Female Reproductive System

Gender and Sex

  • At birth, biological sex is assigned based on external genitalia.

    • Males: penis

    • Females: vagina

    • Intersex: variation in sexual characteristics

  • Biological sex can also be determined by:

    • Sex hormones (X, Y, XX, XXY)

    • Genetic alterations

    • Hormone levels

    • Presence of internal reproductive structures

  • Biological sex exists on a spectrum.

  • Anatomy does not dictate gender identity.

Female Reproductive System: Learning Objectives

  • Identify the structures and functions of the female reproductive system.

  • Describe oogenesis and folliculogenesis.

  • Describe the phases and events of the ovarian and uterine cycles.

  • Identify the steps involved in achieving a successful pregnancy.

Functions of the Female Reproductive System

  • Production of gametes:

    • Oocytes (eggs) in the ovaries

  • Fertilization:

    • Provides passageway and structures for gamete maturation and transportation to the site of fertilization.

  • Development and nourishment of life:

    • Nurtures the developing fetus in the uterus until birth.

    • Provides nourishment (milk) after birth.

  • Production of reproductive hormones:

    • Controls the development of the reproductive system.

    • Controls the development of sex-specific body parts and reproductive behaviors.

Anatomical Structures of the Female Reproductive System

  • Mammary Glands (Breasts):

    • Branched exocrine glands that secrete substances through ducts.

    • Comprised of ducts ending in alveoli.

    • Alveolus and associated duct form a lobule.

    • Lobes culminate at the nipple, secreting milk.

  • Ovaries:

    • Female gonads (paired structures on either side of the uterus).

    • Site of oogenesis (production and maturation of female gametes).

    • Structure:

      • Outer cortex: houses developing follicles.

      • Inner medulla: contains connective tissue, blood vessels, and nerves.

      • Hilum: point of attachment to the broad ligament; where blood vessels, nerves, and lymph enter/leave.

    • Function:

      • Production of female gametes (oocytes).

      • Production of steroid hormones (estrogen, progesterone) and inhibin.

  • Uterine (Fallopian) Tubes:

    • Connect the ovary to the uterus (but are not directly connected).

    • Provide a passageway for sperm to enter and oocytes to be released.

    • Structure:

      • Isthmus: segment closest to the uterus.

      • Ampulla: widest and longest segment; fertilization typically occurs here.

      • Infundibulum: distal, funnel-shaped structure with fimbriae (finger-like projections).

    • Fimbriae help capture the egg released from the ovary.

    • Layers:

      • Mucosal layer: innermost layer lined with ciliated columnar epithelium.

      • Cilia help move the oocyte/embryo towards the uterus.

      • Secretory cells release substances that nourish and protect the developing egg/embryo.

      • Muscularis layer: contains inner circular and outer longitudinal smooth muscle.

    • Contractions (peristalsis) help move the oocyte/egg down the fallopian tube towards the uterus.

  • Uterus:

    • Hollow, thick-walled organ.

    • Site of menstruation and houses a developing fetus.

    • Components:

      • Body: midsection.

      • Fundus: top portion.

      • Cervix: lower portion that narrows and connects the uterus to the vagina.

    • Allows passage of menstrual blood and childbirth.

    • Layers:

      • Serosa (perimetrium): outermost layer.

      • Myometrium: middle layer, composed of smooth muscle.

      • Endometrium: innermost mucosal layer.

    • Endometrium:

      • Stratum functionalis: functional layer that grows during the uterine cycle and sheds during menstruation.

      • Contains glands that secrete nutrient-rich mucus to promote implantation and provide nutrients.

      • Contains spiral arteries for blood supply.

      • Stratum basalis: basal layer of connective tissue cells and stem cells from which the endometrium forms.

      • Stem cells divide and proliferate to regenerate the functionalis layer.

      • Contains the bases for glands and arteries.

      • Does not shed; it's a permanent layer.

  • Cervix:

    • Most distal region of the uterus, separating it from the vagina.

    • Contains glands that secrete mucus.

      • Thick mucus: Stops sperm from entering the uterus and protects from harmful bacteria.

      • Less thick mucus: Allows sperm passage during ovulation.

  • Vagina:

    • Passageway for childbirth and menstrual flow.

    • Acidic environment that helps repel bacteria and prevent infection.

    • Relatively inhospitable environment for sperm.

  • External Genitalia (Vulva):

    • Vulva: the whole structure.

    • Vestibule: space into which the vagina and urethra open.

    • Labia minora: inner lips.

    • Labia majora: outer lips.

    • Clitoris: located at the top of the vulva.

    • Vestibular (Bartholin's) glands: produce lubricating fluid.

Physiology: Production of Gametes

  • Oocytes: Female gametes.

  • Oogenesis: The process by which oocytes are formed.

    • Oocytes are surrounded by supporting cells called follicles.

  • Folliculogenesis: The process by which the follicle grows.

  • Ovarian Follicle: An oocyte and its surrounding follicle (functional unit of the female reproductive system).

