Chapter 43 Animal Reproduction

Biology 102

Tri-County Technical College

Pendleton, SC

Let me answer that one, Doc!!!

Asexual reproduction DOES NOT involve formation of any type (kind) of gametes (sex cells)

Sexual reproduction DOES involve the formation of gametes (sex cells)

How those gametes are gotten together is another matter and can be a real hoot!!!!

Types of Asexual Reproduction

Fission involves separation of parent into two or more individuals of approximately same size (sea anemone)Budding occurs when new individual splits off from existing one (hydra and tunicates)Release of specialized cells that grow into new adults Freshwater sponges produce gemmules which

form from aggregates of several types of cells surround by protective coat

Asexual, cont.

Fragmentation is breaking of body into several pieces, each of which develops into complete animal Must be accompanied by regeneration

(regrowth of lost body parts) Sponges, cnidarians, polychaete annelids, and

tunicates

Regeneration allows many animals to replace lost parts

Asexual Advantages???

Allows production of many offspring in short time thereby exploiting favorable environment with maximum offspring in shortest possible time

Perpetuates successful genotypes precisely

In other words, makes clones

Sexual Advantages???

Offspring have combination of genes inherited from both parents

***Increases genetic variability in the offspring

Highly advantageous in a fluctuating environment

Sociobiology, anyone?

Daring to define…

Parthenogenesis is development of an egg without fertilization Daphnia; male honeybees (drones)

Hermaphroditism: each individual has both functional male and female reproductive parts Some self-fertilize but most mate with another

individual (earthworms are classic example)

Daring to define, cont.

Sequential hermaphroditism describes situation where individual reverses its sex during its lifetime

Some species are protogynous (female first)

Others are protandrous (male first)

Reversal often associated with age and size

Male form and function…

External genitalia includes scrotum and penis (copulatory organ)Internal reproductive organs consist of gonads (testes), accessory glands, and associated ductsTestes comprised of highly coiled tubules (seminiferous tubules) surrounded by layers of connective tissue “Mountain oysters,” anyone???

Male F & F, cont.

Interstitial cells (Leydig cells) scattered among tubules and produce testosterone and other androgens

Sperm passes from seminiferous tubules into tubules of epididymis Sperm are stored and mature (gain motility and

fertilizing power)

At ejaculation, sperm forced through vas deferens (muscular duct connecting epididymis to ejaculatory duct)

Male F & F III

Ejaculatory duct forms by joining of two vas deferens ducts with duct from seminal vesiclesEjaculatory duct opens into urethra (tube that runs through penis and drains both excretory/reproductive systems)There are 3 sets of accessory glands associated with the male systemThey add their secretions to semen

Male F & F IV

Pair of seminal vesicles located below and behind urinary bladder and empty into ejaculatory duct Secrete fluid containing mucus, amino acids

(for coagulation), fructose, and prostaglandins Comprise about 60% of total semen volume

Prostate gland is large gland surrounding upper portion and emptying directly into urethra

Male F & F V

Prostate secretes thin, milky alkaline fluid with enzymes Balances acidity of residual urine in system Buffers acidity of vagina/activates sperm

Bulbourethral glands are pair of small glands below prostate that empty into urethra at base of penis Secrete clear mucus before ejaculation Neutralize remaining acidic urine in urethra

Male F & F V

Head of penis (glans penis) covered with foreskin called prepuce

Is it the “y” or is it just tough being born a male?

Some male mammals possess a baculum ( a penis bone)

Male Visual

Female Form and Function

Internal reproductive organs are gonads (ovaries) and associated ducts and chambers Involved with gamete movement/embryo

development

External genitalia include clitoris/2 sets of labia

Ovaries located in abdominal cavity enclosed in tough protective capsule

Female Form & Function, cont.

Mesentery flanks & attaches each ovary to uterus

Each ovary contains follicles (1 egg cell surrounded by follicle cells which nourish and protect egg cell)

Follicle cells also produce estrogens

During ovulation, egg expelled from follicle

Remaining tissue forms corpus luteum which secretes progesterone (maintains uterine lining) and additional estrogen

Female F & F III

Egg cell expelled into abdominal cavity near opening of oviduct

Cilia lining oviduct draw egg cell in (hopefully) uterus

Uterus (womb) is thick muscular organ that can expand to accommodate a 4-kg fetus

Inner uterine lining (endometrium) is richly supplied with blood vessels

Female F & F IV

Cervix is the neck of the uterus which opens into vagina

Vagina is thin-walled chamber that is repository for semen during copulation, forms birth canal, and is exit for menstrual fluids

Hymen is vascularized membrane usually covering vaginal opening from birth until ruptured by vigorous physical activity or sexual intercourse

Female F & F V

Vestibule: chamberlike area formed by 2 pairs of skin folds covering vaginal orifice/urethral opening

