N-methyl-D-aspartate activates different channels than do kainate and quisqualate

In the mammalian central nervous system, the excitatory amino acid transmitter L-glutamate activates three pharmacologically distinguishable receptors, the N-methyl-D-aspartate (NMDA), kainate, and quisqualate receptors. The present paper addresses the issue of whether these three receptors operate independent channels or whether they share channels that may have several conductance substates. The Xenopus oocyte provides a system for expression of exogenous mRNAs that permits detailed study of receptor structure and function. In oocytes injected with rat brain mRNA, NMDA has a stoichiometry of channel activation different from that for kainate and quisqualate. NMDA activates its own channels as indicated by simple summation or near-summation of currents evoked by NMDA with those evoked by quisqualate or kainate. Deviations from summation are ascribable to lack of selectivity in which an agonist at one receptor acts as a weak antagonist at another receptor. A further indication of separate channels is that block of NMDA channels by Mg2+ or phencyclidine has no effect on kainate or quisqualate responses evoked during the block. Interactions of kainate and quisqualate are more complex, but they can be explained by lack of complete specificity of these agonists for their own receptors.

mRNA from NCB-20 cells encodes the N-methyl-D-aspartate/phencyclidine receptor: a Xenopus oocyte expression study

The mouse neuroblastoma--Chinese hamster brain hybrid cell line NCB-20 is the only clonal cell line in which binding studies indicate the presence of phencyclidine (PCP) receptors. We report here that Xenopus oocytes injected with NCB-20 cell poly(A)+ RNA express N-methyl-D-aspartate (NMDA)-activated channels and that these channels include the PCP receptor site. In injected oocytes, NMDA application evoked a partially desensitizing inward current that was potentiated by glycine, blocked by the competitive antagonist D-2-amino-5-phosphonovaleric acid, blocked by Mg2+ and by Zn2+, and blocked in a use-dependent manner by the PCP receptor ligands PCP and MK-801. There was little or no response to kainate or quisqualate (agonists of the other excitatory amino acid receptors), to gamma-aminobutyric acid (an inhibitory transmitter), or to glycine (an inhibitory transmitter as well as an allosteric potentiator of NMDA channels). Thus, NMDA/PCP receptors expressed from NCB-20 cell mRNA exhibit properties similar to those of the neuronal receptors. The absence of expression of other excitatory amino acid receptors in this system makes it particularly useful for study of NMDA-evoked responses without interference from responses mediated by other receptors. Moreover, NCB-20 mRNA may be an appropriate starting material for cloning the cDNA(s) encoding the NMDA/PCP-receptor complex.

Satellite glial cells penetrate neurosecretory cells to perinuclear position in the goldfish preoptic area

Some goldfish neurosecretory cells have plasma membrane invaginations filled by processes of surrounding satellite glial cells (SCs) that produce trophospongium-like multicellular neuron-glial aggregates. Some penetrating SC processes approach the neuronal nucleus, reaching to within approximately 40 nm of the outer nuclear membrane. Gap junctions were found in one freeze-fracture replica through an apparent neuronal-glial aggregate, suggesting that neuron-glial gap junctions may be present. The extensive covering and penetration of these neurons by SCs suggests trophic relationships and communication by undetermined modalities between neurons and glia. The common close proximity of invaginated SC processes to the neuronal nucleus may indicate that information is transmitted between SCs and the nucleus. Some SCs abut against the basal lamina of large blood vessels and contain dense vesicles, either secretory or lysosomal.

Neuronal and glial gap junctions in the goldfish preoptic area, a thin section and freeze-fracture study

In freeze-fracture, both large macular gap junctions and long thin gap junctions surrounded by a strand of tight junction were found on neurosecretory cells. Preoptic neurons show large areas of soma-to-soma apposition, but thin section showed no evidence for gap junctions between neuronal somata. Neurosecretory cell neurites formed parallel bundles in neuropil lateral to the nucleus, and gap junctions were found between the neurites. These junctions apparently correspond to macular junctions seen on neurosecretory elements in freeze-fracture. Some large macular gap junctions found in freeze-fracture presumably correspond to junctions seen between glial cells in thin section. However, glial membranes lacked characteristics distinguishing them from neuronal membranes. In one instance, a large apparent glial sheet process formed both macular and long thin gap junctions on different surfaces. The long thin gap junctions that were surrounded by a strand of tight junction were formed with a large neurosecretory cell soma. Extensive pinocytosis was observed at some membranes forming gap junctions.

