Female reproductive system. Endocrine and reproductive functions of the ovaries. Regulation of the ovarian and uterine cycle.
The female reproductive system encompasses the ovaries, fallopian tubes, uterus, cervix, and vagina. Its primary functions include oogenesis (egg production), hormone secretion, and supporting pregnancy and childbirth.
The ovaries, the female gonads, have two key functions:
- 1. Oogenesis: The production and maturation of ova (eggs) within ovarian follicles.
- 2. Hormone Secretion: Primarily estrogen (mainly estradiol) and progesterone, synthesized by follicular cells (theca and granulosa cells).
Oogenesis is a complex process that begins during fetal development and continues through a woman's reproductive years until menopause:
- Fetal Development: Primordial germ cells give rise to oogonia, which undergo mitosis to produce millions of primary oocytes. These oocytes begin meiosis but arrest in prophase I until puberty.
- Puberty: With the onset of puberty, hormonal changes stimulate the maturation of a few follicles each menstrual cycle. One follicle typically becomes dominant, while others undergo atresia (degeneration).
- Ovulation: The mature follicle releases the secondary oocyte (which has completed meiosis I) into the fallopian tube.
- Fertilization: If fertilization occurs, the secondary oocyte completes meiosis II, forming a mature ovum.
Hormone Synthesis and Secretion
The ovarian follicles produce estrogen and progesterone through the coordinated actions of theca and granulosa cells:
- Theca Cells: Under the influence of LH, theca cells synthesize androstenedione and progesterone from cholesterol.
- Granulosa Cells: Granulosa cells, stimulated by FSH, convert androstenedione (from theca cells) to estradiol using the enzyme aromatase.
Estrogen (Estradiol)
- Actions: Estradiol has numerous roles in female reproductive physiology, including:
- Growth and development of the female reproductive tract (uterus, fallopian tubes, cervix, vagina).
- Development of secondary sexual characteristics (breast development, fat distribution).
- Regulation of the menstrual cycle, including endometrial proliferation and feedback on the hypothalamic-pituitary axis.
Progesterone
- Actions: Progesterone primarily acts to:
- Prepare the endometrium for implantation and support early pregnancy.
- Maintain pregnancy and suppress new follicle development.
Regulation of the Ovarian and Uterine Cycles: The Hypothalamic-Pituitary-Ovarian Axis
The hypothalamic-pituitary-ovarian axis orchestrates the menstrual cycle, a complex interplay of hormonal and ovarian events that typically spans 28 days:
- 1. Hypothalamus: Secretes GnRH in a pulsatile manner, stimulating the anterior pituitary.
- 2. Anterior Pituitary: Releases FSH and LH in response to GnRH pulses.
- 3. Ovaries: Respond to FSH and LH by developing follicles, producing hormones (estrogen and progesterone), and ultimately ovulating.
Phases of the Menstrual Cycle
The menstrual cycle is divided into three phases:
- 1. Follicular Phase (Days 0-14):
- Dominant follicle selection and maturation: FSH stimulates follicle growth, and estradiol production increases.