The Male Reproductive System

Chapter 27 The Male Reproductive System Anatomy & Physiology

27.1 Sexual Reproduction and Development

  • Expected Learning Outcomes:   - Identify the most fundamental biological distinction between male and female.   - Define primary sex organs, secondary sex organs, and secondary sex characteristics.   - Explain the role of the sex chromosomes in determining sex.   - Explain how the Y chromosome determines the response of the fetal gonad to prenatal hormones.   - Identify which of the male and female external genitalia are homologous to each other.   - Describe the descent of the gonads and explain why it is important.

27.1 Fundamental Sex Distinctions

  1. Gametes and Fertilization    - Male gamete: sperm (spermatozoon)      - Motility: Males are the sex producers of sperm.      - Y chromosome presence indicates male sex.    - Female gamete: egg (ovum)      - Nutritional role for embryo development.      - Lacking a Y chromosome indicates female sex.    - In mammals, females provide an internal environment and nutrition for embryo growth.

  2. Reproductive System Functions    - Male reproductive system's primary functions:      - Produce sperm and introduce them to the female.      - Contains copulatory organ: penis.    - Female reproductive system functions:      - Produce eggs, receive sperm, support fertilization, house fetus, and nurture offspring.      - Contains copulatory organ: vagina.

27.1b Overview of the Reproductive System

  1. Organ Classification    - Primary Sex Organs (Gonads)      - Function: Produce gametes      - Male: testes      - Female: ovaries    - Secondary Sex Organs      - Organs besides gonads necessary for reproduction.      - Male: Ducts, glands; penis delivers sperm.      - Female: Uterine tubes, uterus, vagina receive sperm and support fetal development.

  2. Location-Based Classification    - External Genitalia      - Located mainly in the perineum, visible.      - Accessory glands may be subcutaneous.    - Internal Genitalia      - Located in the pelvic cavity (except testes and some ducts in scrotum).

  3. Secondary Sex Characteristics    - Develop at puberty and contribute to mate attraction:      - Both sexes: body hair (pubic, axillary), scent glands, voice pitch.      - Males: facial hair, coarser torso and limb hair, muscular physique.      - Females: body fat distribution, breast development, finer body hair.

27.1c Chromosomal Sex Determination

  1. Chromosomal Composition    - Human cells contain 23 pairs of chromosomes:      - 22 pairs autosomes.      - 1 pair sex chromosomes (XY males; XX females).    - Male sperm carries both X and Y chromosomes; female eggs carry only X.    - Sex Determination:      - X-carrying sperm fertilizes X-carrying egg: offspring is female (XX).      - Y-carrying sperm fertilizes an X-carrying egg: offspring is male (XY).

27.1d Prenatal Hormones and Sexual Differentiation

  1. Fetal Development    - Initially, the fetus is sexually undifferentiated.    - Gonads develop at 5-6 weeks from gonadal ridges:      - In Males:        - Mesonephric ducts develop into the reproductive tract.        - Paramesonephric ducts degenerate.      - In Females:        - Paramesonephric ducts develop into the reproductive tract.        - Mesonephric ducts degenerate.

   - SRY Gene Functionality      - SRY located on the Y chromosome determines duct differentiation:        - Encodes for Testes-Determining Factor (TDF)        - Initiates testes development.        - Testes secrete testosterone and müllerian-inhibiting factor (MIF):          - Testosterone promotes mesonephric duct development.          - MIF causes paramesonephric ducts' degeneration.      - Female development occurs in the absence of androgens, not due to estrogen presence.

27.1e Sex, Intersex, and Gender

  1. Intersexuality    - Approximately 1% of people are intersexual: a mismatch between genes, gonads, and genitalia.    - Examples of intersex conditions:      - SRY gene transfer can create XY individuals with female characteristics.      - XX fetuses can be masculinized by high prenatal androgen exposure.      - Androgen Insensitivity Syndrome (AIS): XY with testes but exhibiting female phenotype due to testosterone receptor deficiency.

  2. Gender vs. Biological Sex    - Gender: a psychological and social identity that may not align with biological sex.    - Biological sex variances result in a spectrum of anatomy, physiology, and psychology beyond a simple binary classification.

