Essentials of BiologySylvia S. Mader

Chapter 29Lecture Outline

Prepared by: Dr. Stephen EbbsSouthern Illinois University Carbondale

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

29.1 How Animals Reproduce

• Animals generally reproduce sexually (two parents), but some can also reproduce asexually (one parent).– Fragmentation is a mechanism whereby a

new organism can regrow from a fragment of the original organism.

– Parthenogenesis is a modified form of sexual reproduction where an unfertilized egg becomes a whole individual.

Asexual Versus Sexual Reproduction

• Sexual reproduction involves the gamete-producing gonads of animals.– The male sperm-producing testes – The female egg-producing ovaries

• Copulation brings these haploid gametes together, allowing fertilization to produce a diploid zygote.

• Some animals are hermaphroditic, having the reproductive organs of both male and female.

Reproduction in Water Versus on Land

• In most aquatic organisms, external fertilization occurs and the resulting organism is free-living.

• On land, evolutionary modifications arose to provide the embryo and nutrients and to prevent dehydration.– Extraembryonic membranes are present in the eggs

of birds and reptiles. – The mammalian placenta, comprised of shared

tissues between the mother and fetus, provides nutrients, removes wastes, and prevent dehydration.

29.2 Human Reproduction

• The human reproductive tract consists of two components.– The gonads.– The accessory organs that assist reproduction

or, in females, house the embryo.

Male Reproductive System

• There are several components to the male reproductive system.– The testes produce the sperm. – The epididymis is a coiled tubule that receives

sperm produced by the testes.– The vas deferens stores sperm and conducts

the sperm to the urethra in the penis.– The penis is a muscular accessory organ that

facilitates transfer of sperm to the female.

Male Reproductive System (cont.)

Male Reproductive System (cont.)

• The sperm is carried in a fluid (semen) comprised of products from three glands.– The seminal vesicles produces a fluid that

nourishes the sperm.– The prostate gland produces a fluid that is

believed to enhance sperm motility.– The bulbourethral gland produces a

lubricating secretion to facilitate copulation.

Male Reproductive System (cont.)

Male Reproductive System (cont.)

• The release of sperm during copulation occurs in two phases.– During emission, the secretions of the three

glands are released as sperm enter the ejaculatory ducts.

– During expulsion, rhythmic contractions of muscle expel the semen from the penis.

The Testes

• The testes are comprised of coiled seminiferous tubules.

• Sperm production (spermatogenesis) occurs continually within the seminiferous tubules.

The Testes (cont.)

• The sperm produced have three distinct parts.– The head, carrying the

genetic material (acrosome)

– The tail, which provides movement

– The body, which houses mitochondria to provide energy for movement

Hormonal Regulation in Males

• Several glands and hormones regulate sexual function in males.– The hypothalamus controls the secretion of luteinizing

hormone (LH) and follicle-stimulating hormone (FSH) by the anterior pituitary.

– The anterior pituitary also produces FSH to enhance spermatogenesis.

– The anterior pituitary produces LH to promote testosterone production.

– Testosterone, produced by interstitial cells, is essential for normal sexual function.

Female Reproductive System

• There are several components to the female reproductive system.– The egg-producing ovaries– The oviducts (fallopian tubes), which conduct

the egg to the uterus– The uterus, where development of the embryo

occurs.– The cervix and vagina provide openings

through which sperm enter during copulation.

Female Reproductive System (cont.)

The Ovaries

• The ovaries are composed of millions of follicles that contain oocytes for production of eggs.

• When mature, the egg from a follicle erupts and is released (ovulation).

• After ovulation, the follicle develops into the corpus luteum, which degenerates if fertilization does not occur.

The Ovaries (cont.)

The Ovaries (cont.)

• The ovarian cycle can be divided into two phases.– The follicular phase, stimulated by FSH from

the anterior pituitary, results in maturation of the follicle and release of estrogens

– The luteal phase, stimulated by LH from the anterior pituitary, results in development of the corpus luteum and release of progesterone.

The Ovaries (cont.)

The Ovaries (cont.)

• The interactive effects of estrogens and progesterone on the uterus produce the menstrual cycle.

• The follicular phase occurs during the first 14 days of the menstrual cycle.– From days 1-5, hormone levels are low, causing the

endometrium to be sloughed from the uterus (menstruation).

– During days 6-13, release of estrogen promotes development of the endometrium (proliferative phase).

The Ovaries (cont.)

• Ovulation occurs around day 14.

• The luteal phase occurs during the last 14 days of the menstrual cycle.– During days 15-28, progesterone promotes

development of the endometrium to prepare for a fertilized embryo (secretory phase).

– If a fertilized embryo does not implant, endometrium degrades and menstruation occurs.

The Ovaries (cont.)

