Reproductive System Vocabulary

Reproductive System

• Primary sex organs (gonads)

• Gonads produce:

  • Sex cells - gametes

  • Secrete sex hormones

• Sex hormones:

  • Androgens (males)

  • Estrogens and progesterone (females)

Sexual Maturation in Females and Males

• Puberty

  • Onset during adolescence

  • Reproductive organs become functional

  • External sex characteristics develop

    • E.g., breast enlargement in females

    • E.g., pubic hair growth

  • Gametes begin to mature

  • Gonads start to secrete sex hormones

  • Girls typically experience puberty earlier than boys

Hormones Initiating Puberty

• Hypothalamus releases GnRH (Gonadotropin-Releasing Hormone)

• GnRH stimulates the Anterior Pituitary to release:

  • Follicle-Stimulating Hormone (FSH)

  • Luteinizing Hormone (LH)

• Gonads begin to produce significant levels of sex hormones

• This starts the process of gamete and sexual maturation

Gametes

• Female produces and releases a single gamete monthly:

  • Oocyte

• Male produces large numbers of gametes:

  • 100 million/day

  • Stored for a short time only

  • If not expelled from the body, they are resorbed

Male Reproductive System

• Primary reproductive organs:

  • Testes

• Accessory reproductive organs:

  • Ducts and tubules leading from the testes to the penis

  • Male accessory glands

  • Penis

Male Pelvic Region

• Key Structures:

  • Urinary bladder

  • Pubic symphysis

  • Ductus deferens

  • Urogenital diaphragm

  • Urethra

  • Penis

  • Epididymis

  • Testis

  • Glans

  • Scrotum

  • Ureters

  • Ampulla of ductus deferens

  • Seminal vesicle

  • Ejaculatory duct

  • Prostate gland

  • Bulbourethral gland

  • Anus

The Scrotum

• Contains paired testicles

• Separated by a septum

• External positioning maintains a temperature ~3°C lower than core body temperature, which is optimal for spermatogenesis

• Intrascrotal temperature is regulated by:

  • Cremaster muscle

  • Dartos muscle

Scrotum and Testes

• Layers of spermatic cord wall

Testes and Spermatogenesis

• Testes:

  • Produce sperm and androgens

• Tunica albuginea:

  • Thick fibrous capsule covering the testis

• Septa:

  • Internal projections of the tunica albuginea

  • Subdivide the internal space into approximately 250 lobules

Testes and Spermatogenesis

• Seminiferous tubules:

  • Extremely convoluted and elongated

  • Up to 4 per lobule

  • Contain:

    • Non-dividing support cells - sustentacular cells

      • Nourish developing sperm

      • Release hormone inhibin when sperm count is high

        • Inhibits FSH secretion, regulating sperm production

    • Dividing germ cells continuously producing sperm

Testes and Spermatogenesis

• Blood-testis barrier:

  • Protects developing sperm from harmful materials in the blood

• Interstitial cells:

  • Located in interstitial spaces

  • Luteinizing hormone (LH) stimulates them to produce androgens

  • Testosterone

    • Mostly produced by interstitial cells

    • A small amount is produced by the adrenal cortex

Testis Anatomy

• Key Structures:

  • Ductus deferens

  • Spermatic cord

  • Blood vessels and nerves

  • Head of epididymis

  • Duct of epididymis

  • Seminiferous tubule

  • Efferent ductule

  • Mediastinum testis (housing rete testis)

  • Septum

  • Lobule

  • Straight tubule

  • Body of epididymis

  • Tail of epididymis

  • Visceral layer of tunica vaginalis

  • Parietal layer of tunica vaginalis

  • Tunica albuginea

Seminiferous Tubule: Cross Section

• Interstitial cell

• Seminiferous tubule

• Sustentacular cells

• Tubule lumen

• Germ cells

• Sperm

• Spermatids

• Spermatogonia

Duct System in the Male Reproductive Tract

• Ducts within the testis:

  • Rete testis

  • Efferent ductules

• Epididymis

  • Sperm mature and gain motility

• Ductus deferens to ampulla

• Ejaculatory duct

• Urethra

Epididymis

• Storage site for sperm

• Sperm take approximately 20 days to travel through it

• Non-motile sperm become motile here

• Sperm are ejaculated from the epididymis, not directly from the testes

• Smooth muscle contraction moves sperm to the ductus deferens

Ductus Deferens

• Connects the epididymis to the vas deferens

• Contains an ampulla

• Transports sperm from storage to the urethra for delivery during ejaculation

• Has a major layer of smooth muscle to facilitate sperm movement

• Vasectomy:

