ABSTRACT:The purpose of this study was to examine the tracheal structure of the crocodile Caiman latirostris using light microscopy, histochemical and immunocytochemical techniques. The tracheal epithelium of C. latirostris consists of a ciliated pseudostratified columnar epithelium with goblet cells. The respiratory epithelium also includes endocrine cells immunoreactive to serotonin. The histochemical techniques demonstrated the presence of neutral and sulphated mucins secreted by goblet cells. The lamina propria consists of connective tissue with many reticular fibres. The elastic fibres are interspersed among collagen bundles, forming the border between the mucosa and the submucosa. The submucosal layer consists of connective tissue similar to that found in the lamina propria. Serous or mucous glands were not observed. The predominant characteristic in the adventitia is the presence of an incomplete hyaline cartilage ring, in the form of a circle. Dense connective tissue fills the space between the extremities of each cartilage ring. Serotonin-immunoreactive cells frequently had an apical cytoplasmic process directed towards the lumen, and were therefore classified as open type. The α-actin immunohistochemistry revealed smooth muscle cells only in blood vessel walls, confirming the absence of a tracheal muscle.

The broad-snouted caiman, Caiman latirostris, is widely distributed in countries of South America. In Brazil it is considered an endangered species because of natural habitat destruction and illegal hunting. In reptiles, the thyroid gland plays an integral part in ecdysis, reproduction, tail regeneration, growth, endocrine function and metabolism rate. The aim of the present study was to characterize the thyroid gland morphology of C. latirostris, based on histochemical and immunohistochemical techniques. The thyroid glands were fixed in Bouin's fluid and serial cross sections were stained with hematoxylin-eosin, Mallory's trichrome, periodic acid-Schiff (PAS) and Alcian blue (AB pH 1.5 and 2.5). The immunohistochemical technique for 5-HT-IR cells was used. The thyroid gland has a dense irregular connective tissue forms a capsule enveloping the gland. There are several follicular acini of varying size lined by simple cuboidal or columnar epithelium in the thyroid gland. The follicles are connected by interfollicular connective tissue which contains blood vessels. We observed the presence of lymph nodes around the entire gland. There was a positive PAS reaction and negative AB reaction in the colloid. 5-HT-IR cells were detected around the follicle cells. No striking morphological differences were observed between C. latirostris and other domesticated mammals.