The ovaries have two main functions gametogenesis and steroidogenesis The mature haploid gametes are called ova while the main ovarian steroids are hormones called progestogens and estrogens derived from androgenic Should an ovum be fertilized then implantation of the developing blastocyst into the uterine wall requires the simultaneous cyclic development of the Calles estrous cycles in some animals such as rodents in which each cycle is 4 days long or menstrual cycles in primates such as humans in whom each The Gonads 2 Ovaries Introduction precursors endometrial lining cycles lasts approximately 1 month Embryonic development of the ovaries In the absence of SRY gene expression i e in an XX female the gonads become ovaries and the internal reproductive tracts Mullerian ducts develop to form the Fallopian tubes uterus and upper part of the cervix In the female the medullary cords degenerate so no direct contact is made between the developing germ cells and the Mullerian duct There is no tubular transport system along which ova can enter the Fallopian tube In the absence of testosterone the external genitalia automatically develop along the female line with the formation of the outer section of the vagina the labia and the clitoris The female fetus develops without the need for ovarian hormones and the female phenotype develops spontaneously by default The genetic basis of ovum production The initial mitotic divisions in the oogonia result in the formation of approximately 6 million primary oocytes diploid 44XX which then enter the first of the meiotic divisions until the prophase stage when they become arrested The number of oocytes diminishes as they become absorbed into the surrounding stromal tissue atresia From puberty ova within their follicles enter cyclic developmental processes called ovarian cycles By first ovulation there are probably fewer than 250 000 oocytes During each menstrual cycle the number of developing follicles entering further development is around 15 20 from each ovary The reproductive life of a woman is dictated by the diminishing number of follicles By the age of 50 women have no oocytes left reproductive capacity is lost and they are consequently infertile menopause The final maturation of the ovum during each menstrual cycle is associated with the restart of the meiotic process that was arrested at prophase resulting in Both cells are haploid 22XX but one contains most of the cytoplasm the first polar body Secondary oocyte then enters the second meiotic division which again becomes arrested this time at the metaphase stage the formation of two cells Released at ovulation Final completion of the second meiotic division and formation of the second polar body only takes place if when the ovum is fertilized The anatomy and structure of the ovaries and related structures The ovaries are oval structures dimensions approximately 4 X 2 X 1 cm located on either side of the uterus Attached to the posterior abdominal wall by ligaments and to the uterine broad ligament by the ovarian mesentery The area of each ovary which is attached to the mesovarium and through which the blood vessels lymphatics and nerves enter is called the hilum The nerve supply to the ovaries is autonomic mainly sympathetic fibers Each ovary consists of an outer cortex and an inner medullary stroma Aligned along the outer cortical edge of the ovary is the layer of primordial germ cells Also in the cortical region are the primary oocytes each surrounded by a ring of granulosa cells and outer basement membrane primordial follicles Until puberty primordial follicles remain in prophase or undergo atresia After puberty developing follicles accumulate a layer of cells from the stroma called thecal cells at which stage they are called primary preantral follicles cid 127 Granulosa cells begin to secrete a viscous fluid containing mucopolysaccharides antral stage Further development of one of the antral follicles the largest results in that one becoming pre ovulatory or Graafian follicle All other follicles within that cycle stop developing and undergo atresia Development of the preantral and antral follicles and survival of the Graafian follicle all occur due to endocrine signals produced not only by the developing follicles themselves but also from the HPA The menstrual cycle The cyclic production of follicles Each cycle lasts approximately 1 month average length 28 days although can be anywhere from 22 40 The beginning of each cycle day 1 marks the start of the final process of the previous cycle The ovarian cycle Comprised of 3 parts initial follicular phase release of ovum subsequent luteal phase Day 1 of each menstrual cycle is taken as the first day of menstruation when the endometrial lining of the previous cycle is shed cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Soon after the start of the ovarian cycle the start of the follicular phase some of the preantral follicles grow and develop into antral follicles under the influence of increased levels of FSH and LH cid 127 Multiplying thecal cells synthesize LH receptors and synthesize androgens testosterone and androstenedione cid 127 Granulosa cells synthesize FSH receptors and stimulate the synthesis of aromatase which converts androgens to estrogens by aromatization producing 17beta estradiol From day 1 6 estrogen level rise and begin to inhibit release of FSH and LH As FSH levels begin to fall antral follicles undergo atresia The largest antral follicle which is no longer FSH dependent continues to grow under the influence of its own estrogens autocrine paracrine positive feedback Toward the end of the follicular phase if the very high concentrations of estrogen in the circulation are maintained for min 36 hours the feedback influence on the anterior pituitary gonadotrophs changes from negative to positive LH surge occurs Essential for initiating the final stages of the first meiotic division with the formation of the secondary oocyte haploid with nearly all of the cytoplasm and the first polar body waste chromosomes and the process of ovulation itself Secondary oocyte is released directly into the peritoneal cavity where it is caught by the fimbria which bring it inside of