Embryonic maturation of sensory terminals of primate facial hairs

Developmental timing of hair follicle and dorsal skin innervation in mice

The innervation of hair follicles offers an intriguing, yet hardly studied model for the dissection of the stepwise innervation during cutaneous morphogenesis. We have used immunofluorescence and a panel of neuronal markers to characterize the developmental choreography of C57BL/6 mouse backskin innervation. The development of murine skin innervation occurs in successive waves. The first cutaneous nerve fibers appeared before any morphological evidence of hair follicle development at embryonic day 15 (E15). Stage 1 and 2 developing hair follicles were already associated with nerve fibers at E16. These fibers approached a location where later in development the follicular (neural) network A (FNA) is located on fully developed pelage hair follicles. Prior to birth (E18), some nerve fibers had penetrated the epidermis, and an additional set of perifollicular nerve fibers arranged itself around the isthmus and bulge region of stage 5 hair follicles, to develop into the follicular (neural) network B (FNB). By the day of birth (P1), the neuropeptides substance P and calcitonin gene-related peptide became detectable in subcutaneous and dermal nerve fibers first. Newly formed hair follicles on E18 and P1 displayed the same innervation pattern seen in the first wave of hair follicle development. Just prior to epidermal penetration of hair shafts (P5), peptide histidine methionine-IR nerve fibers became detectable and epidermal innervation peaked; such innervation decreased after penetration (P7- P17). Last, tyrosine hydroxylase-IR and neuropeptide Y-IR became readily detectable. This sequence of developing innervation consistently correlates with hair follicle development, indicating a close interdependence of neuronal and epithelial morphogenesis.

Timecourse of development of the wallaby trigeminal pathway. I. Periphery to brainstem

The development of the vibrissae and their innervation and the maturation of the brainstem trigeminal sensory nuclei have been studied in the wallaby, Macropus eugenii, from birth to adulthood. At birth, developing vibrissal follicles consist of solid epidermal pegs surrounded by dermal condensations. The developing follicles and adjacent skin are innervated by trigeminal afferents. Ten days after birth the follicle contains a dermal papilla and the deep vibrissal nerve can be recognised. A hair cone is present at postnatal day (P) 30 and hairs are apparent on the skin surface by P35. By P63 the deep vibrissal nerve can be seen innervating Merkel cells in the outer root sheath; in addition, the first signs of the blood sinus can be recognised. Innervation of the inner conical body and lanceolate and lamellated receptors supplying the mesenchymal sheath and waist region are not seen until P119, when the follicle resembles that seen in the adult. At birth, central processes of the trigeminal ganglion cells have entered the trigeminal tract and extend from the rostral pons to the upper cervical cord. Labelling with a carbocyanine dye at P0 shows afferents extending medially from the tract into the trigeminal subnuclei at all levels. At this stage the trigeminal nuclei appear as areas of increased cell density in the lateral brainstem. By P30-40 the four subnuclei can be distinguished on the basis of shape, cytoarchitecture, and succinic dehydrogenase reactivity. Adult morphology is not fully established until P210. In mature animals, nucleus principalis contains closely packed, polymorphic cells, frequently aligned parallel to thick fibre bundles that traverse the nucleus obliquely. Subnuclei oralis and interpolaris contain sparsely distributed, medium to large cells, randomly oriented, as well as prominent rostrocaudally directed fibre bundles. Subnucleus caudalis consists of the marginal layer, substantia gelatinosa, and magnocellular layers as described in other species. Patches of increased succinic dehydrogenase or cytochrome oxidase reactivity, presumably corresponding to the vibrissae, are present in subnuclei principalis, interpolaris, and caudalis in developing and adult animals, although the pattern is less clear than in rats. The brainstem patches are first seen at P40, approximately 6 weeks before the corresponding vibrissal-related pattern develops in the cortex. This suggests that the onset of patch formation may be regulated independently at different levels of the pathway.

