Female Reproductive System: Functional Anatomy, Oogenesis and Follicular Development

Introduction

Female reproduction follows a cyclical pattern from puberty to menopause. Oocytes begin developing before birth and mature during menstrual cycles. After ovulation, the corpus luteum secretes hormones that support implantation and sustain early pregnancy if fertilization occurs.

• Female reproduction centers on the cyclical release of an ovum.
• Ovarian changes include follicular growth, maturation, ovulation, and hormone secretion.
• The uterus prepares the endometrium for implantation.
• If fertilization does not occur, menstruation follows.
• These events are regulated by the hypothalamic-pituitary-ovarian axis.

Functional Anatomy

External Genitalia

• Female external genital structures are collectively called the vulva.
• They include labia majora, labia minora, clitoris, vaginal vestibule, and vestibular glands.

Labia Majora

• Female external genital structures are collectively called the vulva.
• They include labia majora, labia minora, clitoris, vaginal vestibule, and vestibular glands.

Labia Minora

• Labia minora are paired inner skin folds located medial to the labia majora.
• They form the inner boundaries of the vestibule and help protect the urethral and vaginal openings.

Clitoris

• The clitoris lies at the anterior end of the vestibule and is homologous to the male penis.
• It contains erectile tissue and ends in the glans clitoris.

Vaginal Vestibule

• The vestibule contains the external urethral opening and vaginal opening.
• The hymen may partially cover the vaginal opening.
• Vestibular glands secrete mucus into the vestibule.

Internal Genitalia

Female internal reproductive organs include the vagina, uterus, fallopian tubes, and ovaries.

Vagina

• The vagina is a fibromuscular canal located anterior to the rectum and posterior to the urethra and urinary bladder.
• It extends from the vaginal opening to the cervix of the uterus.
• Average adult length is about 7–9 centimeters, but it is highly distensible because of mucosal folds.
• During sexual arousal, it lengthens and expands to accommodate intercourse.
• During childbirth, it stretches markedly under hormonal and mechanical influences.
• Vaginal muscular contractions may assist movement of semen toward the cervix.
• The vaginal lining is stratified squamous epithelium that responds to ovarian hormones.
• During the proliferative phase, estrogen thickens the epithelium and increases glycogen content.
• Vaginal secretions are usually thinner during estrogen-dominant phases.
• During the luteal phase, progesterone alters epithelial maturation and secretions may become thicker.
• Leukocyte infiltration may increase before menstruation.
• Reduced estrogen causes thinning of vaginal epithelium (atrophic vaginitis risk).
• Vaginal lubrication decreases, causing dryness or discomfort.
• Susceptibility to irritation and infections may increase.

Body of Uterus

The uterus contains a cavity and two main layers: endometrium and myometrium.

Myometrium

The myometrium is composed of multiple layers of smooth muscle.

Endometrium

• The endometrium is the inner lining of the uterus.
• It has two main components: surface epithelium and supporting stroma.
• The stroma contains connective tissue, immune cells, and blood vessels.
• Spiral arteries supply the functional layer and undergo cyclic hormonal changes.
• Uterine glands lined by columnar secretory cells extend into the stroma.
• These glands change in shape and secretion during the menstrual cycle.
• During the proliferative phase, the endometrium regenerates under estrogen influence.
• During the secretory phase, progesterone makes glands tortuous and nutrient rich.
• If pregnancy does not occur, withdrawal of ovarian hormones causes shedding of the functional layer as menstruation.
• During pregnancy, the uterus enlarges markedly to support fetal growth.
• During labor, coordinated uterine contractions expel the fetus.

Uterine Cervix

• The cervix is the lower narrow part of the uterus connecting it to the vagina.
• It contains the internal os, cervical canal, and external os.
• Cervical mucus changes with hormonal fluctuations during the cycle.
• Near ovulation, estrogen makes mucus thin, watery, and stretchable (spinnbarkeit).
• Estrogen-dominant mucus may show a fern pattern on drying.
• Cervical dilation occurs during childbirth and for certain gynecological procedures.

