ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG
141
ovulation is approaching, a presumptive perivitelline cleft develops between the zona pellucida and the peripheral oocyte cytoplasm. The regressing microvilli project into the granular substance that fills the cleft. After
ovulation, the egg is free from its cellular envelopes and is only enclosed
by the vitelline membrane. The latter represents the ground substance
of the zona pellucida after all the villi have withdrawn. At fertilization,
the evidence for the existence of microvilli is still present in the form of
short projections. These disappear by the time of cleavage.
FIG. 7. A three-dimensional representation of a portion of the layers surrounding
the developing oocyte, sen, surface epithelial cell nucleus; fn, fibroblast nucleus;
fen, follicle cell nucleus; zp, zona pellucida; mav, macrovilli; miv, microvilli; hi,
homogeneous layer; zr, zona radiata.
As the egg passes down the oviduct, it becomes surrounded by layers of
mucoid material. These layers are known as egg capsules and were recently
studied by Salthe (1963).
E. Discussion
The application of electron microscopy to the study of the egg envelopes
has established the cellular nature and specific characteristics of all three
outer strata. The most striking findings are related to the formation of the
zona pellucida. In the formation of this acellular stratum, two phenomena
take place simultaneously. One involves the deposition of a homogeneous
granular material between the basal membrane of the follicle cell and
the plasmalemma of the ovum. The other event involves the downward
extension of the basal follicle cell membrane in the form of macrovilli
and the upward projection of the plasmalemma surrounding the oocyte
in the form of microvilli. These two events will now be discussed more fully.
141
ovulation is approaching, a presumptive perivitelline cleft develops between the zona pellucida and the peripheral oocyte cytoplasm. The regressing microvilli project into the granular substance that fills the cleft. After
ovulation, the egg is free from its cellular envelopes and is only enclosed
by the vitelline membrane. The latter represents the ground substance
of the zona pellucida after all the villi have withdrawn. At fertilization,
the evidence for the existence of microvilli is still present in the form of
short projections. These disappear by the time of cleavage.
FIG. 7. A three-dimensional representation of a portion of the layers surrounding
the developing oocyte, sen, surface epithelial cell nucleus; fn, fibroblast nucleus;
fen, follicle cell nucleus; zp, zona pellucida; mav, macrovilli; miv, microvilli; hi,
homogeneous layer; zr, zona radiata.
As the egg passes down the oviduct, it becomes surrounded by layers of
mucoid material. These layers are known as egg capsules and were recently
studied by Salthe (1963).
E. Discussion
The application of electron microscopy to the study of the egg envelopes
has established the cellular nature and specific characteristics of all three
outer strata. The most striking findings are related to the formation of the
zona pellucida. In the formation of this acellular stratum, two phenomena
take place simultaneously. One involves the deposition of a homogeneous
granular material between the basal membrane of the follicle cell and
the plasmalemma of the ovum. The other event involves the downward
extension of the basal follicle cell membrane in the form of macrovilli
and the upward projection of the plasmalemma surrounding the oocyte
in the form of microvilli. These two events will now be discussed more fully.