158
E . N . W I L L M E R
like either epitheliocytes or amoebocytes from any source. The main
problem is to understand the nature of these differences, and there are
several ways of approaching the solution.
In the first place, a little consideration of the normal histology
of the various tissues of the body leads to the conclusion that the tissues
of each organ differ morphologically much more in the arrangement
of their cells than they do in the actual cell-types which they contain.
There are really a rather more limited number of distinguishable
morphological forms of cells or of variations of the cell surface than one
might have expected in relation to the number o f different functions
performed. Moreover, most of these forms of surface are not new
differentiations on the part of the so-called higher animals but are also
to be found in many invertebrates and even among the various groups
of protozoa. Tissues in one of the higher animals differ histologically
from each other mostly in the particular ways and in the various
combinations in which these rather limited numbers of building blocks
are used. Many of the apparent differences between cell types are
quantitative differences, e.g. more or less cilia, mitochondria, rough
endoplasmic reticulum, etc. No doubt this is physiologically a very
gross over-simplification and is made easier in some respects because
of the inadequacy of our histochemical and cytochemical methods for
seeing differences.
Cells which are morphologically indistinguishable may yet differ
in many biochemical and physiological properties just as men in
black coats and white collars may be performing widely differing functions in the human community. Nevertheless the number of morphological types of epithelial cell even in the body is very small. Such cells
may be columnar with a free surface which is smooth, or which has
penetrating infoldings, ruffles or membranes, microvilli, stereocilia
(macrovilli), cilia, flagella, or the cells may be cubical and then overlaid
by the products of their own multiplication as in squamous and keratinizing epithelia or, finally, the cubical cells may be flattened into a
pavement. It is noticeable that most of these differences are, in fact,
differences in the character of the cell surface, and it is interesting to
speculate on the molecular composition of such surfaces. If cell surfaces
are primarily based on phospholipid monolayers, is it possible to explain
some or all of the variant forms in terms of the types of phospholipins,
the proportions of phospholipins between themselves and to cholesterol
or other steroids, neutral fats, etc., or to the adsorption of particular
groups of proteins, carbohydrates or other reactive agents on to the
phospholipid layers? A more detailed study of the various forms assumed
by myelin figures under different conditions might be very illuminating
in this connexion (Revel, Ito and Fawcett, 1958) as are the studies
E . N . W I L L M E R
like either epitheliocytes or amoebocytes from any source. The main
problem is to understand the nature of these differences, and there are
several ways of approaching the solution.
In the first place, a little consideration of the normal histology
of the various tissues of the body leads to the conclusion that the tissues
of each organ differ morphologically much more in the arrangement
of their cells than they do in the actual cell-types which they contain.
There are really a rather more limited number of distinguishable
morphological forms of cells or of variations of the cell surface than one
might have expected in relation to the number o f different functions
performed. Moreover, most of these forms of surface are not new
differentiations on the part of the so-called higher animals but are also
to be found in many invertebrates and even among the various groups
of protozoa. Tissues in one of the higher animals differ histologically
from each other mostly in the particular ways and in the various
combinations in which these rather limited numbers of building blocks
are used. Many of the apparent differences between cell types are
quantitative differences, e.g. more or less cilia, mitochondria, rough
endoplasmic reticulum, etc. No doubt this is physiologically a very
gross over-simplification and is made easier in some respects because
of the inadequacy of our histochemical and cytochemical methods for
seeing differences.
Cells which are morphologically indistinguishable may yet differ
in many biochemical and physiological properties just as men in
black coats and white collars may be performing widely differing functions in the human community. Nevertheless the number of morphological types of epithelial cell even in the body is very small. Such cells
may be columnar with a free surface which is smooth, or which has
penetrating infoldings, ruffles or membranes, microvilli, stereocilia
(macrovilli), cilia, flagella, or the cells may be cubical and then overlaid
by the products of their own multiplication as in squamous and keratinizing epithelia or, finally, the cubical cells may be flattened into a
pavement. It is noticeable that most of these differences are, in fact,
differences in the character of the cell surface, and it is interesting to
speculate on the molecular composition of such surfaces. If cell surfaces
are primarily based on phospholipid monolayers, is it possible to explain
some or all of the variant forms in terms of the types of phospholipins,
the proportions of phospholipins between themselves and to cholesterol
or other steroids, neutral fats, etc., or to the adsorption of particular
groups of proteins, carbohydrates or other reactive agents on to the
phospholipid layers? A more detailed study of the various forms assumed
by myelin figures under different conditions might be very illuminating
in this connexion (Revel, Ito and Fawcett, 1958) as are the studies