Measured femoral density by dual-energy X-ray absorptiometry as a function of rotation

The use of projectional techniques to evaluate bone density of the proximal femur has the potential to be adversely affected by rotation along the head/condyle axis, due to the complex geometry of the region being examined. To experimentally investigate the magnitude of variation attributable to rotation, the bone density of a cadaveric femur, placed in a water bath to simulate soft tissue, was measured as a function of rotation by using a dual-energy x-ray absorptiometry (DXA) system. It was found that 10 to 15 degrees of rotation was the minimum deviation from the baseline of the femoral head perpendicular to the x-ray beam required to produce statistically significant (P = .95) changes in measured bone density. The magnitude of the variation at these rotations ranged from 2.5% for the trochanter to 5.0% for the femoral neck. Even though variation along the axis described is difficult to precisely control in the clinical setting, the magnitude of density errors attributable to these variations should not adversely affect the utility of projectional density measurement techniques.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation phenotype.

Anatomy of the pancreas and Langerhans islets in snakes and lizards

The pancreas of snakes (18 species) was comparatively examined and classified into five major types, based on structure of the lobes and ducts, spatial relationships with the spleen and the gall bladder, and the disposition of islet cells. These types trend toward fusion of the pancreatic lobes and compaction of the pancreas--a progression that coincides with the phylogeny of the snakes. The more primitive pancreas of lizards (17 species) also was surveyed; that of Varanus is of special interest because its structure is intermediate between the extended, tri-lobate pancreas of lizards and the compact pancreas of snakes and may represent a transitional link in the evolution of this organ. Islet tissue is always confined to the dorsal lobe and is concentrated in its distal region adjacent to the spleen. In primitive snakes and in Varanus, a large islet mass is sequestered within a distinct juxtasplenic "islet body" distanced from the dorsal lobe and connected to it by a slender stalk. In some of the most advanced snake species, numerous islets of endocrine cells are found within the spleen. The occurrence and formation of these intrasplenic islets is described in detail. The anatomic "affinity" between spleen and the islet region of the pancreas is discussed. A hypothesis for the development of the pancreas from embryonal placodes on the mid-gut is presented; it proposes that the exocrine and the endocrine components derive from different progenitor cells, and that the endocrine progenitors are located in the center of the dorsal placode. The hypothesis combines embryological and evolutionary views about the origin of the pancreas, and offers a rationale for differences in its structure and in the disposition of the islets.

Quantitative dual-energy radiographic absorptiometry of the lumbar spine: in vivo comparison with dual-photon absorptiometry

Quantitative dual-energy radiographic absorptiometry (DRA) and dual-photon absorptiometry (DPA) were compared to determine the best means of assessing bone density. Both methods were used to evaluate the lumbar spine in 107 women (aged 35-84 years [mean, 64 years]) referred for evaluation of osteoporosis risk. High correlation was documented between measurements derived by the two techniques, with a .95 linear regression coefficient for the total spine density measurement. Age-related regression equations were similar in slope but manifested different intercepts. Bone mineral density values derived with DRA were consistently lower than those obtained with DPA (conversion equation: DPA density = [1.067 X DRA density] + 0.163). Besides the inherent imperfections of each system, it was found that inaccurate identification of intervertebral spaces on the low-resolution DPA images introduced errors in patient data. DRA may replace DPA as the dedicated projectional densitometric procedure of choice for technical reasons, but at present a conversion equation must be used to compare DRA data to DPA data.

Embryonic cell recognition: uncoupling tissue-specific affinities from cell-type specificities

We have experimentally defined the two major aspects of embryonic cell recognition-adhesion (ReAd), tissue type-specific ReAd and cell type-specific ReAd; we showed that they arise consecutively during cell differentiation, and that the former can function in the absence of the latter. Embryonic chick cells (retina and chondroblasts) in which differentiation was arrested by BrdU at an early stage, failed to express cell-type ReAd, yet they continued to display tissue-type ReAd: they distinguished tissue-self from non-self and selectively cohered with self. Our results indicate that tissue-type and cell-type ReAd represent distinct, separately controlled mechanisms. BrdU appears to be useful as a probe for investigating the regulation of these mechanisms, and as an experimental effector of differentiation abnormalities associated with defects in cell recognition.

