Ultrastructure of root cortical cells parasitized by the ring nematodeCriconemella xenoplax

Summary Individuals of the plant-parasitic nematodeCriconemella xenoplax, monoxenically cultured on root expiants of clover, carnation, and tomato, fed continuously for up to 8 days from single cells in the outer root cortex. Individual cortical cells parasitized by nematodes were modified into discrete food cells in all hosts examined. The nematode's stylet penetrated between epidermal cells and frequently through a subepidermal cortical cell. Electron-transparent callose-like material continuous with the cell wall enveloped the portion of the stylet that traversed subepidermal cortical cells. Food cells were typically located in the first or second cell layers of the cortex. The stylet penetrated 5–6 m through the wall of the food cell without penetrating the plasma membrane. Electron-transparent callose-like deposits formed between the invaginated plasma membrane and stylet, except at its aperture. The plasma membrane of the food cell was appressed tightly to the wall of the stylet aperture creating a 130–160 nm hole in the membrane. This opening provided continuity between the lumen of the stylet and the food cell cytosol for ingestion of nutrients by the nematode. Ribosomes were dissociated from the cisternae of the endoplasmic reticulum in food cells and accumulated with other cell organelles in a zone of modified cytoplasm around the stylet. A fibrillar material appeared to form a barrier in the cytosol around the stylet aperture that limited movement of cell organelles toward the aperture. Electron-dense secretory components were secreted into the food cell by the nematode. Clusters of putative nematode secretory components consisting of 20–40 nm diameter, electron-dense particles were dispersed in the densely particulate zone of cytoplasm around the stylet tip. The cytosol immediately around the stylet aperture in the center of the modified cytoplasm was finely granular.Plasmodesmata connecting the cytoplasm of the food cell with the cytoplasm of neighboring cells were greatly modified in a way that could facilitate solute transport into the food cell. The plasma membrane-lined canals of the modified plasmodesmata appeared to be increased in diameter and lacked desmotubules. Additionally, they frequently were lengthened by electron-transparent callose-like deposits projecting from the wall into the cytoplasm of the food cell. An electron-dense cap that formed an apparent tight seal with the plasma membrane developed over the entrance of each modified plasmodesma in the neighboring cells. These caps excluded all cell organelles from the cytosol contained within them. The nucleus of the food cell was usually enlarged and atypically shaped with dense peripheral clumps of condensed chromatin. Our results show thatC. xenoplax induces elaborate cellular modifications in host tissue to support sustained ingestion of nutrients from a single food cell.     

Submicroscopic organization of listeriae during L-transformation]

A study was made of the ultrastructural organization of listeria at the early stages of L-transformation, beginning from the first passage of the bacterial culture on solid nutrient medium with pencillin. The use of potassium benzylpenicillin salt in the capacity of an L-transforming agent permitted to observe the cells at various stages of L-transformation, beginning from the bacterial forms and ending with the typical L-colonies. It was shown that at the earliest stage of L-transformation there occurred not only destruction of the cell wall and the discharge of the mesosomes from the cell, but also significant changes in the nuclear apparatus of the cell. As soon as the second passage the freshly isolated L-forms displayed an internal membrane system in the form of myelin-like structures located under the external membrane, and of individual membranes in the cytoplasm not forming mesosomes. A substance of a medium electrone density resembling the material of the cell wall appeared on the cytoplasmic membrane (in some of its regions).     

Evaluation of membrane-bound black bodies in trophozoites and cysts of Naegleria spp

Various Naegleria strains were examined to determine the possible origin and significance of membrane-bound black bodies that were found in all exponentially growing cell populations. The bodies, 40–80 nm in diameter, were distributed randomly in the cytoplasm of Naegleria with ultrastructural features typical of trophozoites. No evidence was obtained that the contents of the black bodies were synthesized in the rough endoplasmic reticulum (ER) and packaged by membranous components, which could be a primitive “Golgi complex” in these amoebae. Examination of cells in various stages of encystment indicated that at least some of the cyst wall material was synthesized and packaged by the rough ER. After condensation into amorphous granules in the cisternae, the cyst wall material appeared in vesicles of the rough ER; these were frequently seen in close proximity to the cell membrane in the vicinity of developing cyst wall. Amorphous granules (～100 nm in diameter), which had variable densities and did not appear to be membrane bound, were seen in the cytoplasm of encysting cells. The substance of these granules also seemed to be incorporated into the cyst wall. The membrane-bound black bodies appeared to be destroyed in lysosomal elements during encystment. The membrane-bound black bodies were concluded to be characteristic of trophozoites and unrelated to encytment of Naegleria.     

