Characteristics of the cell proliferation in the developing human retina

The retinal primordia were studied in human embryos at developmental weeks 5-8 to examine the expression of proliferation-associated proteins (Ki67 and PCNA), the number of nucleoli in the nuclei of neuroepithelial cells (in respect to their distance to the apical surface) and the distribution of DNA-synthesizing cells after a short-term (20 min) incubation in vitro in serum-free medium containing BrdU. In 5-week embryos, retinal primordium contains the neuroepithelium with a typical structure. BrdU-positive nuclei and nuclei with small number of nucleoli were found in the basal portion of the ventricular zone. However, this organization was disturbed in the initial period of the formation of inner nuclear layer (week 6). At this stage DNA-synthesizing cells were seen even at the apical surface. In retinal primordia of embryos at weeks 6-7 there appeared an additional area of cell proliferation in the Chievitz layer and in the inner nuclear layer. In embryos at week 8 the dividing cells were concentrated in the outer nuclear layer, which has regained the organization, typical for a neuroepithelium.     

Programmed cell death and the morphogenesis of the hindbrain roof plate in the chick embryo

The spatial and temporal distribution of apoptosis in the dorsal midline of the developing chick hindbrain was examined in relation to the development of the neuroepithelium and neural crest using scanning and transmission electron microscopy, immunocytochemistry and in situ hybridization. The pattern of TUNEL labeling and Slug expression in the dorsal midline at stages 10 and 11 differed from that at stages 12-15. At stages 10 and 11, TUNEL labeling and Slug expression were observed in the dorsal part of location II of rhombomere 1/2 (i.e., between the surface ectoderm and the neuroepithelium), but from stage 12 onward, they were observed in both the dorsal and ventral parts of location II. The implication is that whereas apoptosis may be restricted to a subpopulation of the early migrating neural crest at stages 10 and 11, it presumably occurs in subpopulations of both neural crest and neuroepithelial cells from stage 12 onward. Furthermore, as judged by the pattern of TUNEL labeling and Slug expression in r3 and r5, apoptosis in these two rhombomeres likely occurs in subpopulations of both neural crest and neuroepithelial cells. The eminence present in location I of r1/r2 between stages 10 and 12 consisted of both neural crest and neuroepithelial cells. These cells gradually underwent apoptosis until stage 12, when the eminence disappeared in most embryos. The formation of the inner (neuroepithelial) aspect of the hindbrain roof plate involved both cell migration from adjacent neuroepithelium and an alteration in the shapes of the cells, such that cells with flattened surfaces eventually lined the roof plate. During these processes, some of the neuroepithelial cells underwent apoptosis (i.e., in location IV). The results of this study thus demonstrate that subpopulations of both neuroepithelial and neural crest cells may be involved in programmed cell death in the hindbrain. Additionally, apoptosis in the hindbrain contributes significantly to morphogenetic thinning during roof plate formation.     

Characteristics of Cellular Proliferation in the Developing Human Retina

The expression of proliferation-associated proteins Ki67 and PCNA was studied in the retinal rudiments of human embryos at 5-8 weeks of development; studies also addressed the numbers of nucleoli in the nuclei of neuroepithelial cells (with consideration of their distances to the apical surface) and DNA-synthesizing cells after transient (20 min) in vitro incubation in serum-free medium containing BrdU. The retinal rudiment of embryos at five weeks of development had neuroepithelium of the typical structure. BrdU-positive nuclei and nuclei with small numbers of nucleoli were located in the basal part of the ventricular zone. However, this organization was disrupted during the initial period of formation of the inner nuclear layer (six weeks). At this time, DNA-synthesizing cells were found even at the apical surface. Retinal rudiments of embryos at 6-7 weeks of development contained an additional area of cell proliferation in the Chievitz layer and the inner nuclear layer. In eight-week embryos, dividing cells were located in the outer nuclear layer, which again acquired the organization typical of neuroepithelium.     

A morphometric and computer-assisted three-dimensional reconstruction study of neural tube formation in chick embryos.