Fallopian Tubes

• Two fallopian tubes arise from the upper lateral angles of the uterus.
• They are usually about 10–12 centimeters long and are also called oviducts or uterine tubes.
• Main parts are isthmus, ampulla, and infundibulum.
• The infundibulum opens near the ovary and has fimbriae that help capture the ovulated oocyte.
• Cilia and smooth muscle contractions aid transport of the oocyte or embryo.
• Fertilization commonly occurs in the ampulla.
• The early embryo then moves to the uterine cavity for implantation.
• Tubal disease or blockage can cause infertility or ectopic pregnancy.

The Ovaries

• The female reproductive system contains two ovaries, each weighing about 10 grams in adults.
• Each ovary lies on either side of the uterus and is connected to it by the ovarian ligament.
• The ovary has two main regions: an outer cortex and an inner medulla.

Cortex of Ovary

• The cortex is the larger outer portion of the ovary.
• It is covered by a surface epithelium and enclosed by a fibrous capsule called the tunica albuginea.
• The cortex contains numerous oocytes present within ovarian follicles.
• Follicles exist in different stages of growth and maturation throughout reproductive life.
• A fully mature follicle ready for ovulation is called the Graafian follicle.
• Between the follicles lies ovarian stroma made of connective tissue and hormone-producing interstitial cells.

Medulla of Ovary

• The medulla forms the central inner region of the ovary.
• It contains loose connective tissue, interstitial cells, nerves, blood vessels, and lymphatic vessels.
• Blood vessels and lymphatics enter and leave the ovary through the hilum.

Functions of Ovary:

• The ovary is the female gonad and performs both reproductive and endocrine functions.
• It forms primary oocytes during fetal life.
• It supports maturation of follicles and development of the ovum.
• It releases the ovum during ovulation.
• It secretes steroid hormones such as estrogen and progesterone.
• It also produces peptide hormones such as inhibin and relaxin.
• These hormones regulate menstruation, fertility, pregnancy, and metabolic processes.

Oogenesis

• Oogenesis is the process of formation and maturation of female gametes in the ovary.
• It begins during fetal life and usually ends at menopause.
• Unlike spermatogenesis, it does not continue continuously throughout life.
• One mature ovum is generally released during each menstrual cycle.
• Oogenesis proceeds slowly and includes long resting phases.
• The stages are: oogonium to primary oocyte, primary oocyte to secondary oocyte, and secondary oocyte to mature ovum.
• This process ensures periodic release of a female reproductive cell for fertilization.

Oogonia Becoming Primary Oocyte

Oogonia

• Oogonia are the earliest female germ cells that give rise to oocytes.
• Primordial germ cells migrate from the embryonic yolk sac to the genital ridge around the 6th week of gestation.
• After reaching the developing ovary, these cells multiply rapidly by mitosis.
• Their number rises to nearly 6 to 7 million during fetal life.
• When mitotic division stops, oogonia begin differentiation into oocytes.

Primary Oocytes

• The immature cells formed at this stage are called primary oocytes.
• Primary oocytes enter the first meiotic division during fetal development.
• This meiotic process begins around the 8th week of gestation.
• Division is then arrested in prophase I, where it remains for many years.
• During this resting period, the oocyte enlarges and becomes surrounded by follicular cells.
• These surrounding cells help maintain meiotic arrest through local regulatory signals.

First Meiotic Division

• The first meiotic division does not finish before birth.
• It also remains incomplete throughout childhood.
• In each menstrual cycle after puberty, selected primary oocytes resume meiosis.
• Usually, one dominant follicle completes the first meiotic division shortly before ovulation.
• This produces a secondary oocyte and the first polar body.

Oocyte Degeneration

• Degeneration of oocytes starts before birth.
• At birth, only about 1 to 2 million primary oocytes remain in both ovaries.
• By puberty, the number falls to about 300,000 to 400,000.
• By around 30 years of age, the reserve declines further.
• At menopause, functional oocytes are nearly absent.
• The natural degeneration of follicles and oocytes is called atresia.
• During reproductive life, only about 400 to 500 oocytes are ovulated.
• Most remaining oocytes undergo atresia instead of ovulation.
• Unlike males, females do not continuously produce new germ cells after birth.
• Therefore, ovarian reserve decreases progressively with age, reducing fertility over time.

