134
ALBERTO MONROY AND RACHELE MAGGIO
have shown the presence in the uncleaved egg of vesicular
ergastoplasm
with and without ribosomes, and of annulated membranes, which may
represent a special form of ergastoplasm. In the unfertilized egg they are
situated in the perinuclear area or around the heavy bodies and their
structure is indeed similar to that of the nuclear membrane (Afzelius,
1956, 1957; Rebhun, 1956a, b; Swift, 1956; Schultz, 1957). They seem
to be particularly numerous in the fertilized eggs. In other materials
these structures appear 'most prominent in cells that are actively
growing and multiplying' (Porter, 1961). From the embryological point
of view it would certainly be valuable to learn about the morphological
and physiological changes that may possibly take place in these
structures in the course of development. Indeed the available evidence
indicates that in several organisms and in growing tissues the endoplasmic reticulum undergoes far-reaching structural and physiological
rearrangements in connection with the synthesis of specific proteins and
hence of cell differentiation (Birbeck and Mercer, 1961). As we know, the
term microsomes was originally employed to designate a complex of
cytoplasmic vesicles rich in RNA, isolated from homogenates of cells.
It is now established that the microsomes as described above derive
from the breaking up of the endoplasmic reticulum and of all the
membranous structures of the cell. Nevertheless, from an operational
point of view it may be useful to retain this term provided it is made
clear that it is meant to indicate those structures that are sedimented at
105,000g for 1 hour from a sucrose homogenate, after removal of the
heavy particles, and which exhibit the ability to carry out in vitro
incorporation of amino-acids into proteins.
In the sea urchin egg and embryo, the fraction thus obtained is made
up, as in all cell fractionations, of membranes with and without
ribonucleoprotein particles (ribosomes) attached, together with quite a
large population of free ribosomes. The latter are, however, mostly
sedimented after a more prolonged centrifugation. This fraction also
contains the highest percentage of glucose-6-phosphatase activity
(Vittorelli and Maggio, unpublished). It has now been shown that the
ribosomes of unfertilized eggs have a very low ability to carry out
incorporation of
14
C-valine into the proteins, whereas this activity is
already considerably increased a few minutes after fertilization (Hultin
and Bergstrand, 1960; Hultin, 1961). The incorporating activity was
mainly localized in the ribosomes whether free or associated with
lipoprotein membranes. This increase following fertilization is not due to
a more efficient activation by amino-acids in the fertilized eggs (Hultin
and Bergstrand, 1960) nor to the removal of some soluble inhibitor
present in the unfertilized eggs (Hultin, 1961).
A result similar to the one found in the case of mitochondria (Nakano
ALBERTO MONROY AND RACHELE MAGGIO
have shown the presence in the uncleaved egg of vesicular
ergastoplasm
with and without ribosomes, and of annulated membranes, which may
represent a special form of ergastoplasm. In the unfertilized egg they are
situated in the perinuclear area or around the heavy bodies and their
structure is indeed similar to that of the nuclear membrane (Afzelius,
1956, 1957; Rebhun, 1956a, b; Swift, 1956; Schultz, 1957). They seem
to be particularly numerous in the fertilized eggs. In other materials
these structures appear 'most prominent in cells that are actively
growing and multiplying' (Porter, 1961). From the embryological point
of view it would certainly be valuable to learn about the morphological
and physiological changes that may possibly take place in these
structures in the course of development. Indeed the available evidence
indicates that in several organisms and in growing tissues the endoplasmic reticulum undergoes far-reaching structural and physiological
rearrangements in connection with the synthesis of specific proteins and
hence of cell differentiation (Birbeck and Mercer, 1961). As we know, the
term microsomes was originally employed to designate a complex of
cytoplasmic vesicles rich in RNA, isolated from homogenates of cells.
It is now established that the microsomes as described above derive
from the breaking up of the endoplasmic reticulum and of all the
membranous structures of the cell. Nevertheless, from an operational
point of view it may be useful to retain this term provided it is made
clear that it is meant to indicate those structures that are sedimented at
105,000g for 1 hour from a sucrose homogenate, after removal of the
heavy particles, and which exhibit the ability to carry out in vitro
incorporation of amino-acids into proteins.
In the sea urchin egg and embryo, the fraction thus obtained is made
up, as in all cell fractionations, of membranes with and without
ribonucleoprotein particles (ribosomes) attached, together with quite a
large population of free ribosomes. The latter are, however, mostly
sedimented after a more prolonged centrifugation. This fraction also
contains the highest percentage of glucose-6-phosphatase activity
(Vittorelli and Maggio, unpublished). It has now been shown that the
ribosomes of unfertilized eggs have a very low ability to carry out
incorporation of
14
C-valine into the proteins, whereas this activity is
already considerably increased a few minutes after fertilization (Hultin
and Bergstrand, 1960; Hultin, 1961). The incorporating activity was
mainly localized in the ribosomes whether free or associated with
lipoprotein membranes. This increase following fertilization is not due to
a more efficient activation by amino-acids in the fertilized eggs (Hultin
and Bergstrand, 1960) nor to the removal of some soluble inhibitor
present in the unfertilized eggs (Hultin, 1961).
A result similar to the one found in the case of mitochondria (Nakano