P A T T E R N S I N T H E I N T E G U M E N T OF I N S E C T S
79
millipore filter, rather than its porous structure. The implants with epidermal cells migrating on one side only could have had a protective coating of material precipitated from the blood. The epidermis might then
not have come into contact with the filter itself.
However, if we accept the results at their face value, they suggest that
there may normally be continual production of something necessary to
maintain the cells with the order of an epithelium. On the edge of an
excision, or when the cells are separated by a smooth inert surface, the
material is lost and the cells respond by movement. On a millipore filter
the material is trapped within the pores, with or without exchange from
one side to the other, the cells have no sense of leakage and they do not
move. The tentative conclusion is that cells migrate into an excised region because they cease to receive something diffusible from the missing
cells.
If there is something diffusible it may have no connection with gradient
phenomena. In this connection, it is of considerable interest that the
epidermis in Calpodes controls the rate of phenolase activity in abraded
epicuticle by maintaining a very low level of substrate in the cuticle with
a very high turnover (Lai-Fook, 1966a). Her calculations show that the
complete complement of phenolase substrate molecules must be replaced
on average at least every 2 min. This suggests that the cell operates a
fine homeostatic mechanism for controlling its immediate environment,
a property we should have suspected, but for which there is little other
evidence.
Although these two examples may have no connection with gradient
phenomena, they at least point to the possibility of epidermal control of
the concentration of substances which could result in a gradient. The concept of a homeostatic mechanism to maintain the concentration of a
molecule at a particular level in the immediate cellular environment
should not be confused with Stumpfs (1965a,b) notion of a gradient of
slowly diffusing molecules.
9. Structural Features Which Could Result in a Polarity
and Assist in the Maintenance of a Gradient
Any study of the interactions between cells focuses attention upon
their attachments. Three types of junction can be distinguished between
insect epidermal cells (Locke, 1965a). Septate desmosomes have a wide
distribution in invertebrates; tight junctions (zonulae occludentes) and
intermediate junctions (zonulae adhaerentes) are like the equivalent
structures in vertebrates (Farquhar and Palade, 1963; Robertson, 1964).
Maculae adhaerentes may also occur. The distribution of the septate
desmosomes is similar to the maculae adhaerentes or desmosomes of
vertebrate material, but they probably occur in strips or may even ex-
79
millipore filter, rather than its porous structure. The implants with epidermal cells migrating on one side only could have had a protective coating of material precipitated from the blood. The epidermis might then
not have come into contact with the filter itself.
However, if we accept the results at their face value, they suggest that
there may normally be continual production of something necessary to
maintain the cells with the order of an epithelium. On the edge of an
excision, or when the cells are separated by a smooth inert surface, the
material is lost and the cells respond by movement. On a millipore filter
the material is trapped within the pores, with or without exchange from
one side to the other, the cells have no sense of leakage and they do not
move. The tentative conclusion is that cells migrate into an excised region because they cease to receive something diffusible from the missing
cells.
If there is something diffusible it may have no connection with gradient
phenomena. In this connection, it is of considerable interest that the
epidermis in Calpodes controls the rate of phenolase activity in abraded
epicuticle by maintaining a very low level of substrate in the cuticle with
a very high turnover (Lai-Fook, 1966a). Her calculations show that the
complete complement of phenolase substrate molecules must be replaced
on average at least every 2 min. This suggests that the cell operates a
fine homeostatic mechanism for controlling its immediate environment,
a property we should have suspected, but for which there is little other
evidence.
Although these two examples may have no connection with gradient
phenomena, they at least point to the possibility of epidermal control of
the concentration of substances which could result in a gradient. The concept of a homeostatic mechanism to maintain the concentration of a
molecule at a particular level in the immediate cellular environment
should not be confused with Stumpfs (1965a,b) notion of a gradient of
slowly diffusing molecules.
9. Structural Features Which Could Result in a Polarity
and Assist in the Maintenance of a Gradient
Any study of the interactions between cells focuses attention upon
their attachments. Three types of junction can be distinguished between
insect epidermal cells (Locke, 1965a). Septate desmosomes have a wide
distribution in invertebrates; tight junctions (zonulae occludentes) and
intermediate junctions (zonulae adhaerentes) are like the equivalent
structures in vertebrates (Farquhar and Palade, 1963; Robertson, 1964).
Maculae adhaerentes may also occur. The distribution of the septate
desmosomes is similar to the maculae adhaerentes or desmosomes of
vertebrate material, but they probably occur in strips or may even ex-