ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 143
claimed that the endoplasmic reticulum of the follicle cells was associated
with the formation of ground substance, whereas Wartenberg and Stegner
(1960) suggested that special vacuoles of the ovum were involved. Since the
ground substance is a polysaccharide-containing structure, the involvement
of the endoplasmic reticulum, which is primarily associated with protein
synthesis, is questionable.
The diameters of the two types of processes that pass into the zona
pellucida are 0.2 μ for the macrovilli and 0.1 μ for the microvilli. It is
therefore apparent from these measurements that only the macrovilli
should be visible with the light microscope. This is borne out in Fig. 4
where, in a l-μ section of material prepared by electron-microscope techniques and examined with the phase-contrast microscope, macrovilli are
faintly evident extending down from the follicle cells. The presence of
radial striations in the zona pellucida, noted by early microscopists, would
presumably represent macrovilli. However, because these striations are
illustrated as being spaced very closely together and are restricted to the
zona pellucida itself, it is conceivable they represent microvilli. The microvilli, because of possible swelling associated with preparation, would then
become resolvable with the light microscope when, in actuality, they lie
below its limit of resolution.
The microvilli are very numerous, thin, and extend through the greater
part of the zona pellucida. At the time of maximum development in
urodeles, they reach the top of the zona radiata. These fingerlike projections are, in reality, extensions of the oocyte plasmalemma. They disappear
completely after cleavage.
The zona pellucida of mammals has the same basic organization as that
of lower vertebrates. It consists of a dense matrix into which microvilli
and macrovilli project. Electron-microscope studies of mammalia ova have
not resolved the problem of the origin of the ground substance either. Some
investigators (Trujillo-Cenoz and Sotelo, 1959; Chiquoine, 1960) feel that
their studies support the contention that the zona pellucida is a product of
the follicle cells, and others (Wartenberg and Stegner, 1960) feel that it is
a derivative of the ovum. In reality, the problem has not been resolved by
such morphological studies (Odor, 1960). What is probably needed is the
application of autoradiographic and histochemical techniques, in combination with electron microscopy. Since this approach is now becoming technically feasible (Caro and van Tubergen, 1962; Albersheim and Killias,
1963), it is reasonable to expect that resolution of the problem of the formation of the zona pellucida ground substance will be forthcoming.
The second process associated with zona pellucida formation involves
the formation of villi on both sides. These are represented by the striations
seen with the light microscope on the inner portion of the zona pellucida.
Some of the early investigators thought that these striations consisted
claimed that the endoplasmic reticulum of the follicle cells was associated
with the formation of ground substance, whereas Wartenberg and Stegner
(1960) suggested that special vacuoles of the ovum were involved. Since the
ground substance is a polysaccharide-containing structure, the involvement
of the endoplasmic reticulum, which is primarily associated with protein
synthesis, is questionable.
The diameters of the two types of processes that pass into the zona
pellucida are 0.2 μ for the macrovilli and 0.1 μ for the microvilli. It is
therefore apparent from these measurements that only the macrovilli
should be visible with the light microscope. This is borne out in Fig. 4
where, in a l-μ section of material prepared by electron-microscope techniques and examined with the phase-contrast microscope, macrovilli are
faintly evident extending down from the follicle cells. The presence of
radial striations in the zona pellucida, noted by early microscopists, would
presumably represent macrovilli. However, because these striations are
illustrated as being spaced very closely together and are restricted to the
zona pellucida itself, it is conceivable they represent microvilli. The microvilli, because of possible swelling associated with preparation, would then
become resolvable with the light microscope when, in actuality, they lie
below its limit of resolution.
The microvilli are very numerous, thin, and extend through the greater
part of the zona pellucida. At the time of maximum development in
urodeles, they reach the top of the zona radiata. These fingerlike projections are, in reality, extensions of the oocyte plasmalemma. They disappear
completely after cleavage.
The zona pellucida of mammals has the same basic organization as that
of lower vertebrates. It consists of a dense matrix into which microvilli
and macrovilli project. Electron-microscope studies of mammalia ova have
not resolved the problem of the origin of the ground substance either. Some
investigators (Trujillo-Cenoz and Sotelo, 1959; Chiquoine, 1960) feel that
their studies support the contention that the zona pellucida is a product of
the follicle cells, and others (Wartenberg and Stegner, 1960) feel that it is
a derivative of the ovum. In reality, the problem has not been resolved by
such morphological studies (Odor, 1960). What is probably needed is the
application of autoradiographic and histochemical techniques, in combination with electron microscopy. Since this approach is now becoming technically feasible (Caro and van Tubergen, 1962; Albersheim and Killias,
1963), it is reasonable to expect that resolution of the problem of the formation of the zona pellucida ground substance will be forthcoming.
The second process associated with zona pellucida formation involves
the formation of villi on both sides. These are represented by the striations
seen with the light microscope on the inner portion of the zona pellucida.
Some of the early investigators thought that these striations consisted