13. D I F F E R E N T I A T I O N
471
requirements of complex differentiating systems, but that they do not
necessarily provide information on the intimate mechanisms of control
of the differentiative process within component tissues and cells.
Intriguing in this connection are studies of the effect of vitamin A on
the differentiation of chick skin, since it has been suggested that this may
bear on the switch mechanisms which decide between alternative
differentiative pathways. Fell and Mellanby (1953) first noted that
ectoderm from the trunk and limbs of the 6-7-day chick embryo
keratinizes relatively normally under the conditions of culture employed. On addition of high levels of vitamin A, however, keratinization
was suppressed and a mucous-secreting, sometimes ciliated, epithelium
differentiated instead. On withdrawal of vitamin A the mucosa-like
epithelium was pushed outward and shed by a new wave of keratinization from the epidermal basal layer. Weiss and James (1955) reported
that the vitamin A effect could register, following exposures of less than
an hour, on cells in a disaggregated state. The brief duration of action,
on the assumption that no storage of vitamin occurred, suggested to the
authors an inductive or switching action rather than a growth-regulating effect. Lasnitzki (1961), however, refers to data indicating prolonged
storage of vitamin A under these conditions—up to 2, and perhaps 3
days, at declining levels. She finds that vitamin A suppresses keratinization in mouse vagina in vitro and preserves, and stimulates proliferation
in, the original epithelium. Fell (1961) has speculated that the vitamin
A may block keratinization by promoting proteolytic activity in epidermal cells. (See Chapters 11 and 17.)
V . H O M O T Y P I C F A C T O R S
The designation "homotypic
55 is used here to refer to interactions
between cells of like developmental history, and not known to be of
unlike type in other senses. It distinguishes interactions which are
thought of as occurring within a tissue, from those which occur between
tissues. That interactions of this kind occur, and are important in controlling differentiative behavior, is suggested by scattered observations
on developing systems ranging from coelenterates to vertebrates
(Grobstein, 1955a). In recent years a number of observations demonstrating the phenomenon have been made in vitro.
A case in point is the effect of culture on differentiative behavior of
mouse and chick blastoderms (Grobstein, 1952; Grobstein and Zwilling,
1953). A standard piece of the chick blastoderm may be selected which
is known regularly to differentiate nervous tissue on grafting to the
chorio allantoic membrane. Culturing this piece intact for 5 days at a
glass-clot interface does not in itself affect subsequent neural differentia-
471
requirements of complex differentiating systems, but that they do not
necessarily provide information on the intimate mechanisms of control
of the differentiative process within component tissues and cells.
Intriguing in this connection are studies of the effect of vitamin A on
the differentiation of chick skin, since it has been suggested that this may
bear on the switch mechanisms which decide between alternative
differentiative pathways. Fell and Mellanby (1953) first noted that
ectoderm from the trunk and limbs of the 6-7-day chick embryo
keratinizes relatively normally under the conditions of culture employed. On addition of high levels of vitamin A, however, keratinization
was suppressed and a mucous-secreting, sometimes ciliated, epithelium
differentiated instead. On withdrawal of vitamin A the mucosa-like
epithelium was pushed outward and shed by a new wave of keratinization from the epidermal basal layer. Weiss and James (1955) reported
that the vitamin A effect could register, following exposures of less than
an hour, on cells in a disaggregated state. The brief duration of action,
on the assumption that no storage of vitamin occurred, suggested to the
authors an inductive or switching action rather than a growth-regulating effect. Lasnitzki (1961), however, refers to data indicating prolonged
storage of vitamin A under these conditions—up to 2, and perhaps 3
days, at declining levels. She finds that vitamin A suppresses keratinization in mouse vagina in vitro and preserves, and stimulates proliferation
in, the original epithelium. Fell (1961) has speculated that the vitamin
A may block keratinization by promoting proteolytic activity in epidermal cells. (See Chapters 11 and 17.)
V . H O M O T Y P I C F A C T O R S
The designation "homotypic
55 is used here to refer to interactions
between cells of like developmental history, and not known to be of
unlike type in other senses. It distinguishes interactions which are
thought of as occurring within a tissue, from those which occur between
tissues. That interactions of this kind occur, and are important in controlling differentiative behavior, is suggested by scattered observations
on developing systems ranging from coelenterates to vertebrates
(Grobstein, 1955a). In recent years a number of observations demonstrating the phenomenon have been made in vitro.
A case in point is the effect of culture on differentiative behavior of
mouse and chick blastoderms (Grobstein, 1952; Grobstein and Zwilling,
1953). A standard piece of the chick blastoderm may be selected which
is known regularly to differentiate nervous tissue on grafting to the
chorio allantoic membrane. Culturing this piece intact for 5 days at a
glass-clot interface does not in itself affect subsequent neural differentia-