Development of chromaffin modules during postnatal ontogenesis

BACKGROUND: The principle of modularity, which implies organization based on morphofunctional units, is a fundamental feature of living systems at all levels. It has only recently been established that the adrenal medulla comprises adrenaline-producing (A) and noradrenaline-producing (NA) modules, raising questions about the ontogenetic development of these structures. AIM: To investigate the postnatal development of chromaffin modules in the adrenal medulla. METHODS: A cross-sectional, observational, non-controlled study was conducted on adrenal medulla samples from rats. To reliably identify A and NA cells and evaluate the structure of the modules they form, adrenal tissue specimens were processed using the Onore method and conventional protocols recommended for transmission electron microscopy. RESULTS: Adrenal glands from rats were examined at the following postnatal ages: newborn, 6–8 days, 14 days, 21 days, 1 month, and 6–8 months. For each age group, 5 to 8 samples were analyzed using each of the two tissue processing methods (n = 80). Adrenal medullae of adult rats contain mature A and NA modules. A-modules are rounded complexes of tightly packed A-cells, with characteristic expanded intercellular spaces at the center. NA-modules consist of more loosely associated NA-cells, forming ampullary intercellular enlargements and merging into large polygonal arrays. Polygonal NA-modules tend to interpose between rounded A-modules. Each chromaffin cell is found exclusively within a module of its respective type. In newborn rats, the adrenal medulla contains rounded complexes of chromaffinoblasts referred to as medullary spheres. Most of these complexes consist of tightly packed, electron-lucent cells, although some less rounded complexes of more electron-dense and loosely arranged cells are also observed. These features suggest that medullary spheres represent precursors of A and NA modules. By postnatal days 6–8, chromaffinoblasts differentiate into A and NA cells, each forming modules of their respective type, which already display characteristic features: rounded shape, compact arrangement, and central intercellular spaces for A-modules; polygonal shape and looser intercellular contacts with ampullary expansions for NA-modules. These morphological distinctions become more pronounced with age. The size of both module types increases linearly throughout the postnatal period. A-modules maintain a compact, slightly elongated ellipsoid shape, whereas NA-modules and the arrays they form become increasingly angular and extended. These changes are particularly active between postnatal days 14–21 and from 1 to 8 months of age, reflecting overall growth patterns of chromaffin tissue in the adrenal medulla. CONCLUSION: The simultaneous appearance of A and NA modules alongside the differentiation of chromaffinoblasts into A and NA cells, as well as the morphological features of their formation, support the concept of modular organization as the morphofunctional basis of the adrenal medulla.