ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 145
formation by the cells making up this layer of macrovilli is undoubtedly
a reflection of their significant synthetic activity. This activity is supported
by the facts that these cells have a high ribonucleic acid content, that
relatively large amounts of arginine and tyrosine are present in their nuclei
(Brächet, 1947), and that they exhibit a strong alkaline phosphatase
reaction (Osawa, 1951). These facts suggest that the follicle epithelium
may play an important role in the nutrition of the oocyte. This may
involve the uptake of low molecular weight substances from the environment (blood supply of the adjacent theca layer) and synthesis of these
substances into more complex compounds. These would be transmitted
into the oocyte where they would serve as precursors for the formation of
the inclusion bodies (see Section III,C). Final synthesis into these definitive
cytoplasmic elements takes place, probably under nuclear supervision,
within the oocyte itself.
III. Cytoplasm
A relatively large number of studies have been carried out on the fine
structure of the cytoplasm of the animal ova. Such studies have usually
dealt with only one of the cytoplasmic elements. Recently, extensive electron-microscope studies covering the multiple intracytoplasmic changes
taking place during oogenesis have been made by Wartenberg (1962) and
by Balinsky and Devis (1963). In order to appreciate these changes, the
growth of the oocyte, as seen with the light microscope, will be surveyed
first.
A. Oocyte Development
The maturation of the amphibian oocyte is an extremely time-consuming
process that may extend up to 3 years (Grant, 1953; Gabaieva, 1963). The
major portion of oocyte growth occurs during diplotene of the first meiotic
division. Both the nucleus and cytoplasm are actively involved in the events
of maturation. During this time the nucleus undergoes reduction division
of its chromosome complement to insure genetic continuity of the species.
In addition, intranuclear changes occur which reflect its participation in
the morphogenetic events in progress in the cytoplasm. During maturation,
the cytoplasm is involved in the elaboration of ground substance, organelles,
and inclusion bodies such as yolk platelets. Various systems of classification
have been used as a scale of reference to depict oocyte development and
growth (Gersch, 1940; Grant, 1953; Kemp, 1953; Rosenbaum, 1958;
Wischnitzer, 1962a). Fundamentally, the amount of yolk present in the
cytoplasm is used as the index of the development in these schemes. Five
basic stages are usually identified.
formation by the cells making up this layer of macrovilli is undoubtedly
a reflection of their significant synthetic activity. This activity is supported
by the facts that these cells have a high ribonucleic acid content, that
relatively large amounts of arginine and tyrosine are present in their nuclei
(Brächet, 1947), and that they exhibit a strong alkaline phosphatase
reaction (Osawa, 1951). These facts suggest that the follicle epithelium
may play an important role in the nutrition of the oocyte. This may
involve the uptake of low molecular weight substances from the environment (blood supply of the adjacent theca layer) and synthesis of these
substances into more complex compounds. These would be transmitted
into the oocyte where they would serve as precursors for the formation of
the inclusion bodies (see Section III,C). Final synthesis into these definitive
cytoplasmic elements takes place, probably under nuclear supervision,
within the oocyte itself.
III. Cytoplasm
A relatively large number of studies have been carried out on the fine
structure of the cytoplasm of the animal ova. Such studies have usually
dealt with only one of the cytoplasmic elements. Recently, extensive electron-microscope studies covering the multiple intracytoplasmic changes
taking place during oogenesis have been made by Wartenberg (1962) and
by Balinsky and Devis (1963). In order to appreciate these changes, the
growth of the oocyte, as seen with the light microscope, will be surveyed
first.
A. Oocyte Development
The maturation of the amphibian oocyte is an extremely time-consuming
process that may extend up to 3 years (Grant, 1953; Gabaieva, 1963). The
major portion of oocyte growth occurs during diplotene of the first meiotic
division. Both the nucleus and cytoplasm are actively involved in the events
of maturation. During this time the nucleus undergoes reduction division
of its chromosome complement to insure genetic continuity of the species.
In addition, intranuclear changes occur which reflect its participation in
the morphogenetic events in progress in the cytoplasm. During maturation,
the cytoplasm is involved in the elaboration of ground substance, organelles,
and inclusion bodies such as yolk platelets. Various systems of classification
have been used as a scale of reference to depict oocyte development and
growth (Gersch, 1940; Grant, 1953; Kemp, 1953; Rosenbaum, 1958;
Wischnitzer, 1962a). Fundamentally, the amount of yolk present in the
cytoplasm is used as the index of the development in these schemes. Five
basic stages are usually identified.