The origin of the driving forces for neural tube formation remains uncertain but is currently thought to involve the participation of microfilament bundles situated in the apical ends of neuroepithelial cells. In the work presented here, we show how morphometric measurements that map local variations in the apical geometry of neuroepithelial cells (especially apical constriction) can provide information on the distribution of motive forces within the neuroepithelium during neural tube formation. When used in combination with computer-assisted, three-dimensional reconstruction, it becomes possible to analyze the morphometric data from a dynamic, three-dimensional perspective. As an example application of this method, we have used morphometry to evaluate the effects of ionomycin on the developing neuroepithelium. Treatment of early (stages 6-8) chick embryos with 5 microM ionomycin was found to cause rapid bending of the neuroepithelium within 1 min of exposure and a dramatic acceleration of the normal sequence of neural tube formation. Electron microscopy and morphometry revealed that this acceleration was coincident with a marked increase in the local degree of apical constriction of neuroepithelial cells, presumably a consequence of enhanced contractile activity of apical microfilament bundles. This work shows that transient elevation of free calcium levels can accelerate the usual sequential phases of NT formation. The rapidity of the response (hours of normal development reduced to minutes), increased prominence of apical microfilament bundles, and the enhanced degree of apical constriction strongly support a direct causal role for apical microfilament bundles and apical constriction of neuroepithelial cells in bending of the neuroepithelium.     

The effect of cytochalasin B on the neuroepithelial cells of the mouse embryo

Eleven-day mouse embryos were cultured in a medium containing cytochalasin B (10 μg/ml) to examine the effect of the drug on the developing CNS. The embryos were exposed to the drug for two hours. After culturing, the embryos were prepared for light and electron microscopy and the neuroepithelium of the cerebral vesicles was examined. The following abnormalities were noted in the cytochalasin B-treated embryos: (1) mitotic figures were situated in the middle of the neuroepithelial layer instead of on the lumen, indicating that premitotic nuclear migration had been prevented; (2) binucleate cells were found in the neuroepithelial layer, indicating that cytochalasin B does not interfere with mitosis but prevents cytokinesis; (3) the microfilaments usually seen in the apex of the neuroepithelial cells were disrupted and formed an amorphous mass of filamentous material; (4) some neuroepithelial cells were free in the lumen, whereas others protruded into the ventricle but appeared to remain attached to the internal limiting border by their junctional complexes; and (5) in some regions the neuroepithelial cells had broken away from their end-feet at the basal lamina. These morphological changes were associated with a change in the internal limiting boundary from concave to convex and vacuolation of the external processes of the cells. The relationship between cytochalasin B, microfilaments and these morphological changes is discussed.     

Generation of developmental patterns in the neuroepithelium of the developing mammalian eye: the pigment epithelium of the eye

Culture experiments with eye anlages of mouse embryos were performed to study developmental traits of the neuroepithelial cells of the prospective pigment epithelium in the eye anlage of pigmented mice. Between the neural plate stage on embryonic day 8 (ED 8, developmental stage 12) and the neural tube stage on embryonic day 9 1/2 (stage 15), the cultured neuroepithelium of the eye generated neurons and glia, identified by morphological and immunocytochemical evidence, but no pigmented cells. In contrast, eye anlages did produce pigment epithelium when cultured in their natural position in a head tissue fragment. A minority of developing neurons displayed tyrosine hydroxylase immunoreactivity, whereas GABAergic, serotoninergic and substance P-ergic neurons, which are common in the mature neuroretina, were not observed. When neuroepithelial cells from embryonic eyes older than stage 15 (ED 9 1/2) were cultured, they differentiated into pigment cells but not into nerve cells or glia. This developmental sequence indicates that the pigment cells derive from the neural lineage. Pigment cell fate dominates over the neural fate beginning at about stage 15 (ED 9 1/2-10). That is at least 2 days before the pigment cell phenotype becomes apparent in vivo (ED 11 1/2-12).     

Studies on wound healing in the neuroepithelium of the chick embryo

Wound healing has been studied by light microscopy, SEM, and TEM in the neuroepithelium of the early neurula (stages 6 and 8) and advanced neurula (stages 10 and 12) chick embryos. Healing involves two major events: (1) apposition of the wound edges and (2) restitution of the neuroepithelium at the wound site (i. e., restoration of the epithelial integrity of neuroepithelium). Apposition of the wound edges occurs within the first 15 minutes of re-incubation and involves the entire lenght of the wound. The main event during restoration is a change in the shapes of the rounded cells to elongated forms (i. e., spindle, wedge, and inverted wedge shapes). Wounds of younger embryos heal faster than those of older ones. © 1992 Wiley-Liss, Inc.     

