190
SAUL WISCHNITZER
nucleus into the cytoplasm and thus, transmit nuclear information
(Merriam, 1959).
The ultrastructure of the nuclear envelope was most recently reviewed
by Beams (1964), Gall (1964), N0rrevang (1965), and Sichel (1966).
The present review has pointed out the limitations of the electron
microscope as a tool for understanding the morphology of the lampbrush chromosome. This approach has, however, served to provide confirmatory evidence in support of the chromosome model (Fig. 13F) developed from phase-contrast microscope data.
The inadequacy of the electron microscope for chromosomal studies
is possibly due to several factors. These include deficiencies of our
present techniques of nuclear fixation, lack of limiting membranes
around the chromosomes, and absence of evidence of organization within
the chromosome, as seen in thin sections (Fig. 21). Any organization
present is distributed in depth in the whole nucleus, and the reconstruction of three-dimensional images is a very formidable task dependent
upon ultrathin serial sections. Moreover, the use of smear or spread
techniques, the classical methods of light microscopy, has also not
proven useful. Technical improvements along these lines are being made
(Gall, 1963a), and more fruitful results from chromosomal studies may
be anticipated.
The formulation of a theory of lampbrush chromosome morphology,
with the aid of electron-microscope data has served to provide a stiuctural framework useful in clarifying their physiological activities. Their
unusual structural modification is a reflection of physiological functions
which apparently are of such vital significance that the meiotic process
itself needs to be suspended for a prolonged period.
An understanding of the functional activities of the chromosomes is
based on an integration of our knowledge of their morphology and their
chemical composition (see review by Callan, 1963). The latter information was initially obtained by specific staining methods and was confirmed and extended by subsequent studies based on enzymatic digestion.
Thus, it was first established (Dodson, 1948) that the chromomeres are
Feulgen-positive while the lateral loops are Feulgen-negative. The presence of basic staining protein in association with the DNA of the
chromomeres was established by the treatment of the chromosomes with
trichloroacetic acid and subsequent staining with fast green at alkaline
pH. The presence of ribonucleic acid (RNA) on the lateral loops of
fixed preparations was inferred from their stainability with azure B or
toluidine blue at an acid pH. This staining affinity is lost after RNase
digestion. In unfixed preparations, RNase serves to remove the matrix
material and thus, uncovers the loop axes. The presence of DNA on the
axis of the loops and interchromomeric segments has been inferred from
SAUL WISCHNITZER
nucleus into the cytoplasm and thus, transmit nuclear information
(Merriam, 1959).
The ultrastructure of the nuclear envelope was most recently reviewed
by Beams (1964), Gall (1964), N0rrevang (1965), and Sichel (1966).
The present review has pointed out the limitations of the electron
microscope as a tool for understanding the morphology of the lampbrush chromosome. This approach has, however, served to provide confirmatory evidence in support of the chromosome model (Fig. 13F) developed from phase-contrast microscope data.
The inadequacy of the electron microscope for chromosomal studies
is possibly due to several factors. These include deficiencies of our
present techniques of nuclear fixation, lack of limiting membranes
around the chromosomes, and absence of evidence of organization within
the chromosome, as seen in thin sections (Fig. 21). Any organization
present is distributed in depth in the whole nucleus, and the reconstruction of three-dimensional images is a very formidable task dependent
upon ultrathin serial sections. Moreover, the use of smear or spread
techniques, the classical methods of light microscopy, has also not
proven useful. Technical improvements along these lines are being made
(Gall, 1963a), and more fruitful results from chromosomal studies may
be anticipated.
The formulation of a theory of lampbrush chromosome morphology,
with the aid of electron-microscope data has served to provide a stiuctural framework useful in clarifying their physiological activities. Their
unusual structural modification is a reflection of physiological functions
which apparently are of such vital significance that the meiotic process
itself needs to be suspended for a prolonged period.
An understanding of the functional activities of the chromosomes is
based on an integration of our knowledge of their morphology and their
chemical composition (see review by Callan, 1963). The latter information was initially obtained by specific staining methods and was confirmed and extended by subsequent studies based on enzymatic digestion.
Thus, it was first established (Dodson, 1948) that the chromomeres are
Feulgen-positive while the lateral loops are Feulgen-negative. The presence of basic staining protein in association with the DNA of the
chromomeres was established by the treatment of the chromosomes with
trichloroacetic acid and subsequent staining with fast green at alkaline
pH. The presence of ribonucleic acid (RNA) on the lateral loops of
fixed preparations was inferred from their stainability with azure B or
toluidine blue at an acid pH. This staining affinity is lost after RNase
digestion. In unfixed preparations, RNase serves to remove the matrix
material and thus, uncovers the loop axes. The presence of DNA on the
axis of the loops and interchromomeric segments has been inferred from