Animal Reproduction Nancy G. Morris Volunteer State Community College Campbell, Chapter 46

Animal ReproductionAnimal Reproduction

Nancy G. MorrisNancy G. Morris

Volunteer State Community CollegeVolunteer State Community College

Campbell, Chapter 46Campbell, Chapter 46

Animal Reproduction:Animal Reproduction:

Asexual reproduction is the creation of new individuals whose genes all come from one parent without the fusion of egg and sperm.

Sexual Reproduction is the creation of offspring by the fusion of haploid gametes to form a zygote (fertilized egg), which is diploid.

Animal Reproduction:Animal Reproduction:

Sexual Reproduction is the creation of offspring by the fusion of haploid gametes to form a zygote (fertilized egg), which is diploid.

Gametes are formed by meiosis.

Female gamete: egg or ovumrelatively large & nonmotile

Male gamete: spermatozoongenerally, a small motile cell

Diversity in asexual reproduction:Diversity in asexual reproduction:

Fission Budding Gemmules Fragmentation Regeneration

Advantages of asexual Advantages of asexual reproduction:reproduction:

Animals in isolation will not die out

Rapid – ideal for colonizing a a habitat quickly

Most advantageous in stable, favorable environments because it perpetuates successful genotypes precisely

Control of Reproductive Control of Reproductive Cycles:Cycles:

Hormones Environmental cues

(temperature, rainfall, day length) Lunar cycles

Variation in reproductive Variation in reproductive patterns:patterns:

Parthenogenesis– Daphnia, aphids, rotifers, social

insects, whiptail lizards

Hermaphroditism– Tapeworms, earthworms,

sessile, burrowing animals

Variation in reproductive Variation in reproductive patterns:patterns:

Sequential hermaphroditism– individual reverses its sex during its

lifetime– either protogynous – female first– or protandrous – male first– Reef fishes called wrasse p.915

Mechanisms of Mechanisms of Sexual Reproduction Sexual Reproduction

Fertilization: union of egg and sperm

External fertilization: eggs shed by the female & fertilized by the male

Internal fertilization: requires cooperative behavior

External FertilizationExternal Fertilization

Requires environment where zygotes can develop without heat or stress

Many amphibians & fishes use specific mating behaviors resulting in one male fertilizing the eggs of one female

External FertilizationExternal Fertilization

Courtship behavior is a trigger for release of gametes. There are two effects:

1) the probability of successful fertilization is increased &

2) the choice of mates may be somewhat selective.

PhermonesPhermones

Chemical signals produced by one organism that influence the behavior of another organism of the same species

Small, volatile or water-soluble molecules Active in very small amounts (like hormones) Many function to attract mates Powerful:Powerful:Some insects can detect a mile

away Gypsy moth attracts in concentration as low

as 1 molecule in 107 molecules of other gases in the air

SurvivalSurvival

All species produce more offspring than survive to reproduce.

Species with internal fertilization produce fewer zygotes but provide more parental protection than species with external fertilization.

Species with external fertilization usually produce enormous numbers of zygotes, but the number that survive and develop further is small.

SurvivalSurvival

Amniotic eggs have calcium & protein shells which resist water loss & physical damage.

Eggs of amphibians & fishes have only a gelatinous coat.

Marsupials retain the embryos rather than secreting a shells around them.

Placental mammals develop entirely within the uterus, nourished by the mother’s blood supply.

Evolution of complex reproductive

systems Most complex reproductive system in the

animal kingdom is in the parasitic flatworms (Platyhelminthes). Figure 46.6

The least complex system is found in the polychaete worms (Annelida).

In some species, the female possesses a spermatheca, a sac in which sperm are stored for a year or more.

In many nonmammaliam vertebrates, excretory, digestive, and reproductive products are eliminated through a cloaca.

Figure 46.6Reproductive anatomy of a parasitic worm.

Human Male Reproductive Human Male Reproductive SystemSystem

• external genitalia

•Scrotum

•Penis

• internal genitalia

•Gonads – testes

•Accessory glands

•Associated ducts

Male Reproductive Male Reproductive Anatomy Anatomy

Figure 46.8

Male Reproductive Male Reproductive AnatomyAnatomy

Figure 46.8

Male Reproductive Male Reproductive ProcessProcess

Testes develop in abdomen & descend into the scrotum just before birth.

Sperm cannot develop at normal body temperature.

Placing the testes outside the abdominal cavity in the scrotum, reduces the temperature by 20 C.

