huamn bio week 11 lecture 2

Female Reproductive System

Overview

• The female reproductive system includes organs related to offspring production.
• Main organs:
  • Ovaries: Site of oogenesis (egg production) and hormone secretion (estrogen and progesterone).

Anatomy

• Ovaries:
  • Located in the pelvic cavity (left and right sides).
  • Attached to the pelvic wall via ligaments.
• Uterine Tube (Fallopian Tube):
  • Connects the ovaries to the uterus.
  • Hollow tube.
• Uterus:
  • Pear-shaped hollow organ in the pelvic cavity.
  • Where the fetus develops.
• Cervix:
  • Narrow part of the uterus connecting it to the vagina.
• External:
  • Labia.

Ovaries in Detail

• Two ovaries on either side of the pelvic cavity.
• Connected to the uterus via uterine tubes.
• Size: Approximately the size of a walnut.
• Made of dense connective tissue.
• Contain developing follicles.
• Follicles contain oocytes (female gametes).
• Oocyte Number is Finite: Females are born with all the oocytes they will ever have.
• Follicles and oocytes start developing around three months after fertilization during gestation.
• Ovaries undergo monthly cyclic changes (menstrual cycle).
• Each cycle: a small group of follicles develops, preparing for ovulation and mature ovum production.
• Important Cells Within Each Follicle:
  • Granulosa cells: Secrete estrogen.
  • Theca cells: Secrete androgens (steroid hormones).
• Follicle Development Goal: Reach maturity, release a mature oocyte via ovulation.
• Mature oocyte is released into the uterine tube (fertilization may occur).
• Remaining follicle becomes the corpus luteum.
  • Corpus luteum secretes high levels of progesterone.
  • This cycle repeats monthly.

Uterine Tubes

• Function: Collect the oocyte after ovulation, conduit to the uterus.
• Also called fallopian tube or oviduct.
• Fertilization occurs in the uterine tube.
• Fertilized egg (zygote) spends first few days in the uterine tube for initial development.
• Zygote takes about four days to travel to the uterus for implantation and development into a fetus.
• Ectopic Pregnancy:
  • Implantation occurs in the uterine tube (rare).
  • Life-threatening if not detected early.
  • May lead to rupture of the uterine tube, potential loss of the tube and/or ovary.

Uterus

• Uterine cavity is where the fetus develops.
• Three Main Layers:
  • Endometrium (innermost layer):
    • Responds to estrogen and progesterone from the ovaries.
    • Undergoes cyclic changes: thickening for pregnancy, shedding if no pregnancy occurs.
  • Myometrium (thick layer of smooth muscle):
    • Contracts during childbirth to push the baby through the birth canal.
  • Perimetrium (outer layer):
    • Connective tissue.

Hormonal Regulation

• Similar to male reproductive system (hypothalamus and anterior pituitary gland involved).
• Hypothalamus secretes gonadotropin-releasing hormone (GnRH).
• GnRH acts on the anterior pituitary gland.
• Anterior pituitary gland secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Target of FSH and LH: Ovaries.
  • FSH: Binds to granulosa cells in the developing follicle.
    • Stimulates oogenesis (development and maturation of the oocyte).
  • LH: Binds to theca cells in the developing follicle.
    • Stimulates androgen production.
    • Androgens bind to granulosa cells and stimulate more estrogen production.
• Goal: Produce a lot of estrogen.
• Negative Feedback Loops:
  • Granulosa cells secrete inhibin.
    • Inhibits FSH production in the anterior pituitary gland.
  • Estrogen inhibits GnRH, FSH, and LH production in the hypothalamus and anterior pituitary gland.

Menstrual Cycle

• Monthly cycle influenced by hormones from the hypothalamus, anterior pituitary, and ovaries.
• Prepares the uterus for pregnancy.
• Involves FSH, LH, estrogen, and progesterone.
• Changes in both the ovaries and uterus.
  • Ovarian cycle.
  • Uterine cycle.
  • These changes occur simultaneously.

Ovarian Cycle

• Two Main Stages (separated by ovulation):
  • Follicular Phase (pre-ovulation):
    • Development and maturation of follicles.
    • One follicle becomes mature and ovulates a mature oocyte.
    • Dominated by high estrogen levels.
  • Luteal Phase (post-ovulation):
    • Involves the corpus luteum.
    • Corpus luteum secretes high levels of progesterone to maintain the uterus for potential pregnancy.

Follicular Phase in Detail

• Follicles respond to FSH secreted by the anterior pituitary gland.

• Small number of follicles are activated to begin maturation each menstrual cycle.

• Stages:

  • Primordial follicles develop into primary follicles.
  • Some primary follicles progress into secondary follicles.
  • Granulosa cells and theca cells proliferate.
  • One secondary follicle becomes the dominant follicle.
  • Dominant follicle develops into a fully mature Graafian follicle.
  • Other follicles degenerate.
  • Mature Graafian follicle releases the mature oocyte (ovulation).

• Even though many follicles begin developing, only one reaches maturity.

Graafian Follicle Components

• Oocyte: The maturing egg released during ovulation (can be fertilized).
• Zona Pellucida: Protective protein barrier surrounding the oocyte.
  • Protects the oocyte during ovulation.
  • Ensures only one sperm can penetrate and fertilize the egg.
  • Role in maintaining the correct number of chromosomes in the resulting zygote.
• Granulosa cells: Secrete estrogen.
• Theca cells: Secrete androgens.
• Antrum: Fluid-filled cavity containing nutrients for oocyte development.

