Developmental studies on the rat vomeronasal organ: vascular pattern and neuroepithelial differentiation. I. Light microscopy

Structural and ultrastructural studies on the developing vomeronasal sensory epithelium in the grass snake Natrix natrix (Squamata: Colubroidea)

The sensory olfactory epithelium and the vomeronasal sensory epithelium (VSE) are characterized by continuous turnover of the receptor cells during postnatal life and are capable of regeneration after injury. The VSE, like the entire vomeronasal organ, is generally well developed in squamates and is crucial for detection of pheromones and prey odors. Despite the numerous studies on embryonic development of the VSE in squamates, especially in snakes, an ultrastructural analysis, as far as we know, has never been performed. Therefore, we investigated the embryology of the VSE of the grass snake (Natrix natrix) using electron microscopy (SEM and TEM) and light microscopy. As was shown for adult snakes, the hypertrophied ophidian VSE may provide great resolution of changes in neuron morphology located at various epithelial levels. The results of this study suggest that different populations of stem/progenitor cells occur at the base of the ophidian VSE during embryonic development. One of them may be radial glia-like cells, described previously in mouse. The various structure and ultrastructure of neurons located at different parts of the VSE provide evidence for neuronal maturation and aging. Based on these results, a few nonmutually exclusive hypotheses explaining the formation of the peculiar columnar organization of the VSE in snakes were proposed.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

The rodent accessory olfactory system

The accessory olfactory system contributes to the perception of chemical stimuli in the environment. This review summarizes the structure of the accessory olfactory system, the stimuli that activate it, and the responses elicited in the receptor cells and in the brain. The accessory olfactory system consists of a sensory organ, the vomeronasal organ, and its central projection areas: the accessory olfactory bulb, which is connected to the amygdala and hypothalamus, and also to the cortex. In the vomeronasal organ, several receptors-in contrast to the main olfactory receptors-are sensitive to volatile or nonvolatile molecules. In a similar manner to the main olfactory epithelium, the vomeronasal organ is sensitive to common odorants and pheromones. Each accessory olfactory bulb receives input from the ipsilateral vomeronasal organ, but its activity is modulated by centrifugal projections arising from other brain areas. The processing of vomeronasal stimuli in the amygdala involves contributions from the main olfactory system, and results in long-lasting responses that may be related to the activation of the hypothalamic-hypophyseal axis over a prolonged timeframe. Different brain areas receive inputs from both the main and the accessory olfactory systems, possibly merging the stimulation of the two sensory organs to originate a more complex and integrated chemosensory perception.

Stereologic evaluation of the vasculature in a MX1 xenotransplanted tumour model after combinations of treatment with ifosfamide, hyperthermia and irradiation

The vascularization of tumours is a critical parameter of their growing and metastatic behaviour. However, little is known about the morphologic reactions of the microvasculature, especially the capillary bed of tumours and the adjacent tissue. In this study, the vessels in MX1 xenotransplants in athymic nu/nu nude mice were quantified and the angioarchitecture was visualized with the aim of presenting stereologic parameters of vessels based on a morphometric analysis of post mortem tissue blocks which were processed by standard histological procedures. In order to study changes of the microvasculature of MX1 tumours, the xenotransplanted nude mice were treated by different therapeutic regimens. Standardized hyperthermia, ifosfamide and irradiation therapies were applied. Special interest was focused on early changes of capillaries and of the pre- as well as post-terminal vascular bed. The stereologic evaluation of capillaries and larger vessels immediately after the therapy with ifosfamide and hyperthermia shows an increase of the mean capillary sizes. Furthermore, tumour samples after the 5th day of irradiation (5 x 2 Gy) and combinations of irradiation and chemotherapy treatment have been investigated. After 5 days of irradiation, a significant decrease of the vascular density was found. The results presented here clearly show that the timing and the mode of therapy influence the capillary morphology and periterminal vasculature of xenotransplanted MX1 tumours.

