No abstract

The eyes of Sprague-Dawley rats were irradiated with doses of 2.5-10 Gy 250-kVp X rays, 1.25-2.25 Gy fission-spectrum neutrons (approximately 0.85 MeV), or 0.1-2.0 Gy 600-MeV/A 56Fe particles. Lens opacifications were evaluated for 51-61 weeks following X and neutron irradiations and for 87 weeks following X and 56Fe-particle irradiations. Average stage of opacification was determined relative to time after irradiation, and the time required for 50% of the irradiated lenses to achieve various stages (T50) was determined as a function of radiation dose. Data from two experiments were combined in dose-effect curves as T50 experimental values taken as percentages of the respective T50 control values (T50-% control). Simple exponential curves best describe dose responsiveness for both high-LET radiations. For X rays, a shallow dose-effect relationship (shoulder) up to 4.5 Gy was followed at higher doses by a steeper exponential dose-effect relationship. As a consequence, RBE values for the high-LET radiations are dose dependent. Dose-effect curves for cataracts were compared to those for mitotic abnormalities observed when quiescent lens epithelial cells were stimulated mechanically to proliferate at various intervals after irradiation. Neutrons were about 1.6-1.8 times more effective than 56Fe particles for inducing both cataracts and mitotic abnormalities. For stage 1 and 2 cataracts, the X-ray Dq was 10-fold greater and the D0 was similar to those for mitotic abnormalities initially expressed after irradiation.

Quiescent (G0) cells of the central zone region of the rat lens epithelium were recruited into the cell cycle by a wound stimulus. Cells were pulsed with labeled DNA precursor at several different times after the initiation of the DNA synthesis response to wounding and allowed to progress into the mitotic phase. Analysis of mitotic figures resulted in PLM (percentage labeled mitoses) curves that indicated a G2 duration of about 6 h. Double isotopic labeling ([3H]thymidine followed by [14C]thymidine) was utilized to demonstrate the completion of DNA synthesis in earliest responders. Cells completed DNA synthesis in less time (3-5 h) than reflected by the approximately 8-h widths of PLM curves. This discrepancy is attributed to the uptake and retention of labeled precursor by the stimulus-responsive cells while they are still in a pre-S phase condition. Based on a comparison of transit times through G2 and of labeling times to midpoint appearances of labeled mitotic figures, earlier responders do not appear to have faster rates of cell cycle progression than cells responding 2-4 h later. G2 transit time is also comparable for central zone lens cells responding to the relatively strong stimulus of wounding and for the nonperturbed cells previously studied in the germinative zone of the lens epithelium.

Quiescent cells of the central zone region of the rat lens epithelium were stimulated to enter the proliferation cycle by wounding. RNA synthesis and a corresponding increase in poly(A)+/total RNA reached a peak by Hour 4. Cells progressed into the G1B compartment by Hour 10. A rise in protein synthesis began at Hour 8, and onset of DNA synthesis occurred by Hour 14. The timing of cell cycle progression that allowed escape from a dose of X irradiation that completely inhibited DNA synthesis was investigated. A growth-arrest point was identified at Hour 9 where 10 GY of X irradiation given before, but not after, completely inhibited earliest responding cells from entering DNA synthesis on schedule. Increased quantities of cells entered DNA synthesis on schedule as timing of the X irradiation was moved closer to the end of G1. Based on time relationships, the rise in protein synthesis is correlated with the "sufficient" event for the escape.

The mammillary body (MB) comprises several nuclei lying at the posterior end of the hypothalamus that have been implicated in spatial memory function. The MB receives hippocampal input and projects to the thalamus and tegmentum, thus linking together forebrain and midbrain areas. Here we show that the LIM-HD transcription factor Lhx5 is expressed in the MB primordium early in mouse embryogenesis. Targeted inactivation of Lhx5 results in a dramatic reduction of MB area and a concomitant absence of efferent projections perinatally. The early specification of the MB area appeared severely altered, as shown by the lack of expression of Foxb1, Sim2, and other novel MB markers by in situ hybridization. Interestingly, Lhx5 À/À embryos expressed Lhx2 and Lhx9 ectopically in this area, suggesting an intricate regulation of gene expression between LIM-HD members during mammillary body development.