Reproductive Physio 2 Flashcards
Describe the anatomy of the ovaries of the female reproductive system
The female gonads.
* Singular: ovary
* Paired ovals, each about 2 to 3 cm in length, located within the pelvic cavity and attached to the uterus via the ovarian ligament.
Comprises:
* an epithelium,
* a dense connective tissue covering (tunica albuginea),
* cortex with stroma and
* medulla.
* Oocytes develop within the outer layer of this stroma, each surrounded by supporting cells.
* This grouping of an oocyte and its supporting cells are called follicles
What is the purpose of the ovaries?
Serve a dual purpose:
1. Oogenesis, the production of gametes (the ova).
2. Secretion of the female steroidal sex hormones, oestrogen and progesterone, as well as the peptide hormone inhibin.
Describe the ovarian follicle
Follicles are present in a number of different stages of growth:
- primordial follicles (resting)
- primary, secondary, and antral follicles
- preovulatory (Graafian) follicles
These stages of growth begin in utero, during foetal development, and eventually cease at the onset of menopause.
Unlike the continuous sperm production of the male, the production of the oocyte, followed by its reléase from the ovary-ovulation-is cyclical.
Outline the menstrual cycle
• The reproductive system of women, unlike that of men, shows regular cyclic changes.
• In humans and other primates, the cycle is a menstrual cycle, and its most conspicuous feature is the periodic vaginal bleeding that occurs with the shedding of the uterine mucosa (menstruation).
• The length of the cycle is notoriously variable in women, but an average figure is 28 days from the start of one menstrual period to the start of the next. By common usage the days of the cycle are identified by number, starting with the first day of menstruation
• Comprises the ovarian cycle and the uterine cycle.
What is the ovarian cycle?
A set of predictable changes in oocytes and ovarian follicles.
The cycle includes two interrelated processes:
Oogenesis (the production of gametes) and folliculogenesis (the growth and development of ovarian follicles).
Outline the process of oogenesis
• At birth, a female’s ovaries contain an estimated total of 2 to 4 million eggs, and no new ones appear after birth.
• Thus, in marked contrast to the male, the newborn female already has all the germ cells she will ever have with only a few (= 400) destined to be ovulated.
• All the others degenerate at some point in their development so that few, if any, remain by the time a woman reaches approximately 50 years of age.
One result of this development pattern is that the eggs that are ovulated near age 50 are 35 to 40 years older than those ovulated just after puberty. It is likely that certain defects more common among children born to older women are the result of aging changes in the egg.
The female germ cells, like those of the male, have different names at different stages of development.
• The process begins with the ovarian stem cells, or oogonia which are formed during foetal development, and divide via mitosis.
• Unlike spermatogonia, however, oogonia form primary oocytes in the foetal ovary prior to birth.
• These primary oocytes are then arrested in this stage of meiosis I, only to resuming it years later, beginning at puberty and continuing until the person is near menopause (the cessation of a female’s reproductive functions).
• The transition from primary to secondary oocyte is initiated by a surge of LH, followed by it’s release from the ovary (ovulation).
• The initiation of ovulation marks the transition from puberty into reproductive maturity.
• From then on, throughout the reproductive years, ovulation occurs approximately once every 28 days.
• Just prior to ovulation, a surge of LH triggers the resumption of meiosis in a primary oocyte
The larger amount of cytoplasm contained in the ovum supplies the developing zygote with nutrients during the period between fertilization and implantation into the uterus.
• Sperm contribute only DNA at fertilization - not cytoplasm.
• Thus, the cytoplasm and all of the cytoplasmic organelles in the developing embryo are of egg-derived origin, including mitochondria, which contain their own DNA.
• Scientific research in the 1980s determined that mitochondrial DNA was maternally inherited, meaning that you can trace your mitochondrial DNA directly to your biological mother, her mother, and so on back through your female ancestors.
Outline the process of folliculogenesis
The process by which follicles (oocytes and their supporting cells) grow and develop, leading to ovulation of one follicle (= every 28 days) and death of multiple other follicles (called atresia).