  • Involves cutting and sealing the ductus deferens

  • Nearly 100% effective form of birth control

Ejaculatory Duct

• Formed by the union of the vas deferens and the seminal vesicle duct

• Located within the prostate gland

Urethra

• Conveys both urine and semen, but not at the same time

• Consists of three regions:

  • Prostatic urethra

  • Membranous urethra (urogenital diaphragm)

  • Spongy or penile urethra

Duct System and Accessory Glands in Male Reproductive Tract

• Key Structures:

  • Urinary bladder

  • Ureter

  • Ampulla

  • Seminal vesicle

  • Ejaculatory duct

  • Prostate gland

  • Prostatic urethra

  • Bulbourethral gland

  • Urogenital diaphragm

  • Ductus deferens

  • Epididymis

  • Testis

  • Bulb

  • Crus

  • Corpus cavernosum

  • Membranous urethra

  • Penis

  • Corpus spongiosum

  • Spongy urethra

  • Glans

Accessory Glands

• Seminal vesicles

• Prostate gland

• Bulbourethral glands

Seminal Vesicles

• Secrete approximately 60% of the volume of semen

• Secretion is viscous and alkaline

• Contains fructose

• Joins the vas deferens to form the ejaculatory duct

• Sperm and seminal fluid mix in the ejaculatory duct

• Also contains major smooth muscle

Accessory Glands: Prostate Gland

• Doughnut-shaped gland that surrounds the urethra inferior to the bladder

• Secretes a milky, slightly acidic fluid that contains

  • Citric acid, a nutrient for sperm health

  • Seminalplasmin, an antibiotic that helps prevent UTIs

  • PSA (prostate-specific antigen), an enzyme to liquefy semen following ejaculation

Accessory Glands: Bulbourethral Glands (Cowper’s Glands)

• Pea-sized glands located inferior to the prostate gland

• Produce a thick, clear mucus prior to ejaculation

• This mucus likely serves to lubricate and neutralize the acidity of the urethra

Semen

• Milky white, sticky mixture of sperm and secretions from the male reproductive glands.

• Functions:

  • Transports sperm

  • Provides nutrients for sperm

  • Protects and activates sperm

  • Facilitates sperm movement

• Contains Prostaglandins:

  • Decrease the viscosity of mucus in the cervix

  • Stimulate reverse peristalsis in the uterus

  • Help sperm move through the female reproductive tract

Semen Volume

• Typical ejaculate is only 2-5 ml of semen

• Contains 50-130 million sperm/ml

Clinical View: Benign Prostatic Hyperplasia

• Noncancerous enlargement of the prostate gland

• Common in older men

• Related to hormonal changes associated with aging

• The enlarged prostate compresses the urethra, leading to:

  • Frequent urination at night

  • Frequent and painful urination

• Treatment includes medications

• May require surgical removal of the prostatic enlargement in severe cases

Clinical View: Prostate Cancer

• One of the most common malignancies in men over 50

• Characterized by the formation of hard, solid nodules within the prostate

• Symptoms are often asymptomatic early on, but can later include urinary symptoms

• Screening involves:

  • Digital rectal exam

  • PSA (prostate-specific antigen) test

• Treatment:

  • Radiation therapy

  • Surgical removal of the prostate

Penis

• Forms the external genitalia along with the scrotum

• Regions:

  • Root:

    • Internally attached portion of the penis

  • Body (shaft):

    • Elongated movable portion

  • Glans:

    • Tip of the penis

    • Contains the external urethral orifice

  • Prepuce (foreskin):

    • Circular fold of skin

    • Attached to the raised edge of the glans

Penis

• Composed of 3 cylindrical erectile bodies:

  • Paired corpora cavernosa

    • Located dorsolaterally

    • Terminate in the shaft of the penis

  • Corpus spongiosum

    • Ventral to the corpora cavernosa in the midline

    • Contains the spongy urethra

    • Continues within the glans

Anatomy of the Penis: Anterolateral View

• Key Structures:

