Drawing illustrates the steps of oocyte development and corresponding follicular maturation. In the fetal period, once the primordial germ cells arrive in the gonad, they differentiate into oogonia. Mitotic division of oogonia increases the population. Many oogonia further differentiate into primary oocytes, which begin meiosis. However, the process arrests during prophase. A primary oocyte with its surrounding epithelial cells is called a primordial follicle. In childhood, primary oocytes remain suspended in prophase I. Beginning in puberty and extending through the reproductive years, several primordial follicles mature each month into primary follicles. A few of these continue development to secondary follicles. One or two secondary follicles progress to a tertiary or Graafian follicle stage. At this stage, the first meiotic division completes to produce a haploid secondary oocyte and a polar body. During this process, cytoplasm is conserved by the secondary oocyte. Consequently, the polar body is disproportionately small. The secondary oocyte halts meiosis at its second metaphase. One of the secondary oocytes is then released at ovulation. If the oocyte is fertilized, completion of the second meiotic division follows. If fertilization fails to occur, then the oocyte degenerates prior to completion of the second meiotic division. Source: Chapter 15. Reproductive Endocrinology, Williams Gynecology, 2e Citation: Hoffman BL, Schorge JO, Schaffer JI, Halvorson LM, Bradshaw KD, Cunningham F, Calver LE. Williams Gynecology, 2e; 2012 Available at: http://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/books/hoff2/hoff2_c015f021.png&sec=41726221&BookID=399&ChapterSecID=41722304&imagename= Accessed: December 21, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved