Ultrastructure and distribution of epidermal sensory receptors in the beak of the echidna, Tachyglossus aculeatus

Knowing when to stick: touch receptors found in the remora adhesive disc

Remoras are fishes that piggyback onto larger marine fauna via an adhesive disc to increase locomotor efficiency, likelihood of finding mates and access to prey. Attaching rapidly to a large, fast-moving host is no easy task, and while research to date has focused on how the disc supports adhesion, no attention has been paid to how or if remoras are able to sense attachment. We identified push-rod-like mechanoreceptor complexes embedded in the soft lip of the remora adhesive disc that are known in other organisms to respond to touch and shear forces. This is, to our knowledge, the first time such mechanoreceptor complexes are described in fishes as they were only known previously in monotremes. The presence of push-rod-like mechanoreceptor complexes suggests not only that fishes may be able to sense their environment in ways not heretofore described but that specialized tactile mechanoreceptor complexes may be a more basal vertebrate feature than previously thought.

Knowing when to stick: touch receptors found in the remora adhesive disc

Remoras are fishes that piggyback onto larger marine fauna via an adhesive disc to increase locomotor efficiency, likelihood of finding mates and access to prey. Attaching rapidly to a large, fast-moving host is no easy task, and while research to date has focused on how the disc supports adhesion, no attention has been paid to how or if remoras are able to sense attachment. We identified push-rod-like mechanoreceptor complexes embedded in the soft lip of the remora adhesive disc that are known in other organisms to respond to touch and shear forces. This is, to our knowledge, the first time such mechanoreceptor complexes are described in fishes as they were only known previously in monotremes. The presence of push-rod-like mechanoreceptor complexes suggests not only that fishes may be able to sense their environment in ways not heretofore described but that specialized tactile mechanoreceptor complexes may be a more basal vertebrate feature than previously thought.

Evolutionary Specialization of Tactile Perception in Vertebrates

Evolution has endowed vertebrates with the remarkable tactile ability to explore the world through the perception of physical force. Yet the sense of touch remains one of the least well understood senses at the cellular and molecular level. Vertebrates specializing in tactile perception can highlight general principles of mechanotransduction. Here, we review cellular and molecular adaptations that underlie the sense of touch in typical and acutely mechanosensitive vertebrates.

Timing of mammalian peripheral trigeminal system development relative to body size: A comparison of metatherians with rodents and monotremes

Specializations of the trigeminal sensory system are present in all three infraclasses of mammals (metatheria, eutheria, prototheria or monotremata). The trigeminal sensory system has been suggested as a critically important modality for sampling the path to the pouch and detecting the nipple or milk patch, but the degree to which that system may be required to function at birth varies significantly. Archived sections of the snout and brainstem of embryonic and postnatal mammals were used to test the relationship between structural maturity of the two ends of the trigeminal nerve pathway and the body size of mammalian young in metatherians, rodents and monotremes. A system for staging different levels of structural maturity of the vibrissae and trigeminal sensory was applied to embryos, pouch young and hatchlings and correlated with body length. Dasyurids are born at the most immature state with respect to vibrissal and trigeminal sensory nucleus development of any available metatherian, but these components of the trigeminal system are also developmentally advanced relative to body size when dasyurids are compared to other metatherians. Vibrissal and trigeminal sensory nucleus development is at a similar stage of development at birth and for a given body size in non-dasyurid metatherians; and trigeminal sensory nucleus development in monotremes is at a similar stage at birth to metatherians. Rodents reach a far more advanced stage of vibrissal and trigeminal sensory nucleus development at birth than do metatherians, and in the case of the mouse have a more developmentally advanced trigeminal system than all available metatherians at any given body length. Precocious development of the trigeminal sensory pathway relative to body size is evident in dasyurids, as might be expected given the small birth size of those metatherians. Nevertheless, the trigeminal sensory system in metatherians in general is not precocious relative to body size when these species are considered alongside the pace of trigeminal somatosensory development in rodents.