ONTOGENY AND CYTOCHEMISTRY OF ALKALOIDAL VESICLES IN LATICIFERS OF PAPAVER SOMNIFERUM L. (PAPAVERACEAE)

Development of alkaloidal vesicles in laticifers of opium poppy, Papaver somniferum L., was investigated at the ultrastructural level. Laticifer initials possessed abundant endoplasmic reticulum throughout their dense cytoplasm. During differentiation the endoplasmic reticulum organized into long, folded sheets that were parallel to the longitudinal walls along the periphery of the cell. Vesicles appeared to be derived from dilation of endoplasmic reticulum. This relationship was confirmed through cytochemical data obtained with zinc iodide-osmium tetroxide and osmium tetroxide impregnation. Alkaloidal vesicles had electron-dense regions or caps that occurred early in laticifer differentiation, but these caps became less conspicuous in mature cells. Caps appeared to be derived from small particles which condensed along the inner surface of the vesicle membrane and subsequently accumulated at one or two positions along the membrane of the vesicle.     

Tracheid differentiation in tobacco pith cultures

Summary Sterile pith cultures of Nicotiana tabacum have been induced to form localized regions of differentiating tracheids. These localized regions have been examined by phase, fluorescence, and electron microscopy, and polarization optics. Fixation for electron microscopy was with glutaraldehyde-osmium. The differentiating tracheids develop characteristic thick cell walls which are eventually lignified. The lignifications appear to be uniform throughout the secondary wall and little or no lignin appears to be deposited in the primary walls or intercellular layer. At all stages of secondary wall deposition, the peripheral cytoplasm contains a system of microtubules which form a pattern similar to that of the developing thickenings. Within this system the microtubules are oriented, the direction of orientation mirroring that of the fibrils in the most recently deposited parts of the wall. The observations support the view that the microtubules are somehow involved in microfibril orientation. The microtubules appear to be attached to the plasma membrane which has a triple layered structure. The two electron dense layers of the plasma membrane have a particulate structure. In the differentiating tracheids at regions where secondary wall thickening has not yet been deposited numerous invaginations of the plasma membrane are observed which contain loosely organized fibrillar material. It is suggested that these are areas of localized activity of the plasma membrane and that the enzymes concerned with the final organization of the cellulose microfibrils are situated at the surface of the plasma membrane. Dictyosomes in the differentiation cells give rise to vesicles which contain fibrous material and the contents are incorporated into the cell wall. Numerous profiles characteristic of plasmodesmata are evident in sections of the secondary thickenings.Part of this work was carried out at the Osborne Memorial Laboratories, Yale University.     

Developmental sequence of bone-resorbing cells induced by intramuscular implantation of mineral-containing bone particles into rainbow trout,Oncorhynchus mykiss

Mineral-containing bone particles (BPs) were implanted intramuscularly into rainbow trout (Oncorhynchus mykiss) to investigate the sequence of appearance of bone-resorbing cells. A fibrous substance first surrounded the implanted BPs and was gradually replaced by connective tissue containing capillaries. Two weeks after BP implantation, relatively small multinucleated cells (type-1 cells), whose cytoplasm stained deeply with hematoxylin, appeared along the surfaces of the BPs. At later stages (after 4–8 weeks), the majority of cells which appeared to be resorbing the BPs were multinucleated cells whose cytoplasm stained deeply with eosin (type-2 cells). Type-2 cells contained more nuclei than type-1 cells. Electron-microscopical observations revealed that type-2 cells had the characteristic features of osteoclasts: the presence of numerous mitochondria, vacuoles and granules, and a differentiation of the cell membrane and cytoplasm into a ruffled border and clear zone, respectively. A tartrate-resistant acid phosphatase activity, which is an established characteristic of osteoclasts in terrestrial vertebrates, but which had not previously been examined in teleosts, was demonstrated histochemically in the type-2 cells. Development of type-2 cells was closely correlated with the development of connective tissue. These findings suggest that the development of a capillary network around the implanted BPs enables circulating osteoclast-progenitors to reach the surface of the BPs.     