  • Membranous urethra

  • Bulb of penis

  • Pubic symphysis

  • Right crus of penis

  • Body of penis

  • Corpora cavernosa

  • Corpus spongiosum

  • Scrotum

  • Glans

  • External urethral orifice

Anatomy of the Penis

• Deep dorsal vein

• Corpus cavernosum

• Corpus spongiosum

• Skin

• Spongy urethra

• Dorsal vein (blue), artery (red), and nerve (yellow)

• Tunica albuginea

• Central artery

• Venous spaces

• Deep fascia

• Superficial fascia

• Prepuce

Male Sexual Response

• Excitement phase

  • Blood fills the erectile tissues leading to erection

  • PNS (parasympathetic nervous system) stimulates release of nitric oxide

  • Near the end of this phase, heart rate, blood pressure, and respiratory rate increase

• Orgasm

  • Ductus deferens moves sperm toward the urethra

  • Accessory glands secrete seminal fluid components

    • These combine with sperm to form semen

  • Internal urethral sphincter of bladder contracts to prevent the backflow of urine into urethra

  • Ejaculation occurs at the end of orgasm

    • Semen expelled due to contractions of smooth muscle

    • Controlled by SNS (sympathetic nervous system)

Male Sexual Response

• "Point and shoot" (PNS for erection, SNS for ejaculation)

• Resolution phase

  • SNS constricts central artery, reducing blood flow to penis

  • Muscles around erectile tissue constrict

• Refractory period:

  • Period during which erection cannot be achieved

  • Lasts minutes to hours

Testes and Spermatogenesis

• Hormonal regulation

  • Hypothalamus secretes GnRH

    • Stimulates anterior pituitary to secrete FSH and LH

  • FSH and LH stimulate spermatogenesis and androgen production:

    • LH stimulates interstitial cells to secrete testosterone

    • FSH stimulates sustentacular cells to secrete androgen-binding protein (ABP)

      • ABP binds to testosterone, ensuring high levels in the testes

Testes and Spermatogenesis

• Hormonal regulation (continued)

  • Increased testosterone has multiple effects

    • Facilitates spermatogenesis

    • Inhibits GnRH secretion and reduces pituitary sensitivity to GnRH

    • Has a negative feedback effect

  • Sustentacular cells release inhibin

    • In response to rising sperm count levels

    • Inhibits FSH secretion from the anterior pituitary

    • Additional negative feedback mechanism

  • Testosterone stimulates libido and the development of secondary sex characteristics

    • Hair growth in axillary and pubic regions, deeper voice, facial hair

Hormonal Regulation of Spermatogenesis and Androgen Production

• Hypothalamus

  • GnRH

• Anterior pituitary

  • FSH

    • Sustentacular cells

      • Inhibin

      • ABP

    • Spermatogenesis

  • LH

    • Interstitial cells

      • Testosterone

Gametogenesis

• Process of forming human sex cells, gametes

• Female gametes: Secondary oocytes (“eggs”)

• Male gametes: Sperm

• Begins with cell division: Meiosis

  • Similar process in females and males, with a few differences

A Brief Review of Heredity

• Hereditary information

  • Diploid cell

    • 23 pairs of chromosomes

    • 2n chromosomes

      • (where n is the unpaired chromosome number)

  • Haploid cell

    • 23 chromosomes (not 23 pairs)

    • Chromosome number is n

    • Gametes from either sex

    • Ensures offspring do not receive 4n total chromosomes

Overview of Meiosis

• Meiosis

  • Starts with a diploid parent cell

  • Produces haploid daughter cells (gametes)

• Key Difference: Mitosis vs. Meiosis

Cell Division Terminology

• Replicated chromosome (also known as a double-stranded chromosome, duplicated chromosome)

  • A chromosome that initially has two identical sister chromatids joined at the centromere

• Single chromosome (also known as a single-stranded chromosome)

  • A chromosome consisting of a single chromatid and a centromere

Meiosis – Prophase I

• Synapsis:

  • Homologous maternal and paternal replicated chromosomes (each composed of sister chromatids) pair up (synapsis), and the pair forms a tetrad. Crossing over occurs between homologous maternal and paternal chromosomes, increasing genetic diversity.

Meiosis - Metaphase I

• Tetrads line up at the spindle equator during metaphase I

  • Homologous replicated chromosomes line up above and below the equator of the cell, forming a double line of chromosomes. Spindle fibers attach to the centromeres.