Proliferation in the vomeronasal organ of the rat during postnatal development

We investigated proliferation of sensory cell precursors in the rat vomeronasal organ (VNO) at various postnatal ages from birth (P1) to P666. In the rat, which continues to grow during most of its adult life, proliferation might be related to growth and/or replacement. Proliferating cells were labelled by BrdU injection, and histological sections of the VNO were evaluated after immunohistochemical detection of BrdU. Proliferation density (number of proliferating cells/section) decreased dramatically from 115 at P1 to 27.2 at P21, although the area increased. Adult values were reached at P66-P333 (10.3 cells/section); at P400-P666 the value was 8.6 cells/section. Distribution of labelled cells changed considerably with age: in neonates the cells were nearly equally distributed throughout the sensory epithelium, whereas from P21 onwards most proliferating cells were concentrated in clusters near the boundaries with non-sensory epithelium. Labelled cells in the sensory neuronal layer were adjacent to the undulating basement membrane-bordering capillaries that intrude into the sensory epithelium, indicating that they were true basal cells. The volume of the sensory epithelium increased between P1 and P66, and remained constant thereafter, although the length still increased. Length and volume of the sensory epithelium were related to body size, not to sex; males and females of the same body size had the same VNO size. The complex changes in proliferation pattern during postnatal development indicate differential growth and replacement. We suggest that in adults the labelled cell clusters near the boundaries are a pool for growth, whereas proliferation in the central parts represents a replacement pool.

Vomeronasal organ and its associated structures in the opossum Monodelphis domestica

The opossum Monodelphis domestica possesses a well-developed vomeronasal system. Uptake of chemical stimuli into the vomeronasal organ (VNO) involves a stereotypical "nuzzling" behavior. In the present study, ten animals were examined by light and electron microscopy. The peripheral oro-nasal structures that apparently enhance access of solutes into the VNO include: (1) two lateral grooves of the ventral rhinarium and a network of channels leading into them, (2) dental gap adjacent to the grooves, (3) butterfly-shaped incisive papilla, and (4) unique bristle/cup-shaped filiform papillae on the tongue. The longitudinal axis of the vomeronasal complex is composed of the VNO proper at its rostral end, and an extensive compound serous gland at its caudal end with a distinct transition zone in between. The transition zone is characterized by the following features: merging of the main excretory duct of the large vomeronasal gland with the VNO lumen and drainage of auxiliary glandular clusters into the lumen, irregularities in the sensory epithelium ("rosette" appearance), and the ending of the cartilaginous support surrounding the VNO. Multiple elongated bundles of smooth muscle are positioned between the sensory epithelium and the cartilaginous capsule and more caudally are intertwined with the glandular parenchyma. These bundles become more numerous at the transition zone. Contraction and extension of these muscles may function to enhance the flow of solutes and glandular secretion within the lumen. Two extensive venous sinuses are associated with the opossum VNO complex: the internal vein bordering the sensory epithelium at its rostral end, and the external vein alongside the nonsensory epithelium. This arrangement suggests a unique dual pumping mechanism.

Innervation of blood vessels in the vomeronasal complex of the rat

We have made an immunohistochemical study of the vomeronasal (VN) complex of 12-day-old rats to characterize the innervation of its blood vessels. The VN complex can be subdivided into rostral, middle and caudal segments, each one with a particular vascularization pattern. Several small vessels were associated with the rostral segment, whereas a large venous sinus ran along the middle and caudal segments. Immunostaining for alpha-smooth muscle actin demonstrated that the muscular sheath was asymmetric, with more cells layers in its lateral than in its medial walls. Nerves were demonstrated with antisera against protein gene product 9.5 (PGP), and against several molecules associated with specific classes of nerve fibers: the C-terminal peptide of neuropeptide Y (CPON), calcitonin gene-related peptide (CGRP), substance P (SP), galanin (GAL), vasoactive intestinal peptide (VIP) and neuronal nitric oxide synthase (NOS). The latter, was also studied with NADPH-diaphorase. Vascular associated fibers exhibited NOS-, CPON-, GAL-, CGRP-, SP- and VIP-immunoreactivity. Only the vessels of the rostral segment showed VIP-immunoreactive fibers. Each wall of the venous sinus exhibited different types of nerve fibers. CPON-, GAL-, CGRP- and SP-immunoreactive fibers concentrated in the medial wall, whereas NOS-immunoreactive ones concentrated in the lateral wall. This distribution of vascular fibers, plus the presence of sensory fibers exhibiting CGRP-, SP- and GAL-immunoreactivity within the pseudostratified epithelium of the VN tube, would be relevant to understand the operation of the pumping mechanism regulating influx and efflux from the VN tube.