Ontogeny of nerves and neuropeptides in skin of rat: an immunocytochemical study

The sequence of maturation of nerves and appearance of neuropeptides was investigated in skin from fetal and neonatal rats by immunocytochemistry using antisera to protein gene product 9.5, substance P, calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY). Immunoreactivity for PGP 9.5 appeared on fetal day 16 in face and nose, somewhat later (fetal day 19) in paws and tail. The sensory neuropeptides, CGRP/substance P (fetal day 19 and postnatal day 1, respectively) appeared earlier than the autonomic peptides VIP and NPY (postnatal day 7). Thus, the study shows that neuropeptides do not appear simultaneously with nerves and that the development is rostrocaudal.

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.

The differentiation of the skin and its appendages. I. Normal development of papillary ridges

In the present study, the normal development of papillary ridges was studied in the volar pads of both fore and hindpaws of the opossum, Monodelphis domesticus. At birth, the developmental state of the opossum's paws is equivalent to that of a six-week human embryo. The development of papillary ridges in the opossum occurs entirely postnatally and the hindpaw lags behind the forepaw by at least four days in most developmental parameters. Papillary ridge formation is preceded by four events: skin innervation, Merkel cell differentiation, mesenchymal condensation, and epidermal proliferation. The apical pads at the tips of the digits and the interdigital pads between the heads of the metacarpals (or metatarsals) have a unique pattern of innervation and mesenchymal content as compared to the non-pad skin. Each pad is innervated by a prominent nerve trunk and axons ascend towards the epidermis providing a density of innervation that exceeds that in the non-pad epidermis. Merkel cells are absent in non-pad epidermis but present in the pads prior to the onset of formation of papillary ridges. A loose aggregation of mesenchyme forms the core of the pads and the superficial dermis is more cellular in the pads as compared to the equivalent dermis in surrounding non-pad skin. Developing papillary ridges always contained Merkel cell-axon complexes. Merkel cell axon complexes serve as the anatomical substrate of slowly adapting (SA) mechanoreceptors. The presence of these complexes during early skin differentiation is consistent with the use of the opossum's forepaw in climbing to the nipple, but also suggests other possible functions. We hypothesize that the nervous system might play a role in the timing or patterning of the formation of papillary ridges.

The pattern and timing of cutaneous hair follicle innervation in the rat pup and human fetus

The postnatal development of hair follicle innervation was studied in the rat hindlimb using a silver stain which detects large and medium calibre cutaneous nerve fibres. The pattern and timing of innervation in relation to postnatal changes in follicle growth were studied providing new data on nerve-target interactions in the developing peripheral nervous system. Sensory axons begin to leave the dermal plexus and grow towards follicles at P (postnatal day) 3 but do not start to innervate them until P7 or achieve an adult appearance until P19. The first terminals are circumferential, followed some days later by the appearance of palisade endings. The number of axons innervating a hair follicle increases steadily with age until P19 and there is no evidence of exuberant innervation of follicles during development. Hair follicle density in the rat is maintained during development due to waves of small, vellus follicle growth later in postnatal life as the skin grows. The percentage of follicles innervated however, decreases from the second postnatal week onwards presumably because late developing vellus hairs do not become innervated. Comparative analysis in human fetal abdominal skin using the same silver stain reveals a similar sequence and pattern of innervation to the rat over the period of 22 to 35 weeks EGA (estimated gestational age). Human skin does not, however, undergo the late waves of follicle growth seen in the rat. Follicular density decreases and the percentage of innervated follicles increases in the third trimester of fetal life.

Myotome and early neurogenesis in chick embryos

The present study was undertaken in order to verify the identification of profiles of presumptive growth cones in vivo. The developing spinal nerves of chick embryos were studied by light and electron microscopy. We traced the onset of efferent and afferent innervation of the myotome in 2- to 4-day-old chick embryos in order to be sure that we were examining the growing tips of axons. In the process of studying these growing axons, we were able to observe some unique relationships of neural tube, myotome, and differentiating spinal nerves. The neural tube tightly abuts the myotome in Hamburger and Hamilton's (HH) stage 14 chick embryos and cytoplasmic projections from the myotome directly abut the neural tube. The first ventral roots could be identified in HH stage 15 embryos and dorsal roots in HH stage 16 embryos, both under 2 1/2 days of age. The advancing spinal nerve courses toward the anterior or cranial half of the myotome, and growth cones directly contact the medial wall of the myotome. The spinal nerves continue to abut tightly the myotome during the succeeding day of embryonic life, and growth cones enter the substance of the myotome by 3 days, or HH stage 19 embryos. These dorsolaterally directed axons will form the dorsal ramus of the spinal nerves and the ventral ramus continues to be contiguous with the myotome. Invasion of the myotome by axons (putative innervation), and thus innervation of myotomal cells in the 3-day chick embryos, was a totally unexpected finding. The myotome and its potential derivatives thus have extensive neural contact by 3 days of embryonic life in the chick. These findings document a parallel differentiation of afferent and efferent elements of the nervous system and confirm previous accounts identifying growth cones in an intact organism. These findings suggest that afferent as well as efferent nerves may have critical roles in the differentiation of the mesodermal as well as ectodermal derivatives.