Control of Reproduction

• There are several techniques available to prevent pregnancy.– Abstinence (avoid copulation)– Oral contraceptives (inhibition of FSH and

LSH synthesis)– Contraceptive implants (disrupt ovarian cycle)– Intrauterine devices (alter uterine conditions)– Barrier methods (block sperm)– Spermicides (kill sperm)

Control of Reproduction (cont.)

Infertility

• Infertility is a failure to achieve pregnancy.

• In the US, about 15% of couples experience infertility problems.– 40% of cases are attributed to the male.– 40% of cases are attributed to the female.– 20% of cases are attributed to both.

• One means of overcoming infertility is through assisted reproductive technologies (ART).

Assisted Reproductive Technologies

• Artificial insemination by donor involves insertion of harvested sperm into the vagina.

• For in vitro fertilization, an egg is fertilized outside the woman’s body and implanted later.

• Gamete intrafallopian transfer is the same as in vitro fertilization except that the egg and sperm are placed into the fallopian tubes immediately.

• A single sperm is injected directly into an egg in intracytoplasmic sperm injection.

Sexually Transmitted Diseases

• There are several causes of sexually transmitted diseases (STDs).– Viruses are the cause of AIDS, genital warts,

genital herpes, and a form of hepatitis.– Bacteria can cause chlamydia, pelvic

inflammatory disease, gonorrhea, and syphilis.

– Yeast and protozoa can cause infections.

29.3 Human Development

• Human development includes all of the events from fertilization to the full formation of a child.

• The stages of development in humans are similar to those in other land animals, including the presence of nourishing extraembryonic membranes.

29.3 Human Development (cont.)

• There are four extraembryonic membranes.– The chorion is part of the placenta and

facilitates exchange of gases, nutrients, and wastes.

– Blood cells form in the yolk sac.– The blood vessels of the allantois are part of

the umbilical cord.– The amnion contains fluid that cushions and

protects the embryo.

29.3 Human Development (cont.)

Fertilization

• Fertilization of an egg by sperm forms a diploid zygote.– The acrosome of a sperm secretes enzymes

that digest the zona pellucida of the egg.– The sperm then binds to the plasma

membrane of the egg and enters the cell.– The genetic information of the sperm fuses

with the egg nucleus to complete fertilization.

Early Embryonic Development

• The embryonic development involves developmental events prior to implantation.– Ovulation releases a mature egg.– Fertilization occurs to form a zygote.– The zygote undergoes cleavage (cell division)

to form a morula. – The morula develops into a fluid-filled ball of

cells called a blastocyst. – The blastocyst implants in the uterus.

Early Embryonic Development (cont.)

Later Embryonic Development

• The next step in development is gastrulation.– Cells of the embryo begin to invaginate.– The amnion begins to develop. – After the initial development of the outer

ectoderm and inner endoderm, the mesoderm develops.

– Gastrulation is complete when the three germ layers are present.

Later Embryonic Development (cont.)

Later Embryonic Development (cont.)

Neurulation

• The nervous system forms during the neurulation by induction.– The mesoderm cells form a dorsal supporting

rod called the notochord.– The neural plate forms dorsal to the notochord.– The neural tube forms as the neural plates

begin to fold.– Other mesoderm cells consolidate into somites

to give rise to vertebrae and muscles.

Neurulation (cont.)

Organ Formation Continues

• Human embryos at five weeks have flippers called limb buds which give rise to arms and legs.

• At the same time the head and sensory organs develop.

• The umbilical cord has formed, connecting the embryo to the chorion.

• The allantois forms and the connection to the placenta is completed.

Organ Formation Continues (cont.)

Placenta

• The placenta is comprised of the embryonic fetus and the uterine wall.

• The chorionic villi of the chorion project into the uterine wall to facilitate exchange between fetus and mother.

• The umbilical cord carries fetal blood to and from the placenta.

• While fetal and maternal blood do not mix, a variety of chemicals can cross the placenta into the fetus.

Placenta (cont.)

Fetal Development and Birth

• Human fetal development occurs during the last six months of gestation. – The fetus increases in size and weight.– The genitalia develop during the third month.– Hair develops after the third month.– The heartbeat can be detected after 2.5

months.– Fetal movement begins.

The Stages of Birth

• Birth is triggered when the fetal hypothalamus triggers the pituitary to stimulate the release of androgens into the bloodstream.

• The placenta uses the androgens to form estrogens, which stimulate formation of oxytocin and prostaglandin.

• The estrogens, oxytocin, and prostaglandin cause the uterus to contract.

The Stages of Birth (cont.)

• The movement of the fetus out of the uterus is called parturition.– During the first stage of parturition, the cervix

dilates and the amnion bursts.– The baby is born and the umbilical is severed

during the second stage.– The placenta (afterbirth) is expelled from the

uterus in the third stage.

The Stages of Birth (cont.)