Cell contacts are required for induction by cortisol of glutamine synthetase gene transcription in the retina

In embryonic neural retina the enzyme glutamine synthetase [GS; L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] is a glia-specific differentiation marker inducible with cortisol. We show that cortisol elicits GS mRNA accumulation by stimulating transcription of the GS gene and that this stimulation requires cell contacts: in dissociated and separated retina cells GS gene transcription was not induced; when the separated cells were reassembled into multicellular aggregates, restoring cell contacts, accumulation of GS mRNA was again inducible. In cells dissociated from retina tissue that had been preinduced with cortisol, GS gene transcription rapidly declined, despite continued hormone availability. In the separated cells transcription of the histone H3.3 gene and accumulation of carbonic anhydrase II mRNA were unaffected; therefore, cell separation selectively precluded induction of the GS gene. These findings provide direct evidence for the regulatory role of cell contacts in hormonal control of gene transcription.

Developmental regulation of glutamine synthetase and carbonic anhydrase II in neural retina

Glutamine synthetase (GS) is expressed in the neural retina only in Muller glia cells and is inducible with cortisol. A chicken genomic clone that contains at least part of the coding region for the GS enzyme was used to investigate developmental changes in the level of GS mRNA in embryonic chicken retina. A major GS transcript (approximately equal to 3 kilobases) detected by the probe begins to accumulate sharply on day 15 of embryonic development. When cortisol is prematurely supplied to early embryonic retina, it induces precocious accumulation of GS mRNA and of the GS enzyme. At later ages, these effects of cortisol are significantly greater, which suggests that competence to transcribe or stabilize GS mRNA in response to stimulation with cortisol increases with development. Carbonic anhydrase II (CA-II) is expressed in early retina in all the cells, but it becomes later restricted to Muller glia. Using cloned CA-II cDNA, we detected a high level of CA-II mRNA in early retina, followed by a decline due to arrest of CA-II mRNA accumulation in differentiated neurons. As glia cells mature, CA-II mRNA and the enzyme increase to a new high level. Therefore, changes in CA-II gene expression during retina development reflect differentiation-dependent cell-type-specific control of CA-II mRNA accumulation.

Accumulation of c-src mRNA is developmentally regulated in embryonic neural retina

Accumulation of c-src mRNA gradually increased during early development of the neural retina in chicken embryos and reached a peak by days 11 to 13 of embryonic life. Thereafter, its amount declined to a low level which persisted also in adult retina. The early increase in c-src mRNA correlated inversely with the decrease in the amount of H3.2 replication histone mRNA and with the decline in the rate of cell growth. The accumulation profile of c-src mRNA corresponded to that of pp60c-src protein, suggesting that the latter is regulated at the level of transcription.

Retinoic acid inhibits conversion of dissociated Müller glia into lens-like cells

In monolayer cultures of dissociated retina cells, Müller gliocytes undergo conversion into a lens-like cell type (lentoidal cells): they become unable to adhere to neurons and accumulate lens antigens. Retinoic acid (RA) retards these changes. We present evidence that RA prevents the rapid loss of gliocyte adhesivity to neurons; and that, by promoting restoration of glia cell contacts with neurons, RA protects the gliocytes from phenotype alteration.

Formation of lentoids from retina gliocytes: ultrastructural study

In primary monolayer cultures of dispersed neural retina cells from 13-day chick embryo, gliocytes (Müller glia cells) multiply and rapidly change into a lentoidal (lens-like) phenotype. They express lens proteins, including MP26 (a lens plasma-membrane antigen) and ultra-structurally appear to resemble lens cells. A significant aspect of this modification is that the glia-derived lentoidal cells no longer display contact-affinity for neurons but become preferentially adhesive to each other; in aggregates, they assemble into compact lentoids. A likely explanation for this change in cell affinities is that the modified gliocytes express little or no R-cognin, a retinal cell-surface antigen implicated in mutual recognition and adhesion of retina cells. Although lentoidal cells express MP26, a gap-junction component in the lens, no gap junctions could be found in the lentoids.

