ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 169
3. In urodeles, yolk platelet formation has not been shown to be associated with mitochondria. Thus, another mechanism, such as one
involving the interim formation of complex cytosomes, may possibly
be responsible in this group.
It was demonstrated earlier (see Section III,C,1) that in amphibians,
following the appearance of lipochondria, protein yolk is deposited in the
form of platelets. Light-microscope studies of yolk formation in various
species were carried out by Hibbard (1928), Nath (1931), Saguchi (1932),
Wittek (1952), and Kemp (1953). These studies demonstrated that initial
platelet formation takes place in the peripheral cytoplasm. Such platelets
increase in size and migrate inward until they entirely fill the cytoplasm.
Wittek (1952) has claimed that when the platelets reach the nucleus a new
phase of yolk platelet synthesis begins in the perinuclear region.
The newly formed platelets are usually very small spheres, which with
further growth become oval, elongated, or polygonal. The mode of formation of the platelets as suggested by light-microscope studies is conflicting.
It has been thought that platelet formation may or may not occur in relationship to the cytoplasmic organelles. In recent years, electron-microscope
investigations have been of some help in clarifying this difficult problem.
Lanzavecchia (1961), working with oocytes from Rana esculenta, has
suggested a dual mode of origin for yolk platelets (Fig. 24). The first method
involved budding of a mitochondrion, followed by detachment of the bud,
and then transformation of the detached body into a yolk platelet. The
transformation apparently involves the apposition of the cristae to the
peripheral membranes to form a four-layered envelope. Deposition of an
opaque material inside the detached body then takes place and serves to
form the dense core. The core increases in volume, filling the body and
converting it into an oval structure that can be considered a small yolk
platelet.
The second method of platelet formation involves the transformation
of multivesicular bodies. These masses which consist of vesicles, 200-300 A
in diameter, are embedded in a dense matrix. These masses take on a round
shape, and the vesicles disappear to produce a body that has a homogeneous
periphery and particulate central core. Subsequently, a granular outer
layer comes to envelop the structure, and it, in turn, is bounded by a
membrane. This complex mass also is thought to become transformed into
a small yolk platelet.
The next electron-microscope study of the problem of yolk formation
was carried out by Ward (1962) with Rana pipiens tadpoles. In his study,
mitochondria were found to be the only structure implicated in yolk formation, and the process appears to involve the elaboration of intramitochondrial hexagonal bodies (Fig. 24). Such bodies frequently appear near
3. In urodeles, yolk platelet formation has not been shown to be associated with mitochondria. Thus, another mechanism, such as one
involving the interim formation of complex cytosomes, may possibly
be responsible in this group.
It was demonstrated earlier (see Section III,C,1) that in amphibians,
following the appearance of lipochondria, protein yolk is deposited in the
form of platelets. Light-microscope studies of yolk formation in various
species were carried out by Hibbard (1928), Nath (1931), Saguchi (1932),
Wittek (1952), and Kemp (1953). These studies demonstrated that initial
platelet formation takes place in the peripheral cytoplasm. Such platelets
increase in size and migrate inward until they entirely fill the cytoplasm.
Wittek (1952) has claimed that when the platelets reach the nucleus a new
phase of yolk platelet synthesis begins in the perinuclear region.
The newly formed platelets are usually very small spheres, which with
further growth become oval, elongated, or polygonal. The mode of formation of the platelets as suggested by light-microscope studies is conflicting.
It has been thought that platelet formation may or may not occur in relationship to the cytoplasmic organelles. In recent years, electron-microscope
investigations have been of some help in clarifying this difficult problem.
Lanzavecchia (1961), working with oocytes from Rana esculenta, has
suggested a dual mode of origin for yolk platelets (Fig. 24). The first method
involved budding of a mitochondrion, followed by detachment of the bud,
and then transformation of the detached body into a yolk platelet. The
transformation apparently involves the apposition of the cristae to the
peripheral membranes to form a four-layered envelope. Deposition of an
opaque material inside the detached body then takes place and serves to
form the dense core. The core increases in volume, filling the body and
converting it into an oval structure that can be considered a small yolk
platelet.
The second method of platelet formation involves the transformation
of multivesicular bodies. These masses which consist of vesicles, 200-300 A
in diameter, are embedded in a dense matrix. These masses take on a round
shape, and the vesicles disappear to produce a body that has a homogeneous
periphery and particulate central core. Subsequently, a granular outer
layer comes to envelop the structure, and it, in turn, is bounded by a
membrane. This complex mass also is thought to become transformed into
a small yolk platelet.
The next electron-microscope study of the problem of yolk formation
was carried out by Ward (1962) with Rana pipiens tadpoles. In his study,
mitochondria were found to be the only structure implicated in yolk formation, and the process appears to involve the elaboration of intramitochondrial hexagonal bodies (Fig. 24). Such bodies frequently appear near