Libia minora: slender skin folds bordering vestibule

Libia majora: pair of thick, fatty ridges enclosing and protecting libia minora & vestibule

Female F & F VI

Clitoris: bulb of erectile tissue at front edge of vestibule which is covered by a prepuce (small hood)

Bartholin’s glands: small glands located near vaginal opening that secrete mucus into vestibule during sexual arousal, facilitates intercourse by lubricating vagina

Mammary glands: important to reproduction but NOT part of reproductive system

Female Visual

Hormones are flowing…

Androgens directly responsible for formation of primary sex characteristics (reproductive organs) & secondary sex characteristics (deepening of voice, hair growth patterns, muscle growth)

Are steroid hormones produced primarily by Leydig cells of testes

Testosterone most important androgen

Male hormones

Androgens are potent determinants of sexual and aggressive behavior

It’s the testosterone that’s killing the guys…but one can go to the front of the line

GnRH from hypothalamus stimulates anterior pituitary to release LH (stimulates androgen production) and FSH (acts on seminiferous tubules to > sperm production)

Comparing cycles

Menstrual cycle characterized by endometrium break down, sloughing off of lining, and discharge from body via vagina if fertilization has NOT occurred From “menses” which is Latin for “months”

Uterine cycles of mammals other than humans does NOT include menstruation Uterine lining is reabsorbed into body

Cycles, cont.

Ovarian cycle characterized by state of sexual receptivity called estrus (correlated with ovulation)

Female solicits male attention and may be aggressive to other females

Time for sociobiology

Female Hormones

Hormones coordinate menstrual and ovarian cycles

Ovulation synchronized with uterine preparation for possible implantation of embryo

FIVE hormones participate in elaborate positive and negative feedback scheme

Female Hormones, cont.

GnRH from hypothalamus

FHS (responsible for follicle maturation) and LH (responsible for egg release) from anterior pituitary

Estrogens (family of closely related hormones) and progesterone (female sex hormone secreted by ovaries)

Female Hormones Visual

Sync’ing the Cycles

Cycles coordinated and timed by same hormones that initiate sexual maturationIn response to FSH and LH, ovarian tissue grows and produces estrogenEstrogen causes development of secondary sexual characteristicsMenstruation marks beginning of uterine/ovarian cycles

Cycles, cont.

Few days before menstruation begins, anterior pituitary > secretion of FSH and LHSome follicles begin to mature in ovariesGenerally only one follicle continues with processDeveloping follicle secretes > amounts of estrogen which causes endometrium to grow

Cycles III

Estrogen exerts negative feedback control on gonadotropin release by anterior pituitary (FHS & LH) during first 12 days of cycleOn day 12, estrogen exerts positive feedback on pituitary and there is surge of LH and slight surge of FSHLH triggers mature follicle to rupture and release its eggAlso stimulates follicle to become corpus luteum which secretes estrogen and progesterone

Cycles IV

Estrogen and progesterone CRITICAL to continued growth and maintenance of endometrium

These sex steroids exert negative feedback on pituitary inhibiting gonadotropin release thereby preventing new follicles from beginning to mature

Cycles V

Egg NOT fertilized, corpus luteum degenerates on about day 26 of cycle

Without progesterone from corpus luteum, endometrium sloughs off and menstruation occurs

Decrease in circulating steriods allows hypothalamus/anterior pituitary to release GnRH, FSH, and LH and next cycle begins

Spermatogenesis

Spermatogonia are diploid cells that are precursors of spermLocated near outer wall of seminiferous tubules, spermatogonia undergo repeated mitoses which produce large populations of potential spermIn mature male, about 3 million spermatogonia per day differentiate into primary spermatocytesPrimary spermatocytes undergo meiosis I to produce two secondary spermatocytes which are now haploid but still have duplicated chromosomes

Spermatogenesis, cont.

Secondary spermatocytes undergo meiosis II Separates sister chromatids and produces four

spermatids

Spermatids mature into mature spermatozoa (sperm cells)

Involves association with Sertoli cells which transfer nutrients to spermatids

Spermatogenesis III

During spermatogenesis, developing sperm gradually pushed toward center of seminiferous tubule

Make their way to epididymis where they acquire motility

From spermatogonia to motile sperm = 65 to 75 days

Spermatogenesis Visual

Oogenesis

Oogenesis is development of ova (mature, unfertilized egg cells)Begins in embryonic female when primordial germ cells undergo mitotic divisions to produce diploid oogoniaEach oogonium develops into primary oocyte by time of birthAll potential ova present in ovaries at birth

Oogenesis, cont.

