The micromorphology of zeiotic blebs in cultured human epithelial (HEp) cells

Correlative light and electron microscopy: from live cell dynamic to 3D ultrastructure

Correlative light and electron microscopy (CLEM) aims at combining data acquired from the same sample through both imaging modalities. Many combinations can be found in the literature where almost any kind of light microscopy (LM) has been associated to different processing in electron microscopy (EM) and applied to a wide variety of specimen, from cultured cells to multicellular organisms. In this chapter, we focus on a technique that intends to combine LM acquisition on living cells with transmission EM (TEM) analysis. A specific attention is given to the description of a method to bring precise coordinates to the object of interest, to allow a straightforward correlation between LM and EM. Moreover, we describe how, by using high-pressure freezing as a fixation technique, dynamic events observed at the LM are captured and studied at the ultrastructural level.

Compression and dilation of the membrane-cortex layer generates rapid changes in cell shape

A cyclic process of membrane-cortex compression and dilation generates a traveling wave of cortical actin density that in turn generates oscillations in cell morphology.

Rapid changes in cellular morphology require a cell body that is highly flexible yet retains sufficient strength to maintain structural integrity. We present a mechanism that meets both of these requirements. We demonstrate that compression (folding) and subsequent dilation (unfolding) of the coupled plasma membrane–cortex layer generates rapid shape transformations in rounded cells. Two- and three-dimensional live-cell images showed that the cyclic process of membrane-cortex compression and dilation resulted in a traveling wave of cortical actin density. We also demonstrate that the membrane-cortex traveling wave led to amoeboid-like cell migration. The compression–dilation hypothesis offers a mechanism for large-scale cell shape transformations that is complementary to blebbing, where the plasma membrane detaches from the actin cortex and is initially unsupported when the bleb extends as a result of cytosolic pressure. Our findings provide insight into the mechanisms that drive the rapid morphological changes that occur in many physiological contexts, such as amoeboid migration and cytokinesis.

Cell motility through plasma membrane blebbing

Plasma membrane blebs are dynamic cytoskeleton-regulated cell protrusions that have been implicated in apoptosis, cytokinesis, and cell movement. Influencing Rho–guanosine triphosphatase activities and subsequent actomyosin dynamics appears to constitute a core component for bleb formation. In this paper, we discuss recent evidence in support of a central role of nonapoptotic membrane blebbing for cell migration and cancer cell invasion as well as advances in our understanding of the underlying molecular mechanisms. Based on these studies, we propose that in a physiological context, bleb-associated cell motility reflects a cell's response to reduced substratum adhesion. The importance of blebbing as a functional protrusion is underscored by the existence of multiple molecular mechanisms that govern actin-mediated bleb retraction.

Suppression of bleb formation, locomotion, and polarity of Walker carcinosarcoma cells by hypertonic media correlates with cell volume reduction but not with changes in the F-actin content

The putative role of cellular or solvent volume in protrusive activity and locomotion has been investigated in blebbing Walker carcinosarcoma cells using hypertonic media. Blebbing, locomotion, and cell polarity are completely suppressed by 0.2 M sorbitol. The response occurs in two steps. In a first step, i.e. within 10 sec after the addition of sorbitol, blebbing and locomotion are inhibited and this is associated with an average cell volume reduction by 17% (corresponding to a reduction in solvent volume by 38%). It clearly precedes suppression of cell polarity (pre-existing protrusions, tail) occurring in a second step within 5 to 10 min after addition of sorbitol without additional reduction in the cell or solvent volume. The relative amount of F-actin does not correlate with the decrease in cell volume, suppression of blebbing, locomotion, and cell polarity. A significant decrease in the relative amount of F-actin is found only at volume reductions which are higher than those required to completely suppress blebbing, locomotion, and cell polarity. F-actin staining occurs preferentially along the cell membrane in isotonic as well as in hypertonic media. The results are best compatible with the hypothesis that hydrostatic pressure rather than actin polymerization at the front is the direct force driving the membrane forward during bleb formation. Cells with lamellipodia show a similar response to hypertonic media, suggesting that basically similar mechanisms may operate in both forms of protrusions.

