Learning Outline

Reproductive Systems

A&P 2

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Warning: This part of the course includes graphic sexual content. If you really can't handle that, then don't come to class. And seriously reconsider whether you want to work in a health profession—if that's the path you are on.

Because some graphic images will appear on Previews and online tests and exams, it is also important that you are sensitive to your surroundings. Avoid viewing these materials in inappropriate circumstances.

The importance and nature of sexual reproduction

Survival of genes

Despite what you've heard, the prime importance is not survival of the individual or species

In The Selfish Gene, biologist Richard Dawkins explains the theory that reproduction, indeed all the mechanisms of life in general, can be explained in terms of the continued survival of genes (brief video of Dawkins)

Genes are information —NOT strands of nucleotides (any more than the information contained in these notes is a string of letters; the information in the notes can be copied [multiplied] and even changed to another form such as speech or digital code or Swahili)

Thus genes build organisms to live in just as humans build houses to live in

Likewise, organisms have homeostatic control mechanisms to maintain constancy just as homes have automatic control systems such as heating/cooling systems to maintain constancy

Organisms provide a vehicle for the genes and the apparatus to make copies (children) that outlive the organism

Thus, genes can be almost "immortal"

Individuals come and go; species come and go; genes remain [theoretically] forever

Sexual mode of reproduction

Sexual / two-parent (rather than asexual/one-parent) reproduction allows more variation among offspring

Individual genes want to survive and through sexual reproduction can form coalitions with different genes to improve their chances that at least some gene copies will survive (those genes that end up in combinations that turn out to be successful)

It's more fun that way

Basics

Gonads

Gonads are the primary sex organs

Produce gametes and sex hormones

Male gonad: testis (pl. testes)

Female gonad: ovary

Only one gonad is needed, but most of us have two so that we have a "spare" in case of injury or disease

Gametes: reproductive cells

Have half the usual number of nuclear DNA molecules (chromosomes)

Gametes have the haploid number (23) whereas all other cells have the diploid number (46)

Male gamete: sperm (spermatozoan [plural, spermatozoa])

Female: ovum (pl. ova) (oo- is a word part that means "egg" and is pronounced "oh-oh" NOT "oo" as in "zoo")

Similarity of reproductive tract

Both systems have paired gonads and tubes to carry gametes from the gonads and out of the body

Both systems have gonads and tubes in a Y-shaped structure

Many reproductive organs (male/female) are derived from the same tissue and thus are analogous structures

For example, the folds on the outer rim of the female genitals (labia) exist in the male, except that in the male the testes have dropped down into the folds and the folds have fused at the midline to form the scrotum; thus, the labia and scrotum are analogous structures

cover

Life's Greatest Miracle, a film from PBS's Nova series, is required as a preview/review of the structure and function of human reproduction and development.

You can click on NOVA to view it online (free) or you can view a videotape or DVD (click here).

Male Reproductive System

Functional anatomy

Testes (sing. testis)

The male gonads (paired)

Spermatogenesis - making of sperm (gamete) cells

Secrete testosterone

Originate near kidneys

Descend through inguinal canal

Gubernaculum (literally, "governor") is a ligament that guides the way

Temperature is lower outside the body cavity (low temp req'd for spermatogenesis)

Cryptorchidism (crypt- "hidden" -orchid- "testis" -ism "condition") occurs when one or both testes fail to descend completely into scrotum

Structure

Within scrotum

Scrotum is fold of skin into which testes descend, stretching it into a pouch

Dartos muscle in wall of scrotum & cremaster muscle around each testis regulate position of testes relative to body

Temperature regulation

Fibrous capsule (tunica albuginea)

Surrounded by serous membranes forming the fluid-filled tunica vaginalis

Lobules

About 250 lobules per testis

Contain seminiferous tubules (semin- "seed" -ifer- "carry" -ous "pertaining to")

Semiferous tubules are made of germinal epithelium (70 cm each)

Site of spermatogenesis

Interstitial cells (of Leydig)

Found between the seminiferous tubules

Endocrine cells that secrete androgens (primarily testosterone)

Rete testis (literally "network of the testis")