Gap junctions in goldfish preoptic ependyma: regional variation in cellular differentiation

Ependyma adjacent to the goldfish preoptic neurosecretory nucleus was examined with transmission electron microscopy. Ependymal cells adjoining the rostroventral end of the nucleus were spindle-shaped with their long axes perpendicular to the ventricular surface. Gap junctions and desmosomes were common near the apical (ventricular) ends of these cells, and less frequent laterally in the ependymal layer. Ependymal cells in more caudodorsal preoptic regions (adjacent to large neurosecretory cells) were progressively more pleomorphic. The frequent occurrence of apparently internalized gap junctions and of gap junction fragments enclosed within lysosome-like organelles indicated extensive turnover of these junctions, or uncoupling. Ependymal cells in the caudodorsal region formed gap junctions on their lateral and basal (abluminal) surfaces with glial processes containing bundles of intermediate filaments. Subependymally, these processes (presumptive radial glia) were parallel to one another and coupled together by gap junctions. Neurites containing dense core vesicles occasionally invaginated into ependymal cells in the caudal region, but did not appear to form gap junctions. Previous observations indicate continuing maturation and growth of the goldfish preoptic area with neurosecretory cell formation rostroventrally and a rostroventral to caudodorsal gradient of maturation. The present findings suggest a parallel and related gradient in preoptic ependyma. Ependymal cell differentiation possibly involves loss of gap junctions, and radial migration or differentiation into underlying neurons and glia.

The elasmobranch spiracular organ. I. Morphological studies

The spiracular organ is a lateral line derived receptor associated with the first gill cleft (spiracle). Its functional morphology was studied in the little skate, Raja erinacea, and a shark, the smooth dogfish, Mustelus canis, with light and electron microscopy. The spiracular organ is a tube (skate) or pouch (shark) with a single pore opening into the spiracle. The lumen is lined with patches of sensory hair cells, and filled with a gelatinous cupula. In the little skate, hair cells form synapses with afferents but apparently not with efferent fibers. In both species, the spiracular organs are deformed by flexion of the hyomandibular cartilage at its articulation with the cranium. The hyomandibula is a suspensory element of the jaws; hyomandibular flexion results in jaw protrusion. The little skate spiracular organ is anchored at one end to the cranium and at the other to the hyomandibula so that it is stretched or relaxed during hyomandibular extension and flexion, respectively. In Mustelus, the effects of hyomandibular flexion on the spiracular organ are mediated indirectly by the superior post-spiracular ligament which inserts on the distal end of the hyomandibula. Deformation of the dogfish shark cupula during hyomandibular movement was observed. In the little skate, as revealed by transmission electron microscopy, there is a measurable deflection of the hair cell ciliary bundles from spiracular organs fixed with the hyomandibula in the flexed relative to the extended positions. In both species, hyomandibula flexion should result in hair cell depolarization, and sensory afferent excitation, based on the direction of the observed (skate) or expected (shark) deflection of hair cell cilia.

Coexpression of N-methyl-D-aspartate and phencyclidine receptors in Xenopus oocytes injected with rat brain mRNA

