Reproduction and Development Figures 26.1 – 26.2 CHAPTER 26 Reproduction and Development Figures 26.1 – 26.2

During ejaculation, a man releases up to 500 million sperm, only one of which may fertilize an egg You have trillions of cells in your body, and they all arose from one original cell

An American woman is 4 times more likely to deliver triplets today than 25 years ago Even though a woman is born with up to half a million developing gametes, she will only use about 500 during her lifetime

BIOLOGY AND SOCIETY: RISE OF THE SUPERTWINS On November 19, 1997, news reports heralded the arrival of the Iowa septuplets Figure 26.1

Other sets of multiple births soon followed What accounted for the sudden rash of multiple births?

All of these multiple births were by women who had taken fertility drugs because they couldn’t become pregnant naturally Couples turn to fertility drugs to overcome their natural reproductive limitations

UNIFYING CONCEPTS OF ANIMAL REPRODUCTION Reproduction is the creation of new individuals from existing ones

In asexual reproduction One parent produces genetically identical offspring

Binary fission Is the simplest type of asexual reproduction Involves a single parent cell splitting through mitosis into two genetically identical offspring cells

Some multicellular organisms reproduce by a similar means called fission, in which one organism splits into two or more individuals Figure 26.2a

Fragmentation is the breaking of a parent body into several pieces Regeneration, which follows fragmentation, is the regrowth of a whole animal from the pieces

Budding Is the splitting off of new individuals from existing ones Figure 26.2b

Asexual reproduction has a number of advantages It allows a species to perpetuate itself if its individual members are sessile or isolated from one another It allows organisms to multiply quickly

One potential disadvantage of asexual reproduction is that it produces genetically uniform populations

Sexual Reproduction Sexual reproduction Involves the fusion of gametes (sperm and egg) from two parents Increases the genetic variability among offspring

Some animals can reproduce both sexually and asexually Figure 26.3a

Some species are hermaphrodites with both male and female reproductive systems Figure 26.3b

The mechanics of fertilization play an important part in sexual reproduction

Many organisms use external fertilization, in which parents discharge their gametes into the water, where fertilization occurs Figure 26.3c

Other organisms use internal fertilization, which occurs within the female’s body Internal fertilization requires copulation, or sexual intercourse

Both sexes of humans have HUMAN REPRODUCTION Both sexes of humans have A pair of gonads, the organs that produce gametes Ducts to store and deliver the gametes Structures to facilitate copulation

Female Reproductive Anatomy The ovaries Are the site of gamete production in human females Ovaries Oviduct Follicles Corpus luteum Uterus Wall of uterus Endometrium (lining of uterus) Cervix (“neck” of uterus) Vagina Figure 26.4

The ovaries contain follicles Each follicle consists of a single developing egg cell surrounded by layers of cells that nourish and protect it The follicles also produce estrogen, the female sex hormone

Ovulation Is the process by which an egg cell is ejected from the follicle Figure 26.5

The egg enters the oviduct, which is a tube in which cilia sweep the egg toward the uterus

The uterus is the actual site of pregnancy The cervix, the narrow neck at the bottom of the uterus, opens into the vagina, or birth canal During copulation, the vagina serves as a repository for sperm

Female reproductive anatomy Oviduct Ovary Rectum (digestive system) Uterus Bladder (excretory system) Pubic bone Cervix Urethra (excretory system) Vagina Shaft Glans Clitoris Prepuce Labia minora Labia majora Vaginal opening Figure 26.6

Male Reproductive Anatomy The penis Contains erectile tissue

The testes Are the male gonads, enclosed in a sac called the scrotum Produce sperm

Several glands Contribute to the formation of the fluid that carries, nourishes, and protects sperm Semen Consists of this fluid and sperm

Male reproductive anatomy, side view Bladder (excretory system) Seminal vesicle Pubic bone Rectum (digestive system Erectile tissue of penis Vas deferens Urethra Prostate gland Vas deferens Glans of penis Epididymis Testis Prepuce Scrotum Figure 26.7a

Male reproductive anatomy, front view Bladder (excretory system) Seminal vesicle (behind bladder) Prostate gland Urethra Erectile tissue of penis Vas deferens Scrotum Epididymis Testis Glans of penis Figure 26.7b

Gametogenesis Gametogenesis Human gametes Is the production of gametes Are haploid cells that develop by meiosis

