10.2 Female reproductive anatomy and physiology

The female reproductive system is a complex network of organs working together to support fertility and pregnancy. From the ovaries producing eggs to the uterus nurturing a developing fetus, each structure plays a crucial role in the reproductive process.

Hormonal regulation orchestrates the menstrual cycle, preparing the body for potential pregnancy each month. Understanding these intricate processes helps us appreciate the remarkable adaptations that enable human reproduction and the continuation of our species.

Organs of female reproductive system

External genitalia and internal organs

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• The female reproductive system consists of external genitalia (vulva) and internal organs located in the pelvic cavity
• The vulva includes the mons pubis, labia majora, labia minora, clitoris, vestibule, vaginal orifice, and external urethral orifice
• The internal organs of the female reproductive system include the vagina, uterus, fallopian tubes (oviducts), and ovaries

Uterus and fallopian tube anatomy

• The uterus is a muscular, pear-shaped organ that consists of the fundus, body, and cervix
• The fallopian tubes extend laterally from the uterus and have funnel-shaped endings called infundibula, which are fringed with finger-like projections called fimbriae
• The ovaries are small, almond-shaped glands located on either side of the uterus that produce oocytes and secrete female sex hormones (estrogens and progesterone)

Functions of reproductive structures

Ovaries and fallopian tubes

• The ovaries have two primary functions: the production of oocytes (female gametes) and the secretion of female sex hormones, primarily estrogens and progesterone
• The fallopian tubes transport the oocyte from the ovary to the uterus and provide a site for fertilization
  • Cilia lining the fallopian tubes help to move the oocyte towards the uterus

Uterus and vagina

• The uterus serves as the site for implantation and development of the fertilized ovum (zygote) throughout pregnancy
  • The endometrium, the inner lining of the uterus, thickens in preparation for implantation and is shed during menstruation if fertilization does not occur
• The vagina is a muscular, elastic canal that serves as the receptacle for the penis during sexual intercourse and acts as the birth canal during childbirth
  • The vagina also provides a pathway for menstrual flow to exit the body

Hormonal regulation of menstruation

Hypothalamic-pituitary-gonadal axis

• The menstrual cycle is a series of physiological changes in the female reproductive system that occur in preparation for pregnancy, typically lasting 28 days
• The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

Menstrual cycle phases

• The menstrual cycle consists of three phases: the follicular phase, ovulation, and the luteal phase
  • The follicular phase begins on the first day of menstruation and involves the development of ovarian follicles in response to FSH
  • Ovulation occurs around day 14 of the cycle, triggered by a surge in LH, causing the release of a mature oocyte from the ovary
  • The luteal phase follows ovulation, during which the remnants of the ovarian follicle transform into the corpus luteum, which secretes progesterone to prepare the endometrium for implantation
• If fertilization does not occur, the corpus luteum degenerates, causing a decline in progesterone and estrogen levels, leading to the shedding of the endometrium (menstruation) and the beginning of a new cycle

Oogenesis and oocyte development

Oogenesis overview

• Oogenesis is the process of forming mature female gametes (oocytes) within the ovaries
• Oogenesis begins during fetal development with the formation of primary oocytes, which are arrested in prophase I of meiosis until puberty
• At birth, a female has approximately 1-2 million primary oocytes, but this number decreases to about 400,000 by puberty

Oocyte maturation

• During each menstrual cycle, a cohort of primary oocytes resumes meiosis I under the influence of FSH, with each primary oocyte dividing into a secondary oocyte and a first polar body
• The secondary oocyte proceeds to meiosis II but is arrested in metaphase II until fertilization occurs
• If fertilization occurs, the secondary oocyte completes meiosis II, producing a mature ovum and a second polar body

Reproduction in conception and pregnancy

Fertilization and early embryonic development

• Conception, or fertilization, occurs when a sperm cell penetrates and fuses with a secondary oocyte, usually in the ampulla of the fallopian tube
• The fertilized ovum, now called a zygote, undergoes cleavage as it moves through the fallopian tube towards the uterus
• Around 6-7 days after fertilization, the developing embryo, now called a blastocyst, implants into the endometrium of the uterus

Uterine changes during pregnancy

• The uterus provides a nurturing environment for the developing embryo and fetus throughout pregnancy, with the endometrium transforming into the decidua to support the placenta
• The placenta, which develops from both fetal and maternal tissues, allows for the exchange of nutrients, gases, and waste products between the maternal and fetal circulation
• The cervix remains closed throughout pregnancy, helping to maintain the integrity of the uterine environment, and dilates during labor to allow for childbirth through the vagina