CH27 Ovarian Follicles

Exam 5 Review: Chapter 27 Ovarian Follicles

ovarian follicles - The functional unit of the ovary containing a primary oocyte = egg = ovum surrounded by one or more layers of supportive cells; they develop during embryonic life, but remain dormant until puberty; at puberty, under the influence of FSH and LH from the anterior pituitary, they begin a sequential developmental cycle, the ovarian cycle, and a few each month, begin the cycle leading to ovulation; the developmental stages consist of the primordial follicle, primary follicle, secondary follicle, and mature follicle = vesicular follicle = Graafian follicle.

follicular cells - Any of the cells forming protective, supportive layers around the egg = ovum in the cortex of the ovary; they provide nutrients to the egg = ovum and secrete estrogens and progesterone in response to blood levels of FSH and LH from the anterior pituitary; their shape and number vary with the phase of the ovarian cycle and are major criteria for defining the the developmental stages of the ovarian follicles; once a follicle has begun development during a menstrual cycle, beginning with the primary follicle stage and onward, its supporting cells enlarge, develop a granular cytoplasm, and begin secreting estrogens and progesterone, and these active secretory follicular cells may also be called granulosa cells (see below).

granulosa cells - The lining cells of all ovarian follicles except the primordial follicle, and of the corpus luteum; the granularity of the cytoplasm of these follicular cells is due to the increase in the production and secretion of estrogens and progesterone; their endocrine activity is regulated by FSH from the anterior pituitary.

oogenesis - The formation, development, and maturation of an egg = ovum; a long and complex process which (1) begins in the fetal period with the development of oogonial stem cells by mitosis, and (2) is followed by the initiation of meiosis division I which produces primary oocytes which are arrested in development in meiotic prophase I; (3) at puberty, in response to increasing blood levels of FSH and LH from the anterior pituitary, meiosis continues in certain follicles each month during the menstrual cycle, and (4) one egg = ovum and its surrounding follicle develop to the point of being mature follicle = vesicular follicle = Graafian follicle, (5) this egg = ovum. completes meiosis division I at the time of ovulation, and then meiosis is arrested until further external stimuli are provided; Note: due to unequal cytoplasmic division, at the end of meiosis division I, one secondary oocyte and one polar body has been generated; (6) only if fertilization occurs will the secondary oocyte complete meiosis division II to form a mature egg = ovum (7) which will then immediately become a fertilized egg = zygote by the fusion of the haploid egg = ovum nucleus with the sperm nucleus.

oogonia - The diploid cells which arise from a primordial germ cell and differentiate into oocytes in the ovary; they divide by mitosis beginning in the third month of fetal development; they grow in size to become diploid primary oocytes surrounded by a single layer of squamous follicular cells in primordial follicles, the primary oocytes will begin the first meiotic division while the female is still a fetus.

atresia - (1) The process of degeneration and resorption of one or more ovarian follicles before a state of maturity has been reached. (2) An abnormal developmental condition in which a normal opening or tube in the body (e.g., the urethra) is closed or absent.

primordial follicle - The initial stage in the development of ovarian follicles, occurring the third month of fetal development in the ovary of females; the structure is a diploid primary oocyte, derived by mitosis from an oogonium, surrounded by a single layer of squamous follicular cells; the oocyte is considered primary because it has yet to carry out meiosis division I; primordial follicles continue to increase by mitosis and peak at a number of ~7,000,000 at seven months of fetal life; before birth, the diploid primary oocytes enter a final round of DNA synthesis and are then arrested in prophase I of the first meiotic division until shortly before ovulation; thus, a primary oocytes may remain in prophase I between ~12 years (the onset of puberty) and ~45 years (the onset of menopause).

primary follicle - The second stage in the development of ovarian follicles, occurring only after puberty, by which time more than 95% of the original primordial follicles have degenerated by atresia, and ~40,000 remain; at puberty, some15-20 primordial follicles develop further at the onset of each ovarian cycle; the diploid primary oocyte, derived by mitosis from an oogonium during embryonic life, begins to grow and enlarge and begins to accumulate nutrient stores, the future yolk; the single layer of follicular cells surrounding the primary oocyte also begin to grow, becoming cubiodal, and eventually columnar; this process of growth is stimulated by FSH from the anterior pituitary; the zona pellucida, the thick layer of glycoproteins separating the primary oocyte from the follicular cells also develops at this stage; the follicular cells begin to produce progesterone and somewhat larger quantities of estrogen; the follicular cells also begin to reproduce by mitosis, so that two and then more layers of follicular cells develop with time; the primary oocyteis still arrested in prophase I of the first meiotic division.

primary oocytes - The diploid cell, derived from the oogonial stem cell, which is located in the primordial and primary follicles within the cortex of the ovary; it grows in size during the transition from the primordial follicle through the primary, secondary, and vesicular follicle stages, but remains arrested in prophase I of the first meiotic division.

zona pellucida - The thick, solid, transparent outer membrane of glycoproteins. immediately external to the oocyte's cell membrane, separating the developing oocyte from the granulosa, i.e., follicular, cells of the primary, secondary, and vesicular follicles (the cells which will form the corona radiata); after ovulation, it forms a physical barrier through which sperm must penetrate in order to achieve fertilization; the acrosomal proteolytic enzymes of the sperm head can digest the glycoproteins of the zona pellucida.