Brain and behaviour of living and extinct echidnas

The Tachyglossidae (long- and short-beaked echidnas) are a family of monotremes, confined to Australia and New Guinea, that exhibit striking trigeminal, olfactory and cortical specialisations. Several species of long-beaked echidna (Zaglossus robusta, Zaglossus hacketti, Megalibgwilia ramsayi) were part of the large-bodied (10 kg or more) fauna of Pleistocene Australasia, but only the diminutive (2-7 kg) Tachyglossus aculeatus is widespread today on the Australian mainland. We used high-resolution CT scanning and other osteological techniques to determine whether the remarkable neurological specialisations of modern echidnas were also present in Pleistocene forms or have undergone modification as the Australian climate changed in the transition from the Pleistocene to the Holocene. All the living and extinct echidnas studied have a similar pattern of cortical gyrification that suggests comparable functional topography to the modern short-beaked form. Osteological features related to olfactory, trigeminal, auditory and vestibular specialisation (e.g., foramina and cribriform plate area, osseous labyrinth topography) are also similar in living and extinct species. Our findings indicate that despite differences in diet, habitat and body size, the suite of neurological specialisations in the Tachyglossidae has been remarkably constant: encephalisation, sensory anatomy and specialisation (olfactory, trigeminal, auditory and vestibular), hypoglossal nerve size and cortical topography have all been stable neurological features of the group for at least 300,000 years.

Distinct development of the trigeminal sensory nuclei in platypus and echidna

Both lineages of the modern monotremes have been reported to be capable of electroreception using the trigeminal pathways and it has been argued that electroreception arose in an aquatic platypus-like ancestor of both modern monotreme groups. On the other hand, the trigeminal sensory nuclear complex of the platypus is highly modified for processing tactile and electrosensory information from the bill, whereas the trigeminal sensory nuclear complex of the short-beaked echidna (Tachyglossus aculeatus) is not particularly specialized. If the common ancestor for both platypus and echidna were an electroreceptively and trigeminally specialized aquatic feeder, one would expect the early stages of development of the trigeminal sensory nuclei in both species to show evidence of structural specialization from the outset. To determine whether this is the case, we examined the development of the trigeminal sensory nuclei in the platypus and short-beaked echidna using the Hill and Hubrecht embryological collections. We found that the highly specialized features of the platypus trigeminal sensory nuclei (i.e. the large size of the principal nucleus and oral part of the spinal trigeminal nuclear complex, and the presence of a dorsolateral parvicellular segment in the principal nucleus) appear around the time of hatching in the platypus, but are never seen at any stage in the echidna. Our findings support the proposition that the modern echidna and platypus are derived from a common ancestor with only minimal trigeminal specialization and that the peculiar anatomy of the trigeminal sensory nuclei in the modern platypus emerged in the ornithorhynchids after divergence from the tachyglossids.

Distinct development of peripheral trigeminal pathways in the platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus)

The extant monotremes (platypus and echidnas) are believed to all be capable of electroreception in the trigeminal pathways, although they differ significantly in the number and distribution of electroreceptors. It has been argued by some authors that electroreception was first developed in an aquatic environment and that echidnas are descended from a platypus-like ancestor that invaded an available terrestrial habitat. If this were the case, one would expect the developmental trajectories of the trigeminal pathways to be similar in the early stages of platypus and short-beaked echidna development, with structural divergence occurring later. We examined the development of the peripheral trigeminal pathway from snout skin to trigeminal ganglion in sectioned material in the Hill and Hubrecht collections to test for similarities and differences between the two during the development from egg to adulthood. Each monotreme showed a characteristic and different pattern of distribution of developing epidermal sensory gland specializations (electroreceptor primordia) from the time of hatching. The cross-sectional areas of the trigeminal divisions and the volume of the trigeminal ganglion itself were also very different between the two species at embryonic ages, and remained consistently different throughout post-hatching development. Our findings indicate that the trigeminal pathways in the short-beaked echidna and the platypus follow very different developmental trajectories from the earliest ages. These findings are more consistent with the notion that the platypus and echidna have both diverged from an ancestor with rudimentary electroreception and/or trigeminal specialization, rather than the contention that the echidna is derived from a platypus-like ancestor.