Prenatal development of the mammalian vomeronasal organ

The vomeronasal organ (VNO) originates from the medial wall of the olfactory pit shortly after the middle of the embryonic period in mammals. The Anlage stage consists of a cellular bud that grows dorsally, caudally, and towards the midline leaving a groove. The following stage, Early Morphogenesis, includes the closure of the vomeronasal groove to form a parasagittal blind-ended tube in the nasal septum, which opens into the nasal and/or oral cavities. The lumen adopts a crescent shape while the epithelial lining differentiates into an increasingly wider epithelium on the concave side and a gradually thinner epithelium on the convex side. The former goes on to occupy a medial position and develops neuroblasts among supporting and undifferentiated cells, with supporting cell nuclei tending to align in the upper rows. The lateral "non-sensory" epithelium furrows, giving a kidney-shaped appearance to the VNO cross section. The next stage, Late Morphogenesis is extended up to a difference in thickness between both epithelia becomes similar to the adult, generally by birth. An increasing number of ciliary generation complexes, larger and more abundant microvilli, and an evident glycocalyx are observed in the neuroepithelium at the luminal surface, while enzymatic activities become more intense. The non-sensory epithelium appears quite mature save for its luminal surface, which is still devoid of cilia. Blood capillaries penetrate the most basal region of the neuroepithelium and vomeronasal glands are very few and immature. At birth, some neurons appear well developed to support certain functionality; however, persistence of architectural, histochemical, and ultrastructural signs of immaturity, suggests that full performance of the VNO does not occur in newborn mammals, but in prepubertal ages.

Origin and regional distribution of the arterial vessels of the vomeronasal organ in the sheep. A methodological investigation with scanning electron microscopy and cutting-grinding technique

The origin and location of the arteries of the vomeronasal organ (VNO) in the sheep were studied by means of dissection, scanning electron microscopy of corrosion casts, and the cutting-grinding technique after injection with Araldite CY23-HY2967 via one of the carotid arteries. Dissection revealed that the most ventral of the three main branches of the sphenopalatine artery is responsible for the blood supply to the VNO. Scanning electron microscopy of corrosion casts revealed that the arterioles of the vomeronasal organ form a microvascular network. Cross sections of the region of the nasal cavity containing the VNO, obtained by the cutting-grinding technique, showed that the arterioles of the vomeronasal plexus are located medial and ventral to the vomeronasal duct. These results confirm the usefulness of the cutting-grinding technique as a complementary procedure in morphological studies of structures containing hard tissues.

Distribution of the arterial supply to the vomeronasal organ in the cat

BACKGROUND

The main goal of this work was to investigate the general distribution of arterial blood around and inside the vomeronasal organ (VNO) of the cat.

METHODS

Macro- and microdissection methods together with light and scanning electron microscopy were used. Heads were injected with an India ink/agar mixture (the VNO subsequently being cut in transverse, sagittal and horizontal sections), with clear latex (the VNO subsequently being cut in transverse sections), or with an epoxy resin to obtain casts for examination by scanning electron microscopy.

RESULTS

Dissection and microdissection show that the infraorbital, minor palatine, and descending palatine arteries have a common origin, rostral to the Rete mirabile arteria maxillaris. In transverse series and in the rostral half of the VNO, an arteriole is consistently observed between the vomeronasal duct and the lateral sheet of the vomeronasal cartilage. In this same segment, arterial branches with different orientations (perpendicular, horizontal, or transverse with respect to the main axis of the organ) are observed. Scanning electron microscopy of arterial casts shows that arterial vessels of the mucosa of the nasal septum have a direct relationship with the VNO.

