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Chapter 46 Animal Reproduction. Asexual and Sexual Reproduction Asexual reproduction is the creation of new individuals – Whose genes all come from one.

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Presentation on theme: "Chapter 46 Animal Reproduction. Asexual and Sexual Reproduction Asexual reproduction is the creation of new individuals – Whose genes all come from one."— Presentation transcript:

1 Chapter 46 Animal Reproduction

2 Asexual and Sexual Reproduction Asexual reproduction is the creation of new individuals – Whose genes all come from one parent Sexual reproduction is the creation of offspring – By the fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote Animals may reproduce exclusively asexually or sexually or they may alternate between the two

3 Mechanisms of Asexual Reproduction Many invertebrates reproduce asexually by fission – The separation of a parent into two or more individuals of approximately the same size

4 Asexual Reproduction Budding – In which new individuals arise from outgrowths of existing ones Fragmentation – Is the breaking of the body into several pieces, some or all of which develop into complete adults – Must be accompanied by regeneration, the regrowth of lost body parts Hydra Planaria

5 Reproductive Cycles and Patterns

6 Most animals exhibit cycles in reproductive activity – Often related to changing seasons Reproductive cycles – Are controlled by hormones and environmental cues

7 Parthenogenesis Some animals reproduce by parthenogenesis – A process in which an egg develops without being fertilized – The resulting organism is haploid and may produce haploid eggs without meiosis – Male honeybees are produced by parthogenesis

8 Parthenogenesis Among vertebrates, several genera of fishes, amphibians, and lizards, including whiptail lizards (photo) r eproduce exclusively parthenogenesis Time Ovary size Hormones Behavior Ovulation Progesterone Estrogen Female- like Male- like Female- like Male- like (a) Both lizards in this photograph are C. uniparens females. The one on top is playing the role of a male. Every two or three weeks during the breeding season, individuals switch sex roles. (b) The sexual behavior of C. uniparens is correlated with the cycle of ovulation mediated by sex hormones. As blood levels of estrogen rise, the ovaries grow, and the lizard behaves like a female. After ovulation, the estrogen level drops abruptly, and the progesterone level rises; these hormone levels correlate with male behavior. Ovulation is stimulated by mating behavior in this all female species. These lizards having evolved from species with 2 sexes still need certain sexual stimuli for maximum reproductive success There is a doubling of chromosomes after meiosis to create a diploid “zygote”

9 Hermaphroditism Sexual reproduction presents a special problem for certain organisms that seldom encounter a mate One solution to the problem is hermaphroditism – In which each individual has both male and female reproductive systems – Some hermaphrodites can mate with themselves, but most must find a partner (as with mating earthworms where both donate and receive sperm)

10 Hermaphroditism Sequential hermaphroditism: where an individual reverses its sex during its lifetime Some species are male-first, other species are female-first Caribbean bluehead wrasse. All members of this species are born female, but the oldest, largest fish complete their life as males female male

11 Mechanisms of Fertilization

12 External Fertilization Some species have external fertilization, in which – Eggs shed by the female are fertilized by sperm in the external environment Eggs

13 Internal fertilization Some species have internal fertilization – Sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract

14 In either situation, fertilization requires critical timing – Often mediated by environmental cues, pheromones, and/or courtship behavior Internal fertilization – Requires important behavioral interactions between male and female animals – Requires compatible copulatory organs

15 Ensuring the Survival of Offspring All species produce more offspring than the environment can handle – But the proportion that survives is quite small – The more of your offspring that survive to reproduce …the greater the influence of your genes on the next generation

16 Ensuring the Survival of Offspring The embryos of many terrestrial animals develop in shelled eggs that can withstand harsh environments Many animals retain the embryo, which develops inside the female Many different types of animals exhibit parental care to ensure survival of offspring Egg brooding in Giant Water Bug

17 Gamete Production and Delivery Animals must have systems that produce gametes to reproduce sexually The least complex reproductive systems – Do not even contain distinct gonads, the organs that produce gametes in most animals The most complex reproductive systems – Contain many sets of accessory tubes and glands that carry, nourish, and protect the gametes and the developing embryos

