© 2016 Pearson Education, Inc.

28.4 Embryonic Development
Organogenesis Gastrulation sets stage for organogenesis, formation of body organs and systems At week 8, embryo is ~22 m (1 inch) long from crown to rump All organ systems are recognizable Embryo begins as flat plate, but as it grows, it achieves cylindrical body resembling three stacked sheets of paper folding laterally into tube, and at both ends © 2016 Pearson Education, Inc.

Figure 28.9 Folding of the embryonic body, lateral views.
Tail Head Amnion Yolk sac Ectoderm Mesoderm Trilaminar embryonic disc Endoderm Future gut (digestive tube) Lateral fold Somites (seen through ectoderm) Tail fold Head fold Yolk sac Neural tube Notochord Primitive gut Hindgut Yolk sac Foregut © 2016 Pearson Education, Inc.

Organogenesis (cont.) Specialization of the endoderm
Primitive gut is formed from endodermal folding Forms epithelial lining of GI tract Organs of GI tract become apparent, and oral and anal openings perforate Mucosal lining of respiratory tract forms from pharyngeal endoderm (foregut) Glands arise further along tract © 2016 Pearson Education, Inc.

Figure 28.9a Folding of the embryonic body, lateral views.
Tail Head Amnion Yolk sac Ectoderm Mesoderm Trilaminar embryonic disc Endoderm © 2016 Pearson Education, Inc.

Figure 28.9b Folding of the embryonic body, lateral views.
Future gut (digestive tube) Lateral fold © 2016 Pearson Education, Inc.

Figure 28.10 Endodermal differentiation.
Pharynx Parathyroid glands and thymus Thyroid gland Esophagus Trachea Connection to yolk sac Right and left lungs Stomach Liver Umbilical cord Pancreas Gallbladder Small intestine Allantois Large intestine 5-week embryo © 2016 Pearson Education, Inc.

Organogenesis (cont.) Specialization of ectoderm Neurulation
First major event of organogenesis Gives rise to brain and spinal cord Induced by chemical signals from notochord Ectoderm over notochord thickens, forming neural plate Neural plate folds inward as neural groove with neural folds © 2016 Pearson Education, Inc.

Figure 28.9d Folding of the embryonic body, lateral views.
Neural tube Notochord Primitive gut Hindgut Yolk sac Foregut © 2016 Pearson Education, Inc.

Organogenesis (cont.) Specialization of ectoderm (cont.)
By day 22, neural folds fuse into neural tube Anterior end of tube will form brain; rest of tube forms spinal cord Neural crest cells migrate widely to form: Cranial, spinal, and sympathetic ganglia and nerves Adrenal medulla chromaffin cells Pigment cells of skin Contributes to some connective tissues By end of month 1, forebrain, midbrain, and hindbrain are formed Brain waves can be recorded by end of month 2 © 2016 Pearson Education, Inc.

Figure 28.11a Neurulation and early mesodermal differentiation.
Head Amnion 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Amniotic cavity Neural plate Left Right Ectoderm Cut edge of amnion Primitive streak Mesoderm Notochord Tail Endoderm Yolk sac © 2016 Pearson Education, Inc.

Figure 28.11b Neurulation and early mesodermal differentiation.
20 days. The neural folds form by folding of the neural plate, which then deepens, producing the neural groove. Three mesodermal aggregates form on each side of the notochord (somite, intermediate mesoderm, and lateral plate mesoderm). Neural groove Somite Neural fold Intermediate mesoderm Neural crest Lateral plate mesoderm Coelom © 2016 Pearson Education, Inc.

Figure 28.11c Neurulation and early mesodermal differentiation.
22 days. The neural folds have closed, forming the neural tube which has detached from the surface ectoderm and lies between the surface ectoderm and the notochord. Embryonic body is beginning to undercut. Surface ectoderm Neural crest Neural tube Somite Notochord © 2016 Pearson Education, Inc.

Figure 28.11d Neurulation and early mesodermal differentiation.
End of week 4. Embryo undercutting is complete. Somites have subdivided into sclerotome, myotome, and dermatome, which form the vertebrae, skeletal muscles, and dermis respectively. Body coelom present. Neural tube (ectoderm) Dermatome Somite Myotome Epidermis (ectoderm) Sclerotome Gut lining (endoderm) Kidney and gonads (intermediate mesoderm) Lateral plate mesoderm • Limb bud • Smooth muscle of gut • Visceral serosa Peritoneal cavity (coelom) • Parietal serosa • Dermis © 2016 Pearson Education, Inc.

Organogenesis (cont.) Specialization of mesoderm
First evidence of mesodermal differentiation is appearance of notochord Eventually replaced by vertebral column Three aggregates appear laterally of notochord: Somites: 40 pairs with three functional parts Sclerotome cells produce vertebra and rib at each level Dermatome cells form dermis of skin on dorsal part of body Myotome cells form skeletal muscles of neck, trunk, and limbs (via limb buds) © 2016 Pearson Education, Inc.

Figure 28.9c Folding of the embryonic body, lateral views.
Somites (seen through ectoderm) Tail fold Head fold Yolk sac © 2016 Pearson Education, Inc.