Ultrastructural and replicative features of foot-and-mouth disease virus in persistently infected BHK-21 cells

Persistent foot-and-mouth disease (FMD) virus infection in vitro has been studied in a chronically infected cloned BHK-21 cell line. Virus growth during serial cell passages was followed by infectivity assay and immunocytochemical staining. Only a small percentage of cells (0.006-6%) was found to harbour virus during persistence. Light and electron microscopy showed the presence of cytoplasmic protuberances ("blebs") at the surface of persistently infected cells. The curing of cell cultures was achieved by passaging them in the presence of polyvalent immune serum. The absence of virus in cured cells was confirmed by infectivity assay and immunocytochemistry. This finding, together with the low percentage of infected cells in cultures confirms that persistently infected BHK-21 cells satisfy the definitions of a carrier culture. The characteristics of the in vitro system and its relevance to the study of FMD carrier state in vivo are discussed.

Cell surface extensions associated with overexpression of Alzheimer beta/A4 amyloid

Deposition of beta/A4 amyloid in Alzheimer disease (AD) brain parenchyma and vasculature occurs by mechanisms that are currently undefined. Similarly the potential consequences of amyloid accumulation for disrupting cellular integrity have not been addressed in detail. To investigate the possible significance of amyloid deposits for cellular viability, PC12 cells were permanently transfected with DNA coding for the beta/A4-C terminal region of the amyloid precursor protein. The DNA represented 97 amino acids of the amyloid precursor protein of which 40 amino acids were derived from the beta/A4 region. Transfected clonal cell lines and controls were examined at both the light and electron microscopic levels for morphological abnormalities. beta/A4 amyloid accumulated in the cell membrane where the peptide was located at cellular processes resembling blebs and microvilli. These specialized structures at the cell surface were over-abundant in transfected cells that overexpressed the beta/A4 peptide but not in controls. Membranous processes may be involved in the delivery of the beta/A4 peptide to the external surface of the cell of origin and release into the extracellular space. Similar surface features of cells in the AD brain, should they occur, may indicate a role for membrane-associated processes in the pathophysiology of the disorder.

Diversity of ultrastructure in different phenotypes of cultured microvessel endothelial cells isolated from bovine corpus luteum

Five different types of cultured microvessel endothelial cells defined by use of light microscopy and scanning electron microscopy in a preceding study were investigated by transmission electron microscopy. Type-1 cells displayed a deep invagination of the cell membrane or a single cilium. Granules of low electron density were abundant. A perinuclear ring of intermediate filaments occurred. Cultures of type-2 cells were subdivided into phenotype A, reminiscent of cell-type 1, and into phenotype B, assumed to be vascular smooth muscle cells. Many highly electron-dense granules appeared in late postconfluent cultures of both phenotypes. Cell-type 3 was conspicuous because of a large intracytoplasmic vacuole. Lysosomes with curvilinear bodies were found in cell-types 3 and 4. Both cell types developed a peripheral regular network of microfilaments. Cell-type 5 showed vesiculation of the rough endoplasmic reticulum, lipid droplets and a peripheral felt-like belt of microfilaments. Tubular forms seen in late postconfluent cultures of cell-types 1 to 3 displayed a core of extracellular matrix. Pseudotubular forms of cell-type 4 contained apoptotic bodies. Thus, as seen at the ultrastructural level, different features are maintained by cultured microvessel endothelial cells, suggesting that they have different inherent properties.

Endocardial surface and atrial morphological changes during development and aging

Light, scanning, and transmission electron microscopic observations related to morphological changes of the right atrium as well as the atrial endocardium during development (15th embryonic day and 1 day old) and aging (560 days old) in the Syrian hamster were described and correlated. From the fetus to the adult, the atrial endocardium differentiates in parallel with, or in response to, the subjacent proliferating myocytes in the atrial wall and the trabeculae. Simultaneously, the atrium compartmentalizes grossly into a main chamber and an appendicular region. There is a progressive differentiation from a rudimentary, open chamber with primitive mural ridges in the fetal atria to a distinct, separate, atrial main chamber and appendage with a dense network of trabeculae in the adult. The fetal and neonatal endocardial, endothelial cells are convex with a central nuclear bulging and attenuated cytoplasmic extensions; the adult endocardium shows a squamous endothelium. Two cell surface specializations were observed in all age groups: microvilli and blebs or cytoplasmic protrusions. The general atrial morphology and surface endocardial changes were correlated with growth and the role of the endocardial endothelium as a barrier which controls metabolic exchanges, including the transport of atrial natriuretic factor, between the myocytes and the blood. This endothelial function appears to be essential in the fetal and neonatal age groups since no blood vessels are detected in these groups.