Male Reproductive Male Reproductive ProcessProcess

Penis serves as the copulatory organ

Ejaculatory duct joins the urethra and opens at tip of the penis.

Movement of semen through the urethra results in the sperm being deposited directly in the female system (internal fertilization).

Figure 42.8 “Parts”

Testes are comprised of highly coiled tubes surrounded by layers of connective tissue.

Seminiferous tubules- the tubes where the sperm form.

Interstitial cells are scattered between the tubules & produce testosterone & androgens.

Sperm pass from the seminiferous tubules to the epididymis.

Figure 42.8 “Parts”

Epididymis contains coiled tubes where sperm are stored & mature- gain motility

Sperm are forced through the vas deferens to the ejaculatory duct.

Ejaculatory duct forms by the joining of two vas deferens ducts and the duct from the seminal vesicles. It opens into the urethra.

Penis is composed of 3 cylinders of spongy erectile tissue that fill with blood during erection.

3 sets of Accessory Glands

These glands add their secretions to the semen:

SEMINAL VESICLES

PROSTATE GLAND

BULBOUREHRAL GLAND


SEMINAL VESICLESSEMINAL VESICLES- secrete - secrete fluid containingfluid containing 1) mucus 2) amino acids – causes semen to coagulate after

deposited in female3) fructose – energy

4) prostaglandins – stimulate uterine contractions to help

move semen to the uterus


PROSTATE GLAND – secretes a thin, milky alkaline fluid containing enzymes to balance acidity in vagina.

BULBOUREHRAL GLAND – secretes viscous fluid before sperm ejaculation which may neutralize any acid urine remaining in urethra.

Female Human Reproductive Anatomy

• More complicated than male

• Structures not only for production of gametes, but also to house the embryo and fetus.

•INTERNAL

Gonads – Ovaries & associated ducts

•EXTERNAL

Clitoris & two sets of labia that surround it & and the vaginal opening


• Ovaries located in the abdominal cavity

• Each ovary contains many follicles (one egg cell surrounded by follicle cells, which nourish and protect the developing egg.)

•All EGGS are formed prior to birth!

•Follicle cells produce estrogens.

•Starting at puberty and continuing until menopause, one follicle matures and releases its egg cell during each menstrual cycle.


• During ovulation, the egg is expelled from the follicle. The follicular tissue becomes the corpus luteum, which secretes progesterone (maintains uterine lining) and additional estrogen.

• If the egg is not fertilized, the corpus luteum degenerates.

• The egg is release into the abdominal cavity near the opening of the oviduct.

•Cilia lining the oviduct draw the egg in and convey it to the uterus.


•The uterus (or womb) is a thick muscular organ 7 cm long and 5 cm wide.

• The inner uterine lining, the endometrium, is richly supplied with blood vessels.

•


•The remaining reproductive structures are:

• Cervix – neck of the uterus opening into the vagina

•Vagina – thin walled chamber; the repository for semen during copulation; forms the birth canal.

•The hymen, a vascularized membrane, usually covers the vaginal opening from birth until ruptured by vigorous physical activity or sexual intercourse.


•Labia minora -slender skin folds bordering the vestibule

• Labia majora – a pair of thick, fatty ridges enclosing & protecting the labia minora

• Clitoris – bulb of erectile tissue covered by a prepuce (small hood of skin)

Female Reproductive Female Reproductive AnatomyAnatomy

Figure 46.9

Female Reproductive Female Reproductive AnatomyAnatomy

Figure 46.9

Human Sexual ResponseHuman Sexual Response

Although variable, human sexual behavior is based in a common physiological pattern, the sexual response cycle.

Physiological reactions which predominate in both sexes can be divided into two types:– 1) Vasocongestion – increased blood flow

through the arteries of that tissue

– 2) Myotonia – both smooth & skeletal muscles may show sustained or rhythmic contractions


1) Excitement

2) Plateau

3) Orgasm

4) Resolution

There are four phases in the sexual response cycle in human males & females:


Excitement the penis & vagina are prepared for coitus (sexual intercourse)

1) Vasocongestion of the penis & clitoris; enlargement of testes, labia, breasts

2) Vaginal lubrication & myotonia occur


Plateau breathing & heart rates increase from stimulation of the autonomic nervous system

1) vagina depresses to receive the sperm

2) the outer third becomes vasocongested, the inner third slightly expands & the uterus elevates


