The compound eye of<Emphasis Type="Italic"> Scutigera coleoptrata</Emphasis> (Linnaeus, 1758) (Chilopoda: Notostigmophora): an ultrastructural reinvestigation that adds support to the Mandibulata concept

The lateral compound eye of Scutigera coleoptrata was examined by electron microscopy. Each ommatidium consists of a dioptric apparatus, formed by a cornea and a multipartite eucone crystalline cone, a bilayered retinula and a surrounding sheath of primary pigment and interommatidial pigment cells. With reference to the median eye region, each cone is made up of eight cone segments belonging to four cone cells. The nuclei of the cone cells are located proximally outside the cone near the transition area between distal and proximal retinula cells. The connection between nuclear region and cone segment is via a narrow cytoplasmic strand, which splits into two distal cytoplasmic processes. Additionally, from the nuclear region of each cone cell a single cytoplasmic process runs in a proximal direction to the basement membrane. The bilayered rhabdom is usually made up of the rhabdomeres of 9–12 distal retinula cells and four proximal retinula cell. The pigment shield is composed of primary pigment cells (which most likely secrete the corneal lens) and interommatidial pigment cells. The primary pigment cells underlie the cornea and surround, more or less, the upper third of the crystalline cone. By giving rise to the cornea and by functioning as part of the pigment shield these pigment cells serve a double function. Interommatidial pigment cells extend from the cornea to the basement membrane and stabilise the ommatidium. In particular, the presence of cone cells, primary pigment cells as well as interommatidial pigment cells in the compound eye of S. coleoptrata is seen as an important morphological support for the Mandibulata concept. Furthermore, the phylogenetic significance of these cell types is discussed with respect to the Tetraconata.     

柚木野螟成虫复眼外部形态和超微结构观察

【目的】柚木野螟Eutectona machaeralis主要取食危害珍贵树种柚木。本研究旨在观察研究柚木野螟成虫复眼的形态、组织结构和超微结构,分析其复眼结构特征,为更好了解该物种复杂的视觉行为与感光、趋光机制的关系奠定基础。【方法】运用光学显微镜以及扫描和透射电子显微镜技术观察了柚木野螟成虫复眼的形态、组织结构和超微结构。【结果】柚木野螟成虫复眼着生于头部触角基部,呈椭球形,属对称性复眼。雌、雄成虫复眼分别有2 300~2 755和1 950~2 316个小眼。小眼呈正六边形,表面密被角膜乳突,间隙偶有感觉毛。每个小眼由1个角膜、4个晶锥细胞、1对初级色素细胞,6个次级色素细胞、不同水平面分布的12个视网膜细胞和基膜等组成。沿小眼纵轴11个视网膜细胞的向心侧细胞膜特化成细丝状微绒毛,形成放射状排列的视小杆,组合呈融合型视杆;第12个视网膜细胞位于小眼基部。基膜上方,视网膜细胞和次级色素细胞末端膨大,以轴突形式穿过基底膜。【结论】柚木野螟复眼为典型的重叠像眼,雌、雄成虫小眼排列方式及内部结构无明显差异,但雌、雄虫小眼数量和大小具有明显的性二型现象。     

Basement Membrane Proteins: Structure,Assembly, and Cellular Interactions

Abstract

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N-and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.     

Basement membrane proteins: structure, assembly, and cellular interactions.

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.     

Comparative morphology of the epidermis of seven species of polyclad flatworms (Platyhelminthes: Rhabditophora)

