Interdigitating Processes Research Articles

Phagocytosis of sperm cytoplasmic droplets by a specialized region in the epididymis of the brushtailed possum, Trichosurus vulpecula.

During sperm maturation in the brushtailed possum, Trichosurus vulpecula, cytoplasmic droplets are shed from maturing spermatozoa in the distal regions of the head of the epididymis. Examination of luminal contents from various regions of the epididymis showed that the proportion of detached droplets in the luminal contents was reduced from about 45% in the proximal corpus epididymidis to less than 10% in the distal corpus and cauda epididymides. In contrast, the proportion of droplet-free spermatozoa increased from about 45% to more than 90% in the luminal contents. Disappearance of detached cytoplasmic droplets from the lumen was found to be associated with a region of specialized principal cells lining Regions 6 and 7 of the epididymis which selectively sequester and phagocytose free droplets from the luminal milieu. The luminal surfaces of these cells are characterized by a complex system of interdigitating processes which appear as waves of microfolds . These processes contrast with the stereocilia which cover the luminal surfaces of principal cells in adjacent, nonphagocytic regions of the duct. Cytoplasmic droplets are phagocytosed with their limiting membrane intact and gradually become condensed as they are transported deeper into the cell. Membrane lamellae are gradually compacted, transformed into concentrically arranged membrane stacks and then condensed into small electron-dense vesicles, which are probably degraded by the epithelial cells. The presence of a specific recognition factor on cytoplasmic droplets is suggested by the observation that phagocytic principal cells are able to selectively remove detached cytoplasmic droplets from the lumen in the presence of sperm-associated droplets and spermatozoa.

Multicystic transformation of kidneys in chronic renal failure.

With real-time sonography, 120 nondialyzed uremic patients prior to hemodialysis, 108 patients on maintenance hemodialysis and 9 patients postdialysis after successful homotransplantation were examined for the presence of renal cysts. Even in incipient renal failure, multiple cysts were demonstrable in some patients (at a serum creatinine of 3 mg/dl in 22% of patients), particularly in patients with analgesic nephropathy. When hemodialysis was started (serum creatinine approximately 10 mg/dl), 35% of the patients had multiple cysts. On hemodialysis, the prevalence, number and size of cysts rose progressively with time. After 8 years of hemodialysis, 92% of the patients had multiple cysts. However, enlargement of the kidneys was observed in only 2/108 patients. No major clinical complications were noted with the possible exception of 1 case of renal cell carcinoma. No correlation was noted between hematocrit and presence or extent of cystic transformation, but the 2 patients with cystic enlargement of the kidneys were polyglobulic. In 8/9 patients after transplantation, cysts were demonstrable in the patient's own kidneys after a median follow-up of 16 months. On light microscopy, cysts were lined by cuboidal or columnar epithelial cells with frequent papillary or adenomatous proliferations. The cyst lumen was filled with amorphous or lamellated organic material, which exhibited microfibrillar structure on electron microscopy. One kidney examined after ex vivo perfusion fixation showed multiple interconnected cavities on scanning electron microscopy. Ultrastructural studies showed epithelia with either the characteristics of proximal tubular cells (i.e. numerous microvilli, interdigitations and abundant lysosomes or mitochondria) or distal tubular cells (i.e. highly interdigitating processes) or finally collecting duct cells (i.e. no interdigitations and few microvilli).

The pig mesonephros. II. The proximal tubule: SEM, TEM and freeze-fracture images.

The ultrastructure of the proximal tubule of mature mesonephric nephrons was studied in perfusion-fixed pig embryos of the 41st gestational day. Despite its 8-12 mm long course, the proximal tubule possesses no cytologically different subsegments except its very last cells at the abrupt transition into the distal tubule. The first brush-bordered proximal tubule cells stand considerably within Bowman's capsule, abuting its attenuated cells. In SEM specimens, the average luminal cell diameters are 8 X 13 micron. The cells are 6-11 micron in height with overlying microvilli 2-4 micron long. Lateral faces of perfectly disjoined cells exhibit plate-like interdigitating processes projecting more than 5 micron deep into the neighboring cells. The basal cell face is completely covered with microvilli. The TEM pictures reveal an endocytic apparatus largely matching its metanephric counterpart. Mitochondria account for 23% of the cytoplasm and together with the many basolateral cell membranes indicate a high capacity for energy-dependent transport processes. Small basal lipid droplets represent a species peculiarity. Freeze-fracture specimens show an electrocoupling of the cells by gap junctions. The tight junction, with only 1-2 strands, characterizes a "leaky" epithelium. In most features, this tissue is as mature as its metanephric counterpart.