Network of tubules into which the seminiferous tubules carry the sperm

Forms a sort of "exit lobby"

Efferent ductules lead from rete testis to the epididymis

Reproductive tract

Epididymis

Within scrotum; coiled tubule alongside testis (6 m)

Storage and conduction of semen (sperm and spermatic fluid)

Where sperm learn to swim

Secretes <5% of seminal fluid

Contains glycogen (to nourish sperm)

Ductus (vas) deferens (vas "duct" or "vessel" deferens "detour")

Muscular tube

Last part widens to form ampulla

Conducts semen from epididymis through inguinal canal into pelvic cavity, where it turns back down to behind the bladder to join the urethra (during emission)

Left and right ductus deferens each join with a duct from a seminal vesicle (exocrine gland) to form an ejaculatory duct

The left and right ejaculatory duct extend through the prostate gland and to the urethra

Urethra

Shared by reproductive & urinary systems

Extends through penis, delivering semen (potentially) to the female's vagina

Two main parts: prostatic urethra, penile urethra

Accessory glands

Produce seminal fluid

Seminal vesicles

Paired exocrine glands

Duct joins ductus deferens behind bladder, just before the ductus deferens joins its partner to form the ejaculatory duct

Contributes about 60% of seminal fluid at ejaculation

Contains prostaglandins, fructose, etc.

Prostate

Unpaired (single) exocrine gland

Surrounds first part of urethra (under bladder) like a donut

Prostatitis or benign prostatic hypertrophy (BPH) can constrict urethra, interfering with urination

BPH occurs in about 3 of every 4 men over 50 yrs old

Has many ducts leading into prostatic urethra

Contributes about 30% of seminal fluid at ejaculation

High pH to neutralize male urethra and female reproductive tract

Highly viscous and slippery to aid swimming of sperm

Bulbourethral (Cowper's) glands

Tiny, paired exocrine glands

Located near base of penis

Duct leads into penile urethra

Secrete <5% of seminal fluid

Secretion released just before the rest of the semen arrives

Therefore, called "pre-ejaculate"

Lubricates penis/vagina

Neutralizes pH

Pre-ejaculate MAY contain sperm (thus fertilization may occur even if "ejaculate" never enters female body)

Penis

Urethra extends through penis

Erectile columns

Vascular reservoirs: arteries dilate, veins constrict during sexual arousal

Corpora cavernosa (two) and corpus spongiosum

Insertion into female tract: delivery of semen/sperm

Coitus (sexual intercourse; copulation)

Glans penis

Distal enlargement of corpus spongiosum

Highly sensitive skin on surface - sexual stimulation

Foreskin (prepuce) is collar of skin over the glans

Often removed by circumcision

MGM=male genital mutilation

Spermatogenesis

Germinal epithelium

Spermatogenic cells - produce sperm by means of meiotic cell division (NOT mitotic cell division)

Sertoli (nurse) cells - support/nourish sperm cells

Development of sperm cells

Meiosis

One parent cells produces four daughter cells

Diploid parent produces haploid daughter cells

Structure

Head

Acrosome is "cap"

Enzymes (to get through "zona" coating around egg) and receptors (including olfactory receptors to "sniff out" an egg)

Body or midpiece

Mitochondria provide fuel (ATP) for movement (swimming)

Acts as "engine house" because microtubules of the tail undergo reactions here that are similar to actin-myosin reactions (except here, it is a back-and-forth action that causes microtubules to "wiggle" the tail)

Tail (flagellum)

Several times longer than the diameter of the head

Provides swimming ability

Seminal fluid (semen)

Combined secretions of epididymes, vesicles, prostate, bulbourethral glands

pH 7.5

Ball (no pun intended) park figures:

120 million sperm per ml

2-6 ml per ejaculation

Numbers vary with time since last ejaculation, hydration, general health and nutrition, who the partner is (sex with spouse has lower sperm count than sex with infrequent partner), stress and emotions, etc

Release of semen

Emission — release of sperm from epididymis and through vas deferens

Ejaculation — release of sperm from the body (through urethra)