CONCLUSIONS

Branches of the sphenopalatine artery are the chief route of blood supply to the VNO. The vomeronasal parenchyma has few arterial vessels, and these are usually situated in the same position. Differences observed between the arteries inside and outside the VNO and the dilation of both by isoproterenol support the idea that the VNO is similar to erectile tissue organs and that it may act as a physiological pump.

Development of substance P immunoreactivity in the mouse vomeronasal organ

We investigated the development of substance P immunoreactivity in mouse vomeronasal organs in embryos, juveniles, and adults. In all stages, substance P fibers were found in the receptor-free epithelial area, but never in the neuroepithelium. Substance P fibers were found sparsely in the lamina propria of 15-day-old embryos. Although buds of the vomeronasal glands in the cavernous tissue were observed in 17-day-old embryos, and gradually grew in size and numbers, the substance P fibers around them decreased after about the 13th day. Thus, substance P may be a trophic factor for the development of the vomeronasal glands in the cavernous tissue. We first recognized substance P fibers reaching the surface of the receptor-free epithelium in 13-day-old pups. In 21-day-old mice, substance P fibers were as well developed as in adult mice. Considering the development of the substance P fibers in the receptor-free epithelium and the cavernous tissue, they probably cause the vasodilation of the cavernous tissue via local axon reflexes. These structures may then act as a defense system, eliminating noxious stimulus substances sucked into the vomeronasal organ.

Olfactory experience modulated apoptosis in the developing olfactory bulb

Early sensory stimulation plays a key role in shaping the structure and function of the developing olfactory system. Here, we provide the first direct evidence for apoptotic cell death in the olfactory bulbs of rat pups during normal development and we also demonstrate that olfactory deprivation by unilateral naris occlusion causes a dramatic increase in apoptotic cell death in the glomerular and granule cell layers of the deprived bulb. The accessory olfactory bulbs displayed a remarkably high basal level of apoptosis but the occluded accessory bulb did not differ in that regard from the control accessory bulb. These results suggest that apoptosis may be an important mechanism by which the olfactory system can adjust its cell numbers in response in sensory stimuli experienced in early life, thereby underlying one form of plasticity in the developing olfactory system.

Immunocytochemical study of the differentiation process of the septal organ of Masera in developing rats

The septal organ of Masera is a small patch of olfactory epithelium located near the base of the nasal septum. Using the growth-associated protein B-50/GAP-43 as neuronal marker, we have studied the differentiation process of this organ from the olfactory sheet in embryonic and newborn rats. Results show that the septal organ first appeared at embryonic day 16. Even though it was included in the olfactory sheet, the presumptive septal organ could be distinguished by a higher density of B-50/GAP-43-positive neurons. Concomitantly to its morphological development, the septal organ progressively isolated from the main olfactory epithelium. This isolation resulted from the extension of a transitional area which progressively lost its typical features of olfactory epithelium to become a putative respiratory epithelium in late embryonic stages. Results strongly suggest that the septal organ should be a proper chemosensory system with its own time-course of development.

Stimulus access to the accessory olfactory system in the prenatal and perinatal rat

The fetal rat can detect odors in its amniotic fluid. Indirect evidence suggests that the accessory olfactory system may be mediating chemoreception prenatally. The primary goal of this study was to determine if stimuli in the amniotic fluid gain access to the receptor neurons of the accessory olfactory system that are sequestered inside the vomeronasal organ. Other goals of the study were to compare the access of stimuli to the vomeronasal organ in the prenatal and young postnatal rat and to examine the role of the autonomic nervous system in this process. On the day before birth fluorescent beads (0.95 microns diameter) were injected into the amniotic sacs of rat fetuses from seven dams. After 4-6 h one group of mothers received an i.p. injection of epinephrine (either 60 or 75 micrograms) and another group received no injection. One hour later pups were collected and processed for histological examination using fluorescence microscopy. A portion of the fetuses from both treatment groups had significant numbers (< 50) of beads in their nasal passages. Some of these subjects also had beads in the vomeronasal organ. There was no significant difference in the proportion of subjects with beads in the nasal cavities or vomeronasal organ across the two treatments. In a second experiment, five- to seven-day-old rat pups had beads infused into one naris. After 3-4 h one group was injected i.p. with epinephrine (2-4 micrograms), while a second group received no injection. As expected, virtually all the subjects had large numbers of beads in their nasal cavities upon post mortem examination.(ABSTRACT TRUNCATED AT 250 WORDS)

