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PubMed2000

Isolation of Helicobacter mustelae from ferrets in New Zealand.

AIMS: The bacterial genus Helicobacter contains over 20 species, including the human gastric pathogen H. pylori, and the mustelid-specific H. mustelae. A previous study in this country failed to isolate H. mustelae from a captive breeding colony of ferrets. We sought to confirm whether or not H. mustelae was present in this country. METHODS: A combination of bacterial culture, phenotypic testing and molecular techniques were used to isolate and identify gastric bacteria from captive and wild populations of ferrets in the New Zealand North Island. RESULTS: Bacteria were isolated from captive and wild ferrets which were phylogenetically identical to the type strain of H. mustelae. A mild to moderate gastritis was seen in five of six animals examined, and an antibody response to H. mustelae proteins was demonstrated. CONCLUSIONS: Helicobacter mustelae is not exotic to New Zealand, but is present in two populations of ferrets tested in the North Island.

Journal Article↗
PubMed1990

The Course of Fibre Diameter Classes Through the Chiasmatic Region in the Ferret.

The present study has examined the transition of fibre order from the optic nerve through the optic chiasm and into the optic tract in the ferret's retinofugal pathway. Semi-thin sections through the chiasmatic region were examined in normal and in monocularly enucleated ferrets in order to display the segregation of the different axon diameter classes as the fibres pass from the optic nerve into the optic tract, and to determine, for each diameter class, where the crossed and uncrossed components become separated in the chiasmatic region. As demonstrated in the preceding manuscript, fine and coarse optic axons begin to segregate from the medium optic axons rostral to the fusion of the two optic nerves. This segregation continues in the chiasmatic region where the different axon diameter classes decussate at different rostro-caudal levels: the fine and coarse diameter axons decussate rostrally, accumulating along the ventral, superficial surface of the contralateral half-chiasm, while the medium diameter axons continue caudally before crossing the midline. These two populations, a ventral, crossed (fine and coarse) population and a dorsal, yet-to-cross (medium) population are discriminable not only by their size in the chiasmatic region, but also by a thin invagination of hypothalamic neuropil separating them at their lateral extreme. The population of ipsilaterally projecting fibres is composed of both fine and medium optic axons. No coarse optic axons project ipsilaterally in the ferret: these fibres all decussate rostrally in the optic chiasm, intermingled with many of the decussating fine fibres. The fibre ordering in the adult ferret's optic chiasm is interpreted as reflecting a gradient of axonal arrival during development, with successively later arriving optic axons entering one of the two optic tracts at a progressively more superficial, rostral and ventral, location in the chiasmatic region. A fibre's position and time of arrival may influence its subsequent crossed or uncrossed course during the period of axonal ingrowth in development.

Journal Article↗
PubMed1996

Development of local connections in ferret somatosensory cortex.

Ferrets have become recognized as a useful and interesting model for study of neocortical development. Because of their immaturity at birth, it is possible to study very early events in the ontogeny of the brain. We used living slices of ferret somatosensory cortex to study the formation and development of intrinsic elements within the neocortex. A small number of fixed, hemisected brains injected with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were also used. The slices were obtained from ferret kits aged postnatal day (P)1 to P62 and maintained in a chamber; each slice received injections of fluorescent-labeled dextrans. The injections were made at different ages in several distinct sites, which included the proliferative ventricular zone, the intervening white matter (or intermediate zone), and different sites of developing cortex, including the deeper cortical plate, which incorporated the subplate in young animals, and more superficial cortical sites depending on the age of the animal. Several animals also received injections into the ventrobasal thalamus. Injections into young animals (P1-7) produced a dominant radial pattern that extended from the ventricular zone into the cortex. Injections into the ventricular zone labeled many cells that appeared morphologically like radial glia as well as presumptive neurons. Although the predominant pattern was radial, injections in the ventricular zone often produced tangentially oriented cells and horizontally arranged fibers at the outer edge of the proliferative zone. These cells and fibers may provide a substrate for tangential dispersion of neurons within the neocortex. More superficial injections within the slice labeled lines of cells that appeared to be stacked upon one another in a radial pile in the cortex; the cortical plate received very few lateral projections. Data obtained from more mature slices indicated that although the overall pattern of staining remained radial, the precise character of the pattern changed to include more lateral spread into surrounding cortex, which eventually refined and developed into distinct patches by P28, when the overall cortical architecture appeared adult like. The data involving thalamocortical connections were more limited, but they indicated that the thalamus projects precisely to the somatosensory cortex in a point-to-point fashion from the earliest date studied (P0) and that the ventrobasal nucleus terminates upon the somatosensory cortex in a patchy manner during the early postnatal days of development. This study of the development of the somatosensory cortex confirms the ubiquitous nature of column-like connections throughout the neocortex and provides a novel view of the radial nature of early neocortical maturation.