Reversible plasma membrane ultrastructural changes correlated with electropermeabilization in Chinese hamster ovary cells

Chinese hamster ovary cells (CHO) grown in monolayers were permeabilized to molecules with molecular weight up to 1000 by high intensity 100 mus square wave electric field pulses. This permeability was transient and the cell viability was not affected. It was not possible for molecules with molecular weight larger than 1500 to penetrate inside the cytoplasm if lytic pulsing conditions were not used. In order to investigate the ultrastructural changes associated with this transient and limited permeabilization, cells were chemically fixed a few seconds after their pulsation and observed by electron microscopy. By scanning electron microscopy, numerous microvilli and blebs were observed almost immediately after application of the field. No other membrane changes were observed. Permeabilization of the membrane was visualized at the electron microscopic level by penetration of Ruthenium red. The appearance of osmotic pressure-dependent 'blebs' was indicative of local weakening of the plasma membrane. Most of these effects were fully reversible and disappeared within 30 min at 37 degrees C with the formation of huge polykaryons when cells were in contact before pulsing.

Ultrastructure of the choroid plexus epithelium of pigeons treated with drugs: II. Effect of cytochalasin D and colchicine

A remarkable projection of bleblike protrusions, the expulsion of organelles into the protrusions formed on the apical surface, and the separation into the ventricular lumen of these protrusions was the general cellular response of choroidal epithelial cells to intravenous injection of cytochalasin D (CD). The compact microfilament mass and agglomeration of microtubules at the base of the cluster of protrusions reflect the results of cell contraction and displacement of microfilaments induced by CD. In earlier stages after intravenous injections of colchicine, an obvious increase in the number of various-sized vesicles, vacuoles, and lysosomes in the Golgi region was detected. In the later stages, these organelles were seen to accumulate in the basal portion of the epithelial cells. These changes were accompanied by an increase in vacuoles and the disorganization and displacement of the Golgi complex, and they coincided with a decrease in the number of microtubules in apical and basal cytoplasm. These findings suggest that the action of colchicine results in destruction of the three-dimensional architecture between cytoskeletal network and cell organelles. The present results suggest that the cytoskeletal network plays a role in the spatial coordination of the three-dimensional architecture of cell organelles. The study also indicates that the structural differences in the ventricles of the choroid plexus in drug-treated pigeons are manifestations of regional functional specialization in different parts of the ventricular system.

Cytochalasin D and cationized ferritin as probes for the morphological investigation of blebbing in two human cell lines

The potent fungal metabolite cytochalasin D (CD) and cationized ferritin (CF) are used in combination to test for negative charge distribution on blebs (knobs). Two established human epithelial cell lines, WISH and HeLa, that display blebs in various phases of the cell cycle or under certain culture conditions are investigated. CD alone, applied at a low concentration (1.0 micrograms/ml) and for a short time period (3 min), causes blebs to appear as the prevalent surface feature. These are filled mainly with free ribosomes. Additionally, feltlike mats, presumed to be disorganized, compacted microfilaments, are formed directly beneath the cell membrane. These are especially evident in the cortical cytoplasm below the blebs or bleb clusters. CF (0.345 mg/ml), applied for a 5-min period after CD administration (1.0 microgram/ml) for 3 min, appears along the surface of microvilli, at the base of blebs, and in vesicles beneath the bleb clusters. In some cases, microfilaments (6 nm in diameter) are closely related to the vesicles. CF does not preferentially bind to the apical cell membrane of blebs. Above areas of the subplasmalemmal microfilaments, CF membrane binding is apparent, even under circumstances where the filaments are disorganized by cytochalasin treatment. These results seem to show the following: bleb membranes are different from the remainder of the cell and do exhibit a loss of negative charge and surface charge may be dependent on the presence or structural integrity of membrane-related 6-nm microfilaments.