18 Many animals with relatively simple body plans possess highly complex reproductive systems Male organs: Female organs: Genital pore (Excretory pore) Seminal receptacle (Digestive tract) Testis 1 Vas efferens 2 Sperm duct (vas deferens) 3 Seminal vesicle 4 Ovary 1 Oviduct 2 Yolk duct Yolk gland 3Uterus

19 Most insects have separate sexes with complex reproductive systems (a) Male honeybee. Sperm form in the testes, pass through the sperm duct (vas deferens), and are stored in the seminal vesicle. The male ejaculates sperm along with fluid from the accessory glands. (Males of some species of insects and other arthropods have appendages called claspers that grasp the female during copulation.) (b) Female honeybee. Eggs develop in the ovaries and then pass through the oviducts and into the vagina. A pair of accessory glands (only one is shown) add protective secretions to the eggs in the vagina. After mating, sperm are stored in the spermatheca, a sac connected to the vagina by a short duct. Testis 1 Accessory gland 3 Seminal vesicle Vas deferens 2 Penis 5 Ejaculatory duct 4 Accessory gland Spermatheca Ovary 1 Vagina 3 Oviduct

20 Human Reproductive System

21 Female Reproductive Anatomy Prepuce (Rectum) Cervix Vagina Bartholin’s gland Vaginal opening Ovary Oviduct Labia majora Labia minora (Urinary bladder) (Pubic bone) Uterus Urethra Shaft Glans Clitoris

22 Female Reproductive Anatomy Vagina Uterus Cervix Ovaries Oviduct Uterine wall Endometrium Follicles Corpus luteum

23 Ovaries The female gonads, the ovaries, lie in the abdominal cavity Each ovary – Is enclosed in a tough protective capsule and contains many follicles A follicle – Consists of one egg cell surrounded by one or more layers of follicle cells

24 Ovulation The process of ovulation – Expels an egg cell from the follicle The remaining follicular tissue then grows within the ovary – To form a solid mass called the corpus luteum, which secretes hormones, depending on whether or not pregnancy occurs

25 Oviducts The egg cell is released into the abdominal cavity near the opening of the oviduct, or fallopian tube Cilia in the tube help draw the ovulated egg into the tube – Cilia also convey the egg to the uterus Fertilization usually occurs in the oviduct The oviduct is the site of ectopic (tubal) pregnancies

26 Uterus A thick muscular organ Lining is the endometrium Cervix is the neck of the uterus that opens into the vagina Vagina Uterus Cervix Ovaries Oviduct Uterine wall Endometrium Follicles Corpus luteum

27 Vagina and Vulva The vagina is a thin-walled chamber – That is the repository for sperm during copulation – Serves as the birth canal through which a baby is born The vagina opens to the outside at the vulva – Which includes the hymen, vestibule, labia minora, labia majora, and clitoris

28 Mammary Glands Not part of the reproductive system but are obviously important to mammalian reproduction Within the glands small sacs of epithelial tissue produce and secrete milk High levels of estrogen during pregnancy stimulate growth and development of mammary glands

29 Male Reproductive Anatomy External reproductive organs in most mammalian species are the scrotum and penis The internal organs – gonads, which produce sperm and hormones – accessory glands

30 Male Reproductive Anatomy Seminal vesicle (Rectum) Vas deferens Ejaculatory duct Prostate gland Bulbourethral gland (Urinary bladder) (Pubic bone) Erectile tissue of penis Urethra Glans penis Prepuce Vas deferens Epididymis Testis Scrotum

31 Male Reproductive Anatomy Erectile tissue of penis Prostate gland (Urinary bladder) Bulbourethral gland Vas deferens Epididymis Testis Seminal vesicle (behind bladder) Urethra Scrotum Glans penis

32 Testes The male gonads, or testes – Consist of many highly coiled tubes (seminiferous tubules ) surrounded by several layers of connective tissue Sperm is formed in the seminiferous tubules Testosterone is produced in the interstitial cells (cells of Leydig)

33 Production of normal sperm – Cannot occur at the body temperatures of most mammals The testes of humans and many mammals – Are held outside the abdominal cavity in the scrotum, where the temperature is lower than in the abdominal cavity – A testicle not in the scrotum is a cryptorchid