Inhibition of cephalic neural tube closure by 5-azacytidine in neurulating rat embryos in vitro

Summary Head-fold stage rat embryos (9.5 days of gestation) were cultured for 48 h in rat serum with or without 0.8 M 5-azacytidine. Incomplete closure of the cephalic neural tube was observed in 5-azacytidine-treated embryos cultured for 48 h (25-somite stage). Control embryos showed complete fusion of cephalic neural folds at 33 h (16-somite stage) in culture. Drug administration or removal experiments revealed that embryos were sensitive to 5-azacytidine during 6–12 h of culture (three to five somite stages). Electron microscopical studies indicated that the arrangement and fine structure of cephalic neuroepithelial cells were almost the same in control and treated embryos. There was no significant difference in DNA and protein contents between control and treated embryos cultured for 36 h. Immunocytochemical observations using 5-methylcytosine-specific antibody revealed that the staining of neuroepithelial cells in the median part of the transversely sectioned cephalic neural plate, and of mesenchymal cells near the apices of the plate, was suppressed by 5-azacytidine. These results suggest that DNA methylation of these cells plays an important role in closure of the cephalic neural tube.     

Inhibition of mammary gland lactose secretion by colchicine and vincristine

The possible role of microtubules in milk production was examined in mammary gland slices from lactating guinea pigs. Colchicine, 10(-5)-10(-4) M, depressed lactose secretion within 15 min, maintaining maximal inhibition over 2.5 h, accompanied by retention of lactose within the slices. In colchicine dose-response studies (2 h, 10(-8)-10(-5) M), secretion was depressed 22% by 10(-5) M, whereas tissue lactose increased with dose up to +25% at 10(-5) M. Lactose synthesis was inhibited 3-19% without correlation to colchicine concentration. In another study, incubation with 10(-5) M lumicolchicine yielded one-third less inhibition of secretion than 10(-5) M colchicine with no increase in tissue lactose. Both drugs depressed synthesis 31%. Lactose secretion showed a negative correlation with 10(-7)-10(-4) M vincristine yielding a maximal 66% inhibition at 10(-4) M, whereas tissue retention showed a linear increase with concentration up to 151% of control at 10(-4) M. Effects on synthesis were sporadic. These data suggest that microtubules have a role in facilitating the transport and/or secretion of lactose and perhaps other milk components.     

Effect of deuterium oxide on leukotriene C4 generation in immunologically stimulated human leukocytes

Addition of deuterium oxide (D2O), 6-36%, resulted in a dose-dependent increase in allergen- or anti-IgE-induced leukotriene C4 (LTC4) generation from human basophils. In the presence of 36% D2O, the enhancement was 260 +/- 135% for allergen stimulation and 480 +/- 152% for anti-IgE stimulation as compared with the control incubated in normal buffer. The increasing effect of D2O on LTC4 generation from basophils was completely reversed by washing the cells before incubation with allergen. Vinblastine as well as colchicine, at a concentration of 100 microM, counteracted the effect of D2O. The enhanced release of histamine and LTC4 from basophils challenged with allergen was suppressed by Dimaprit, a histamine H2 receptor agonist, at a concentration required to inhibit the release by 50% of 5 X 10(-5) M for histamine and 10(-5) M for LTC4. These observations suggest that microtubules may be involved in LTC4 generation from immunologically stimulated basophils.     

The influence of excess vitamin A on neural tube closure in the mouse embryo

Summary The effect of excess vitamin A on the closure of the neural tube in mouse embryos was examined with light microscopy, transmission and scanning electronmicroscopy. The embryos were treated with the vitamin just before closure of the brain vesicles and examined during the following 24 h, a period during which under normal conditions the brain completely closes.At 18–24 h after treatment the external features of the treated specimens began to differ from those of the controls. In the treated embryos the neural walls folded laterally and became widely separated, whereas those of the controls folded dorsomedially and fused in the midline. Histologically, the first difference between treated and control embryos was noted at two hours after treatment, when large intercellular spaces appeared between the neuroepithelial cells of the treated embryos. These spaces were mainly present between the apical ends of the wedge-shaped neuroepithelial cells. This accumulation of intercellular spaces interfered with the normal morphogenetic movement of the neural walls, which remained convex instead of becoming concave. This convex bending resulted in non-closure of the neural tube.In addition to the appearance of large intercellular spaces some neuroepithelial cells as well as some mesenchymal, endothelial, and surface ectoderm cells showed swelling and degeneration as a result of the vitamin A treatment. This cell degeneration probably contributes to failure of the neural tube to close due to loss of cohesion at the luminal surface and the lack of mesenchymal support needed for the elevation of the neural walls. However, the increase of intercellular spaces at the apical side of the neuroepithelium is in all probability the major cause for the failure of the neural tube to close.     

