Conductance and dye permeability of a rectifying electrical synapse

Electrical coupling and its channels

Harris explores the development of our current understanding of electrical coupling between cells and the channels that mediate it, highlighting the contributions of the Journal of General Physiology.

As the physiology of synapses began to be explored in the 1950s, it became clear that electrical communication between neurons could not always be explained by chemical transmission. Instead, careful studies pointed to a direct intercellular pathway of current flow and to the anatomical structure that was (eventually) called the gap junction. The mechanism of intercellular current flow was simple compared with chemical transmission, but the consequences of electrical signaling in excitable tissues were not. With the recognition that channels were a means of passive ion movement across membranes, the character and behavior of gap junction channels came under scrutiny. It became evident that these gated channels mediated intercellular transfer of small molecules as well as atomic ions, thereby mediating chemical, as well as electrical, signaling. Members of the responsible protein family in vertebrates—connexins—were cloned and their channels studied by many of the increasingly biophysical techniques that were being applied to other channels. As described here, much of the evolution of the field, from electrical coupling to channel structure–function, has appeared in the pages of the Journal of General Physiology.

A structural and functional comparison of gap junction channels composed of connexins and innexins

Methods such as electron microscopy and electrophysiology led to the understanding that gap junctions were dense arrays of channels connecting the intracellular environments within almost all animal tissues. The characteristics of gap junctions were remarkably similar in preparations from phylogenetically diverse animals such as cnidarians and chordates. Although few studies directly compared them, minor differences were noted between gap junctions of vertebrates and invertebrates. For instance, a slightly wider gap was noted between cells of invertebrates and the spacing between invertebrate channels was generally greater. Connexins were identified as the structural component of vertebrate junctions in the 1980s and innexins as the structural component of pre-chordate junctions in the 1990s. Despite a lack of similarity in gene sequence, connexins and innexins are remarkably similar. Innexins and connexins have the same membrane topology and form intercellular channels that play a variety of tissue- and temporally specific roles. Both protein types oligomerize to form large aqueous channels that allow the passage of ions and small metabolites and are regulated by factors such as pH, calcium, and voltage. Much more is currently known about the structure, function, and structure-function relationships of connexins. However, the innexin field is expanding. Greater knowledge of innexin channels will permit more detailed comparisons with their connexin-based counterparts, and provide insight into the ubiquitous yet specific roles of gap junctions. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 522-547, 2017.

Functional asymmetry and plasticity of electrical synapses interconnecting neurons through a 36-state model of gap junction channel gating

We combined the Hodgkin–Huxley equations and a 36-state model of gap junction channel gating to simulate electrical signal transfer through electrical synapses. Differently from most previous studies, our model can account for dynamic modulation of junctional conductance during the spread of electrical signal between coupled neurons. The model of electrical synapse is based on electrical properties of the gap junction channel encompassing two fast and two slow gates triggered by the transjunctional voltage. We quantified the influence of a difference in input resistances of electrically coupled neurons and instantaneous conductance–voltage rectification of gap junctions on an asymmetry of cell-to-cell signaling. We demonstrated that such asymmetry strongly depends on junctional conductance and can lead to the unidirectional transfer of action potentials. The simulation results also revealed that voltage spikes, which develop between neighboring cells during the spread of action potentials, can induce a rapid decay of junctional conductance, thus demonstrating spiking activity-dependent short-term plasticity of electrical synapses. This conclusion was supported by experimental data obtained in HeLa cells transfected with connexin45, which is among connexin isoforms expressed in neurons. Moreover, the model allowed us to replicate the kinetics of junctional conductance under different levels of intracellular concentration of free magnesium ([Mg²⁺]_i), which was experimentally recorded in cells expressing connexin36, a major neuronal connexin. We demonstrated that such [Mg²⁺]_i-dependent long-term plasticity of the electrical synapse can be adequately reproduced through the changes of slow gate parameters of the 36-state model. This suggests that some types of chemical modulation of gap junctions can be executed through the underlying mechanisms of voltage gating. Overall, the developed model accounts for direction-dependent asymmetry, as well as for short- and long-term plasticity of electrical synapses. Our modeling results demonstrate that such complex behavior of the electrical synapse is important in shaping the response of coupled neurons.