The ultrastructure of the epidermis of seven species of polyclad flatworms (Phaenocelis medvedica, Phaenocelis peleca, Pleioplana atomata, Boninia divae, Pericelis orbicularis, Enchiridium periommatum, and Cycloporus variegatus) representing six families is described. In all seven species, the epidermis consists of a single layer of columnar cells that rests on a bipartite basement membrane. Epithelial cell surfaces are covered by numerous microvilli and cilia. Cilia contain microtubules arranged in the 9 + 2 pattern, and from their basal bodies two striated rootlets arise, a rostrally directed one running parallel to the apical cell membrane, and a vertical one at a right angle to the first rootlet. Numerous epithelosomes and mitochondria occupy the apical parts of the cells. The basal part of the cells is highly folded, forming a cell web that connects to the basement membrane. The basement membrane consists of a thin basal lamina and a thick, multilayered reticular lamina. The number of layers in the reticular lamina varies among the different species and appears to be correlated with body size. Numerous canals containing either pigment granules or nervous processes perforate the basement membrane. We have identified four different types of glands: rhabdite glands, rhabdoid glands, mucoid glands containing vacuoles filled with flocculent material, and mucoid glands resembling thread cells of hagfish slime glands. The latter have been found only in P. orbicularis. Pigment cells were found in all species examined with the exception of C. variegatus, which takes its coloration from its ascidian prey. Our results further support the unique taxonomic status of Boniniidae.     

Developmental expression and cellular origin of the laminin alpha2, alpha4, and alpha5 chains in the intestine.

Laminins are extracellular matrix glycoproteins that are involved in various cellular functions, including adhesion, proliferation, and differentiation. In this study, we examine the expression patterns and the cellular origins of the laminin alpha2, alpha4, and alpha5 chains in the developing mouse intestine and in in vitro mouse/chick or chick/mouse interspecies hybrid intestines. In situ hybridization and Northern blot analysis revealed that mRNA levels for all three laminin alpha chains are highest in the fetal intestine undergoing intense morphogenetic movements. Laminin alpha4 mRNA and polypeptide are associated with mesenchyme-derived cell populations such as endothelium and smooth muscle. In contrast, laminin alpha2 and alpha5 chains participate in the structural organization of the subepithelial basement membrane and, in the mature intestine, show a complementary pattern of expression. All three laminin alpha chains occur in the smooth muscle basement membrane, with a differential expression of laminin alpha5 chain in the circular and longitudinal smooth muscle layers. The cellular origin of laminin alpha2 and alpha5 chains found in the subepithelial cell basement membrane was studied by immunocytochemical analysis of mouse/chick or chick/mouse interspecies hybrid intestines at various stages of development using mouse-specific antibodies. Laminin alpha2 was found to be deposited into the basement membrane exclusively by mesenchymal cells, while the laminin alpha5 chain was deposited by both epithelial and mesenchymal cells in an apparently developmentally regulated pattern. We conclude that (1) multiple laminin alpha chains are expressed in the intestine, implying specific roles for individual laminin isoforms during intestinal development, and (2) reciprocal epithelial/mesenchymal interactions are essential for the formation of a structured subepithelial basement membrane.     

The relation between connective tissue cells and intercellular substances, including basement membranes.

Basement membranes are distributed widely in the body forming an extracellular matrix for epithelial and endothelial cells. The collagenous and glycoprotein constituents of basement membranes are synthesized by these two cell types. Disturbance of the interactions between basement membranes and their associated epithelial and endothelial cells can lead to the pathological changes seen in diseases involving basement membranes. These changes are illustrated here by reference to glomerulonephritis induced by the deposition of immune complexes in the glomerulus of the kidney, and chronic inflammatory changes occurring in the lung after inhalation of asbestos. In these diseases basement membrane changes can occur in several ways. Hydrolytic enzymes released from inflammatory cells degrade basement membranes while other constituents by epithelial and endothelial cells. Alternatively the physical separation of epithelial and endothelial cells from their basement membrances by space-occupying substances such as immune complexes can interfere with feedback mechanisms leading to synthesis of basement membrane constituents and cell proliferation. Studies of these pathological changes at a cellular level should shed new light on the ways in which cells interact with their pericellular environment.     

