Male Reproductive System

Overview and Main Structures

The male reproductive system is specialized for the production, maturation, and delivery of sperm. It consists of several organs and accessory glands that work together to ensure successful fertilization.

• Testes: Paired organs that produce sperm, testosterone, and inhibin.

• Scrotum: Sac of skin and smooth muscle that holds the testes and maintains them at a slightly lower temperature, which is optimal for sperm production.

• Seminiferous tubules: Located within the testes; the site of sperm production through cell division.

• Epididymis: Sperm mature and are stored here.

• Ductus deferens: Transports sperm from the epididymis to the ejaculatory duct.

• Ejaculatory duct: Formed by the joining of the ductus deferens and the duct from the seminal vesicle; transports sperm and glandular secretions.

• Urethra/Penis: The final passageway for sperm during ejaculation; the penis is also the erectile organ for sexual intercourse.

Accessory Glands and Their Functions

Accessory glands produce secretions that help sperm survive and facilitate their transport.

• Seminal vesicles: Secrete fructose (energy source for sperm) and produce most of the seminal fluid.

• Prostate gland: Secretes a watery alkaline fluid that raises vaginal pH, aiding sperm survival.

• Bulbourethral gland: Secretes lubricating mucus and cleanses the urethra.

Table: Male Reproductive Organs and Accessory Glands

Sperm Production and Maturation

Sperm are produced in the seminiferous tubules through a series of cell divisions, including mitosis and meiosis. This process ensures genetic diversity and the correct chromosome number in gametes.

• Spermatogonia (2n): Stem cells that divide by mitosis.

• Primary spermatocyte (2n): Undergoes meiosis I.

• Secondary spermatocyte (n): Undergoes meiosis II.

• Spermatids (n): Differentiate into mature sperm.

• Sertoli cells: Provide support and nourishment to developing sperm.

Key equation:

Hormonal Regulation of Male Reproduction

Testosterone, produced by interstitial cells in the testes, is the main steroid hormone controlling male reproductive functions.

• Testosterone: Controls growth and function of male reproductive tissues, stimulates aggression and sexual behavior, and determines the rate of sperm formation.

• Gonadotropin-releasing hormone (GnRH): From the hypothalamus; stimulates release of LH and FSH from the anterior pituitary.

• Luteinizing hormone (LH): Stimulates testosterone production.

• Follicle-stimulating hormone (FSH): Enhances sperm formation by stimulating Sertoli cells.

• Inhibin: Produced by Sertoli cells; inhibits FSH secretion, maintaining a negative feedback loop.

Feedback control equation:

Female Reproductive System

Overview and Main Structures

The female reproductive system produces eggs (oocytes), supports pregnancy, and undergoes cyclic changes to prepare for fertilization.

• Ovaries: Release oocytes and secrete estrogen and progesterone.

• Oviduct (Fallopian tube): Transports oocyte from ovary to uterus; site of fertilization.

• Uterus: Hollow organ where the fertilized egg grows and develops; consists of endometrium (implantation site) and myometrium (muscular layer).

• Cervix: Lower part of uterus; allows sperm entry and fetal exit during birth.

• Vagina: Organ of sexual intercourse and birth canal.

• External genitalia (vulva): Includes labia majora, labia minora, and clitoris (important in sexual response).

Table: Components of the Female Reproductive System

Mammary Glands and Lactation

Mammary glands are specialized for milk production and are regulated by hormones.

• Prolactin: Stimulates milk production.

• Oxytocin: Stimulates contractions that eject milk.

Menstrual Cycle

Ovarian and Uterine Cycles

The menstrual cycle consists of two linked cycles: the ovarian cycle (oocyte maturation) and the uterine cycle (endometrial changes). It is controlled by hormones from the pituitary gland and ovaries.

• Ovarian cycle: Series of changes in ovaries associated with oocyte maturation; controlled by FSH and LH.

• Uterine cycle: Changes in the endometrial lining; controlled by estrogen and progesterone.

Ovarian Cycle Details

• At birth, about 1 million primary oocytes are stored in each ovary; no more are produced.

• By puberty, most oocytes have been resorbed; about a dozen resume development each month, but typically only one is released at ovulation.

