Chapter 27: Reproduction and Embryonic Development
27.1 Sexual and asexual reproduction are both common among animals
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Reproduction is the creation of new individuals from existing ones. Animals reproduce in a variety of ways, but the two principal ways are asexual and sexual.
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Asexual – without sex. It is the creation of an offspring whose genes all come from one parent without the fusion of egg and sperm. Several types of asexual reproduction exist: o
Budding – splitting off new individuals from existing ones o Fission – separation of a parent into two or more individuals of about equal size. o Fragmentation – breaking of parent body into several pieces which may develop into adults, but this must result in regeneration
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Asexual has several advantages:
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Allows animals that live in isolation to produce offspring
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Enables animal to produce many offspring quickly without energy loss in gamete production
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Perpetuates a particular genotype precisely and rapidly
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It has some disadvantages
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Produces genetically uniform populations
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If the environment changes, then the population could die out because of no genetic variety.
27.2 Sexual reproduction results in the generation of genetically unique offspring
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Sexual reproduction – creation of an offspring by fusion of two haploid gametes. o Sperm – the male gamete. A relatively small cell that is propelled by a flagellum o
Egg (ovum) – the female gamete. A larger cell that is not self propelled o
Zygote – a new individual containing a unique combination of the parents’ genes
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Sexual reproduction increases genetic variation among the offspring. The variability increases adaptability to a changing environment. In theory, if the environment changes drastically, then some offspring will survive and reproduce thereby keeping the population going.
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Many animals can reproduce by both methods, benefiting from both. While sexual reproduction has its advantages, it can be a problem for nonmotile individuals, or for those who live solitary lives. In those cases, hermaphroditism may develop. In this case, an individual may have both male and female reproductive systems. Although some may fertilize themselves, most of them do require a mate. In this encounter, both animals donate and receive sperm so that mating will result in twice as many offspring.
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The mechanisms of fertilization are very important to reproduction. In many aquatic vertebrates, such as fish and amphibians, external fertilization is used. Parents discharge their gametes into the water, sometimes with no contact between parents. Timing is critical because the eggs must be ripe for fertilization.
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Internal fertilization is when sperm is deposited in or close to the female tract and the gametes unite within the tract. Internal fertilization protects the eggs from drying out.
27.3 Reproductive anatomy of the human female
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Both sexes have a pair of gonads where the gametes are produced, a system of ducts that house and conduct the gametes, and structures that aid in copulation (intercourse).
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Ovaries are about an inch long and covered with a bumpy surface. The bumps are follicles that each consist of a single developing egg surrounded by one or more layers of cells that nourish and protect it. The ovaries are also responsible for hormone production , specifically estrogen. Most of the 400,000 follicles a woman will have are thought to form before her birth but only several hundred will actually be released. One follicle matures and releases its egg every 28 days in the process of ovulation. After ovulation, the remaining tissue grows within the ovary to make the corpus luteum that secretes progesterone that will maintain uterine lining during pregnancy. If the egg is not fertilized, then the corpus luteum degenerates and the cycle resumes.
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The oviduct, fallopian tube, lies next to the opening of the ovary. The oviduct opening has fringed projections that touch the surface of the ovary. When ovulation occurs, the egg is swept into the oviduct and cilia channel it to the uterus. Fertilization usually occurs within the oviduct and the egg travels to the uterus.
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The uterus, womb, is the site of pregnancy. In a woman who has never been pregnant, the uterus is about 3 inches long, but during pregnancy it expands to hold the baby. It has a thick muscular wall, and the inner lining, endometrium, is supplied with blood. The embryo implants into the endometrium and development occurs there.

o
Embryo is the term for the stage of development that starts with the first division of the zygote and ends at nine weeks, when the offspring becomes a fetus. o If implantation is not in the uterus, then the pregnancy is ectopic and requires surgical removal.
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The neck of the uterus is the cervix, which opens into the vagina. The vagina is a thin-walled muscular chamber that serves as the repository for sperm during copulations and the birth canal for the baby.
