14. CELL A G G R E G A T E S
523
and cell groups, the question of their directiveness or of the nature of
"guiding" factors remains unanswered. It is unlikely that a single effect
can provide a complete explanation, particularly since a similar result
can be arrived at differently: epidermal cells may converge singly
towards the surface of a heterotypic aggregate and form patches which
eventually become continuous; or clusters, preformed internally, may
break out to the surface and then spread out. Furthermore, as mentioned before, their behavior in this respect must be affected by the type
of neighbouring cells as well as by their state of differentiation, resulting
in quite different patterns in the aggregates.
According to Trinkaus (1961) regionalization of cells in aggregates
cannot be explained simply on the basis of random cell movements.
Curtis (1960) suggested that layering of cells in aggregates was due to
cessation of movement with recovery of contact inhibition (Abercrombie
and Heaysman, 1953) at different times for each tissue type. However,
Trinkaus points out that, apart from contact inhibition which could
play a role in stabilizing emergent cellular groupings, Curtis's view is
difficult to fit with the facts. The possibility that the cells become
differentially arranged in response to gradients of metabolites, 0 2 ,
C O 2 or each other's products has not been adequately examined.
O f obvious interest in this connexion is the "contact guidance"
hypothesis (P. Weiss, 1958, 1961) based on the demonstration that cell
movements can be oriented by molecular configurations of their
operational substratum. Studies on aggregation of cells upon variously
polarized and structured surfaces should be of much interest. In
aggregates, the substrates for cellular manoeuvres are adjacent cells and
their extracellular products; according to the working hypothesis of this
reviewer (Moscona, 1960) such cell-linking products "may provide a
dynamically organized substratum, an orientating and integrating
framework for the kinetic and aggregative manoeuvres of cells. . . . "
Similar views, in a somewhat different context, have been voiced by
P. Weiss (1947) and Grobstein (1961). Evidence for and inferences bearing on this hypothesis have been referred to repeatedly in this article
and its speculative nature has been stressed. The justification for
advancing it is that it may provide a methodologically useful approach
to the nexus of problems represented by cell adhesion, contact selectivity
and grouping phenomena, and that work along lines suggested by it
might bring us closer to understanding cell interactions in development.
A C K N O W L E D G M E N T
Work by the author and co-workers was supported by research grants
from the National C ancer Institute, U.S. Public Health Service; the
523
and cell groups, the question of their directiveness or of the nature of
"guiding" factors remains unanswered. It is unlikely that a single effect
can provide a complete explanation, particularly since a similar result
can be arrived at differently: epidermal cells may converge singly
towards the surface of a heterotypic aggregate and form patches which
eventually become continuous; or clusters, preformed internally, may
break out to the surface and then spread out. Furthermore, as mentioned before, their behavior in this respect must be affected by the type
of neighbouring cells as well as by their state of differentiation, resulting
in quite different patterns in the aggregates.
According to Trinkaus (1961) regionalization of cells in aggregates
cannot be explained simply on the basis of random cell movements.
Curtis (1960) suggested that layering of cells in aggregates was due to
cessation of movement with recovery of contact inhibition (Abercrombie
and Heaysman, 1953) at different times for each tissue type. However,
Trinkaus points out that, apart from contact inhibition which could
play a role in stabilizing emergent cellular groupings, Curtis's view is
difficult to fit with the facts. The possibility that the cells become
differentially arranged in response to gradients of metabolites, 0 2 ,
C O 2 or each other's products has not been adequately examined.
O f obvious interest in this connexion is the "contact guidance"
hypothesis (P. Weiss, 1958, 1961) based on the demonstration that cell
movements can be oriented by molecular configurations of their
operational substratum. Studies on aggregation of cells upon variously
polarized and structured surfaces should be of much interest. In
aggregates, the substrates for cellular manoeuvres are adjacent cells and
their extracellular products; according to the working hypothesis of this
reviewer (Moscona, 1960) such cell-linking products "may provide a
dynamically organized substratum, an orientating and integrating
framework for the kinetic and aggregative manoeuvres of cells. . . . "
Similar views, in a somewhat different context, have been voiced by
P. Weiss (1947) and Grobstein (1961). Evidence for and inferences bearing on this hypothesis have been referred to repeatedly in this article
and its speculative nature has been stressed. The justification for
advancing it is that it may provide a methodologically useful approach
to the nexus of problems represented by cell adhesion, contact selectivity
and grouping phenomena, and that work along lines suggested by it
might bring us closer to understanding cell interactions in development.
A C K N O W L E D G M E N T
Work by the author and co-workers was supported by research grants
from the National C ancer Institute, U.S. Public Health Service; the