Development of the dorsal and ventral thalamus in platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus)

The living monotremes (platypus and echidnas) are distinguished from therians as well as each other in part by the unusual structure of the thalamus in each. In particular, the platypus has an enlarged ventral posterior (VP) nucleus reflecting the great behavioural importance of trigeminosensation and electroreception. The embryological collections of the Museum für Naturkunde in Berlin were used to analyse the development of the dorsal thalamus and ventral thalamus (prethalamus) in both species. Prosomeric organization of the forebrain emerged at 6 mm crown-rump length (CRL), but thalamic neurogenesis did not commence until about 8-9 mm CRL. Distinctive features of the dorsal thalamus in the two species began to emerge after hatching (about 14-15 mm CRL). During the first post-hatching week, dense clusters of granular cells aggregated to form the VP of the platypus, whereas the VP complex of the echidna remained smaller and divided into distinct medial and lateral divisions. At the end of the first post-hatching week, the thalamocortical tract was much larger in the platypus than the echidna. The dorsal thalamus of the platypus is essentially adult-like by the sixth week of post-hatching life. The similar appearance of the dorsal thalamus in the two species until the time of hatching, followed by the rapid expansion of the VP in the platypus, is most consistent with ancestral platypuses having undergone changes in the genetic control of thalamic neurogenesis to produce a large VP for trigeminal electroreception after the divergence of the two lineages of monotreme.

Vascular actions of estrogens: functional implications

The impact of estrogen exposure in preventing or treating cardiovascular disease is controversial. But it is clear that estrogen has important effects on vascular physiology and pathophysiology, with potential therapeutic implications. Therefore, the goal of this review is to summarize, using an integrated approach, current knowledge of the vascular effects of estrogen, both in humans and in experimental animals. Aspects of estrogen synthesis and receptors, as well as general mechanisms of estrogenic action are reviewed with an emphasis on issues particularly relevant to the vascular system. Recent understanding of the impact of estrogen on mitochondrial function suggests that the longer lifespan of women compared with men may depend in part on the ability of estrogen to decrease production of reactive oxygen species in mitochondria. Mechanisms by which estrogen increases endothelial vasodilator function, promotes angiogenesis, and modulates autonomic function are summarized. Key aspects of the relevant pathophysiology of inflammation, atherosclerosis, stroke, migraine, and thrombosis are reviewed concerning current knowledge of estrogenic effects. A number of emerging concepts are addressed throughout. These include the importance of estrogenic formulation and route of administration and the impact of genetic polymorphisms, either in estrogen receptors or in enzymes responsible for estrogen metabolism, on responsiveness to hormone treatment. The importance of local metabolism of estrogenic precursors and the impact of timing for initiation of treatment and its duration are also considered. Although consensus opinions are emphasized, controversial views are presented to stimulate future research.