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External structures of the female tract include the labia minora, labia majora, clitoris, glans, and prepuce.
All of which are responsible for copulation.
27.4 Reproductive anatomy of the human male
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The male gonads are housed outside of the abdominal cavity in a sac, the scrotum. Sperm cannot develop at body temperature, their location outside of the body keeps the temperature low enough for production.
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From each testis, sperm pass through the epididymis, which stores them while they develop. During ejaculation, the sperm leave the male body. When this happens, the sperm leave the epididymis and travel through another duct, the vas deferens that joins a short duct, the seminal vesicle. The two ducts unite into the ejaculatory ducts that empty into the urethra and allow sperm to leave the body.
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In addition to the testes and ducts, the male reproductive system has three glands: o
Seminal vesicles secrete a fructose containing fluid that give the sperm energy o Prostate gland that secretes a thin fluid used to nourish the sperm o Bulbourethral glands secrete a clear alkaline mucus that balances any acidity left by urine in the urethra o
Together, these secretions make semen, the fluid released during ejaculation. These secretions are
95% of the fluid released. 50-130 million sperm make up the other 5%
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The penis consists of tissue that can fill with blood during arousal and allow for copulation.
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Hormones control sperm production. The hypothalamus secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH increases sperm production while LH promotes secretion of androgens, like testosterone, that stimulate sperm production.
27.5 The formation of sperm and ova requires meiosis
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Both sperm and ova are haploid cells that develop by meiosis from diploid cells in the gonads. Formation of gametes is termed gametogenesis.
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Spermatogenesis is the formation of sperm cells and takes about 65-75 days. Sperm develop in the tested in coiled tubes called the seminiferous tubules. Diploid cells that start the process are near the outer wall of the tubules. These cells multiply constantly by mitosis, and each day about 3 million of them differentiate into primary spermatocytes. Meiosis I of these cells produces secondary spermatocytes, each of which are haploid. Meiosis II forms four cells with this haploid number. A sperm cells develops by differentiation of each of these cells and passes through to the center of the seminiferous tubule. From there, the cells pass to the epididymis, where they mature and become motile.
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Oogenesis is the development of mature ova, eggs. Oogenesis begins before birth when a diploid cell in each developing follicle begins meiosis. At birth, each follicle contains a dormant primary oocyte, a diploid cell resting in prophase of meiosis I. At puberty, FSH from the pituitary stimulates one dormant follicle to develop. The follicle enlarges and the primary oocyte completes meiosis I and begins meiosis II. Meiosis II halts at metaphase II. Cytoplasmic division during oogenesis is unequal, with one cell gaining all of the cytoplasm, the secondary oocyte, and the second cell, a polar body, receiving none. The secondary oocyte is released during ovulation and enters the oviduct. If fertilized, the secondary oocyte completes meiosis II and releases a second polar body along with the actual ovum. The haploid nucleus of the ovum fuses with the haploid sperm to make a diploid zygote.
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Spermatogenesis and oogenesis are similar in that they both produce haploid gametes. They differ in that spermatogenesis produces 4 viable cells, where oogenesis only produced one ovum. Sperm development continues throughout the man’s life, while this is not the case for egg development. Oogenesis also has long resting periods while spermatogenesis occurs in an uninterrupted sequence.
27.6 Hormones synchronize cyclic changes in the ovary and uterus
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Oogenesis is one part of the female cycle that produces gametes, makes them available for fertilization and prepares the body for pregnancy. This cycle involves two organs, the ovaries and uterus. The ovarian cycle occurs every 28 days and releases an egg for fertilization. Hormones synchronize the ovarian cycle with events in the uterus, leading to the menstrual cycle.
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The ovarian cycle can be explained in two phases: o Pre-ovulatory phase – follicle is growing and secondary oocyte is developing

o
Post-ovulatory phase – the follicle has become the corpus luteum
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Events of the menstrual cycle are coordinated with the ovarian cycle. Day one of the menstrual cycle is designated by the first day of the period, or menstruation, which also corresponds to the beginning of the pre—ovulatory phase. During menstruation, the endometrium breaks down and leaves the body.
