Immunocytochemical detection of mitochondria-rich cells in the brood pouch epithelium of the pipefish, Syngnathus schlegeli : structural comparison with mitochondria-rich cells in the gills and larval epidermis

The brood pouch of the male pipefish (Syngnathus schlegeli) is a ventral organ located on the tail, with the anterior region closely associated with the genital pore. The embryos in the pouch are attached to highly vascularized placenta-like tissue which seals the pouch folds from inside during incubation. The epithelium of the placenta-like tissue consists of mitochondria-rich cells (MRCs) and pavement cells. Differences in MRC morphology in the brood pouch epithelium, the gills and the larval epidermis of the pipefish were examined by light and electron microscopy. Transmission electron microscopy revealed that the MRCs in the brood pouch and the gills shared common characteristics: the presence of numerous mitochondria packed among a well-developed tubular system and the close association of the basal parts with the capillaries running underneath the epithelia. The size of the apical opening of the elongate, flask-shaped brood pouch MRC was about one-tenth that of the apical pit of the gill MRC. The gill and larval epidermal MRCs formed a multicellular complex, in contrast to solitary brood pouch MRCs. The brood pouch MRCs were intensively stained by immunocytochemistry with an antiserum specific for Na⁺,K⁺-ATPase. The Na⁺ concentrations in the brood pouch were maintained near those in the serum rather than seawater during incubation. We conclude that the brood pouch MRCs function as an ion-transporting cell, absorbing ions from the brood pouch lumen, perhaps to protect the embryos from the hyperosmotic environment.     

Neuronal systems immunoreactive with antiserum to lamprey gonadotropin-releasing hormone in the brain of Petromyzon marinus

Summary The wall structure of arteriovenous anastomoses in the rabbit ear was investigated. (1) Clusters of epithelioid smooth muscle cells form 3–4 longitudinally oriented plicae. The channel shows a single, irregularly outlined lumen, and its wall is very thin between adjacent plicae. (2) Endothelial cells covering the plicae protrude into the lumen, thus suggesting active contraction or shortening of the plicae. (3) The tunica adventitia is composed of 4–6 sheaths of flat fibroblasts, which may serve as a barrier to prevent loss of neurotransmitters. Processes of some of the fibroblasts also extend into the tunica media. (4) The tunica media is composed of an outer circular layer of typical smooth muscle cells, and an inner longitudinally running plica of ramified smooth muscle cells. Wide intercellular spaces between these ramified cells are filled with collagen fibrils, microfibrils, amorphous intercellular substances, and fibroblasts. Fibroblasts form close membrane contacts with each other, and with the smooth muscle cells. (5) Fibroblasts and other connective tissue components may function as an elastic support during active motility of the anastomotic channel.     

Osmoregulatory role of the brood pouch in the euryhaline Gulf pipefish, Syngnathus scovelli

Many species of pipefish exhibit a reversal of parental roles, in which females insert eggs into the brood pouch of the male where they are incubated until the end of embryonic development. While the significance of the male brood pouch has been examined for over a century, the role of the pouch is still unclear. One possible function is to aid in osmoregulation by buffering embryos from the external environment. To investigate this role, the euryhaline Gulf pipefish, Syngnathus scovelli, was collected and maintained in either a low salinity or a saltwater environment. Changes in plasma and pouch fluid osmolality and morphological changes of the pouch were examined. Brood pouch fluid was similar to male plasma during the early and late stages of the brooding period for low salinity males, but was significantly hyperosmotic during the middle of the brooding period. In saltwater males, brood pouch fluid was similar to plasma during early brooding, but became hyperosmotic as brood time progressed. The brood pouch epithelium of both low salinity and saltwater males contained mitochondria-rich cells. In early brooding saltwater males these cells contained an apical opening into the pouch lumen. Osmotic and morphological differences observed suggest that the brood pouch plays an active role in regulating osmotic concentration of the pouch fluid. Additionally, pouch fluid concentration may be regulated more during early stages of embryonic development.     