Usually occurs at orgasm

Capacitation

Stages of development of sperm after ejaculation and before fertilization

Complete development of the sperm does not occur until (and unless) the sperm nucleus joins with the egg nucleus to form the first cell of a new offspring (first cell is called the zygote)

Endocrine regulation

Gonadotropin releasing hormone (GnRH) from hypothalamus (to the anterior pituitary)

Stimulates the anterior pituitary to release the gonadotropins

Gonadotropins are hormones that stimulate the gonads

Gonadotropins are FSH and LH

FSH (follicle stimulating hormone)

from anterior pituitary (to testis)

Named for its function in the female (to stimulate ovarian follicles/eggs) but also exists in males

Promotes spermatogenesis

LH (luteinizing hormone)

from anterior pituitary (to testis)

Named for its function in the female (to stimulate dev/secretion of the corpus luteum of the ovary) but also exists in males

Promotes male hormone (androgen = "maleness maker") production in testis

Testosterone (literally "steroid from the testis") is the primary androgen in humans

Development of primary/secondary sex characteristics

Primary sex characteristics are those needed to produce gametes and get them out of the body

Secondary sex characteristics are those helpful but not needed, such as male pattern of fat distribution, male hair growth, male's larger larynx, and so on

Anabolic steroid - testosterone (and its synthetic equivalents) stimulates manufacture of new protein filaments in muscles (anabolism) and thus increases strength

Controlled by long and short loop negative feedback

Long loop means feedback on testosterone levels goes all the way back to the hypothalamus

Short loop means feedback on FSH levels goes back to next higher level (the hypothalamus)

Relatively small peaks and valleys and short, daily cycles compared to female

Females have slow response to changes in hormone levels, meaning that the hormones can get to high peaks over many days before being reversed, then dip to very low valleys over many days before being reversed again

This means that males are fertile all day every day (females, in contrast, are fertile only a few days out of every 28-day hormonal cycle)

Female reproductive system

Functional anatomy

Ovary

Located in pelvic cavity (paired organs)

Descend from position near kidney during fetal development (as do the testes, but don't drop as far)

Suspended by ligaments

Structure

Medulla (inner region)

Cortex (outer region)

Ovarian follicles are pockets in which eggs (pl. ova, sing. ovum) develop

All follicles are "started" before birth, but during each adult reproductive cycle a group of follicles resume their development

Some studies now challenge the idea that new egg cells are not produced during adulthood

Only one [usually] follicle/ovum per cycle reaches point of development where follicle bursts open, releasing the egg (ovulation)

Some studies now challenge the idea that ovulation only occurs once per cycle. It may occur more often.

Fallopian (uterine) tubes

Paired muscular tubes lined with folded, ciliated mucosa

Fimbriae form a fringe of fingerlike processes at distal end of tube

Help pull ovum into tube

Conduct ovum toward uterus via action of cilia (& peristalsis & fimbriae)

Site of gamete fusion (fertilization)

Takes 3 days for egg to travel down fallopian tube, but egg can only be fertilized during first 24 hours after ovulation so fertilization must take place in first one-third of fallopian tube

Uterus

Unpaired (single) organ

Wall

Endometrium

Mucous lining

Sensitive to sex hormones estrogen(s), progesterone

Highly vascular

Uterine glands (exocrine)

Hormones cause endometrium to thicken and change through reproductive (menstrual) cycle, then when hormones drop off so do the new, outer layers of the endometrium

Menses, menstruation from Latin menses meaning "month" after the 28-day lunar month which seems to be used as a time keeping signal by the pineal gland, which in turn influences reproductive hormone levels

Endometriosis occurs when membranes outside the uterus (for example, in the vagina or peritoneum) also respond to hormonal changes, thickening then dropping off

Myometrium

Middle, muscular coat

Smooth muscle contracts when stretched, which is a problem during pregnancy

hormones inhibit uterine contraction during pregnancy, then at end of gestation [time of pregnancy] hormones change and labor contractions begin

Contracts during menstruation

Contracts during orgasm

Perimetrium

Part of the peritoneum (lining of the abdominopelvic cavity)