Vinblastine-induced autophagocytosis: the effect of disorganization of microfilaments by cytochalasin B

The effects of disorganization of cellular microfilaments by cytochalasin B on vinblastine-induced autophagocytosis was studied in Ehrlich ascites tumor cells in vitro. Incubation with vinblastine induced a formation of autophagic vacuoles in the cytoplasm. The disorganization of microfilaments by cytochalasin B failed to inhibit vinblastine-induced autophagocytosis. Incubation with cytochalasin B alone induced a rapid formation of blebs on the cell surface. These contained cytoplasmic organelles and were connected by a narrow shaft to the main part of the cell. Thin subcortical microfilaments seen in the control cell cytoplasm were apparently relocated after cytochalasin B treatment and formed amorphous masses deeper in the cytoplasm. Vinblastine did not affect the formation of blebs after cytochalasin B treatment.

Conversion of blebs to microvilli: cell surface reorganisation after trypsin

The blebbed surface morphology produced by trypsinisation of Chinese hamster ovary cells is subsequently reorganized to a microvillous topography, even in the continued presence of trypsin. Scanning and transmission electron microscope (SEM and TEM) observations of this transition showed the initial formation of a "crown' of densely clustered microvilli at one pole of the cell. At the periphery of this region the blebs coalesced to form ridges which subsequently extended over the entire cell surface. Long, and occasionally branched microvilli were generated from the ridges. Large numbers of membrane associated vesicles were also characteristic of these areas of surface reorganisation.

Isolated heart myocytes: ultrastructural case study technique

We have developed a method for performing case studies of heart muscle cells enzymatically isolated from the ventricular walls of rats that is a simple and inexpensive adaptation of procedures developed for the examination of monolayers of attached, cultured cells. The technique represents a marked departure from published accounts of electron microscopic studies of pellets or monolayers from a population of potentially heterogeneous isolated myocytes. Here we report the method, which we have used under controlled conditions with 0 mmol and 1 mmol added CaCl2, to correlate sarcomere length and electrical stimulatibility in the living state with ultrastructural features that include the relative disposition of myofilaments and the integrity of the cell coat. The degree of shrinkage during the preparative steps in less than 5%, as directly determined from photographs of striations in the living, fixed, and embedded states.

Ultrastructure of blebbing phenomenon and phagocytosis of blebs in laryngeal carcinoma

The epithelial-connective tissue junction of invasive squamous cell laryngeal carcinoma was examined electron microscopically. The pleomorphic cytoplasmic protrusions known as blebs were observed on the lateral and basal surfaces of malignant keratinocytes. These blebs were pinched off from the malignant epithelial cells and were then observed in the connective tissue or in the intercellular spaces. After the pinching-off process some blebs were seen to be closely surrounded by pseudopods of phagocytizing histiocytic cells in the lamina propria or by adjacent malignant keratinocytes in the intercellular spaces. Since blebs are believed to be intact parts of viable cells this engulfment can be interpreted as cytophagocytosis. During cytophagocytosis the zeiotic blebs exhibited varying degrees of enzymatic digestion. The process of blebbing was discussed in respect to cellular locomotion of malignant keratinocytes during tumor invasion. This phenomenon seems to occur only in the preliminary stages of malignant tumor growth in the larynx.

Surface configuration of mesothelial cells in effusions. A comparative light microscopic and scanning electron microscopic study

Surface configuration of mesothelial cells identified by light microscopy (LM) has been studied by scanning electron microscopy (SEM). It has been shown that mesothelial cells may have a variable SEM appearance. The surfaces of a small proportion of mesothelial cells are covered by regular microvilli (MV) and show openings of the pinocytotic vesicles. The surfaces of the majority of these cells are covered by vesicles or blebs. An intermediate population of mesothelial cells, i.e., cells displaying side-by-side blebs and MV, has also been observed. The latter cells no longer display pinocytotic vesicles. Occasional mesothelial cells have smooth surfaces. It has been shown by LM and transmission electron microscopy that cells with blebs are viable and capable of mitotic activity. It is concluded that mesothelial cells, detached from their epithelial setting, lose microvilli and pinocytotic vesicles and acquire surface blebs. The possible relationship between mesothelial cells and macrophages based on surface features has been discussed.

