EARLY DEVELOPMENT OF THE SEA URCHIN
133
general occurrence. However, this may not necessarily be true. Actually
we still know too little about mitochondria from a comparative point of
view and indeed most of our knowledge is limited to that of rat liver and
there seems to be no reason to assume that all mitochondria should
behave similarly. The fact, for instance, that the mitochondria of rabbit
liver can oxidize propionate whilst kidney mitochondria lack this ability
(Huennekens et al., 1951) indicates a physiological differentiation of
mitochondria in the various types of tissues. The recent work of Weber
and Boell (1962) has shown that in the course of development of
Xenopus even such enzymes as cytochrome oxidase, ATPase and acid
phosphatase which are known to be present in all mitochondria, undergo
activity changes different and specific for each type of tissue. It seems
reasonable to assume therefore that in the course of evolution mitochondria may have undergone various degrees and types of specialization in the different animal species and hence have acquired or lost
properties according to their genetic environment. After all, if it is
accepted that cell differentiation is under genetic control, there is no
reason to suppose that mitochondria have escaped it. In much the same
way as different proteins are synthesized in different cell types, it seems
reasonable to presume that mitochondria may be different as well.
Hence it seems not unlikely to the present writers that mitochondria
play a more or less important morphogenetic role in the different animal
species and hence the lack of a differential distribution of mitochondria
in some embryos cannot be taken as evidence against this.
In the course of the last few years evidence has been produced to show
that mitochondria may also be able to carry out protein synthesis. In
the liver and heart mitochondria this seems to be confined to the synthesis
of the proteins of the mitochondrial membrane (Roodyn et al., 1962;
Truman and Korner, 1962a, b). In the case of the mitochondria of the
sea urchin embryo also, it has been shown that they can carry out
incorporation of radioactive amino-acids in their proteins (Giudice,
1960). However, the situation may very well be different in the embryo
and in the different animal groups. It would indeed by very interesting
if it could be shown that in some stages of development mitochondria
participate in a more extensive way in the synthesis of specific proteins.
It may be added that the ability of mitochondria to synthesize
membrane proteins is interesting in connection with the problem of the
origin of mitochondria. Indeed, these observations may support the
view that mitochondria are formed from pre-existing ones.
C. The Microsomes
As previously mentioned (Section IV, A), the investigations of
Afzelius (1956, 1957), Pasteels (1958) and Pasteels et al. (1958, 1959)
E2
133
general occurrence. However, this may not necessarily be true. Actually
we still know too little about mitochondria from a comparative point of
view and indeed most of our knowledge is limited to that of rat liver and
there seems to be no reason to assume that all mitochondria should
behave similarly. The fact, for instance, that the mitochondria of rabbit
liver can oxidize propionate whilst kidney mitochondria lack this ability
(Huennekens et al., 1951) indicates a physiological differentiation of
mitochondria in the various types of tissues. The recent work of Weber
and Boell (1962) has shown that in the course of development of
Xenopus even such enzymes as cytochrome oxidase, ATPase and acid
phosphatase which are known to be present in all mitochondria, undergo
activity changes different and specific for each type of tissue. It seems
reasonable to assume therefore that in the course of evolution mitochondria may have undergone various degrees and types of specialization in the different animal species and hence have acquired or lost
properties according to their genetic environment. After all, if it is
accepted that cell differentiation is under genetic control, there is no
reason to suppose that mitochondria have escaped it. In much the same
way as different proteins are synthesized in different cell types, it seems
reasonable to presume that mitochondria may be different as well.
Hence it seems not unlikely to the present writers that mitochondria
play a more or less important morphogenetic role in the different animal
species and hence the lack of a differential distribution of mitochondria
in some embryos cannot be taken as evidence against this.
In the course of the last few years evidence has been produced to show
that mitochondria may also be able to carry out protein synthesis. In
the liver and heart mitochondria this seems to be confined to the synthesis
of the proteins of the mitochondrial membrane (Roodyn et al., 1962;
Truman and Korner, 1962a, b). In the case of the mitochondria of the
sea urchin embryo also, it has been shown that they can carry out
incorporation of radioactive amino-acids in their proteins (Giudice,
1960). However, the situation may very well be different in the embryo
and in the different animal groups. It would indeed by very interesting
if it could be shown that in some stages of development mitochondria
participate in a more extensive way in the synthesis of specific proteins.
It may be added that the ability of mitochondria to synthesize
membrane proteins is interesting in connection with the problem of the
origin of mitochondria. Indeed, these observations may support the
view that mitochondria are formed from pre-existing ones.
C. The Microsomes
As previously mentioned (Section IV, A), the investigations of
Afzelius (1956, 1957), Pasteels (1958) and Pasteels et al. (1958, 1959)
E2