Animals↗
PubMed1996

Maturational gradients in the retina of the ferret.

In the present set of studies, we have examined the site for the initiation of retinal maturation in the ferret. A variety of maturational features across the developing inner and outer retina were examined by using standard immunohistochemical, carbocyanine dye labelling, and Nissl-staining techniques, including 1) two indices of early differentiation of the first-born retinal ganglion cells, the presence of beta-tubulin and of neuron-specific enolase; 2) the receding distribution of chondroitin sulfate proteoglycans within the inner retina; 3) the distribution of the first ganglion cells to grow axons along the optic nerve; 4) the emergence of the inner plexiform layer; 5) the emergence of the outer plexiform layer and 6) the onset of synaptophysin immunoreactivity within it; 7) the differentiation of calbindin-immunoreactive horizontal cells; and 8) the cessation of proliferative activity at the ventricular surface. Although we were able to define distinct maturational gradients that are associated with many of these features of inner and outer retinal development (each considered in detail in this report), with dorsal retina maturing before ventral retina, and with peripheral retina maturing last, none showed a clear initiation in the region of the developing area centralis. Rather, maturation began in the peripapillary retina dorsal to the optic nerve head, which is consistent with previous studies on the topography of ganglion cell genesis in the ferret. These results make clear that the order of retinal maturation and the formation of the area centralis are not linked, at least not in the ferret.

Animals↗
PubMed1996

Development of axonal arbors of layer 6 pyramidal neurons in ferret primary visual cortex.

We followed the development of axonal arbors of layer 6 pyramidal neurons in ferret striate cortex to determine whether early developing axon collaterals are formed specifically in the correct target layers from the outset or achieve their adult specificity by the elimination of initially exuberant projections. These neurons were chosen for study because they are amongst the first to be generated in the developing ferret's striate cortex, and, in mature animals, these cells have axonal arbors that are highly specific for layer 4 and to a lesser extent layers 2/3 but have few collateral branches in layer 5. The axonal arbors of individual layer 6 pyramidal neurons were reconstructed following labeling in living slices prepared from the striate cortex of ferrets aged 13-35 days postnatal (P13-35). At the earliest ages (P13-15), axonal arbors consisted of a simple axon extending from the base of the cell body into the subplate or white matter and usually forming a few collateral branches but never ascending into layer 5. By P19-20, about one-half of the cells had extended axon collaterals into layer 5 or higher, and these already appeared to branch preferentially in layer 4. All of the cells from older animals had substantial axonal arbors in layers 2-4. By P26-28, there were approximately ten times as many axonal branches in layer 4 as in layer 5. Between P26-28 and P35, there was no significant change in the number of branches in layer 5, but the numbers of both branches and of axon collateral terminations in layer 4 approximately doubled. Thus, the extent of axonal arborization in layer 4 increases dramatically between P13 and P35, and growth is highly specific for correct target layers, with few branches formed in layer 5.

Animals↗
PubMed2000

Development of inhibitory circuitry in visual and auditory cortex of postnatal ferrets: immunocytochemical localization of calbindin- and parvalbumin-containing neurons.