Afterwards, it regrows during the 20-25 day time between cycles. There are five hormones that regulate the cycle and synchronize the ovarian cycle and the menstrual cycle.
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Hormonal Events before Ovulation –
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Hypothalamus stimulates the anterior pituitary to increase output of FSH and LH
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FSH stimulates the growth of an ovarian follicle, starting the ovarian cycle. The follicle secretes little estrogen
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As the follicle grows, it secretes more estrogen to exert negative feedback to the pituitary. As ovulation approaches, hormone levels increase, with estrogen reaching peak levels just before ovulation.
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The pituitary secretes bursts of FSH and LH and ovulation starts
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Hormonal Events at Ovulation and After –
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LH stimulates the completion of meiosis, transforming primary oocyte into the secondary, and signals enzymes the follicle to rupture. High levels of estrogen and progesterone in the blood have a strong influence on the ovary and uterus. The combination produces negative feedback on the hypothalamus and pituitary producing falling FSH and LH levels
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Drops in these hormones prevent follicles and ovulation from occurring. Unless an embryo has implanted, the follicle stops secreting estrogen and progesterone.
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As blood levels decline, the pituitary will secrete more FSH and LH
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Control of the menstrual cycle – o Menstrual cycle is controlled by estrogen and progesterone only. The endometrium thickens in response to rising levels of estrogen and progesterone. When the levels drop, the endometrium slough off and menstruation begins.
27.7 Sexual activity can transmit disease
Sexually transmitted diseases (STDs) are contagious diseases transmitted by sexual activity. Viral STDs are not curable. Others are usually curable by medication if diagnosed in time. They are most prevalent among teens and young adults, with 2/3 of cases in people under 25.
27.8 Contraception can prevent unwanted pregnancy
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Contraception is the deliberate prevention of pregnancy. Complete abstinence is the only totally effective method of birth control.
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Women can undergo a tubal ligation, or males a vasectomy, as sterilization processes. Both are very reliable and may be reversible.
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Temporary abstinence, or natural family planning (rhythm method), or barrier methods may be employed with varying levels of success.
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Hormonal birth control can be some of the most effective methods of birth control.
27.9 Fertilization results in a zygote and triggers embryonic development
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Embryonic development begins with fertilization, the union of sperm and egg to form a diploid zygote.
Fertilization combines two haploid sets of chromosomes from two individuals and activates the egg by triggering metabolic changes to start embryonic development.
 Only one sperm enter and fertilize the egg. The sperm’s shape is adapted to reach the egg and fertilize it.
The head of the sperm is tipped with a vesicle, the acrosome, which contains enzymes to allow for entrance into the egg. The neck of the sperm contains a mitochondrian which provides ATP for tail movement. o The acrosome releases a cloud of enzymes to digest a cavity into the jelly coat of the egg o The sperm head reaches the vitelline layer and proteins bind with the specific receptors of the vitelline layer. After this binding, the sperm proceeds through the layer and its plasma membrane fuses with the egg membrane. o The sperm nucleus is now able to enter the egg. Fusion of sperm and egg triggers important changes in the egg. About one second after fusion of the membranes, the egg membrane becomes impenetrable to other sperm. o
The vitelline layer hardens and separates from the plasma membrane and becomes the fertilization envelope. o Egg and sperm nuclei fuse to produce the diploid zygote

27.10 Cleavage produces a ball of cells from the zygote
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First two major phases of embryonic development: cleavage and gastrulation
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Cleavage is a succession of cell divisions that produces a ball of cells from the zygote. Cell division occurs rapidly and partitions the zygote into many smaller cells.
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The number of cells doubles with each division. Each cell is much smaller than the zygote. As cleavage continues, a fluid filled cavity, blastocoels, forms in the center of the embryo. The hollow ball of cells is called a blastula.
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Cleavage makes two important contributions: o It creates multicellular embryo o It organizes and partitions the embryo into developmental regions
27.11 Gastrulation produces a three-layered embryo
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Gastrulation is the second major phase. It adds more cells to the embryo and sorts the cells into distinct layers. The embryo goes from a hollow ball of cells into a three layered gastrula.