Morphological organization of the male brood pouch epithelium of Syngnathus abaster Risso (Teleostea, Syngnathidae) before, during, and after egg incubation

The morphological organization of the male brood pouch epithelium of Syngnathus abaster, before, during, and after the breeding period, was observed by light and electron microscopy. Before gestation, the epithelium of the pouch wall was compact and consisted of three kinds of cells: typical epithelial cells (pavement cells), mitochondria-rich cells (MR cells), and, presumably, differentiating MR cells. In this stage, very few capillaries were observed beneath the epithelium. During egg incubation, the capillaries increased in number and size, large intercellular spaces formed among epithelial cells at their basal sides, MR cells were abundant, and differentiating MR cells were only occasionally observed. After incubation, MR cells degenerated by necrosis and apoptosis. The intercellular spaces between the epithelial cells disappeared and the number and size of the capillaries beneath the epithelium decreased. The presence of MR cells during gestation and their degeneration after incubation suggest that these cells play a pivotal role in the physiological functions of the brood pouch. The similar cytological characteristics of syngnathid pouch MR cells and chloride cells of the teleostean gills suggests that the Syngnathidae brood pouch is involved in osmoregulation of the fluid surrounding the developing embryos.     

Interembryonic regions of the uterus of the viviparous lizard Mabuya brachypoda (Squamata: Scincidae)

Analysis of the structure and physiology of the uterine incubation chambers of viviparous squamates has provided insight concerning adaptations for gestation. However, the literature addressing the biology of the interembryonic regions of the uterus is very limited, presumably because it has been assumed that this area has little role in the development and support of embryos in viviparous squamates. This study was undertaken to examine the histology of the interembryonic regions of Mabuya brachypoda, a viviparous lizard with microlecithal ova and consequently substantial matrotrophic activity. The incubation chambers are oval, distended zones of the uterus, adjacent to the interembryonic regions. The wall of the interembryonic regions includes: mucosa, formed by a cuboidal or columnar epithelium with ciliated and nonciliated cells, and a lamina propria of vascularized connective tissue containing abundant acinar glands; myometrial smooth muscle consisting of inner circular and outer longitudinal layers; and serosa. The segment of the interembryonic region adjacent to the incubation chamber forms a transitional segment that displays folds of the mucosa that protrude into the uterine lumen. The limit of the incubation chamber is well defined by the long mucosal folds of the transitional segment. Long and thin extensions of extraembryonic membranes are present in the lumen of the transitional segment, outside of the incubation chamber region. The presence of abundant uterine glands and extraembryonic membranes in the interembryonic regions during gestation suggests uterine secretory activity and histotrophic transfer of nutrients to embryos in these regions.     

Ultrastructure of the ovary and oogenesis in six species of patellid limpets (Gastropoda: Patellogastropoda) from South Africa

Abstract. The ultrastructural features of the ovary and oogenesis have been described in 6 species of patellid limpets from South Africa. The ovary is a complex organ that is divided radially into numerous compartments or lacunae by plate-like, blind-ended, hollow trabeculae that extend from the outer wall of the ovary to its central lumen. Trabeculae are composed of outer epithelial cells, intermittent smooth muscle bands, and extensive connective tissue. Oocytes arise within the walls of the trabeculae and progressively bulge outward into the ovarian lumen during growth while partially surrounded by squamous follicle cells. During early vitellogenesis, the follicle cells lift from the surface of the underlying oocytes and microvilli appear in the perivitelline space. Follicle cells restrict their contact with the oocytes to digitate foot processes that form desmosomes with the oolamina. When vitellogenesis is initiated, the trabecular epithelial cells hypertrophy and become proteosynthetically active. Yolk synthesis involves the direct incorporation of extraoocytic precursors from the lumen of the trabeculae (hemocoel) into yolk granules via receptor-mediated endocytosis. Lipid droplets arise de novo and Golgi complexes synthesize cortical granules that form a thin band beneath the oolamina. A fibrous jelly coat forms between the vitelline envelope and the overlying follicle cells in all species.     

Evidence of paternal nutrient provisioning to embryos in broad-nosed pipefish Syngnathus typhle

In two experiments, radioactively labelled nutrients (either (3)H-labelled amino-acid mixture or (14)C-labelled glucose) were tube-fed to brooding male Syngnathus typhle. Both nutrients were taken up by the males and radioactivity generally increased in the brood pouch tissue with time. Furthermore, a low but significant increase of (3)H-labelled amino acids in embryos was found over the experimental interval (48 h), whereas in the (14)C-glucose experiment the radioactivity was taken up by the embryos but did not increase over the experimental time (320 min). Uptake of radioisotopes per embryo did not differ with embryo size. A higher uptake mg(-1) tissue of both (3)H-labelled amino acids and (14)C-labelled glucose was found in smaller embryos, possibly due to a higher relative metabolic rate or to a higher surface-area-to-volume ratio compared to larger embryos. Uptake in embryos was not influenced by male size, embryonic developmental advancement or position in the brood pouch. It is concluded that brooding males provide amino acids, and probably also glucose, to the developing embryos in the brood pouch.     