Gangliosides in postmitotic retina of chick embryo: changes in vivo and in cell cultures

Developmental changes in ganglioside composition of postmitotic neural retina of chick embryo were analyzed by thin-layer chromatography. Gangliosides were identified by comparing their chromatographic mobilities with reference standards. The outstanding changes are decrease in the concentration of GD3L and increase in GD1a and GM1 concentrations. By depleting Müller glia cells from retina tissue of 13- and 16-day embryos (R13, R16) we determined that the bulk of the major gangliosides is associated with the neurons. Analysis of gangliosides in monolayer cultures of R13 and R16 cells highly enriched for Müller cell-derived gliocytes indicated that these cells express the same types of gangliosides as neurons, but in somewhat different concentrations and relative proportions; however, after time in culture these cells showed ganglioside types and changes in ganglioside profile that are not characteristic of normal retina. The latter observation is consistent with other evidence that the phenotype of Müller glia cells becomes altered in monolayer culture. In contrast to cultures of early embryonic retina, in organ cultures of later postmitotic retina, ganglioside composition did not continue to change as in normal development. This suggests that in postmitotic retina, normal developmental progression of ganglioside changes requires systemic and/or other conditions which are missing or altered when this tissue is isolated and cultured in vitro.

Antiserum to lens antigens immunostains Müller glia cells in the neural retina

Antiserum to a lens fraction enriched for alpha-crystallin selectively immunostains Müller glia cells in the neural retina of several vertebrate species. Also, in embryonic retina (chicken), this antiserum reacts with Müller cells and, at early stages of development, with their apparent precursors. Thus, antibodies to a lens product(s) detect a Müller glia cell marker that begins to be expressed very early in their ontogeny and can be useful in studies on differentiation, function, and pathologies of this cell type.

Cortisol receptors and inducibility of glutamine synthetase in embryonic retina

Glutamine synthetase (GS) is a marker enzyme for Müller glia cells in neural retina. In chick embryo retina GS begins to increase sharply on the 16th day of development, but can be precociously induced by premature supply of the inducer, cortisol, already on the 8th day. At this stage GS inducibility is low, but it increases progressively with embryonic age. We investigated whether there was a corresponding age-dependent increase of cortisol-binding molecules (cortisol receptors) and found that their level is highest in the early retina and decreases with development. In light of this inverse relationship, we examined whether functional characteristics of these receptors change with age, but detected no differences. In in vitro tests, receptors from older retina translocated cortisol into nuclei from young retina, and vice versa, with similar effectiveness. Also, cortisol receptors from liver cells (which differ from retina receptors) can translocate the hormone into retina nuclei, and vice versa. These findings indicate that translocation of cortisol receptors is neither tissue-specific or age-dependent, nor is it conditional on the total amount of receptors normally present in cells. Therefore, the age-dependent increase of GS inducibility in embryonic retina cannot be directly related to quantitative or functional differences of cortisol receptors and is evidently controlled primarily at the gene level. The very large amount of cortisol-binding molecules in early embryonic retina raises the possibility that they play some role in early differentiation of retina cells unrelated to hormone binding.

Comparative characterization of monoclonal antibodies to carbonic anhydrase

Monoclonal antibodies (Mabs) were generated to avian carbonic anhydrase-C and characterized; their reactivity with human, murine, bovine, chicken and fish erythrocyte carbonic anhydrase-C, and with human carbonic anhydrase-B was investigated by ELISA and electroblot techniques. Reactivity of the Mabs with native and SDS-denatured carbonic anhydrase was compared. Mabs that recognize antigenic determinants shared by all the carbonic anhydrases examined were identified. The results demonstrate the potential usefulness of these particular probes for investigating various aspects of function, evolution, development and regulation of this important, but not well understood group of enzymes.

Cell disorganization and malformation in neural retina caused by antibodies to R-cognin: ultrastructural study

Retina tissue from 6-day chick embryos was organ-cultured for 3 days in the presence of antibodies to R-cognin, a surface antigen of retina cells. The antibodies which are known to bind to this antigen caused a striking malformation: interruption of the outer limiting membrane and extensive cell disorganization resulting in exteriorization of many cells and forming of chaotic masses on the surface of the tissue. Controls did not show these effects. These results further confirm that R-cognin is involved in the mechanism of histotypic contacts and recognition of retina cells, and that it plays an essential role in cell organization and histogenesis in the retina.

