Reproductive System: Meiosis and Male

Human Chromosomes and Sexual Reproduction

• Humans are diploid → each somatic cell has 46 chromosomes (23 pairs)

  • 22 pairs of homologous chromosomes (autosomes)

    • 1 from mom, 1 from dad

  • homologs are similar but NOT identical → there’s about 0.15% genetic difference per 1000 base pairs

• 1 pair of sex chromosomes:

  • XX = female

  • XY = male

• Gametes (sex cells) are haploid

  • formed by meiosis in the gonads which are also known as primary reproductive organs

  • each contains 23 chromosomes, 22 autosomes + 1 sex chromosomes (X or Y)

• Fertilization: Union of sperm and egg → restores diploid # (46 chromosomes)

  • forms a zygote which then divides by mitosis to form all the cells in the body

Structure of The Reproductive System

• Systems are “blind-ended tubes”

  • Gametes are produced proximally (at the beginning, near the gonads)

  • they travel through ducts towards distal ends

• Proximal end = gonads (testes or ovaries)

• Distal end = external genitalia for gamete release or reception

Homologous but Structurally Different Organs

• though male and female reproductive organs differ in structure, they are functionally similar

• Gonads:

  • produce gametes via meiosis

    • Male → Testes make sperm

    • Female → Ovaries make eggs

  • secrete sex hormones (steroids made from cholesterol)

    • Testes → testosterone

    • Ovaries → Estrogen + Progesterone

    • These regulate sexual development and secondary sex characteristics

• Duct Systems (Excurrent Paths):

  • Male: Epididymis → Vas deferens → Ejaculatory duct → Urethra

  • Female: Uterus → Vagina (occytes are not actively transported like sperm → go to fallopian tubes)

• Accessory Glands:

  • provide secretions that aid fertilization:

    • Male: Seminal vesicles, prostate gland, and bulbourethral (Cowper’s) glands

    • Female: Uterus (secretory function) and Bartholin’s glands

• Erectile Tissue:

  • filled with blood during arousal to aid in copulation

    • Male: Penis

    • Female: clitoris

Female Specialization for Reproduction

• The female system supports zygote development and infant nutrition:

  • Uterus: where the zygote implants and develops into a fetus via support from the placenta

  • Mammary Glands: produce milk for postnatal infant nutrition

Mitosis vs. Meiosis

• Mitosis: produces two genetically identical daughter cell → used for growth and repair

• Meiosis: Reduces chromosome number by half to produce gametes (sperm or egg) with genetic variation

Chromosome Basics

• Humans are diploid (2n=46): 22 pairs of autosomes + 1 pair of sex chromosomes (XX or XY)

• Gametes (haploid, n=23): made by meiosis, contain 22 autosomes + 1 sex chromosome (X or Y)

• Fertilization combines gametes → zygote (diploid)

Reproductive System Overview

• Blind ended tubes: Gametes made at one end and released at the other

• Gonads (testes/ovaries): make gametes and sex hormones (testorone, estrogen/progesterone)

• Male ducts: epididymis → vas deferens → ejaculatory duct → urethra

• Glands assist fertilization (e.g. prostate in males, Bartholin’s gland in females)

• Erectile tissue differs but serves similar roles (penis vs clitoris)

• Females specialize in nurturing the embryo/fetus (uterus/placenta) and feeding infants (mammary glands)

Cell Cycle and Gametogenesis

• Cell cycle:

  • G1: growth

  • S: DNA replication

  • G2: prep for division

• Gametes develop from germ cells (spermatogonia/oogonia):

  • Males: meiosis starts after puberty

  • Females: starts meiosis in fetus, pause in meiosis I, resume after puberty

Meiosis I (Reduction Division)

• Prophase I:

  • chromosomes condense

  • Homologous chromosomes pair (tetrads)

  • crossing over occurs → genetic diversity

• Metaphase I: Tetrads line up randomly

• Anaphase I: Homologs pulled apart (sisters stay together)

• Telophase I + Cytokinesis: 2 haploid cells form (23 chromosomes, each with sister chromatids)

Interkinesis

• a brief pause, centrioles replicate, DNA does not

Meiosis II

• Prophase II → chromosomes condense again

• Metaphase II → chromosomes align

• Anaphase II → sisters separate

• Telophase II + Cytokinesis → 4 haploid gametes (genetically unique)

Molecular Events

• Crossing over:

  • controlled DNA breaks and repairs create recombination

  • produces haplotypes (inherited DNA segments)

• Recombination hotspots: Non-coding regions where crossing over commonly occurs

• Used in genetic testing, ancestry and disease gene mapping

Errors in Meiosis

• Nondisjunction

  • failure to separate chromosomes (Meiosis I) or sister chromatids (Meiosis II)

• Results in aneuploidy:

  • Monosomy (XO) → Turner Syndrome

  • Trisomy 21 → Down Syndrome

  • XXY → Klinefelter Syndrome

  • XYY → Jacobs Syndrome

Uniparental Disomy

• Both copies of a chromosome come from 1 parent

• Can cause imprinting disorders due to epigenetic regulation

• explains cloning difficulties due to disrupted epigenetic marks

Testes Location and Temperature Control

• Testes are located in the scrotum, which hangs below the abdominal cavity at the base of the penis

