ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 139
The fact that a fine double membrane exists at these interfaces, rather
than a single, thick partition, was demonstrated by Wischnitzer (1963a).
As seen with the light microscope, the follicular epithelium does not
seem to be composed of distinct cells since their lateral boundaries are not
evident after conventional fixation and staining. This led some early
workers to conclude that this layer was a syncytium. With the electron
microscope, it has been established that a pair of closely associated membranes are present where adjacent follicle cells meet (Wartenberg, 1962;
Wischnitzer, 1963a, 1964b) and that the cells may be bound to each other
by desmosomes (Wartenberg, 1962). The upper cell boundary is adjacent
to the theca; the lower or basal cell boundary forms the interface overlying
the zona pellucida.
The nuclei of the follicle cells are ovoid, and their chromatin is more
heavily concentrated near the inner surface of the nuclear envelope. The
cytoplasm contains considerable numbers of mitochondria and short
ergastoplasmic strands.
The formation of the zona pellucida in Triturus is diagrammatically
illustrated in Fig. 6. In very young oocytes (below ^100 μ in diameter), it
is absent. Thus, in these oocytes the basal membrane of the follicle cell
and the plasmalemma of the oocyte directly appose each other and reportedly are even connected by desmosomes (Wartenberg, 1962). In
slightly older oocytes (~200 μ in diameter), undulation of the plasmalemma, presumably along the interdesmosomal segments, takes place.
This is followed, in oocytes of ~300 μ diameter, by the deposition of the
matrix material of the zona pellucida in these segments. In urodeles,
where the zona pellucida has two strata, the initial ground substance
deposited here reportedly belongs to the homogeneous layer (Wartenberg,
1962). The matrix material that is gradually being layed down projects
upward against the follicle cells and becomes compressed into hillocks which
are bound on either side by club-shaped extensions of the follicle cells,
presumably at the sites of desmosome attachment. At this time (oocyte
diameter of ^400 μ), the undulation of the plasmalemma in the interdesmosomal segments apparently results in the formation of microvilli
which extend up through part of the matrix material. As growth continues
(oocyte diameter of ^500 μ), the club-shaped extensions of the follicle
cells become compressed into cylinders that take the shape of macrovilli.
These extend into the zona radiata close to the level of the oocyte itself.
bundles of collagen fibers as well as a fibroblast (fn) whose nucleus is very prominent. A portion of a follicle, epithelial cell nucleus (fen) is present. The zona
pellucida (ZP) is well developed and consists of the homogeneous layer (hi) and
zona radiata (zr). Both macrovilli (mav) and microvilli (miv) are evident in the
zona radiata. Beneath this layer is the peripheral cytoplasm (PC). X 16,000.
The fact that a fine double membrane exists at these interfaces, rather
than a single, thick partition, was demonstrated by Wischnitzer (1963a).
As seen with the light microscope, the follicular epithelium does not
seem to be composed of distinct cells since their lateral boundaries are not
evident after conventional fixation and staining. This led some early
workers to conclude that this layer was a syncytium. With the electron
microscope, it has been established that a pair of closely associated membranes are present where adjacent follicle cells meet (Wartenberg, 1962;
Wischnitzer, 1963a, 1964b) and that the cells may be bound to each other
by desmosomes (Wartenberg, 1962). The upper cell boundary is adjacent
to the theca; the lower or basal cell boundary forms the interface overlying
the zona pellucida.
The nuclei of the follicle cells are ovoid, and their chromatin is more
heavily concentrated near the inner surface of the nuclear envelope. The
cytoplasm contains considerable numbers of mitochondria and short
ergastoplasmic strands.
The formation of the zona pellucida in Triturus is diagrammatically
illustrated in Fig. 6. In very young oocytes (below ^100 μ in diameter), it
is absent. Thus, in these oocytes the basal membrane of the follicle cell
and the plasmalemma of the oocyte directly appose each other and reportedly are even connected by desmosomes (Wartenberg, 1962). In
slightly older oocytes (~200 μ in diameter), undulation of the plasmalemma, presumably along the interdesmosomal segments, takes place.
This is followed, in oocytes of ~300 μ diameter, by the deposition of the
matrix material of the zona pellucida in these segments. In urodeles,
where the zona pellucida has two strata, the initial ground substance
deposited here reportedly belongs to the homogeneous layer (Wartenberg,
1962). The matrix material that is gradually being layed down projects
upward against the follicle cells and becomes compressed into hillocks which
are bound on either side by club-shaped extensions of the follicle cells,
presumably at the sites of desmosome attachment. At this time (oocyte
diameter of ^400 μ), the undulation of the plasmalemma in the interdesmosomal segments apparently results in the formation of microvilli
which extend up through part of the matrix material. As growth continues
(oocyte diameter of ^500 μ), the club-shaped extensions of the follicle
cells become compressed into cylinders that take the shape of macrovilli.
These extend into the zona radiata close to the level of the oocyte itself.
bundles of collagen fibers as well as a fibroblast (fn) whose nucleus is very prominent. A portion of a follicle, epithelial cell nucleus (fen) is present. The zona
pellucida (ZP) is well developed and consists of the homogeneous layer (hi) and
zona radiata (zr). Both macrovilli (mav) and microvilli (miv) are evident in the
zona radiata. Beneath this layer is the peripheral cytoplasm (PC). X 16,000.