Surface-replica topography of retinal pigment epithelium during phagocytosis

The initial step in retinal phagocytosis may be mediated by complementary recognition molecules on the pigment epithelial microvilli and outer segment membrane surfaces. These molecules may display a surface morphology which is related topographically to phagocytic events. In order to explore this, we have developed a method for replicating the membrane surfaces of rat pigment epithelial explants during phagocytosis of latex beads. The explants are fixed in a mixed aldehyde and osmium fixative and the basal surface glued to a coverslip. After dehydration and critical point drying, the tissue is replicated with platinum and carbon in a freeze-fracture apparatus using either rotary or unidirectional coating and the replicas are examined by transmission electron microscopy. The membrane surfaces of the pigment epithelial cells and their microvilli are studded with numerous surface particles varying in size from 20 to 50 nm that are closely packed and give a cobblestone appearance to the membrane topography. During the early stages of phagocytosis, microvillous processes appear to spread over the beads and form branched processes as they contact bead surfaces. During bead engulfment the microvilli have shortened into flattened sheets with interdigitating processes overlapping the beads. These uptake sites resemble Venus' fly-traps, which as they close over the beads create craters in the membrane surfaces of the pigment epithelium. The crater-like uptake sites appear to flatten out in the later stages and form flattened membrane domains which are surrounded by crenulated membrane that displays an irregular particulate morphology.

Ultrastructural findings in so-called ependymal rat tumors induced by transplacental administration of ethylnitrosourea (ENU).

Tumors of the central nervous system (CNS) in Sprague-Dawley and Lewis rats were induced transplacentally by a single intraperitoneal (IP) injection of 10–25 mg/kg ethylnitrosourea (ENU) on days 16–20 of gestation. Light-microscopic examination revealed that 22% of these tumors could be diagnosed as ependymomas, anaplastic ependymomas (ependymoblastomas), or mixed tumors revealing both oligodendroglioma and anaplastic ependymoma tissues if the criteria of accepted classifications were followed. Electron-microscopic examination, however, demonstrated that the ependymoma and the anaplastic ependymoma-like tissue in ENU-induced tumors lacked ependymal features, such as basal bodies, cilia, complicated junctional complexes, microvilli, etc. This tissue type was repetitious, always being composed of cells arranged in groups, cords, and rosette-like (pseudorosette) formations. In the pseudorosettes, the cell nuclei were polarized at the periphery and the cytoplasm contained numerous polyribosomes, occasional short microtubules and usually a few small dense-core vesicles. The center of the pseudorosettes showed numerous slender interdigitating processes interconnected by maculae adherens. The tips of these processes showed vesicular degeneration. The cells arranged in groups or cords and perivascular rosettes revealed identical ultrastructure, but they were not polarized. The present findings indicate that the socalled ENU-induced ependymomas and anaplastic ependymomas are not true ependymal tumors, but rather primitive neuroepithelial neoplasms with some features of oligodendroglioma.

Scanning electron microscopic study of normal human glomerulogenesis and of fetal glomeruli in congenital nephrotic syndrome of the Finnish type

Kidneys from 5 normal fetuses and 3 with congenital nephrotic syndrome gestational age of 13 to 24 weeks, were studied under a scanning electron microscope. Differentiation of the visceral epithelial cells was observed in normally developing glomeruli. The originally cuboidal, smooth epithelial cells first flattened and acquired cytoplasmic buds on the cell margin. The buds gradually grew longer and branched out to form regular interdigitating foot processes. In the most mature glomeruli the majority of the visceral epithelial cells were trabeculated and the cytoplasmic processes were tightly wound around the glomerular capillaries. In the glomeruli of specimens from fetuses with congenital nephrotic syndrome the epithelial differentiation was different. The individual epithelial cells varied in size and shape and formed irregular cell groups. The cytoplasmic processes of visceral epithelial cells were shortened and flattened. Also, there was a marked loss of interdigitation of adjacent processes. The findings confirm previous transmission electron microscopic observations of pathological foot processes in glomeruli of fetuses with congenital nephrotic syndrome and show that the differentiation of the visceral epithelial cells is abnormal prior to the formation of the final interdigitating processes.

Heterogeneity of the MtTw15 mammosomatotropic tumor. II. Characterization of parenchymal cells by superimposition immunocytochemistry and electron microscopy.