34 Ducts From the seminiferous tubules of a testis – The sperm pass into the coiled tubules of the epididymis During ejaculation – Sperm are propelled through the muscular vas deferens, the ejaculatory duct, and exit the penis through the urethra

35 Glands Three sets of accessory glands add secretions to the semen, the fluid that is ejaculated A pair of seminal vesicles contributes about 60% of the total volume of semen Prostate gland – Secretes its products directly into the urethra through several small ducts Bulbourethral gland – Secretes a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra

36 Semen in the Female Reproductive Tract Once in the female reproductive tract – A number of processes, including contractions of the uterus, thinning of cervical mucus, and swimming of the spermatozoa, help move the sperm up the uterus

37 Penis The human penis – Is composed of three cylinders of spongy erectile tissue (distensible blood spaces) During sexual arousal – The erectile tissue fills with blood from the arteries, causing an erection – Viagra works by increasing blood flow to the genital region

38 Human Sexual Response Two types of physiological reactions predominate in both sexes – Vasocongestion, the filling of tissue with blood – Myotonia, increased muscle tension The sexual response cycle can be divided into four phases – Excitement, plateau, orgasm, and resolution

39 Gametogenesis The process of gametogenesis – Is based on meiosis, but differs in females and males

40 Ovary Primary germ cell in embryo Differentiation Oogonium in ovary Mitotic division Primary oocyte, arrested in prophase of meiosis I (present at birth) Completion of meiosis I and onset of meiosis II Primary oocyte within follicle Secondary oocyte, arrested at meta- phase of meiosis II First polar body Ovulation Entry of sperm triggers completion of meiosis II Ovum Growing follicle Mature follicle Ruptured follicle Ovulated secondary oocyte Corpus luteum Degenerating corpus luteum 2n2n 2n2n n n n n Oogenesis Oogenesis is the development of mature ova

41 Spermatogenesis The production of mature sperm Epididymis Seminiferous tubule Testis Cross section of seminiferous tubule Sertoli cell nucleus Lumen of Seminiferous tubule Spermatogonium Primary spermatocyte (in prophase of meiosis I) Secondary spermatocyte Early spermatids Spermatids (at two stages of differentiation) Differentiation (Sertoli cells provide nutrients) Meiosis II Meiosis I completed Mitotic division, producing large numbers of spermatogonia Sperm cells Acrosome Nucleus Mitochondria Neck Tail Plasma membrane HeadMidpiece 2n2n 2n2n n n n nnn n n n n Differentiation and onset of meiosis I

42 Oogenesis differs from spermatogenesis First – during the meiotic divisions of oogenesis cytokinesis is unequal, with almost all the cytoplasm monopolized by a single daughter cell, the secondary oocyte Second, – sperm are produced continuously throughout a male’s life which is not the case in oogenesis Third, – oogenesis has long “resting” periods, while spermatogenesis produces sperm in uninterrupted sequence

43 The Reproductive Cycles of Females The secretion of hormones and the reproductive events they regulate are cyclic

44 Menstrual Versus Estrous Cycles Two different types of cycles occur in females Humans and other primates have menstrual cycles Other mammals have estrous cycles In both cases ovulation occurs at a time in the cycle after the endometrium has started to thicken in preparation for implantation

45 Menstrual Versus Estrous Cycles In menstrual cycles – The endometrium is shed from the uterus in a bleeding called menstruation – Sexual receptivity is not limited to a specific timeframe In estrous cycles – The endometrium is reabsorbed by the uterus – Sexual receptivity is limited to a “heat” period – In estrous animals that bleed (dogs & cats), it is due to high estrogen that accompanies receptive period

46 The Human Female Reproductive Cycle The female reproductive cycle – Is one integrated cycle involving two organs, the uterus and ovaries Cyclic secretion of GnRH from the hypothalamus – And of FSH and LH from the anterior pituitary orchestrates the female reproductive cycle