Bridging structure with function: structural, regulatory, and developmental role of laminins

The basement membrane is a highly intricate and organized portion of the extracellular matrix that interfaces with a variety of cell types including epithelial, endothelial, muscle, nerve, and fat cells. The laminin family of glycoproteins is a major constituent of the basement membrane. The 16 known laminin isoforms are formed from combinations of alpha, beta, and gamma chains, with each chain containing specific domains capable of interacting with cellular receptors such as integrins and other extracellular ligands. In addition to its role in the assembly and architectural integrity of the basement membrane, laminins interact with cells to influence proliferation, differentiation, adhesion, and migration, processes activated in normal and pathologic states. In vitro these functions are regulated by the post-translational modifications of the individual laminin chains. In vivo laminin knockout mouse studies have been particularly instructive in defining the function of specific laminins in mammalian development and have also highlighted its role as a key component of the basement membrane. In this review, we will define how laminin structure complements function and explore its role in both normal and pathologic processes.     

The fine structure of the retina of the honey bee drone

Summary The eye of the honey bee drone is composed of approximately 8,000 photoreceptive units or ommatidia, each topped by a crystalline cone and a corneal facet. An ommatidium contains 9 visual or retinula cells whose processes or axons pierce a basement membrane and enter the optic lobe underlying the sensory retina. The visual cells of the ommatidium are of unequal size: six are large and three, small. In the center of the ommatidium, the visual cells bear a brush of microvilli called rhabdomere. The rhabdome is a closed-type one and formed mainly by the rhabdomeres of the six large retinula cells. The rhabdomeric microvilli probably contain the photopigment (rhodopsin), whose modification by light lead to the receptor potential in the retinula cells. The cytoplasm of the retinula cells contains various organelles including pigment granules (ommochromes), and peculiar structures called the subrhabdomeric cisternae. The cisternae, probably composed of agranular endoplasmic reticulum undergo swelling during dark adaptation and appear in frequent connection with Golgi cisternae. Three types of pigment cells are associated with each ommatidium. The crystalline cone is entirely surrounded by two corneal pigment cells. The ommatidium, including its dioptric apparatus and corneal pigment cells, is surrounded by a sleeve of about 30 elongated cells called the outer pigment cells. These extend from the base of the corneal facet to the basement membrane. Near the basement membrane the center of the ommatidium is occupied by a basal pigment cell. Open extracellular channels are present between pigment cells as well as between retinula cells. Tight junctions within the ommatidium are restricted to the contact points between the rhabdomeric microvilli. These results are discussed in view of their functional implications in the drone vision, as well as in view of the data of comparative morphology.This work was supported by a grant from the Fonds National Suisse de la Recherche Scientifique.     

Basement membrane stimulates the polarized distribution of integrins but not the Na,K-ATPase in the retinal pigment epithelium.

The basement membrane stimulates the differentiation and polarity of simple transporting epithelia. We demonstrated for the retinal pigment epithelium (RPE) of chicken embryos that polarity develops gradually. Although the RPE and an immature basement membrane are established on embryonic day 4 (E4), the distribution of the Na,K-ATPase and a family of basement membrane receptors containing the beta 1 subunit of integrin is nonpolarized. The percentage of polarized cells increases gradually until cells in all regions of the epithelium are polarized on E11. During this time, the basement membrane increases in size and complexity to form Bruch's membrane. To study the ability of the basement membrane to stimulate the polarized distribution of the beta 1 integrins or the Na,K-ATPase, RPE was harvested from E7, E9, or E14 embryos and cultured on Bruch's membrane isolated (in association with the choroid) from E14 embryos. As a control, the RPE was plated on the side of the choroid lacking a Bruch's membrane. The distribution of the beta 1 integrins and the Na,K-ATPase was determined by indirect immunofluorescence. Bruch's membrane stimulated the polarized distribution of the beta 1 integrins regardless of the developmental age of the RPE even though E7 RPE is nonpolarized in vivo. To examine the role of individual matrix components, RPE was plated on matrix-coated filters. The polarized distribution of the beta 1 integrins was stimulated by laminin, collagen IV, and Matrigel but not by fibronectin. Interestingly, laminin and collagen IV are present in the basement membrane on E4 when RPE is not polarized in vivo. Under no circumstances was the distribution of the Na,K-ATPase polarized. These data indicate that the basement membrane influences the distribution of a subset of plasma membrane proteins but that other factors are required for full polarity.     