Recent evidence suggest that the N-methyl-D-aspartate (N-Me-D-Asp) channel is functionally and structurally associated with the phencyclidine (PCP) receptor, which mediates the psychotomimetic effects of PCP, sigma opioids, and dioxalanes. To investigate the relationship between N-Me-D-Asp and PCP receptors on a molecular level, we injected mRNA isolated from adult rat brain into Xenopus oocytes. In injected oocytes N-Me-D-Asp application (with glycine) evoked a partially desentizing inward current that was potentiated by glycine and blocked by D-(-)-amino-5-phosphonovaleric acid (D-APV), by Zn2+ and, in a voltage-dependent manner, by Mg2+. These results show that the distinguishing features of rat brain N-Me-D-Asp channels are reproduced in this translation system. In addition, kainic acid elicited a nondesensitizing inward current at short latency, and quisqualate elicited a delayed oscillatory inward current, presumably mediated by a second-messenger system. Responses to glutamate had both short-latency and delayed components. The PCP derivative N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) blocked the N-Me-D-Asp-evoked current, and its potency was comparable to its binding affinity in rat brain membranes. Onset of block required the presence of antagonist. Antagonism was stereoselective in that the active ligand dexoxadrol was a more effective blocker than its relatively inactive stereoisomer levoxadrol. adrol. Other PCP receptor ligands, (+)SKF-10,047 and MK-801, also blocked. Potencies of compounds active at N-Me-D-Asp and PCP receptors in oocytes were comparable to those obtained previously in electrophysiological and binding assays on neural tissues. These results indicate the coexpression of neuronal PCP and N-Me-D-Asp receptors in Xenopus oocytes.

The elasmobranch spiracular organ. II. Physiological studies

The spiracular sense organs of the little skate, Raja erinacea, and the smooth dogfish, Mustelus canis, respond to movements of the hyomandibula-cranial joint. Afferent activity was recorded from the spiracular organ nerve in isolated preparations consisting of at least part of the cranium, the hyomandibula, and the spiracular organ and nerve. Afferents are excited by hyomandibular flexion at its joint with the cranium. Single unit recordings in the little skate revealed a single class of units that were slowly adapting, and had a regular firing pattern. Single unit firing rate increased up to about 70 spikes/s during hyomandibular flexion from a spontaneous rate at rest of 15-20 spikes/s, and could often be silenced by hyomandibular extension. The direction of excitation is consistent with the orientation of the hair cell ciliary bundles observed in morphological studies (Barry et al. 1988). Local deformations of the cupula are sufficient to excite or inhibit primary afferent firing, and volume changes in the spiracular organ as a whole are not necessary. The spiracular organs are relatively insensitive to electrical stimuli, vibration, or water movement. In conclusion, the spiracular organ functions as a sensitive joint receptor.

Carbon tetrachloride at hepatotoxic levels blocks reversibly gap junctions between rat hepatocytes

Electrical coupling and dye coupling between pairs of rat hepatocytes were reversibly reduced by brief exposure to halogenated methanes (CBrCl3, CCl4, and CHCl3). The potency of different halomethanes in uncoupling hepatocytes was comparable to their hepatotoxicity in vivo, and the rank order was the same as that of their tendency to form free radicals. The effect of carbon tetrachloride (CCl4) on hepatocytes was substantially reduced by prior treatment with SKF 525A, an inhibitor of cytochrome P-450, and by exposure to the reducing reagent beta-mercaptoethanol. Halomethane uncoupling occurred with or without extracellular calcium and did not change intracellular concentrations of calcium and hydrogen ions or the phosphorylation state of the main gap-junctional protein. Thus the uncoupling appears to depend on cytochrome P-450 oxidative metabolism in which free radicals are generated and may result from oxidation of the gap-junctional protein or of a regulatory molecule that leads to closure of gap-junctional channels. Decreases in junctional conductance may be a rapid cellular response to injury that protects healthy cells by uncoupling them from unhealthy ones.

Superoxide dismutase protects cultured neurons against death by starvation

Brief substrate deprivation resulted in high mortality of superior cervical ganglion neurons in culture, assayed 2 hr later by trypan blue exclusion. Involvement of superoxide anions was indicated by several observations. Survival was increased significantly by prior treatment that induced cells to take up superoxide dismutase. During starvation, neurons reduced nitroblue tetrazolium to form the blue precipitate formazan, and the color change was blocked in neurons preloaded with superoxide dismutase. The incidence of staining was comparable to the mortality. In many cells, brief starvation caused the appearance of fluorescence due to oxidation of 2',7'-dichlorofluorescin to dichlorofluorescein, which indicates that oxidants were generated intracellularly. In some cells fluorescence was transient, as would be caused by membrane breakdown, and these cells were then shown to be dead. Superoxide generation caused by substrate deprivation may contribute importantly to cell damage in a variety of pathological conditions.

