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SAUL WISCHNITZER
The annulate lamellae consist of a number of parallel, double-membraned
sheets, each of which closely resembles the nuclear envelope in structure.
In cross sections, it has been found that the individual membranes are
from 120 to 170 Â in diameter and are interrupted by discontinuities across
which a diaphragm may extend (Balinsky and Devis, 1963). In tangential
sections, annuli whose rims appear to contain subunits (Kessel, 1963) are
seen. These have an inside diameter of 600 A and occasionally contain central granules. These fine structural details are in essential agreement with
those pertaining to the nuclear envelope.
The morphological resemblance of the annulate lamellae and their
proximity to the nuclear envelope have long suggested that the former
arise in some manner from the latter. Fragmentation or delamination from
the nuclear envelope were offered as possible modes of origin. Until recently, direct evidence for such a morphogenesis has been lacking.
Recently, blebbing of the nuclear envelope has been observed in oocytes
from the amphibian Triturus (Wischnitzer, 1963b). A significant result of
this phenomenon is strikingly evident in Necturus oocytes (Kessel, 1963).
In this form most of the outer nuclear membrane of smaller oocytes is
involved in bleb formation (Fig. 18). The blebs are apparently released
simultaneously into the cytoplasm as groups of vesicles arranged in a long
chain or row adjacent to the nuclear membrane. Repetition of this phenomenon results in the formation of chains of vesicles linearly arrayed in
the cytoplasm. With the deposition of more such chains, the distance
between individual rows diminishes and a complex vesicular system is
organized. Simultaneously, some of the vesicles undergo fusion to form
short, flattened cisternae. This process continues, and the narrowing rows
of vesicles are gradually converted to narrowing rows of cisternae. With
continued fusion and addition of other rows, the complex vesicular system
becomes converted into the annulate lamellae.
Balinsky and Devis (1963) have studied the formation of annulate
lamellae during the maturation of the oocyte Xenopus. At the time of early
yolk formation, they found an organized array of membranous sheets to
be present, but did not describe their origin. In both these and older oocytes,
the lamellae consist of concentrically arranged double-membraned sheets.
In more mature oocytes, the sheets are predominantly aligned parallel
and are located in the more peripheral parts of the cytoplasm. In the mature
egg, the annulate lamellate have almost reached the surface and lie just
beneath the layer of cortical granules. In ovulated eggs, the membranes
making up the annulate lamellae have broken down into a cluster of vesicles
located just below the cortical layer. After fertilization, when the cortical
granules have disappeared, the vesicles are no longer arranged in clusters
but rather are distributed as a continuous layer beneath the plasmalemma.
SAUL WISCHNITZER
The annulate lamellae consist of a number of parallel, double-membraned
sheets, each of which closely resembles the nuclear envelope in structure.
In cross sections, it has been found that the individual membranes are
from 120 to 170 Â in diameter and are interrupted by discontinuities across
which a diaphragm may extend (Balinsky and Devis, 1963). In tangential
sections, annuli whose rims appear to contain subunits (Kessel, 1963) are
seen. These have an inside diameter of 600 A and occasionally contain central granules. These fine structural details are in essential agreement with
those pertaining to the nuclear envelope.
The morphological resemblance of the annulate lamellae and their
proximity to the nuclear envelope have long suggested that the former
arise in some manner from the latter. Fragmentation or delamination from
the nuclear envelope were offered as possible modes of origin. Until recently, direct evidence for such a morphogenesis has been lacking.
Recently, blebbing of the nuclear envelope has been observed in oocytes
from the amphibian Triturus (Wischnitzer, 1963b). A significant result of
this phenomenon is strikingly evident in Necturus oocytes (Kessel, 1963).
In this form most of the outer nuclear membrane of smaller oocytes is
involved in bleb formation (Fig. 18). The blebs are apparently released
simultaneously into the cytoplasm as groups of vesicles arranged in a long
chain or row adjacent to the nuclear membrane. Repetition of this phenomenon results in the formation of chains of vesicles linearly arrayed in
the cytoplasm. With the deposition of more such chains, the distance
between individual rows diminishes and a complex vesicular system is
organized. Simultaneously, some of the vesicles undergo fusion to form
short, flattened cisternae. This process continues, and the narrowing rows
of vesicles are gradually converted to narrowing rows of cisternae. With
continued fusion and addition of other rows, the complex vesicular system
becomes converted into the annulate lamellae.
Balinsky and Devis (1963) have studied the formation of annulate
lamellae during the maturation of the oocyte Xenopus. At the time of early
yolk formation, they found an organized array of membranous sheets to
be present, but did not describe their origin. In both these and older oocytes,
the lamellae consist of concentrically arranged double-membraned sheets.
In more mature oocytes, the sheets are predominantly aligned parallel
and are located in the more peripheral parts of the cytoplasm. In the mature
egg, the annulate lamellate have almost reached the surface and lie just
beneath the layer of cortical granules. In ovulated eggs, the membranes
making up the annulate lamellae have broken down into a cluster of vesicles
located just below the cortical layer. After fertilization, when the cortical
granules have disappeared, the vesicles are no longer arranged in clusters
but rather are distributed as a continuous layer beneath the plasmalemma.