In most computational models of neuronal networks, it is assumed that electrical synapses have a constant and ohmic conductance. However, numerous experimental studies demonstrate that connexin-based channels expressed in neuronal gap junctions can change their conductance in response to a transjunctional voltage or various chemical reagents. In addition, electrical synapses may exhibit direction-dependent asymmetry of signal transfer. To account for all these phenomena, we combined a 36-state model of gap junction channel gating with Hodgkin–Huxley equations, which describes neuronal excitability. The combined model (HH-36SM) allowed us to evaluate the kinetics of junctional conductance during the spread of electrical signal or in response to chemical factors. Our modeling results, which were based on experimental data, demonstrated that electrical synapses exhibit a complex behavior that can strongly affect the response of coupled neurons. We suggest that the proposed modeling approach is also applicable to describe the behavior of cardiac or other excitable cell networks interconnected through gap junction channels.

Hemichannel composition and electrical synaptic transmission: molecular diversity and its implications for electrical rectification

Unapposed hemichannels (HCs) formed by hexamers of gap junction proteins are now known to be involved in various cellular processes under both physiological and pathological conditions. On the other hand, less is known regarding how differences in the molecular composition of HCs impact electrical synaptic transmission between neurons when they form intercellular heterotypic gap junctions (GJs). Here we review data indicating that molecular differences between apposed HCs at electrical synapses are generally associated with rectification of electrical transmission. Furthermore, this association has been observed at both innexin and connexin (Cx) based electrical synapses. We discuss the possible molecular mechanisms underlying electrical rectification, as well as the potential contribution of intracellular soluble factors to this phenomenon. We conclude that asymmetries in molecular composition and sensitivity to cellular factors of each contributing hemichannel can profoundly influence the transmission of electrical signals, endowing electrical synapses with more complex functional properties.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Ca2+ mobilization and interlayer signal transfer in the heterocellular bilayered epithelium of the rabbit ciliary body

1. 'Ratiometric' fura-2 methodology in slice preparations and 'intensitometric' fluo-3 measurements of confocal images were used to simultaneously monitor Ca2+ mobilization in the two distinct, apically joined cell layers which constitute the ciliary body epithelium (CBE): the non-pigmented (NPE) and pigmented (PE) epithelia. 2. Both methods yielded comparable results regarding Ca2+ responses in the syncytium upon stimulation with adrenergic and cholinergic agonists. 3. The alpha 1-adrenoceptor agonist phenylephrine elicited a moderate [Ca2+]i increase in the PE, whereas NPE [Ca2+]i remained unchanged or exhibited a slight diminution. 4. In combination with carbachol, the alpha 2-adrenoceptor agonist brimonidine elicited large Ca2+ increases (> 10-fold) in both the NPE and PE cell layers, even though previous studies indicated the absence of an alpha 2-adrenergic effect on [Ca2+]i in the PE. The onset, as well as the peak of the Ca2+ responses in PE cells frequently exhibited a small delay with respect to adjacent NPE cells. No such time difference was observed between adjacent NPE cells. 5. Pre-incubation of the ciliary body in Ca(2+)-free solution under conditions known to elicit overt NPE-PE separation abolished the alpha 2-adrenocholinergic response in the PE. 6. Addition of heptanol to the perfusate, to block gap-junctional communication, caused a small [Ca2+]i decrease in the NPE and a slight increase in PE[Ca2+]i. Subsequently, the Ca2+ mobilization in the Pe in response to the brimonidine and carbachol combination was either blocked or showed a substantial delay. The Ca2+ mobilization in the NPE, in contrast, remained unchanged. 7. We conclude that the heterocellular syncytium exhibits rectificatory behaviour with respect to Ca2+ mobilization; responses originating within the NPE are easily transferred to the PE, while the reverse does not occur.