Large MtTw15 tumors, which secrete growth hormone (GH) and prolactin (PRL), are composed of ovoid, elongated, and angular cells which demonstrated interdigitating processes and junctional complexes. The majority of the cells were essentially agranular, but two types of granulated cells were identifiable. One class of granulated cells contained moderate to sparse populations of round dense-cored granules measuring up to 250 nm in diameter. Rod-shaped to filamentous mitochondria with an electron-dense matrix were characteristic of a second class of granulated cells with plemorphic granules of various sizes and electron densities. Images of exocytotic release of the round dense-cored granules were frequently seen, but were not observed with the pleomorphic granules, many of which were judged to be lysosomes. Superimposition immunocytochemistry revealed hormones only in the granulated cells with round to ovoid granules. Morphometry indicated that hormone specific subpopulations of tumor cells can be identified since PRL secretory granules were significantly smaller than GH secretory granules (149 +/- 6 nm for PRL versus 221 +/- 9 nm for GH, P less than 0.001). The vast majority of immunopositive cells contained only GH or PRL, but a few were observed containing both hormones. Ovoid to irregular-shaped nuclei, large lipid inclusions, numerous free ribosomes and polyribosomes, moderate development of the rough endoplasmic reticulum, and prominent Golig profiles were characteristics of all cell types. Irrespective of the presence or absence of cytoplasmic granular elements, particles resembling viruses were encountered in many tumor cells, and these frequently appeared to be budding into the cisternae of the endoplasmic reticulum.

Regional analyses of the reversal process in the neonatal galactose cataract

Reversal of the galactose cataract in neonatal rat lenses involves a decrease in lenticular galactitol, a recovery of existing fiber contour and interdigitation, and a production of new fibers. The onset of neonatal cataractogenesis appeared at 7 days when the young were nursed by mothers placed on a 50% galactose diet at the time of delivery. On the 7th day, the mothers were returned to a normal diet and reversal of the neonatal cataract was monitored for the remainder of the nursing period. After 5 days on a normal diet, cataract reversal was incomplete; this is evidenced by an abnormal fiber contour with decreased number of interdigitating processes, the presence of galactitol and decreased lens size. Fiber alterations were most pronounced in the anterior superficial cortex (0–200 μm from the capsule). As the reversal period progressed for an additional 8 days, lenticular galactitol declined and normalization of anterior superficial and supranuclear cortical fibers occurred. Growth of the normal and reversal lenses was compared by determining the number of fiber lamellae produced per day. As new fibers were formed, by the anterior lens epithelium, the radius of the lenses increased with time; the increase in lens radius and the average fiber width were measured for the juvenile lenses (12–63 days) and the reversal lenses (12–20 days). The number of fiber lamellae produced during each growth period was calculated. Normal lens growth was a consistent 1 fiber lamella/day. During the reversal period, lens growth was increased with respect to normal; the source of the rapid generation of new fibers may be the differentiation of equatorial, multilayered epithelial cells which were induced by the galactose feeding.

Fine structure of the secretory and non‐secretory ameloblasts in the frog. II. Fine structure of the non‐secretory ameloblast

The non-secretory ameloblasts present at the enamel-free surfaces of maxillary teeth in the frog Rana pipiens were examined by electron microscopy at different stages of tooth development. Their main fine structural features seem to reflect a transport function. During early tooth development, the non-secretory ameloblasts adjacent to odontoblasts and predentin exhibit extensive lateral surface specializations and numerous cytoplasmic vesicles. During late tooth development, the non-secretory ameloblasts adjacent to mineralizing dentin show numerous cellular junctions, well-developed intercellular channels with numerous interdigitating processes and labyrinthine configurations at their distal surfaces. An intact basal lamina is present between the non-secretory ameloblasts and the dentin surface until the dentin becomes fully mineralized. At this stage the adjacent cells no longer exhibit surface specializations. It is suggested that the non-secretory ameloblasts may participate in the mineralization of adjacent dentin at the enamel-free surfaces. This surface dentin becomes fully mineralized at a later stage of development than the underlying dentin.

The fine structure of the interrenal cells of the duck (Anas platyrhynchos) with evidence for possible exocytotic release of steroids.