Fine structure of the erythrocytic stages of Plasmodium knowlesi

Summary The fine structure of erythrocytic stages of Plasmodium knowlesi was compared with that of the same parasite isolated from its host cell by a saponin technique. Rhesus monkeys experimentally infected with Plasmodium knowlesi were the source of parasitized red cells. The erythrocytic stages of this Plasmodium showed all the organelles described in other mammalian forms; the nucleus lacked a typical nucleolus but contained a cluster of granules. P. knowlesi did not have protozoan-type mitochondria as do the avian and reptilian forms, but had double-membrane-bounded bodies as observed in other mammalian malarial parasites.The isolation procedure caused a slight swelling of the parasite, but in general, the structure and structural relationships of the parasite were preserved. However, the isolation technique gave a new insight into the connection of the host cell cytoplasm with the large, so-called food vacuoles of the parasite. The parasite freed from its host cell showed clear spaces where the large vacuoles had been. The content of these vacuoles had been removed together with the red cell cytoplasm. As the nature of the isolation procedure precluded any disruption of the parasite itself, these findings support our view that the vacuoles are not true food vacuoles. If these were true food vacuoles, they would be completely enclosed by a parasite membrane within the parasite cytoplasm. However, we have demonstrated that they represent extensions of host cell cytoplasm in direct communication with the rest of the red cell. The outer membrane surrounding the intra-erythrocytic parasites disappeared after isolation of the parasite from the host cell. This strongly suggested that the outer membrane is of host cell origin. The budding process of the merozoites from a schizont was also described and discussed.This paper is contribution No. 558 from the Army Research Program on Malaria and was supported in part by Research Grant AI 08970-01 from the United States Public Health Service.     

Cell surface proteins in the early embryogenesis of Pleurodeles waltlii

Surface proteins in the first embryonic stages (8–32 cells, morula, blastula, early and late gastrula) of Pleurodeles waltlii were selectively labelled by ¹²⁵I using lactoperoxidase and glucose/glucose oxidase. Iodination was effected either on non-dissociated embryos or after their dissociation with EDTA. On the outer surface of non-dissociated embryos the two-dimensional electrophoresis revealed only three groups of ¹²⁵I-labelled proteins which did not change during all studied stages. Quite different results were obtained with the cells of dissociated embryos. In addition to the iodinated proteins of the embryonic outer surface seven major iodinated proteins were identified. These proteins originate from the regions of cell-cell contacts in intact embryo. Their two-dimensional pattern in dissociated cells changes between stages 8–32 cells and morula. The next important difference was observed during gastrulation, which corresponds in Pleurodeles waltlii to the first morphogenetic movements. Therefore the outside and inside cell surfaces of embryo are different already at stage 8–32 cells (and probably earlier), before the first step of morphogenesis. The changes of cell surface proteins at early embryonal development take place inside the embryo, in the regions of cell-cell interactions.     

Ultrastructural studies of neutral lipid localisation inStreptomyces

Triacylglycerol is accumulated by Streptomyces spp. when grown in submerged culture. Ultrastructural studies using transmission electron microscopy (TEM), staining and freeze-fracture/freeze-etch procedures, and light microscopy confirmed the accumulation of neutral lipid by S. lividans and S. coelicolor during the stationary phase and its storage within membrane-bound globular structures within the cytoplasm. These structures were of various sizes and occupied up to approximately 80% of the total cell volume at that time. There was no evidence of such material within cells examined during the early exponential phase of growth. The globules visualised by TEM were electron-transparent since they comprised lipids containing saturated fatty acids that did not react with osmium tetroxide. The globules appeared to be bounded by a single membrane. Received: 6 June 1995 / Accepted: 4 September 1995     

Effects of Treatment with Sodium Dodecyl Sulfate on the Ultrastructure of Escherichia coli

