Nucleolar organizer regions: their significance in protein synthesis and consequent release of factors that attract immature lymphocytes in different types of thymic epitheliocytes and in different stages of thymic development.
The thymus is a lymphoepithelial organ that has the central role in T-lymphocyte development. Unlike other lymphoid structures, where the supportive framework is chiefly collagenous reticular tissue, the thymus is permeated by a network of interconnected epithelial cells (thymic epitheliocytes) between which lodge lymphoid and other cells of the organ. There is much evidence that many distinctive functional roles are subserved by the thymic epitheliocytes such as, the differentiation of T lymphocytes, the production of soluble thymic factors or hormones, supportive functions, or their role in MHC restriction of T-cell immune responses. Nucleolar organizer regions (NORs), which are important for regulating protein synthesis, were identified in 30 fetal thymuses in different stages of development ( 10th, 14th, 15th, 19th, 20th, 23rd, 31st, and 35th week), by means of a silver (Ag) staining technique (AgNOR). The aim of our study was to estimate the AgNOR counts in the six different types of fetal thymic epitheliocytes, following suggestions that there may be a possible association between AgNOR values and consequent protein synthesis in the different types of these cells and in different stages of thymic development. The results showed that: First Type I epitheliocytes (subcapsular-perivascular) of the cortex represent a higher number of AgNORs in comparison with the other cellular types, a difference that was observed every week of our study, and especially between the 10th and 15th week of development (p < 0.01). The increased number of AgNORs in Type I epitheliocytes reflect their intense protein synthesis, a fact that explains the increased secretion of factors, e.g. beta2-microglobulin, that release immature lymphocytes from the yolk sac and the liver. Second, gradual increase of the average AgNOR in all thymic epitheliocytes from the 10th till the 35th week, without any statistical variation. This increase might be due to the intense functional activity of the whole number of epitheliocytes that participate in the proliferation, differentiation and issue in the circulation of mature T lymphocytes, which takes place after the 17th week of development. The 17-week thymus appears fully differentiated, and after this time it produces the main type of thymocyte also present throughout life (designated TdT +).
Papadopoulos
N
N
Department of Histology-Embryology, Democritus University of Thrace, Alexandroupolis, Greece.
Kotini
A
A
Lambropoulou
M
M
Tamiolakis
D
D
D005314:Embryonic and Fetal Development / Q000502:physiology*; D015622:Epithelioid Cells / Q000502:physiology*; D005865:Gestational Age; D006801:Humans; D009697:Nucleolus Organizer Region / Q000502:physiology*; D014176:Protein Biosynthesis; D016622:Silver Staining; D013950:Thymus Gland / Q000196:embryology*