Primary Oocyte Converted to Secondary Oocyte

• During fetal life, oogonia develop into primary oocytes.
• Primary oocytes begin the first meiotic division but stop in prophase I.
• This meiotic arrest continues from birth until puberty.
• At birth, all ova are primary oocytes containing 46 chromosomes.
• In each menstrual cycle, the selected primary oocyte resumes meiosis before ovulation.
• Completion of the first meiotic division forms two cells.
• The larger cell is the secondary oocyte, containing 23 chromosomes.
• The smaller cell is the first polar body.
• Cytoplasm is distributed unequally, so the secondary oocyte retains most cellular contents for future embryonic development.

Secondary Oocyte Forming Ovum

Second Meiotic Division

• The secondary oocyte begins the second meiotic division after ovulation.
• This division stops at metaphase II until fertilization occurs.
• Entry of a sperm triggers completion of meiosis.
• The process then forms a mature ovum with 23 chromosomes.
• A second polar body is also produced and expelled.
• Therefore, each primary oocyte normally gives rise to only one functional ovum.

Development of Ovarian Follicle

• Folliculogenesis is the growth and maturation of ovarian follicles containing developing oocytes.
• The oocyte enlarges within the follicle until ovulation releases the ovum.
• Follicular development begins during fetal life and continues through reproductive years until menopause.
• At the start of each menstrual cycle, several follicles begin to grow.
• Usually, only one becomes the dominant follicle and reaches full maturity.
• The dominant follicle releases the ovum during ovulation.
• Most remaining follicles undergo degeneration called atresia.
• Nearly all follicles present at birth are lost by atresia over time.

Stages of Follicular Development

• Folliculogenesis is the progressive maturation of ovarian follicles.
• It is commonly described in four developmental stages.
• Early stages include the primordial follicle and primary follicle.

Stage 1 (Primordial Follicular Stage)

• This is the earliest recognizable follicle in the ovary.
• It contains a primary oocyte at the center.
• The oocyte is surrounded by a single layer of flattened pregranulosa cells.
• A basement membrane is present outside these cells.
• The oocyte measures about 25 micrometers in diameter.
• Primordial follicles form the resting reserve of follicles present from fetal life.
• Their number gradually declines from before birth until menopause.
• The oocyte has already entered the first meiotic division.
• Meiosis remains arrested in prophase I until just before ovulation many years later.

Stage 2 (Primary Follicular Stage)

• Some primordial follicles are recruited to continue development.
• The flattened pregranulosa cells become cuboidal granulosa cells.
• These cells multiply to form one or more layers around the oocyte.
• The oocyte enlarges to about 80 to 140 micrometers.
• Granulosa cells and the oocyte produce the zona pellucida, a glycoprotein layer surrounding the oocyte.
• The zona pellucida is important for sperm binding and fertilization.
• Development into primary follicles begins during fetal life and continues after puberty.
• The oocyte still remains arrested in prophase I at this stage.

Stage 3 (Secondary Follicular Stage)

• In the secondary follicular stage, the primary follicle enlarges further.
• Granulosa cells multiply and form several layers around the oocyte.
• This causes marked increase in follicular size to about 500 micrometers.
• At this stage, the follicle is commonly called a preantral follicle.
• A small fluid-filled cavity may begin to appear.
• Spindle-shaped cells develop outside the basement membrane.
• These cells later differentiate into the theca layer.
• Much of this growth occurs slowly before puberty.
• The oocyte remains arrested in prophase I of the first meiotic division until ovulation approaches.

Stage 4 (Tertiary Follicular Stage)

• The tertiary follicular stage is the final phase of follicle maturation.
• It includes the early antral stage and the Graafian follicle stage.

Early Tertiary or Antral Follicular Stage

• Peripheral cells differentiate into theca interna and theca externa.
• Theca interna cells become steroid-producing and help synthesize estrogens.
• Theca externa forms a supportive outer connective tissue layer.
• Blood vessels, lymphatics, and nerves supply the theca layer.
• Granulosa cells remain avascular because vessels do not cross the basement membrane.
• A fluid-filled cavity called the antrum develops among granulosa cells.
• Further growth depends mainly on follicle-stimulating hormone.

Late Tertiary or Graafian Follicular Stage

• The late tertiary stage is also called the Graafian follicular stage.
• It is the fastest phase of follicular growth and occurs only after puberty.
• About 5 to 7 days after menstruation begins, one follicle usually becomes dominant.
• This dominant follicle continues growth, while others undergo atresia.
• Normally, one dominant follicle develops in one ovary during each cycle.