Gap junctions in excitable cells

Gap junction channels are an integral part of the conduction or propagation of an action potential from cell to cell. Gap junctions have rather unique gating and permeability properties which permit the movement of molecules from cell to cell. These molecules may not be directly linked to action potentials but can alter nonjunctional processes within cells, which in turn can affect conduction velocity. The data described in this review reveal that, for the majority of excitable cells, there are two limiting factors, with respect to gap junctions, that affect the conduction/propagation of action potentials. These are (1) the total number of channels and (2) the selective permeability of the channels. Interestingly, voltage dependence and the time course of voltage inactivation (kinetics) are not rate limiting steps under normal physiological conditions for any of the connexins studied so far. Only specialized rectifying electrical synapses utilize strong voltage dependence and rapid kinetics to permit or deny the continued propagation of an action potential.

Gap junctions formed by connexins 26 and 32 alone and in combination are differently affected by applied voltage

Gap junctions are formed by a family of homologous proteins termed connexins. Their channels are dodecamers, and homomeric forms differ in their properties with respect to control by voltage and other gating stimuli. We report here the properties of coupling from expression of connexin complementary RNAs (cRNAs; sense to mRNA, antisense to cDNA) in Xenopus oocyte pairs in which endogenous coupling was blocked by injection of DNA oligonucleotides antisense to the mRNA of Cx38, the principal endogenous connexin. We found that a connexin recently sequenced from rat liver, Cx26, formed functional gap junctions whose conductance exhibited voltage dependence with unusual characteristics suggestive of two gating mechanisms. Junctional conductance (gj) was increased to a small degree by depolarization and decreased by hyperpolarization of either cell in a coupled pair, indicating dependence on the potential between the inside and outside of the cells (Vi-o). These changes were fast compared with the resolution of their measurement (ca. 10 ms). On a slower timescale, large transjunctional potentials (Vj) of either sign caused a more substantial decrease in conductance similar to that previously reported for several other gap junctions. Homotypic junctions formed of another connexin, Cx32, exhibited a similar slow dependence on Vj but no dependence on Vi-o. In contrast, heterotypic junctions between an oocyte expressing Cx26 and one expressing Cx32 were electrically asymmetric; they exhibited a greater fast change in gj, which depended, however, on Vj, such that gj increased with relative positivity on the Cx26 side and decreased with relative negativity on the Cx26 side. There was also a large slow decrease in gj in response to Vj for relative positivity on the Cx26 side but not for Vj of the opposite sign. These data indicate that properties of the hemichannels contributed by the two connexins in the heterotypic case were changed from their properties in homotypic junctions. The fast change in gj may involve a mechanism analogous to that at fast rectifying electrical synapses. Experiments in which oocytes expressing Cx32 were paired with oocytes expressing both Cx26 and Cx32 demonstrated that asymmetric junctions would form between oocytes expressing both connexins, thereby confirming their potential relevance in vivo, where the same coupled cells are known to express both proteins.

Postembryonic development of rectifying electrical synapses in crayfish: physiology

In a previous paper we showed that the ultrastructure of the giant fibre to motor giant synapse of crayfish changes in the first few weeks after hatching from having predominantly the appearance of a chemical synapse to having the appearance of an electrical synapse. This is paralleled by a behavioural change from non-giant fibre-mediated to giant fibre-mediated tailflips. In this paper we describe the physiology of the giant fibre to motor giant synapse over this period. We find the following: (1) The giant fibre to motor giant synapse usually transmits spikes 1:1 from the day of hatching. (2) The synapse operates by electrical transmission from the day of hatching, when no connexons are apparent at the ultrastructural level. (3) The synapse has no detectable chemical component, even at an age when the predominant type of junctional apposition has the ultrastructural appearance of a chemical synapse. (4) Inhibitory chemical synapses occur onto the motor giant at the day of hatching, and these show similar physiological characteristics to those which occur onto the motor giant in adults. (5) In some preparations, the giant fibre to motor giant electrical synapse shows rectification similar to that in the adult, but in most cases both depolarizing and hyperpolarizing current injected into the medial giant spreads to the motor giant. (6) Current spread from the medial giant to the motor giant is increased by hyperpolarizing the motor giant neuron, even when medial giant to motor giant transmission is apparently non-rectifying. (7) Both the giant fibre and the motor giant have resting potentials of about -90 mV. There is no standing difference in resting potential as there is in the adult. This may explain the apparent lack of medial giant to motor giant rectification observed in most preparations.