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is thought to play an important role in activity-dependent stages of brain development. Previous studies have shown that different functional subclasses of cortical GABA-containing neurons can be distinguished by antibodies to the calcium-binding proteins parvalbumin and calbindin. Thus insight into the development of distinct subsets of inhibitory cortical circuits can be gained by studying the development of these calcium-binding protein-containing neurons. Previous studies in several mammalian species have suggested that calcium-binding proteins are upregulated in sensory cortex when thalamocortical afferents arrive. In ferrets, the ingrowth of thalamic axons into cortex occurs well into postnatal development, allowing access to early stages of cortical development and calcium-binding protein expression. We find in ferrets that both parvalbumin- and calbindin-immunoreactivity are present in primary visual and primary auditory cortex long before thalamocortical synapse formation, but that there is a sharp decline in immunoreactivity by postnatal day 20. Day 20 in ferrets corresponds to postnatal day 1 in cats, and thus previous studies in postnatal cats would have missed this early pattern of calcium-binding protein distribution. Another surprising finding is that the proportion of parvalbumin- and calbindin-immunoreactive neurons peaks secondarily late in development, between P60 and adulthood. This result suggests that the parvalbumin- and calbindin-containing subclasses of nonpyramidal neurons remain immature until late in the critical period for cortical plasticity, and that they are positioned to play an important role in experience-dependent modification of cortical circuits.

Animals↗
PubMed1999

An autoradiographic assessment of epithelial cell proliferation and post-natal maturation of the tracheal epithelium in infant ferrets.

The tracheal epithelium of infant ferrets undergoes rapid postnatal maturation over the first month of life to achieve the pseudostratified columnar configuration characteristic of the large airways of other mammals. We have used in vivo pulsing with tritiated thymidine ((3)HT) to elicit autoradiographic labeling of cells synthesizing nucleic acids in order to characterize more fully the contribution to development of different cell types comprising the nascent epithelial layer during this period of rapid growth. These studies indicate that two distinct populations of epithelial cells possess proliferative potential and contribute to the establishment of the mature adult epithelial layer. These investigations further confirm the mitotic potential of basal cells during a period of rapid postnatal growth and development of the tracheal epithelial layer. These studies also document the contribution to early airway development by non-ciliated cells, which predominate on the luminal border of the ferret trachea at birth. The temporal and histologic patterns of airway epithelial maturation during post-natal life in the ferret as contained in this study exhibit similarities to those which occur with recovery from injury by infection and irritant exposure in mature airways. Thus, the characterization of epithelial cell compartments having proliferative potential may provide insights into the mechanisms whereby normal airway epithelial organization is established and maintained during development as well as the possible recapitulation of these mechanisms during times of epithelial regeneration following injury.

Animals↗
PubMed1998

Ontogeny of postcanine tooth form in the ferret, Mustela putorius (Carnivora: Mammalia), and the evolution of dental diversity within the Mustelidae.

This study describes dental development within the ferret, Mustela putorius, through study of the form of the carnassial teeth and the upper first molar at progressive growth stages. Primordial teeth were serially sectioned in sagittal and transverse planes and three-dimensional reconstructions of tooth primordia were generated using MacReco software. Regional growth of the crown and asynchronous maturation of the dental tissues were observed in each tooth. The upper carnassial blade develops early and the tooth increases in length rapidly. Lingual growth of the upper carnassial is less pronounced and the protocone and its surrounding region mature late. The lower carnassial blade develops early and the talonid is late to mature. Development of the upper first molar differs from carnassial development in the early emphasis upon transverse growth and reduced lengthwise expansion. The early development of the carnassial blades in the ferret is shared with other carnivores, and may reflect the functional significance of this feature. Later stages of tooth ontogeny differ among carnivoran taxa and the specialized morphology of ferret teeth results from an apparently truncated period of late tooth ontogeny. This suggests that carnivoran species may share a common path of early development that specifies the ontogeny of homologous tooth features and that in later stages developmental differences result in species-specific tooth forms.

Animals↗
PubMed1996

Neurogenesis and cell migration into the sexually dimorphic preoptic area/anterior hypothalamus of the fetal ferret.

