Sexual Reproduction and Human Anatomy

An Evolutionary Survey

• Sexuality, involving the union of parts to create new genetic combinations, is a common feature in the living world.
  • Even viruses exhibit recombinations of their nucleic acid.
  • Gametes (specialized cells) unite to form a zygote during sexual union.
• In algae, gametes can be similar (isogametes) or structurally alike but functionally different (functional heterogamy).
• Advanced plants and animals show highly specialized gametes: motile sperm and a passive, food-bearing egg.
• Hunger theory of sex: An older idea suggesting that spore-like cells united as gametes to share food.
• Sperm Variation:
  • Sperm shape and activity vary considerably.
  • Nematode worms, arachnids, centipedes, and some crabs have amoeboid sperm that move via undulations.
  • Higher animals (vertebrates) have elongated sperm with flagella and a midpiece rich in mitochondria for ATP production.
  • Mitochondria may be individual (mammals) or fused (other vertebrates and most invertebrates).
  • Flagella usually exhibit a 9 + 2 microtubular structure.
• Sperm Structure:
  • Golgi apparatus, endoplasmic reticulum, and ribosomes may be sequestered into a bud that breaks away from the sperm.
  • Sperm nuclei, containing the payload for fertilization, are relatively small.
  • Nucleus and cytoplasm show a microtubular structure; chromatin may be in a tubular array.
  • Histones are abundant, but in fish sperm, they're replaced by protamines (highly basic proteins).
  • Chromosomes may be oriented as longitudinal fibers along the head.
  • In primate sperm, fructose is the primary monosaccharide fuel instead of glucose.
• Sperm Lifespan:
  • Sperm lifespan varies greatly.
  • Insect sperm (drone bees) can live over a year in the queen bee's body.
  • Human sperm can remain active up to a week in the female reproductive tract and for 3 days in a dead male's body.
• Advantages and Disadvantages of Sexuality:
  • Advantage: Provides variation in progeny, allowing selection of better-adapted individuals.
  • Disadvantage: Requires the meeting of two different organisms (in dioecious forms).

Fertilization

• External Fertilization:
  • Gametes are shed into the water, requiring only a rendezvous of gametes.
  • Common in aquatic animals and amphibians.
  • Example: Salamanders - males deposit sperm packets on the forest floor, which females pick up.
  • Example: Frogs - males use amplexus (embrace) to squeeze eggs out of the female and then urinate sperm-rich fluid on them.
• Internal Fertilization:
  • Adaptation required for terrestrial habitats to maintain an aqueous environment for gametes and embryos.
  • Occurs in reptiles and birds, where sperm are deposited internally and the zygote is covered by albumin and a shell.
  • Coitus: Penis is inserted into the female reproductive tract for internal sperm deposition.
  • Birds: Cloacal kiss - male and female place their cloacae together.
  • Reptiles: Use an imperfect hemipenis to introduce sperm into the vagina.
  • Most insects accomplish fertilization through copulation.
  • Example: Bedbugs - Males deposit sperm packets into specialized pouches on the female's abdomen; sperm burrow through tissues to reach eggs.

Internal Development

• Mammals have evolved an internal development strategy where the embryo develops within the mother's body in a parasitic fashion, eliminating dependency on water for reproduction.
• Some insects, spiders, and fish exhibit modifications for internal development, usually brooding fertilized eggs in the reproductive tract.
• Changes in the female reproductive tract are necessary for carrying the young:
  • Uterus (womb) for embryo development.
  • Placenta for exchange of food, oxygen, and waste between mother and child.
  • Hormones to regulate changes in the uterus and other structures.

Male Reproductive System in Humans

• Functions:
  1. Produces sperm (male gamete).
  2. Delivers sperm to the female reproductive tract.
  3. Produces androgens (male sex hormones) like testosterone.
     • Prepares glands and tubular conduits of the male reproductive tract for functioning.
     • Produces secondary sexual characteristics (facial hair, increased muscular strength, libido).
• Testes:
  • Male gonads where sperm are produced and androgens synthesized.
  • Descend from the kidney region to the scrotum (external pouch).
• Spermatic Tubules:
  • Sperm are formed through meiotic divisions within coiled spermatic tubules.
• Interstitial Cells:
  • Located among the tubules, these cells secrete testosterone.
  • Androgen synthesis and secretion increase significantly after puberty.
• Epididymis:
  • Sperm are stored in this coiled tubular structure atop each testicle.
• Vas Deferens:
  • A long tube carrying sperm through the abdominal cavity to the urethra near the prostate gland.
• Urethra:
  • Exits through the penis, carrying urine or seminal fluid.
• Accessory Glands:
  • Seminal vesicles, prostate gland, and Cowper's glands contribute to the fluid carrying the sperm.
• Penis:
  • An efficient tool for delivering sperm to the female reproductive tract.
  • Erection occurs during sexual excitation when blood engorges the spongy erectile tissues due to parasympathetic nerve impulses, causing vasodilation.
  • Ejaculation: Seminal fluid is expelled from the penis in spurts due to rhythmic contractions along the lower genital tract (periodicity of $0.8$ second).