Cyclic histological changes of the oviductal-cloacal junction in the viviparous snake Toluca lineata

The structure and seasonal changes of the oviductal-cloacal junction remain poorly understood in most squamates. This study was undertaken to describe the histology of the oviductal-cloaca junction of a female viviparous snake Toluca lineata, during gestation, previtellogenesis, and vitellogenesis. The oviductal-cloacal junction exhibits a wider lumen and thicker layers of connective tissue, smooth muscle layers, and total wall width compared to the posterior vagina. The lining is characterized by thick, short longitudinal mucosal folds. The luminal epithelia differ morphologically from anterior to posterior portions of the oviductal-cloacal junction. The anterior portion is lined with a simple columnar epithelium composed of nonciliated cells. The middle portion is lined with stratified epithelium that contains an apical columnar cell layer that undergoes morphological changes coincident with the reproductive cycle. The posterior portion is lined with a stratified squamous epithelium. The connective tissue underlying the epithelium contains numerous ovoid cells having abundant acidophilic cytoplasmic granules—eosinophils. Copulation occurs during the previtellogenic stage, as evidenced by the presence of abundant spermatozoa in the lumen of the anterior portion and of a copulatory plug in the middle and posterior portion of the oviductal-cloacal junction. J. Morphol. 237:91–100, 1998. © 1998 Wiley-Liss, Inc.     

Unusual modes of oogenesis and brooding in bivalves: the case of Gaimardia trapesina (Mollusca: Gaimardiidae)

Abstract. I describe an unusual case of follicular oogenesis in a bivalve, Gaimardia trapesina , a common marine bivalve from the Magellan Region and adjacent Sub-Antarctic waters, whose members brood their developing larvae. The gonad in G. trapesina is an acinus organ that infiltrates the perivisceral connective tissue; the walls of the acini are formed by tall, slender cells with distal nuclei, supported by a thin conjunctive tissue layer. At the onset of vitellogenesis, each developing oocyte becomes surrounded by a one-cell-thick layer of follicle cells, which may originate from the wall of the acinus. The cells form a follicle that completely encompasses single oocytes, except at the basal zone, where oocytes are in contact with the acinus wall. The follicle persists beyond the end of vitellogenesis and spawning. After gamete release, the persistent follicle participates in the attachment of ova and developing embryos to the interfilamental junctions of the inner and outer demibranchs of the gill, where embryos are incubated until hatching as late-stage pediveliger larvae. Ripe eggs are large (∼250 μm diameter), suggesting that development is entirely lecithotrophic. The follicle cells that mediate connections between developing embryos and the maternal individual probably have a mechanical role only, providing support and possibly facilitating the accommodation of a large number of embryos to maximize the branchial space available for brooding.     

Functional morphology of the Conus proboscis (Mollusca: Gastropoda)

The extended proboscis of the toxoglossan gastropod Conus may exceed four times its contracted length and 1.5 times the shell length. The proboscis wall consists of cuboidal epidermis and circular, crossed helical and longitudinal muscle layers. Between the proboscis wall and its lumen, free longitudinal muscles and nerves course through haemocoel. A thick layer of connective tissue and columnar epithelium surround the proboscis lumen. In C. Catus , muscle comprises about 70% of the volume of the proboscis (exclusive of its lumen) and haemocoel about 20%, in both moderately extended and contracted states. Differentiation along the length of the proboscis includes gradual replacement of muscle by connective tissue distally in the proboscis wall, and a subapical sphincter muscle that probably prevents back-slippage of the detached radular tooth prior to its use in feeding and aids injection of the tooth into the prey during capture.     