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Three layers are produced in gastrulation. These three layers develop into all layers of the adult animal. o
Ectoderm – forms the outer layer (skin) of the gastrula. Nervous system and epidermis o Endoderm – forms the embryonic digestive tract o Mesoderm – fills the space between the other layers. Most organs and tissues, kidneys, heart and muscles form from mesoderm.
27.12 Organs start to form after gastrulation
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Once the layers begin to form, cells of each layer begin to differentiate into tissues and embryonic organs.
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The notochord has developed from the mesoderm. It is made from something similar to cartilage, runs most of the length of the embryo, and provides support for other developing tissues. It will serve as a core that other mesoderm cells gather around to make the backbone.
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A thickened region of ectoderm, the neural plate, allows for a rising of a pair of ridges, the neural folds.
The neural plate rolls up to form the neural tube which becomes the brain and spinal cord. The neural tube lies directly above the notochord, and the spinal cord will lie within extensions of the dorsal surface of the backbone, which replaces the notochord.
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Somites are a series of internal ridges. They are blocks of mesoderm that give rise to segmental structures such as vertebrae and associated muscles of the backbone. The mesoderm next to the somites is developing a hollow space, coelom, which is a basic feature of chordates.
27.13 Multiple processes give form to the developing animal
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Signals pass between cells and cell layers telling embryonic cells what to do when. Induction is the mechanism where one group of cells influences the development of other cells. It may be mediated by diffusible signals or direct cell-surface interactions. Induction plays a major role in the development of virtually all tissues and organs. It switches on a set of genes whose expression makes cells differentiate into specific tissues. It also leads to increasingly greater specialization of cells as organs take shape.
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Cell migration is essential for development. Migrating cells follow chemical trails to their specific destination. Once it reaches that destination, surface proteins allow it to recognize similar cells. The cells join together and secrete a glycoprotein to hold them together. This allows them to differentiate into a particular tissue.
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Programmed cell death, apoptosis, is timely death of cells. Genes code for proteins that kill the cell that produces them. This is critical for normal development of fingers and toes to create the spaces between them. It also occurs in the nervous system and immune system.
27.14 Pattern formation during embryonic development is controlled by ancient genes
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Pattern formation is the emergence of a body form with specialized organs and tissues in the right place.
Master control genes respond to chemical signals that tell a cell where it is relative to other cells. Positional signals determine which master control genes will be expressed and which body parts will form. Positional signals affect the development of vertebrate limbs, which develop from limb buds. For limbs to form, they must get the correct signals identifying position on the embryonic body. Only with this information can it form normally.
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Homeotic genes help direct pattern formation is many organisms. Researchers have found that a gene sequence of 180 genes in every homeotic gene they have studied. They have named these sequences homeoboxes. Each one is translated into a protein product of the homeotic gene. These polypeptides

segments bind to a certain sequence in DNA, enabling homeotic proteins that contains it to turn on or off groups of genes during development.
27.15 The embryo and placenta take shape during the first month
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Pregnancy, gestation, is carrying of developing young within the female reproductive tract. It begins at conception and continues until birth. Gestation varies among species.
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Conception occurs in the oviduct. Cleavage starts about 24 hours after fertilization and the embryo will continue moving down the oviduct to the uterus. By 6-7 days, the embryo has reached the uterus and cleavage has made around 100 cells. The embryo is a hollow ball of cells, the blastocyst. In a human, the blastocyst has a fluid filled cavity, an inner cell mass that becomes the baby, and an outer layer of cells called the trophoblast. The trophoblast secretes enzymes to help the blastocyst implant into the uterus. This occurs about a week after conception.
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Extensions of the trophoblast spread into the endometrium. The trophoblast will eventually form part of the placenta, the organ that provides nourishment and oxygen to the embryo and disposes of metabolic waste.
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Many cells of the trophoblast give rise to 4 structures, the extraembryonic membranes, which help support the embryo. These membranes include the amnion, chorion, yolk sac, and the allantois.