Variable CA II compartmentalization in vertebrate retina

We have generated a series of polyclonal and monoclonal antibodies to mammalian, avian, and osteichthian CA II for the purpose of studying its distribution in vertebrate nervous systems. In mature chicken retina, CA II is immunohistochemically detectable only in Müller glial cells. However, during embryonic development, CA II expression is suddenly "switched-on" early as a general constituent of all retinoblasts, later becoming restricted to Müller cells and transiently to a distinct type of amacrine neuron. A similar developmental pattern occurs in mouse. However, at maturity high CA II levels remain in certain amacrine neurons in addition to Müller cells. Comparative analyses of mature retinas of lower vertebrates show that reptiles parallel chicken with high CA II only in Müller cells, certain amphibians show CA II staining in Müller cells, amacrine neurons as in mouse, and in horizontal neurons, teleost and elasmobranch fish possess high CA II in Müller cells and the horizontal neurons, and lamprey eel shows CA II staining primarily in horizontal cells. An evolutionary sequence that will be discussed is thus suggested.

Effect of embryonic age on aggregability, histogenesis and biochemical differentiation in the embryonic chick and quail neural retina

In this paper, the different ages of the chicken and quail embryonic retina cell aggregates are used to examine the aggregation, histogenesis, and the induction of glutamine synthetase (GS) elicited by hydrocortisone. The results show that the GS inducibility in the retina cells increases at a slow rate, then rises sharply on the 9th day, and after 9 days, the GS inducibility of aggregates decreases with the increase of embryonic age. The observed value of induced GS activity in the chick retina cells coaggregate with quail embryonic retina cells is less than the expected value. Bispecific cell coaggregates were observed when the 9-day chicken embryonic retina cells and the 71/2-day quail embryonic retina cells were combined and coaggregated, the chicken and quail cells formed three retinal rosettes. When the 9-day chicken embryonic retina cells were commingled with the 13-day quail embryonic retina cells, in the coaggregates, only one retinal rosette was formed.

Cellular compartmentalization of carbonic anhydrase-C and glutamine synthetase in developing and mature mouse neural retina

Using immunohistochemical methods, we have determined the cellular localization of the enzymes, glutamine synthetase (GS) and carbonic anhydrase-C (CA-C), in mouse neural retina during development and in the mature tissue. GS is always confined exclusively to the Müller glial cells; it is first detectable in these cells post-natally on about day 12, i.e. shortly before the eyes open. Also CA-C in the mature retina is localized in the Müller cells but, in addition, it is found in certain amacrine neurons as well. CA-C is first detectable in the retina already several days before birth; at that time it is found in most of the cells, with the exception of the emerging ganglion cells. However, with advancing differentiation, CA-C becomes progressively restricted to Müller cells and to a sub-category of amacrine neurons, and persists only in these cells in the mature retina. The present results extend our previous studies on these enzymes in the avian retina; they demonstrate that also in mammalian retina, different temporal and cellular patterns of GS and CA-C expression and localization earmark distinct phases of structural and functional differentiation of the retina. The striking developmental changes in the cellular localization of CA-C, and the finding of this enzyme in certain amacrine neurons as well as in Müller cells, raise questions about the role of CA-C in the retina, and about mechanisms regulating its expression in specific cell types.

Transformation of retinal glia cells into lens phenotype: expression of MP26, a lens plasma membrane antigen

We describe experiments in which dissociated cells from differentiated, post-mitotic neural retina of late chicken embryos (13 and 16 days) rapidly and consistently transform (transdifferentiate) in vitro into lens-like phenotype and form spherical lentoids. Using immunohistochemical and other tests, we have established that the lentoids arise from the progeny of definitive retinal glia cells (Müller cells). An early event in their transformation is the appearance in the cell surface of MP26, a plasma membrane protein characteristic of lens but not found in the retina. The results support the hypothesis that the phenotype of definitive glia cells in the retina is stabilized by contact-mediated interactions with neurons; disruption of cell contacts and cell separation alter surface properties of the glia cells, decontrol their phenotype, and predispose them to phenotype transformation.

Localization of retina cognin in embryonic neural retina tissue by immuno-scanning electron microscopy

The retina cognin (a retina-specific cell-surface glycosylated protein that mediates self-recognition and morphogenetic contact associations of embryonic retina cells) was visualized by immunolabeling and scanning electron microscopy on the surface of cells within retina tissue of 9- and 16-day chick embryos. The photoreceptor processes which are free of contact with cells in the neural retina, were found to be devoid of surface cognin from early on in their development. These results extend previous studies on cognin localization and regeneration on separated retina cells in vitro and conclusively correlate its presence and surface topology with its postulated role.

