Fig. 3

Histological and ultra-structural characterization of the stolon, budding nest, and spherule. a Transversal semi-thin section of a stolon. b A stolonial epithelial cell, with strongly electron-dense basal lamina (bl) and several mitochondria (m). The cells are connected by elaborated intercellular contacts (black arrows). c Detail of a stolonial epithelial cell, showing Golgi apparatus (g), basal lamina (bl), and mitochondria (m). d Longitudinal semi-thin section of a young nest. e Epithelial cells of the stolonial vessel, containing extensive rough endoplasmic reticulum (RER), large electron-dense granules (red arrows), and glycogen granules (green arrows). Tight junctions (black arrows) connect the neighboring cells. f Detail showing the intercellular junctions (black arrow), some small electron-dense granules (red arrow) and glycogen granules (green arrow). g Semi-thin sections of a mature nest. h Epithelial cell from a budding nest, containing spherical electron-dense granules (red arrow) and heterogeneous inclusions (yellow star). i Detail of the inclusions’ content. j Semi-thin section of a spherule. k An epithelial cell from a spherule, completely filled by heterogeneous inclusions (yellow stars) and electron-dense granules (red arrows). l Detail showing the two different types of inclusions present in the epithelial cells. (Scale bars. Semi-thin sections: 10 µm; b: 2 µm; c: 1 µm; e: um; f: 2 µm; h: 5 µm; i: 1 µm; k: 5 µm; l; 5 µm) (bl basal lamina, g Golgi, l vessel lumen, m mitochondria, n nucleus, rer rough endoplasmic reticulum, t tunic)