Structural Changes

• The fluid-filled antrum enlarges markedly during this stage.
• Increasing fluid volume pushes the oocyte toward the follicle wall.
• Follicular fluid contains proteins, electrolytes, glycosaminoglycans, enzymes, and regulatory factors.
• It also contains hormones such as estrogen, progesterone, follicle-stimulating hormone, and luteinizing hormone.
• Blood supply to the theca layer increases to support rapid growth and hormone synthesis.

Granulosa Cell Compartments

• Granulosa cells become organized into functional groups.
• Mural granulosa cells line the follicular wall and actively produce steroid hormones.
• Cumulus granulosa cells surround the oocyte and support its maturation.
• The cluster carrying the oocyte into the antrum is called the cumulus oophorus.
• Granulosa cells facing the antral cavity also participate in hormone secretion.

Pre-ovulatory Follicle

• The mature follicle reaches about 2 to 2.5 centimeters in diameter.
• It is then called the pre-ovulatory follicle.
• Just before ovulation, the primary oocyte completes the first meiotic division.
• This forms a secondary oocyte and the first polar body.
• The secondary oocyte immediately enters the second meiotic division and arrests at metaphase II.

Ovulation

• Enzymatic weakening and rupture of the follicular wall release the secondary oocyte.
• This event is known as ovulation.
• The oocyte is expelled with surrounding cumulus cells.
• It is then captured by the fimbriae of the uterine tube.

Fate After Ovulation

• If fertilization occurs, sperm entry triggers completion of meiosis II.
• A mature ovum and second polar body are formed.
• Fusion of male and female pronuclei forms the zygote.
• If fertilization does not occur, the oocyte usually degenerates within 24 to 48 hours.
• The ruptured follicle later transforms into the corpus luteum.

Corpus Luteum Formation

Luteinization

• After ovulation, the ruptured ovarian follicle briefly fills with blood.
• At this stage, it is called the corpus hemorrhagicum.
• Soon, blood is replaced by proliferating luteal cells rich in lipids.
• The structure then becomes the corpus luteum, marking the start of the luteal phase.

Structure of Corpus Luteum

• The corpus luteum is a temporary yellow endocrine gland in the ovary.
• It is composed mainly of granulosa lutein cells, theca lutein cells, and connective tissue cells.
• Granulosa cells enlarge and develop abundant mitochondria, smooth endoplasmic reticulum, and lipid droplets.
• These changes support active synthesis of steroid hormones.
• The process of transformation into luteal cells is called luteinization.

Vascular Changes

• New blood vessels grow rapidly into the corpus luteum.
• Unlike follicular granulosa cells, luteal granulosa cells become highly vascular.
• Increased blood flow supplies cholesterol needed for steroid hormone production.
• Good vascularity is essential for maintaining luteal endocrine activity.

Hormone Secretion

• The corpus luteum secretes mainly progesterone.
• It also produces smaller amounts of estrogen and androgens.
• Progesterone levels usually peak about 7 days after ovulation.
• This timing corresponds to full functional maturity of the corpus luteum.
• Progesterone prepares the endometrium for implantation and early pregnancy.

Regulation of Luteinization

• Luteinizing hormone is the principal hormone that stimulates luteinization.
• It maintains survival and secretory function of the corpus luteum.
• This is why it is called luteinizing hormone.
• Follicle-stimulating hormone helps by increasing luteinizing hormone receptors on ovarian cells.

Clinical Importance

• If pregnancy does not occur, the corpus luteum regresses after about 14 days.
• If pregnancy occurs, human chorionic gonadotropin maintains it during early gestation.

Luteal Regression

• If fertilization does not occur, the corpus luteum usually regresses about 12 to 14 days after ovulation.
• Its endocrine cells degenerate and are replaced by connective tissue cells.
• This process is called luteolysis or luteal regression.
• White blood cells and fibroblasts participate in tissue breakdown and repair.
• The exact mechanism is complex and involves reduced luteinizing hormone support.
• Local mediators such as prostaglandins may contribute to regression.
• The remaining scar becomes the corpus albicans, a nonfunctional fibrous structure.
• After implantation, the trophoblast secretes human chorionic gonadotropin.
• This hormone acts similarly to luteinizing hormone.
• It maintains the corpus luteum and prevents regression.
• The maintained gland is called the corpus luteum of pregnancy.