The development of Meissner corpuscles in primate digital skin

Digital skin from fetal and neonatal primates was examined using light and electron microscopic techniques to document the development of the Meissner corpuscle. Generation of the receptor was initiated early in the third trimester by fine neurites which left the superficial dermal nerve plexus, ascended the dermal papillae and entered the basal epidermis. As maturation of the Meissner corpuscle continued, the axons were confined to the apex of the dermal papilla, where they were oriented parallel to the surface to the surface of the skin and terminated among cytoplasmic extensions of presumptive lamellar cells. During late fetal life the complexity of the lamellar cell stacking increased and the lamellar cell bodies were found solely at the base of the receptor. Numerous axon terminals were evident between the cytoplasmic lamellae. The appearance of the neonatal Meissner corpuscle was indistinguishable from that of the adult, indicating that the complete cycle of development is concluded before birth.

The early ontogeny of the afferent nerves and papillary ridges in human digital glabrous skin

The present study examines the early ontogeny of afferent nerves in human embryonic glabrous digital skin and documents the onset of cutaneous innervation and papillary (sweat duct) ridge formation by light and electron microscopy. The skin examined in this study was taken from 3 developmental stages of decreasing embryonic age: embryos older than 10 weeks estimated gestational age (EGA) representing the period of primary ridge formation, embryos of 8-9 weeks EGA representing the period immediately prior to ridge formation; and embryos 6-8 weeks EGA representing the period weeks before the onset of ridge formation. The earliest papillary ridges are present in 10 week EGA embryos, with small ridges present in two sites: the center of the proximal third and also at the tip of the distal phalangeal or apical pad. These papillary ridges typically contained Merkel cells. Papillary ridges formed progressively in a radial manner from these central foci. The proximal focus corresponds to the geometric center of the mature dermatoglyphic pattern of loops, arches, or whorls. This radial wave of ridge differentiation is discontinuous with the abrupt cessation of ridge formation responsible for the discontinuities in the mature papillary ridges and the corresponding dermatoglyphic print. Skin over the proximal and middle phalanges developed papillary ridges beginning in the 12th week. No papillary ridges could be identified in embryos of 8-9 weeks EGA, but a large number of growth cones are present in the superficial dermis subjacent to differentiating Merkel cells. The basal lamina of the epidermis was discontinuous wherever growth cones abutted Merkel cells. Merkel cells not directly associated with axons were also present in the epidermis of embryos of 8-9 weeks EGA. The embryos of 6-8 weeks EGA lack any sign of Merkel cells and/or melanocytes, but developing neurovascular bundles with axonal growth cones near the epidermis could be identified by light and electron microscopy. Presumptive Schwann and perineural cells are also seen in the dermis. We conclude that the developing afferent nerve fibers provide a grid which influences the temporal and/or spatial factors involved in the sequential onset and cessation of formation of papillary ridges. Thus the dermatoglyph can reflect the ontogeny of the afferent nervous system that occurred prior to papillary ridge development. These observations lend support to the concept that successive waves of afferent neural development have an important role in the spatial and temporal sequence of papillary ridge formation and thus the formation of both the dermatotopic map of the digits and the dermatoglyph.