Response properties of primary afferents supplying Eimer's organ

The mole's nose is covered with mechanosensory structures called Eimer's organs. Each organ contains Merkel cell-neurite complexes, Paciniform corpuscles and intraepidermal free nerve endings. The function of Eimer's organ has been the subject of speculation since the 1800s, but responses from the afferents have never been investigated. Our goal was to explore the function of Eimer's organ by recording primary afferent responses to a range of mechanosensory stimuli. Unit activity from the trigeminal ganglion was recorded from coast (Scapanus orarius) and star-nosed (Condylura cristata) moles, while stimulating the nose with a Chubbuck mechanosensory stimulator, a piezo-electric sweeping stimulator, and hand-held probes. Stimuli included static indentations, sinusoidal displacements, different indentation velocities, displacement amplitudes, and directional stimuli across the skin. Receptive fields were small, sometimes restricted to single Eimer's organs. Responses were consistent with a slowly adapting Merkel cell-neurite complex-like receptor class and a dynamically sensitive Pacinian-like rapidly adapting class. A second rapidly adapting class was hypothesized to represent activity of prominent free nerve endings within a central cell column. Some receptors were most sensitive to stimuli applied in particular directions across the skin. Most receptors relayed mechanosensory input with high temporal fidelity. In addition some receptors were tuned to respond best when stimulated at a velocity matching the velocity of the nose during foraging. These results support the hypothesis that Eimer's organ functions to detect small surface features and textures by encoding and integrating deflection information for multiple Eimer's organs during brief touches.

Fine structure of Eimer's organ in the coast mole (Scapanus orarius)

Eimer's organ is a small, densely innervated sensory structure found on the glabrous rhinarium of most talpid moles. This structure consists of an epidermal papilla containing a central circular column of cells associated with intraepidermal free nerve endings, Merkel cell neurite complexes, and lamellated corpuscles. The free nerve endings within the central cell column form a ring invested in the margins of the column, surrounding 1-2 fibers that pass through the center of the column. A group of small-diameter nociceptive free nerve endings that are immunoreactive for substance P surrounds this central ring of larger-diameter free nerve endings. Transmission electron microscopy revealed a high concentration of tonofibrils in the epidermal cells of the central column, suggesting they are more rigid than the surrounding keratinocytes and may play a mechanical role in transducing stimuli to the different receptor terminals. The intraepidermal free nerve endings within the central column begin to degrade 15 microm from the base of the stratum corneum and do not appear to be active within the keratinized outer layer. The peripheral free nerve endings are structurally distinct from their counterparts in the central column and immunocytochemical double labeling with myelin basic protein and substance P indicates these afferents are unmyelinated. Merkel cell-neurite complexes and lamellated corpuscles are similar in morphology to those found in a range of other mammalian skin.

Cyto- and Chemoarchitecture of the Cerebellum of the Short-Beaked Echidna (Tachyglossus aculeatus)

The monotremes (echidnas and platypus) have been claimed by some authors to show 'avian' or 'reptilian' features in the gross morphology and microscopic anatomy of the cerebellum. We have used Nissl staining in conjunction with enzyme histochemistry to acetylcholinesterase and cytochrome oxidase and immunohistochemistry to non-phosphorylated neurofilament protein (SMI-32 antibody), calcium binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase to examine the cyto- and chemoarchitecture of the cerebellar cortex and deep cerebellar nuclei in the short-beaked echidna. Immunoreactivity for non-phosphorylated neurofilament (SMI-32 antibody) was found in the deep cerebellar nuclei and in Purkinje cells of most regions except the nodule. Purkinje cells identified with SMI-32 immunoreactivity were clearly mammalian in morphology. Parvalbumin and calbindin immunoreactivity was found in Purkinje cells with some regional variation in staining intensity and in Purkinje cell axons traversing cerebellar white matter or terminating on Lugaro cells. Calbindin immunoreactivity was also present in inferior olivary complex neurons. Calretinin immunoreactivity was found in pontocerebellar fibers and small cells in the deep granule cell layer of the ansiform lobule. We found that, although the deep cerebellar nuclei were much less clearly demarcated than in the rodent cerebellum, it was possible to distinguish medial, interposed and lateral nuclear components in the echidna. As far as we can determine from our techniques, the cerebellum of the echidna shows all the gross and cytological features familiar from the cerebellum of therian mammals.

Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat

We have examined the cyto- and chemoarchitecture of the trigeminal nuclei of two monotremes using Nissl staining, enzyme reactivity for cytochrome oxidase, immunoreactivity for calcium binding proteins and non-phosphorylated neurofilament (SMI-32 antibody) and lectin histochemistry (Griffonia simplicifolia isolectin B4). The principal trigeminal nucleus and the oralis and interpolaris spinal trigeminal nuclei were substantially larger in the platypus than in either the echidna or rat, but the caudalis subnucleus was similar in size in both monotremes and the rat. The numerical density of Nissl stained neurons was higher in the principal, oralis and interpolaris nuclei of the platypus relative to the echidna, but similar to that in the rat. Neuropil immunoreactivity for parvalbumin was particularly intense in the principal trigeminal, oralis and interpolaris subnuclei of the platypus, but the numerical density of parvalbumin immunoreactive neurons was not particularly high in these nuclei of the platypus. Neuropil immunoreactivity for calbindin and calretinin was relatively weak in both monotremes, although calretinin immunoreactive somata made up a large proportion of neurons in the principal, oralis and interpolaris subnuclei of the echidna. Distribution of calretinin immunoreactivity and Griffonia simplicifolia B4 isolectin reactivity suggested that the caudalis subnucleus of the echidna does not have a clearly defined gelatinosus region. Our findings indicate that the trigeminal nuclei of the echidna do not appear to be highly specialized, but that the principal, oralis and interpolaris subnuclei of the platypus trigeminal complex are highly differentiated, presumably for processing of tactile and electrosensory information from the bill.

Sensory receptors in monotremes

This is a summary of the current knowledge of sensory receptors in skin of the bill of the platypus, Ornithorhynchus anatinus, and the snout of the echidna, Tachyglossus aculeatus. Brief mention is also made of the third living member of the monotremes, the long-nosed echidna, Zaglossus bruijnii. The monotremes are the only group of mammals known to have evolved electroreception. The structures in the skin responsible for the electric sense have been identified as sensory mucous glands with an expanded epidermal portion that is innervated by large-diameter nerve fibres. Afferent recordings have shown that in both platypuses and echidnas the receptors excited by cathodal (negative) pulses and inhibited by anodal (positive) pulses. Estimates give a total of 40,000 mucous sensory glands in the upper and lower bill of the platypus, whereas there are only about 100 in the tip of the echidna snout. Recording of electroreceptor-evoked activity from the brain of the platypus have shown that the largest area dedicated to somatosensory input from the bill, S1, shows alternating rows of mechanosensory and bimodal neurons. The bimodal neurons respond to both electrosensory and mechanical inputs. In skin of the platypus bill and echidna snout, apart from the electroreceptors, there are structures called push rods, which consist of a column of compacted cells that is able to move relatively independently of adjacent regions of skin. At the base of the column are Merkel cell complexes, known to be type I slowly adapting mechanoreceptors, and lamellated corpuscles, probably vibration receptors. It has been speculated that the platypus uses its electric sense to detect the electromyographic activity from moving prey in the water and for obstacle avoidance. Mechanoreceptors signal contact with the prey. For the echidna, a role for the electrosensory system has not yet been established during normal foraging behaviour, although it has been shown that it is able to detect the presence of weak electric fields in water. Perhaps the electric sense is used to detect moving prey in moist soil.

The development of the electroreceptors of the platypus (Ornithorhynchus anatinus)

A series of developmental stages of the platypus were examined to obtain an anatomical description of the development of the periphery of the electroreceptive system. Putative electroreceptors, composed of modified mucous glands, were observed to appear at 10 days post hatching (p.h.). The typical striped arrangement of peripheral electroreceptors in the platypus was seen at 12 days p.h. The arrangement of the stripes was modified during development with a range of additions and divisions of stripes occurring until the adult pattern is obtained, approximately 6 months p.h. After appearing at 10 days p.h., the number of electroreceptors increases rapidly until sometime between 24 and 28 days p.h. when there is massive death of electroreceptors, the number present at 28 days p.h. being 60% of the number present at 24 days p.h. This massive death of receptors is coincident with the appearance of other sensory structures in the epidermis of the bill skin, the push-rod mechanoreceptors and the sensory serous glands. Histological examination of a range of developmental stages demonstrated poorly differentiated innervation at 28 days p.h., which became differentiated and reached the adult configuration between 11 weeks p.h. and 6 months p.h., the time at which nestling platypuses leave the burrow. Lamination of the cells lining the duct of the electroreceptors showed a similar developmental profile. This study indicates that the electroreceptive system of the developing platypus is not functional, in a similar manner to the adult, until it is time for the platypus to leave the nesting burrow. However, the system may be functional in the developing platypus, and may be used speculatively in the location of the mammary region for suckling.