A voltage-dependent gap junction in Drosophila melanogaster

Steady-state and kinetic analyses of gap junctional conductance, gi, in salivary glands of Drosophila melanogaster third instar larvae reveal a strong and complex voltage dependence that can be elicited by two types of voltages. Voltages applied between the cells, i.e., transjunctional voltages, Vj, and those applied between the cytoplasm and the extracellular space, inside-outside voltages, Vi,o, markedly alter gj. Alteration of Vi-o while holding Vj = O,i.e., by equal displacement of the voltages in the cells, causes gj to increase to a maximum on hyperpolarization and to decrease to near zero on depolarization. These conductance changes associated with Vi-o are fit by a model in which there are two independent gates in series, one in each series, one in each membrane, where each gate is equally sensitive to Vi-o and exhibits first order kinetics. Vj's generated by applying voltage steps of either polarity to either cell, substantially reduce gj. These conductance changes exhibit complex kinetics that depend on Vi-o as well as Vj. At more positive Vi-o's, the changes in gj have two phases, an early phase consisting of of a decrease in gj for either polarity of Vj and a later phase consisting of an increase in gj on hyperpolarizing either cell and a decrease on depolarizing either cell. At negative Vi-o's in the plateau region of the gj-Vi-o relation, the later slow increase in gj is absent on hyperpolarizing either cell. Also, the early decrease in gj for either polarity of Vj is faster the more positive the Vi-o. The complex time course elicited by applying voltage steps to one cell can be explained as combined actions of Vi-o and Vj, with the early phase ascribable to Vj, but influenced by Vi-o, and the later phase to the changes in Vi-o associated with the generation of Vj. The substantially different kinetics and sensitivity of changes in gj by Vi-o and Vj suggests that the mechanisms of gating by these two voltages are different. Evidently, these gap-junction channels are capable of two distinct, but interactive forms of voltage dependence.

Voltage-dependent properties of electrical synapses formed between identified leech neurones in vitro

1. The voltage-dependent properties of rectifying and non-rectifying electrical synapses formed between identified leech neurones were quantified during their regeneration in vitro. 2. Junctional conductance increased with time in culture. This was evaluated by making comparisons between cell pairs maintained in vitro for differing amounts of time, as well as by taking repeated measurements from a single cell pair at different time intervals. 3. Non-rectifying electrical synapses were formed between certain identified neurones of the same type. Thus, Leydig cells cultured with Leydig cells established non-rectifying electrical connections, as did Retzius cells, longitudinal motoneurones (L cells) and anterior pagoda (AP) cells, each paired with its own cell type. 4. Rectifying synapses developed when sensory neurones (P cells or N cells) were paired with the other neurones mentioned above that form non-rectifying connections between themselves. The cell combinations examined were L cell-P cell. Leydig cell-N cell, and AP cell-P cell. The direction of current flow across these rectifying synapses was consistently from the sensory neurone to the other cell in the pair. 5. Non-rectifying connections early in the process of synapse regeneration (1-3 days) showed non-linearities greater than those observed in established non-rectifying synapses. There was a subtle, but clear, voltage dependence even at the later stages of synapse formation (4-18 days). 6. In contrast to non-rectifying connections, rectifying synapses formed between cells at early times in culture showed less voltage dependence than those observed at later times. 7. The marked non-linearities of the non-rectifying connections at early stages in synapse formation along with the reduced voltage dependence of the rectifying connections within the same time period revealed unexpected similarities between the two. At the early stages of synapse formation, the two types of electrical synapse were essentially indistinguishable for one direction of junctional current.