• Scrotum is thin-skinned and has many BV which help regulate temperature

• there’s a septum between the right and left testes

• keeping the testes outside the body is essential because sperm develop best at 34 degrees Celsius which is cooler than normal body temperature

Protective Layers

• two layers around the testis:

  • Tunica vaginalis → a slippery outer layer that helps the testis move smoothly inside the scrotum which comes from the peritoneum (a body membrane)

  • Tunica albuginea → a tough protective layer (closer to the testes) made of dense connective tissue

Spermatic Cord

• cord contains important structures like nerves, BV, vas deferens, and muscles

• helps move sperm but doesn’t store them

• wrapped in connective tissue and travels through a passage called the inguinal canal

Blood Supply

• arterial blood flows in from the abdominal aorta

• blood drains out into the inferior vena cava or left renal vein

• a pampiniform plexus which is a network of veins surrounds the testicular artery and helps cool incoming blood to protect sperm

Temperature Regulation

Two muscles keep the testis temperature just right

• Dartos Muscle → wrinkles the scrotal skin to reduce surface area and heat loss

• Cremaster muscle → lifts the testis closer to the body in cold conditions

Inside the Testis: Parenchyma and Tubules

• Parenchyma is the functional part of the testis. It includes:

  • Seminiferous Tubules → tiny tubes where sperm is made (contain Sertoli cells and spermatogenic cells)

  • Lobules → compartments in the testis that contain 1-4 seminiferous tubules each

  • Connective Tissue septa → walls that divide the lobules

  • Myoid cells → muscle-like cells that surround each tubule and help move sperm along

Interstital Tissue

• located between the tubules

• Contains:

  • Leydig cells → make testosterone

  • vasculature

Pathway of Sperm

Sperm travel through a series of ducts before reaching the epididymis

• seminiferous tubule (origin) → straight tubules → rete testes ( a network of tubes) → efferent ductules

  • which are Sertoli cells that line the straight tubules and rete testis

  • Efferent ductules leave the testis and lead to the epididymis

Spermatogenesis (making sperm)

• happens inside the seminiferous epithelium

• Stages (from outer to inner side of the tubule)

  • Spermatogonium B (stem cell made from mitosis with type A, B is used for meiosis after a few rounds of mitosis) → primary spermatocyte → secondary spermatocyte (become meitoic cells primary (meiosis I) and most abundant and secondary (product of meiosis I) → meiosis II) → early spermatid → late spermatid → spermatozoa (final sperm cells)

  • base → apical side

Role of Sertoli Cells

• Support cells inside the tubules that:

  • form tight junctions to protect developing sperm from immune attacks (blood testis barrier)

  • create different compartments for different stages of sperm development

  • produce androgen-binding protein (ABP) to keep testosterone levels high for sperm growth

  • secrete nutrients and fluid to feed developing sperm

  • clean up leftover cytoplasm from sperm cells during their maturation

Spermiogenesis (Maturation of Spermatids into Sperm)

• The final step where spermatids become mature sperm (spermatozoa)

  • Nucleus becomes more compact and head-shaped

  • Golgi apparatus forms the acrosome, which covers the head and contains enzymes needed for fertilization

  • The extra cytoplasm is discarded

  • A flagellum (tail) develops for movement

  • midpiece forms and fills with mitochondria to power the tail

  • sperm are now mature in shape but still non-motile

  • They are released into the lumen of the tubule and moved by peristalsis from myoid cells to the epididymis via:

    • Seminiferous tubule

    • Straight tubule

    • Rete testis

    • Efferent ductules

    • Epididymis

Functional Anatomy of the Excurrent Path

General Function

• duct system connects the testes to the penis

• helps sperm mature, receive nutrients, get stored and be propelled out during ejaculation

Structure of the Ducts

• the ducts are made of:

  • Epithelium: lining varies from simple to pseudostratified and contains stereocilia (long microvilli) for absorption and secretion

  • Submucosa: a thin connective layer under the epithelium

  • Muscularis: layers of smooth muscle that contract to push sperm forward

• Main Ducts included:

  • Epididymis

  • Vas deferens

  • Urethra

Epididymis

• located on the back of the testes

• has three parts:

  • Head: receives sperm from the efferent ductules

  • Body: where sperm mature

  • Tail: where sperm are stored

• Functions:

  • feed sperm

  • releases substances for sperm maturation (especially motility)

  • Removes old or extra sperm

  • uses the muscle to push sperm into the vas deferens for ejaculation

Vas Deferens

• connect epididymis → ejaculatory duct (near the bladder and prostate)

• Path:

  • starts above and behind the testis

  • travels in the spermatic cord through the inguinal canal into the pelvis

  • loops over the bladder and reaches the ejaculatory duct

• Structure:

