520
A . A . M O S C O N A
proteins (Katchalsky, Danon, Nevo and DeVries, 1959; Easty and
Mutolo, 1960) since in such clumps there is no progression towards
histological order. For similar reasons, it is also not comparable with
typical serological agglutination which immobilizes cells. It may be
argued that lack of contact stability between non-matching cells is
evidence of repulsive forces due to which the cells continue to migrate
(L. Weiss, 1961); however, repulsion need not be necessary to account
for lack of stable attachment between structures with different surface
configurations. The intercellular distances in first stage aggregates
suggest that the making of contact may be mediated either by extracellular "surface coat-like" materials, or by rather long cellular processes the retraction of which might pull the cells together. Filopodial
extensions have been shown to play an important role in the attachment
of primary mesenchymal cells to the ectoderm in sea-urchin embryos
(Gustafson and Wolpert, 1961). Since cells aggregating in rotating
cultures are round and, in the light microscope, show no long processes,
such extensions, if present, might be of submicroscopic diameter.
Suitable electron-microscopic studies on dissociated cells to examine
these and other possibilities are not yet available.
There is considerable evidence suggestive of a role of the lipid—
protein cell membrane in cell behavior and in certain aspects of cell
contact (Willmer, 1961). There are, however, increasing indications
that the cell surface extends both structurally and functionally outside
this double membrane into an extracellular region consisting of
materials thought to be of mucoidal nature (Robertson, 1961; Brandt
and Pappas, 1960). It has been suggested that such extracellular
materials may be involved in the dynamics of cell interactions (Grobstein and Dal ton, 1957) and that their chemical and structural constituents may be related to cell adhesion and the phenomena of
contact-specificity (Fawcett, 1961). The integrity of such cell-surface
materials is probably markedly affected at dissociation by enzymic
degradation and cation depletion; re-aggregation may therefore depend
on their regeneration and orderly organization. Transition from nonselective to selective attachment of cells in aggregates could thus be
related to the sequence of synthesis of such materials at the cell surface
and their molecular ordering; such a sequence would involve, first,
initial "precursor" products at the cell surface, followed by their
arrangement into configurations that can make discriminatory, more
stable linkages with corresponding molecular arrays. According to this
working assumption, the characteristic properties of such extracellular
products might be a function of the biosynthetic pattern of the cells,
and thus susceptible to modifications by conditions affecting these
patterns. It is of interest that in monolayer-grown cells there is a decline
A . A . M O S C O N A
proteins (Katchalsky, Danon, Nevo and DeVries, 1959; Easty and
Mutolo, 1960) since in such clumps there is no progression towards
histological order. For similar reasons, it is also not comparable with
typical serological agglutination which immobilizes cells. It may be
argued that lack of contact stability between non-matching cells is
evidence of repulsive forces due to which the cells continue to migrate
(L. Weiss, 1961); however, repulsion need not be necessary to account
for lack of stable attachment between structures with different surface
configurations. The intercellular distances in first stage aggregates
suggest that the making of contact may be mediated either by extracellular "surface coat-like" materials, or by rather long cellular processes the retraction of which might pull the cells together. Filopodial
extensions have been shown to play an important role in the attachment
of primary mesenchymal cells to the ectoderm in sea-urchin embryos
(Gustafson and Wolpert, 1961). Since cells aggregating in rotating
cultures are round and, in the light microscope, show no long processes,
such extensions, if present, might be of submicroscopic diameter.
Suitable electron-microscopic studies on dissociated cells to examine
these and other possibilities are not yet available.
There is considerable evidence suggestive of a role of the lipid—
protein cell membrane in cell behavior and in certain aspects of cell
contact (Willmer, 1961). There are, however, increasing indications
that the cell surface extends both structurally and functionally outside
this double membrane into an extracellular region consisting of
materials thought to be of mucoidal nature (Robertson, 1961; Brandt
and Pappas, 1960). It has been suggested that such extracellular
materials may be involved in the dynamics of cell interactions (Grobstein and Dal ton, 1957) and that their chemical and structural constituents may be related to cell adhesion and the phenomena of
contact-specificity (Fawcett, 1961). The integrity of such cell-surface
materials is probably markedly affected at dissociation by enzymic
degradation and cation depletion; re-aggregation may therefore depend
on their regeneration and orderly organization. Transition from nonselective to selective attachment of cells in aggregates could thus be
related to the sequence of synthesis of such materials at the cell surface
and their molecular ordering; such a sequence would involve, first,
initial "precursor" products at the cell surface, followed by their
arrangement into configurations that can make discriminatory, more
stable linkages with corresponding molecular arrays. According to this
working assumption, the characteristic properties of such extracellular
products might be a function of the biosynthetic pattern of the cells,
and thus susceptible to modifications by conditions affecting these
patterns. It is of interest that in monolayer-grown cells there is a decline