Meiosis – Anaphase I

• Homologous chromosomes are distributed to opposite ends of the cell

  • Maternal and paternal pairs of replicated chromosomes are separated and pulled to the opposite ends of the cell, a process called reduction division. Note that the sister chromatids remain attached in each replicated chromosome.

Meiosis – Telophase I

• Nuclear membrane forms around chromosomal masses

• Two haploid daughter cells

  • Nuclear division finishes and nuclear envelopes re-form. The cytoplasm divides and two new cells are produced, each containing 23 replicated chromosomes only. The replicated chromosomes are still composed of sister chromatids.

Meiosis I

• Maternal ("mom") chromosomes

• Paternal ("dad") chromosomes

• Replicated chromosome

• Sister chromatids

Meiosis II

• Essentially mitosis without prior chromosomal replication

• Meiosis accomplishes two tasks:

  • It reduces the chromosome number by half (2n to n)

  • It introduces genetic variability

Meiotic Cell Division: Meiosis II

• Sister chromatids separate

• Cells separate into four haploid daughter cells

Boy or Girl?

• Sex determination of offspring is determined by the male gamete

• Sex chromosomes = X and Y in males

  • Gametes can be either X or Y

• Females (XX) à gametes can only be “X”

  • X (mom) + X (dad) = girl

  • X (mom) + Y (dad) = boy

SRY Gene – Sex Determination

• Sex-determining region Y (SRY) gene

  • Located on the Y chromosome

• In a genetic male with a Y chromosome:

  • SRY gene is expressed

  • Produces proteins to stimulate androgens

  • Androgens initiate male phenotypic development

• If the Y chromosome is absent, lacks, or has an abnormal SRY gene, female phenotypic sex results

Clinical View: Nondisjunction

• Failure of separation of homologous chromosomes or sister chromatids during meiosis

• Results in a gamete with either 22 or 24 chromosomes instead of 23

• Resulting gamete:

  • May have 47 chromosomes (trisomy)

    • Individual with 3 copies of one chromosome

  • May have 45 chromosomes (monosomy)

    • Individual with 1 copy of a chromosome

    • Most monosomies are lethal

  • Trisomy 21 (Down syndrome) is an example of a non-lethal trisomy

Spermatogenesis

• Seminiferous tubule cells are at various stages of cell division

• Spermatogenic cells give rise to sperm through:

  • Mitosis of spermatogonia to form spermatocytes

  • Meiosis to form spermatids

  • Spermiogenesis to form sperm

Testes and Spermatogenesis

• Spermatogenesis and spermiogenesis

  • Spermatogenesis

    • Sperm development within the seminiferous tubule

  • Spermatogonia

    • Germ cells - diploid

    • Diploid cells near the base of the seminiferous tubule

    • Undergo mitosis to produce a spermatogonium and a primary spermatocyte

Spermatogenesis and Spermiogenesis

• Interstitial space

• Interstitial cells

• Developing acrosome cap

• Spermatid nucleus

• Mitochondria

• Spermatogonium

  • Mitotic division

• Sustentacular cell

• Primary spermatocyte

• Meiosis I

• Secondary spermatocyte

• Tight junctions

• Meiosis II

• Spermatids

• Developing flagellum

• Sperm

• Excess cytoplasm

Seminiferous Tubule: Cross Section

• Interstitial cell

• Seminiferous tubule

• Sustentacular cells

• Tubule lumen

• Germ cells

• Sperm

• Spermatids

• Spermatogonia

Testes and Spermatogenesis

• Primary spermatocyte:

  • Diploid (46 chromosomes)

• Secondary spermatocytes:

  • Formed after Meiosis I

  • Haploid (23 chromosomes)

• Spermatid:

  • Formed after Meiosis II

  • Haploid (23 chromosomes)

• Spermiogenesis:

  • Spermatid differentiates into sperm

  • Excess cytoplasm is shed, and the nucleus elongates

  • Acrosome cap (containing digestive enzymes) develops

  • Midpiece containing mitochondria forms

  • Tail (flagellum) forms from organized microtubules within the cell

Clinical View: Paternal Age Risks for Disorders in the Offspring

• Males over 45 years of age:

  • Are approximately 4 times more likely to pass on genetic mutations than men in their 20s

  • Risk increases further with age

  • Increased risk of:

    • Bipolar disorder, schizophrenia, autism, depression, attention deficient hyperactivity disorder

