifteen inferior rectal nerves pierced through the sacrospinous ligament, perhaps making it prone for entrapment. We measured the average diameter of the main trunk of the pudendal nerve to be 4.67 +/- 1.17 mm. We also measured the average length of the pudendal nerve trunks before terminal branching to be 25.14 +/- 10.29 mm. There was no significant statistical difference in the average length, average diameter, number of trunks, and pudendal nerve variations between male and female or right or left sides of the pelves. A detailed study of pudendal nerve trunking in relationship to the sacrospinous ligament would be useful for instruction in basic anatomy courses and in relevant clinical settings as well.     

Sensory supply of the anterior uvea: A light and electron microscope study

Summary We have studied the ultrastructural features of sensory nerve fibers in the ciliary body and the iris and their parent trigeminal ganglion cells, using intra-axonally transported horseradish peroxidase as a tracer. Unmyelinated nerve fibers of ipsilateral trigeminal origin were found in stroma of both the ciliary body and the iris. Most appeared in bundles of nerve fibers; but some were found singly. Two distinct types of varicosities were found, one containing both a few irregularly shaped vesicles and mitochondria, the other containing mitochondria only. The thin profiles of the axons between these varicosities or endings contained neurotubules and filaments. The trigeminal ganglion cells supplying the anterior eye were of a relatively small size (15–50 m in diameter) and confined to the anteromedial part of the ipsilateral ganglion. These cells could not be classified according to the arrangement of their subcellular organelles. The morphological characteristics of the sensory nerve fibers and ganglion cells supplying the anterior uvea are consistent with the view that pain may be the predominant sensation mediated by these neurons.     

Multifunctional features of a gastrodermal sensory cell in Hydra: three-dimensional study

Summary Computer-assisted, three-dimensional reconstructions of two gastrodermal sensory cells from transmission electron micrographs of serial sections ofHydra revealed a unipolar morphology with the nucleus near an apical cilium and a simple unbranched axon with a widened terminal. The sensory cells were similar in size and shape to a unipolar sensory cell isolated from macerated gastrodermis and examined with scanning electron microscopy. In thin sections, the cells were characterized by the presence of numerous dense-cored vesicles in the axon and its terminal. A few dense-cored vesicles were aligned at electron-dense synaptic foci in the axon terminal of the sensory cell, which formed an axo-axonal synapse with a nearby centrally located ganglion cell and a neuromuscular synapse with the basal myoneme of a digestive cell. The ganglion cell possessed a perikaryal cilium and a slender axon that extended adjacent to the sensory cell terminal, where it formed anen passant axo-axonal synapse in reciprocal arrangement with that of the sensory cell. In addition, the ganglion cell axon formed a neuromuscular synapse in sequence with the sensory cell axo-axonal synapse. The presence of a large number of neurosecretory-like granules, apical cilium and reciprocal interneuronal and neuromuscular synaptic loci suggests that this gastrodermal sensory cell, characterized ultrastructurally for the first time, represents a third type of multifunctional neuron inHydra. Thus,Hydra may contain primitive stem-like neurons, which are sensory-motor and also function in both neurosecretion and neurotransmission.     

Ultrastructure studies on the papillae and the nonciliated sensory receptors of adultSpirometra erinacei (Cestoda,Pseudophyllidea)

The small numerous papillae on the ventral surface of the gravid proglottid of adultSpirometra erinacei were studied by scanning electron microscopy. The arrangement of clumps of papillae was recognized on the surface of the central portion around the genital atrium, with lateral clumps being located above a pair of longitudinal nerve cords and marginal ones, on both sides of the proglottid. By transmission electron microscopy, two types of nonciliated sensory receptors were observed within the papillae. The type I, single receptor was embedded within a papilla. This dome-like sensory receptor contained two electron-dense collars and four rootlets surrounded by numerous thin filaments. The type II receptor was found arranged in groups in the area between the papillae, and the apical end was exposed to the external environment. This simple, club-like sensory receptor contained electron-lucent vesicles and microtubules. We believe that the papillae play an important role in crossinsemination.     