A sexually dimorphic male nucleus (MN) of the preoptic area/anterior hypothalamus (POA/AH), comprising large, estradiol-receptor containing neurons, is formed in male ferrets due to the action of estradiol, derived from the neural aromatization of circulating testosterone, during the last quarter of a 41-day gestation. Two experiments were conducted to compare the birthdates and the migration pattern of cells into the sexually dimorphic portion of the dorsomedial POA/AH as well as the nondimorphic ventral nucleus (VN) of the POA/AH of males and females. In experiment 1 the thymidine analog, bromodeoxyuridine (BrdU), was injected into the amniotic sacs of fetuses of different mothers between embryonic (E) days 18 and 30. Kits from all mothers were sacrificed on E38, and brains were processed to localize BrdU immunoreactivity (IR) for determining the birthdates of neurons in the POA/AH. Cells in the MN-POA/AH of males and in a comparable region of females were born between E22 and E28; cells in the nondimorphic VN-POA/AH of both sexes were born between these same ages. These results suggest that cells in the sexually dimorphic as well as the nondimorphic subdivision of the ferret POA/AH are born during the same embryonic period. This is well before the ages (E30-E41) when administering testosterone to females can stimulate, and blocking androgen aromatization in males can inhibit, MN-POA/AH differentiation. In experiment 2 BrdU was injected on E24, and kits from different litters were perfused on E30, E34, or E38. Brains were processed for BrdU-IR as well as glial fibrillary acidic protein (GFAP), which served as a marker for radial glial processes. The orientation of radial glial processes in fetal brains of both sexes suggested that cells migrate into the dorsomedial POA/AH from proliferative zones lining the lateral as well as the third ventricles. Quantitative, computer-assisted image analysis of BrdU-IR in groups of male and female brains supported this hypothesis. There were no significant sex differences in the distribution of BrdU-IR over the three ages studied, suggesting that formation of the MN-POA/AH in males cannot be attributed to an effect of estradiol on the migration of those cells born on E24 into this sexually dimorphic structure. Finally, total BrdU-IR did not change significantly in the POA/AH of male and female kits killed at E30, E34, or E38 while the area of the POA/AH increased more than 2.5-fold over this period, suggesting that few of the POA/AH cells born on E24 die during this period in either sex. In the absence of evidence that formation of the male ferret's MN-POA/AH depends on steroid-induced changes in neurogenesis, cell migration, or death, we suggest that the specification of a particular neuronal phenotype (e.g., large somal size; capacity to produce some undetermined neurotransmitter or neuropeptide) may be responsible.

Animals↗
PubMed1983

The early development of the AV node and bundle in the ferret heart.

The development of the atrioventricular (AV) junctional tissues in the ferret embryonic heart was studied on days 16, 18, and 21 of gestation. This important region of the heart was examined with PAS and toluidine-blue staining at the light microscope level and with transmission electron microscopy at the ultrastructural level. By day 16 of gestation the ferret heart was in the initial stages of convolution. The heart was at the primitive four-chamber stage by 18 days postcoitum. On day 21 of gestation the heart was in the process of being septated. The AV nodal primordia were first observed as two clusters of cells in the dorsal wall of the common atrium of the 16 day ferret embryo heart. These nodal primordial cells were morphologically different from working myocardial cells or cells of the AV canal. The AV canal cells were particularly numerous in the dorsal wall of the canal and eventually gave rise to the AV bundle in this region. On day 18 of gestation the morphological differences between the AV nodal, AV canal, and myocardial cells were readily apparent. By 21 days postcoitum, the AV node with its two regions had reached its definitive anatomic position. The AV bundle was also present in its normal adult location. The AV nodal cells were distinctly different when compared to the ventricular or atrial myocytes at this stage in development. In addition, the AV bundle cells were morphologically different from the AV nodal cells and working myocardial cells. A discussion of these findings relates this information to current descriptions of how the AV node and bundle develop.

Animals↗
PubMed1989

Terminal arbors of single ON-center and OFF-center X and Y retinal ganglion cell axons within the ferret's lateral geniculate nucleus.