• Maternal age is not correlated with an increase in these disorders

• Age of both sexes carries risk of development problems

Male Climacteric

• Male climacteric (andropause)

  • Men experience decreased testosterone levels typically in their 50s

    • Related to a decreased number of interstitial cells

  • Most men experience few symptoms

  • Some may have mood swings, decreased sex drive, hot flashes

• Prostate enlargement often occurs

  • Interferes with sexual and urinary functions

• Erectile dysfunction may occur

  • Inability to achieve or maintain an erection

  • Associated with aging and other risk factors such as diabetes, heart disease, and smoking

Female Reproductive Anatomy

• Ovaries:

  • Produce female gametes (ova)

  • Secrete female sex hormones (estrogen and progesterone)

• Ducts:

  • Uterine tubes, uterus, and vagina

• Internal genitalia:

  • Ovaries and the internal ducts

• External genitalia:

  • External sex organs (vulva)

Female Pelvic Region

• Key Structures

  • Urinary bladder

  • Pubic symphysis

  • Urethra

  • Clitoris

  • External urethral orifice

  • Vaginal orifice

  • Labium minus

  • Labium majus

  • Ureter

  • Uterine tube

  • Ovary

  • Fimbriae of uterine tube

  • Vesicouterine pouch

  • Rectouterine pouch

  • Cervix of uterus

  • Rectum

  • Vagina

  • Anus

Ovaries

• Ovarian follicles

  • Oocyte production

• Sex hormone release

• Paired almond shaped organs

Ovaries

• Broad ligament

  • Drape of peritoneum hanging over the uterus

• Ovarian ligament

  • Anchors the ovary to the posterior aspect of the broad ligament

  • Superior portion of the round ligament of the uterus

• Suspensory ligament

  • Attaches to the lateral edge of each ovary

  • Ovarian blood vessels and nerves are housed here

Internal Organs of the Female Reproductive System: Posterior View

• Key Structures:

  • Ovarian artery

  • Ovarian vein

  • Suspensory ligament

  • Infundibulum

  • Fimbriae

  • Ovary

  • Mesosalpinx (part of broad ligament)

  • Ovarian ligament

  • Uterine tube

  • Uterine artery

  • Uterine vein

  • Broad ligament

  • Ureter

  • Cervix

  • Vagina

  • Uterus

  • External os

Ovaries

• Tunica albuginea

  • Dense CT capsule

• Deep to tunica albuginea

  • Outer cortex

    • Contains highly cellular CT and ovarian follicles

  • Inner medulla

    • Composed of areolar CT

    • Contains branches of ovarian blood vessels, lymph vessels, and nerves

Internal Organs of the Female Reproductive System: Lateral Section of Ovary

• Uterine tube

• Tunica albuginea

• Mesosalpinx

• Medulla

• Cortex

• Mesovarium

• Broad ligament

• Hilum

• Germinal epithelium

Ovaries

• Ovarian follicles

  • Consist of an oocyte surrounded by follicle cells

  • Support the oocyte

  • Thousands are found in the cortex

  • 6 main types of ovarian follicle representing different stages of development:

Ovaries

• Primordial follicle

  • Most primitive type of ovarian follicle

  • Consists of a primary oocyte and a single flattened layer of follicle cells

  • Primary oocyte is arrested in the first meiotic prophase

  • Approximately 1.5 million are present at birth

Ovaries

• Primary follicle forms from a maturing primordial follicle

  • Primary oocyte with layers of cuboidal follicular cells (granulosa cells)

  • Granulosa cells secrete estrogen as it matures

    • Estrogen stimulates changes in the uterine lining

  • Thecal cells are present on the periphery of the follicle

    • Help control follicle development

    • Secrete androgens, which are converted to estrogen by the granulosa cells

Ovary: Primary Follicle

• Primary follicle

• Thecal cells

• Primary oocyte

• Granulosa cells

Ovaries

• Secondary follicle forms from primary follicle

  • Primary oocyte with many layers of granulosa cells and a fluid-filled space called the antrum

    • In the antrum, serous fluid increases as ovulation nears

  • Oocyte is forced to one side of follicle

    • Surrounded by a cluster of follicle cells called the cumulus oophorus

  • Zona pellucida and corona radiata are protective structures surrounding the primary oocyte