Female Reproductive System in Humans

• Ovaries:
  • Paired gonads within the abdominal cavity.
  • Contain over 400,000 potential eggs, but only about 400 will complete meiosis.
  • Held in place by ligaments and mesenteries.
• Fallopian Tubes (Oviducts):
  • Extend from the ovaries to the uterus.
  • Fertilization occurs here; eggs are swept along by cilia into the uterus.
• Uterus:
  • A thick, muscular organ where embryo development occurs.
  • Connects to the vagina via the cervix.
• Vagina:
  • A corrugated tube extending from the cervix to the exterior.
• Vulva:
  • External sex organs: labia majora, labia minora, and clitoris.
  • Labia majora are homologous to the scrotum in males; the clitoris is homologous to the penis.

Menstrual Cycle

• Menstrual Cycle vs. Estrous Cycle:
  • Menstrual cycle: Series of events that periodically modifies the female reproductive tract in humans and advanced primates.
  • Estrous cycle: Cycle of receptivity to sexual activity in other mammals, less complex than the menstrual cycle.
    • Estrus: A strong, short-lived compulsion to mate; animals are "in heat."
    • Prepares the female reproductive tract for copulation but lacks the elaborate uterine lining development of the menstrual cycle.
    • If fertilization doesn't occur, any preparatory thickening of the uterine wall is reabsorbed.
    • Environmental influence: Ovum release may depend on copulation.
• Hormonal Regulation:
  • Regulated by hormones from the hypothalamus, pituitary gland, and ovaries.
  • Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
  • FSH and LH stimulate the ovaries to produce estradiol and progesterone.
  • Prolactin, produced by the pituitary, acts on the breast and influences mineral and fluid balance.
  • Estradiol and progesterone cause menstrual changes in the ovaries, uterus, and throughout the body and may also interact with the nervous system.
  • Menstruation often ceases under fasting conditions or during rigorous athletic training.
• Cycle Phases:
  • Menses (flow of blood): lasts about 5 days.
    • GnRH levels are low to moderate.
    • FSH is low and will decrease further at ovulation.
    • LH is at a low plateau.
    • Estradiol and progesterone levels are low.
    • The endometrium (inner lining of the uterus) is shed along with blood and deteriorated vascular tissue.
    • Blood loss rarely exceeds $1.5$ cups.
    • Menstrual blood is declotted due to the enzyme fibrinolysin or plasmin.
  • Follicular (or proliferative) phase: Lasts about 9 days after the cessation of flow.
    • Follicles develop in the ovaries under the influence of FSH and LH.
    • Hypothalamic production of GnRH is appreciable.
    • A single growing follicle produces increasing amounts of estradiol, promoting growth and vascularization of the endometrium.
  • Luteal phase: Comprises the last 2 weeks of the cycle.
    • Ovulation: Marked by a sharp drop in estradiol and a surge in LH levels.
    • The ruptured follicle converts to the corpus luteum, which produces a small amount of estradiol but mainly secretes high levels of progesterone.
    • Progesterone causes the thickened endometrium to become highly glandular, increasing glycogen levels to prepare for implantation and pregnancy.
    • Very little GnRH is produced.
    • Progesterone suppresses follicular development in the ovary and is the main ingredient in birth control pills.
    • If pregnancy fails, the corpus luteum breaks down around day 21, the endometrial lining degenerates, and a new flow begins.
    • The boundary between luteal and flow phases is marked by a sharp drop in progesterone.
    • With the loss of the corpus luteum, GnRH buildup occurs, and estrogen buildup is renewed by newly developing follicles.
• Menstruation and the Moon:
  • The term "menstruation" derives from the root for "moon" because the menstrual period approximates the lunar month (28 days).
• Menopause:
  • Menstruation usually ceases during pregnancy and trails off when older women enter menopause, the period of slackening hormone secretions that control the menstrual cycle.
  • The cessation of the menstrual cycle at menopause is not necessarily correlated with a decrease in sexual desire.

Sexual Response in Humans

• Physiological Responses:
  • Both genders share similar responses: vasocongestion and myotonia.
  • Vasocongestion: Movement and pooling of blood, causing ventral sex flush, erection of the penis and clitoris, erection of the nipples, and vaginal sweating (transudation).
  • Myotonia: Spasmodic muscle contractions causing waves of contractions along the reproductive tracts and anus during orgasm, tremors, and superficial contractions during resolution.
• Sexual Response Phases:
  • Excitement phase: Erotic stimuli induce engorgement and increased tumescence.
  • Plateau phase: High degree of excitation is sustained at peak levels.
  • Orgasmic phase: Feelings of release accompanied by myotonia of orgasm.
  • Resolution phase: A return to the ground level occurs, followed by the possibility of renewed arousal.
• Gender Differences:
  • Women: Each phase tends to be drawn out.
  • Women: Greater variety of postplateau responses, including multiple orgasm.
  • Men: Single consecutive stages; orgasm is usually followed by resolution; older men may have an inordinately long resolution phase.
  • First sign of sexual excitation:
    • Male: Phallic erection.
    • Female: Vaginal sweating phenomenon.
  • In females, orgasm is associated with a bright red coloration of the vulval region.
  • The menstrual cycle influences fluctuations in the physical and emotional parameters of sexuality in females; fluctuations in males are less well-defined.