• FSH and LH stimulate follicle development and hormone secretion.

• Ovulation is triggered by a surge in LH.

• Corpus luteum forms and produces progesterone to prepare the endometrium for implantation.

Uterine Cycle Phases

• Menstrual phase (Days 1-5): Estrogen and progesterone decrease; endometrial lining degenerates and menstruation occurs.

• Proliferative phase (Days 6-14): Estrogen and progesterone increase; endometrial lining proliferates.

• Ovulation (Day 14): Release of secondary oocyte.

• Secretory phase (Days 15-28): Corpus luteum produces progesterone and estrogen; endometrium continues to proliferate and uterine glands mature.

Hormonal Feedback in the Menstrual Cycle

• Positive feedback: Increasing estrogen during the proliferative phase causes a surge in LH, leading to ovulation.

• Negative feedback: Steady levels of estrogen and progesterone during the secretory phase inhibit LH and FSH release.

Human Sexual Response and Fertilization

Phases of Sexual Response

• Excitement: Increased sexual awareness and arousal.

• Plateau: Intense and continuing arousal.

• Orgasm: Peak of sexual sensations; ejaculation in males, rhythmic muscular contractions in females.

• Resolution: Abatement of arousal.

Fertilization

• Ejaculate may contain several hundred million sperm.

• Sperm may reach the egg within hours to a day or more; viable for up to 5 days in the female tract.

• One sperm penetrates the egg within the oviduct.

Birth Control Methods

Surgical Sterilization

• Vasectomy (males): Cutting and tying off ductus deferens.

• Tubal ligation (females): Cutting and tying off oviducts.

• Hysteroscopy: Cauterizing oviducts to seal them.

Hormonal Birth Control

• Pills, injections, patches, rings, implants: Synthetic hormones inhibit FSH and LH, blocking ovulation.

• Advantages: May reduce cramps, menstrual flow, and risk of certain cancers; may reduce acne.

• Disadvantages: Side effects include acne, headaches, fluid retention, high blood pressure, blood clots; do not protect against STDs.

Other Birth Control Methods

• Intrauterine devices (IUDs): Prevent fertilization or implantation; some release hormones.

• Diaphragms and cervical caps: Prevent sperm from entering cervix; effectiveness improved with spermicides.

• Chemical spermicides: Kill sperm cells.

• Condoms: Trap ejaculated sperm; also protect against STDs.

• Natural alternatives: Fertility awareness, withdrawal.

• Emergency contraception: Pills taken after intercourse to prevent ovulation or implantation.

Table: Approximate Failure Rates of Various Contraceptive Methods

Additional info: Actual numbers may vary; refer to CDC or WHO for current statistics.

Infertility and Assisted Reproductive Technologies

Infertility

• Male infertility: Low sperm count or poor sperm quality; causes include low testosterone, immune disorders, radiation, anabolic steroids, mumps, gonorrhea.

• Female infertility: Abnormal FSH/LH production, irregular cycles, blocked oviducts (often due to pelvic inflammatory disease), thick cervical mucus, endometriosis, acidic vaginal secretions, age-related decline, miscarriage.

Enhancing Fertility

• Artificial insemination: Sperm placed into vagina or uterus near ovulation.

• Assisted reproductive technologies (ARTs): Eggs and sperm handled outside the body; includes in vitro fertilization (IVF).

• IVF: Fertilization in a petri dish; embryo inserted into uterus.

• Fertility-enhancing drugs: Boost egg production; may result in multiple births.

• Surrogate motherhood: Another woman carries a baby for prospective parents.

Sexually Transmitted Diseases (STDs)

Overview

STDs are transmitted by sexual contact and can be caused by viruses, bacteria, protozoa, fungi, and arthropods. Some are asymptomatic, while others can cause serious health problems.

Table: Common STDs and Their Features

Prevention of STDs

• Choose partners wisely

• Communicate openly

• Use suitable barriers (e.g., condoms)

• Get tested and treated regularly

• Get vaccinated (Hepatitis B, HPV)

Additional info: For more details on reproductive anatomy, physiology, and disease, consult your course textbook or instructor.