CHAPTER 27 Reproduction and Embryonic Development Overview Animal

CHAPTER 27 Reproduction and Embryonic Development Overview: Animal Reproduction Asexual & Sexual Human Reproductive System Female Male Female Reproductive Cycle Fertilization Pregnancy

• 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 • ASEXUAL AND SEXUAL REPRODUCTION Sexual and asexual reproduction are both common among animals

Asexual Reproduction • 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

• 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

• 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

• Advantages of sexual reproduction – Increases genetic variation – Enhances reproductive success in changing environments • Disadvantage of sexual reproduction – Locating a mate

• Some animals can reproduce both sexually and asexually

• Some species are hermaphrodites with both male and female reproductive systems

• 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

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

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

• 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 • Oviducts –Convey eggs to the uterus

• Uterus - Actual site of pregnancy – Development of fertilized egg – Opens into the vagina • Vagina – Receives penis during intercourse – Forms the birth canal

• Ovulation – Is the process by which an egg cell is ejected from the follicle – An egg cell is released from a follicle at the surface of an ovary – The orange mass below the ejected oocyte is part of the ovary

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

• There are two stages of ejaculation – First stage of ejaculation

– Second stage of ejaculation (expulsion stage)

• Androgens stimulate sperm production – They also maintain homeostasis by a negative feedback mechanism that inhibits the secretion of FSH (follicle-stimulating hormone) and LH (luteinizing hormone)

The formation of sperm and ova requires meiosis • Gametogenesis – Is the production of gametes • Human gametes – Are haploid cells that develop by meiosis • Spermatogenesis – Produces sperm in the male • Oogenesis – Produces ova in the female

• Spermatogenesis – Increases genetic variation – Primary spermatocytes are produced throughout a male’s reproductive years – Diploid cells undergo meiosis to form four haploid sperm

• Oogenesis – Most of the process occurs within the ovaries – Lifetime supply of primary oocytes is present at birth – One primary oocyte matures each month to form a secondary oocyte – If the secondary oocyte is fertilized, it completes meiosis and becomes a haploid ovum

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 human sexual response occurs in four phases • Excitement – Sexual passion builds – Penis and clitoris become erect – Testes, labia, nipples swell – Vagina secretes lubricating fluid – Muscles of arms and legs tighten

• Plateau – Continuation of excitement responses – Increase in breathing and heart rates • Orgasm – Rhythmic contraction of the reproductive structures – Extreme pleasure – Ejaculation by the male • Resolution – Reverse previous phase responses – Structures return to normal size – Muscles relax – Passion subsides

REPRODUCTIVE HEALTH • Two issues of human reproductive health – Contraception – Transmission of disease Contraception • Contraception – Is the deliberate prevention of pregnancy

• Contraception prevents pregnancy in one of three ways – Blocking the release of gametes – Preventing fertilization – Preventing implantation

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

• Contraceptive methods and their effectiveness

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

REPRODUCTIVE TECHNOLOGIES • Reproductive technologies – Can help solve problems related to the inability to conceive a child • 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

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

PRINCIPLES OF EMBRYONIC DEVELOPMENT Fertilization results in a zygote and triggers embryonic development • The shape of a human sperm cell is adapted to its function

• 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

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

• 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

Gastrulation produces a three-layered embryo • Gastrulation is the second major phase of embryonic development – It adds more cells to the embryo – It sorts all cells into three distinct cell layers – The embryo is transformed from the blastula into the gastrula

• The three layers produced in gastrulation – Ectoderm, the outer layer – Endoderm, an embryonic digestive tract – Mesoderm, which partly fills the space between the ectoderm and endoderm

• Development of frog gastrula

Organs start to form after gastrulation • Embryonic tissue layers begin to differentiate into specific tissues and organ systems • In chordates – the notochord develops from the mesoderm – the neural tube develops from the ectoderm • The neural tube becomes the brain and spinal cord

• Somites are blocks of mesoderm that will give rise to segmental structures • The body cavity, or coelom, also develops from the mesoderm

Changes in cell shape, cell migration, and programmed cell death give form to the developing animal • Tissues and organs take shape in a developing embryo as a result of – cell shape changes – cell migration

Embryonic induction initiates organ formation • Induction is the mechanism by which one group of cells influences the development of tissues and organs from ectoderm, endoderm, and mesoderm – Adjacent cells and cell layers use chemical signals to influence differentiation – Chemical signals turn on a set of genes whose expression makes the receiving cells differentiate into a specific tissue

Pattern formation organizes the animal body • Pattern formation is the emergence of a body form with structures in their correct relative positions – It involves the response of genes to spatial variations of chemicals in the embryo

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

• Gestation is pregnancy – It begins at conception and continues until birth – Human gestation is 266 days (38 weeks or 9 months) – Mouse gestation is 1 month – Elephant gestation is 22 months

• 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

• 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 placenta allows for a variety of substances to pass from mother to fetus – Protective antibodies – German measles virus – HIV – Drugs (prescription and nonprescription) – Alcohol – Chemicals in tobacco smoke

The Stages of Pregnancy • Pregnancy is divided into three trimesters

The First Trimester • A human embryo about 5 weeks after fertilization

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

• 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

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

• 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

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)

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

• There are three stages of labor • Dilation of the cervix is the first stage – Cervix reaches full dilation at 10 cm – Longest stage of labor (6 -12 hours or longer)

• Expulsion is the second stage – Period from full dilation of the cervix to delivery of the infant – Uterine contractions occur every 2 -3 minutes – Mother feels urge to push down with her abdominal muscles – Infant is forced down and out of uterus and vagina within a period of 20 minutes

• The delivery of the placenta is the final stage of labor – Usually occurs within 15 minutes after the birth of the baby

• Hormones continue to be important after the baby and placenta are delivered – Decreasing progesterone and estrogen levels allow the uterus to return to its pre-pregnancy state – Oxytocin and prolactin stimulate milk secretion