Primordial Follicle	Early Primary Follicle	Later Primary Follicle	Zona Pellucida at arrow

corona radiata - The single layer of low columnar granulosa, i.e., follicular, cells of the secondary and vesicular ovarian follicles which surround and anchor the the diploid primary oocyte to the inner follicular wall.

antrum - The open space or chamber, filled with a clear watery fluid, which develops as the primary ovarian follicle continues to grow by mitotic divisions of the granulosa, i.e., follicular, cells; the presence of an antrum which occupies less than half of the volume of the follicle (a somewhat arbitrary distinction) defines the attainment of the stage of secondary follicle; a larger antrum defines the mature follicle = vesicular follicle = Graafian follicle.

secondary follicle - The third stage in the development of ovarian follicles; in terms of appearance, it is defined by the presence of an antrum which occupies less than half of the volume of the follicle (a somewhat arbitrary distinction); in terms of reproductive development, it is defined by the progression toward completion of the first meiotic division to produce a haploid egg = ovum nucleus and the haploid nucleus of a polar body. [Note: Although some15-20 primordial follicles begin development at the onset of each ovarian cycle, by the middle of the second week, many of those have been lost to degeneration = atresia, and less than half a dozen or less reach the stage of secondary follicle in each cycle; this tructure is actively secreting female steroid reproductive hormones, estrogens and progesterone.

Early Secondary Follicle	Secondary Follicle	Late Secondary Follicle	Corona Radiata at Arrow

secondary oocyte - The haploid oocyte, produced by the first meiotic division, a process which is being continued as the developing ovarian follicle transforms from the secondary follicle to the mature follicle = vesicular follicle = Graafian follicle (meiosis division I will not reach completion until ovulation); it retains the bulk of the cytoplasm while the other daughter cell, the polar body, has minimal cytoplasm. [Note: Over the course of a lifetime, ~500 oocytes are ovulated.]

mature follicle = vesicular follicle = Graafian follicle - The fourth stage in the development of ovarian follicles, which is not reached until shortly before ovulation; in terms of appearance, it is defined by its large size, occupying essentially the entire width of the ovarian cortex, and the external surface bulging away from the normal border of the ovarian wall (a somewhat arbitrary distinction which will not be obvious in every microscopic section); the primary oocyte inside is nearing completion of meiosis division one, although it will not reach completion until ovulation); by this point in the ovarian cycle, usually only a single ovarian follicle remains, the rest having been lost to degeneration = atresia; ovulation will be triggered by the pulses of LH (larger) and FSH (smaller) from the adenohypophysis in midcycle; the remnant tissue, after ovulation becomes the corpus luteum secreting female steroid reproductive hormones, estrogens and progesterone.

Early Vesicular Follicle	Vesicular Follicle	Ovulation

polar body - A minute cell produced and ultimately discarded in the development of an oocyte, containing little or no cytoplasm but having one of the haploid nuclei derived from the first or second meiotic division; the first polar body formed is usually larger than the second one, and may divide into two after its separation from the ovum; the unequal cytoplasmic divisions permit the formation of a large ovum = egg with larges nutrient stores (yolk).

ovum - The haploid cell, produced by the second meiotic division, a process which begins when the secondary oocyte is ovulated, but will not becompleted unless the cell is penetrated by a sperm nucleus at fertilization; it contains a large amount of cytoplasm with nutrient stores to provision the developing embryo until implantation and placentation can occur.

ovulation - The discharge of a secondary oocyte through the germinal epithelium into the pelvic cavity from a mature follicle = vesicular follicle = Graafian follicle in response to the LH surge or pulse (and lesser simultaneous FSH surge or pulse) from the anterior pituitary about midway in the ovarian cycle; the discharge is accomplished by smooth muscle fiber contractions around the mature follicle = vesicular follicle = Graafian follicle.

corpus luteum - A yellow glandular mass of active follicular cells (luteal cells) in the cortex of the ovary, formed after a mature follicle = vesicular follicle = Graafian follicle discharges its secondary oocyte in response to the LH surge or pulse (and lesser simultaneous FSH surge or pulse) from the anterior pituitary about midway in the ovarian cycle; it secretes large quantities of estrogens and progesterone in response to blood levels of FSH and LH from the anterior pituitary; it will shrink and degenerate into a corpus albicans at the end of the menstrual cycle; however, it a pregnancy occurs, it will continue to secretes large quantities of estrogens and progesterone until partruition.

Early Corpus Luteum	Corpus Luteum with Blood Clot	Mature Corpus Luteum

Corpus Luteum low power	Corpus Luteum high power	Corpus Albicans

corpus albicans - The pale shrunken remnant of a corpus albicans in the cortex of the ovary, consisting primarily of dense fibrous connective tissue which develops at the end of a menstrual cycle or a pregancy.

Sketch and label:

10. a cross section of the ovary including the various stages of the ovarian cycle. Indicate the stages of meiosis corresponding to the follicular stages.

A Cross Section of the Ovary Including the Various Stages of the Ovarian Cycle.

The Stages of Meiosis Corresponding to the Follicular Stages.

During Any One Menstrual Cycle:

All primordial, all primary, and all but one of the secondary follicles remain in early meiosis I.

The most developed of the secondary follicles, which will become the vesicular follicle, continues with meiosis I. It will complete meiosis I in the last few hours before ovulation.

Only an egg which is fertilized by a sperm will complete meiosis II.