The lateral geniculate nucleus of the ferret contains not only eye-specific layers, but a further subdivision of layers A and A1 into inner and outer sublaminae that contain, respectively, ON-center and OFF-center cells (Stryker and Zahs, '83). To study how the arbors of single retinal ganglion cell axons correlate with these cellular divisions, we have examined the morphology of physiologically classified retinal axons in the ferret's lateral geniculate nucleus. As in cats, we could classify retinal axons as X or Y on the basis of a number of physiological criteria. X and Y axons have distinct patterns of termination in the lateral geniculate nucleus. Contralateral X axons innervate lamina A and ipsilateral axons lamina A1. X axons are further segregated in these laminae so that ON-center axons terminate in the inner sublamina, and OFF-center axons in the outer sublamina. We did not observe any branches of X axons innervating the medial interlaminar nucleus or the midbrain. Y axons have much larger terminal arbors and exhibit greater variation in their terminations. Generally, within layers A and A1, ON-center Y axons innervate the inner sublamina and OFF-center Y axons innervate the outer sublamina. However, they often innervate both sublaminae, and occasionally have a few boutons in the inappropriate lamina as well. Y axons also terminate in the dorsal C laminae, the interlaminar zones, and the media interlaminar nucleus; branches of these axons course toward the midbrain, presumably to innervate the superior colliculus. Thus, whereas the Y pathway in the ferret is one of high divergence, the X pathway appears to be the substrate for segregated ON and OFF channels through the lateral geniculate nucleus.

Animals↗
PubMed1991

Serotoninergic innervation of the ferret cerebral cortex. II. Postnatal development.

We have investigated the serotoninergic innervation of the ferret cortex from the day of birth to adulthood with immunohistochemical techniques. Due to the premature birth of ferrets, this period spans the entire generation of cells located within the upper cortical layers and their subsequent migration to their final positions. Already at birth, serotoninergic fibers innervate the developing cortex. This innervation is most dense within the marginal zone, the subplate region, and the lower portion of the cortical plate. As long as cell migration continues, serotoninergic fibers enter the expanding portions of the cortex. Only the region just below the marginal zone where newly arriving cells are added to the cortical plate is not innervated by the ingrowing fibers. When the bulk of cell migration ceases, during the third postnatal week, this gap disappears and the fibers gradually form a continuous innervation from the pia to the ventricle. As the cortex matures, the serotoninergic fibers become successively confined to the upper layers, to generate the adult pattern. In the adult ferret cortex, the highest innervation density is found within layers 1, 2, and 3, with a much sparser innervation within the lower layers (Voigt and de Lima, J. Comp. Neurol. 314:403-414, 1991). The dense innervation in the deep cortical layers is only transient, virtually disappearing toward adulthood. These results suggest that serotoninergic axons innervate cortical layers as soon as newly arriving cells reach their final positions within the cortex. This early innervation lends support to the idea that serotonin may play a role during development of the cerebral cortex.

Aging↗
PubMed1992

Position of growth cones within the retinal nerve fibre layer of fetal ferrets.

Optic axons are added to the retinal nerve fibre layer of fish along its vitreal border in a chronotopic manner. Likewise, the optic tract of all vertebrate species acquires axons preferentially along the superficial surface of the pathway. We have examined the developing retina of fetal ferrets (Mustela putorius furo) aged between embryonic day 27 (E27) and E34 to see whether a similar segregation of growth cones is apparent within the mammalian retinal nerve fibre layer. The distributions of growth cone, "wrist" (thick trailing portion of the growth cone), axonal, and glial profiles were determined from electron micrographs, and expressed as a percentage of neural profiles for several retinal locations. The retinal nerve fibre layer of fetal ferrets contains radially elongated bundles of fibres composed of axonal, wrist, and growth cone profiles. Glial processes of varying density divide the adjacent bundles, occasionally subdividing them in the plane of the retina, and give rise to endfeet lining the basal lamina and separating the optic axons from the latter. Growth cones within the developing fibre layer represented about 2.4% of profiles at E28, while at later developmental stages (E34), this value fell to about 0.6%. During this period of axonal outgrowth, growth cones were not preferentially segregated toward the vitreal basal lamina or the glial endfeet within the nerve fibre layer. Rather, they were found scattered throughout the axon bundles of the fibre layer. While there were differences in the proportion of immature profiles found within the vitreal half compared to the scleral half of the fibre layer, such that more growth cones and wrists were found vitreally, there was no clear accumulation of them in association with features of the vitreal margin. The present results show that young and old optic axons course together throughout the depth of the nerve fibre layer. A chronotopic mode of pathway genesis such as seen in the optic fibre layer of fish or in the optic tract of mammals is not present in the nerve fibre layer of ferrets. Differences in growth cone behaviour in the optic fibre layer and tract indicate that the mechanisms governing pathway formation differ along its course.