A unique neuronal glycolipid defines rostrocaudal compartmentalization in the accessory olfactory system of rats

A monoclonal antibody, CC6, reacts with a complex glycolipid whose expression in the rat is restricted to the olfactory system. Structural analysis reveals that the glycolipid contains two alpha-fucose branches; one each at internal and external beta-galactose residues. Immunocytochemistry demonstrates in rat embryos that CC6 glycolipid expression is restricted to a subset of neurons in the vomeronasal organ (VNO) and their corresponding axon projections in the caudal accessory olfactory bulb (AOB). This pattern of expression in the accessory olfactory system is the converse of the pattern revealed by a previously characterized antiglycolipid antibody that reacts with VNO neurons projecting to the rostral AOB. The CC6-reactive glycolipid is also expressed on a subset of neurons in the main olfactory epithelium. Postnatally, axons from these CC6 positive sensory neurons converge to form a limited number of axon bundles running longitudinally through the nerve layer of the main olfactory bulb. These data provide further evidence that groups of vomeronasal and olfactory neurons expressing unique surface molecules project axons that terminate in selected targets in the AOB and OB, respectively.

The vomeronasal duct has a protracted postnatal development in the mouse

Recent evidence suggests that the accessory olfactory system (AOS) may mediate chemoreception before birth. Such a capability may allow the fetus to begin to sample chemical stimuli from the outside world, a possibility that has important developmental and evolutionary implications. Herein we describe the development in the mouse of the duct that connects the vomeronasal organ (VNO), containing the primary receptor neurons of the AOS, with the nasal cavity and thus with external stimuli. Twenty-four mice, four at each of six different ages from the last day of gestation through 25 days of age, were fixed and embedded in glycol methacrylate. Serial sections were examined under the light microscope so that the VNO duct could be reconstructed in three dimensions. Results confirm an earlier study which demonstrated that the VNO duct is not patent before birth. The duct becomes patent sometime after the first day of life but remains in an immature condition throughout the normal prepubertal period. During this period the duct is characterized by an internal surface that is rapidly desquamating such that the lumen of the duct contains sloughed tissue. These results suggest that the VNO is unlikely to function in the prenatal period, since the route for external stimuli to reach its receptor surface is blocked. The protracted period of VNO duct development reported here is consistent with the great bulk of data on the AOS of mammals which firmly establishes its role in the detection of pheromones which coordinate reproduction.

Endocytic pathways in the olfactory and vomeronasal epithelia of the mouse: ultrastructure and uptake of tracers

Mammalian olfactory neurons possess a well-developed system of endocytic vesicles, endosomes, and lysosomes in their dendrites and perikarya. Vomeronasal neurons are similar and also contain much perikaryal agranular endoplasmic reticulum (AER). Olfactory supporting cells contain endocytic vesicles and endosomes associated closely with abundant fenestrated AER, and vesicles and numerous large dense vacuoles are present basally. Vomeronasal supporting cells have little AER, and few dense vacuoles occur in their bases. In olfactory neurons, ultrastructural tracers (0.08% horseradish peroxidase, thorium dioxide, ferritin) are endocytosed by olfactory receptor endings and transported to the cell body, where their movement is halted in lysosomes. Higher concentrations (1%) of horseradish peroxidase penetrate olfactory receptor plasma membranes and intercellular junctions. In olfactory supporting cells, endocytosed tracers pass through endosomes to accumulate in dense basal vacuoles. These observations indicate that olfactory sensory membranes are rapidly cycled and that endocytosed materials are trapped within the epithelium. It is proposed that in the olfactory epithelium, endocytosis presents redundant odorants to the enzymes of the supporting cell AER to prevent their accumulation, whereas in the vomeronasal epithelium the receptor cells carry out this activity.

