Complex sentence: After that, the epithelium remains thin with only a few layers of cells without glycogen until puberty, when the epithelium thickens and glycogen containing cells are formed again, under the influence of the girl's rising estrogen levels.
Split into simpler sentences: After that, the epithelium remains thin with only a few layers of cells without glycogen. When puberty begins, the epithelium thickens and glycogen containing cells are formed again, under the influence of the girl's rising estrogen levels.