  • Oogenesis begins before the fourth month of embryonic development.

    • Oogonia (primordial stem cells) develop into oocytes.

    • Ovaries initially have about 5,000,000 oogonia during embryonic development.

    • Many oogonia degenerate by birth.

    • At birth, a female has approximately 1-2 million oocytes.

    • From birth to puberty, oocytes continue to undergo atresia (degeneration).

    • By puberty, the number of oocytes reduces to about 300-400,000.

  • Oocytes are produced by mitosis and meiosis.

    • Mitosis: Produces two genetically identical daughter cells (how many oogonia are produced).

    • Meiosis: A single cell divides twice to produce four cells containing half the genetic material.

Mitosis and Meiosis in Oogenesis

  • Mitosis: Germ cells undergo multiple rounds of mitosis to produce oogonia.

  • Meiosis:

    • Around the fifth month of fetal development, meiosis I is initiated, and oogonia differentiate into primary oocytes.

    • Meiosis I pauses and resumes at the onset of puberty.

    • At puberty, the hypothalamus increases the production of sex hormones (e.g., follicle-stimulating hormone).

    • Each month, before a female ovulates, one dominant cell resumes meiosis I, and the primary oocyte becomes a secondary oocyte.

      • One secondary oocyte (containing one set of chromosomes) is produced, along with a polar body (to discard the other half of the chromosomes).

    • When ovulation occurs, the secondary oocyte is released into the fallopian tube, initiating meiosis II.

      • Meiosis II pauses and only resumes/completes if a sperm fertilizes the egg.

      • The sperm causes the egg to finish meiosis II.

      • When the sperm and egg fuse, they form a zygote (two sets of chromosomes).

  • Puberty: Triggered by the maturation of the hypothalamus, generally begins between ages 11-13.

    • The hypothalamus triggers the release of hormones that stimulate the growth/enlargement of the vagina, uterus, uterine tubes, external genitalia, adipose tissue deposition in the breasts and hips, and the growth of pubic/axillary hair.

  • Menarche: The onset of puberty in a female, signified by the first onset of menstrual bleeding.

Oogenesis: A Summary

  • Mitosis: Begins before birth, producing many germ cells that become oogonia.

  • Meiosis I: Begins before birth around the fifth month of development; oogonia differentiate into primary oocytes.

  • Infancy: N/A

  • Puberty: Meiosis I (previously arrested) restarts.

    • During each menstrual cycle, one dominant oocyte will undergo and complete meiosis I.

      • Forms a secondary oocyte (one set of chromosomes) and a polar body (which degenerates).

  • Meiosis II: Triggered by ovulation.

    • The secondary oocyte is released from the ovary (mature follicle).

    • If the egg meets a sperm, meiosis II resumes to form a zygote (two sets of chromosomes) and another polar body.

Folliculogenesis

  • Primary Oocyte: Single layer of flat granulosa cells surrounds it to form the primary follicle.

  • Primary Follicle: Arrested until puberty.

  • Secondary follicle: At puberty that single layer will begin to enlarge to form several layers.

    • Zona pellucida: A darker pink substance around the primary oocyte contains glycoproteins that help to protect the egg and/or the embryo. It is also important as well in restricting the passage of sperm.

  • Tertiary Follicle: In amongst those granulosa cells, get these little fluid filled spaces and they eventually fuse to form a single chamber that's called the antrum.

    • Antrum: The site of production of those steroid hormones in the egg, such as estrogen.

  • Ovulation: Follicle matures to where we have this really large antrum, and then our secondary oocyte inside begins to be pushed off to the site before its release is triggered.

    • Follicle ruptures, and the secondary oocyte is released into the fallopian tubes.

  • Corpus Luteum: The remaining follicle after the oocyte heads off.

    • If a sperm meets the egg, it forms a zygote and the corpus luteum will remain until implantation occurs.

    • If the egg is not fertilized, the corpus luteum will begin to degenerate to form what's called the corpus albicans.

Timeline: Oogenesis and Folliculogenesis

  • Oogenesis:

    • Germ cells become oogonium.

  • Folliculogenesis:

    • Oogonium are surrounded by a primordial follicle.

  • Before Birth:

    • Oogonium differentiate into primary oocytes.

  • Primary Oocytes:

    • Single layer of granulosa cells that form around our oocyte.

  • Infancy: N/A

  • Puberty:

    • The primary oocyte finishes meiosis I.

    • The follicle begins to grow.

      • We have multiple layers of cells.

      • The development of an antrum.

  • Meiosis I:

    • Puberty hits Meiosis one is finished and we see the formation of our secondary oocyte and also our polar body.

  • Secondary Oocyte:

    • Our follicle continues to mature around what's inside, which is our secondary oocyte.

    • We have our zona pellucida surrounding it, and we have our antrum producing our steroid hormones.

  • Ovulation:

    • Follicle ruptures, the secondary oocyte is released.

    • If a sperm meets it, meiosis II is complete, and we have the formation of a zygote.