Hormonal induction of glutamine synthetase in cultures of embryonic retina cells: requirement for neuron-glia contact interactions

Cortisol induces glutamine synthetase (GS) in gliocytes of chick embryo neural retina. Using adherent cultures of retina cells we have demonstrated that responsiveness of the gliocytes to GS induction by the hormone requires contact with neurons. GS is not inducible in high-density cultures depleted of neurons and consisting only of gliocytes. In neuron-containing cultures, induced GS was detected immunohistochemically only in those gliocytes that were closely juxtaposed with clusters of neurons. Unlike the induction of GS, the expression of carbonic anhydrase-C (which does not require cortisol) persisted in these glia cells also in the absence of neurons. The nature and role of glia-neuron interactions in the hormonal induction of GS are briefly discussed.

Changes in patterns and synthesis of proteins in embryonic neural retina studied by 2-dimensional gel electrophoresis

Changes in protein patterns during early differentiation of embryonic neural retina (chick) were studied by 2-dimensional gel electrophoresis. The procedures employed here made it possible to visualize the overall population of proteins present in the tissue at a given time and, on the same gel to distinguish labeled from unlabeled proteins. 2-Dimensional gels were stained by a highly sensitive silver stain to visualize, map and quantitate proteins (and polypeptides) resolved by electrophoresis; the same gels were then autoradiographed in order to differentiate between actively synthesized and pre-existing proteins at each development stage. The effectiveness of this combinative analysis was first verified by identifying and localizing glutamine synthetase, an inducible enzyme marker of retina differentiation. Next, protein patterns in retina tissue at 2 embryonic ages were compared. Of the large number of spots visualized by the above methods approximately 10% showed distinct qualitative-quantitative developmental changes; these were grouped into 7 classes representative of major modes of alteration of protein patterns during cell differentiation.

Development of differential affinities and positional information in embryonic retina cells: inhibition by BrdU

Embryonic neural retina cells fail to develop surface properties for type-specific cell recognition if they are transiently exposed to BrdU during an early, critical age. Such cells do not proceed with histogenetic positioning, organization and differentiation and, instead, form a malformed cell mass. This effect of BrdU is correlated with BrdU incorporation into DNA, and it can be prevented by simultaneous treatment of the cells with cytosine arabinoside. The proposed working hypothesis is that, in this system, BrdU interferes with expression of genes controlling cognitive specification of the cell surface.

Carbonic anhydrase C in the neural retina: transition from generalized to glia-specific cell localization during embryonic development

The developmental profile and cellular localization of carbonic anhydrase C (carbonate dehydratase; carbonate hydro-lyase, EC 4.2.1.1) in the neural retina of chicken embryos and adults were investigated by immunochemical and immunohistochemical methods. Carbonic anhydrase C is present in the retina by the 3rd day of embryonic development. In the undifferentiated retina, it is detectable in virtually all the cells; however, as cell specialization progresses, its level declines rapidly in the emerging neurons and increases in Müller glia cells. An exception is certain amacrine neurons that contain carbonic anhydrase C to about the 16th day of development. In the adult retina, the enzyme is confined exclusively to Müller cells (the only gliocytes in the retina). Their identification was confirmed by immunostaining for glutamine synthase, an established Müller cell "marker." The presence in the mature retina of both these enzymes in Müller cells indicates that retinal gliocytes combine functional features that, in the brain, are segregated in astrocytes and oligodendrocytes. In the embryonic retina, carbonic anhydrase C and glutamine synthase differ markedly in their developmental profiles, cellular distribution, and susceptibility to regulation by cortisol and by cell interactions. Such differences make these two enzymes an attractive "marker team" for studying developmental mechanisms in embryonic retina and specific functions of Müller cells.

Normal development and precocious induction of glutamine synthetase in the neural retina of the quail embryo

The developmental pattern of glutamine synthetase (GS) in the neural retina of the quail embryo is described and correlated with retina growth and differentiation. We show that GS in the quail retina can be precociously induced by cortisol, and that the enzyme is localized in Müller glia cells. The results are compared with the development and induction of GS in chick retina.

