Ultrastructural changes in thyroid perifollicular capillaries during normal postnatal development and after infrared laser radiation.

This study was carried out on the perifollicular capillaries of the thyroid gland during postnatal (PN) development in normal conditions and after irradiation (46.8 J/cm(2)) with an infrared (IR) laser (904 nm). This was done using 11-, 21-, and 35-day-old Wistar rats. The changes in the capillaries were determined using quantitative methods as well as electron microscopy. During normal PN development the most relevant changes were an increase in the size of the capillaries, especially the lumen, thinning of the endothelium and an increase in the size of pinocytotic vesicles. Our results suggest that during PN development, the capillaries undergo some growth and maturation processes until they reach the optimal morphological conditions for their exchange functions. IR laser irradiation seems to stimulate the growth and maturation of endothelial cells in the youngest rats, while in older ones it causes irregular thickening of the endothelium and a reduction of the capillary lumen. These changes could be a sign of functional alterations in follicular cells caused by exposure to IR laser.