Induced glial differentiation of fetal rat brain cells in culture: an ultrastructural study

Primary and secondary cultures of fetal rat brain cells (FBC) from 18th day of gestation have been investigated by scanning and transmission electron microscopy. Primary cultures consisted of a monolayer of flat, undifferentiated epithelioid cells, with some oligodendrocytes, astrocytes and immature neuronal cells. In secondary cultures, cells with glia morphology disappeared. Following addition of extracts from adult rat brains to secondary cultures, a dramatic change of the epithelioid cells took place. They detached from the palstic surface, extruded long cytoplasmic processes with numerous microvilli and cytoplasmic blebs as well as parallel arrays of microtubules and filaments. The differentiated cells resembled astrocytes, and characteristic glia filaments were also observed. An increase of ribosomes and rough endoplasmatic reticulum suggested enhancement of protein synthesis. At the same time S-100 protein and glial fibrillary acidic protein accumulated within the cells. The morphological changes were mostly reversible within 48 h of removal of the brain extract.

Plasma membrane vesiculation: a cellular response to injury

The shedding of plasma membrane vesicles has been shown to result from exposure of monolayer cell cultures to formaldehyde and other sulfhydryl blocking agents. Incubation of cells in concentrations of these agents as low as 5 to 10 mM for intervals as brief as fifteen minutes is effective (Scott, 1976). Plasma membrane vesiculation has been shown to be an energy-dependent process that requires Ca++ and physiological temperature. Following plasma membrane vesiculation, cell monolayers appear intact by phase microscopy and show only slight evidence of cell injury by electron microscopy. In view of these observations, the question has been raised whether plasma membrane vesiculation is compatible with continued cell growth and metabolism. The experiments described in this paper were designed to answer these questions. We pulse exposed 3T3 mouse embryo cells to concentrations of formaldehyde, between 2.5 and 250 mM, for intervals 15, 30 or 60 min. Cell momolayers were then washed in a variety of different media in an attempt to reverse the effect of formaldehyde on cells. Cell monolayers were thereafter assayed for the shedding of plasma membrane vesicles and for their ability to transport 2-deoxy-D-glucose. Cells were also replated in serum-containing medium and their ability to grow was assayed over a seven day interval. The results show an inverse relationship between the shedding of plasma membrane vesicles and the ability of the cells to transport nutrients and to grow. We interpret these data to suggest that the process of plasma membrane vesiculation results from a form of cell injury which blocks cellular metabolism and growth.

Scanning electron microscopy of neoplastic neurogenic rat cell lines in culture

The surface structure of a series of malignant neurogenic rat cell lines in culture has been investigated by scanning electron microscopy. Neoplastic transformation was induced by a single transplacental administration of the carcinogen ethylnitrosourea (ENU) to BD-IX rats on the 18th day of gestation. The malignant cell lines were established either by explantation into culture of cells from solid gliomas or neurinomas developed in the offspring, or by transfer of fetal brain cells to culture where they subsequently underwent malignant transformation. A high degree of surface activity was observed, as evidenced by microvilli, filopodia, ruffling membranes and zeiotic blebs. Surface activity was highest in cell cultures giving rise to glioma-like tumours upon re-implantation into syngeneic hosts, and low in those giving rise to neurinoma-like tumours, with one exception. The lowest surface activity was seen in a cell line which was not tumourigenic. High surface activity was mostly correlated with a high degree of aneuploidy. No correlation was apparent with other properties of the neoplastic cell lines, e.g., stem line ploidy and population doubling time in cell culture.