Animals↗
PubMed1993

Trigeminal nuclear complex of the ferret: anatomical and immunohistochemical studies.

In order to establish the ferret as an animal model for studies of trigeminal pain, we describe the cytoarchitecture and neurochemistry of the trigeminal nuclear complex in the ferret and compare them to those of the cat and rat. The complex was divided as previously described, but the ferret differed in the extent of the nuclear boundaries. The neuroanatomical istribution of substance P-, calcitonin gene-related peptide-, galanin-, enkephalin-, serotonin-, somatostatin-, neuropeptide Y-, and neurotensin-immunoreactivity was determined throughout the rostrocaudal extent of the complex. In subnucleus caudalis, substance P-, calcitonin gene-related peptide-, enkephalin-, serotonin-, somatostatin-, neuropeptide Y-, and galanin-immunoreactivity was densest in laminae I and II. In subnucleus interpolaris, immunoreactivity for all the above neurochemicals was most dense along the lateral border and the ventral third of the caudal part of the subnucleus. Enkephalin-immunoreactive cell bodies were present in subnucleus caudalis and interpolaris. In subnucleus oralis, labelling for substance P, calcitonin gene-related peptide, galanin, enkephalin, and serotonin was most prominent in the dorsomedial part of the subnucleus. Somatostatin-immunoreactive cell bodies were distributed throughout the spinal nucleus. Labelling of serotonin, substance P, calcitonin gene-related peptide, galanin, enkephalin, and somatostatin was present in the main sensory nucleus. The motor nucleus contained fibers immunoreactive for substance P, enkephalin, serotonin and neuropeptide Y, and cell bodies immunoreactive for calcitonin gene-related peptide. The majority of neurotensin-immunoreactivity was found at the level of subnucleus caudalis, where it was densest in the trigeminal extension of the lateral cervical nucleus. The distribution of peptides in this species throughout the spinal nucleus is consistent with the notion that all the subnuclei may be involved in the processing of nociceptive inputs.

Animals↗
PubMed1993

Laterality of superior olive projections to the inferior colliculus in adult and developing ferret.

The laterality of projections from the lateral superior olivary nucleus (LSO) to the inferior colliculus was studied in adult and immature postnatal ferrets. In the adult ferret, large unilateral injections of horseradish peroxidase (HRP) in the inferior colliculus labeled about equal proportions of cells in the ipsilateral and contralateral lateral superior olivary nuclei. The contralateral labeled cells consistently were more densely labeled than those on the ipsilateral side. Double labeling experiments using fluorescent dyes indicated that only about 3% of LSO cells in the adult give rise to collaterals ending in the inferior colliculus on both sides. As expected, the distribution of labeled cells varied topographically in the LSO as a function of the injection site in the inferior colliculus. Dorsolateral inferior collicular injections labeled cells in the lateral limb of the LSO, whereas ventromedial injections labeled cells in the medial limb of the LSO. The proportion of ipsilateral and contralateral labeled cells also varied across the lateral-medial axis of the LSO in some cases. A gradient in laterality was observed in these cases with the lateral limb of the LSO containing the highest proportion of contralateral labeled cells, and the medial limb, the highest proportion of ipsilateral labeled cells. Larger inferior collicular injections resulted in greater proportions of ipsilateral labeling in LSO than smaller injections. Finally, ipsilateral labeled cells tended to be in the marginal region of the LSO, whereas contralateral labeled cells were more common within the core region of the LSO, irrespective of the location along the lateral-medial axis of LSO. The contralateral predominance of labeled cells, greater density of labeling in contralateral cells, different topographic distribution, and regional segregation of ipsilateral and contralateral labeled cells were typical of the LSO in ferret kits by birth, one month before the onset of hearing. Nevertheless, the relative proportion of ipsilateral and contralateral projection cells appears to change during postnatal development.

Animals↗
PubMed1993

Sequential differentiation of sensory innervation in the mystacial pad of the ferret.

