Regression of the epithelium in late previtellogenic follicles ofPodarcis sicula: A case of apoptosis

Abstract

We investigated the reorganization of the follicular epithelium at the end of previtellogenesis in the lizard Podarcis sicula. In particular, we determined the mechanism of intermediate and pyriform cell regression and the fate of their different subcellular constituents. Morphological and biochemical analyses revealed the presence of hallmarks of apoptosis such as nuclear changes, chromatin condensation, protein scaffolds, RNA synthesis, and DNA fragmenta- tion. It was therefore concluded that the remodelling of the follicular epithelium is associated with programmed cell death. Evidence was also obtained that during regression, the cytoplasmic con- stituents of intermediate and pyriform cells are transferred into the oocyte while the remnants of the nuclei are recycled by the small cells. The functional significance of these events is discussed. 0 1996 Wiley-Liss, Inc. In Podarcis siculu, as in most Squamata (Hubert, '77; Klosterman, '87; Andreuccetti, ,921, the follicu- lar epithelium of previtellogenic oocytes is poly- morphic, being characterized by the presence of three distinct cell types: small, intermediate and pyriform cells. The small stem cells are gathered immediately below the connectival theca; the in- termediate cells are located close to the zona pel- lucida, while the large, flask-shaped pyriform cells are located in the inner portion of the epithelium (Filosa, '73; Andreuccetti et al., '79). The small cells differentiate during primary fol- licle formation, constituting a homomorphic and unilayered epithelium. When the follicle reaches about 150 pm in diameter, the membrane of the small cell fuses with the oocyte membrane to form an intercellular bridge. This event triggers the de- velopment of the cell into an intermediate cell and, following a further differentiation, into a pyriform cell (Andreuccetti et al., '78). The progression of such processes of proliferation and differentiation of the small cells will determine an increase in the total number of follicle cells that parallels the increase in surface occurring in the growing oo- cyte (Filosa et al., '79). The intermediate and pyriform cells play a fun- damental role in oogenesis since they synthesize various materials then transferred to the oocyte via intercellular bridges. RNAs (Motta et al., '95) and probably also a large variety of different sub- cellular components (Taddei, '72; Andreuccetti et al., '79) reach the oocyte, thus significantly con- tributing to its growth. The intermediate and py-