Orbital passage of pterygopalatine ganglion efferents to paranasal sinuses and nasal mucosa in man

Parasympathetic nerves of pterygopalatine ganglion origin are considered to enter the orbit and distribute to the nasal mucosa with the anterior ethmoidal nerve. As their distribution has never been demonstrated the present study was undertaken to seek evidence of their passage and to identify their relationship with the ethmoidal nerves. The soft tissues of the pterygopalatine fossa and orbit from sixteen sides of twelve cadavers were removed in one piece and either dissected or cut coronally into slabs and prepared histologically using montages of thin resin-embedded sections at intervals suitable for nerve path tracing. Several of the rami orbitales passing mediodorsally from the ganglion enter the orbit apically, branch and enter the posterior ethmoidal foramen terminating in the lining of the paranasal sinuses and others advance to enter the anterior ethmoidal canal to reach the nasal mucosa. No junctions were made with ethmoidal nerves within the orbit or the canal. Failure of surgical lesions of the anterior ethmoidal nerve as a treatment for vasomotor rhinitis may be attributed to the sparing of the separate parasympathetic nerves. Appropriate chemical lesions, on the other hand, could ensure destruction of isolated parasympathetic nerves while limiting damage to the larger anterior ethmoidal nerve.     

The distribution of ganglion cells in the posterior cricoarytenoid muscle of the normal adult rat. A light and electron microscopic study

We employed by light and electron microscopy to examine the innervation of the posterior cricoarytenoid muscle of the adult rat. The laryngeal nerve was found to often bifurcate into two different bundles. One contained large myelinated (motor) nerve fibers, which were located along the frontal (ventral) muscle surface and entered the muscle at its middle portion to form neuromuscular contacts with individual muscle fibers. The other nerve bundle consisting of clustered ganglion cells (20-30 microm in diameter) and their associated nonmyelinated and small-sized myelinated nerve fibers were mainly found on the dorsal side of the muscle and often ran along the peripheral clefts or depressions of the muscle surface. The nerve bundle often extended side branches, which entered the muscle to be distributed among muscle fibers and near arterioles. Some ganglion cells are considered to enter the muscle, accompanied by branched nerves. Intramuscular ganglion cells and their associated nerve fibers examined by electron microscopy were similar in fine structure to perimuscular ganglion cells and their associated nerve fibers. Nerve fibers contained abundant clear synaptic vesicles which were cholinergic in nature, and often formed synapses with both neighboring axons and the cell body of the ganglion cells. These findings suggest that, in the rat posterior cricoarytenoid muscle, perimuscular and intramuscular ganglion cells exist and may be involved in innervating and contracting smooth muscle cells of the arterioles, thus regulating the blood flow or intravascular pressure.     

The growth of cochlear fibers and the formation of their synaptic endings in the avian inner ear: a study with the electron microscope

The developmental sequence of nerve-epithelial cell contacts, leading up to the formation of the mature receptoneuronal synapse, has been studied in the basilar papilla of chick embryos with electron microscopy. The receptor epithelium before innervation, on embryonic days 3-4, consists of a homogeneous population of primitive cells; hair cells and supporting cells cannot be distinguished. During innervation of the epithelium (embryonic days 5-7), the invading peripheral fibers of cochlear ganglion cells penetrate the basal lamina and form nerve-epithelial attachments with the epithelial cell bases. Once within the epithelium some fibers turn and spread in the transverse dimension across the basilar papilla through channels formed between the basal epithelial processes. Subsequently, nerve-epithelial attachments are observed more superficially within the epithelium. Hair cells and supporting cells differentiate during early synaptogenesis (embryonic days 8-9). Receptoneural synapses, possibly derived from the nerve-epithelial attachments formed during the innervation stage, are first seen during this period. They are characterized by symmetrical or asymmetrical membrane densities, separated by a cleft containing a dense material. At many of these junctions synaptic bodies, as well as dense-cored and coated vesicles, gather in the hair cells. During mid-synaptogenesis (embryonic days 11-13) the hair cells proliferate synaptic bodies, many of which are not located at receptoneural junctions. The preterminal portions of the sensory endings form large swellings, containing flocculent material, endoplasmic reticulum and vesicles. Late in synaptogenesis (embryonic days 15-17) the swellings disappear, while synaptic endings are transformed to foot-shaped terminals. In the hair cells, synaptic bodies not associated with junctions disappear. Efferent synapses are first seen during this period. This sequence of ultrastructural changes, which the developing sensory nerve endings and their target cells undergo in parallel, can be correlated with observations of Golgi preparations from a companion study. These correlations suggest that the innervation of the cochlea involves the following developmental processes. Initially the peripheral fibers of the ganglion cells grow directly toward the otocyst in fascicles. Having reached the base of the primitive receptor epithelium, the axonal endings, including some with growth cones, encounter a barrier in the basal lamina. When they enter some of the fibers attach to the basal end-feet of the primitive epithelial cells.(ABSTRACT TRUNCATED AT 400 WORDS)