ULTRASTRUCTURE OF THE AMPHIBIAN EGG
181
is made most conspicuous by the elongation of the lateral loops from
the chromomeric axis (Figs. 11 and 12). The nuclear envelope has lost
its even contour and has become undulated. Nucleoli are located almost
exclusively near the periphery. By the end of this phase, the nucleus
has attained a diameter of 250-350 μ. By the late diplotene phase,
the nucleus has attained its maximal size of 350-500 μ, and the
chromosomes have become contracted. At this time, the chromosomes
are aggregated near the center of the nucleus, and their lateral loops
are only slightly evident. The nucleoli are arranged around the mass
of chromosomes and are absent in the region adjacent to the undulated
nuclear envelope.
The most dramatic event occurring during prophase, as has been
described, is the formation of the lampbrush (lateral loop) chromosomes
during the mid-diplotene phase. Their fine structure will now be
discussed.
IV. Lampbrush Chromosomes
An understanding of the current concept of the ultrastructure of the
lampbrush chromosomes can best be appreciated from a consideration
of the subject by means of a historical review.
The study of lampbrush chromosomes has a long history dating from
1882, when they were first seen by Flemming (see details in review by
Callan, 1957). The highlights of these investigations are shown in Fig. 13.
Riickert (1892), who studied sectioned elasmobranch oocytes, was the
first to recognize the nuclear structures as chromosomes (Fig. 13A). He
described them as consisting of lateral filaments that in some stages
extend from axial micromeres. The filaments, which were responsible
for what he described as the "lampbrush" appearance of the chromosomes, were sometimes seen as loops. Born (1892), at the same time
described each of these chromosomes as being made up of one continuous,
long, chromosomal filament projecting laterally in the form of loops
(Fig. 13B).
The studies of lampbrush chromosomes for the next 45 years were all
on sectioned preparations. The inherent deficiencies of this approach was
the inability to reconstruct a reliable three dimensional model, which
resulted in limited contributions of such studies to an understanding of
the morphology of the chromosomes. In 1937, Duryee showed that lampbrush chromosomes could be isolated easily from the nuclei of amphibian oocytes and studied in an unfixed state. He described each
chromosome as a "cylinder" containing a series of paired chromosomes
from which granular loops were thought to arise (Fig. 13C).
Further significant advances in the study of lampbrush chromosomes
awaited the development of phase-contrast microscopy. By using such
181
is made most conspicuous by the elongation of the lateral loops from
the chromomeric axis (Figs. 11 and 12). The nuclear envelope has lost
its even contour and has become undulated. Nucleoli are located almost
exclusively near the periphery. By the end of this phase, the nucleus
has attained a diameter of 250-350 μ. By the late diplotene phase,
the nucleus has attained its maximal size of 350-500 μ, and the
chromosomes have become contracted. At this time, the chromosomes
are aggregated near the center of the nucleus, and their lateral loops
are only slightly evident. The nucleoli are arranged around the mass
of chromosomes and are absent in the region adjacent to the undulated
nuclear envelope.
The most dramatic event occurring during prophase, as has been
described, is the formation of the lampbrush (lateral loop) chromosomes
during the mid-diplotene phase. Their fine structure will now be
discussed.
IV. Lampbrush Chromosomes
An understanding of the current concept of the ultrastructure of the
lampbrush chromosomes can best be appreciated from a consideration
of the subject by means of a historical review.
The study of lampbrush chromosomes has a long history dating from
1882, when they were first seen by Flemming (see details in review by
Callan, 1957). The highlights of these investigations are shown in Fig. 13.
Riickert (1892), who studied sectioned elasmobranch oocytes, was the
first to recognize the nuclear structures as chromosomes (Fig. 13A). He
described them as consisting of lateral filaments that in some stages
extend from axial micromeres. The filaments, which were responsible
for what he described as the "lampbrush" appearance of the chromosomes, were sometimes seen as loops. Born (1892), at the same time
described each of these chromosomes as being made up of one continuous,
long, chromosomal filament projecting laterally in the form of loops
(Fig. 13B).
The studies of lampbrush chromosomes for the next 45 years were all
on sectioned preparations. The inherent deficiencies of this approach was
the inability to reconstruct a reliable three dimensional model, which
resulted in limited contributions of such studies to an understanding of
the morphology of the chromosomes. In 1937, Duryee showed that lampbrush chromosomes could be isolated easily from the nuclei of amphibian oocytes and studied in an unfixed state. He described each
chromosome as a "cylinder" containing a series of paired chromosomes
from which granular loops were thought to arise (Fig. 13C).
Further significant advances in the study of lampbrush chromosomes
awaited the development of phase-contrast microscopy. By using such