Shaping of the chick neuroepithelium during primary and secondary neurulation: role of cell elongation

Cell elongation is a cardinal event in formation and shaping of the neuroepithelium during both primary and secondary neurulation. This study had three purposes. The first was to clarify the role of microtubules in maintaining the elongated configurations of chick neuroepithelial cells. Neuroepithelial cells of the neural plate (the rudiment of the primary neural tube) and medullary cord (the rudiment of the secondary neural tube) reduced their heights an average of approximately 25% when their microtubules were depolymerized, but most cells remained considerably elongated and columnar. Complete rounding up occurred only as cells entered metaphase where they arrested. These results suggest that microtubules as well as other factors are required to maintain the fully elongated configurations of cells composing epithelial sheets. The second purpose of this study was to evaluate the hypothesis that neuroepithelial cell elongation plays a major role in narrowing of the neural plate. To do this, the width of the neural plate was examined after microtubule depolymerization and repolymerization. As the heights of neuroepithelial cells decreased with loss of their microtubules, the width of the neuroepithelium increased roughly proportionately; subsequent repolymerization with concomitant cell elongation resulted again in neural plate narrowing. Thus, the hypothesis is supported. The third purpose of this study was to examine the roles of cell rearrangement and change in neuroepithelial cell or extracellular volume in neural plate narrowing and extension. Extensive cell rearrangement, resulting in net cell loss from the width of selected, representative levels of the neural plate, does not seem to play a major role in plate narrowing, but decreases in cell or extracellular volume are likely involved. Further studies are necessary to complete our understanding of the mechanisms driving neural plate shaping and bending.     

Vimentin and neuroepithelial cell differentiation in the spinal cord of chick embryos: an immunohistochemical study]

Using vimentin immunohistochemical staining, the differentiation processes of neuroepithelium in the neural tube were examined in chick embryos from stages 8 through 28. At an early stage of the neural groove, stage 8, no morphological differences could be found among the neuroepithelial cells, but vimentin staining allowed us to identify four different regions in the groove wall. The epithelial cells in the ventral wall exhibited moderate staining of vimentin, and vimentin was detected in the basal and middle cytoplasmic areas. A weak staining limited to the basal cytoplasm was observed in the dorsal wall. In contrast, epithelial cells in the median hinge region and in the lateral edge of the neural groove had little vimentin. On the basis of this vimentin staining, four similar regions could also be observed in the neuroepithelium of the neural tube at stage 12; the ventral wall, dorsal wall, floor plate and roof plate. Prior to the morphological changes in the neuroepithelial cells, vimentin expression showed dramatic changes, and our immunohistochemical data suggest that cell differentiation into motor areas and sensory areas starts at an early stage of the neural groove.     

Cell cycle and neuroepithelial cell shape during bending of the chick neural plate

Neuroepithelial cells change shape from spindle-like to wedge-like within three restricted areas (hinge points) of the bending neural plate. The mechanisms underlying these localized cell shape changes and the specific role that these changes play in bending are unclear. This study was designed to determine whether changes in neuroepithelial cell shape involve basal cellular expansion owing to alteration of the cell cycle. Neurulating chick embryos were treated with colchicine to arrest and accumulate cells in metaphase, and colchicine indices and cell generation times were calculated for the neural plate. During bending of the neural plate, cell generation time in the median hinge point, which contains predominantly wedge-shaped cells, was significantly longer than that in adjacent lateral areas of the neural plate, which contain predominantly spindle-shaped cells. In addition, cell generation time in the flat neural plate, which contains predominantly spindle-shaped cells and has not yet differentiated into the median hinge point and lateral subdivisions, was identical to that in lateral areas of the bending neural plate but was significantly shorter than that in the median hinge point. These results support the hypothesis that changes in neuroepithelial cell shape from spindle-like to wedge-like involve basal cellular expansion owing to alteration of the cell cycle. Additional tests of this hypothesis and studies on the role of localized cell shape changes in neurulation are in progress.     