  • Remaining Follicle:

    • Then that remaining follicle that's been left behind is going to become the corpus luteum.

    • The corpus luteum: It is really important for producing another hormone in the cycle, which we'll talk about in a moment.

Ovarian Cycle

  • The development of those ovarian follicles occurs over three main phases: the follicular phase, the ovulation phase, and the luteal phase.

  • After puberty, folliculogenesis typically leads to ovulation of one follicle approximately every twenty eight days.

  • Progression across these phases and also the progression of that dominant follicle is all under hormonal control.

  • Follicular Phase: Days 1 – 14 on a 28 day cycle

    • Hypothalamus begins to release increased levels of gonadrotropin-releasing hormone.

    • The presence of gonadrotropin-releasing hormone (GnRH) is going to stimulate the anterior pituitary to release: Follicle Stimulating hormone (FSH) and Luteinizing Hormone (LH).

    • Follicle stimulating hormone (FSH) is going to stimulate the growth of that follicle.

    • Luteinizing hormone (LH) will also stimulate the growth of that follicle and the production of estrogen.

    • Estrogen will send initial negative feedback to the hypothalamus, not needing any more GnRH.

  • Ovulation Phase:

  • That surge of luteinizing hormone in the end of the follicular phase is going to stimulate the release of the oocyte from the follicle.
    Remember that ovary doesn't actually anatomically connect to the fallopian tubes, so it gets swept up by the fimbrae and then heads down the fallopian tubes.

  • The remaining follicle is going to become the corpus luteum.

  • This LH surge leads to Luteinization.

  • The corpus luteum will begin to produce another hormone that you've probably heard of called progesterone, begins to peak.
    Triggers the luteal period.

  • Luteal Phase

    • The surge of progesterone that occurs keeps the corpus luteum alive and it can continue to produce progesterone to improve the chances of the success of a pregnancy.

    • In a menstrual cycle when fertilization does not occur, after about ten to twelve days, that corpus luteum will begin to die off.

    • The supply of progesterone will decrease and the degradation of the corpus luteum into corpus albicans (dies).

Hormonal Graph

  • Follicular phase: 0-14 days

    • Gonadotropin-releasing hormone increases.

    • FSH and LH increase.

    • Estrogen increases.

    • Negative feedback: Rising estrogen dips in FSH and LH.
      Dominant follicle.
      Becomes stimulatory.

  • That estrogen rising with that maturing follicle of the dominant follicle actually tells sends positive feedback to the hypothalamus saying, hey, release more GnRH, and that stimulates this peak here, this spike in luteinizing hormone and follicle stimulating hormone.

  • Surge of luteinizing hormone stimulates the release of the oocyte
    After the oocyte is released, luteinization (the creation of the corpus luteum)

  • Corpus luteum begins to produce progesterone.

After about ten to fifteen days, fertilization does not occur.

  • Progesterone dies off and becomes corpus albicans

  • Hypothalamus says produce more gonadotropic releasing hormone, to simulate the production of LH and FSH, and the process starts again.

Ovarian Cycle Recap

*Follicle Maturity:
* Increased gonadotropic releasing hormone
* LH and FSH stimulates the follicle to mature and develop.
* Estrogen produces in antrum.
Estrogen Rise:

  • Sends negative feedback to hypothalamus.

    • One Dominant Follicle Survives: Estrogen is stimulatory.

    • Triggered Surge :Triggers a pulse: releases the oocyte and into the fallopian tubes.

    • Corpus Luteum: that forms will produce more and more levels of progesterone (occurs in the luteal phase)

    • High Levels: levels after ten to fourteen will trigger the release/start of Progesterone

Menstrual Cycle: Cyclical Changes

*Occur in the lining of the uterus to make it a more hospitable environment for that oocyte to hopefully implant, to be fertilised and implant and grow a developing fetus.

  • Hormonal changes trigger changes in the lining of the uterus. *Three Main Phases *Menstrual Phase, *Proliferative Phase

    • Secretory Phase
      These coincide with phases of Ovarian Cycle

Phases of the Menstrual Cycle

*Influence hormonal changes: these will trigger Menstrual Phases
*Ovarian Cycles
*3 Main Phases *Menstrual Phase,
*Proliferative Phase

  • Secretory Phase

Phase Explanation

*Proliferative Phase: days 5-14 with the follicular phase of the ovarian cycle.
*The Presence of FSH + LH : will lead to the production of estrogen (in the maturing follicle).
Specifically will stimulate the stratum functionalis layer to begin to proliferate and regenerate.
We will see the elongation and the regeneration of spinal arteries

Ovulation

Oocyte is released, remaining follicle becomes corpus luteum
*
*Secretory Phase *the luteal: of the ovary
Progesterone release: promotes the thickening of the stratum functionalis layer.
Promotes these glands to secrete nutrient rich fluid

Menstrual Phase

*Shedding Lining
*Reduction in Progesterone: will trigger and the FSH and LH will shed the layer (commonly referred to as “period”)