27.1f Development of the External Genitalia

  • Male and female external genitalia develop similarly. Major events include:   - Genital tubercle forms glans of the penis or glans of the clitoris.   - Urogenital folds form the male urethra or labia minora.   - Labioscrotal folds become the scrotum or labia majora.

  • By week 12, distinct male or female genitalia forms, indicating homologous organs between sexes:   - Penis homologous to clitoris.   - Scrotum homologous to labia majora.

27.1g Descent of the Gonads

  1. Gonadal Migration    - Gonads develop high in abdominal cavity and migrate:      - Ovaries descend slightly to the pelvic cavity.      - Testes migrate into the scrotum.    - Gubernaculum: a connective tissue cord that aids descent, forming the inguinal canal, a common site for hernias.    - Testes descend around the 7th month of gestation, guided by the gubernaculum.    - Cryptorchidism: condition in 3% of males at birth where the testes do not descend.    - Gubernaculum transforms into supportive ligaments for the ovary and uterus after descent.

27.2 Male Reproductive Anatomy

  • Expected Learning Outcomes:   - Describe the anatomy of the scrotum, testes, and penis.   - Trace the pathway of sperm from formation to ejaculation.   - Identify male accessory reproductive glands' names, locations, and functions.

27.2a The Scrotum

  1. Structure and Function    - Contains external genitalia: scrotum and penis located in the perineum.    - Scrotum: pouch of tissue housing testes, comprised of skin, muscle, and connective tissue.    - Typically, the left testicle hangs lower than the right for temperature regulation (ideal: 35°C).    - Temperature Regulation Mechanisms:      - Cremaster muscle adjusts distance from the body based on temperature.      - Dartos muscle wrinkles skin to reduce surface area and heat loss.      - Pampiniform plexus: venous network that cools arterial blood to the testes.

27.2b The Testes

  1. Testicular Structure    - Shape: Each testis is oval and slightly flattened, averaging 4 cm in length and 2.5 cm in diameter.    - Encased by the tunica vaginalis and covered by the tunica albuginea, which divides into lobules.    - Lobules: contain 1-3 seminiferous tubules per lobule, where sperm is produced.    - Interstitial endocrine cells produce testosterone between tubules.

27.2c The Spermatic Ducts

  1. Pathway of Sperm    - After leaving testes, sperm pass through several ducts:      - Efferent Ductules: Collect sperm from the rete testes and transport to epididymis.      - Ductus Deferens: Connects epididymis to the seminal vesicle, where it enlarges to form an ampulla.      - Ejaculatory Duct: Formed by uniting ductus deferens with seminal vesicle duct, passing through prostate to urethra.

27.2d The Accessory Glands

  1. Glandular Contributions    - Seminal Vesicles: Glands that produce 60% of semen volume, emptying into the ejaculatory duct.    - Prostate Gland: Surrounds urethra, contributes around 30% of semen volume, contains thin milky secretion.    - Bulbourethral Glands: Produce a pre-ejaculate fluid that protects sperm by neutralizing residual urinary acidity in urethra.

27.2e The Penis

  1. Penile Structure    - Composed of a root (internal) and shaft/glands (external).    - Length varies (4 in flaccid; 5-7 in erect).    - Erectile Tissues:      - Corpus Spongiosum: Contains the spongy urethra.      - Corpora Cavernosa: Paired structures that engorge with blood during erection.    - Skin on the penis loosely adheres to allow expansion and covers the glans as the foreskin (prepuce).

27.3 Puberty, Hormonal Control, and Climacteric

  • Expected Learning Outcomes:   - Describe hormonal control of puberty.   - Describe changes in the male body due to puberty.   - Define and describe male climacteric and effects of aging on male reproductive function.

27.3a Puberty and Adolescence

  1. Definitions    - Puberty: Onset of pituitary gonadotropin secretion until the first ejaculation of viable sperm.    - Adolescence: Extends from puberty to full adult height.    - Hormones involved in changes:      - Androgens like testosterone and DHT promote growth of sex organs, sperm production, body growth, libido, and development of hair in various body locations.

27.3b Hormonal Control of Male Reproductive Function

  1. Hypothalamo-Pituitary-Gonadal Axis    - Hypothalamus produces gonadotropin-releasing hormone (GnRH) activating the anterior pituitary:      - FSH: Triggers nurse cells to produce ABP, stimulating spermatogenesis.      - LH (or ICSH): Stimulates interstitial endocrine cells for androgen production, primarily testosterone.