The sensory world of the platypus

Vision, audition and somatic sensation in the platypus are reviewed. Recent work on the eye and retinal ganglion cell layer of the platypus is presented that provides an estimate of visual acuity and suggests that platypus ancestors may have used vision, as well as the bill organ, for underwater predation. The combined electroreceptor and mechanoreceptor array in the bill is considered in detail, with special reference to the elaborate cortical structure, where inputs from these two sensory arrays are integrated in a manner that is astonishingly similar to the stripe-like ocular dominance array in primate visual of cortex, that integrates input from the two eyes. A new hypothesis, along with supporting data, is presented for this combined mechanoreceptive-electroreceptive complex in platypus cortex. Bill mechanoreceptors are shown to be capable of detecting mechanical waves travelling through the water from moving prey. These mechanical waves arrive after the electrical activity from the same prey, as a function of distance. Bimodal cortical neurones, sensitive to combined mechanical and electrical stimulation, with a delay, can thus signal directly the absolute distance of the prey. Combined with the directional information provided by signal processing of the thousands of receptors on the bill surface, the stripe-like cortical array enables the platypus to use two different sensory systems in its bill to achieve a complete, three-dimensional 'fix' on its underwater prey.

Distribution and putative function of autonomic nerve fibres in the bill skin of the platypus (Ornithorhynchus anatinus)

The electroreceptors located in the bill skin of the platypus are modified secretory glands. The electroreceptive nerve terminals form bare endings in close proximity to the duct of these glands. In this study, we describe the autonomic innervation of the glands and a separate specialized autonomic innervation of the epidermal portion of the glandular duct. A range of immunohistochemical labels showed that the gland cells of the electroreceptors have a non-noradrenergic (putative parasympathetic) innervation. Phalloidin labelling revealed a 'sphincter' of epidermal luminal cells that labelled strongly for actin. These actin-dense keratinocytes were seen to have a noradrenergic (putative sympathetic) innervation. Fine-diameter sensory fibres containing substance P (presumably C-fibre thermoreceptors or polymodal nociceptors) were observed to terminate in the superficial epidermis surrounding the pore of the gland. When the bill of the platypus is dry these pores were closed. However, when room temperature water was washed over the bill, the pores opened. It is proposed that this autonomic and sensory innervation, along with the actin sphincter, mediates the opening and closing of the pores. By doing this, the platypus prevents the desiccation of the bare electrosensory nerve terminals when it is out of the water, and it may also be a way to regulate the impedance of the internal electrical circuit presented to the water at the pores.

Electroreception and the feeding behaviour of platypus ( Ornithorhynchus anatinus : Monotremata: Mammalia)

It has previously been shown that platypus are sensitive to small electrical fields. It was predicted that platypus use their electrosensitivity to locate the source of foodstuffs on the bottom of the freshwater river systems in which they live, because the platypus are nocturnal, and close their eyes, ears and nostrils while underwater. In this paper we demonstrate for the first time that platypus are indeed sensitive to electrical waveforms that imitate the electromyogenic potential’s of fleeing prey, and following stimulation show interest in area surrounding the electrodes. We also show that platypus respond with a reflex after stimulation with a square wave, and show that this reflex is directionally tuned to the origin of the electrical pulse, with a preferential sensitivity axis 40 times more sensitive than non-preferred axes. The strong directional sensitivity explains previous discrepancies in the lowest threshold for platypus electroreception, which we find to be 50 μV cm -1 . Platypus are also sensitive to galvanic fields. We present the data in the light of standardized feeding strategies of the platypus, and discuss the integration of the findings into these feeding strategies. We surrounded our platypus enclosure with a Faraday cage, thereby eliminating excess electrical noise, a suggested new addition to the husbandry regime of platypus.