An electron microscopy study has been made of the effects of dissolution of the plasma membrane of Escherichia coli with sodium dodecyl sulfate (SDS) on the organization of the nucleoplasm and the cytoplasm. The alterations observed in time course experiments were related to absorbance changes and to release of macromolecules from the cells. As the cells became plasmolyzed, under the conditions used, the first visible effect of SDS was a collapse of the plasmolysis spaces. This was accompanied by a displacement of the nuclear material which then appeared in broad contact with the redeployed plasma membrane. This initial displacement of nuclear material to the cell border may indicate an association between the nucleoplasm and the plasma membrane. Upon further dissolution of the plasma membrane, the nuclear material receded from the cell margin and contracted into an axial filament. Meanwhile, the cytoplasm dissociated into an amorphous, Pronase-sensitive component and an electron-opaque, granular one sensitive to ribonuclease. The latter represented one continuous area of ribosomal structures surrounding the nucleoplasm, an organization which did not occur when the cells were inhibited with rifamycin before SDS treatment. During prolonged SDS interaction, approximately 65% of the cellular protein, 25% of the ribonucleic acid and 40% of the deoxyribonucleic acid were released from the cells concomitant with the disappearance of the amorphous cytoplasmic part, expansion of the ribosomal aggregate, and rearrangement of the nuclear material at the cell periphery. The observations support the contention that all ribosomal structures bear a direct relationship with the nucleoplasm.     

Behavior of silver stainable material during mitosis inVicia faba

To reveal the behavior of silver stainable material localized mainly in the nucleoli and nucleolar organizing regions (NORs), the somatic cells ofVicia faba were investigated by silver staining throughout the mitotic cell cycle. Nucleoli of interphase and early prophase nuclei were darkly stained. From late prophase to anaphase the secondary constrictions were discriminated as silver stained NORs and many silver grains appeared throughout the cytoplasm. At late prophase the NOR condensed at the same rate as the chromosome arm. Small spherical bodies and two new nucleoli appeared in telophase nuclei and at the same time the cytoplasmic grains disappeared. On the basis of the above observations on the silver stainable material during each mitotic phase, the behavior of silver stainable material is interpreted.     

Structure and possible function(s) of charasomes; complex plasmalemma-cell wall elaborations present in some characean species

Summary Charasomes, complex membrane structures, were found along the longitudinal walls of internodal and lateral branch cells ofChara corallina andC. braunii, but not along their transverse walls or in other cell types. Charasome-complexes were larger and more numerous in the lateral branch cells than in internodal cells. InC. corallina, a dioecious species, especially large elaboration of charasome material occurs in the lateral branch cells of the female plant, sometimes reaching a cross-sectional width which is as great as that of the adjacent cell wall. Chara internodes transport hydroxyl (OH^–) out of the cell and bicarbonate (HCO₃ ^–) into the cell. Spatial distribution of charasomes along the cell was examined with respect to these transport phenomena, which occur at specific identifiable regions along the cell. Charasome-complexes were always found in regions in which HCO₃ ^– transport occurs but were often fewer, reduced in size or absent in areas of OH^– efflux.Nitella flexilis exhibited similar patterns of OH^– and HCO₃ ^– transport along the cell; however, there was a complete absence of charasomes. Ultrastructural examinations onNitella translucens indicated that charasomes were also absent in this species. The observation that charasomes are present in both transport regions ofChara but are totally lacking in the twoNitella spp. indicates that the charasome-complex is not involved in transport of either substance. Other possible functions for the charasomes, including a role in osmoregulation, are discussed.Charasome substructure is the same in bothChara species, consisting of a mass of short (50 nm average length) anastomosing tubules (30 nm average diameter) derived from the plasmalemma. The interior of the tubules is open to the cytoplasm while the area surrounding the tubules is ultimately open to the wall and thus can be considered to be wall space. Charasomes are quite variable in size and shape, but are roughly globular, with the bulk of the structure projecting into the cell cytoplasm. Tubular components of the charasome were sometimes seen to extend into the microfibrillar wall matrix. A three dimensional model of the charasome-complex presented details the great complexity of this membrane system.     