Degeneration and regeneration of peripheral nerve in the rat trigeminal system: III. Abnormal sensory reinnervation of rat guard hairs following nerve transection and crush

The present study was undertaken in an attempt to better understand the abnormalities of cutaneous sensibility that are present in patients following nerve injury with concomitant cutaneous denervation and subsequent reinnervation. Reinnervated intervibrissal pelage of the rat mystacial pad was studied in silver-impregnated sections 3 and 5 months after transecting and 2 and 5 months after crushing the infraorbital nerve. The sensory terminals on guard and vellus hairs were analyzed in serial paraffin sections and in thick frozen sections. In normal rat mystacial skin, approximately nine/ten of innervated guard hairs have a typical piloneural complex consisting of a palisade of highly regular lanceolate terminals surrounded by circularly arranged Ruffini terminals and free nerve endings (FNEs). The remaining one of ten innervated guard hairs has only circularly arranged presumptive FNEs and Ruffini terminals. Vellus hairs, either singly or in clusters, typically have only circularly arranged terminals that in many cases are simple FNEs. We first recognized abnormalities in innervation of hairs following nerve transection and fully expected nerve terminals to be completely normal following nerve crush. Almost all reinnervated sensory nerve terminals associated with guard hairs were markedly abnormal following nerve transection and quantitatively abnormal following nerve crush. Following nerve transection, lanceolate terminals were almost completely absent, and they were remarkably reduced in number following nerve crush. Vellus hairs when reinnervated typically lacked the complex circular presumptive Ruffini terminals. These findings may be in part the basis for the abnormal cutaneous sensory perceptions (dysasthesias and paresthesias) noted in human subjects following damage to nerves with subsequent sensory reinnervation of the skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Cryostat sections for coexistence studies and preembedding electron microscopic immunocytochemistry of central and peripheral nervous system tissue

Perfusion-fixed tissue blocks were incubated in high molar sucrose solutions, shock frozen in melting isopentane, and sectioned on a conventional cryostat. Semithin sections (2-4 microns) alternatingly stained for parvalbumin and glutamate decarboxylase enabled us to demonstrate the coexistence of both antigens in the same cell. Thick sections (40 microns) of central and peripheral nervous system tissue were immunostained and processed for correlated light and electron microscopic studies. At the electron microscopic level, the preservation of ultrastructural features such as membranes and synaptic contacts was comparable to that normally seen in vibratome sectioned material. Hence, this technique can successfully be used for preembedding coexistence studies and electron microscopic preembedding immunocytochemistry when vibratome sectioning is problematic.

Neural modulation of cutaneous differentiation: epidermal hyperplasia and precocious hair development following partial neuralectomy in opossum pups

The original intent of the present study was to evaluate the compensatory response of the nervous system to areas of denervation. A portion of the spinal cord in the lumbosacral region of one-day opossum pups (Monodelphis domesticus) was removed by cauterization. This partial neuralectomy produced an expected compensatory response of neurons in the dorsal root ganglia, but in addition produced unexpected abnormalities of cutaneous differentiation. At 4-6 days following surgery, an increase in the thickness of the epidermis resembling glabrous palmar or plantar skin was seen. This hyperplastic epidermis appeared to be associated with an abnormally dense innervation of the dermis and epidermis. Eight days following partial neuralectomy most animals showed areas of precocious hair development. Nerve fibers were always seen in the dermis associated with these precocious hairs and were seen to penetrate the basal lamina in the region of the epidermal-hair shaft boundary. These results imply a critical role for afferent nerves in the normal development of the skin and its appendages.

Successive waves of differentiation of cutaneous afferents in rat mystacial skin

The present study has traced the sequence of maturation of sensory receptors in the mystacial pad of postnatal rats. At birth the follicle-sinus complexes (F-SC) are well innervated by deep vibrissal nerves although the number of axons entering the sinus is less than that in the adult. The innervation of the F-SC by the conus or superficial vibrissal nerves derived from skin nerves that form the superficial dermal nerve plexus is limited to the Merkel rete ridge collar at birth, and the innervation to the inner conical body is conspicuously absent. The inner conical body innervation begins to appear 3-4 days after birth and rapidly matures over the week. By 3 weeks of age the F-SCs have a mature sensory innervation. At birth small guard hairs are present in the intervibrissal pelage and are associated with scant axons of the superficial dermal nerve plexus, but no mature sensory terminals are present. The sensory innervation of the intervening pelage begins to differentiate during the second week and mature piloneural complexes can be recognized by 3 weeks of age. Innervation to vellus hairs is still developing at 3-4 weeks of age. These maturational changes in peripheral sensory innervation correlate with gradual changes in the structure of barrels in the first somatosensory cortex (SI). Sequential waves of differentiation of sensory receptors appear to be a general feature of neural development.