Male: Emission – forcing the semen into the urethra

due to contraction of the glands & ducts of the reproductive system

Ejaculation - expels the semen due to contraction of the urethra

Female: Contraction of the uterus & outer vagina

Orgasm is characterized by rhythmic, involuntary contractions in the reproductive systems of both sexes


Resolution reverses responses of earlier phases & completes the cycle

1) Vasocongested organs return to normalcy

2) Muscles relax

Spermatogenesis

•Spermatogenesis – production of mature sperm cells in the adult male (Figure 42.12)

• Continuous process in adult males; 250- 400 million sperm cells per ejaculate

• Occurs in the seminiferous tubules

• Begins with differentiation of germ cells (2N) into spermatogonia (2N) in the embryonic testes

• Maturation of spermatogonia begins at puberty and continues until death

Spermatogenesis

•Primary spermatocytes undergo meiosis I to produce two haploid secondary spermatocytes

• Each secondary spermatocyte undergoes meiosis II to form two spermatids

• Result is 4 haploid spermatids through meiotic division

• Each spermatid becomes attached to a Sertoli cell from which it receives nutrients

•All four spermatids differentiate into mature spermatozoa

Figure 46.12Figure 46.12SpermatogenesisSpermatogenesis

Figure 46.12Figure 46.12SpermatogenesiSpermatogenesiss

Spermatozoon

• Thick head contains the haploid nucleus

• Acrosome at tip contains enzymes to aid in egg penetration

• Many mitochondria are present behind the head to provide ATP for flagellum movement

Figure 46.11 Structure of a Sperm Figure 46.11 Structure of a Sperm CellCell

Oogenesis

• Oogenesis is the development of ova (mature unfertilized eggs) Figure 42.13

• Begins at puberty and ends at menopause

• Begins in embryo -- germ cell undergoes mitotic divisions to produce a diploid oogonium.

•By birth, oogonia exist as primary oocytes, thus ALL potential ova at present at the time of birth!

Oogenesis

• Between birth & puberty, primary oocytes enlarge & their surrounding follicles grow.

• They replicate their DNA and enter prophase I & remain there until activated by hormones – one per month beginning at puberty.

• After puberty, during each ovarian cycle, FSH stimulates a follicle to enlarge & the primary oocyte inside completes meiosis I.

• The result is a haploid secondary oocyte and the first polar body.

Oogenesis

• Meiosis then stops again

• LH triggers ovulation & the secondary oocyte is released from the follicle.

• If a sperm cell penetrates the secondary oocyte’s membrane, meiosis II will occur and the second polar body will separate form the ovum.

• This completes oogenesis.

Figure 46.13 OOGENESISFigure 46.13 OOGENESIS

Figure 46.13 OOGENESISFigure 46.13 OOGENESIS

Important Differences:

• 1) In spermatogenesis, all four products of meiosis become spermatozoa, whereas

•1) In oogenesis, unequal cytokinesis occurring in meiosis I & II results in most of the cytoplasm being distributed to a single daughter cell which will form the ovum. The other cells (polar bodies) will degenerate.


• 2) Spermatogenesis is a continuous process throughout reproductive life of the male, whereas

• 2) in oogenesis, all potential ova are present as primary oocytes in the ovaries at the time of the female’s birth.


• 3) Spermatogenesis occurs continuously, whereas

• 3) in oogenesis, long “resting” periods occur between the formation of the initial steps and final production of the ovum

Mammalian female cycles:

• Estrous cycles occur in non-primate mammals.

• Ovulation occurs after the endometrium thickens & is vascularized

• If pregnancy does not occur, the endometrium is reabsorbed by the uterus.

• Involves pronounced behavioral changes: seasonal & climate changes effect the estrous cycle more than the menstrual cycle.

• ESTRUS is the period of sexual activity surrounding ovulation & is the only time most mammals will copulate. The length & frequency varies widely among species.

Mammalian female cycles:

• Menstrual cycles occur in humans and many other primates.

• Ovulation occurs after the endometrium thickens & is vascularized

• If pregnancy does not occur, the endometrium is shed form the uterus through the cervix & the uterus.

• The term menstrual cycle refers to changes that occur in the uterus during the reproductive cycle. (Figure 46.15)

•In humans, cycle is on average 28 days.

Figure 46.15Reproductive Cycle of the human female

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Animal Reproduction Nancy G. Morris Volunteer State Community College Campbell, Chapter 46