The duck interrenal cell possesses ultrastructural characteristics common to other steroid-secreting cells. Lipid droplets and mitochondria are abundant and lie principally at the apical end of the cell. Lipid droplets are not membrane-limited. Cisternae of smooth endoplasmic reticulum that are occasionally continuous with the less abundant rough endoplasmic reticulum are a prominent feature of the interrenal cell. Tubular profiles of rough endoplasmic reticulum often lie tangentially to mitochondria and ribosomes are either free, grouped in polyribosomal clusters, or bound to the endoplasmic reticulum. Mitochondria possess tubular cristae in the inner regions of the gland and frequently contain a paracrystalline array of small 10nm (o.d.) tubules and less frequently a hexagonal array of 40 nm trilaminar rings. Other cytoplasmic components include dense bodies, residual bodies, microtubules, microfilaments and specialized single membrane-bound vesicles. Gap junctions, intermediate junctions and interdigitating processes constitute the main intercellular associations. No tight junctions were identified. The single membrane-bound vesicles which are occasionally filled with a low electron-dense, lipid-like material form septate-like "junctions" with the plasma membrane. The septa bridge an intracellular gap of 15-17 nm. The vesicles are usually located near the subendothelial space at the basal and basilateral regions of the cell. Occasionally, vesicles fuse with the plasma membrane. It is suggested that these vesicles represent morphological evidence for the exocytotic release of steroid hormones.

Intercellular gap junctions in the developing retina and pigment epithelium of the chick.

Gap junctions are found in the pigment epithelium, between retina and pigment epithelium and in the retina of 5-14 day chick embryos, they are identified using block staining and extracellular tracer techniques. In the pigment epithelium gap junctions are found between cell bodies and interdigitating processes and many change their position during development. Gap junctions between retina and pigment epithelium are only made by undifferentiated retinal ventricular cells and may provide intercytoplasmic pathways important for photoreceptor differentiation. Retinal gap junctions are found in an outer zone next to the pigment epithelium and inner zone near the vitreous, they are only seen between ventricular cells but may provide pathways for ganglion cell specification. The role of gap junctions in the generation of retinal neurons is discussed.

So-called “minute pulmonary chemodectoma”.A tumor not related to paragangliomas

Twenty-six cases of so-called "minute pulmonary chemodectoma" are presented. The patient population showed a marked female preponderance, and there appeared to be an association of the lesion with pulmonary injury from a variety of causes including cardiac failure, chronic bronchitis and emphysema, and thromboemboli. Half the cases had multiple tumors. Microscopically, the tumors consisted of nests of cells in the interstitial tissue near small veins. Argentaffin and argyrophil stains failed to demonstrate cytoplasmic granules in any case. By electron microscopy, the nests were composed of large cells with broadly interdigitating processes connected by many well-formed desmosomes. The cytoplasm was filled with numerous 60-A filaments. The Golgi apparatus was prominent, while other organelles were sparse. No secretory granules were identified. It is concluded that the fine structure and lack of silver-positive granules are inconsistent with the morphology of previously reported paragangliomas, but that there is a resemblance at the light and electron microscopic level to meningeal arachnoed cells and the cells of meningiomas.

Intercellular relationships in the external glial limiting membrane of the neocortex of the cat and rat.

The external glial limiting membrane of the cerebral cortex appears to be a complete astrocytic mantle covering the pial surface of the molecular layer. It consists of flattened cell bodies arranged singly or in small groups spaced about 100 mu apart and multitudes of interdigitating processes arrayed in layers. The glial mantle is thicker in the sulci than on the gyri. It is covered externally by a basal lamina which is associated with collagenous fibrils and cells of the pia mater. The extracellular space in aldehyde-perfused material appears as a regular, electron-lucent interval 150 A wide between adjacent cell membranes. Gap junctions are frequently encountered in the external glial limiting membrane; desmosomes are present between astrocytic processes but are seen much less often.

Septate-like junctions in abnormal erythroblasts: cytochemical, ultrastructural and freeze-etch studies.

Ultrastructural studies of the bone marrow of a patient with refractory anemia revealed aberrant erythroblasts with unique cell junctions. Periodic structures linked adjacent processes on the same cell as well as surfaces of neighboring erythroblasts. Inclusions circumscribed by similar complexes were also present in the cytoplasm. The junction appeared in cross sections as two rigidly parallel unit membranes separated by 250-300 A interspace with two regular arrays of facing 70 A particles at intervals of 160-200 A. These intracellular specializations were present between erythroblasts at various stages of maturation and between, mono, bi or multinucleated erythroblasts. Junctions were permeable to lanthanum and many that appeared to be intracellular were demonstrated to be continuous with the extracellular space. The fact that others were not penetrated by lanthanum indicates that detachment of the interdigitating processes from cells of origin could have occurred. In freeze-etched replicas, distribution of membrane particles was random in areas of septate-like junctions, although parallel rows were sometimes observed both on A and B inner hydrophobic faces of the membrane leaflets. Junctional complexes in tissue culture appeared to have been disrupted and were not reestablished; however, inclusions resembling internalized junctions were observed associated with multivesicular bodies. Ineffective erythropoiesis and the resulting refractory anemia appear to be associated with the presence of the described anomalous junctional complexes.

