Early Meiotic Prophase Research Articles

P32 autoradiographs of mouse testis; preliminary observations of the timing of spermatogenic stages.

(1) Testis material from mice killed 2, 5, 9 and 12 days after injection of 200 μC of P32 was autoradiographed by the stripping film technique and examined cytologically. (2) Cells at different stages of spermatogenesis were found to take up P32 differently. Some preliminary estimates as to the time taken by various stages of spermatogenesis are (a) from spermatogonial metaphase to early meiotic prophase, between 3 and 9 days; (b) from diakinesis to second metaphase, 4 days or less; (c) from diakinesis to immature sperm, 7 days or more; (d) from spermatogonial metaphase to immature sperm, more than 10 days. (3) Leydig cells were observed to show a strong uptake of P32 within the first 2 days. Some possible explanations for this observation are put forward.

THE STRUCTURE AND DEVELOPMENT OF THE CHROMOSOME SPIRALS IN MICROSPORES OFTRILLIUM

The structure and development of somatic spirals in microspore chromosomes of Trillium have been investigated. The chromonemata in each metaphase chromatid and each anaphase chromosome form a large-gyred, hollow spiral. This spiral develops gradually during prophase by an increase in gyre diameter and a decrease in gyre number and in chromatid length. Its development is associated with the elimination not the production of chromatid relational coiling. At later stages an irregular waviness or "minor" somatic spiral is visible along its "major" gyres in which reversals of direction can also be discerned. Where the spiral can be seen to be double-stranded it is plectonemic (as early as mid-prophase).The prophase to metaphase chromatid contraction ratio is not less than 6:1. Mean chromonema length increases from 650 ± 17.2 at metaphase to 977 ± 28.3 at anaphase. This latter length is approximately that estimated for early meiotic prophase. Chromosome volume also increases (about twofold) during the interval between metaphase and anaphase. Mean chromonema length and gyre number in microspore anaphase chromosomes are more than twice as great as those of meiotic anaphase chromosomes. Since the chromosomes at these stages are of approximately the same mean length the gyres of the somatic spiral are thus more tightly "packed".In Trillium, microspore anaphase chromosomes are considered to be of essentially the same spiral structure as meiotic second division chromosomes, i.e., a single coil (but not single-stranded), rather than two or more independently coiled chromatids. The process of reducing this plectonemic spiral into parallel, freely-separable chromatids begins in one prophase as a reduction in gyre number and continues as relational uncoiling in the next. Paradoxically, therefore, a spiralization cycle such as that described above can be interpreted as an uncoiling process in which successive cycles overlap in prophase.

Marsupial Spermatogenesis

In the three animals studied the total number of chromosomes in the male is as follows : Phascolarctus 16 (14 autosomes + XY). Sarcophilus 14 (12 autosomes + XY). Dasyurus 14 (12 autosomes + XY). In the female the number of chromosomes is as follows : Phascolarctus 16 (14 autosomes + XX). Sarcophilus 14 (12 autosomes + XX). In all animals dealt with in this paper the Y-chromosome is very minute in size compared with the other chromosomes; also the X-chromosome is much smaller than any of the autosomes. Chromomeres are conspicuous during syndesis, early pachytene, and early diplotene stages. The early pachytene stage is followed by a late pachytene stage in which the threads become diffuse and lose their capacity for taking up the stain. Except in the early meiotic prophase the sex chromosome remains compact and deeply stained and does not thread out like the autosomes. In all the above animals the first meiotic division is reductional, separating the X- and the Y-chromosomes, and the second division is equational, in each cell the sex chromosome dividing. The spermatozoa are therefore of two kinds, one containing an X-chromosome and the other containing a Y-chromosome. No further reduction in the number of chromosomes takes place during the second meiotic division. The Y-chromosome could not be identified during the meiotic phase until the metaphase of the first meiotic division. At this stage in Phascolarctus the sex chromosomes are separate and do not form a bivalent. The archoplasm seems to exert some influence on the chromatin threads at synizesis and during the early pachytene stage. In the former case the contraction takes place to that side of the nucleus at which the archoplasmic mass is situated; in the latter the chromosomes are in the form of thick loops with the ends of the chromosomes pointing towards the archoplasmic mass. In Phascolarctus the Sertoli cells are very large and possess peculiar rod-like bodies, the origin and function of which was not arrived at. The result of experiments seem to show that the rods are not affected by the action of digestive fluids.