Effects of rapid cerebellectomy on adaptive gain control of the vestibulo-ocular reflex in alert goldfish

In goldfish, adaptive gain control of the vestibuloocular reflex (VOR) is blocked by cerebellectomy. The operation was rapidly performed on alert goldfish before and after extended periods of adaptive gain training of the VOR produced by sinusoidal oscillation in the horizontal plane. The VOR in these conditions was abolished by sectioning the horizontal semicircular canals. Removal of the cerebellum from naive goldfish resulted in VOR gains significantly greater than 1 at all frequencies tested, with an average value near 1.4 at 1/8 Hz. This value represents an increase of about 65% over the initial VOR gain of 0.85. Changes in phase of the reflex were negligible. Cerebellectomy in animals previously trained to higher or lower gains immediately produced the same mean gain as in cerebellectomized naive animals; gains were increased in animals trained to lower gains and decreased in animals trained to higher gains. As little as 1 min separated aspiration and subsequent gain measurements. These results suggest that the cerebellum not only acts on extra cerebellar circuitry during the training, but that it is also involved in retaining the altered VOR gain. Adaptive gain control could not be achieved with prolonged training after cerebellectomy; in addition, cerebellectomy did not affect the response to visual stimulation at the onset of training to decrease or increase gain.

Pharyngeal movements during feeding sequences of Navanax inermis (Gastropoda: Opisthobranchia) in successive stages of dissection

Feeding in Navanax inermis Cooper was filmed and analysed after various dissections. In preparations with a cut through the body wall exposing the pharynx and buccal ganglia, completely normal feeding was observed. In addition to seven motor acts previously described in intact animals, an eighth act, peristalsis, was observed. In preparations with the pharynx excised from the animal but attached to the buccal ganglia, four motor acts were observed: flaring, expansion, contraction and peristalsis. In addition to increasing information about the nature of feeding movements in Navanax, these data indicate that preparations suitable for neurophysiological studies are capable of producing a variety of feeding acts.

Gap junctional conductance and permeability are linearly related

The permeability of gap junctions to tetraethylammonium ions was measured in isolated pairs of blastomeres from Rana pipiens L. and compared to the junctional conductance. In this system, the junctional conductance is voltage-dependent and decreases with moderate transjunctional voltage of either sign. The permeability to tetraethylammonium ions was determined by injecting one cell of a pair with tetraethylammonium and monitoring its changing concentration in the prejunctional and postjunctional cells with ion-selective electrodes. Junctional conductance was determined by current-clamp and voltage-clamp techniques. For different cell pairs in which the transjunctional voltage was small and the junctional conductance at its maximum value, the permeability to tetraethylammonium ions was proportional to the junctional conductance. In individual cell pairs, a reduction in the junctional conductance induced by voltage was accompanied by a proportional reduction in the permeability of the gap junction over a wide range. The diameter of the tetraethylammonium ion (8.0 to 8.5 A, unhydrated) is larger than that of the potassium ion (4.6 A, hydrated), the predominant current-carrying species. The proportionality between the permeability to tetraethylammonium ions and the junctional conductance, measured here with exceptionally fine time resolution, indicates that a common gap junctional pathway mediates both electrical and chemical fluxes between cells, and that closure of single gap junction channels by voltage is all or none.

Isolated liver gap junctions: gating of transjunctional currents is similar to that in intact pairs of rat hepatocytes

We have shown previously that conductance of rat liver gap junctions is blocked by an affinity-purified polyclonal antibody generated against rat liver junctional membranes, is not affected by moderate transjunctional or transmembrane potentials, and is reversibly decreased by cytoplasmic acidification and perfusion with octanol. We have now recorded currents from isolated liver gap junctions using patch electrodes dipped through a layer of mixed lipids whose concentrations match those of isolated liver appositional membranes. These currents are blocked by the same polyclonal antibody, are insensitive to moderate voltages imposed across the pipette tip, and are reversibly blocked by similar concentrations of H ions and octanol as are junctions in situ. The currents are likely to be gap junctional in origin; their block by low pH and other agents indicates that the gating mechanisms are intrinsic to the gap junctions themselves and presumably result from conformational change in the channel-forming protein.