Studies of mechanoreceptors in skin of the snout of the echidna Tachyglossus aculeatus

The echidna Tachyglossus aculeatus, together with the platypus, belongs to the monotremes, a group of mammals with a number of reptilian characteristics. A structure unique to the skin of monotremes is the push rod-a compacted column of epidermal cells that is 20 microns wide and 100 microns long with its tip at the skin surface, and that is able to move relatively independently of adjacent tissue. At the base of each push rod is a cluster of encapsulated nerve endings. Push rods are common in skin of the snout and have been postulated to have a mechanosensory function. Experiments were carried out on four anesthetized echidnas with the aim of determining the function of push rods. Recordings made from the infraorbital nerve, which supplies the skin of the upper jaw, yielded responses from a total of 46 afferents. Two were electroreceptors; the others were mechanoreceptors. Within the group of mechanoreceptors with rapidly adapting responses, three responded to high-frequency vibration and resembled pacinian corpuscles. There were 26 slowly adapting (SA) mechanoreceptors, which, based on the regularity of their discharge, could be divided into two groups: SA I or Merkel type, and SA II or Ruffini type. SA I receptors had very discrete receptive fields with diameters of 100 microns. The receptive fields of two SA I receptors were marked, and after histological processing, one was seen to lie near two push rods. It is concluded that mechanoreceptor responses in the echidna's snout skin resemble those in other mammals in many aspects. We could not unequivocally associate responses to mechanical stimulation with the push rods.

Organization of somatosensory cortex in monotremes: in search of the prototypical plan

The present investigation was designed to determine the number and internal organization of somatosensory fields in monotremes. Microelectrode mapping methods were used in conjunction with cytochrome oxidase and myelin staining to reveal subdivisions and topography of somatosensory cortex in the platypus and the short-billed echidna. The neocortices of both monotremes were found to contain four representations of the body surface. A large area that contained neurons predominantly responsive to cutaneous stimulation of the contralateral body surface was identified as the primary somatosensory area (SI). Although the overall organization of SI was similar in both mammals, the platypus had a relatively larger representation of the bill. Furthermore, some of the neurons in the bill representation of SI were also responsive to low amplitude electrical stimulation. These neurons were spatially segregated from neurons responsive to pure mechanosensory stimulation. Another somatosensory field (R) was identified immediately rostral to SI. The topographic organization of R was similar to that found in SI; however, neurons in R responded most often to light pressure and taps to peripheral body parts. Neurons in cortex rostral to R were responsive to manipulation of joints and hard taps to the body. We termed this field the manipulation field (M). The mediolateral sequence of representation in M was similar to that of both SI and R, but was topographically less precise. Another somatosensory field, caudal to SI, was adjacent to SI laterally at the representation of the face, but medially was separated from SI by auditory cortex. Its position relative to SI and auditory cortex, and its topographic organization led us to hypothesize that this caudal field may be homologous to the parietal ventral area (PV) as described in other mammals. The evidence for the existence of four separate representations in somatosensory cortex in the two species of monotremes indicates that cortical organization is more complex in these mammals than was previously thought. Because the two monotreme families have been separate for at least 55 million years (Richardson, B.J. [1987] Aust. Mammal. 11:71-73), the present results suggest either that the original differentiation of fields occurred very early in mammalian evolution or that the potential for differentiation of somatosensory cortex into multiple fields is highly constrained in evolution, so that both species arrived at the same solution independently.