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Gastrulation starts by the 9 th day after conception. The three embryonic layers are already taking form. The embryo develops from the inner cell layers.
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Roles of the extraembryonic membranes – o Amnion – amniotic cavity is filled with fluid to protect the embryo and contains a large mass of yolk. In shelled eggs, the yolk provides nourishment. In mammals, the yolk sack produces the embryo’s first blood cells and germ cells. It will also give rise to the gamete forming cells of the gonads. o Allantois – forms part of the umbilical cord. It will also form part of the urinary bladder. In birds and reptiles, the allantois will expand to function in waste disposal. o Chorion – becomes part of the placental. Chorion secretes human chorionic gonadotropin (HCG) that maintains the production of estrogen and progesterone to maintain the pregnancy. o Placenta – knobby outgrowths of the chorion are the chorionic villi that contain blood vessels formed from mesoderm. The placenta is a composite organ composed of the chorionic villi that are associated with the mother’s blood vessels in the endometrium. The villi absorb nutrients and oxygen from the mother and pass them to the embryo through the chorionic blood vessels. The placenta takes care of the embryonic needs. It allows protective antibodies to pass from mother to child.
27.16 Human development from conception to birth is divided into three trimesters
First trimester – a time of radical change for mother and child.
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In five weeks, the highly organized multicellular embryo has developed from a single cell. The embryo is about 7 mm long, has a notochord and a coelom. The brain and spinal cord has begun to develop. Four limb buds, a short tail, and gill pouches are evident. The gill pouches will develop into the throat and middle ear.
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By 9 weeks, the placenta is attached to the fetus by the umbilical cord and the amnion surrounds the fetus.
It is about 5.5 cm and has all of the organs and major body parts even though the head is disproportionately large. The limbs have become tiny arms and legs, and the muscles and bones of the back and ribs have developed. At this time, the fetus can move its arms and legs, turn its head, frown and make sucking motions with the lips. The heartbeat can be detected with a stethoscope and the gender is evident.
Second trimester – main developmental changes during this time involve an increase in size and refinement of human features.
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By 14 weeks, the fetus is about 6 cm. Placenta begins to secrete progesterone.
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At 20 weeks, the fetus is about 19 cm and weighs about 1 pound. The face is evident and complete with eyebrows and eyelashes. Arms, legs, fingers, and toes have lengthened. Fingernails and toenails are in place and the fetus is quite active. By the end of this trimester, the eyes are open and the teeth are forming.
Third trimester – time of rapid growth to give the fetus the strength it needs to survive outside of the womb
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The fetal circulatory system and respiratory system undergo changes that will allow air breathing. The bones harden and muscles thicken. The fetus gains the ability to maintain its own temperature and the head changes proportion. At birth, babies average 20 inches and 6-8 pounds.

27.17 Childbirth is hormonally induced and occurs in three stages
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Birth is brought about by strong rhythmic contractions of the uterus called labor. Several hormones play a role in this: o Estrogen triggers the formation of oxytoxin receptors on the uterus. Oxytocin stimulates the contractions in the smooth muscle of the uterine wall. It stimulates the placenta to make prostaglandins to result in intense contractions that propel the baby from the womb.
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Labor occurs in three stages: o Dilation – the time it takes for the cervix to reach full dilation, opening. Usually between 6&12 hours. o Expulsion – from full dilation to delivery of the infant. Usually 20-60 minutes. o Placental delivery – usually occurs within 15 minutes after the birth.
27.18 Reproductive technology increases our reproductive options
About 15% of couples are not able to conceive. Reproductive technologies may help many cases of infertility.
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Drug therapy can be used to treat impotence.
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If either the sperm or egg is nonfunctional, a donor cell can be obtained from a bank.
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If the mother is unable to carry her own offspring, a surrogate could be used.
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Assisted reproductive technology (ART) procedures to surgically remove eggs, fertilize them, and return them to the body. The extra embryos can be frozen for later use. In vitro fertilization (IVF) is the most common procedure.