Covers outside of uterus

Cervix

Cervix is Latin for "neck" —cervix of uterus is neck of uterus (as in neck of a beer bottle)

Cervical canal has mucus-producing glands

Mucus forms a "plug" during nonfertile times, then becomes slippery during fertile times

Internal os & external os of cervix (os is "opening")

Main functions

Conducts sperm

Peristalsis during orgasm helps move semen into and through uterus

Protects and supports embryo/fetus during gestation

Endometrium joins with placenta of offspring to form a physiological connection

Vagina (literally "sheath")

Extends from uterine cervix to the outside of the body (vulva)

Receives penis/semen during intercourse

Muscular walls reflexively adjust to accommodate various penis sizes

Rhythmic contractions at orgasm

Vaginal sweat - lubricating fluid released by vaginal wall during sexual arousal

Birth canal - rhythmic contractions at birth

Menstrual flow

Vulva

All external genitals are grouped under the single term "vulva"

Labia (literally "lips")

Two pairs of skin folds

Labia majora — outer, larger folds covered by pubic hair (singular, labium majus)

Labia minora — inner, thinner folds with no hair and are highly vascular (singular, labium minus)

Vestibule — space between the labia minora

Vestibular glands (function uncertain; may release lubricants during sex)

Vaginal and urethral openings

Margin of vaginal orifice (opening) may be a fold of skin called the hymen

Clitoris (glans clitoris)

Includes erectile tissue similar to that in penis

Covered with sensitive "sexual" touch receptors

Foreskin is a "hood" of skin that partly or entirely covers clitoris

May be cut or removed in any of various ways in "female circumcision"

FGM — female genital mutilation

Oogenesis

Meiotic division

Produces four daughter cells: one ovum and four polar bodies

Primary follicles

Contains primary oocyte

Stops mid-meiosis I — before birth

Continues, one at a time, at puberty (resumes Meiosis I)

Secondary follicles

Contains secondary oocyte (ovum) (Meiosis I is completed)

Ovulation

Mature follicle ruptures, releasing ovum from surface

Meiosis II is complete once a sperm enters

The "crater" left behind after ovulation becomes the endocrine gland called the corpus luteum (lit. "yellow body") that secretes hormones

Ovum that is released is surrounded by layers of cells

Some studies now challenge the notion that oogenesis cannot begin in the adult ovary and the notion that ovulation only occurs once per cycle

Hormonal regulation

Gonadotropin releasing hormone (GnRH) from hypothalamus

FSH - follicle-stimulating hormone from anterior pituitary

Stimulates maturation of a follicle

Maturing follicle secretes estrogen

Maintains female sex characteristics

Causes endometrium to thicken

LH - luteinizing hormone from anterior pituitary

Stimulates ovulation and formation of corpus luteum

Corpus luteum secretes progesterone

Promotes endometrial vascularization

More blood vessels so that offspring can implant successfully and build a placenta

Placenta is circulatory and endocrine structure that forms a connection with the maternal blood supply

Promotes endometrial glandularization

Uterine glands are like sweat glands in the endometrium that secrete a fluid that nourishes the offspring until the placenta is well established (fluid sometimes called "uterine milk")

Menstruation

Corpus luteum degenerates

Estrogen/progesterone levels decrease

Loss of hormonal "support" of endometrium causes sloughing (surface layers are shed; bleeding occurs)

Low estrogen/progesterone levels trigger another rise in LH/FSH secretion (cyclic negative feedback)

Uterine contractions help shedding; help stem bleeding

Pregnancy

Corpus luteum maintained by HCG (human chorionic gonadotropin) early in pregnancy

Later, the placenta takes over supplying hormones from the ovaries (thus, responsibility for hormonal control shifts more and more to offspring rather than mother)

Placental hormones have a variety of effects and combined effects

Estrogen/progesterone

Maintain uterine lining

Inhibit LH/FSH (a new cycle would start otherwise)

Progesterone/relaxin

Inhibit uterine contractions

Estrogen/relaxin

Softens pelvic joints

Enlarges vagina

Placental lactogen

Stimulates milk production

Increased aldosterone increases water retention

Extra precaution against dehydration

Tissues of developing offspring are higher in water content than adult tissues

Increased PTH (parathyroid hormone)