Ultrastructural and cytochemical study on the epithelium lining ductuli efferentes in Equus asinus

The ultrastructure and cytochemistry of the epithelium lining ductuli efferentes in adult Equus asinus was studied. The epithelium is constituted of randomly distributed ciliated and non-ciliated cells extending from the basement membrane to the lumen. A third cellular type appearing to be in a degenerating state may also occur with variable frequency and is basally located. Mainly "non-ciliated cells" display ultrastructural and cytochemical features which can be related to strong resorptive activity; lysosomes and pigment granules, lipofuscinic in nature, are also present. Pigment masses exhibiting the same morphological and histochemical features fill the cytoplasm of degenerating cells. The results are compared with those obtained by previous authors in other Mammals and in Birds and are related to the functions commonly attributed to ductuli efferentes. The origin of degenerating cells, which are peculiar of Equidae, is discussed.     

大草蛉成虫复眼的外部形态及其显微结构

用扫描电镜和光学显微镜观察了大草蛉Chrysopa pallens Ramber成虫复眼的外部形态及明、暗适应和性别对其显微结构的影响。结果发现：（1）其复眼呈半球形,位于头部两侧,略成“八”字形排列,单个复眼约由3 600个小眼组成,最前和最后小眼之间的夹角约为180°,最上和最下小眼之间的夹角约200°;（2）小眼主要由角膜、晶锥和6～8个小网膜细胞、基膜组成,外围环绕有2个初级虹膜色素细胞和6个次级虹膜色素细胞,基膜处有色素颗粒分布;（3）暗适应时,晶锥开裂程度较大,远端5～7个网膜细胞核向远端移动,与晶锥近端相接或接近,次级虹膜色素颗粒亦向远端移动包围晶锥;明适应时,晶锥开裂程度小或闭合,远端网膜细胞核向近端移动,透明带显现,大部分次级虹膜色素颗粒亦向近端移动分布在小网膜细胞柱周围,包被透明带;（4）在相同的明、暗适应下,雌、雄成虫复眼的显微结构无明显差异。结果表明大草蛉复眼为透明带明显的重叠象眼,其小眼不但具有次级虹膜色素颗粒纵向移动的常规调光机制,还存在晶锥开闭、远端网膜细胞核移动和基膜色素颗粒纵向扩散的调光新机制。     

Distribution of myoepithelial cells and basement membrane proteins in the resting, pregnant, lactating, and involuting rat mammary gland

Using antisera to specific proteins, the localization of the rat mammary parenchymal cells (both epithelial and myoepithelial), the basement membrane, and connective tissue components has been studied during the four physiological stages of the adult rat mammary gland, viz. resting, pregnant, lactating, and involuting glands. Antisera to myosin and prekeratin were used to localize myoepithelial cells, antisera to rat milk fat globule membrane for epithelial cells, antisera to laminin and type IV collagen to delineate the basement membrane and antisera to type I collagen and fibronectin as markers for connective tissue. In the resting, virgin mammary gland, myoepithelial cells appear to form a continuous layer around the epithelial cells and are in turn surrounded by a continuous basement membrane. Antiserum to fibronectin does not delineate the basement membrane in the resting gland. The ductal system is surrounded by connective tissue. Only the basal or myoepithelial cells in the terminal end buds of neonatal animals demonstrate cytoplasmic staining for basement membrane proteins, indicating active synthesis of these proteins during this period. In the secretory alveoli of the lactating rat, the myoepithelial cells no longer appear to form a continuous layer beneath the epithelial cells and in many areas the epithelial cells appear to be in contact with the basement membrane. The basement membrane in the lactating gland is still continuous around the ducts and alveoli. In the lactating gland, fibronectin appears to be located in the basement membrane region in addition to being a component of the stroma. During involution, the alveoli collapse, and appear to be in a state of dissolution. The basement membrane is thicker and is occasionally incomplete, as also are the basket-like myoepithelial structures. Basement membrane components can also be demonstrated throughout the collapsed alveoli.     