Developmental changes in the distribution of S-100 in avian neural retina

Localization of S-100 in cells of embryonic and mature chick neural retina was studied by immunostaining of tissue sections with antiserum to S-100 and indirect immunofluorescence. Immunostaining was found to be localized predominantly in the neurons. In embryonic retina, the intensity of immunostaining increased transiently in each class of neurons at the time when they were undergoing organization within their stratum. Late embryonic and mature retina reacted weakly with the antiserum, with the exception of certain 'giant' ganglion cells that immunostained intensely. Müller cells, the only kind of glia cells in the chicken transiently in each class of neurons at the time when they were undergoing organization within their stratum. Late embryonic and mature retina reacted weakly with the antiserum, with the exception of certain 'giant' ganglion cells that immunostained intensely. Müller cells, the only kind of glia cells in the chicken transiently in each class of neurons at the time when they were undergoing organization within their stratum. Late embryonic and mature retina reacted weakly with the antiserum, with the exception of certain 'giant' ganglion cells that immunostained intensely. Müller cells, the only kind of glia cells in the chicken retina, did not react prominently with the antiserum at any of the stages examined and could not be discerned by immunostaining either in embryonic or in mature retina. The results indicate that in chick neural retina S-100 is found predominantly in neurons, and it increases and declines in different subpopulations of neurons coordinately with changes in the development and maturation if this tissue.

Induction of glutamine synthetase in embryonic neural retina: its suppression by the gliatoxic agent alpha-aminoadipic acid

Competence for cortisol-mediated induction of glutamine synthetase (GS) is a differentiation marker of embryonic neural retina. Earlier work has indicated that the induction and accumulation of GS is localized in the Müller glia cells. This localization was presently confirmed by the finding that the gliatoxin D,L-alpha-amino-adipic acid (AAA) reduces responsiveness to GS induction by 60--90% due to preferential damage to Müller cells. The tests were performed on organ cultures of retina tissue from chick embryos, and on retina cell aggregates in which there is tissue reconstruction. The presence of GS-inducible Müller cells was monitored by immuno-staining of tissue sections with anti-GS antiserum. Reduction of GS inducibility due to pretreatment with AAA resulted in virtual absence of cells that immunostained for GS. The preferential toxicity of AAA for Müller cells was also demonstrated by cell viability tests; it was further corroborated by the finding that treatment with AAA greatly reduced the level of carbonic anhydrase activity, another enzyme localized predominantly in Müller cells, but did not affect gamma-aminobutyric acid transaminase and choline acetyl transferase, neuronal enzymes. Susceptibility of Müller cells to AAA was found to increase with embryonic development of the retina. We suggest that acquisition of susceptibility for AAA represents another differentiation marker of embryonic Müller cells.

Age-dependent differences in cognin regeneration on embryonic retina cells: immunolabeling and SEM studies

The retina cognin (a glycoprotein isolated from the surface membrane of neural retina cells of chick embryos and postulated to mediate self-recognition and histogenetic association of retina cells) has been visualized by SEM on the surface of embryonic retina cells in vitro following immunolabeling of the cells with antibodies to the purified cognin and with polystyrene latex microbeads. Trypsin dissociation of retina tissue into separated cells resulted in cognin depletion from the cell surface; following incubation at 37 degrees C the cells regenerated the cognin. Regeneration was fastest and most abundant on cells from the youngest retinas examined; it declined markedly with the embryonic age of the cells, suggesting an age-dependent decrease in cell capacity for cognin formation. Evidence is discussed that the rate and amount of cognin regeneration on the cell surface are temporally-causally correlated with the capacity of the cells to reaggregate into retinotypic tissue. The results support the suggested role of the cognin in the mechanism of self-affinity and morphogenetic association of embryonic neural retinal cells.

Induction of glutamine synthetase in embryonic neural retina: localization in Müller fibers and dependence on cell interactions

The cellular localization of glutamine synthetase [GSase; L-glutamate:ammonia ligase(ADP)-forming), EC 6.3.1.2] induced by cortisol in the neural retina of chicken embryos was investigated by immunostaining with GSase-specific antiserum and indirect immunofluorescence. In organ cultures of retina tissue, and in the retina in vivo, hormone-induced GSase was found to be confined only to the Müller fibers (retinoglia). Also, in mature chicken retina, which contains a very high level of GSase, the enzyme was detected solely in Müller fibers. In short-term monolayer cultures of dispersed embryonic retina cells, there was no GSase induction and no immunodetectable increase in enzyme level. However, when the dispersed cells were reaggregated and they restituted retinotypic cell associations, GSase could be induced and it was localized in Müller fibers. The results suggest that, in addition to the hormonal stimulus, contact-dependent interactions between Müller glia cells and retina neurons are involved in the mechanism of GSase induction in the retina.