Distribution patterns of the paraneuronal endocrine cells in the skin, gills and the airways of fishes as determined by immunohistochemical and histological methods

The neuro-endocrine cells of fish skin and respiratory surfaces, and their bioactive secretion as far as is known, are reviewed, and compared with similar elements in tetrapods, particularly amphibians. In the skin of teleost fish, immunohistochemistry has shown that Merkel cells react for serotonin, neuron-specific enolase and enkephalins. The pharmacology is not established in dipnoans or lampreys. In some teleosts, neuromasts react for substance P and leu-enkephalins; substance P is also reported from some ampullary organs (electroreceptors). Taste buds of teleosts may react for enkephalin and substance P. Basal cells of taste buds react for serotonin and neuron-specific enolase. Some unicellular skin glands of teleosts express bioactive compounds, including serotonin and some peptides; this ectopic expression is paralleled in amphibian skin glands. The dipnoan Protopterus has innervated pulmonary neuro-endocrine cells in the pneumatic duct region with dense-cored vesicles. In Polypterus and Amia the lungs have serotonin-positive neuro-endocrine cells that are apparently not innervated. In fish gills, a closed type of neuro-endocrine cell reacts for serotonin, an open type for enkephalins and some calcium-binding proteins (calbindin, calmodulin and S-100 protein). The functions of neuro-endocrine cells in fishes await investigation, but it is assumed they are regulatory.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

The evolution of the dorsal pallium in the telencephalon of amniotes: cladistic analysis and a new hypothesis

The large body of evidence that supports the hypothesis that the dorsal cortex and dorsal ventricular ridge of non-mammalian (non-synapsid) amniotes form the dorsal pallium and are homologous as a set of specified populations of cells to respective sets of cells in mammalian isocortex is reviewed. Several recently taken positions that oppose this hypothesis are examined and found to lack a solid foundation. A cladistic analysis of multiple features of the dorsal pallium in amniotes was carried out in order to obtain a morphotype for the common ancestral stock of all living amniotes, i.e., a captorhinomorph amniote. A previous cladistic analysis of the dorsal thalamus (Butler, A.B., The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis, Brain Res. Rev., 19 (1994) 29-65; this issue, previous article) found that two fundamental divisions of the dorsal thalamus can be recognized--termed the lemnothalamus in reference to predominant lemniscal sensory input and the collothalamus in reference to predominant input from the midbrain roof. These two divisions are both elaborated in amniotes in that their volume is increased and their nuclei are laterally migrated in comparison with anamniotes. The present cladistic analysis found that two corresponding, fundamental divisions of the dorsal pallium were present in captorhinomorph amniotes and were expanded relative to their condition in anamniotes. Both the lemnothalamic medial pallial division and the collothalamic lateral pallial division were subsequently further markedly expanded in the synapsid line leading to mammals, along with correlated expansions of the lemnothalamus and collothalamus. Only the collothalamic lateral pallial division--along with the collothalamus--was subsequently further markedly expanded in the non-synapsid amniote line that gave rise to diapsid reptiles, birds and turtles. In the synapsid line leading to mammals, an increase in the degree of radial organization of both divisions of the dorsal pallium also occurred, resulting in an 'outside-in' migration pattern during development. The lemnothalamic medial division of the dorsal pallium has two parts. The medial part forms the subicular, cingulate, prefrontal, sensorimotor, and related cortices in mammals and the medial part of the dorsal cortex in non-synapsid amniotes. The lateral part forms striate cortex in mammals and the lateral part of dorsal cortex (or pallial thickening or visual Wulst) in non-synapsid amniotes. Specific fields within the collothalamic lateral division of the dorsal pallium form the extrastriate, auditory, secondary somatosensory, and related cortices in mammals and the visual, auditory, somatosensory, and related areas of the dorsal ventricular ridge in non-synapsid amniotes.