Cell invasion through basement membranes: an anchor of understanding

To metastasize, cancer cells must acquire the ability to breach several basement membrane barriers. Cell invasions through basement membranes also occur during normal development and immune system function, enabling organ formation and cell dispersal. The mechanisms that cells use to cross basement membranes in vivo remain elusive. In cancer and development, these invasions occur in complex and inaccessible environments, which are difficult to study in vivo. Anchor-cell invasion in Caenorhabditis elegans is a simple, visually and experimentally accessible model of basement membrane invasion that is beginning to reveal a network of cellular and molecular control mechanisms that regulate the fundamental cellular process of invasion through basement membranes.     

Interleukin-1β and tumor necrosis factor-α have opposite effects on fibroblasts and epithelial cells during basement membrane formation

Interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) are typical proinflammatory cytokines that influence various cellular functions, including metabolism of the extracellular matrix. We examined the roles of IL-1beta and TNF-alpha in basement membrane formation in an in vitro model of alveolar epithelial tissue composed of alveolar epithelial cells and pulmonary fibroblasts. Formation of the basement membrane by immortalized rat alveolar type II epithelial (SV40-T2) cells, which ordinarily do not form a continuous basement membrane, was dose-dependently upregulated in the presence of 2 ng/ml IL-1beta or 5 ng/ml TNF-alpha. IL-1beta or TNF-alpha alone induced increased secretion of type IV collagen, laminin-1, and nidogen-1/entactin, all of which contributed to this upregulation. In contrast, while SV40-T2 cells cultured with a fibroblasts-embedded type I collagen gel were able to form a continuous basement membrane, they failed to form a continuous basement membrane in the presence of IL-1beta or TNF-alpha. Fibroblasts treated with IL-1beta or TNF-alpha secreted matrix metalloproteinase (MMP)-9 and MMP-2, and these MMPs inhibited basement membrane formation and degraded the basement membrane architecture. Neither IL-1beta- nor TNF-alpha-treated SV40-T2 cells increased the secretion of MMP-9 and MMP-2. These results suggest that IL-1beta participates in basement membrane formation in two ways. One is the induction of MMP-2 and MMP-9 secretion by fibroblasts, which inhibits basement membrane formation, and the other is induction of basement membrane component secretion from alveolar epithelial cells to enhance basement membrane formation.     

Ultrastructure of the eye of a euphausiid crustacean

The compound eye of the Antarctic euphausiid Euphausia superba is a spherical clear zone eye. The dioptric system consists of a hexagonally-faceted cornea, two corneagenous cells, two crystalline cone cells which form the bipartite crystalline cone, and two accessory cone cells. The dioptric system of each ommatidium is separated from that of adjacent ommatidia by six distal pigment cells and a basement membrane. The proximal tip of the crystalline cone is cupped by the distal ends of the seven retinula cells whose nuclei are arranged in a staggered array slightly distal to the middle of the clear zone. In the distal half of the clear zone, each narrow retinula cell column is surrounded by large proximal extensions of the six distal pigment cells. The pigment cells narrow more proximally and terminate at the proximal basement membrane. A specialized axial channel complex extends from the crystalline cone through the clear zone, and is continuous with a conical refractive element which caps the distal end of the rhabdom. The rhabdom is fused, and made up of alternating highly birefringent layers of orthogonally-oriented microvilli. It is surrounded by a narrow extra-cellular space which is continuous with the distal refractive element and a second conical refractive element at the proximal end of the rhabdom.     

Larval development in the Homoscleromorpha (Porifera, Demospongiae)