Contraception

• Humans have no specific season for sexual activity; coitus can occur at any time.
• Coitus serves to maintain a permanent bond between partners.
• Maintaining sexual interest throughout the year may have had evolutionary advantages for humans.
• Contraception serves social functions:
  • Reduces tensions and provides gratification and enhanced self-worth.
  • Controls family size.
• Contraceptive Technologies:
  • Effective Methods:
    • Vasectomy: Surgical cutting and tying of the vas deferens in males, preventing sperm from being ejaculated.
    • Tubal ligation: Tying of the fallopian tubes in females, preventing sperm from reaching the eggs.
      • These methods are usually irreversible and require extensive counseling.
    • Birth control pill: Relatively effective but requires regular use and some sophistication.
      • Some increased risk of stroke has been reported.
    • Condom: Effective in preventing venereal disease and AIDS, in addition to contraception.
    • Diaphragm with spermicidal jelly: Highly effective if fitted carefully by gynecologists.
      • No side effects are associated with this approach.
  • Less Effective Methods:
    • Vaginal foams, withdrawal, rhythm method (avoiding sex around ovulation), and the douche are less reliable.
    • The Natural Family Planning method is a newer, more effective application of the rhythm method based on examinations of cervical mucous.
  • Male Contraceptives:
    • Promising research involves the injection of testosterone derivatives to reduce sperm production.

Induced Abortion

• In some countries, induced abortion is used to limit population size.
• Abortion is common in the West, but there is increasing opposition.
• Controversies:
  • Debate over when life begins and ends.
  • Religious views differ on when life begins (sperm entering egg vs. moment of birth).
  • Quality of life considerations, such as whether artificial maintenance is justified for vegetative functions without feelings and thoughts.

Sexual Preference

• Libido: A generalized propensity for tension-reducing activities.
• Influences:
  • Genetic makeup, early training, and experience determine sexual choices.
• Alternatives:
  • Heterosexual intercourse is often cited as natural, but alternatives include masturbation, homosexual intercourse, and abstinence.
• Societal Condemnation:
  • Many societies condemn some or all of these practices.
• Animal Behavior:
  • Masturbation and homosexual activity can be found in other mammals, especially under stress.
• Alfred Kinsey's Study:
  • Discovered tremendous variation in the nature of sexual outlet chosen and the amount of sexual activity.
  • Sexual activity eludes the establishment of a norm.
  • At least six different kinds of outlets were repeatedly cited.

Sexual Dysfunction

• Several functional levels influence sexual fulfillment and reproductive success.
• Impotency:
  • Failure to achieve or maintain an erection.
  • Long-term failure is a cause for medical concern.
  • Often has a psychogenic cause (emotional conflict or psychic confusion).
  • Mobilization for danger suppresses vegetative functions like sex, under control of the sympathetic nervous system.
  • Underlying organic dysfunction should not be ignored.
• Sterility:
  • Inability to become a biological parent.
  • Male: Failure to produce sperm or producing too few.
  • May be associated with defective sperm, particularly if mobility is impaired.
  • Impotency and sterility are separable phenomena.
  • Female: Failure to produce eggs or eggs not discharging from the ovary.
  • A block to fertilization occurs in the tubes, usually associated with chronic inflammation.
  • Some women may produce lethal factors which destroy or inactivate sperm.
• Lack of Receptivity:
  • A failure to respond to sexual stimuli does not preclude copulation in women but may make it less pleasurable.
• Orgasm:
  • Failure to achieve orgasm is a sexual dysfunction, particularly in women.
• Premature Ejaculation:
  • Orgasm and expulsion of semen occur within an inappropriately short span of time.
• Treatment:
  • Sexual expression is a complex of emotional and physical responses.
  • Increasing availability of skilled professionals and the growing willingness of societies to deal with sexuality.
• Masters and Johnson:
  • Began studies of human sexual response in 1954.
  • Developed a conceptual framework for understanding human sexuality and clinical techniques for dealing with sexual inadequacy.
  • Discovered the vaginal sweating phenomenon.
  • Their contributions have been seminal to the fields of comparative physiology, ethology, and sexual dysfunction.
  • Spawned centers that have been highly successful in curing dysfunctions.
  • Brought a profound measure of respect for the scientific study of human sexuality.