14. CELL AGGREGATES
507
aggregates may also be considerably influenced by cultural procedures.
Organ-culture methods (Fell, 1953, 1956) can effectively supply the
conditions required by most aggregates up to the point where further
progress depends on vascularization, whereupon grafting to suitable
hosts can lead to advanced, even organotypic development (Weiss and
Taylor, 1960).
The need for precise criteria in assigning identities to tissues formed in
aggregates should be stressed here. The situation is simple when the
histology of the product is unequivocal, as in the case of cartilage,
striated muscle fibres, sensory retinal cells, keratinizing skin, etc. In
other cases, functional tests may be essential, particularly when, due to
experimental conditions, there is a possibility of changes in phenotypic
traits of the cells. Secretory activity in aggregates of glandular cells,
concentration of phenol red in aggregates of nephric cells, the presence
of glycogen in aggregates of liver cells, etc., must be demonstrated
before the identity and functional normality of the cells can be
ascertained. In other cases, the presence of typical enzymes or enzyme
patterns should be demonstrated, before assuming a functional similarity between the aggregated cells and their tissue of origin.
B. DEVELOPMENT OF ISOTYPIC CELL AGGREGATES
The term "isotypic" indicates that the cells were derived from one
kind of tissue or organ rudiment and were thus functionally identical or
complementary; in contrast, "heterotypic" aggregates are compounded
of cells from two or more different tissues or organs. Isotypic aggregates
can be compounded of homologous cells from the same or from
different species. Many ofthe available references to aggregation studies
on avian and mammalian cells are collected in Table I.
In the majority of cases the dissociated cells retain their original distinctive histoformative capacities and pursue them in aggregates.
This is evident in the reconstruction of the relatively simple cartilage
fabric in aggregates of chondroblasts or chondrocytes, as well in the
rebuilding of complex nephric, retinal, glandular, etc., structures and
their progressive development. In some cases the dissociated and reaggregated cells may retain the capacity to reestablish the kind of gross
morphology that is characteristic of their tissue of origin: for instance,
aggregated scleral chondroblasts form chondrified sheets resembling
normal sclera, in sharp contrast with the lumpy masses of cartilage in
aggregates of skeletal chondroblasts (Weiss and Moscona, 1958;
Moscona, 1962b).
A suspension of cells freshly dissociated from an organ or tissue com-
507
aggregates may also be considerably influenced by cultural procedures.
Organ-culture methods (Fell, 1953, 1956) can effectively supply the
conditions required by most aggregates up to the point where further
progress depends on vascularization, whereupon grafting to suitable
hosts can lead to advanced, even organotypic development (Weiss and
Taylor, 1960).
The need for precise criteria in assigning identities to tissues formed in
aggregates should be stressed here. The situation is simple when the
histology of the product is unequivocal, as in the case of cartilage,
striated muscle fibres, sensory retinal cells, keratinizing skin, etc. In
other cases, functional tests may be essential, particularly when, due to
experimental conditions, there is a possibility of changes in phenotypic
traits of the cells. Secretory activity in aggregates of glandular cells,
concentration of phenol red in aggregates of nephric cells, the presence
of glycogen in aggregates of liver cells, etc., must be demonstrated
before the identity and functional normality of the cells can be
ascertained. In other cases, the presence of typical enzymes or enzyme
patterns should be demonstrated, before assuming a functional similarity between the aggregated cells and their tissue of origin.
B. DEVELOPMENT OF ISOTYPIC CELL AGGREGATES
The term "isotypic" indicates that the cells were derived from one
kind of tissue or organ rudiment and were thus functionally identical or
complementary; in contrast, "heterotypic" aggregates are compounded
of cells from two or more different tissues or organs. Isotypic aggregates
can be compounded of homologous cells from the same or from
different species. Many ofthe available references to aggregation studies
on avian and mammalian cells are collected in Table I.
In the majority of cases the dissociated cells retain their original distinctive histoformative capacities and pursue them in aggregates.
This is evident in the reconstruction of the relatively simple cartilage
fabric in aggregates of chondroblasts or chondrocytes, as well in the
rebuilding of complex nephric, retinal, glandular, etc., structures and
their progressive development. In some cases the dissociated and reaggregated cells may retain the capacity to reestablish the kind of gross
morphology that is characteristic of their tissue of origin: for instance,
aggregated scleral chondroblasts form chondrified sheets resembling
normal sclera, in sharp contrast with the lumpy masses of cartilage in
aggregates of skeletal chondroblasts (Weiss and Moscona, 1958;
Moscona, 1962b).
A suspension of cells freshly dissociated from an organ or tissue com-