Abstract. Embryonic development from coeloblastula to fully developed larva was investigated in 8 Mediterranean homoscleromorph species: Oscarella lobularis, O. tuberculata, O. microlobata, O. imperialis, Plakina trilopha, P. jani, Corticium candelabrum , and Pseudocorticium jarrei. Morphogenesis of the larva is similar in all these species; however, cell proliferation is more active in species of Oscarella than in Plakina and C. candelabrum. The result of cell division is a wrinkled, flagellated larva, called a cinctoblastula. It is composed of a columnar epithelium of polarized, monoflagellated cells among which are scattered a few non-flagellated ovoid cells. The central cavity always contains symbiotic bacteria. Maternal cells are also present in O. lobularis, O. imperialis , and P. jarrei. In the fully developed larva, cell shape and dimensions are constant for each species. The cells of the anterior pole have large vacuoles with heterogeneous material; those of the postero-lateral zone have an intranuclear paracrystalline inclusion; and the flagellated cells of the posterior pole have large osmiophilic inclusions. Intercellular junctions join the apical parts of the cells, beneath which are other specialized cell junctions. A basement membrane underlying the flagellated cells lines the larval cavity. This is the first observation of a basement membrane in a poriferan larva. The basal apparatus of flagellated cells is characterized by an accessory centriole located exactly beneath the basal body. The single basal rootlet is cross striated. The presence of a basement membrane and a true epithelium in the larva of Homoscleromorpha—unique among poriferan clades and shared with Eumetazoa—suggests that Demospongiae could be paraphyletic.     

Fine structure of light- and dark-adapted eyes of desert ants,Cataglyphis bicolor (Formicidae,Hymenoptera)

Among ants, Cataglyphis bicolor shows the best performance in optical orientation. Its eye is of the apposition type with a fused rhabdom. Morphological studies on the general struture of the eye as well as the effect of light have been carried out with transmission and scanning electron microscopy. An ommatidium is composed of a dioptric apparatus, consisting of a cornea, corneal process and a crystalline cone, the sensory retinula, which is made up of eight retinula cells in the distal half and of an additional ninth one in the proximal half. The ommatidia are separated from each other by two primary pigment cells, which surround the crystalline cone and an average of 12 secondary pigment cells, which reach from cornea to the basement membrane. The eye of Cataglyphis bicolor possesses a light intensity dependent adaptation mechanism, which causes a radial and distal movement of the pigment granules within the retinula cells and a dilatation of cisternae of the ER along the rhabdom. Until now, no overall order in arrangement of retinula cells or direction of microvilli has been found from ommatidium to ommatidium. Such an order, however, must exist, either on the retina or the lamina level, since we have proven the ant's capacity for polarized light analysis.     

Immunocytochemical identification of neural elements in the central nervous systems of a snail,some insects,a fish,and a mammal with an antiserum to the molluscan cardio-excitatory tetrapeptide FMRF-amide

Summary The choriocapillaris is the fenestrated capillary network that supplies a large portion of the nutrients required by the retinal pigment epithelium, photoreceptor cells, and other cells of the outer neural retina. The permeability of these capillaries was investigated in the rat by the use of ferritin (mol. wt. approx. 480,000; mol. diam. 110Å) as a tracer. Ninety minutes after intravascular ferritin administration, a high concentration of tracer particles was distributed uniformly in the capillary lumina but few particles were present in Bruch's membrane, the multilayered basement membrane that separates the choriocapillary endothelium from the retinal pigment epithelium. The bulk of the tracer remained in the capillary lumina with a definite blockage seen at fenestral, channel, and vesicle diaphragms. These results indicate that the rat choriocapillary endothelium, unlike the fenestrated endothelia lining other capillary beds, constitutes an important barrier to the passage of ferritin and presumably of circulating native molecules of similar size.Supported by NIH grants EY 01889 and EY 07034 from the National Eye Institute and a grant-in-aid from Fight for Sight, Inc., of New York City     

The vascular basement membrane: a niche for insulin gene expression and Beta cell proliferation

Endocrine pancreatic beta cells require endothelial signals for their differentiation and function. However, the molecular basis for such signals remains unknown. Here, we show that beta cells, in contrast to the exocrine pancreatic cells, do not form a basement membrane. Instead, by using VEGF-A, they attract endothelial cells, which form capillaries with a vascular basement membrane next to the beta cells. We have identified laminins, among other vascular basement membrane proteins, as endothelial signals, which promote insulin gene expression and proliferation in beta cells. We further demonstrate that beta1-integrin is required for the beta cell response to the laminins. The proposed mechanism explains why beta cells must interact with endothelial cells, and it may apply to other cellular processes in which endothelial signals are required.