80
MICHAEL LOCKE
tend round the cell. They occur between the cells towards the cuticular
side but usually not immediately next to it. On the haemocoel side there
may be a large intercellular space and the desmosomes may have the
mechanical function of preventing cells from parting further. Septate
desmosomes also occur densely on the processes which epidermal cells extend between one another, and on tracheolar extensions. Near to the
cuticle the membranes may be closely adpressed in tight junctions and
intermediate junctions. The intercellular material in the tight junctions
is frequently discontinuous with a repeat pattern, like the junctions in
synaptic disc-membrane complexes (Robertson, 1964).
The permeability of junctions between salivary gland cells in Drosophila has been studied by Loewenstein and Kanno (1964) and Weiner
et al. (1964). They found that the electrical resistance across these junctions with many septate desmosomes was only slightly greater than that
in cytoplasm alone. In contrast, the diffusion resistence along the
intercellular space to the exterior was very high, so that functionally it is
part of the cell. Small ions and molecules such as fluorescein can move
freely from one cell to the next with no loss to the outside.
If the septate desmosomes in the epidermis have similar properties to
those in Drosophila salivary glands, we can see how a selective arrangement of septa on certain faces of the cells could influence the distribution
of molecules. High permeability would reduce differences in concentration
in line with the septa. In the direction at right angles to this, lower permeability could allow differences in concentration to be maintained efficiently
with little leakage. A logical line of research would seem to be to look for
a correlation between the distribution of junctions and the orientation of
the gradient.
If junctions are concerned in polarity then we are faced with the problem of what determines the distribution of the junctions. The problems
posed are similar to those concerned with the specificity of nerve pathways, is the junction formed in response to an impulse, or does the impulse span that gap because a particular junction is already there? It
may be significant that the tight junctions between insect epidermal cells
are similar to those in some nervous connections (Robertson, 1964), but
then so are the tight junctions between cells in many epithelia (Farquhar
and Palade, 1963).
If junctions are concerned in both gradient phenomena and electrical
activity, it would help to explain the reversal of morphological polarity
by the external application of electrical fields in regenerating flatworms
(Marsh and Beams, 1952; Marsh, 1962). Such fields might affect first
the junctions and their polarity, in an analogous way to nerve impulses
acting upon nerve junctions, and this in turn might influence the direction
of a gradient and regeneration.
MICHAEL LOCKE
tend round the cell. They occur between the cells towards the cuticular
side but usually not immediately next to it. On the haemocoel side there
may be a large intercellular space and the desmosomes may have the
mechanical function of preventing cells from parting further. Septate
desmosomes also occur densely on the processes which epidermal cells extend between one another, and on tracheolar extensions. Near to the
cuticle the membranes may be closely adpressed in tight junctions and
intermediate junctions. The intercellular material in the tight junctions
is frequently discontinuous with a repeat pattern, like the junctions in
synaptic disc-membrane complexes (Robertson, 1964).
The permeability of junctions between salivary gland cells in Drosophila has been studied by Loewenstein and Kanno (1964) and Weiner
et al. (1964). They found that the electrical resistance across these junctions with many septate desmosomes was only slightly greater than that
in cytoplasm alone. In contrast, the diffusion resistence along the
intercellular space to the exterior was very high, so that functionally it is
part of the cell. Small ions and molecules such as fluorescein can move
freely from one cell to the next with no loss to the outside.
If the septate desmosomes in the epidermis have similar properties to
those in Drosophila salivary glands, we can see how a selective arrangement of septa on certain faces of the cells could influence the distribution
of molecules. High permeability would reduce differences in concentration
in line with the septa. In the direction at right angles to this, lower permeability could allow differences in concentration to be maintained efficiently
with little leakage. A logical line of research would seem to be to look for
a correlation between the distribution of junctions and the orientation of
the gradient.
If junctions are concerned in polarity then we are faced with the problem of what determines the distribution of the junctions. The problems
posed are similar to those concerned with the specificity of nerve pathways, is the junction formed in response to an impulse, or does the impulse span that gap because a particular junction is already there? It
may be significant that the tight junctions between insect epidermal cells
are similar to those in some nervous connections (Robertson, 1964), but
then so are the tight junctions between cells in many epithelia (Farquhar
and Palade, 1963).
If junctions are concerned in both gradient phenomena and electrical
activity, it would help to explain the reversal of morphological polarity
by the external application of electrical fields in regenerating flatworms
(Marsh and Beams, 1952; Marsh, 1962). Such fields might affect first
the junctions and their polarity, in an analogous way to nerve impulses
acting upon nerve junctions, and this in turn might influence the direction
of a gradient and regeneration.