Thyroid gland comprises two parts which are different in structure, function and origin, namely follicular and parafollicular compartments with thyrocytes and calcitoninocytes as the main cellular populations in them accordingly. Thyrocytes constitute simple epithelial lining of the follicles and originate from the endodermal epithelial lining of the pharyngeal floor, while calcitoninocytes may either be present interfollicularly or included in the follicular epithelial lining being separated from the colloid by the intertwining processes of the neighboring thyrocytes, they originate from the 4th pharyngeal pouch populated by the neural crest cells and are considered to be APUD-cells (K.Pacak et al., 2001; J.Seidel et al, 2003; J.Dadan et al., 2004; M.K.Irmak et al., 2004; Y.Kameda et al., 2007). For a long time calcitoninocytes were thought to have no significance for the calcium metabolism control, but later investigations revealed their important role in stress-related activation of the hypothalamo-hypophyseo-thyroid axis (N.Pondel et al., 2000; V.Rajkovic et al., 2001; M.A. Titova et al., 2003; V.I.Loginov, 2007). Recently evidence was provided that close functional interactions between the two cellular types are controlled by the paracrine mechanism (B.Sawicki et al., 2002; R.L.Zbuckie et al., 2007; M.Gutiérrez-Mariscal et al., 2008). After birth thyroid gland undergoes significant structural and functional changes in the follicular compartment (S.K.Banu et al., 2001; 2002; D.G.Moreira et al., 2005). The dynamics of the parafollicular cell population and their interaction with the follicular cells after birth was not described before.
The objective of the present study is to evaluate age-related changes in the follicular and parafollicular compartments of the thyroid gland in the growing body.
Thyroid gland of the Spargue-Dawley rats aged 14 days (preweaning period), 21 days (weaning period) and 30 days (infant period) was sampled, fixed in formalin, embedded in paraffin, serially sectioned and stained by hematoxylin-eosin and immunohistochemically for thyroglobulin (marker of thyrocytes) and calcitonin (marker of calcitoninocytes) using biotin-streptavidin-peroxidase complex technology with subsequent image analysis of the immunohistochemically stained sections by the Leica image analyser (Germany) with Leica QWin sofware (Great Britain).
The results obtained demonstrated that during early postnatal ontogenesis in rats both follicular and parafollicular compartments undergo significant morphological and immunohistochemical changes which may be quantitatively evaluated. These changes include morphometric parameters of the follicles and distribution of the calcitoninocytes and thyrocytes in the parenchyma of the thyroid gland. It was shown that in the follicular compartment the developmental changes include an increase of the average thyrocyte height and follicle diameter (difference between the 14-day and 30-day old rats being significant, p<0,05), while the numeric density of the follicles was significantly reduced from preweaning to infant period, and changes of the activation index with age did not reach a level of significance. The volume density of the thyrogobulin-immunoreactive cells was slightly higher in the weaning rats compared to the preweaning pups and in the infant rats compared to the weaning one, while in the infant rats it was meaningfully increased (p<0,05) compared to the preweaning animals. The volume density of the calcitoninocytes was increasing with age reaching the level of significance by the infant period. Correlation analysis demonstrated that in the preweaning and weaning rats negative correlation between the number of calcitoninocytes and thyrocytes was insignificant, while by the infant period this correlation became strong and significant (r=-0,71; p<0,05). These observations demonstrate that by the infant period the thyroid gland of the growing rats reaches certain level of functional maturity, which may contribute to the developing adaptational potential of the body in the changing environmental conditions to which thyroid gland is extremely sensitive. This observation should be taken into consideration in evaluation of the adaptational changes of the thyroid gland as a peripheral link of the hypothalamo-hypophyseo-thyroid axis under stress conditions.
The work was submitted to international scientific conference «The development of the scientific potential of high school», UAE (Dubai), March 4-11, 2009. Came to the editorial office on 23.01.2009.