The female reproductive system is the birthplace of a new human life. It produces female gametes, creates a favorable environment for fertilization and development of embryos, and ultimately it nurtures the growing fetus during 40 weeks of gestation.
Women are born with two ovaries, located on both sides of the uterus in the abdominal area. They play a complex role in the regulation of the menstrual cycle, producing hormones and giving one mature egg every month, which is ready for fertilization, which subsequently gives the couple a chance of pregnancy. The purpose of this blog post and several subsequent blog posts is to describe to our patients the structure of a single ovarian follicle, which is the place of origin of the human oocyte, its development at various stages of a woman’s life and menstrual cycle. Furthermore, we will explain to our patients the effect of hormones on the development of the ovarian follicle and tissue and how they affect the oocytes maturation, what occurs with the ovarian follicle after the rupture, and what the value of various growth factors and chemical signals in the physiology of the ovarian follicle.
Gametogenesis, oogenesis in this particular case, represents developmental stages of oocytes.
Oogenesis starts before birth when primordial germ cells migrate do gonadal ridge and start their differentiation to oogonia. Oogonia divide by mitosis and reach a number of 7 million by the fifth month of gestation and at the same time some of them differentiate into primary oocytes. By the seventh month of gestation, most of them become atretic and primary oocytes that do survive and enter in prophase of the first meiotic division and become dormant in that form all the way to puberty. Primary oocyte together with its layer of follicular cells makes a primordial ovarian follicle. Also, it is well known that a female newborn carries approximately 1-2 million of primary oocytes and that number decreases to 400,000 at puberty and about 400 hundred reach maturation and ovulate.
It is interesting to see what happens to all the oocytes that become atretic. The survival of human ovarian follicles from fetal to adult life and the mechanism of apoptosis in human ovaries show that a large number of oocytes degenerate during fetal life through apoptosis, and it is already evident in the 13th week of gestation. It is also showed that the rate of apoptosis in the adult ovary increased with growing follicular size and only slightly affected early growing follicles. It is a mechanism for eliminating recruited follicles that do not reach dominant ovarian follicle stage. It is also important to notice that during fetal stage apoptosis is a mechanism for eliminating oocytes, but in adult life apoptosis is located in granulosa cells and they play a major role in follicular demise.
Ovaries are specific by their structure since at a monthly basis they go through extreme changes and reorganization. Follicles that contain oocytes go through various stages of development that include primordial, primary, secondary, preantral and antral follicles that ultimately give rise to a large ovarian follicle called Graafian follicle. This process is controlled by various hormones and growth factors.
Environment in which oocyte matures has been proven to influence the quality of the oocyte and subsequently entire embryo and its implantation potential. This is especially interesting to observe from assisted reproductive techniques perspective where a mature oocyte with excellent metabolic activity plays a crucial role in embryo development and its morphology.
A relationship between follicular fluid and cumulus cells and oocyte health was investigated. It was found that signaling between oocytes and somatic cells changed intrafollicular environment that controlled ovarian follicle growth and which antral follicle was to be selected to ovulate a healthy oocyte. Cellular metabolism is key to a normal meiotic resumption. Maternal ageing or metabolic disease perturb cellular mechanisms within the oocyte, alter macromolecules, and induce mitochondrial mutations which hurts the oocyte.
It is clear that the environment that brings an oocyte to its maturity plays a crucial role and not just in maturity but also in oocyte shape and quality. Therefore, there will be a great emphasis on folliculogenesis and how the actual follicles change and what structural changes give rise to a mature oocyte.