Vascularization of the rat cornea after prolonged zinc deficiency

Neovascularization of the anterior stroma of the rat cornea was associated with prolonged zinc deficiency (in this model). There was also an increase in the myelinated nerves of the cornea. Blood vessels were not observed in the corneas of the pair-fed and ad-libitum-fed control animals. The invading blood vessels were frequently associated with Schwann cells and neurites. Unmyelinated nerves were observed in the corneal stroma of all three experimental groups.

The early embryogenesis of papillary (sweat duct) ridges in primate glabrous skin: the dermatotopic map of cutaneous mechanoreceptors and dermatoglyphics

The present study documents the early innervation of the epidermis prior to the onset of differentiation of the papillary (sweat duct) ridge in glabrous digital skin of rhesus monkey embryos measuring 45, 50 and 55 mm (crown-rump) length. We observed small papillary ridges, spaced at a distance of approximately 40 microns, projecting into the dermis in the center of the distal glabrous digital pad of digits 2-5 in the 55-mm embryo. The other digital pads lacked any sign of ridge formation. A two-dimensional, approximately hexagonal grid of afferent nerves was present in the superficial dermis of all digital and palmar pads. At regular intervals of approximately 40 microns, afferent nerves ascended from the superficial dermal nerve plexus and innervated the overlying epidermis. By electron microscopy, axonal growth cones were identified contacting Merkel cells that projected several microns down into the superficial dermis in the digital pad of digit 3. Thus, the earliest wave of differentiated dorsal root ganglion neuroblasts innervates Merkel cells. Schwann cells partially encircled these growing axon tips and could be identified by the presence of rough endoplasmic reticulum and free ribosomes. The youngest embryo studied had no sign of ridge formation; however, axons ascended from the superficial dermal nerve net at 30-40-microns intervals to innervate the epidermis. We conclude that afferent nerve fibers provide a two-dimensional grid that could modulate the spacing and arrangement of the papillary or sweat duct ridges of successive digits. Such an interaction is possible between digits based on the overlapping dermatotopic maps of each rete ridge. An abnormal fingerprint could thus reflect abnormal dorsal root ganglion neuroblasts expressed through mesenchyme and epidermis.

Early differentiation of the afferent nervous system in glabrous snout skin of the opossum (Monodelphis domesticus)

Early differentiation of afferent fibers innervating the snout skin of Monodelphis domesticus was studied by electron microscopy and by light-microscopic silver staining techniques. This study was undertaken to investigate the relationship between dermal and epidermal innervation in the neonate opossum while correlating these findings with behavioral responses. The advantage of using this species is that the neonate is born in a very immature stage and has a rostrocaudal development of the peripheral nervous system. Glabrous snout skin from young opossums was studied at birth (0 day) and postnatal days 1, 3, and 5. Neurite bundles were seen within the dermis, with axons and growth cones approaching the epidermis. Some axons penetrated the dermal-epidermal junction in newborn pups. Merkel cells were consistently numerous during the time spanned by this study. Mature Merkel cells had granules polarized toward an associated neurite and were often located in the base of the rete pegs. Immature Merkel cells were characterized by a lack of polarization of granules and absence of an apposed neurite, suggesting that these cells can differentiate without an associated neurite. Contiguous Merkel cells had junctional complexes in 0- and 1-day animals. Schwann cells, identified by their contact with neurite bundles, were present in large numbers, especially in the superficial dermis. Melanocytes could be identified in the epidermis in 5-day pups only. Developing rete pegs could be recognized in 0-day animals and became prominent in 5-day pups. These observations suggest that afferent fibers are present at a very early age and that some of these fibers are anatomically mature. These findings support the concept that the ability of the neonate to locate a teat and suckle requires only the presence of mature epidermal innervation, while dermal receptors appear later during postpartum development.