27.3c Aging and Sexual Function

  1. Effects of Aging    - Testosterone levels decline with age leading to various physiological changes:      - Reduced libido and arousal, loss of muscle mass, bone density decline, slower beard growth, and genital shrinkage.      - Seminal fluid volume, sperm count, and motility decrease, leading to potential erectile dysfunction.

27.4 Sperm and Semen

  • Expected Learning Outcomes:   - Describe stages of meiosis, spermatogenesis, sperm structure, and composition of semen.

27.4a Meiosis

  1. Meiosis vs. Mitosis    - Mitosis: A diploid cell produces two identical daughter cells (2n).      - Essential for growth and tissue repair.    - Meiosis: A diploid cell yields four haploid gametes (n) with genetic variation, occurring in two successive divisions (Meiosis I and II).    - Meiosis I involves:      1. Prophase I: Homologous chromosomes form tetrads, crossing over.      2. Metaphase I: Tetrads align at the equator.      3. Anaphase I: Homologous chromosomes separate.      4. Telophase I: Two haploid cells form.    - Meiosis II mirrors mitosis, producing four haploid cells.

27.4b Sperm Production

  1. Spermatogenesis Sequence    - Initiated by rising testosterone levels at puberty.    - Primordial germ cells become spermatogonia:      - Each spermatogonium divides, leading to a production of sperm through various stages, ultimately resulting in four spermatids per primary spermatocyte via meiosis.      - Each spermatid undergoes spermiogenesis to mature into a spermatozoon with flagella.

27.4c The Spermatozoon

  1. Structure of Sperm Cell    - Head: Contains the nucleus with haploid chromosomes, acrosome over the nucleus for penetrating the egg.    - Tail: Comprises three areas:      - Midpiece with mitochondria.      - Principal piece as the main length of the flagellum.      - Endpiece as the narrower tip.

27.4d Semen

  1. Composition and Function    - Semen expelled during ejaculation consists primarily of seminal plasma from accessory glands.    - Normal sperm count: 50 to 120 million/mL; lower counts indicate infertility.    - Semen's stickiness helps keep sperm closer to uterine entrance, while enzymes contribute to its structural integrity and facilitate motility.

27.5 Male Sexual Response

  • Expected Learning Outcomes:   - Describe blood and nerve supply to the penis.   - Explain erection and ejaculation processes.

27.5a Anatomical Foundations

  1. Blood Supply    - Internal pudendal artery branches:      - Dorsal artery supplies the skin, fascia, corpus spongiosum.      - Deep artery fills cavernous spaces to achieve erection.    - Deep dorsal vein drains penile blood.

27.5b Excitement and Plateau Phases

  1. Phases of Sexual Response    - Excitement: Vasocongestion (blood swelling, increased heart rate), bulbourethral glands secrete fluid.    - Plateau: Increase in physiological parameters continue just before orgasm.

27.5c Orgasm and Ejaculation

  1. Ejaculation Process    - Orgasm: Short severe reaction leading to semen release.    - Ejaculation Phases:      - Emission: Propulsion of sperm through ductal system, aided by peristalsis.      - Expulsion: Muscle contractions activate for semen release while ensuring urine retention.

27.5d Resolution Phase

  1. Post-Ejaculatory Return to Baseline    - Body function returns to normal; penis detumescences.    - Refractory period follows, where multiple orgasms are physically unlikely.

27.5e Treating Erectile Dysfunction

  • Treatments such as Viagra®, Levitra®, and Cialis® are phosphodiesterase inhibitors to enhance blood flow and sustain erection by prolonging cGMP action.

27.6 Sexually Transmitted Diseases

  1. STDs Overview    - Infectious diseases through sexual contact; incubation without symptoms can affect future pregnancies or neonates.    - Bacterial STDs:      - Gonorrhea: painful discharge, can lead to sterility.      - Chlamydia: causes urethral discharge and pain.      - Syphilis: stages ranging from apparent sores to systemic organ damage.    - Viral STDs:      - Genital herpes: causes painful sores, common in the U.S.      - Genital warts (HPV): promote tumor growth, prevention via vaccine available.      - Hepatitis B and C leading to thyroid disease affecting liver function.