Pressure effects on the ADH-induced initiation of water flow in toad bladder

Earlier studies employed colchicine to demonstrate the need for microtubules in the ADH-induced initiation of increased water permeability in toad bladder. We have used colchicine and hydrostatic pressure together to determine whether formed or growing microtubules are required for initiation of the ADH response in Bufo marinus. When ADH and 8,000 psi were administered simultaneously, the ADH-induced increase in water flow was inhibited while under pressure by 107 +/- 7% (n = 6). Application of 8,000 psi for 10 min before ADH administration resulted in an increased initiation of the ADH osmotic response over the non-pressure-treated control (average acceleration, rate of water flow increase during first 3 min after ADH stimulation, 1.25 +/- 0.27 vs. 0.43 +/- 0.12 mg X cm-2 X min-2, n = 6). In addition, the inhibition of the ADH response brought about by colchicine incubation was overcome with pretreatment of the colchicine-incubated bladders with 8,000 psi for 10 min (average 3-min acceleration, 0.18 +/- 0.04 vs. 1.13 +/- 0.06 mg X cm-2 X min-2, respectively). Repeating the experiments with dibutyryl cAMP gave similar results. We interpret these data as suggesting that growing microtubules are required for initiation. The proposed model is as follows. Pressure removes colchicine inhibition by introducing, through disassembly of formed microtubules, more colchicine-free tubulin subunits. These subunits are then available following decompression to reassemble when the tissue is challenged with hormone.     

Differentiation of the mammalian retinal pigment epithelium in vitro: Influence of presumptive retinal neuroepithelium and head mesenchyme

The ancestor cells of the pigment epithelium of the mammalian eye are derived from the neuroepithelial cells of the neural plate. They are neurally determined in the process of neurulation but finally decide to follow the pigment cell lineage, whereas the adjacent tissue develops into the neuroretina and the optic stalk. This decision is most probably made in the developmental stage of eye cup formation. The pigment epithelium becomes restricted to the outer leaf of the eye cup and does not encroach on the adjacent neuroepithelial tissues of the internal leaf and the eye stalk. It is therefore supposed to be channelled by a locally confined determinant factor that has not yet been identified. In the present study, development of the mammalian eye and the neural versus pigment cell decision were investigated in mouse embryos. Three approaches were used to discover the source of the putative determinant involved in the process of neuroepithelial decision. First, eye primordia were cultured from stage 11 embryos (0 somites, early neural plate stage, embryonic day 7 1/2–8) to stage 16 embryos (34 somites, neural tube stage, ed 10); this is prior to pigment cell induction. The eye primordia were first cultured in head segments and their natural position. In these experiments, 50% of the ocular neuroepithelia developed along the nerve cell and glial cell lineage. However, the other 50% of the cultured specimens partly developed into pigment epithelia. In these specimens the determinant factors had obviously remained functionally intact in vitro. In the second type of experiment, the eye primordia were also cultured within the head segments, but with the prospective neuroretina selectively removed. This experiment should show whether the inner layer of the eye cup (the prospective neuroretina) is involved in the neuroepithelial lineage decision. In these experiments 90% of the cultured eye primordia failed to develop pigmented cells. The prospective neuroretina was therefore considered as a candidate for the production of an inductive factor. Finally, eye primordia from stage 14–15 embryos (13–29 somites, ed 9–9 1/2) were either transplanted into heterotopic tissues, such as mesenchymal organs, neuroepithelium or heterochronic muscle, or grown as controls in their natural position and tissue environment. In these conditions both transplanted eye primordia and controls bore pigmented epithelium. Hence, the lineage decision, whether to form neural or pigment cell, remained undisturbed in all epitopes tested. On the basis of these experiments, it seemed unlikely that the development of pigment cells was initiated by a mesenchyme-derived factor exclusively produced near the eye vesicle.     

Effects of colchicine and vinblastine on axonal transport of choline acetyltransferase in rat sciatic nerve.

The effects of colchicine (0.5-10(-2) M) and vinblastine (10(-2)-10(-5) M) Upon axonal transport of choline acetyltranserase (CAT) and on nerve impulse conduction have been investigated in the rat sciatic nerve. High concentrations of colchicine (0.5 M) and vinblastine (10(-2) M) blocked completely both axonal transport of CAT and impulse conduction. 10(-3) M vinblastine did not affect impulse conduction until 20-22 h after injection, but this concentration of vinblastine did block CAT transport but not impulse conduction. 10(-2) M and 10(-1) M colchicine were without effect on impulse conduction, but did produce substantial, although incomplete, block of CAT transport. The results are discussed in relation to the possible involvement of microtubules in transport of CAT.     