Vascular ontogeny of the septal region in rats

The development of the arterial and venous systems of the septum was studied in rat brains injected daily with India ink, from the 11th embryonic (E) until the first postnatal day. Arterial blood is supplied to the septum by the unpaired hemispheric artery, the stem and septal branches of which are to be recognized on the 14th and 15th embryonic days respectively. At earlier stages, e.g. on E12, a capillary network, the hemispheric plexus, can be seen between the two hemispheres contributing to the blood supply of the septum during the early phase (E14 to E18) of development. From E18 onwards, the arterial supply of the septum is derived only from direct branches of the hemispheric artery; one group of them being dorsal (infracallosal) and the other ventral (subcallosal). The venous drainage of the septum is bidirectional: 1) Veins of the ventral group leading to the interperioptic sinus are seen on E14. At first they collect blood only from a small rostral portion of the septum, but later their territory expands to include the anteroventral two-thirds of the septum. 2) The dorsal septal veins drain into the great cerebral vein (of Galen), or into the superior sagittal sinus directly. Initially, twigs run directly into the great cerebral vein. These later become the tributaries of the internal cerebral vein, which appears on E17 or E18. Until E18 this dorsally-directed drainage predominates, whereas at birth it becomes restricted to one third of the septum as a result of a gradual regression. The development of both arterial and venous circulations of the septum is complete at birth.

Supra-neuroectodermal cells and fibers on the primary nasal cavity and in the fourth ventricle of mouse and human embryos: scanning and transmission electron microscopic studies

Neuroectoderm-derived epithelia of the primary nasal cavity and the fourth ventricular floor and roof were observed by scanning (SEM) and transmission electron microscopy (TEM) and SEM-TEM correlative views in mouse embryos of 9th to 13th days of gestation, and in 38 externally normal human embryos ranging at Carnegie stages from 13 to 18 (about 5 to 7 weeks of gestation). Smooth-surfaced spindle-shaped cells with one or more cytoplasmic processes and cord-like cytoplasmic structures were observed by SEM on the wall of the primary nasal cavity of both species. They had morphological features similar to those of neuronal type 1 supraependymal (SE) cells and SE fibers on the floor and roof of the fourth ventricle in both species. Type 1 SE cells, SE fibers, and corresponding structures in the primary nasal cavity were localized in relation to the underlying developing nerve and vascular systems. Furthermore, their processes and fibers ran roughly parallel to these underlying structures and they penetrated the epithelial layer at the ends, suggesting a connection with underlying structures. From TEM and SEM-TEM correlative observations, SE fibers in the fourth ventricle and cord-like structures in the primary nasal cavity, both with a larger diameter, were deduced as single axon-like processes or bundles of processes. Those fibers and cord-like structures of smaller diameters were interpreted as elongated telophase bridges; both contained parallel packed microtubules and connected distant cells. Since these processes and fibers were generally longer and became fewer at later developmental stages, they appeared to be transient neuronal structures. They may play a development-related role in such morphogenetic cell movements as in the developing nerve and vascular systems in the epithelial and/or subepithelial layers, but not as direct rudiments of adult nerve tissues.

Postnatal development of the vomeronasal epithelium in the rat: an ultrastructural study

Three basic types of cells are distinguished in the rat vomeronasal epithelium at birth: bipolar neurons, supporting cells, and basal cells. Neurons at this time include both immature and differentiated cells. By the end of the first postnatal week, all neurons show morphological signs of maturity in their cytoplasm, including abundant granular and smooth endoplasmic reticulum, neurotubules, dense lamellar bodies, apical centrioles, and tufts of microvilli. During the third week microvilli are more frequently encountered and appear to be longer and more branched. Supporting cells appear well-developed by the second day after birth. During the first ten days of life, supporting cells lose their centrioles and all of the complex associated with ciliary generation in the apical zone. Basal cells appear to be more numerous in newborns than in older animals. Protrusions projecting into the lumen are frequently observed in the epithelium of newborn animals, both on the dendrites of neurons and on supporting cells. After the third week, such protrusions are only observed in the transitional zone between the sensory and the non-sensory epithelia of the vomeronasal tubes. In this transitional zone, a fourth cell type showing apical protrusions with microvilli differentiates. Cytoplasm in this type resembles that of neighboring ciliated cells but has no cilia or centrioles. These transitional cells are considered to be cells in an intermediate state of differentiation, between that of the differentiated neurons and supporting cells of the sensory epithelium and that of the predominate ciliated cells of the non-sensory epithelium. The results suggest that by the end of the third week the vomeronasal epithelium is morphologically mature.

