accessory  organs  include  both  internal  and  external reproductive organs (fig. 1-61). OVARIES The  ovaries,  as  stated  earlier,  are  the  primary female reproductive organs, producing the female sex cells and sex hormones (fig. 1-61). Structure The ovaries, or female gonads, are two almond-shaped glands suspended by ligaments in the upper pelvic cavity. There is one ovary on each side of the  uterus.  The  ligaments  that  suspend  the  ovaries contain ovarian blood vessels and nerves. The  tissues  of  an  ovary  are  divided  into  two regions,  an  inner  medulla  and  an  outer  cortex.  The ovarian   medulla   is   largely   composed   of   loose connective  tissue,  numerous  blood  vessels,  lymph vessels, and nerves. The ovarian cortex is composed of compact tissue containing tiny masses of cells called ovarian  (primordial)  follicles. The follicles contain the female sex cells or ova. The external surface of the ovary  is  covered  by  a  layer  of  cuboidal  epithelium cells.  Beneath  the  epithelium  is  a  layer  of  dense connective tissue. Primordial Follicle In   the   outer   region   of   the   ovarian   cortex, microscopic   groups   of   cells   are   referred   to   as primordial follicles. The primordial follicles consist of a   single   large   cell,   called   an   oocyte,   which   is surrounded  by  a  layer  of  flattened  epithelial  cells called follicular cells. The oocyte is an immature egg cell.  Follicular  cells  surround  a  developing  egg  cell and   secrete   female   sex   hormones.   There   are approximately 400,000 primordial follicles at puberty. Of  these,  probably  fewer  than  500  will  be  released from the ovary during the reproductive life of a female. At  puberty,  the  anterior  pituitary  gland  secretes increased   amounts   of   FSH   (follicle-stimulating hormone). In response, the ovaries enlarge and many of the primordial follicles begin to mature. During this maturation process, the oocyte enlarges and the follicle cells multiply until there are 6 to 12 layers. Fluid-filled spaces begin to appear among the follicle cells. These spaces join to form a single cavity called the antrum. Ten  to  fourteen  days  after  this  process  begins,  the primordial   follicle   reaches   maturity.   The   mature primordial follicle (preovulatory or graafian follicle) and its fluid-filled cavity bulges outward on the surface of the ovary, like a blister. Ovulation Ovulation  is  the  process  by  which  the  mature oocyte is released from the primordial follicle (fig. 1-62). Ovulation is stimulated by hormones from the anterior  pituitary  gland.  These  hormones  cause  the mature follicle to swell rapidly and its walls to weaken. Eventually the wall ruptures, permitting the oocyte and 1 or 2 layers of follicle cells to be released from the ovary's surface. After ovulation, the oocyte is usually propelled to the opening of a nearby fallopian tube. If the oocyte is not fertilized by a sperm cell within a relatively short time, it will degenerate. This  process  of  ovulation  occurs  once  a  month. Each ovary normally releases an ovum every 56 days. The right and left ovary alternately discharge an ovum approximately every 28 days. The menstrual cycle in most women is therefore approximately 28 days. Female Sex Hormones Female sex hormones are produced by the ovaries and various other tissues, such as the adrenal glands, pituitary   gland,   and   placenta   (during   pregnancy). These   female   sex   hormones   are   estrogen   and progesterone. The primary source for estrogen is the ovaries. At puberty,  estrogen  stimulates  enlargement  of  various accessory  organs,  which  include  the  vagina,  uterus, fallopian  tubes,  and  external  structures.  Estrogen  is also responsible for the development and maintenance 1-60 HM3F0161 ANUS SYMPHYSIS PUBIS URETHRA CERVIX RECTUM VAGINA VAGINAL ORIFICE LABIA MINORA LABIA MAJORA CLITORIS URINARY BLADDER UTERUS OVARY UTERINE TUBE Figure 1-61.—The female reproductive system.