Ultrastructural study of the mature egg of Tethya citrina sarà and melone (porifera, demospongiae)

Tethya citrina is an oviparous demosponge in which eggs are distributed in clumps within the choanosome. The cytoplasm of the mature egg presents a peripheral cortex consisting of a slightly granular layer sandwiched between two densely granular, vesiculated ones. The cortex probably has a specialized, trophic function. Mesohyl bacteria are phagocyted at the egg surface, included in vacuoles, and transferred across the cortical sheath toward the inner cytoplasm. The region of the egg extending between the cortex and the nucleus shows a lacunary system mostly developed beneath the cortical envelope. The noncortical cytoplasm also contains lipid droplets, dense rodlike bodies, and phagosomelike granules. Most of the latter are probably autophagosomes, forming lacunae and supporting autosynthetic vitellogenesis. Rodlike inclusions are probably proteinaceous; they likely originate within the phagosomes and represent the actual yolk material.