ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 137
no longer striated, the zona radiata will be the major component of the
true vitelline membrane. Completion of the development of this membrane,
which is the end product of zona pellucida formation in the frog, takes
more than two breeding seasons (Grant, 1953; Gabaieva, 1963).
D. Ultrastructure of the Egg Envelopes
The study of the egg envelopes by means of the electron microscope has
not only elucidated the fine structure of these layers but has also provided
information concerning the formation and nature of the zona pellucida,
especially about its radial striations.
Low-power electron micrographs (Fig. 5) reveal that the three cellular
layers are demarcated from one another by double membranes. The surface epithelium contains spindle-shaped cells (see Fig. 7) which bulge
outward at sites where oval nuclei are present. The chromatin of these
nuclei is, to a large extent, concentrated on the inner surface of the nuclear
membrane. Only a small amount of cytoplasm is located around the nucleus.
At the extremities of such a cell where the nucleus is absent, the cytoplasm
contains some mitochondria, randomly distributed ergastoplasmic membranes, and granule-free vesicles.
The theca or connective tissue layer is characterized by the presence of
bundles of collagen fibers, as well as blood vessels that contain nucleated
red blood cells. As would be expected, fibroblasts are also present in this
layer. They contain oval nuclei and are enclosed by a thin layer of cytoplasm in which mitochondria and ergastoplasmic elements are present.
Wartenberg (1962) claims that the collagen fibrils of the theca are condensed as basal membranes against the inner and outer cellular layers.
FIG. 2. A light-microscope photomicrograph of the peripheral portion of a yolkfilled oocyte. The cellular layers containing an array of follicle cell nuclei (fen)
have, in the course of preparation, become partially separated from the acellular
layer (zp). X 1200.
FIG. 3. A light-microscope photomicrograph of the peripheral portion of a yolkfilled oocyte in which the elements that characterize the individual layers are evident. Two surface epithelial cell nuclei (sen) are seen in the outermost layer. A
portion of a fibroblast nucleus (fn) and red blood cells (rbc) are seen in the middle
layer. Two follicle cell nuclei (fen) are present in the innermost cellular layer. The
acellular zona pellucida (zp) shows faint striations. X 1200.
FIG. 4. A phase-contrast photomicrograph of the peripheral portion of an oocyte.
A blood vessel containing several nucleated red blood cells (rbc) is present in the
thecal layer, as are several follicle cell nuclei (fen) in the innermost cellular layer.
The acellular zona pellucida (zp) consists of a thin, light layer (homogeneous layer)
and a dense, thicker inner layer (zona radiata). Both of these acellular layers are
perforated by projections, the macrovilli (mav) which extend down from the follicle
cells. X 1500.
no longer striated, the zona radiata will be the major component of the
true vitelline membrane. Completion of the development of this membrane,
which is the end product of zona pellucida formation in the frog, takes
more than two breeding seasons (Grant, 1953; Gabaieva, 1963).
D. Ultrastructure of the Egg Envelopes
The study of the egg envelopes by means of the electron microscope has
not only elucidated the fine structure of these layers but has also provided
information concerning the formation and nature of the zona pellucida,
especially about its radial striations.
Low-power electron micrographs (Fig. 5) reveal that the three cellular
layers are demarcated from one another by double membranes. The surface epithelium contains spindle-shaped cells (see Fig. 7) which bulge
outward at sites where oval nuclei are present. The chromatin of these
nuclei is, to a large extent, concentrated on the inner surface of the nuclear
membrane. Only a small amount of cytoplasm is located around the nucleus.
At the extremities of such a cell where the nucleus is absent, the cytoplasm
contains some mitochondria, randomly distributed ergastoplasmic membranes, and granule-free vesicles.
The theca or connective tissue layer is characterized by the presence of
bundles of collagen fibers, as well as blood vessels that contain nucleated
red blood cells. As would be expected, fibroblasts are also present in this
layer. They contain oval nuclei and are enclosed by a thin layer of cytoplasm in which mitochondria and ergastoplasmic elements are present.
Wartenberg (1962) claims that the collagen fibrils of the theca are condensed as basal membranes against the inner and outer cellular layers.
FIG. 2. A light-microscope photomicrograph of the peripheral portion of a yolkfilled oocyte. The cellular layers containing an array of follicle cell nuclei (fen)
have, in the course of preparation, become partially separated from the acellular
layer (zp). X 1200.
FIG. 3. A light-microscope photomicrograph of the peripheral portion of a yolkfilled oocyte in which the elements that characterize the individual layers are evident. Two surface epithelial cell nuclei (sen) are seen in the outermost layer. A
portion of a fibroblast nucleus (fn) and red blood cells (rbc) are seen in the middle
layer. Two follicle cell nuclei (fen) are present in the innermost cellular layer. The
acellular zona pellucida (zp) shows faint striations. X 1200.
FIG. 4. A phase-contrast photomicrograph of the peripheral portion of an oocyte.
A blood vessel containing several nucleated red blood cells (rbc) is present in the
thecal layer, as are several follicle cell nuclei (fen) in the innermost cellular layer.
The acellular zona pellucida (zp) consists of a thin, light layer (homogeneous layer)
and a dense, thicker inner layer (zona radiata). Both of these acellular layers are
perforated by projections, the macrovilli (mav) which extend down from the follicle
cells. X 1500.