Malformation of embryonic neural retina elicited by BrdU

Exposure of neural retina tissue from early chick embryos (5 and 6-days) to 5-bromo-2'-deoxyuridine (BrdU) for 24 h irreversibly prevented normal histogenesis and resulted in the formation of chaotically disorganized tissue. The sensitivity of the retina to this effect decreased with embryonic age and declined sharply after the commencement of cell stratification. Examination by electron microscopy revealed the following progressive morphologic changes resulting from BrdU treatment: complete breakdown of the outer limiting membrane due to disappearance of its constituent tight junctions which normally anchor cells at the outer retinal surface; collapse and endocytosis of cilia, resulting in the absence of photoreceptor processes; increasing disorganization of the cells which commenced at the outer surface of the retina and progressed inward, resulting in chaotic distortion of the histologic architecture of the retina. Ultrastructural differences were noted between cells in the malformed retina, indicating the presence of several cell types. Possible mechanisms of this BrdU-elicited malformation are considered in the Discussion.

The development of inducibility for glutamine synthetase in embryonic neural retina: inhibition by BrdU

The hydrocortisone-mediated induction of glutamine synthetase (GS) in the neural retina of the chick embryo is a characteristic and unique feature of differentiation of this tissue. The induction involves genomic activity elicited by the inducer resulting in synthesis and accumulation of the enzyme. We describe correlations between the growth of embryonic retina tissue in vivo and in vitro and the development of its inducibility for GS, and demonstrate that this development proceeds through two phases: competence-acquisition phase (before the 7th day of development), and maturation phase. BrdU applied for 24 h to retinas of 5-day embryos irreversibly suppresses the development of induction-competence. However, BrdU does not affect the progressive maturation of inducibility when applied to retinas that already are fully induction-competent (8 days and older). The short treatment with BrdU of 5-day retinas also causes defective histogenesis resulting in drastic malformation of the tissue. The nature of the processes involved in competence-acquisition and in the maturation of inducibility for GS are examined. Possible mechanisms by which BrdU prevents the development of induction-competence for GS in the early embryonic retina and elicits defective histogenesis are discussed.

Effects of cytosine arabinoside on differential gene expression in embryonic neural retina. II. Immunochemical studies on the accumulation of glutamine synthetase

Cytosine arabinoside (Ara-C) elicits a significant increase in the level of the enzyme glutamine synthetase (GS) while it markedly reduces overall RNA and protein synthesis in cultures of embryonic chick neural retina. This increase was analyzed by radioimmunochemical procedures and compared with the induction of GS by hydrocortisone (HC). Accumulation of GS in Ara-C-treated retinas was found to be due to de novo synthesis of the enzyme; however, unlike the induction of GS by HC, Ara-C caused no measurable increase in the rate of GS synthesis. The results indicate that Ara-C facilitates GS accumulation largely by preventing degradation of the enzyme. Even though Ara-C inhibits the bulk of RNA synthesis in the retina, it does not stop the formation of GS-specific RNA templates. However, the progressive accumulation of these templates does not result in an increased rate of GS synthesis unless Ara-C is withdrawn from such cultures under suitable experimental conditions. Thus, it is suggested that the continuous presence of Ara-C imposes a reversible hindrance at the translational level which limits the rate of GS synthesis. The results demonstrate that the increase in retinal GS elicited by Ara-C is achieved through mechanisms which are quite different from those involved in the hydrocortisone-mediated induction of this enzyme.

