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LAURI SAXEN ET AL.
ing tissue when morphogenetic movements have brought the two tissue
components into close contact. This determinative tissue interaction has
been defined by Grobstein (1956) as follows:
. . inductive tissue interaction takes place whenever in development
two or more tissues of different history and properties become intimately
associated and alteration of the developmental course of the interactants
results."
The existence of such interactive systems has posed one of the central
problems of modern embryology, the nature of the mechanism of these
environmental control systems. At the same time, however, they have
provided a powerful tool for experimental study of the problem of differentiation. The interactants can be separated and brought together
again in vitro, with the great advantage that model systems can be
built in which the onset of differentiation can be determined with great
accuracy and the subsequent events followed in controlled conditions. In
addition to the classic model systems of lens induction (Spemann, 1912)
and the determination of the central nervous system (CNS) (see Saxen
and Toivonen, 1962), a great many such inductive systems have been
detected and used as in vitro model systems of differentiation. The following examples may be mentioned: dermal-epidermal interaction in the
development of the skin and its appendages (Sengel, 1961; Wessells, 1962);
epitheliomesenchymal interaction in the development of certain glandular
organs (Grobstein, 1953, 1964; Wessells, 1965); inductive processes in
vertebral chondrogenesis (Holtzer, 1961; Lash, 1963); mutual interactions of the components of the limb bud (Saunders, 1948; Zwilling,
1956); and the inductive processes involved in the development of the
thymus (Auerbach, 1960a).
Such interactive processes seem to be one of the basic mechanisms
controlling morphogenesis, yet our understanding of their nature is still
too fragmentary for any generalizations, although certain common features can be discerned in the different instances of inductive processes so
far analyzed. It is thought that a further survey of these events and
their sequential consequences may help to elucidate the basic mechanism
of induction and subsequent differentiation. The excellent model systems
that have been developed make it easy to establish the temporal relationships between the different developmental events, and when these
are known, it may be possible to draw conclusions about their causal
relationship.
The present review deals with one such model system, the development
LAURI SAXEN ET AL.
ing tissue when morphogenetic movements have brought the two tissue
components into close contact. This determinative tissue interaction has
been defined by Grobstein (1956) as follows:
. . inductive tissue interaction takes place whenever in development
two or more tissues of different history and properties become intimately
associated and alteration of the developmental course of the interactants
results."
The existence of such interactive systems has posed one of the central
problems of modern embryology, the nature of the mechanism of these
environmental control systems. At the same time, however, they have
provided a powerful tool for experimental study of the problem of differentiation. The interactants can be separated and brought together
again in vitro, with the great advantage that model systems can be
built in which the onset of differentiation can be determined with great
accuracy and the subsequent events followed in controlled conditions. In
addition to the classic model systems of lens induction (Spemann, 1912)
and the determination of the central nervous system (CNS) (see Saxen
and Toivonen, 1962), a great many such inductive systems have been
detected and used as in vitro model systems of differentiation. The following examples may be mentioned: dermal-epidermal interaction in the
development of the skin and its appendages (Sengel, 1961; Wessells, 1962);
epitheliomesenchymal interaction in the development of certain glandular
organs (Grobstein, 1953, 1964; Wessells, 1965); inductive processes in
vertebral chondrogenesis (Holtzer, 1961; Lash, 1963); mutual interactions of the components of the limb bud (Saunders, 1948; Zwilling,
1956); and the inductive processes involved in the development of the
thymus (Auerbach, 1960a).
Such interactive processes seem to be one of the basic mechanisms
controlling morphogenesis, yet our understanding of their nature is still
too fragmentary for any generalizations, although certain common features can be discerned in the different instances of inductive processes so
far analyzed. It is thought that a further survey of these events and
their sequential consequences may help to elucidate the basic mechanism
of induction and subsequent differentiation. The excellent model systems
that have been developed make it easy to establish the temporal relationships between the different developmental events, and when these
are known, it may be possible to draw conclusions about their causal
relationship.
The present review deals with one such model system, the development