HA5 shows a portion of a mature or Graafian follicle. Note the large antrum and the well-defined cumulus oophorus (oocyte mound) that projects into the antrum and surrounds the oocyte (arrow). It is part of the membrana granulosa. Note the rich blood supply in the theca interna (ti). There is a portion of the antrum of another Graafian follicle at the right.