Oogenesis is the development of eggs within the ovaries Diploid cell in embryo Differentiation and onset of meiosis I Ovary Primary oocyte, arrested in prophase of meiosis I; present at birth Completion of meiosis I and onset of meiosis II Corpus luteum First polar body Secondary oocyte, Growing follicle arrested at metaphase of meiosis II; released from ovary Entry of sperm triggers completion of meiosis II Mature follicle Second polar body Ovulation Ovum Ruptured follicle (haploid) Sperm Figure 26.8

Spermatogenesis Spermatogenesis Is the formation of sperm cells

Differentiation and onset of meiosis I Seminiferous tubule Epididymis Penis Testis Scrotum Diploid cell Testis Differentiation and onset of meiosis I Seminiferous tubule Primary spermatocyte Cross section of seminiferous tubule Meiosis I completed Secondary spermatocyte Meiosis II Developing spermatids Differentiation Sperm cells (haploid) Center of seminiferous tubule Figure 26.9

The Female Reproductive Cycle Human females have a reproductive cycle, a recurring series of events that produces gametes, makes them available for fertilization, and prepares the body for pregnancy

The female reproductive cycle involves two sets of changes The ovarian cycle controls the growth and release of an ovum The menstrual cycle prepares the uterus for possible implantation of an embryo

Hormones Synchronize cyclical changes in the ovaries and uterus

The female reproductive cycle Control by hypothalamus Inhibited by combination of estrogen and progesterone Hypothalamus Stimulated by high levels of estrogen Releasing hormone Anterior pituitary (a) 1 Pituitary hormones in blood 4 (b) 2 FSH stimulates follicle to grow LH peak triggers ovulation Ovarian cycle 5 Growing follicle Mature follicle Ovulation Corpus luteum Degenerating corpus luteum (c) Estrogen secreted by growing follicle Progesterone and estrogen secreted by remnant of follicle Figure 26.10a–c

(d) (e) Ovarian hormones in blood Peak causes LH surge 3 7 Estrogen Progesterone 6 (d) Progesterone and estrogen promote thickening of endometrium Low levels trigger menstruation Menstrual cycle Endometrium (e) Menstruation Figure 26.10d, e

Two issues of human reproductive health Contraception Transmission of disease

Contraception Contraception Is the deliberate prevention of pregnancy

There are many forms of contraception, each with varying degrees of reliability Figure 26.11

Contraceptive methods and their effectiveness Table 26.1

Sexually Transmitted Diseases Sexually transmitted diseases (STDs) Are contagious diseases spread by sexual contact

Viral STDs, such as AIDS, genital herpes, and genital warts, cannot be cured but can be controlled by medications

STDs caused by bacteria, protozoans, and fungi are generally curable with drugs Table 26.2

REPRODUCTIVE TECHNOLOGIES Can help solve problems related to the inability to conceive a child

Infertility Infertility Is the inability to have children after one year of trying Is most often due to problems in the man, such as underproduction of sperm or impotence

Female infertility can result from a lack of eggs or a failure to ovulate There are technologies available to help treat the many forms of infertility

In Vitro Fertilization In vitro fertilization (IVF) Happens under artificial, laboratory conditions

IVF Begins with the surgical removal of eggs and the collection of sperm Involves fertilization of eggs in a petri dish Figure 26.12

IVF Offers choices that nature does not Raises many moral and legal issues

Embryonic development HUMAN DEVELOPMENT Embryonic development Begins with fertilization, the union of sperm and egg to form a zygote

Fertilization Copulation releases hundreds of millions of sperm into the vagina, but only a few hundred survive the trip to the egg, and only one will fertilize it Figure 26.13

A mature human sperm Has a streamlined shape that suits its need to swim through fluids in the vagina, uterus, and oviduct Head Plasma membrane Mitochondria Nucleus Acrosome Figure 26.14

The events of fertilization 3 The plasma membranes of the sperm and egg fuse. 2 The sperm’s acrosomal enzymes digest the zona pellucida. 1 The sperm squeezes through cells left over from the follicle. 4 The sperm enters the egg cytoplasm. Acrosomal enzymes Sperm Nucleus Acrosome Sperm nucleus 5 The nuclei of sperm and egg join. Follicle cell Cytoplasm Plasma membrane Zona pellucida Egg nucleus Egg cell Zygote nucleus Figure 26.15

Basic Concepts of Embryonic Development The key to development in all organisms is that each stage of development takes place in a highly organized fashion

Zygote 2 cells Cleavage 4 cells 8 cells Many cells (solid ball) Inner cell mass Blastocyst (hollow ball) Cross section of blastocyst Ectoderm Gastrulation Mesoderm Gastrula (cross section) Endoderm Figure 26.16