47 Reproductive cycle of the human female Control by hypothalamus Inhibited by combination of estrogen and progesterone Stimulated by high levels of estrogen Inhibited by low levels of estrogen Hypothalamus Anterior pituitary GnRH FSH LH Pituitary gonadotropins in blood LH FSH FSH and LH stimulate follicle to grow LH surge triggers ovulation Ovarian cycle Growing follicle Mature follicle Corpus luteum Degenerating corpus luteum Estrogen secreted by growing follicle in increasing amounts Progesterone and estrogen secreted by corpus luteum Follicular phase Luteal phaseOvulation Ovarian hormones in blood Peak causes LH surge Estrogen Progesterone Estrogen level very low Progesterone and estro- gen promote thickening of endometrium Uterine (menstrual) cycle Endometrium Menstrual flow phase Proliferative phase Secretory phase Days 1 (a) (b) (c) (d) (e) Refer back to this figure to integrate your thinking of ovarian and uterine events

48 The Ovarian Cycle The female has a set of primary follicles containing primary oocytes that have become arrested in early meiosis With each cycle a small # of primary follicles are stimulated by FSH to grow and mature (generally one follicle completes maturation) In the Follicular Phase of the ovarian cycle – FSH stimulates follicle growth – The growing follicle secretes ever increasing amounts of estrogen (which ultimately leads to the LH surge)

49 Ovulation A surge in LH levels results in ovulation

50 The Ovarian Cycle In the Luteal Phase of the ovarian cycle – The follicular tissue left behind transforms into the corpus luteum – The corpus luteum secretes progesterone and estrogen – Over time the corpus luteum regresses and sex hormone production declines. This leads to menstruation

51 The Uterine (Menstrual) Cycle Cycle after cycle – The maturation and release of egg cells from the ovary are integrated with changes in the uterus If an embryo has not implanted in the endometrium by the end of the secretory phase – A new menstrual flow commences

52 The Uterine (Menstrual) Cycle The classic 28-day cycle Menstruation (days 1-5) – Regression of the C.L. from the previous cycle results in low hormone levels and shedding of the endometrium Proliferative Phase (days 6-13) – Estrogen from the growing ovarian follicle stimulates growth of the endometrium Ovulation (day 14) Secretory Phase (days 15-28) – Progesterone from the C.L. stimulates development of uterine glands – Estrogen and Progesterone levels continue to thicken the endometrium

53 Menopause After about 450 cycles, human females undergo menopause – The cessation of ovulation and menstruation – Due to decrease in sensitivity of the ovary to FSH?

54 Hormonal Control of the Male Reproductive System Testosterone and other androgens are directly responsible for the primary and secondary sex characteristics of the male

55 Androgen secretion and sperm production – Are both controlled by hypothalamic and pituitary hormones Stimuli from other areas in the brain Hypothalamus GnRH from the hypothalamus reg- ulates FSH and LH release from the anterior pituitary. FSH acts on the Sertoli cells of the seminiferous tubules, promoting spermatogenesis. LH stimulates the Leydig cells to make testosterone, which in turn stimulates sperm production. Anterior pituitary Negative feedback Leydig cells make testosterone Primary and secondary sex characteristics Sertoli cells Spermatogenesis Testis

56 Embryo Growth in Placental Mammals

57 Conception, Embryonic Development, and Birth In humans and most other placental mammals – Pregnancy, or gestation, is the condition of carrying one or more embryos in the uterus After fertilization – The zygote undergoes cleavage and develops into a blastocyst before implantation in the endometrium

58 Fertilization of an egg by a sperm, conception occurs in the oviduct Ovary Uterus Endometrium From ovulation to implantation Endometrium Inner cell mass Cavity Blastocyst Trophoblast (a) Implantation of blastocyst(b) Ovulation releases a secondary oocyte, which enters the oviduct. 1 Fertilization occurs. A sperm enters the oocyte; meiosis of the oocyte finishes; and the nuclei of the ovum and sperm fuse, producing a zygote. 2 Cleavage (cell division) begins in the oviduct as the embryo is moved toward the uterus by peristalsis and the movements of cilia. 3 Cleavage continues. By the time the embryo reaches the uterus, it is a ball of cells. It floats in the uterus for several days, nourished by endometrial secretions. It becomes a blastocyst. 4 The blastocyst implants in the endometrium about 7 days after conception. 5

59 First Trimester Human gestation – Can be divided into three trimesters of about three months each The first trimester – Is the time of most radical change for both the mother and the embryo