Changes in gain of the vestibulo-ocular reflex induced by sinusoidal visual stimulation in goldfish

The effects of sustained sinusoidal visual stimulation on the vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) were investigated. Goldfish were held stationary inside a striped drum rotating sinusoidally about the vertical axis for 3 h. The VOR gain, the ratio of eye to head rotational velocities, was measured in the dark with passive sinusoidal rotation of the fish and showed modest increases that were greatest at the stimulation frequency. Furthermore, the fish generated spontaneous sinusoidal eye movements at approximately the stimulation frequency, and these movements summated with the response to other frequencies of vestibular stimulation in the dark. It is hypothesized that the pathways of OK and VO stimuli converge and that the animal increases gain in a common part when it attempts to stabilize the visual image by increasing its response to the OK signal. Thus increases in gain of both OKR and VOR are produced.

Changes in gain of the vestibulo-ocular reflex induced by combined visual and vestibular stimulation in goldfish

Adaptive changes in the vestibulo-ocular reflex (VOR) of goldfish were produced in a few hours by sinusoidally rotating restrained fish in the horizontal plane inside a vertically striped drum. The drum could also be sinusoidally rotated so that the gain of the VOR (the ratio of eye to head angular velocity) would have to increase to two or decrease to zero in order to maintain a stable retinal image. During 'training' towards two VOR gain measured at the stimulation frequency of 0.125 Hz increased rapidly over 6 h of stimulation to about 1.5 from an initial gain of 0.7. Half of that change occurred in the first 30 min. During training towards zero VOR gain measured at the stimulation frequency decreased to 0.15. About one-third of that change occurred in the first 30 min. Testing at different sinusoidal frequencies after 6 h stimulation showed that increases in VOR gain were generated across a 6-octave range; however, reductions in gain were produced over a narrow frequency range close to the training frequency. Gain reductions occurred more rapidly on a second day of stimulation. In a paradigm simulating reversing prisms, partial reversal of the VOR was observed in some fish. However, these fish also demonstrated spontaneous slow sinusoidal eye movements that may have represented a different means of adjusting eye movements to stabilize the retinal image. Goldfish provide a useful preparation for the study of adaptive gain changes in vertebrate oculomotor systems.

Sensitivity of gap junctional conductance to H ions in amphibian embryonic cells is independent of voltage sensitivity

In vertebrate embryos gap junctional conductance (gj) is reduced by transjunctional voltage (Vj) and by cytoplasmic acidification; in each case sensitivity is comparable to those of other channels gated by voltage and ligand-receptor binding. We show here that the mechanisms by which Vj and intracellular pH (pHi) gate gj are apparently independent. Partial reduction of gj by lowering pHi neither attenuates nor enhances further reduction by Vj. Certain drugs irreversibly (glutaraldehyde, 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) or reversibly (retinoic acid) abolish dependence of gj on pHi without appreciably affecting kinetic properties of voltage dependence or the shape of the steady-state Vj-gj relation. These findings suggest that the mechanisms by which pHi and Vj act on the gap junction are at least partially distinct and presumably involve separate regions of the junctional macromolecules.

cAMP increases junctional conductance and stimulates phosphorylation of the 27-kDa principal gap junction polypeptide

Membrane-permeant cAMP derivatives (dibutyryl- and 8-bromo-cAMP) increase gap-junctional conductance within minutes when applied to voltage-clamped pairs of rat hepatocytes. Glucagon also increases junctional conductances, but the response has a more rapid onset and is more rapidly reversible. The glucagon effect can be prevented by intracellular injection of the protein inhibitor of the cAMP-dependent protein kinase (Walsh inhibitor), indicating that the catalytic subunit of cAMP-dependent protein kinase is directly involved. The 27-kDa major gap junction polypeptide is phosphorylated when liver cells dissociated into small groups are incubated with 32P. Addition of 8-bromo-cAMP to cells increases the incorporation of 32P into the 27-kDa junctional protein. Serine is the amino acid residue that is phosphorylated. When isolated liver gap junctions are incubated in the presence of catalytic subunit of the cAMP-dependent protein kinase, the 27-kDa gap junction polypeptide is phosphorylated with low stoichiometry on serine. The rapid increases in gap junctional conductance caused by agents that elevate cAMP and phosphorylation of the gap junction protein by cAMP-dependent protein kinase suggest that cAMP-dependent phosphorylation of the gap junction channel modulates the conductance of liver gap junctions.

