Electron microscopical analysis of meiotic chromosomes from human spermatocytes during and after treatment with steroid hormones

1980

Serial sectioning and three dimensional reconstruction of 82 nuclei divided into 15 well characterized substages covering the period from early zygotene to metaphase I in diploid Bombyx spermatocytes have permitted the following observations and conclusions: 1) At late zygotene, a mean of four chromosome and bivalent interlockings was found per nucleus. 2) During the transition from zygotene to pachytene, all interlockings resolve by breakage and reunion Of chromosomes or bivalents. 3) Recombination nodules first appear at early zygotene, reach a maximum of 91 per nucleus by late zygotene, decrease to 55 per nucleus at mid pachytene and increase again to 70-75 towards the end of pachytene. 4) At late z~cgotene and pachytene, nodules are essentially distributed at random among bivalents as well as along bivalents. 5) Small, but important deviations from randomness were, however, revealed: Three percent of the bivalents were without nodules at late zygotene, 8 % at early pachytene but at mid and late pachytene, only 2 % were devoid of nodules. 6) From mid pachytene, an increasing fraction of the recombination nodules becomes larger and more irregular in shape. These are termed chromatin nodules and at the pachytene-diplotene transition, nearly all nodules are of this type. 7) During early diplotene, the bulk of the chromatin decondenses leaving only about 60 major condensed domains. These domains originate from chromatin nodules and are frequently associated with a piece of synaptonemal complex which in its central region contains a dense core resembling a recombination nodule. 8) At late diplotene, the condensed domains of the bivalents consist of two dense chromatin segments, one on each of the homologues, and bridged by a complex circular structure giving the regions a characteristic tripartite appearance. 9) At mid diakinesis, the circular structures are located in chromatin bridges -chiasmata -between the homologous chromosomes. 10) The number of chiasmata per nucleus is constant from mid diplotene to mid diakinesis and is distributed among the bivalents similar to the distribution of recombination nodules at mid and late pachytene. 11) The circular chiasma components are eliminated from late diakinesis until metaphase I. 12) Finally, each metaphase I bivalent contains four distinct localized centromeres, two by two facing opposite spindle poles. 485

Section: General Aspectssupporting

confidence: 65%

“…The distribution of these bridges was very similar to the distribution of recombination nodules in sectioned material (5,66) and it is conceivable that the bridges represent recombination nodules or their derivatives.…”

Section: General Aspectsmentioning

confidence: 51%

Chromosome pairing, recombination nodules and chiasma formation in diploid Bombyx males

1980

“…These observations are at vailance with the much lower number of bars reported by BERTH ELSEN et al (1) and neither of the investigations explored the relationship between recombination nodules, bars and chiasmata. With the exception of the zygotene stage and the very first period ofpachytene (13) the progressive changes in structure, number and distribution of the recombination nodules during human spermatogenesis have thus not been satisfactorily mapped.…”

Section: Introductioncontrasting

confidence: 44%

“…The ultrastructural changes in nuclear morphology during pachytene have been described in several papers (1,2,7,16,17,18,19) and the following discussion will therefore concentrate upon aspects not adequately accounted for by previous investigations.…”

Section: The Course Of the Pachytene Stagementioning

confidence: 99%

Human meiosis V. Substages of pachytene in human spermatogenesis

1983

The pachytene stage of human spermatogenesis has been investigated by serial sectioning of spermatocytes and three dimensional reconstruction of nuclei from electron micrographs. The analysis of 73 completely reconstructed normal nuclei from late zygotene to early diplotene revealed a continuous sequence of changes in the fine structure and morphology. The developmental sequence permits the recognition of 7 substages on the basis of morphological changes of autosomal chromatin, centromeric chromatin, the XY bivalent, the secondary constriction on bivalents l, 9 and 16, the nucleolus and the centrioles. Of particular interest is the observation that the disappearance of the synaptonemal complex segment between the X and Y chromosomes at mid pachytene is only temporary, a distinct complex segment being present in nearly all analysed XY bivalents by the end of pachytene. The number of recombination nodules and recombination bars in nuclei of the different substages is recorded. The total number of nodules decreases from 75 at the beginning of pachytene to l or none at the end of this stage. The bars appear at early pachytene, reach a number of 35 at mid pachytene and disappear following the nodules towards the end ofpachytene. The morphology of pachytene nuclei at various stages of degeneration as well as the structural characteristics of two tetraploid spermatocytes are described.

“…Morphologically identical bars have previously been implicated in crossing over at pachytene in rat spermatocytes (37) and human spermatocytes (3,4,46) and are also present in human oocytes (6). Analogous structural changes in recombination nodules during pachytene have, however, also been described in Bombyx (25) and Coprinus (26) and may be inferred from observations in Drosophila (10) and Neurospora (19).…”

Section: The Total Number Of Crossovers Per Nucleusmentioning

confidence: 60%

Human meiosis VI. Crossing over in human spermatocytes

1983

The structure, number and distribution of recombination nodules in human spermatocytes have been analyzed by serial sectioning and three dimensional reconstruction of 10 late zygotene and 73 pachytene nuclei, the pachytene nuclei being allocated to seven substages. The analysis has permitted the following observations and conclusions. 1) Recombination nodules bind to the central region of the synaptonemal complex at zygotene, increase in size and transform, at pachytene stage 2, into a bridge-like structure termed a bar. During pachytene stages 3 and 4, the bars become more spindle shaped and are often oriented obliquely to the synaptonemal complex. By pachytene stages 5 -7 the bars appear to resolve into filaments and disappear eventually. 2) The number of nodules decreases from a mean of 101 at late zygotene (72 percent pairing) to 75 at pachytene stage I. At pachytene stage 2, 58 nodules and 16 bars are present, the corresponding figures for pachytene stages 3, 4 and 5 being ll and 35, 6 and 35 and 7 and 24. At the later stages very few nodules and bars are seen. 3) Nodules are distributed at random among the bivalents and the bivalent arms at zygotene while at pachytene stage 3 + 4, 99 percent of the bivalents and 75 percent of the bivalent arms have at least one nodule or bar. By stage 5 the nodulebar distribution is to a large extent random. 4) The structural, numerical and distributional changes of nodules and bars suggest that the transformation of a nodule into a bar reflects a crossing over event. 5) As not all crossovers of a nucleus can be observed at the same time, the total crossover distribution was constructed by cumulating the frequencies of bars at stage 2 and the nodules and bars at stage 3-7. The frequency of crossing over is high in the distal segments and low near the centromeres. In longer bivalent arms crossing over is also frequent in one or more interstitial segments. Crossing over occurs in 75 percent of the XY bivalents as judged by the frequency of nodules in the synaptonemal complex which combines the homologous regions of the X and Y chromosomes. 6) The total number of crossovers is estimated to range from 69 to 73 per nucleus and compares with a mean of 50 chiasmata at diakinesis. 7) The distribution ofchiasmata at diakinesis and that of crossovers at pachytene differs, especially in the short arms. 8) One crossover (rarely two) in the form of a recombination structure is present in the short bivalent arms. Several crossovers are present in the longer arms, but preferentially placed in the middle of two or three domains. Once a crossover has occurred in a domain, the probability for a second crossover to be realized in the same domain is decreased. The limitation to the placement of a single crossover in a short arm and in a domain of a long arm is a source for positive interference in genetic linkage analyses.Springer-Verlag 0105 -1938/83/0048/0385/$05.80