• These follicles begin from primordial (resting state), and progress to primary, to secondary and tertiary stages prior to ovulation-with the oocyte inside the follicle remaining as a primary oocyte until right before ovulation.
• Atresia can occur at any point during follicular development.
Explain the hormonal control of the ovarian cycle
• The process of development during the ovarian cycle from primordial follicle to early tertiary follicle, takes approximately two months in humans.
• The final stages of development of a small cohort of tertiary follicles, ending with ovulation of a secondary oocyte, occur over a course of approximately 28 days.
• These changes are regulated by many of the same hormones that regulate the male reproductive system, including GRH, LH, and FSH.
• GRH from the hypothalamus stimulates the secretion of FSH and LH.
• FSH stimulates the follicles to grow (hence its name of follicle-stimulating hormone), and the five or six tertiary follicles expand in diameter.
• LH also stimulates granulosa and theca cells of the follicles to produce the sex steroid hormone oestradiol, a type of ostrogen.
• This phase of the ovarian cycle, when the tertiary follicles are growing and secreting ostrogen, is known as the follicular phase.
• The larger and more developed a follicle is, the more ostrogen it will produce in response to LH stimulation.
• As a result of these large follicles, systemic plasma ostrogen concentrations increase.
• A negative feedback loop reduces the production of GRH, LH, and FSH.
• This decline in FSH leads to atresia of most tertiary follicles!
• Typically only one follicle, now called the dominant follicle, will survive this reduction in FSH, and this follicle will be the one that releases an oocyte
• The dominant follicle that has remained in the ovary begins to secrete large amounts of ostrogen.
• This time, it has a positive effect on the anterior pituitary gland, causing it to increase its secretion of FSH and LH.
• This burst of LH (called LH surge) causes:
• Ovulation of the dominant follicle.
• Resumption of meiosis of the primary oocyte to a secondary oocyte.
• Degradation of the polar body.
The LH surge also triggers proteases to breakdown the structural proteins in the ovary wall on the surface of the dominant follicle. This degradation of the wall combined with pressure from the large fluid filled antrum results in the expulsion of the oocyte surrounded by granulosa cells into the peritoneal cavity. This releases ovulation.
• The follicle that ruptures at the time of ovulation promptly fills with blood, forming what is sometimes called a corpus hemorrhagicum.
• Minor bleeding from the follicle into the abdominal cavity may cause peritoneal irritation and fleeting lower abdominal pain (“mittelschmerz”).
• LH surge also stimulates a change in the granulosa and theca cells, that remain in the follicle after the oocyte has been ovulated, called luteinisation.
• This transforms the collapsed follicle into a new endocrine structure called the corpus luteum.
• The luteinized granulosa and theca cells begin to produce large amounts of progesterone.
• This is a sex steroid hormone that is critical for the establishment and maintenance of pregnancy.
• Progesterone triggers negative feedback at the hypothalamus and pituitary, which keeps GRH, LH, and FSH secretions low, so no new dominant follicles develop at this time.
• This phase of progesterone secretion is known as the luteal phase.
• If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and degrade into the corpus albicans that will disintegrate in the ovary over a period of several months.
• During this time of reduced progesterone secretion, FSH and LH are once again stimulated, and the follicular phase begins again with a new cohort of early tertiary follicles.
List the stages of folliculogenesis
- Primordial follicle
- Primary follicle
- Secondary follicle
- Tertiary follicle
- Ovulating follicle
- Corpus luteum
What are the sites of ovarian hormone secretion?
• Oestrogen is secreted during the follicular phase mainly by the granulosa cells; following ovulation, it is secreted by the corpus luteum.
• Progesterone is secreted in very small amounts by the granulosa and theca cells just before ovulation; but its major source is the corpus luteum.
• Inhibin, a peptide hormone, is secreted by both the granulosa cells and corpus luteum.
How is oestrogen secretion controlled during the early and middle follicular phases?