Primary oocytes (2N) proceed to Prophase I between 3rd and 7th month of fetal development

Then enter state of suspended development until female reaches puberty

After puberty, with each ovarian cycle, FSH stimulates a follicle to enlarge & primary oocyte within completes Meiosis I to produce haploid secondary oocyte and first polar body

Oogenesis III

Meiosis STOPS again

LH triggers ovulation and secondary oocyte released from follicle

IF fertilization occurs, Meiosis II will occur and second polar body will separate from ovum

Female strategy appears to be QUALITY of reproductive cell whereas male strategy appears to be QUANTITY of reproductive cells

Oogenesis Visual

Comparing the Processes

In spermatogenesis all 4 products of Meiosis I and II become mature spermatozoa

In oogenesis, only one daughter cell will become single ovum, the other cells (polar bodies) will degenerate

Spermatogenesis is continual process throughout reproductive life of male

Comparing, cont.

All potential ova that can be produced by oogenesis are present as primary oocytes in ovaries at time of female’s birthSpermatogenesis occurs as uninterrupted sequence (subject to brain stem influence…time for an Estesism…yeah!!!Oogenesis has long resting periods between formation of initial steps and final production of ovum

What a Miracle….

Human gestation divided into 3 trimesters (about 3 months each)

FIRST trimester is when most radical changes occur for both mother and baby

Fertilization occurs in oviduct and cleavage (cell division) begins in about 24 hours

Zygote develops into a ball of cells

Pregnancy, cont.

Embryo reaches uterus in 3-4 days and develops into hollow ball of cells called blastocyst (about 1 week after fertilization)

Blastocyst will implant in endometrium in next 5 days (critical thinking time)

During implantation, blastocyst bores into endometrium and endometrium begins to grow over blastocyst

Pregnancy III

Embryonic tissues begin to mingle with endometrium to form placenta which functions in respiratory gas exchange, nutrient transfer, and waste removal from embryoAlso main period of organogenesis (organ development)After 8 weeks, embryo becomes a fetusFetus possesses ALL adult organs in rudimentary form

Pregnancy IV

Fetus is about 5 cm in length by end of 1st trimesterEmbryo secretes hormones that signal its presence and control’s mother reproductive system**Human chorionic gonadotropic (HCG) maintains progesterone and estrogen secretion by corpus luteum to prevent menstruation

Pregnancy V

High progesterone levels stimulate formation of protective mucus plug in cervix, growth of maternal part of placenta, uterus enlargement, and cessation of ovulation and menstrual cycling

SECOND TRIMESTER time of rapid growth and fetal activity

Fetus grows to about 30 cm in length

Pregnancy VI

Mother will feel movement (quickening)Hormone levels stablize as HCG declines Corpus luteum degenerates Placenta secretes its own progesterone to

maintain pregnancy

Uterus grows sufficiently for pregnancy to be obviousTHIRD TRIMESTER is time of rapid growth but decreasing fetal activity

Pregnancy VII

Fetus grows to about 50 cm in length, and 3 to 3.5 kg in weight

Maternal abdominal organs become compressed and displaced

Labor induced & regulated by interplay among estrogen, oxytocin, and prostaglandins

High estrogen levels during last weeks of pregnancy trigger formation of oxytocin receptors on uterus

Pregnancy VIII

Oxytocin (from fetus/maternal posterior pituitary) stimulate smooth muscles of uterus to contract

Oxytocin stimulates prostaglandin secretion by placenta (enhance muscle contractions)

Parturition (birth) occurs through series of strong, rhythmic contractions of uterus (called labor)

Pregnancy IX

Three stages to labor

1st stage: involves opening and thinning of cervix until completely dilated

2nd stage: expulsion of baby from uterus Contractions are strong and continuous

3rd stage: expulsion of placenta from the uterus

Positive Feedback Visual

Hormonal Control of Pregnancy

Hormones secreted by embryo necessary to insure pregnancy continuesHCG acts like pituitary LH to maintain secretion of progesterone and estrogens by corpus luteum through 1st trimesterIn absence of hormonal override provided by embryo, decline in maternal LH due to inhibition of pituitary by progesterone would result in menstruation and spontaneous abortion of embryoEPT (and others) based on HCG hormone

Hormonal Control, cont.

Cervix mucus plug, growth of maternal part of placenta, enlargement of uterus, cessation of ovulation and menstruation all result of high levels of progesterone

Contraception

Three major ways to achieve contraception

(1) Prevent fertilization by keeping egg and sperm apart

(2) Prevent implantation of embryo

(3) Prevent release of mature egg and/or sperm from gonads

Contraception, cont.

Keeping egg and sperm from meeting in the female reproductive tract prevents fertilization

Abstinence

Rhythm method

Condoms, diaphragms, cervical caps, contraceptive sponges, and spermicides

Coitus interruptus

Tubal ligation and vasectomy

Contraception III

Preventing implantation can be accomplished by using intrauterine device (IUD) Probably works by irritating endometrium

Chemical contraception prevents release of mature gametes from gonadsBirth control pills, minipills, and male birth control pill????