Lungs of Polypterus and Erpetoichthys

The wall of the asymmetrical saclike lungs of the fishes Polypterus and Erpetoichthys consists of several functionally different tissue layers. Their lumen is lined by a surface epithelium composed of (1) highly attenuated cells, termed pneumocytes I; (2) pneumocytes II with lamellar bodies, presumably indicating surfactant production; (3) mucous cells; and (4) ciliated cells. Underlying the pneumocytes I is a dense capillary net. The thin continuous endothelium of this net, together with the pneumocytes I, constitute the very thin blood-air barrier. The basement membrane of epithelium and endothelium fuse in the area of the blood-air barrier (thickness 210 m?m). Secretory and ciliary cells form longitudinal rows in the epithelium. Below the zone with a gas-exchanging tissue, a layer of connective tissue containing collagen and special elastic fibers occurs. The blood vessels that give rise to or drain the superficial capillary plexus are located in this connective tissue. The outermost layer of the lung consists of muscle cells, a narrow inner zone with smooth muscle cells, and an outer, broader zone with cross-striated muscle cells. The lung is innervated by myelinated and nonmyelinated nerve fibers. The morphology of the gas-exchange tissue in the lungs of these primitive bony fish is fundamentally very similar to that of the lungs of tetrapod vertebrates. The morphologic observations are in close agreement with physiologic data, disclosing well-developed respiratory capacities. Structural simplicity can be regarded as a model from which the lungs of the higher vertebrates derived. In addition to respiratory function, the lungs seem also to have hydrostatic tasks.     

Role of the arterial smooth muscle cell in the pathogenesis of atherosclerosis

The development of an atherosclerotic lesion is characterised by a proliferation of arterial smooth muscle cells and an accumulation of cholesterol, cholesteryl esters and connective tissue. The main connective tissue components of an atherosclerotic lesion, i.e. acidic glycosaminoglycans and collagen, are synthesized by the smooth muscle cells. Cholesterol is chiefly derived from plasma lipoproteins, but there is an enhanced intracellular esterification of cholesterol in the cells of the lesions. The important role of the arterial smooth muscle cell in the development of atherosclerotic lesions has resulted in cultures of these cells being used as experimental models to study the pathogenesis of atherosclerosis. Such studies have revealed many blood-derived and other substances affecting proliferation, as well as lipid and connective tissue metabolism of arterial smooth muscle cells. In this way certain risk factors for cardiovascular disease have turned out to be associated with the metabolic disturbances of atherogenesis at the cellular level. Studies with cultured arterial smooth muscle cells have also demonstrated other factors for example one derived from aggregating platelets that may significantly contribute to the development of atherosclerotic lesions. On the other hand, certain inherent features of the smooth muscle cells of the lesions, such as enhanced proliferation and synthesis of glycosaminoglycans, may also contribute to the pathological changes.     

Dependence of the elastic characteristics and contractile activity of the rat tail artery on the transmural difference of potentials

Change in the diffuse portion of the double electric layer at the blood-vascular wall border under the effect of thorium nitric oxide and heparin affects the tone and contractile ability of the rat tail artery: thorium diminishes the vessel rigidity and the amplitude of its contractile response, whereas heparin augments the rigidity as well as the amplitude of the response. Thorium nitric oxide also diminishes rigidity of the vessel connective tissue skeleton. The difference of potentials between the vessel lumen and its wall seems to be capable of affecting the contractile apparatus of the smooth muscle cells by means of activation of potential-sensitive ionic channels.     

Embryology and cytodifferentiation of the pituitary gland in the lizard Anolis carolinensis

Anolis embryos have limb buds at the time eggs are laid and require about 39 days to complete development at 28°C. Rathke's pouch is present at five days, and the subdivisions of the adenohypophysis are differentiated by ten days after oviposition. The cells of the rostral half of the pars distalis (PD) are derived from the anterior face of Rathke's pouch; cells of the caudal half from the posterior face. Lateral lobe cells differentiate on the lateral margins of the developing caudal PD, and knob-like outgrowths of this tissue attach to the walls of the diencephalon to form the pars tuberalis (PT). Subsequently, the cells of the PT lose their connection with the PD and become a pair of flattened oblong plaques. They reach maximal size in midincubation, and are gradually invaded by nervous elements and incorporated into the walls of the hypothalamus. Electron micrographs demonstrate that the embryonic PT is secretory. Ultrastructurally the pars intermedia (PI) and PD are composed of parenchymous secretory cells in a framework of stellate cells. Stellate cells surround the lumen of Rathke's pouch and are connected laterally by complex junctions that exclude the secretory cells from the luminal surface. They extend in sheet-like processes among the secretory cells to the outer margin of the gland where they form a partial sheath within the basal lamina around the secretory tissue. As development proceeds, the lumen becomes subdivided and the resulting reduced lumina are recognizable as the forerunners of the follicles of the adult adenohypophysis. The cells of the PI are differentiated into secretory or stellate cells halfway through incubation. At this time only half of the cells of the PD can be so classified. Four of the five granulated cell types described in the adult are recognizable by mid-incubation; the fifth cell type (prolactin cell) becomes distinguishable within ten days thereafter, and at hatching appears to be actively synthesizing secretory products.     