The morphology of the buccal apparatus of Oenopota levidensis (Gastropoda, Turridae)

1. The Buccal, radular, and venom apparatus morphology and histology of Oenopota levidensis are described. This is the first report on the functional morphology of the turrid radular sac, the first report on the functional morphology of a north Pacific turrid gastropod, and the first histological study of these structures in the Turridae, in which they are highly modified in connection with the envenomation of prey. 2. The rhynchodaeum is similar to that in Oenopota Curricula, except that the rhynchodaeal sphincter is placed more posteriorly. 3. The proboscis of O. levidensis is intraembolic and similar to, but simpler than the proboscis of O. Curricula and Mangelia attenuata. 4. The Buccal tube, Buccal sac, and Buccal cavity correspond quite closely to O. turricula, except that O. levidensis has a. much larger buccal sphincter. 5. The salivary glands are paired and each has a duct that enters the buccal sac. They may function in removing cellular debris from the lumen of the tooth just prior to use. They may also function to initiate digestion, or to lubricate swallowed prey. 6. The radula is highly modified in the typical toxoglossan manner. It lacks a radular ribbon and a subradular membrane. Evidence is presented, and hypotheses are proposed to explain tooth formation. The teeth are most likely formed by odontoblast cells using a cell possibly of inferior epithelial origin as a template. This internal cell may also secrete the base of the tooth and its remnants may constitute the ligament. The tooth appears to undergo a chemical change in the radular sac that may be caused by secretions from the superior epithelium. The tooth may move down the radula supported by epithelial cells which are fastened to the ligament by interdigitating processes. 7. The venom apparatus is quite typically toxoglossan, very similar to Conus, and to the other turrids that have been studied, Lora, Mangelia, Oenopata, Philbertia, and Polystira. 8. The oesophagus is largely an unmodified wide tube as in other turrids.

Ultrastructure of the placenta of Antarctic seals during the first third of pregnancy.

AbstractThe chorioallantoic placenta from six crabeater seals (Lobodon carcinophagus) with fetuses from 15–180 mm crown rump length (CRL), and two leopard seals (Hydrurga leptonyx) with fetuses 76 and 86 mm CRL was lobulated, zonary, labyrinthine and endotheliochorial. The fine structure of the placenta was essentially similar in both species. Maternal capillaries of the labyrinth had attenuated endothelial cells unlike the fissiped carnivores. They were surrounded by an outer syncytiotrophoblast, incomplete inner cytotrophoblast, and a core of mesenchymal tissue containing fetal vessels. In general, the cytotrophoblastic cells were less electron‐opaque than the syncytium; the former possessed abundant free ribosomes, while the latter had many nuclei, mitochondria, and numerous strands of rough endoplasmic reticulum. Mitotic figures were observed only in the cytotrophoblast. Plasma membranes between the trophoblastic layers had interdigitating processes which showed many desmosomes along the intercellular spaces, whereas membranes surrounding the maternal capillaries were usually smooth. In the limb‐bud stage embryo, the syncytiotrophoblast had thick and thin areas which became progressively attenuated in older fetuses. Instances of intratrophoblastic fetal capillaries were observed in seals with fetuses of 40 mm CRL and older, with consequent greatly thinned placental barriers. By Perl's reactions, Turnbull's method, and electron microscopy, rows of granular bodies positive for iron were demonstrated along the basement membrane of the trophoblastic cells and in the mesenchymal cells of the crabeater seals, but not the leopard seals. Furthermore, it is suggested that the deposits of granular bodies show the sites of iron storage in the placentas. In the limb‐bud stage embryo the hematome borders and isolated pouches of the central hematome were observed. The central hematome was antimesometrial. The hematomes were lined by columnar cytotrophoblastic cells which phagocytized extravasated maternal blood and absorbed histiotrophic material.