Hormone Level Changes

• Mature Graafian follicle secretes high levels of estrogen.
• High estrogen levels switch from inhibiting LH to causing a surge of LH from the anterior pituitary.
• LH surge triggers the Graafian follicle to rupture and release the mature oocyte (ovulation).
• Remaining parts of the Graafian follicle (granulosa cells, theca cells, antrum) develop into the corpus luteum.

Luteal Phase

• After ovulation, progesterone becomes the dominant hormone (secreted by the corpus luteum).
• Corpus Luteum's Role:
  • If pregnancy doesn't occur: Corpus luteum secretes progesterone for about 10 days, then degenerates, causing progesterone levels to drop.
    • Becomes scar tissue (corpus albicans) that persists for a few months.
  • If pregnancy does occur: Developing embryo secretes human chorionic gonadotropin (hCG) to maintain the corpus luteum.
    • Corpus luteum continues secreting progesterone for about three months to maintain the uterus.
    • After three months, the placenta develops and secretes its progesterone, and the corpus luteum degenerates.

Uterine Cycle

• Endometrium responds to estrogen and progesterone.
• Three Phases:
  • Menstrual Phase: Shedding of the endometrial lining from the previous cycle due to a drop in estrogen and progesterone levels.
    • Coincides with the first half of the follicular phase in the ovaries.
  • Proliferative Phase: Endometrium thickens in response to estrogen secreted by the ovaries.
    • Myometrium also thickens.
    • Development of glands and more blood vessels.
    • Coincides with the second half of the follicular phase in the ovaries.
  • Secretory Phase: Increased blood supply to the endometrium in preparation for implantation.
    • Glands secrete nutrients.
    • Coincides with the luteal phase in the ovaries.
• Note: The menstrual phase coincides with the start of the next ovarian cycle, representing a reset or starting point.

Ovarian and Uterine Cycle Coordination

• Follicle development and estrogen secretion lead to regeneration of the uterine lining after menstruation.
• Ovulation triggers progesterone secretion from the corpus luteum.
• Progesterone promotes development of endometrial glands and thickening of the endometrium.
• Hormone Levels:
  • FSH and LH stimulate follicle development in the ovaries.
  • Estrogen is dominant before ovulation and stimulates initial growth of the endometrium.
  • LH surges trigger ovulation.
  • Progesterone is dominant after ovulation and prepares the endometrium for implantation.

Oogenesis

• Development of female gametes (oocytes) into a mature ovum during the ovarian cycle.
• Key Differences from Spermatogenesis:
  • Finite Number of Oocytes: Females have a fixed number of oocytes determined at birth.
  • Timing and Stages: Oogenesis stages occur at specific times across the lifespan.
• Process:
  • Germ cells (oogonia) start oogenesis early in fetal life (around 2-3 months gestation).
  • Oogonia undergo mitosis to produce 2-4 million cells, which differentiate into primary oocytes.
  • Primary oocytes begin meiosis but arrest just after DNA replication, pausing the process.
  • Some primary oocytes degenerate through apoptosis.
  • At the end of this process, there's about one to 2,000,000 left.
  • The arrested oocytes are maintained until puberty.
    • However, not all survive until puberty. Only about 400,000 make it, and the rest degenerate through a process called atresia.
• Puberty Onward:
  • With each menstrual cycle, up to 20 follicles begin to grow and develop.
  • Usually, only one develops into a Graafian follicle.
  • Rarely, two oocytes are ovulated (fraternal twins).
  • Prior to ovulation, meiosis resumes in the oocyte of the Graafian follicle.
    • First round of cell division produces two daughter cells:
      • Secondary oocyte: Continues meiosis if fertilization occurs.
      • Polar body: Receives very little cytoplasm and eventually degenerates.
  • Meiosis Completion (only if fertilization occurs):
    • Sperm penetration triggers the second round of cell division in the secondary oocyte.
    • Produces an ovum (mature egg) and a second polar body (degenerates).
  • Fertilization: Fusion of ovum and sperm, resulting in a zygote with 46 chromosomes (23 from each).

Fertilization and Implantation

• Fertilization occurs in the uterine tube.
• Oocyte is collected by the uterine tube after ovulation; sperm travels up to meet it.
• Sperm Survival: Can survive for about five days in the reproductive tract.
• Oocyte Survival: Only survives for 12-24 hours; short window for fertilization.
• Sperm Count: Each ejaculate contains 100-300 million sperm, but most die, and only about 100 reach the oocyte.
• Acrosomal Reaction:
  • Sperm bind to the zona pellucida, triggering the release of proteins and enzymes.
  • These break down the zona pellucida, allowing entry of one sperm.
• Only One Sperm: Ensures the correct number of chromosomes is present.
• Diploid Zygote: Forms after sperm and egg fuse; starts dividing and developing into an embryo.

Early Development

• Fertilization occurs at the beginning of the uterine tube.
• Zygote: Single cell containing DNA from both sperm and egg.
• Cell Division: Rapid cell division (cleavage) occurs, but the zygote doesn't get bigger; cells get smaller with each division.
• Morula: At around 3-4 days, the dividing cells form a morula.
  • Identical Twins: Rarely, the morula can divide into two, resulting in identical twins.
• Blastocyst: At around 4-5 days, the morula develops into a blastocyst and enters the uterus, ready for implantation.

Blastocyst Structure

• Inner Cell Mass: Develops into the embryo.
• Trophoblasts: Outer layer of cells that invade the endometrium.
  • Initially provide nutrients to the blastocyst.
  • Eventually form the placenta for continued fetal development.