Direct evidence for electrical coupling among rat supraoptic nucleus neurons

Transfer of the fluorescent dye, Lucifer yellow (LY), from an intracellularly injected neuron to one or more other neurons is accepted as indirect evidence of electrotonic interactions among such dye coupled cells. Direct evidence requires that at least two coupled cells be recorded from simultaneously and such evidence in the CNS has been gained only for hippocampal pyramidal neurons. Since interpretations of the functional significance of dye coupling among magnocellular neuroendocrine cells depend upon its relation to electrical coupling, we sought to obtain direct evidence for electrotonic interactions in such neurons. Over 150 pairs of supraoptic nucleus (SON) neurons in hypothalamic slices were recorded from intracellularly using one LY and one potassium acetate electrode in each instance. Of these, 9 pairs were studied in sufficient detail to determine that they were electrically coupled. Most of the remaining pairs were determined not to be coupled. In each coupled pair of cells, membrane voltage changes due to spontaneously occurring or current evoked action potentials, as well as current evoked hyperpolarizations, in one cell were reflected in similar, though attenuated changes in the other cell. All of these changes occurred simultaneously in the two neurons. Spontaneously arising postsynaptic potentials in the two cells were temporally uncorrelated. In each case that electrical coupling was observed, dye coupling resulted from LY injection. Coupling ratios ranged from 0.05 to 0.2. Capacitative coupling between the recording electrodes as an artifact was ruled out since cells in the same tissue penetration as the coupled cell showed no responses to membrane voltage changes in the primary cell; no responses were seen with the second electrode placed extracellularly or in the medium; and similar coupling potentials were also seen when one cell was recorded without a second electrode present. We conclude that electrical coupling exists among magnocellular neurons of the SON and that the incidence of dye coupling is a reasonable estimate of the incidence of electrical coupling. These electrotonic interactions probably play important roles in the coordination of firing among magnocellular neurosecretory neurons.

Voltage-clamp analysis of a crayfish rectifying synapse

1. The rectifying crayfish giant motor synapse has been studied in the second abdominal ganglion, using the double-voltage-clamp technique which allowed direct measurements of junctional current at various fixed transjunctional potentials. 2. The transjunctional potential (Vj), defined as the difference between the voltages recorded in the lateral giant axon and the giant motor fibre, was varied from -70 to +50 mV, the minimum and maximum junctional chord conductances (gmin and gmax, respectively) were found to be 1.2 +/- 1.3 microS (n = 10) and 22.9 +/- 6.3 microS (n = 10), respectively. 3. For a given Vj, changes in the lateral giant axon or giant motor fibre membrane potential over a range of +/- 30 mV around their resting levels did not influence the junctional permeability (gj), indicating that the inside-outside potential of the junctional channel does not control gj. 4. Therefore, the steady-state junctional chord conductances were dependent only upon Vj. 5. The voltage dependence of the chord conductance was well fitted by a modified Boltzmann relation given by the equation (Formula: see text) with the constants: A = 0.15 +/- 0.03 mV-1 (n = 10) and V0 = 28 +/- 4 mV (n = 10); the latter two parameters were also found to be independent of both transmembrane potentials. 6. The junctional currents were already constant 1 ms after step changes in the junctional voltage; this was three orders of magnitude faster than the other known examples of voltage-controlled gap junctions between embryonic cells. 7. Our results may be interpreted by a highly voltage-dependent probability of opening of the junctional channels. They also suggest that the gap-junction channels forming the giant motor synapse respond very rapidly to potential and that the hemi-channels which constitute them may not be symmetric.