Increased blood calcium levels for developing offspring (esp. fetal skeleton)

Often increases risk of tooth cavities and stress fractures in the mother

"Morning sickness" is nausea associated with pregnancy

May subside later in pregnancy (then again, it may not)

Doesn't always occur (but don't count on it)

Probably decreases mother's ingestion of unusual foods or spicy/herbal foods that may contain substances that will adversely affect the development of the offspring

Teratogens are substances that cause developmental errors in developing offspring

Labor and delivery

Triggered by cortisol ( a stress hormone) released by the offspring

Thus, the offspring (not the mother) signals the end of pregnancy

Prostaglandins are also involved

Estrogen increases, progesterone decreases

Allows uterine contractions

Oxytocin (neurohypophysis) increases rate and strength of labor contractions

Positive feedback mechanism: increased vaginal stretch causes more OT release, which stimulates more contraction, which pushes baby, causing more stretch, more OT, more contraction, more stretch, more OT, more contraction (and so on) until baby is delivered and cycle of positive feedback is broken (whew!)

Mammary glands

Integumentary vs. reproductive

Technically, mammary glands are integumentary (skin) structures, NOT reproductive structures

However, mammary glands have a reproductive function in the sense that they provide nutrients to offspring during early development (birth to a year or so)

Functional anatomy

Breasts are mammary glands plus surrounding/supporting fat and ligaments, and skin

Both males and females have breasts (with mammary glands)

Rarely (thank goodness) male mammaries become functional to supply nutrients in the absence of adult females

Anthropologists believe that constantly swollen breasts of human females make them more sexually attractive, even when they are not ovulating

Nipple - central bump with multiple openings of lactiferous ducts (literally "milk-carrying ducts")

Lactiferous ducts have sinuses that can be used like the bulb of an eye-dropper to pump milk when baby's jaw squeezes them

Nipple becomes swollen and erect (by arrector muscles, same as in goose bumps of the skin) when stimulated to help baby find and hold it

Areola is circle of thin skin surrounding nipple

Often becomes darker by action of pregnancy hormones

Infants cannot see well or interpret what they see well, but are known to be attracted to dark circles when they are hungry —thus the areola may help infants locate their food source more easily

Areolar glands (Montgomery's glands)

Enlarged sebaceous (oil) glands surrounding the nipple

Reduces "chapping" of skin caused by wetness of baby's saliva and milk

May also help baby locate nipple by smell

Mammary glands

Exocrine glands

Milk

Mixture of proteins, carbohydrates (esp. lactose), and emulsified fats

Also includes antibodies (Ig) from mother's immune system — passive immunity

Lactation

Production and release of milk

Hormones regulate breast function / lactation

Estrogen stimulates breast development at puberty and then more during pregnancy

Placental lactogen promotes milk production

Prolactin (adenohypophysis) promotes milk secretion

Oxytocin stimulates milk ejection (from ducts)

human sexual response

Masters and Johnson

Physiological processes first verified by Dr. William Masters and his wife Dr. Virginia Johnson at their research institute at Washington University in St. Louis in the mid-twentieth century.

It has essentially 4 parts (in this order)

1. Excitement (arousal)

The name says it all. One is stimulated sexually and becomes excited, showing several physiological changes including increased blood volume in the erectile tissues and other sexual tissues. (parasympathetic effect)

2. Plateau

Once you have reached a peak of excitement, then the rate of physiological change levels off to a "plateau" or flat but raised level.

This continues until stimulation decreases or stops or (if you are lucky) an orgasm occurs. (parasympathetic effect)

3. Orgasm

At orgasm, there is a sudden switch to sympathetic effects, including contractions of the reproductive tract including ejaculation of semen in the male (a heck of a lot of subjective effects, as well).

The classic model includes a single orgasm, but women more often than men can experience multiple orgasms within one sexual response.

4. Resolution

After orgasm(s), continued sympathetic effects essentially "reverse" all the changes that occurred during the Excitement phase, including reduction of blood volume in erectile tissues.