Secondary Follicle

• Secondary follicle

• Corona radiata

• Thecal cells

• Primary oocyte

• Zona pellucida

• Granulosa cells

• Antrum

• Cumulus oophorus

Ovaries

• Mature (large vesicular) follicle forms

  • Secondary oocyte

    • Surrounded by the zona pellucida and corona radiata

  • Numerous layers of granulosa cells

  • Fluid-filled antrum

  • Has completed meiosis I

  • Arrested in second meiotic metaphase

  • Typically, one mature follicle forms per month

Ovaries

• Corpus luteum forms from the remnants of the follicle after the mature follicle ruptures and the oocyte is expelled

  • Secretes the sex hormones progesterone and estrogen

    • These hormones stimulate buildup of the uterine lining

    • Prepare the uterus for possible implantation of a fertilized oocyte

Ovaries

• Corpus albicans forms from the regressed corpus luteum

  • White connective tissue scar

  • Most structures are completely resorbed

Oogenesis and the Ovarian Cycle

• Oogenesis

  • Maturation of a primary oocyte to a secondary oocyte

  • Occurs in stages over the life of a female

• Before birth

  • Ovary contains primordial germ cells called oogonia

    • Oogonia are diploid cells with 46 chromosomes

    • Mitosis of oogonia produces primary oocytes

    • Primary oocytes start meiosis but stop in prophase I

    • Approximately 1.5 million follicles are present within the ovarian cortex at birth

    • These remain arrested in prophase I until after puberty

Oogenesis and the Ovarian Cycle

• Childhood

  • Ovaries are inactive, with no follicles developing

  • Atresia causes the regression of some primordial follicles

  • By puberty, approximately 400,000 follicles remain in the ovaries

Oogenesis and the Ovarian Cycle

• From puberty to menopause

  • Hypothalamus releases gonadotropin-releasing hormone (GnRH)

    • This stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary

    • FSH and LH levels vary in a cyclical pattern

    • Produce a monthly sequence of events called the ovarian cycle

    • The ovarian cycle has 3 phases:

      • Follicular phase

      • Ovulation

      • Luteal phase

Oogenesis and the Ovarian Cycle

• Follicular phase

  • Days 1–13 of a 28-day ovarian cycle

  • Around 20 primordial follicles are stimulated to mature by LH and FSH

    • Unclear why some are stimulated and some are not

  • Follicular cells

    • Release the hormone inhibin

    • Helps inhibit further FSH production

    • Prevents excessive follicle development

  • A few primary follicles develop into vesicular follicles

    • One becomes dominant à oocyte + polar body and preventing the others from continuing development

Oogenesis and the Ovarian Cycle

• Ovulation

  • Release of the secondary oocyte from the mature follicle

  • Occurs on day 14 of a 28-day cycle

  • Usually, only one ovary ovulates each month

  • Induced by a peak in LH secretion

  • Antrum increases in size and swelling

    • Expands until the ovarian surface thins

    • Eventually ruptures, expelling the secondary oocyte

Oogenesis and the Ovarian Cycle

• Luteal phase

  • Remaining follicle cells become the corpus luteum

  • Occurs on days 15–28 of the cycle

  • Corpus luteum:

    • Temporary endocrine gland

    • Secretes progesterone and estrogen

    • Builds up the uterine lining to prepare for a fertilized oocyte

    • Has a life span of 10 to 13 days if the oocyte is not fertilized

  • First menstrual cycle is called menarche

    • Around age 11–12

Oogenesis

• Before birth (embryonic and fetal period)

  • Oogonia are diploid cells (containing 23 pairs of chromosomes, or 46 total) that are the origin of oocytes. Mitotic divisions of oogonia produce primary oocytes, which are diploid cells.

• Primary oocytes start the process of meiosis but are arrested in prophase I.

• Ovary is inactive

• Houses primordial follicles

• Some atresia of primordial follicles occurs.

• Monthly, from puberty to menopause

  • Up to approximately 20 primordial follicles mature into primary follicles every month. Some primary follicles mature into secondary follicles.

• Primary follicles that do not mature undergo atresia.

• Typically, only one secondary follicle matures into a mature follicle, where the primary oocyte completes the first meiotic division to produce a polar body and a secondary oocyte. The secondary oocyte is a haploid cell (containing 23 chromosomes only) that is arrested in the second meiotic metaphase.

• If the secondary