The mechanism of rectification at the electrotonic motor giant synapse of the crayfish

The synapse between the giant interneurone and the motor giant axon of the crayfish is a well-known example of the rare class of current-rectifying electrotonic synapses. One early proposal for the basis of this rectification was that rectifying junctions are like diodes. Biological correlates of diodes can exist, such as constant-field channels which rectify by very high-speed rearrangements of charge carriers, but these require high selectivity and large concentration gradients. Electrotonic synapses are believed to be composed of wide-bore (1-2 nm) gap-junction channels which have poor selectivity and bridge similar intracellular compartments. An alternative mechanism for rectification would be by voltage-dependent gates that sense trans-synaptic potential. These two mechanisms can be distinguished because a diode should rectify instantaneously (on a biological time-scale) while a gated channel should show kinetic processes. Although a gating model is more consistent with the known behaviour of channels than a diode model, previous work has failed to find any time course for the rectification. We have now developed a high-quality voltage clamp and by working at reduced temperatures we are able to demonstrate channel kinetics. These results support the hypothesis that this rectifying synapse contains voltage-dependent gates.

The giant fiber and pectoral fin adductor motoneuron system in the hatchetfish

In the medulla of the hatchetfish each Mauthner fiber forms chemical synapses on a number of large myelinated axons termed giant fibers. The giant fibers form rectifying electrotonic synapses on pectoral fin adductor motoneurons, and in this fish bilateral pectoral fin adduction is an important component of the Mauthner fiber-mediated escape reflex. The branching patterns of giant fibers were determined by intracellular injection of Lucifer yellow. Dye coupling to the motoneuron somata was not observed, although a low level of transfer might have been obscured by autofluorescence. Individual giant fibers terminate primarily on pectoral fin motoneurons contralateral to their cell bodies, but may also send a branch back across the midline to ipsilateral motoneurons. The rostral process of each giant fiber ends on neurons presumably associated with cranial musculature. The number and geometry of the pectoral fin motoneurons were determined using Golgi and Nissl staining and serial reconstruction methods.

Voltage-dependent dye coupling at a rectifying electrotonic synapse of the crayfish

At the crayfish giant motor synapse, the lateral giant axon (l.g.a.) and the giant motor fibre (g.m.f.) form an electrotonic junction which exhibits two states of ionic coupling (Furshpan & Potter, 1959a; Giaume & Korn, 1983). Junctional conductance is low at resting membrane potentials (i.e. with lateral axon more negative than the motor fibre) and high when the polarity of the voltage difference (delta V) across the synapse is reversed. For these two states of conductance, junctional permeability was investigated using the intercellular tracer Lucifer Yellow. The dye was ionophoretically injected into either the presynaptic (l.g.a.) or the post-synaptic (g.m.f.) cell. In the high conductance state (delta V greater than 0), fluorescence was detected in both neurones whether Lucifer Yellow had been injected pre- or post-synaptically. By contrast, at the resting junctional polarization (delta V less than 0) Lucifer Yellow spread from the giant axon to the g.m.f., but not from the g.m.f. to the giant axons. These data demonstrate that dye transfer at the giant motor synapse, like ionic coupling, is sensitive to junctional polarization and is more marked in the high conductance state. Possible explanations for the asymmetry observed in the low conductance state are discussed.

Electrical potentials and cell-to-cell dye movement in mouse mammary gland during lactation

Stable potentials were recorded with microelectrodes in an in vivo preparation of the mammary gland from the anesthetized lactating mouse. Location of the microelectrode tip was determined by ionophoretic injection of the fluorescent dye Lucifer yellow CH. Fifteen dye injections were localized to mammary alveolar cells; the average recorded potential for these penetrations was -49 +/- 2 mV. Cell-to-cell dye transfer between alveolar cells was observed with all intracellular Lucifer yellow injections. Ten dye injections were localized to the alveolar lumina with an average recorded potential of -35 +/- 2 mV. With these penetrations Lucifer yellow spread rapidly to many alveolar lumina. These findings indicate that stable potentials can be obtained from both cells and lumina in the in vivo mammary gland, demonstrating the feasibility of electrophysiological studies of the mammary epithelium. The presence of a large transepithelial potential provides evidence for physiologically tight junctions between mammary alveolar cells. In addition, the distribution of Lucifer yellow shows that mammary alveolar cells are coupled and suggests that milk flows freely between alveolar lumina.