Incorporation of 3H-thymidine in the embryonic vomeronasal and olfactory epithelial of garter snakes

Previous studies have shown that the vomeronasal and olfactory epithelia of adult vertebrates provide good models for studying normal neuronal turnover and regeneration in response to axotomy. However, little is known about the cell dynamics in the embryonic vomeronasal and olfactory epithelia or the origins of different cell types in these structures. By using 3H-thymidine autoradiography, both in vivo and in vitro, the origins of receptor and supporting cells and the survival of labelled cells in the embryonic vomeronasal and olfactory epithelial of garter snakes were examined. The results of this study suggest that the receptor and supporting cells of both epithelial arise from separate stem cells and that two subpopulations of stem cells exist for receptor cells in the embryonic vomeronasal epithelium. One subpopulation generates cells that migrate through the receptor cell columns, while another subpopulation remains at the base of the epithelium for approximately 50 days. Although it is unclear how long receptor cells in the embryonic olfactory epithelium survive, the results of this study suggest that they survive at least 37 days and may survive over 56 days. In addition, the development of these sensory epithelia appears different in early versus late embryos, and regeneration in the vomeronasal and olfactory epithelia of adult garter snakes appears similar to development during late gestation. Cells in the developing receptor cell layer of the olfactory epithelium lose their ability to incorporate 3H-thymidine before those in the vomeronasal epithelium, suggesting that the onset of neuronal maturation occurs earlier in the olfactory epithelium than in the vomeronasal epithelium.

The cranial venous system in the rat: anatomical pattern and ontogenetic development. I. Basal drainage

The anatomical pattern and development of the venous system of the cranial base in the rat is described. The anatomy of the venous system was determined from observations of vascular casts in adult rats; the development of the vascular system was established by examination of ink-injected embryos. A transverse sinus system was found to be present in the basal venous system. The sinus connects the posterior facial veins; its middle section transverses the cranial base through the basisphenoid canal, and it receives the venae ophthalmicae within the basisphenoid bone. The venae ophthalmicae in turn are connected to the perioptic veins and to the sinus interperiopticus intracranially. Dorsally, the venae ophthalmicae anastomose with the paired sinus cavernosus. The term sinus transversus basalis is proposed for the venous connection between the posterior facial veins within the basisphenoid bone of the rat. The anlage of the sinus transversus basalis is established by vascular networks during the final prenatal period, its formation, however, is only completed postnatally. The anlages of the venae periopticae, the venae ophthalmicae, the sinus cavernosus and the rami intercavernosi are already established at early developmental stages. The characteristic pattern is formed before birth.

Embryonic and neonatal development of the vomeronasal and olfactory systems in garter snakes (Thamnophis spp.)

Newborn, unfed garter snakes (Thamnophis spp.) respond preferentially to aqueous extracts of natural prey items, and these responses are mediated by the vomeronasal system (VNS). Since the VNS, and possibly the olfactory system (OS), are functional at birth, we examined the ontogeny of VNS and OS structures in four embryonic stages and two postnatal ages in garter snakes. The results of this study show 1) significant changes in thickness of the receptor epithelia for both systems; 2) temporal differences in the innervation of the telencephalon for each system; and 3) concurrent development of primary and secondary projection sites in both systems. Possible interactions between different cell populations and their significance for morphogenesis are discussed.

Developmental studies on the rat vomeronasal organ: vascular pattern and neuroepithelial differentiation. II. Electron microscopy

The present electron microscopical study demonstrates that the establishment of the typical vascular pattern of the rat vomeronasal organ by the eighteenth day of gestation is accompanied by neither complete maturation of the wall of the blood vessels nor of the vomeronasal neuroepithelium. In the newborn rat, however, the vasculature and the neuroepithelium of the vomeronasal organ present morphological elements suggesting functional capability at birth.