Ultrastructure of blebbing and phagocytosis of blebs by hyperplastic thyroid epithelial cells in vivo

In addition to pseudopods, somewhat pleomorphic blebs were consistently found protruding from the apical surfaces of hyperplastic rat thyroid epithelial cells into the follicular lumens in vivo. Many blebs were knobby, roughly hemispherical protrusions, with a diameter of 2-3 mum. Such blebs had a densely packed microfilamentous core and contained numerous apparent ribosomes. They were morphologically similar to blebs that have been observed in a variety of cultured cells. Other blebs were larger, more elongate, and less knobby, but had a similar ultrastructural organization. Blebs of all sizes appeared to be phagocytosed on some occasions by nearby epithelial cells. The phagocytic process involved partial engulfment of the bleb by a typical epithelial pseudopod, followed by an apparent pinching-off process, presumably resulting in the separation of the bleb from its cells or origin. The pinching-off process was associated with a band of approx. 6-nm diameter microfilaments that developed within the pseudopod cytoplasm surrounding the base of the bleb and is postulated to function as a contractile ring. The finding of blebbing is an intact tissue in vivo indicates that this phenomenon is not restricted to cultured cells, and thus tends to extend the significance of in vitro observations of the process. In relation to their occurrence in the hyperplastic thyroid gland in vivo, possible interconversions are considered between different types of blebs, and between blebs and pseudopods.

Protective action of beta-carotene against lethal photosensitization of fibroblasts in vitro

Cell culture experiments using haematoporphyrin photosensitized bovine hoof fibroblasts and long-wave uv-irradiation revealed two distinct and separable patterns of lethal photosensitization according to two different sensitization procedures: (1) Photosensitization of cell membranes by short exposure (5 min) of cells to haematoporphyrin. (2) Cytoplasmic photosensitization elicited by a 2 h exposure of cells to haematoporphyrin. Cell membrane photosensitization was reversible by incubation of cells in serum which removed surface bound haematoporphyrin; cytoplasmic photosensitization was irreversible. Beta-carotene was tested in these two systems and the following results were obtained: (1) Preincubation of bovine hoof fibroblasts in beta-carotene protects from lethal haematoporphyrin photosensitization. (2) Protection with beta-carotene is achieved against both types of photosensitization. (3) The protective effect of beta-carotene depends upon the duration of pretreatment, reaching a maximum after 7 days. (4) Beta-carotene protection is maintained even after trypsinization of bovine hoof fibroblasts and withdrawal of beta-carotene from the medium for 24 h or more. (5) Haematoporphyrin sensitized bovine hoof fibroblasts show a distinct pattern of red fluorescence for each type of photosensitization. Incubation of bovine hoof fibroblasts in beta-carotene prior to haematoporphyrin photosensitization results in a pronounced reduction of red fluorescence. Some of these data indicate that beta-carotene acts, at least in cell membrane photosensitization, at the level of the cell membrane into which it appears to be incorporated.

Effects of artificial intraocular pressure elevation on the corneal endothelium in the vervet monkey (Cercopithecus ethiops)

Both eyes of anesthetized vervet monkeys were perfused with mock aqueous humor for 3-7 hours. By adjusting the height of a reservoir connected to the anterior chamber of each eye the intraocular pressure in one eye was maintained at 33-44 mmHg and in the other eye it was a few mmHg above the spontaneous level (12-15 mmHg). Morphologically the control eyes appeared normal, whereas pronounced changes were observed in the high pressure eyes. Thus the corneal endothelium showed an uneven surface towards the anterior chamber with vacuolization, bledding and disruption of the cytoplasm. Pycnosis, excaryocytosis and even loss of whole endothelial cells were observed as well. The morphological changes were most pronounced in the peripheral part of the cornea and furthermore differed among neighbouring cells. During the healing process one could observe mitosis, amitosis and cell surface increase.

Mitotic cells and their microappendages in the primitive streak of the chick embryo

Fresh pullet eggs (White Leghorn Strain) were incubated to the primitive streak stage of development. Blastoderms were fixed in situ with isotonic aldehyde fixatives and prepared for scanning electron miscropy by means of post-osmication, critical point drying and gold-palladium coating. Cells judged to be in various stages of mitosis by their surface contours were numerous on the ventral surface of the chick blastoderm. Cells which were in the late preparatory stages for mitosis had rounded up from their surroundings. Microvilli dominated the surface. The degree of separation and number of microvilli increased until late metaphase or anaphase. Mitotic cells did not completely separate themselves from adjacent cells. Ruffles and blebs were not prominent during mitotis and long filopodia were absent. A definite localization of microappendages (microvilli, blebs, ruffles) to the area of cytokinesis was evident in early telophase and persisted through daughter cell formation.