Electrolyte content and ultrastructure of the kidney in hamsters with heart failure

Electrolyte (Na, K, Ca, Mg) concentrations were measured in the urine, serum, and renal tissues of cardiomyopathic (BIO 14.6) hamsters, during various stages of spontaneous congestive heart failure, and correlated with ultrastructural changes in renal tubules. The renal concentration of sodium was significantly elevated during the very early (subclinical) stages of congestive heart failure. Further increases paralleled the progress of the disease. The derived data indicate an intracellular location for the augmented load of sodium in the kidney. The magnesium concentrations in the kidney remained unchanged and the variations in renal potassium proved to be significant only during the advanced stages of cardiocirculatory insufficiency. Young cardiomyopathic hamsters had low renal calcium concentrations in spite of elevated serum calcium levels. However, during the terminal phase of congestive heart failure, the calcium values in the kidneys of cardiomyopathic hamsters were higher despite lower serum calcium values. The proximal and distal convoluted tubules as well as the cortical collecting tubules during severe congestive heart failure were characterized by marked widening of the extracellular spaces between the basilar interdigitating processes. The distal convoluted tubules exhibited edema and mitochondrial alterations and the intercellular spaces of the collecting tubules were dilated. These tubular changes indicate an increased flow of water and ions.

The structure of the trabecular meshwork of the primate eye: A light and electron microscopic study with peroxidase

The trabecular meshwork of the anterior chamber angle is composed of two regions. Adjacent to Schlemm's canal is the meshwork region which is formed by cells with interdigitating processes and large intercellular spaces containing collagen. Between this meshwork region and the anterior chamber is the trabecular region in which the cells closely envelop a collagenous core. The endothelium of Schlemm's canal is a complete, nonfenestrated membrane. Protrusions of meshwork tissue into Schlemm's canal form villi that are invested by the intact endothelium of Schlemm's canal. There are no canals, large pores, or other open channels of communication between the anterior chamber and Schlemm's canal. Peroxidase injected into the anterior chamber is trapped appreciably in the spaces of the trabecular meshwork. The connective tissue cells and infiltrating leukocytes become phagocytic, and within 4 days have disposed of the injected protein. No evidence was found for the direct passage of peroxidase into Schlemm's canal through endothelial cell vacuoles. The results of this study suggest that the anterior chamber and the spaces of the chamberangle connective tissue are structurally delimited from Schlemm's canal and that the trabecular meshwork may function as a biological filter.

Lens of the rat eye: an electron microscope and freeze-etch study

The structure of lens fibers has been reinvestigated utilizing electron microscopy and the freeze-etch technique. Interest was focused particularly on the nature of cellular interfaces in relation to the paucity of extracellular space. While cortical fibers exhibit regular, parallel interfaces with frequent and extensive occluded zones, at the equator and in the lens core the interfaces are complicated by extensive interdigitating processes. In the replicas, two dissimilar membrane surfaces result from fracture of an interface, one with a high particle content, and the other with few associated particles. These findings are discussed in relation to current theories concerning the plane of membrane fracture.

The micromorphology of the nauplius eye of the estuarine calanoid copepod, Sulcanus conflictus Nicholls (Crustacea)

The nauplius eye consists of one median and two lateral ocelli, each within a pigment cup. The three pigment cups are made up from two multi-nucleate pigment cells: each cell forming one lateral cup and half of the median cup. The three cups are lined on the insides by tapetal cells which contain layers of reflectile crystals. Each of the ocelli contains six sensory cells which protrude from the rims of the pigment cups and the protruding parts are sheathed by the conjunctiva cells. The whole eye is enveloped by a thin membrane which also sheaths the proximal parts of the five nerve bundles that leave the eye. All the sensory cells of the lateral ocelli are similar and have rhabdomeric microvilli on the terminal end, and contain phaosomes and a multitude of other organelles and cytoplasmic inclusions. The complex median ocellus contains a superior group of three retinular cells, linked by interdigitating processes, and an inferior group consisting of a large central cell enclosed in two cup-shaped peripheral retinular cells. A two-tiered rhabdome arrangement exists, with a rather complex inferior rhabdome set made up of a central rhabdomere and two hemi-annulate rhabdomeres. The cytoplasm of the retinular cells of the median ocellus lack phaosomes but instead contain double-walled tubular elements, possibly formed by the inpushings of microvilli into adjacent cells. The possible functional significance of the unique arrangement seen in the median ocellus is discussed. The retinular cells are of the inverse type. There are no efferent nerve fibres from the brain nor any nervous connection between the lateral and the median ocelli.