Electrical coupling between embryonic cells by way of extracellular space and specialized junctions

Electrical coupling between fat cells in newt fat body and mouse brown fat

White fat from the newt, Triturus pyrrhogaster, fat body, and brown fat from the interscapular fat pad of newborn mice have been tested for the presence of low-resistance intercellular junctions. 42 pairs of amphibian fat cells and 15 pairs of mammalian brown fat cells were found to be "electrically coupled." In most of these cases intracellular deposition of a dye, Niagara Sky Blue: 6B, was used to supplement and confirm direct observations of impalements. Coupling was often difficult to find in both preparations, but the mechanical disturbance of the tissue during the preparative procedures may have uncoupled many cells. The fact that, in both types of fat, coupling was observed between cells separated by one or more other cells suggests that coupling may be more widespread in vivo. Electron microscopy (provided by Dr. J. -P. Revel and Mrs. K. Wolken) of the brown fat revealed frequent intercellular junctions resembling "gap junctions" but possibly lacking the substructure usually visible with colloidal lanthanum infiltration. The results are discussed in relation to current ideas about the exchange of regulatory molecules via low-resistance junctions and about the control of brown fat by hormones and nerves.

Coexistence of gap and sseptate junctions in an invertebrate epithelium

The intercellular junctions of the epithelium lining the hepatic caecum of Daphnia were examined. Electron microscope investigations involved both conventionally fixed material and tissue exposed to a lanthanum tracer of the extracellular space. Both septate junctions and gap junctions occur between the cells studied. The septate junctions lie apically and resemble those commonly discerned between cells of other invertebrates. They are atypical in that the high electron opacity of the extracellular space obscures septa in routine preparations. The gap junctions are characterized by a uniform 30 A space between apposed cell membranes. Lanthanum treatment of gap junctions reveals an array of particles of 95 A diameter and 120 A separation lying in the plane of the junction. As this pattern closely resembles that described previously in vertebrates, it appears that the gap junction is phylogenetically widespread. In view of evidence that the gap junction mediates intercellular electrotonic coupling, the assignment of a coupling role to other junctions, notably the septate junction, must be questioned wherever these junctions coexist.

Morphological correlates of increased coupling resistance at an electrotonic synapse

Close appositions between axonal membranes are present in the septum between adjacent axonal segments of the septate or lateral giant axons of the crayfish Procambarus. In sections the closely apposed membranes appear separated by a space or gap. The use of lanthanum indicates that there may be structures connecting the apposed membranes. The apparent gap is actually a network of channels continuous with the extracellular space. Adjacent axonal segments are electrotonically coupled at the septa. The coupling resistance is increased by mechanical injury of an axon, immersion in low Cl(-) solutions, and immersion in low Ca(++) solutions, followed by a return to normal physiological solution. Septa at which coupling resistance had been measured were examined in the electron microscope. The induced increases in coupling resistance are associated with separation of the junctional membranes (with the exception of the moderate increases during immersion in low Ca(++) solutions). Schwann cell processes are present between the separated axonal membranes. When nerve cords in low Cl(-) solutions are returned to normal physiological solution, coupling, i.e., electrotonic synapses. A model of an electrotonic synapse is proposed in which tween axonal membranes are again found. The association between the morphological and physiological findings provides further evidence that the junctions are the sites of electrotonic coupling, i.e., electrotonic, synapses. A model of an electrotonic synapse is proposed in which intercytoplasmic channels not open to the extracellular space are interlaced with a hexagonal network of extracellular channels between the apposed junctional membranes.

Nerve terminals and cellular junctions in young and adult mouse submandibular glands

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Experimental alteration of coupling resistance at an electrotonic synapse

Adjacent segments of the septate axon of the crayfish Procambarus are electrotonically coupled by junction located in the septa between them (see Pappas et al. 1970. J. Cell Biol.49:173). The coupling resistance at the septa was changed by several experimental treatments. Mechanical injury to an axon increased coupling resistance (more than 7-fold); no recovery of coupling resistance was observed, although the resting potential and resistance of the injured axon could return to near normal levels. Immersion in salines with Na propionate substituted for NaCl increased coupling resistance (mean: 6.1-fold). On return of the preparation to normal saline, coupling resistance recovered virtually completely. Immersion in low Ca(++) solutions moderately increased coupling resistance (3.5-fold or less), but return to normal saline was followed by large increases in coupling resistance (5-100-fold). 60 nM Ca(++) is near the maximum concentration that leads to increased coupling resistance on return to normal saline. Large increases in coupling resistance are associated with separation of junctional membranes (Pappas et al. 1970. Ibid.); calculations show that the separated membranes greatly increase in resistance. Increase in coupling resistance is probably an important response to injury. Mechanisms underlying changes reported here may be relevant to normal physiological processes of coupling and decoupling.

Differentiation of the junctional complex of surface cells in the developing Fundulus blastoderm

The structure of the junctional complex between surface cells was investigated in blastula, mid gastrula, late gastrula, and early embryo of the teleost fish Fundulus heteroclitus. In blastulae, the intercellular complex is simple and consists of an apical region where the adjacent membranes are closely apposed (40-60 A) and in places touch, an intermediate zone with a wider intercellular space (> 100 A), and incipient desmosomes. In gastrulae, there are frequent points of fusion of membranes along the apical zone of the complex. Dilatations and an increased number of desmosomes in different stages of development are found along the intermediate zone. In mid gastrula, a close or gap junction with an intercellular space of 20 A occurs below the level of the desmosomes. In late gastrula, the gap junction is reduced in extent and desmosomes are better developed. In the early embryo, the basic organization of the complex is the same, although the deeply situated close junctions are no longer apparent and desmosomes and their associated system of filaments are well developed. At this time, the junctional complex is comparable to that of many epithelia and consists of an apical zonula occludens, a short zonula adherens, and deeply situated maculae adherentes.