Development begins with cleavage, a series of rapid cell divisions that results in a multicellular ball Cleavage continues as the embryo moves down the oviduct toward the uterus About 6–7 days after fertilization, the embryo has reached the uterus as a fluid-filled hollow ball of about 100 cells called a blastocyst

The next stage of development is gastrulation, a process that produces the three embryonic tissue layers

Tissues and organs take shape in a developing embryo as a result of many different changes in the cells Outer layer of ectoderm Neural tube Figure 26.17

In the process called induction, one group of cells influences the development of an adjacent group of cells Lens ectoderm Optic cup Cornea Future brain Lens Optic vesicle Future retina Optic stalk 1 2 3 4 Figure 26.18

Pregnancy and Early Development Pregnancy, or gestation Is the carrying of developing young within the female reproductive tract Is measured as 40 weeks from the start of the last menstrual cycle in humans

The early stages of human development begin with fertilization and cleavage in the oviduct Cleavage starts Fertilization of ovum Ovary Oviduct Secondary oocyte Ovulation Blastocyst (implanted) Endometrium Uterus Figure 26.19

About one week after conception The embryo, which has become a blastocyst, implants itself in the uterine wall The outer cell layer, the trophoblast, becomes part of the placenta

Endometrium Inner cell mass Cavity Blood vessel (maternal) Inner cell mass Future embryo Multiplying cells of trophoblast (future placenta) Cavity Future yolk sac Trophoblast Trophoblast Uterine cavity (a) Blastocyst (6 days after conception) (b) Implantation underway (about 7 days) Figure 26.20

The embryo after about a month after conception Mother’s blood vessels Allantois (forms part of the umbilical cord) Placenta Amnion Yolk sac Embryo Chorion Chorionic villi Figure 26.21

Four structures develop that assist the developing embryo The amnion, a fluid filled sac that encloses and protects the embryo The yolk sac, which produces the embryo’s first blood and germ cells

The allantois, which forms part of the umbilical cord The chorion, which becomes part of the placenta

The Stages of Pregnancy Pregnancy is divided into three trimesters

A human embryo about 5 weeks after fertilization The First Trimester A human embryo about 5 weeks after fertilization Figure 26.22

A human embryo, now called a fetus, about 9 weeks after fertilization Figure 26.23

By the end of the first trimester The fetus looks like a miniature human being The sex of the fetus can be determined by ultrasound Figure 26.24

The Second Trimester The main developmental changes during the second and third trimesters involve an increase in size and general refinement of the human features

A fetus at 14 weeks, 2 weeks into the second trimester Figure 26.25

At 20 weeks, the fetus Is about 19 cm (7.6 in.) long and weighs about half a kilogram (1 lb) Has the face of an infant Figure 26.26

The Third Trimester The third trimester Is a time of rapid growth Includes many important physical changes

At birth A typical baby is about 50 cm (20 in.) long and weighs 2.7–4.5 kg (6–10 lb) Figure 26.27

Childbirth The birth of a child is brought about by a series of strong, rhythmic contractions of the uterus called labor

Hormones play a key role in inducing labor Estrogen, oxytocin, and prostaglandins are all involved Estrogen Oxytocin from ovaries from fetus and pituitary Induces oxytocin receptors on uterus Positive feedback Stimulates uterus to contract Stimulates placenta to make Prostaglandins Simulate more contractions of uterus Figure 26.28

The three stages of labor Figure 26.29

EVOLUTION CONNECTION: MENOPAUSE AND THE GRANDMOTHER HYPOTHESIS Typically between the ages of 46 and 54, human females undergo menopause, the cessation of ovulation and menstruation

Why does the reproductive system shut down? One group of researchers proposed that menopause actually increases a woman’s evolutionary fitness in the long run Postreproductive women contribute to raising grandchildren

SUMMARY OF KEY CONCEPTS Gametogenesis Oogenesis Spermatogenesis Primary oocyte Primary spermatocyte Once per month Polar body Secondary oocyte Secondary spermatocyte Spermatid Fertilization Sperm Polar body Ovum Zygote Visual Summary 26.1

In Vitro Fertilization Collected egg Implantation 8-cell embryo Zygote In vitro fertilization Collected sperm Visual Summary 26.2

Basic Concepts of Embryonic Development Cleavage Gastrulation Ectoderm Mesoderm Endoderm Zygote 2-cell embryo Gastrula (cross section) Many-celled solid ball Blastocyst (cross section) Visual Summary 26.3

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