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Abstract
The thyroid gland carnes out highly specific functions such as synthesis, accumulation and release of hormones containing iodine and calcitonin with their various metabolic activities. These functions are reflected on the histological characteristics of the gland. However, there are various hormonal requirements of the body that either related to age, physiological changes or stress conditions. These requirements bring about modifications in the activity of the gland and significant changes in the histological pattern of the thyroid tissue. The thyroid gland plays an important and specific role in the relationship between the endocrine system and aging. It has been recognized that symptoms of aging can easily be confused with hypothyroidism. In the past decreased thyroid function was believed to be one of the hallmarks of the aging process. Thyroid diseases are common in the elderly, but their clinical manifestations are frequently different from those seen in younger patients. So aging may be defined as a time of reduced adaptability to metabolic perturbation. The aim of this work is to throw more light upon histological structure and ultrastructural changes of the thyroid glands in albino rats during postnatal development and aging. The thyroid gland is one of the most important endocrine glands in the body. It is composed of two independent endocrine cell populations concerned with the synthesis of two different classes of hormones. Both hormones have effective role on the body functions and control the activity of many vital organs. The present work was carried out to study the histological changes of the thyroid gland in albino rats during postnatal development and aging. F or this purpose, forty healthy male albino rats were utilized in this study They were classified into four groups (10 animals each) according to their ages. group A composed of neonatal animals that were further subdivided into Al (day-O-old) and A2 (one-week old), group B included infantile rats (one month old), group C was the adult (5-7 months) and group D contained senile animals (18-24 months). Animals were anaesthetized using ether inhalation; their thyroids were completely removed with the adjacent part of the trachea and processed for light and electron microscope examination. For light microscope examination, sections of 7 urn thick were obtained and stained with H&E stains to demonstrate the general structure of the thyroid gland. F or electron microscope examination, the thyroids were cut into small pieces, fixed in glutaraldhyde and osmium tetraoxide and processed to obtain ultrathin sections (90 nm).