306
TH. LENDER
II. The Neoblasts of Fresh-Water Planaria
The number of studies devoted to the neoblasts of the planarians is
very great. Historical surveys and pertinent bibliographies may be
found in the reports of Dubois (1949) and H. V. Brondsted (1955).
The neoblasts are free cells in the parenchyma. They are found in the
regeneration blastema within which the tissues differentiate. At present
everyone accepts the existence of these cells as well as their participation in the regeneration process. However, agreement on their role is
less unanimous. In general, the regeneration of the tissues of the freshwater planarians is attributed to the differentiation of the neoblasts.
A few authors merely assign to these cells a slight and indirect role
during regeneration. Thus, the neoblasts have been considered to be
only carriers of very active enzymatic factors (Lindh, 1957) or reservoirs
of ribonucleic acid (Chandebois, 1960b). Two aspects of the investigations on neoblasts will be dealt with here: first the cytological data, then
the formation of a blastema at the expense of the neoblasts. The role of
these cells in morphogenesis will be reviewed in a second section.
A. Cytological Study of the Neoblasts
In the last few years much information has been gained by cytological
studies of the neoblasts (Dubois, 1949; Pedersen, 1958, 1959a, b ;
Lender and Gabriel, 1960a, b, 1961). It has long been known that the
cytoplasm of the neoblasts is basophilic because it contains ribonucleic acid (Clement, 1944; Br0ndsted, 1955; Lindh, 1957; Pedersen,
1958). It was shown by Pedersen (1959b), working on Planaria vitta,
and by Lender and Gabriel (1960b), working on Dugesia lugubris, that
the neoblasts may be characterized histochemically by methyl greenpyronine staining. The nucleus is stained green and the nucleolus and
cytoplasm stain red. The green colour is due to the high DNA content
of the nucleus, and the red colour to the abundance of RNA in the
nucleolus and cytoplasm. After exposure to ribonuclease, the red stain
fails to appear. This method, therefore, makes it possible to identify with
certainty the neoblasts in the body of a planarian.
The histochemical methods also show that the cytoplasm of the
regeneration cells has sulphhydryl groups in the form of sulphurcontaining proteins (Pedersen, 1959b; Lender and Gabriel, 1960b).
The neoblasts are free cells in the shape of a spindle or a pear (Fig. 1).
Their mean diameter is 11-12/x. Each has a large nucleus, 8-ΙΟμ, in
diameter, with one or two nucleoli. The cytoplasm forms a thin fringe.
The neoblasts exhibit characteristics of undifferentiated cells, as confirmed by electron microscopic observations (Pedersen, 1959b).
Neoblast counts have been made by Lender and Gabriel (1960b) on
TH. LENDER
II. The Neoblasts of Fresh-Water Planaria
The number of studies devoted to the neoblasts of the planarians is
very great. Historical surveys and pertinent bibliographies may be
found in the reports of Dubois (1949) and H. V. Brondsted (1955).
The neoblasts are free cells in the parenchyma. They are found in the
regeneration blastema within which the tissues differentiate. At present
everyone accepts the existence of these cells as well as their participation in the regeneration process. However, agreement on their role is
less unanimous. In general, the regeneration of the tissues of the freshwater planarians is attributed to the differentiation of the neoblasts.
A few authors merely assign to these cells a slight and indirect role
during regeneration. Thus, the neoblasts have been considered to be
only carriers of very active enzymatic factors (Lindh, 1957) or reservoirs
of ribonucleic acid (Chandebois, 1960b). Two aspects of the investigations on neoblasts will be dealt with here: first the cytological data, then
the formation of a blastema at the expense of the neoblasts. The role of
these cells in morphogenesis will be reviewed in a second section.
A. Cytological Study of the Neoblasts
In the last few years much information has been gained by cytological
studies of the neoblasts (Dubois, 1949; Pedersen, 1958, 1959a, b ;
Lender and Gabriel, 1960a, b, 1961). It has long been known that the
cytoplasm of the neoblasts is basophilic because it contains ribonucleic acid (Clement, 1944; Br0ndsted, 1955; Lindh, 1957; Pedersen,
1958). It was shown by Pedersen (1959b), working on Planaria vitta,
and by Lender and Gabriel (1960b), working on Dugesia lugubris, that
the neoblasts may be characterized histochemically by methyl greenpyronine staining. The nucleus is stained green and the nucleolus and
cytoplasm stain red. The green colour is due to the high DNA content
of the nucleus, and the red colour to the abundance of RNA in the
nucleolus and cytoplasm. After exposure to ribonuclease, the red stain
fails to appear. This method, therefore, makes it possible to identify with
certainty the neoblasts in the body of a planarian.
The histochemical methods also show that the cytoplasm of the
regeneration cells has sulphhydryl groups in the form of sulphurcontaining proteins (Pedersen, 1959b; Lender and Gabriel, 1960b).
The neoblasts are free cells in the shape of a spindle or a pear (Fig. 1).
Their mean diameter is 11-12/x. Each has a large nucleus, 8-ΙΟμ, in
diameter, with one or two nucleoli. The cytoplasm forms a thin fringe.
The neoblasts exhibit characteristics of undifferentiated cells, as confirmed by electron microscopic observations (Pedersen, 1959b).
Neoblast counts have been made by Lender and Gabriel (1960b) on