13. D I F F E R E N T I A T I O N
481
(Grobstein and Parker, 1954; Grobstein and Holtzer, 1955) or chick
embryos (Lash, Holtzer and Holtzer, 1957). Clusters of such somites
fail to differentiate cartilage when cultured by themselves, but can be
induced to form cartilage by inclusion of ventral spinal cord, notochord
or differentiating cartilage at the proper stage (Cooper, personal communication). As with the kidney or salivary gland, these effects are
exerted across a membrane filter, but over considerably greater distances than the dorsal cord factor (Cooper, ibid). Saline extracts of
neural tube and notochord have been reported by Strudel (1959, 1962)
to promote cartilage differentiation in such somites. Interestingly,
Benoit (1960a, b) finds that an extract of otic vesicle which promotes
cartilage differentiation of otic mesenchyme fails to act similarly on
somites. Very recently Lash, Hommes and Zilliken (1962) have reported
activity in nucleotide-rich extracts of spinal cord and notochord
obtained with cold perchloric acid. The extract was fractionated on a
charcoal-celite column to yield a sugar phosphate filtrate and a
nucleotide eluate. On further electrophoretic and chromatographic
processing both of these fractions showed characteristic peaks when
prepared from inductive tissues, peaks which were absent in similar
fractions from non-inductive tissues. Only the nucleotide fraction,
however, shov/ed inductive activity. Further purification of this fraction
suggests (Hommes, Van Leeuwen and Zilliken, 1962) that the activity
is associated with a relatively small-molecular component containing
at least cytidine monophosphate, seven amino acids and a reducing
hexosamine.
These exciting, and potentially highly significant findings invite a few
words of caution. It is to be noted that any differentiating system which
is dependent upon a number of inputs is susceptible to control by any
one of them, and hence each may, under suitable circumstances, act as
an ''inducer". Systems in vitro obviously may be deprived of a number
of inputs which are available and non-limiting in vivo. The detection and
identification of such missing inputs is important to culture technology
and useful in analyzing the metabolism of differentiation. It does not,
however, necessarily relate to the inductive control mechanism for
differentiation in vivo. The isolation of a chondrogenic factor for somites
in culture makes a strong claim as to its operation in vivo because the
effect is relatively specific, and correlates with an identifiable biochemical difference between inductive and non-inductive sources.
Further evidence is required, however, that the isolated material
actually is transmitted, and actually operates in biosynthetic control,
before the assertion is justified that "isolation of an embryonal inducer
in a pure state" (Hommes, Van Leeuwen and Zilliken, 1962) has been
accomplished.
R
481
(Grobstein and Parker, 1954; Grobstein and Holtzer, 1955) or chick
embryos (Lash, Holtzer and Holtzer, 1957). Clusters of such somites
fail to differentiate cartilage when cultured by themselves, but can be
induced to form cartilage by inclusion of ventral spinal cord, notochord
or differentiating cartilage at the proper stage (Cooper, personal communication). As with the kidney or salivary gland, these effects are
exerted across a membrane filter, but over considerably greater distances than the dorsal cord factor (Cooper, ibid). Saline extracts of
neural tube and notochord have been reported by Strudel (1959, 1962)
to promote cartilage differentiation in such somites. Interestingly,
Benoit (1960a, b) finds that an extract of otic vesicle which promotes
cartilage differentiation of otic mesenchyme fails to act similarly on
somites. Very recently Lash, Hommes and Zilliken (1962) have reported
activity in nucleotide-rich extracts of spinal cord and notochord
obtained with cold perchloric acid. The extract was fractionated on a
charcoal-celite column to yield a sugar phosphate filtrate and a
nucleotide eluate. On further electrophoretic and chromatographic
processing both of these fractions showed characteristic peaks when
prepared from inductive tissues, peaks which were absent in similar
fractions from non-inductive tissues. Only the nucleotide fraction,
however, shov/ed inductive activity. Further purification of this fraction
suggests (Hommes, Van Leeuwen and Zilliken, 1962) that the activity
is associated with a relatively small-molecular component containing
at least cytidine monophosphate, seven amino acids and a reducing
hexosamine.
These exciting, and potentially highly significant findings invite a few
words of caution. It is to be noted that any differentiating system which
is dependent upon a number of inputs is susceptible to control by any
one of them, and hence each may, under suitable circumstances, act as
an ''inducer". Systems in vitro obviously may be deprived of a number
of inputs which are available and non-limiting in vivo. The detection and
identification of such missing inputs is important to culture technology
and useful in analyzing the metabolism of differentiation. It does not,
however, necessarily relate to the inductive control mechanism for
differentiation in vivo. The isolation of a chondrogenic factor for somites
in culture makes a strong claim as to its operation in vivo because the
effect is relatively specific, and correlates with an identifiable biochemical difference between inductive and non-inductive sources.
Further evidence is required, however, that the isolated material
actually is transmitted, and actually operates in biosynthetic control,
before the assertion is justified that "isolation of an embryonal inducer
in a pure state" (Hommes, Van Leeuwen and Zilliken, 1962) has been
accomplished.
R