The mystacial pad of the ferret has an elaborate sensory innervation provided by three types of terminal nerves that arise from the infraorbital branch of the trigeminal nerve. Deep and superficial vibrissal nerves innervate nearly exclusive targets in the large follicle-sinus complexes (F-SCs) at the base of each tactile vibrissa. Dermal plexus nerves innervate the fur between the vibrissae. Each type of nerve provides a similar variety of sensory endings, albeit to different targets. In this study, Winkelmann and Sevier-Munger reduced silver techniques revealed that most of the endings differentiate postnatally in an overlapping sequence like that observed previously in the rat. Afferents from the deep vibrissal nerves begin to differentiate first, followed successively by those from superficial vibrissal nerves and the dermal plexus. Within each type of nerve, Merkel endings begin to differentiate first, followed successively by lanceolate endings and circumferential endings. In the ferret, the differentiation of the intervibrissal fur and its innervation is slightly delayed but substantially overlaps the development of the vibrissal innervation, whereas in the rat it occurs almost entirely later. There was no evidence of a transient exuberant or misplaced innervation or other secondary remodeling. Differentiating afferents and endings are located only in the sites normally seen in the adult, suggesting a high degree of afferent-target specificity. In the ferret, innervation is virtually lacking in one target--the inner conical body of the F-SCs, which is densely innervated in the rat. This lack was due to a failure of innervation to develop rather than to a secondary elimination of a transient innervation.

Animals↗
PubMed1993

Experimentally induced visual projections to the auditory thalamus in ferrets: evidence for a W cell pathway.

We have previously reported that following specific neonatal brain lesions in ferrets, a retinal projection is induced into the auditory thalamus (Sur et al., Science 242:1437, '88). In these "rewired" ferrets, a novel visual pathway is established through auditory thalamus [the medial geniculate nucleus (MGN)] and primary auditory cortex (A1); cells in both MGN and A1 are visually responsive and exhibit properties similar to those of visual cells in the normal visual pathway. In this paper, we use three approaches--physiological, anatomical, and developmental--to examine which of the retinal ganglion cells project to the MGN in these rewired ferrets. We find that: 1) physiological response properties of postsynaptic visual cells in the MGN are W-like; 2) retinal ganglion cells back-filled from the MGN are small and similar to soma sizes of subsets of the normal retinal W cell population; and 3) subpopulations of these small cells can be preferentially rerouted to the MGN in response to different surgical manipulations at birth, consistent with differential W cell projection patterns in normal animals. These data suggest that retinal W cells come to project to the MGN in rewired animals. These findings not only provide a basis on which to interpret functional properties of this novel visual pathway, but also provide important information about the developmental capabilities of specific retinal ganglion cell classes and the regulation of their projections by target structures in the brain during development.

Animals↗
PubMed1993

Visual projections induced into the auditory pathway of ferrets: II. Corticocortical connections of primary auditory cortex.

Although the development of corticocortical projections has been well studied, less is known about the role of sensory inputs in the specification of these connections. As part of an ongoing series of studies in our laboratory, we have examined the role of thalamic input modality in the development of corticocortical connections. These studies involve making unilateral lesions and inducing retinal inputs into the auditory thalamus (MGN) during early development in ferrets, thereby conferring visual responsiveness on primary auditory cortex (AI). In this way we can examine the role of input identity in cortical specification in general, and connectivity patterns specifically. A previous paper (Pallas et al. [1990] J. Comp. Neurol. 298:50-68) described the pattern of thalamocortical and corticothalamic connections of auditory cortex in normal and lesioned animals. This study compares the pattern of auditory corticocortical connections in normal and lesioned animals. We injected neuroanatomical tracers into AI and mapped out the distribution of retrogradely labelled cells in the cortex. We report that the cortical inputs to ferret AI resembled those in cats, and that the pattern of ipsi- and contralateral corticocortical connections of ferret AI with visual input was similar to the normal pattern. Auditory cortex with visual input did not make ectopic connections with visual cortex, but maintained its connections with other auditory cortical areas. These results suggest that the overall corticocortical connections of an area are not influenced by the modality or activity pattern of its inputs. In particular, altering the input activity to a cortical area does not seem to promote the formation of entirely new connections, although small changes in the strength of existing connections are possible (Sur et al. [1990] Trends Neurosci. 13:227-233).

Animals↗