On the basis of reports that deoxyguanosine is selectively toxic for adult T lymphocytes, the usefulness of this compound in the production of lymphocyte-depleted embryonic thymus rudiments in an in vitro organ culture system was investigated. The results showed that a period of exposure to deoxyguanosine causes depletion of the lymphoid cells while the stromal elements continue to survive, with many of the cells showing an epithelial morphology and expression of I region products of the major histocompatibility complex (MHC). When associated with either fetal liver or another thymus fragment as a source of T cell precursors in transfilter experiments, these "empty" thymuses become recolonized, enabling the production of chimeric thymus with stromal and lymphoid cells of different haplotypes. In combination with functional assays, this system offers an entirely in vitro approach to questions relating to the repertoire potential of T cell precursors from different sources and the role of the thymus in tolerance and MHC restriction.
Immunogold labelling was used to study the distribution of newly synthesized slow muscle myosin (SM) at the ultrastructural level as it replaced fast muscle myosin (FM) in rabbit muscles undergoing stimulation-induced type transformation. Control fast muscle was labelled strongly with antibody to FM and control slow muscle with antibody to SM; label was confined to the A-band. Well-defined differences in the distribution of label within the A-band suggested that the monoclonal antibodies used corresponded to epitopes on different parts of the myosin molecule; this was confirmed by Western blots of subfragments prepared from FM and SM. After 4 weeks of continuous stimulation at 10 Hz, fibres of the tibialis anterior muscle reacted with antibodies to both isoforms; after 6 weeks, labelling was obtained only with antibody to SM. After a 7-week period of stimulation and 3 further weeks of recovery, fibres again reacted with both antibodies. In all positively-labelled sections, the distribution of gold particles was characteristic of the antibody and independent of the origin or history of the fibres. This observation supports the conclusion that newly synthesized myosin is capable of being incorporated throughout the length and cross-section of the A-band.
Ovaries from eighty foetal and neonatal rats (aged 16·0 days
post coitum
to 4 days
post partum
) were examined under the electron microscope. All the normal developmental stages (oogonia and oocytes in the leptotene, zygotene, pachytene, diplotene and dictyate phases of meiotic prophase) were identified. Patterns of degeneration (‘atretic divisions’, ‘
Z
’ cells and atresia at the diplotene phase), whose histological appearance and incidence have been recorded by Beaumont & Mandl (1962), were also recognized. The nuclei of oocytes at the leptotene phase contain single electron dense threads which become aligned in parallel pairs at the following phase (zygotene). A third finer fibril half-way between them appears at pachytene (tripartite ribbon). The longitudinal segments of threads, visible in ultra-thin sections, become shorter, presumably due to coiling. Nuclei at the diplotene phase contain single threads essentially similar to those seen at leptotene. As the oocyte enters the dictyate or resting phase, electron dense threads disappear from the nucleus. These observations suggest that the threads represent chromosomal ‘cores’. Nucleolus-like components persist throughout meiotic prophase, and at the diplotene phase regain the more complex form typical of oogonia. The cytoplasmic organelles become more numerous and complex as the oocyte approaches the dictyate phase. ‘Atretic divisions’ in oogonia are characterized by the persistence of long segments of nuclear membrane and the appearance of vesicles enveloped by a double membrane resembling the nuclear envelope. The dense masses of ‘chromatin’ (mitotic chromosomes) are more rounded than in normal cells at metaphase, and tend to coalesce. Spindle fibres have not been observed. Cytoplasmic organelles tend to increase in number and complexity. ‘
Z
’ cells (cells degenerating largely at the pachytene phase) show heavy ‘chromatin’ condensation around the tripartite ribbons. The major cytoplasmic changes consist in swelling of the endoplasmic reticulum, vacuolation of mitochondria and increase in incidence of multilamellar bodies. Atretic oocytes at the diplotene phase differ markedly from ‘
Z
’ cells in that ‘chromatin’ condensation around electron dense threads (single) is markedly less prominent. Cytoplasmic changes are similar to those of ‘
Z
’ cells, but also include a rise in the incidence of multivesicular and other complex bodies. All three types of degenerating cells are removed from the ovary by the phagocytic activity of neighbouring somatic cells.
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