Cyclosis-related asymmetry of chloroplast–plasma membrane interactions at the margins of illuminated area in <Emphasis Type="Italic">Chara corallina</Emphasis> cells

Cytoplasmic streaming in plant cells is an effective means of intracellular transport. The cycling of ions and metabolites between the cytosol and chloroplasts in illuminated cell regions may alter the cytoplasm composition, while directional flow of this modified cytoplasm may affect the plasma membrane and chloroplast activities in cell regions residing downstream of the illumination area. The impact of local illumination is predicted to be asymmetric because the cell regions located downstream and upstream in the cytoplasmic flow with respect to illumination area would be exposed to flowing cytoplasm whose solute composition was influenced by photosynthetic or dark metabolism. This hypothesis was checked by measuring H⁺-transporting activity of plasmalemma and chlorophyll fluorescence of chloroplasts in shaded regions of Chara corallina internodal cells near opposite borders of illuminated region (white light, beam width 2 mm). Both the apoplastic pH and chlorophyll fluorescence, recorded in shade regions at equal distances from illuminated area, exhibited asymmetric light-on responses depending on orientation of cytoplasmic streaming at the light–shade boundary. In the region where the cytoplasm flowed from illuminated area to the measurement area, the alkaline zone (a zone with high plasma membrane conductance) was formed within 4-min illumination, whereas no alkaline zone was observed in the area where cytoplasm approached the boundary from darkened regions. The results emphasize significance of cyclosis in lateral distribution of a functionally active intermediate capable of affecting the membrane transport across the plasmalemma, the functional activity of chloroplasts, and pattern formation in the plant cell.     

Seasonal changes in the physical nature of the bark phloem parenchyma cells ofPinus strobus

Summary Changes in the vacuome and cytoplasm of phloem parenchyma cells ofPinus strobus were observed from summer to winter over a three year period. These changes were evidence mainly by aggregation of the vacuolar contents with vital stains in summer but not usually in winter. Gelation of the vacuome could be induced in either season, but in winter the solid material appeared to take up more space in the cell when treated with 2M sucrose than in summer. The same phenomenon of aggregation appeared in fixed material in summer but not in winter in most cases. Changes in the cytoplasm included an apparently greater fluidity in winter, either of whole cytoplasm or a part of it. This was suggested by plasmolysis behavior and starch fallin situ. Use of bromphenol blue stain indicated that there was an increased number of sites for attachment of the stain in winter in the cytoplasm. Use of I₂-KI caused a coagulation of the cytoplasm in winter but not in summer. With 1 Textfigure     

The three-dimensional structure of the cyanobacterium Synechocystis sp. PCC 6803

To advance our knowledge of the model cyanobacterium Synechocystis sp. PCC 6803 we investigated the three-dimensional organization of the cytoplasm using standard transmission electron microscopy and electron tomography. Electron tomography allows a resolution of ~5 nm in all three dimensions, superior to the resolution of most traditional electron microscopy, which is often limited in part by the thickness of the section (70 nm). The thylakoid membrane pairs formed layered sheets that followed the periphery of the cell and converged at various sites near the cytoplasmic membrane. At some of these sites, the margins of thylakoid membranes associated closely along the external surface of rod-like structures termed thylakoid centers, which sometimes traversed nearly the entire periphery of the cell. The thylakoid membranes surrounded the central cytoplasm that contained inclusions such as ribosomes and carboxysomes. Lipid bodies were dispersed throughout the peripheral cytoplasm and often juxtaposed with cytoplasmic and thylakoid membranes suggesting involvement in thylakoid maintenance or biogenesis. Ribosomes were numerous and mainly located throughout the central cytoplasm with some associated with thylakoid and cytoplasmic membranes. Some ribosomes were attached along internal unit-membrane-like sheets located in the central cytoplasm and appeared to be continuous with existing thylakoid membranes. These results present a detailed analysis of the structure of Synechocystis sp. PCC 6803 using high-resolution bioimaging techniques and will allow future evaluation and comparison with gene-deletion mutants.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Cryo-electron tomography of the magnetotactic vibrio Magnetovibrio blakemorei: Insights into the biomineralization of prismatic magnetosomes

We examined the structure and biomineralization of prismatic magnetosomes in the magnetotactic marine vibrio Magnetovibrio blakemorei strain MV-1 and a non-magnetotactic mutant derived from it, using a combination of cryo-electron tomography and freeze-fracture. The vesicles enveloping the Magnetovibrio magnetosomes were elongated and detached from the cell membrane. Magnetosome crystal formation appeared to be initiated at a nucleation site on the membrane inner surface. Interestingly, while scattered filaments were observed in the surrounding cytoplasm, their association with the magnetosome chains could not be unequivocally established. Our data suggest fundamental differences between prismatic and octahedral magnetosomes in their mechanisms of nucleation and crystal growth as well as in their structural relationships with the cytoplasm and plasma membrane.     