Bovine ovarian cortical pieces grafted to chick embryonic membranes: a model for studies on the activation of primordial follicles.

BACKGROUND: Little is known about the factors that control the initiation of growth of primordial follicles. Primordial follicles in pieces of fetal bovine ovarian cortex spontaneously activate in vitro and develop to the primary stage, but few follicles develop further. For decades, embryologists have grafted tissue to the chorioallantoic membrane (CAM) of chick embryos to study the development of various organs and structures. METHODS AND RESULTS: To test the hypothesis that grafting cortical pieces beneath the CAM membrane of 6-day-old chick embryos ('in ovo') would support the activation of primordial follicles and the growth of activated follicles to the secondary stage, ovarian cortical pieces from six bovine fetuses (6-8 months gestation) were placed either in ovo or in organ culture in serum-free medium (in vitro). Cortical pieces were retrieved after 0, 2, 4, 7, or 10 days in ovo or in vitro. Histological examination revealed a dramatic infiltration of the CAM-grafted cortical pieces with blood vessels. By day 2 in vitro, the number of primordial follicles had declined by 87% concomitant with a 3.5-fold increase in primary follicles (P < 0.01), providing evidence of the expected activation of primordial follicles. Unexpectedly, primordial follicles were not activated in CAM-grafted tissue, as shown by maintenance of their numbers and lack of increase in primary follicles during 10 days in ovo. In experiment 2, a subset of pieces was switched from culture to CAM grafts and from CAM grafts to culture on day 2. The CAM did not support the growth of primary follicles activated in vitro, apparently because the activated follicles did not survive the transfer (P < 0.05). However, primordial follicles maintained in ovo retained their ability to activate; after their removal from the CAM into culture, primordial follicles decreased in number and primary follicles increased in number within 2 days (P < 0.05). CONCLUSIONS: The CAM graft will provide a useful model for studying the factors involved in activation of primordial follicles.     

Effects of colchicine and cytochalasin B on vasopressin- and cyclic adenosine monophosphate-induced changes in toad urinary bladder

Coincident with an increase in water permeability, the ridge-like surface structures of toad bladder granular cells transform to individual microvilli after stimulation with vasopressin (VP) or cyclic adenosine monophosphate (cAMP) by a mechanism that is yet to be defined. To explore the possible role of microtubules and microfilaments in this cell response, colchicine and cytochalasin B were employed to determine whether interference with the function of these components of the cytoskeletal system would prevent the VP- and cAMP-induced conversion of ridges to microvilli. Incubation of toad urinary bladders in 10(-4) M colchicine for 4 hours or 10(-5) M cytochalasin B for 90 minutes before stimulation with 20 mU. per ml. of VP markedly inhibited osmotic water flow. However, neither agent prevented the striking conversion of ridges to surface microvilli induced by VP and cAMP as seen with scanning electron microscopy. In addition, the ridges characteristic of granular cells were maintained in control bladders incubated with colchicine or cytochalasin B, but left unstimulated. Under the conditions of these experiments, these findings suggest that microtubules and microfilaments are not essential for maintenance of normal surface configuration in granular cells of toad urinary bladder, and that they are not involved in the mechanism responsible for VP- and cAMP-induced surface changes that occur in association with increased water permeability of this epithelium.     

Effects of Colchicine and Vinblastine on Axonal Transport of Choline Acetyltransferase in Rat Sciatic Nerve1

The effects of colchicine (0.5–10^-2 M) and vinblastine (10^-2-10^-5 M) upon axonal transport of choline acetyltransferase (CAT) and on nerve impulse conduction have been investigated in the rat sciatic nerve. High concentrations of colchicine (0.5 M) and vinblastine (10^-2 M) blocked completely both axonal transport of CAT and impulse conduction. 10^-3 M vinblastine did not affect impulse conduction until 20–22 h after injection, but this concentration of vinblastine did block CAT transport; 5 × 10^-5 and 10^-4 M vinblastine blocked CAT transport but not impulse conduction. 10^-2 M and 10^-1 M colchicine were without effect on impulse conduction, but did produce substantial, although incomplete, block of CAT transport. The results are discussed in relation to the possible involvement of microtubules in transport of CAT.