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.

Some electrical and pharmacological properties of gap junctions between adult ventricular myocytes

Ventricular myocytes were isolated from adult rat hearts using the technique of Wittenberg and Robinson (Cell Tissue Res. 216: 231-251, 1981). These cells exhibited morphology, input resistance, time constant, and excitability expected for cells in intact cardiac tissue. Pairs of these cells were electronically coupled, and junctional conductance was unaffected by transjunctional potential or hyperpolarization of both cells. Brief exposure of cell pairs to medium equilibrated with 100% CO2 or containing 0.1 mM octanol quickly and reversibly decreased junctional conductance. We conclude that gap junctions between pairs of ventricular myocytes possess physiological properties like those of junctions in many other tissues. This preparation will be useful in evaluating drug action on junctional communication in heart.

Ultrastructure of the rectifying electrotonic synapses between giant fibres and pectoral fin adductor motor neurons in the hatchetfish

Synapses formed by giant fibres on pectoral fin adductor motor neurons were identified by horseradish peroxidase (HRP) injection. The synapses were distributed in clusters on the somata and proximal dendrites of the motor neurons. All of the labelled synapses contained synaptic vesicles and often had clearly defined active zones characteristic of chemical synapses. Some synapses also showed gap junctions with the motor neuron soma, often directly adjacent to an active zone. The gap junctions were asymmetrical, with a thick layer of electron dense material on the postsynaptic side. Previous electrophysiological data indicate that giant fibre inputs to motor neurons are purely electrotonic and that these electrical synapses rectify.

Regulation of gap junctional conductance

Gap junctional conductance is regulated by the number of channels between coupled cells (the balance between formation and loss of these channels) and by the fraction of these channels that are open (gating mechanisms). A variety of treatments are known to affect junction formation. Adenosine 3',5'-cyclic monophosphate (cAMP) is involved in some cases, and protein synthesis may be required but precursor molecules can also exist. Junction removal occurs both by dispersion of particles and by internalization of junctional membrane. Factors promoting removal are not well understood. A variety of gating mechanisms exist. Coupling may be controlled by changes in conductance of nonjunctional membranes. Several kinds of voltage dependence of junctional conductance are known, but rat ventricular junctions at least are electrically linear. Cytoplasmic acidification decreases conductance of most gap junctions. Sensitivity in rat ventricular myocytes allows modulation of coupling by moderate changes near normal internal pH. Increasing intracellular Ca also decreases junctional conductance, but in the better studied cases sensitivity is much lower to Ca than H. A few data support low sensitivity to Ca in cardiac cells, but quantitative studies are lacking. Higher alcohols such as octanol block junctional conductance in a wide range of tissues including rat ventricular myocytes. An antibody to liver gap junctions blocks junctions between rat ventricular myocytes. Cross reactivity indicates at least partial homology between many gap junctions. Although differences among gap junctions are known, a general physiology is being developed, which may have considerable relevance to normal cardiac function and also to conduction disorders of that tissue.

Cell junctions in early embryos of squid (Loligo pealei)

Squid embryos examined by freeze-fracture and thin-section electron microscopy exhibit identifiable gap junctions during mid-cleavage stages (stages 7-8), and junctional complexes composed of adherent appositions, elaborate septate junctions and gap junctions at slightly later stages (stages 12-13). During germinal layer establishment (stages 12-13) cytoplasmic bridges frequently link the embryonic cells. The presence of gap junctions in cleavage-stage embryos provides the morphological substrate for a demonstrated pathway of direct cell-cell communication that is modifiable by experimental treatments and may be physiologically regulatable. The existence of septate junctions and adherent contacts at later stages suggests that some functional specialization, perhaps the establishment of a strongly joined framework of cells at the surface of the embryo, accompanies the formation of germinal layers.