  • similar epithelial lining to the epididymis

  • thicker smooth muscle layer for strong pulsation

  • ends in a dilated area called the ampulla, which helps sperm move forward

Ejaculatory Duct

• very short duct where vas deferens joins with the seminal vesicle

• passes through the prostate to empty sperm into the prostatic urethra

Urethra

• prostate → intermediate part → penile urethra → external opening

• Epithelium: lined with transitional epithelium (can stretch)

• intermediate part is the like female urethra and contains sphincters that contain urination

Functions of Glands of the Male Reproductive System

• Glands secrete fluids that mix with sperm to form semen

• secretions support:

  • sperm nutrition

  • mobility through the female reproductive tract

  • lubrication

  • protection from acidic environments

What’s in Semen

• semen = sperm + glandular secretions

• glandular secretions make up 90% of semen volume

Seminal Vesicles (most volume of exocrine secretions)

• Location: Behind and below the bladder

• Structure: Coiled, sac-like glands with secretory epithelium and smooth muscle

• Secretes:

  • Mucus - for viscosity

  • Fructose and citrate → energy source for sperm.

  • Coagulating proteins → help semen clot post ejaculation to stay in the vagina

  • Prostaglandins → help sperm move by stimulating female muscle contractions

• Ducts: join with vas deferens to form the ejaculatory ducts

Prostate Gland

• Location: Base of the bladder, surrounds the urethra

• Structure: Alveolar glands with smooth muscle

• Secretes:

  • Citrate → a nutrient

  • Enzymes → that help liquefy the semen post-ejaculation, releasing sperm

• Contribution: The Second largest volume of semen after the seminal vesicle

Bulbourethral (Cowper’s) Glands

• Location: Base of the penis

• Function:

  • secretes alkaline mucus before ejaculation

  • neutralizes acidity in the urethra (from urine) and vagina

  • lubricates the urethra and tip of the penis during intercourse

• important for protection and ease of passage

Structure and Function of the Penis

• The penis is responsible for depositing sperm during reproduction

• consists of three main regions:

  • Root (anchored in the perineum)

  • Body (shaft)

  • Glans Penis (tip)

• bulbospongiosus muscle, made of skeletal muscle, wraps around the base and assists in erection and ejaculation

• Hydrostatic organ → uses blood pressure to become firm for the purpose of depositing sperm (gametes)

• covered with glabrous skin which means its smooht hairless and loaded with sensory nerve endings in the dermis

• has erectile (cavernous) tissue wrapped in a tough outer layer of connective tissue called the tunica albuginea

Anatomy of Erectile Tissue

• covered in sensitive, hairless skin packed with nerve endings

• inside, it contains erectile (cavernous) tissue surrounded by connective tissue (tunica albuginea) and smooth muscle

• contains 3 vascular compartments:

  • 2 corpora cavernosa (paired) (dorsal side)

  • 1 corpus spongiosum (ventral, wraps around the urethra from shaft to tip)

• These spaces fill with blood during erection, enlargening and stiffening the penis

• vasodilation of helicine arteries with cavernous spaces hydrostatically enlarges the penis → erection

How Erection Happens

• helicine arteries supply blood to the cavernous spaces

• when these arteries vasodilate → blood rushes in and fills the spaces → penis enlarges + becomes rigid → erection

• hydrostatic → driven by blood pressure

Nervous System Role in Erection and Ejaculation

• Gamete disperal (getting sperm out) is controlled by a mix of CNS and ANS

• all the nerves that control this process travel through the pudendal nerve (a major pelvic nerve)

• What triggers?

  • Sensory inputs → touch, smell, sight, sound, and emotions → are sent to the brain (CNS) which processes them

  • The brain sends out signal to the ANS, which activates either:

    • The parasympathetic system (for erection)

    • The sympathetic system (for ejaculation and resolution)

Erection (Parasympathetic Control)

• begins with parasympathetic stimulation

• causes release of NO in the penis

• NO causes helicine arteries to dilate, filling the cavernous spaces with blood

• as blood fills in, veins are compressed, so blood can’t leave - this maintains the erection

• Parasympathetic input also triggers the Cowper’s glands to release lubricating mucus, preparing the urethra for sperm travel

Ejaculation (Sympathetic and Somatic Control)

• Occurs once enough stimulation is received → reaching a climax point

• controlled by sympathetic and somatic motor nerves

Sympathetic Responses:

• contract smooth muscles in:

  • internal urethral sphincter → to prevent urine flow

  • Vas deferens → pushes sperm

  • seminal vesicle and prostate → release fluids

• Secretions combine with sperm to form semen in the prostate

Somatic Reponses

• activate the bulbospongiosus muscle (a skeletal muscle around the base of the penis)

• contracts rhythmically to push semen through the urethra and out of the penis (orgasm occurs here)

Resolution Phase (post-ejaculation)

• after ejaculation, the body enters a resolution phase to end the erection

  • The sympathetic nervous system takes over again

  • Helicine arteries constrict → cutting off the blood supply

  • Smooth muscle in the erectile tissue contracts → forcing blood out and returning the penis to a flaccid state

  • refractory period → lasting minutes to hours, where a male cannot get another erection or orgasm → regulates sexual response and recovery