Aortic carboxypeptidase-like protein is expressed in collagen-rich tissues during mouse embryonic development

Aortic carboxypeptidase-like protein (ACLP) was originally identified in vascular smooth muscle cells and contains discoidin and catalytically inactive metallocarboxypeptidase domains. ACLP is a secreted protein that associates with the extracellular matrix and is essential for abdominal wall development and contributes to dermal wound healing. Because of these developmental and adult phenotypes, we examined the expression of ACLP by immunohistochemistry throughout mouse embryonic development. ACLP was not detected in 7.5 days post-coitum (dpc) embryos, however at 9.5 dpc low levels of expression were detected in the somites and dorsal aorta. Expression was detected in both the yolk sac and embryonic vasculature at 10.5d pc. ACLP expression increased in both large and small blood vessels at 11.5 and 13.5 dpc and intense expression was detected within the vascular smooth muscle layer in 16.5 dpc embryos. At later developmental time points, discrete areas of ACLP expression were detected in the mesenchymal cells in the dermal layer, developing skeletal structures, connective tissue, and in the umbilical ring and vessels. The predominance of ACLP immunoreactivity localized with collagen-rich regions including tendons and basement membranes. Overall, the developmental expression pattern is consistent with a regulatory or structural role in the abdominal wall, vasculature, and dermis.     

Structure of the female gonoduct of the viviparous teleost Poecilia reticulata (Poeciliidae) during nongestation and gestation stages

Female teleosts do not have oviducts because Müllerian ducts do not develop. Instead, the caudal region of the ovary, the gonoduct, connects to the exterior. Because of the lack of oviducts in viviparous teleosts, the embryos develop in the ovary, as an intraovarian gestation, unique in vertebrates. This is the first study to address the histology of the gonoduct in a viviparous teleost. The gonoduct of Poecilia reticulata was analyzed during previtellogenesis, vitellogenesis, and gestation. The gonoduct lacks germinal cells. From deep to superficial, the wall has simple cuboidal or columnar epithelium, loose connective tissue, longitudinal layer of smooth muscle, and visceral peritoneum. Cells of the immune system occur in the lumen and in the mucosa. The gonoduct was divided in three regions: 1) cephalic, 2) middle, and 3) caudal. At the initial part of each region, thin mucosal folds extend into the lumen. The cephalic region forms a tubular structure with light and irregular folds. The middle region has a wider lumen and is more irregular due to ventral invaginations and irregular and short mucosal folds; beneath the epithelium there are melano‐macrophage centers. The caudal region is delimited from the middle region by folds; however, they are thinner than these of the other regions. Ventral invaginations form exocrine glands, and the smooth muscle is thicker than in the other regions. During gestation, cells of the immune system are abundant; melano‐macrophage centers become larger and the glands exhibit desquamated cells. These observations suggest roles of the gonoduct in reducing the diameter of the lumen; receiving sperm during vitellogenesis; producing secretions, more abundant during vitellogenesis; and in immunological activity throughout the reproductive cycle. The ciliated epithelium and the thick muscle of the caudal region may be involved during birth. J. Morphol. 275:247–257, 2014. © 2013 Wiley Periodicals, Inc.     

Functional morphology of the gas-gland cells of the air-bladder of Oreochromis alcalicus grahami (teleostei: cichlidae): an ultrastructural study on a fish adapted to a severe, highly alkaline environment

