الفهرس 
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Abstract
In addition to anti-tumor effects, ionizing irradiation causes damage in normal tissues located in the field of radiation. This became particularly evident in the head and neck region, a complex area composed of several dissimilar structures that respond differently to radiation.
Although many authors had studied the effects of ionizing radiation on different head and neck structures, the majority of them concerned with the effects on mucosal linings, skin coverings, subcutaneous connective tissue, salivary gland tissue, bone and cartilage. Only few studies had investigated the effects of radiotherapy on different pulp components. Any disturbance in the pulp tissue leading to infection may pre-dispose to osteoradionecrosis, which is the most serious complication of head and neck radiotherapy.
The present study aimed to investigate the effect of ionizing radiation on the different components of mandibular 1st molar dental pulp of Albino rats.
Sixty adult male albino rats weighing about 200-250 grams were utilized in this study and divided into three groups:
1)Control group: It consisted of 30 non-irradiated rats (each 5 rats were killed at the same time with their correspondent experimental group).
2) Experimental group I (Gp I): It consisted of 15 rats that received 2.5 Gy of gamma rays.
3) Experimental group II (Gp II): It consists of 15 rats received 5 Gy of gamma rays.
Each experimental group was further divided into 3 subgroups (5 rats each) according to the time of killing or sacrifaction:
-Subgroup A: Rats that were killed 2 weeks after irradiation.
-Subgroup B: Rats that were killed 4 weeks after irradiation.
-Subgroup C: Rats that were killed 8 weeks after irradiation.
At the appropriate time for each subgroup, rats were killed by cervical dislocation. After cutting the incisor region from the mandible, the molar areas were dissected and prepared for histological, ultra structural and immunohistochemical examinations.
Histological Results:
The pulp tissue of the control groups showed normal architecture. However, the irradiated groups showed disrupted architecture in the form of: irregularity of the odontoblastic layer, hardly defined CFZ and CRZ, irregular cellular density in pulp core, and areas of connective tissue destruction of variable sizes. The blood vessels appeared narrow in those exposed to 2.5 Gy of γ-rays, while appeared dilated in those with 5 Gy dose. All of the previously mentioned changes were more aggravated in 5 Gy dose and after 4 weeks following irradiation. These changes also showed a gradual improvement upon progression from 4 to 8 weeks following irradiation.
Ultra-structural Results:
Control groups appeared of normal appearance of pulp elements. The experimental groups showed apoptotic changes affecting mainly odontoblasts and fibroblasts, and also the endothelial cells of some blood vessels. These changes appeared as irregularily shaped cell, indistinct cell boundaries, loss of cellular junctions, atrophied cells with pyknotic and fragmented nuclei, degenerated cellular organelles and cytoplasmic clumping &/or vacuolations. Disorganized collagen fibers and thickened basal lamina of blood vessels were also noted. Clearly defined neutrophils, eosinophils, lymphocytes and macrophages observed in 8 weeks after irradiation. Nerve fibers showed affection in the form of myelin sheath disruption, only after 4 weeks of irradiation. Again all of these pulpal changes were more exaggerated with 5 Gy dose and upon progression from 2 to 4 weeks following irradiation, while showed a gradual improvement upon progression from 4 to 8 weeks following irradiation.
Immunohistochemical results:
Immunopositive cells for Anti-active Caspase 3 antibody were observed in the pulp tissues of all groups. Positive cells appeared throughout the odontoblastic layer as well as other areas of pulp zones. The total area of immunopositive cells for each group was calculated and the collected data were tabulated for statistical analysis.
Statistical results:
The least values of total area of positive cells (for Anti active Caspase-3) obtained from image analysis results were observed in the control group. The value gradually increased to reach the highest peek at 4 weeks following irradiation. There was a reduction in the number of immunopositive cells after 8 weeks of irradiation. In all subgroups the total areas of positive cells were higher with 5 Gy dose than those of 2.5 Gy of irradiation dose.