A new method for comparative light and electron microscopic studies of individual cells, selected in the living state

A method is described which permits comparative light and electronmicroscopic studies of cell cultures, cell spreads or single selected cells which have been kept in the Plastic Film Dish (PFD). The PFD is a versatile large surface tissue culture chamber which, for electron microscopy, is mounted with a transparent FEP-Teflon film bottom. Cells are observed, selected and marked on the PFD-bottom with a high power inverted light microscope. The cells are fixed and dehydrated with a semi-automatic device while they are still in situ in the PFD. During the preparation steps for electron microscopy the topographical relationship between individual cells and between cells and cell support is accurately retained. After embedding and polymerization the Teflon film is easily peeled off the polymerized Epon, leaving a replica of the mark around the selected cell. This permits relocation of the selected cells for ultrathin sectioning in a plane plan-parallel to the original cell support. To enable orientated sectioning of selected cells in a plane perpendicular to the cell support, cells are tagged with Letraset-letters after original embedding and polymerization. Subsequently the re-embedded polymerized specimens are orientated in the microtome in a position which permits controlled thin sectioning of the tagged cells in the previously selected plane.

Scanning and transmission electron microscope studies of interkinetic nuclear migration in the cerebral vesicles of the rat

A scanning electron microscope (SEM) was used to examine the morphology and surface texture of neuroepithelial cells during interkinetic nuclear migration in the cerebral vesicles of the rat at 12, 13 and 14 days of gestation. Serial sections of embryonic material of the same age were also prepared for the transmission electron microscope (TEM). Particular attention was paid to the SEM and TEM appearance of mitotic neuroepithelial cells which occur exclusively along the ventricular border of the neural epithelium. Three distinctly-shaped classes of mitotic cells were recognised in scanning micrographs. (1) Pyriform cells. This type of mitotic cell was characterised by the presence of very long, fine processes radiating from the tip and shoulders of a short external (basal) process. These fine processes were termed "intramitotic filopodia." Microvilli were found on the surface of most pyriform cells. (2) Conical cells. These lacked an external process but there were large numbers of intramitotic filopodia at the basal pole of the cell body, and the perikaryal surface was rich in microvilli. (3) Globular cells. It was possible to subdivide this class of cell into large and small sizes, but usually a few short intramitotic filopdia were present at the basal pole. The perikaryal surfaces of the globular population were raised in coarse lumps and bubble-like protrusions. By pooling TEM and SEM information we were able to deduce that pyriform cells probably possess a prophase or prometaphase chromosome morphology, while conical cells exhibit a chromosome morphology somewhere between prometaphase and early anaphase. Large globular mitotic cells were found to be between metaphase and late anaphase and small globular cells were identified as early telophase cells. On the basis of these findings we have proposed that as a bipolar neuroepithelial cell rounds up for mitosis it passes first through a pyriform stage during which the external process is retracted or broken down, and then through a conical stage when the cell consolidates its position on the ventricular surface. Finally, the cell enters a large globular stage before dividing into two small globular telophase cells. It is not known what part, if any, the intramitotic filopodia play in this process of rounding up.

Growth inhibition and morphological changes caused by indomethacin in fibroblasts in vitro

In a definite range of concentrations indomethacin inhibits the multiplication of fibroblasts in vitro without influencing their viability. The effect of lethal doses of indomethacin on the fine structure of normal and transformed fibroblasts was investigated by light and electron microscopy. The cells grown in flat monolayers became roundish and, varying in sequence, small zeiotic blebs were bulging at the external cell membrane. Beside coarse vesicular transformation of the endoplasmatic reticulum and extension of the perinuclear spaces ballooning of the mitochondria occurred. After breaking up of the vesicles of the endoplasmatic reticulum loosening of the cytoplasmic ground substance associated with enlargement of the hyaline plasma margins was observed in further progress of the experiment. The internal structures of the mitochondria were completely disorganized. In the ultrastructure of the fibroblasts altered by lethal doses of indomethacin no specific sites of drug action could be identified.