60 During its first 2 to 4 weeks of development – The embryo obtains nutrients directly from the endometrium Meanwhile, the outer layer of the blastocyst – Mingles with the endometrium and eventually forms the placenta

61 Blood flow at the placenta Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein Placenta Umbilical cord Chorionic villus containing fetal capillaries Maternal blood pools Uterus Fetal arteriole Fetal venule Umbilical cord Maternal portion of placenta Fetal portion of placenta (chorion) Umbilical arteries Umbilical vein Maternal arteries Maternal veins Figure 46.16

62 Organogenesis The first trimester is the main period of organogenesis (the development of the body organs) (a) 5 weeks. Limb buds, eyes, the heart, the liver, and rudiments of all other organs have started to develop in the embryo, which is only about 1 cm long. (b) 14 weeks. Growth and development of the offspring, now called a fetus, continue during the second trimester. This fetus is about 6 cm long. (c) 20 weeks. By the end of the second trimester (at 24 weeks), the fetus grows to about 30 cm in length.

63 Second Trimester During the second trimester – The fetus grows and is very active – The mother may feel fetal movements – The uterus grows enough for the pregnancy to become obvious

64 Third Trimester During the third trimester – The fetus continues to grow and fills the available space within the embryonic membranes

65 Labor A complex interplay of local regulators and hormones induces and regulates labor, the process by which childbirth occurs EstrogenOxytocin from ovaries from fetus and mother's posterior pituitary Induces oxytocin receptors on uterus Stimulates uterus to contract Stimulates placenta to make Prostaglandins Stimulate more contractions of uterus Positive feedback

66 Three stages of labor Placenta Umbilical cord Uterus Cervix Dilation of the cervix Expulsion: delivery of the infant Uterus Placenta (detaching) Umbilical cord Delivery of the placenta Birth, or parturition is brought about by a series of strong, rhythmic uterine contractions Cervical dilation Expulsion Placenta delivery

67 Contraception and Abortion Contraception, the deliberate prevention of pregnancy can be achieved in a number of ways Some contraceptive methods – Prevent the release of mature eggs and sperm from gonads (sterilization, the pill) – Prevent fertilization by keeping sperm and egg apart (barrier methods) – Prevent implantation of an embryo (IUD)

68 RU486 (Mifepristone) RU-486 is the name commonly used for an artificial steroid that blocks progesterone. This hormone is needed to continue a pregnancy When taken alone, RU-486 causes a complete abortion only about 60% of the time. A second drug, a prostaglandin, is given 48 hours later to increase its effectiveness. The prostaglandin causes uterine contractions to help expel the embryo

69 Morning After Pill The active ingredients in morning-after pills are similar to those in birth control pills, except in higher doses. Some morning-after pills contain only one hormone, progestin (Plan B), and others contain two, progestin and estrogen. The morning-after pill is designed to be taken within 72 hours of intercourse with a second dose taken 12 hours later.

70 Mechanisms of some contraceptive methods Male Female Method Event Method Production of viable sperm Production of viable oocytes Vasectomy Combination birth control pill (or injection, patch, or vaginal ring) Sperm transport down male duct system Ovulation Abstinence Condom Coitus interruptus (very high failure rate) Sperm deposited in vagina Capture of the oocyte by the oviduct Abstinence Tubal ligation Spermicides; diaphragm; cervical cap; progestin alone (minipill, implant, or injection) Sperm movement through female reproductive tract Transport of oocyte in oviduct Meeting of sperm and oocyte in oviduct Morning-after pill (MAP) Union of sperm and egg Implantation of blastocyst in properly prepared endometrium Birth Progestin alone A lot of stuff here. Take a look at this figure in your textbook.

71 Modern Reproductive Technology Scientific and technological advances have made it possible to deal with many reproductive problems Amniocentesis and chorionic villus sampling – Are invasive techniques in which amniotic fluid or fetal cells are obtained for genetic analysis Modern technology can help infertile couples by ART (assisted reproductive technology) – in vitro fertilization – Etc.

72 Ultrasound Noninvasive procedures usually use ultrasound imaging to detect fetal condition Head Body Head Body


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