Oreochromis alcalicus grahami is a small cichlid fish that lives in shallow peripheral lagoons of Lake Magadi, Kenya. The internal surface of the air-bladder is highly vascularized, illustrating possible utilization as an accessory respiratory organ. The wall of the bladder consists of five distinct tissue layers. From the outer to the inner surfaces are: a squamous, undifferentiated epithelial cell; a collagen-elastic tissue space; a smooth muscle tissue band; an inner connective tissue space; and columnar gas-gland cells projecting into the lumen. The cell membrane over the perikarya of the gas-gland cells was sporadically broken. The disruptions were ascribed to possible physical damage by discharging gas-bubbles. Juxtaluminally, the gas-gland cells attached across tight junctions. The cells have highly euchromatic nuclei and conspicuously large intracytoplasmic secretory bodies. On the blood capillary facing (basal) aspect, the cell membrane of the gas-gland cells is highly amplified. The cells insert onto a smooth muscle tissue band. The morphological features and the topographical arrangement of the gas-gland cells in O. a. grahami are indicative of an operative exchange of materials between them and the underlying tissue components especially the blood capillaries. For a fish that subsists in hot, highly saline and alkaline water heavily invested by avian predators and where the partial pressure of oxygen diurnally shifts from virtual anoxia to hyperoxia, development of a versatile air-bladder for efficient buoyancy control conforms to the functional demands placed on it by a unique environment. Illustratively, instead of the gas-gland morphology in O. a. grahami resembling that in the freshwater fishes, the group from which the fish has evolved, it compares more closely to that of the marine fish. This similarity suggests amazing parallel evolutionary adaptation to biophysically corresponding aquatic milieus.     

Differentiation of vascular pseudointima under normal and disturbed blood flow conditions: Ultrastructural observations in the rat

Summary To study the effect of haemodynamic stress on the morphological differentiation of pseudointima, the ultrastructure of the cells lining normally shaped and aneurysmal polyurethane vascular prostheses implanted into the abdominal aorta of rats was examined. In the normally shaped vascular prostheses the pseudointima was composed of several layers of smooth muscle cells, which varied in differentiation from normal smooth muscle cells to myofibroblasts, and which were lined by a continuous sheet of endothelial cells. In the aneurysmal vascular prostheses, a pseudointima, composed of only layers of smooth muscle cells had developed. Those smooth muscle cells which lined the lumen had a typical morphology: they were polygonal, flat cells of unequal size, with a distinct organelle-free zone, containing myofilaments, at the luminal peripheral cytoplasmic side. The other smooth muscle cells varied in differentiation from normal smooth muscle cells to myofibroblasts. Under severe haemodynamic stresses, such as occur in the aneurysmal vascular prostheses, the regeneration of endothelial cells is impaired and smooth muscle cells undergo morphological changes to form a pseudoendothelial lining.     

Breeding and post-breeding structure of the vas deferens of the long-toed salamander Ambystoma macrodactylum

The vas deferens of Ambystoma macrodactylum is composed of a peritoneal epithelium, connective tissue layer with fibroblasts, circular smooth muscle, capillaries, cells containing lipid, and a luminal epithelium composed of a single layer of cuboidal cells covered by a net of interconnected ciliated squamous cells. The cuboidal cells have abundant rough endoplasmic reticulum, mitochondria, and PAS + secretory vesicles. Squamous cells of breeding males consistently have tufts of ～100 cilia located at one end of the long axis of each cell. These cilia may help distribute secretory products. The squamous cells, absent in post-breeding males, are apparently sloughed into the lumen. Lipid vesicles are present throughout the cytoplasm of the cuboidal and squamous epithelial cells and are also in some cells of the connective tissue layer. These vesicles increase dramatically in number during the first 4 weeks after breeding and may serve as an energy pool for the next breeding season. Enzyme-histochemical tests for testosterone synthesis were negative. In addition to the accumulation of lipid and the loss of squamous cells in the vas deferens, after breeding PAS + vesicle production is terminated. These alterations appear to represent energy conservation strategies employed by the sperm-depleted vas deferens.     

Further observations on upper urinary tract smooth muscle

Summary Light and electron microscopic techniques have been employed to study the arrangement and distribution of two types of muscle in the upper urinary tract of the rat. An outer layer of cells has been identified in the wall of the renal calix and pelvis. These cells are separated by connective tissue but possess numerous processes which make close contacts with adjacent cells. A layer of similar cells has not been observed in the wall of the upper ureter. The inner layer of muscle in the calix and pelvis is composed of larger cells similar to and apparently continuous with ureteric muscle. These cells are closely related to one another without intervening connective tissue and possess numerous bundles of myofilaments which extend along the length of the cell. The two types of muscle are closely related and, in the junctional region, cells of the outer layer are arranged along the length and make close contacts with one or more of the inner smooth muscle cells. A quantitative estimation has been made of nerve bundles associated with smooth muscle forming the outer layer of the calix and pelvis and with the muscle of the ureter. The results have shown a five fold increase in nerves associated with the caliceal muscle when compared with the ureter. The results are discussed in relation to the concept of a ureteric pacemaker.The authors wish to thank Professor G. A. G. Mitchell for his useful advice and encouragement.