Functions of Corpus Luteum:

• It secretes progesterone, estrogen, inhibin A, and small amounts of androgens.
• Progesterone converts the endometrium into a secretory lining suitable for implantation.
• It supports the early weeks of pregnancy until the placenta becomes hormonally active.
• Adequate progesterone also reduces uterine contractility.
• Inadequate luteal function may reduce endometrial receptivity.
• This can contribute to infertility or early pregnancy loss

Atresia of Follicle

• During reproductive life, only about 400 oocytes usually reach ovulation.
• Most non-dominant follicles undergo degeneration called atresia.
• Atresia begins in fetal life after primordial follicles appear.
• It occurs mainly through apoptosis, a regulated form of programmed cell death.

Regulation of Follicular Development

Regulation of follicular development varies at different growth stages.

Primordial Follicular Stage

• Early growth of primordial follicles is largely independent of gonadotropins.
• It is mainly controlled by local ovarian growth factors.

Primary Follicular Stage

• Signals from the oocyte promote conversion of pregranulosa cells into granulosa cells.
• Granulosa cells support development of surrounding pre-theca cells.
• Low levels of follicle-stimulating hormone and luteinizing hormone help maintain growth during childhood.
• Absence of these hormones can impair follicular progression.

Secondary Follicular Stage

• At puberty, secondary follicles develop more actively.
• Granulosa cells acquire follicle-stimulating hormone receptors.
• These cells begin secreting small amounts of estrogen, which promotes further follicular maturation.

Early Tertiary Stage

• In the early tertiary stage, growing follicles become increasingly responsive to hormones.
• This stage commonly begins after menarche as cycles become established.
• Follicle-stimulating hormone acts on granulosa cells during the follicular phase.
• It increases aromatase activity, converting androgens into estrogens.
• As a result, follicular estrogen concentration rises.
• Estrogen together with follicle-stimulating hormone promotes formation of luteinizing hormone receptors on granulosa cells.
• Estrogen also enhances hypothalamic-pituitary activity needed for normal luteinizing hormone secretion.
• Rising estrogen supports increased synthesis of luteinizing hormone before ovulation.
• In theca cells, estrogen increases receptor number and sensitivity to luteinizing hormone.
• Luteinizing hormone then stimulates androgen production in theca cells.
• These androgens diffuse to granulosa cells and serve as substrate for estrogen synthesis.
• Luteinizing hormone also promotes some progesterone production by granulosa cells.

Late Tertiary Stage

• During days 5 to 7 of the follicular phase, one follicle usually becomes the dominant follicle.
• Other developing follicles typically undergo atresia.

Hormonal Changes in the Dominant Follicle

• Granulosa cells of the dominant follicle develop more follicle-stimulating hormone receptors.
• Aromatase activity also increases, leading to greater estrogen production.
• Rising estrogen supports continued growth of the dominant follicle.

Positive Feedback and Luteinizing Hormone Surge

• At low to moderate levels, estrogen usually suppresses luteinizing hormone release.
• When estrogen remains high for about 36 hours, feedback becomes positive.
• This stimulates the hypothalamus and pituitary gland.
• A marked luteinizing hormone surge then occurs.
• The surge is the key trigger for ovulation.

Events Leading to Ovulation

• Luteinizing hormone increases progesterone secretion within the follicle.
• Progesterone helps activate proteolytic enzymes that weaken the follicular wall.
• Follicular fluid volume rises, increasing pressure inside the follicle.
• Luteinizing hormone also promotes synthesis of prostaglandins and leukotrienes.
• These mediators assist breakdown of the follicular wall.
• Plasmin further digests connective tissue and supports rupture.
• Follicle-stimulating hormone promotes plasmin formation through plasminogen activator.

Ovulation

• Contraction of the follicular wall helps expel the secondary oocyte.
• Release of the oocyte from the ovary is called ovulation.
• It usually occurs about 36 hours after onset of the luteinizing hormone surge.

After the Surge

• Soon after the surge, luteinizing hormone receptor numbers decline.
• This reduces sensitivity of granulosa and theca cells to further stimulation.