An ultrastructural and autoradiographic study of the immune response in Hyalophora cecropia pupae

Summary Membrane-bounded spherical vesicles found in rat Sertoli cells have been examined quantitatively during the cycle of the seminiferous epithelium. Most of the vesicles were localized to the basal and columnar portions of the Sertoli cell cytoplasm. The thin lateral projections of the Sertoli cells contained very few vesicles. Morphometric analysis of the basal portion of the Sertoli cell cytoplasm revealed that the volume density (V _v ) of the vesicles changed markedly during the cycle. The V _v was at its minimum (0.036) at stage VII and maximum (0.117) at stages XI-I. The vesicles were also smaller at stage VII compared to the vesicles at stages IX-V. The stage-dependent difference in the size of the vesicles was found both in the basal and the columnar portions of the Sertoli cells. At stage VII some of the vesicles appeared to be elongated much like the tubular elements of the smooth endoplasmic reticulum (SER) from which they are probably derived. The stage-dependent differences in volume density and size of the Sertoli cell vesicles may be related to cyclic biochemical variations in the Sertoli cells, and are further indications of a variation in Sertoli cell function during the cycle of the seminiferous epithelium. Whether or not this is due to an internal cycle of the Sertoli cell or to influences from adjacent germ cells remains to be determined.     

Ultrastructure of infection-thread development during the infection of soybean by Rhizobium japonicum

The location and topography of infection sites in soybean (Glycine max (L.) Merr.) root hairs spot-inoculated with Rhizobium japonicum have been studied at the ultrastructural level. Infections commonly developed at sites created when the induced deformation of an emerging root hair caused a portion of the root-hair cell wall to press against an adjacent epidermal cell, entrapping rhizobia within the pocket between the two host cells. Infections were initiated by bacteria which became embedded in the mucigel in the enclosed groove. Infection-thread formation in soybean appears to involve degradation of mucigel material and localized disruption of the outer layer of the folded hair cell wall by one or more entrapped rhizobia. Rhizobia at the site of penetration are separated from the host cytoplasm by the host plasmalemma and by a layer of wall material that appears similar or identical to the normal inner layer of the hair cell wall. Proliferation of the bacteria results in an irregular, wall-bound sac near the site of penetration. Tubular infection threads, bounded by wall material of the same appearance as that surrounding the sac, emerge from the sac to carry rhizobia roughly single-file into the hair cell. Growing regions of the infection sac or thread are surrounded by host cytoplasm with high concentrations of organelles associated with synthesis and deposition of membrane and cell-wall material. The threads follow a highly irregular path toward the base of the hair cell. Threads commonly run along the base of the hair cell for some distance, and may branch and penetrate into subjacent cortical cells at several points in a manner analagous to the initial penetration of the root hair.     

Epidermal ultrastructure of Seison nebaliae and Seison annulatus, and a comparison of epidermal structures within the Gnathifera

 The epidermis of both species of Seison is syncytial and has a characteristic internal layer divided into two distinct sublayers. Sublayer I is very thin (0.03 μm) and bounded to the outer cell membrane of the epidermis. Sublayer II is 0.5 μm thick and separated from sublayer I by a thin layer of cytoplasm. Intrusions of the outer cell membrane of the epidermis perforate the internal layer, before terminating within the cytoplasm. The intrusions of the cell membrane of S. annulatus are coated by an electron-dense material, the annulus, when passing through the internal layer. Bundles of filaments are present in the epidermis of S. nebaliae. A comparison of epidermal structures within the Gnathifera Ahlrichs, 1995, confirms phylogenetic relationships earlier proposed by the author. Accepted: 19 December 1996     

A novel intercellular junction between cone cells in the compound eye of Ephestia kuehniella

A previously unidentified intercellular junction between cone cells in the compound eye of the moth Ephestia is described. The junctions are characterized by deposition of granular material, in register, along portions of the membranes of adjacent cone cells during compound eye development and by a constant intercellular space of 8–10 nm. Accumulation of the cone cell material along localized regions of the cell membrane suggests an interaction between a specialized area of the membrane and a specific cytoplasmic constituent, and the exact matching of the regions of deposition between adjacent cells implies intercellular interaction. The junctional nature of these membrane regions is often obscured in the adult crystalline cone but is inferred from observations on developing cone cells.