Glutamine synthetase induction in embryonic neural retina. Interactions of receptor-hydrocortisone complexes with cell nuclei

In the neural retina of the chick embryo, hydrocortisone (HC) elicits differential gene expression resulting in the induction of glutamine synthetase (GS), which is an enzyme marker of differentiation in the retina. The relationship between nuclear binding of receptor-hydrocortisone (R-HC) complexes and GS induction was investigated in cultures of retina tissue from 12-day chick embryos. The number of HC binding sites in the cytoplasm was estimated as 1650+/-200 per retina cell; there are approximately 1500+/-100 acceptor sites for R-HC per retina nucleus. GS induction in the retina became detectable only after R-HC bound to more than 40% of the nuclear acceptors sites; increased binding coincided with higher induction levels, until complete site saturation was attained; Proflavine, which blocks preferentially and completely GS induction in the retina by interfering in the nucleus with the enzyme-inducing action of the hormone, reduced nuclear binding of R-HC by only 20%; thus, only part of the R-HC that binds in the nucleus appears to be directly involved in eliciting the induction of GS. Within one hour after exposure of the retina to an inducing dose of HC, there was translocation of HC and HC-receptors (as R-HC complexes) from the cytoplasm into the nucleus and saturation of nuclear accepegan to decline; in 12 h, it was reduced to 50% of the initial saturation level. Since, during this time, the enzyme activity to increase, persistence of the induced state depends on association of the hormone with only a portion of the sites in the nucleus to which it can bind. The decrease in the amount of bound HC in the nuclei of induced cells was accompanied by an increase in the level of HC receptors in the cytoplasm. About 50% of this increase could be prevented by cycloheximide; this suggests that the reappearance of HC receptors in the cell cytoplasm may be due, at least in part, to de novo synthesis of HC receptors.

Preparation of tissue-specific cell-aggregating factors from embryonic neural tissues

Tissue-specific cell-aggregating factors were previously obtained from the supernatant media of embryonic cell cultures. We describe here that such factors can be extracted from cell-membrane preparations derived directly from embryonic tissues: neural retina, cerebrum, and spinal cord of chick embryos.

Isolation of retina-specific cell-aggregating factor from membranes of embryonic neural retina tissue

There is increasing evidence that developmental associations among embryonic cells are mediated by specific components of the cell surface. Earlier work has indicated that such components are extruded into the medium of primary monolayer cultures of embryonic cells, and that they represent the active constituents of the tissue-spedific cell-aggregating factors isolated fro- the supernatant medium of such cultures. We presently report that tissue-specific cell-aggregating factors can be obtained directly from embryonic tissues, and describe the isolation and partial purification of retina-spedific factor from a cell-membrane preparation derived from retina tissue of the chick embryo. Extraction of the purified membrane preparation with 1-butanol yielded an activity in the aqueous phase which resides in a protein probably a glycoprotein, with an estimated molecular weight of 50,000 in solution. This material could be obtained from retinas of embryos not older than 13 days, and only pre-13-day cells responded to its cell-aggregating activity. By these characteristics, this membrane-derived retina factor closely resembles the retina cell-aggregating glycoprotein previously purified from the supernatant medium of retina cell cultures. It is of special interest that the cell-aggregating protein is obtainable from cellular membranes during those stages of development when retina cells are most actively engaged in histological organization. Work in progress indicates that, by the procedures described herein, tissue-specific cell-aggregating factors can also be obtained from membrane preparations of other embryonic tissues.

Stimulation of DNA synthesis by ouabain and concanavalin A in cultures of embryonic neural retina cells

Ouabain and concanavalin A, agents which bind to specific sites in the cell membrane, stimulate DNA synthesis and cell replication in monolayer cultures of neural retina cells from late chick embryos. The results suggest a relationship between control of retina cell replication and properties of the cell membrane. The experiments involved measurements of 3H-thymidine incorporation in primary monolayer cultures (24-48h) of retina cells from embryos of different ages. Stimulation by ouabain was greatest in cells from 14-day embryos, and its magnitude was similar to that elicited in these cell cultures by concanavalin A. Simultaneous treatment of 14-day retina cells with both agents resulted in a greater than additive stimulation of DNA synthesis. Our results demonstrated that, although during normal embryogenesis cell replication in the neural retina has virtually ceased by day 14 of development, some cells retained a capacity for mitogenesis when exposed to conditions such as provided in these experiments. By autoradiography the responding cells were identified as large epithelioid retina cells (LER cells). Under optimal conditions of simultaneous treatment with ouabain and Con A about 20% of the LER cells showed stimulation of DNA synthesis. The nature of LER cells and other aspects of our findings are discussed.