Organogenesis (cont.) Intermediate mesoderm Lateral plate mesoderm
Forms gonads and kidneys Lateral plate mesoderm Consists of two plates of cells Somatic mesoderm forms dermis of skin in ventral region, parietal serosa of ventral body cavity, and most tissues of limbs Splanchnic mesoderm forms heart and blood vessels, most connective tissues of body, and entire wall of digestive and respiratory organs © 2016 Pearson Education, Inc.

Lateral plate mesoderm
Figure Flowchart showing major derivatives of the embryonic germ layers. Epiblast ECTODERM MESODERM ENDODERM Notochord Somite Intermediate mesoderm Lateral plate mesoderm Somatic mesoderm Splanchnic mesoderm • Epidermis, hair, nails, glands of skin Nucleus pulposus of intervertebral discs • Sclerotome: vertebrae and ribs • Kidneys • Parietal serosa • Wall of digestive and respiratory tracts (except epithelial lining) Epithelial lining and glands of digestive and respiratory tracts • Gonads • Dermis of ventral body region • Brain and spinal cord • Dermatome: dermis of dorsal body region • Neural crest and derivatives (e.g., cranial, spinal, and sympathetic ganglia and associated nerves; chromaffin cells of the adrenal medulla; pigment cells of the skin) • Connective tissues of limbs (bones, joints, and ligaments) • Visceral serosa • Heart • Myotome: trunk and limb musculature • Blood vessels © 2016 Pearson Education, Inc.

Development of Fetal Circulation
First blood cells arise in yolk sac By end of week 3: Embryo has system of paired vessels Two vessels forming heart have fused and bent into “S” shape Heart beats by 3½ weeks © 2016 Pearson Education, Inc.

Development of Fetal Circulation (cont.)
Unique vascular modifications seen only during prenatal development and are occluded at birth include: Umbilical arteries and umbilical vein Three vascular shunts: Ductus venosus: bypasses liver; umbilical vein drains into ductus venosus which empties into inferior vena cava Foramen ovale: opening in interatrial septum bypasses pulmonary circulation Ductus arteriosus: bypasses pulmonary circulation; pulmonary trunk drains into ductus arteriosus, which drains into aorta © 2016 Pearson Education, Inc.

Figure 28.13a Circulation in fetus and newborn.
Aortic arch Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary artery Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres Inferior vena cava Umbilicus Abdominal aorta Common iliac artery Umbilical arteries Medial umbilical ligaments Urinary bladder Umbilical cord Placenta High oxygenation Moderate oxygenation Low oxygenation Very low oxygenation © 2016 Pearson Education, Inc.

Figure 28.13b Circulation in fetus and newborn.
Aortic arch Newborn Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary artery Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres Inferior vena cava Umbilicus Abdominal aorta Common iliac artery Umbilical arteries Medial umbilical ligaments High oxygenation Urinary bladder Moderate oxygenation Low oxygenation Very low oxygenation © 2016 Pearson Education, Inc.

Figure 28.13 Circulation in fetus and newborn.
Aortic arch Newborn Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary artery Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres Inferior vena cava Umbilicus Abdominal aorta Common iliac artery Umbilical arteries Medial umbilical ligaments Urinary bladder Umbilical cord Placenta High oxygenation Moderate oxygenation Low oxygenation Very low oxygenation © 2016 Pearson Education, Inc.

Events of Fetal Development
At end of embryonic period: weeks 1–8 Bones have begun to ossify, skeletal muscles are well formed and contracting, metanephric kidneys are developing, and gonads are formed Lungs, digestive organs are attaining final shape and body position Blood delivery to/from placenta is constant and efficient Heart and liver bulge on ventral surface All this happening in a embryo that is less than 1 inch from crown to rump and weighs 2 g (0.06 ounces) © 2016 Pearson Education, Inc.

Events of Fetal Development (cont.)
Fetal period: weeks 9 through 38 Time of rapid growth of body structures established in embryo Fetus grows to 360 mm (14 inches) and 3.2 kg (7 lbs +) Changes in fetal appearance are dramatic Greatest amount of growth occurs in first 8 weeks, when embryo grows from one cell to 1-inch-long fetus © 2016 Pearson Education, Inc.

Figure 28.14a Photographs of a developing fetus.
Amniotic sac Umbilical cord Umbilical vein Chorionic villi Yolk sac Cut edge of chorion Embryo at week 7, about 17 mm long. © 2016 Pearson Education, Inc.

Figure 28.14b Photographs of a developing fetus.
Fetus in month 3, about 6 cm long. © 2016 Pearson Education, Inc.

Figure 28.14c Photographs of a developing fetus.
Fetus late in month 5, about 19 cm long. © 2016 Pearson Education, Inc.

Table 28.1 Developmental Events of the Fetal Period
© 2016 Pearson Education, Inc.

Table 28.1 Developmental Events of the Fetal Period (continued)
© 2016 Pearson Education, Inc.

Effects of Pregnancy on the Mother: Anatomical Changes
Reproductive organs become engorged with blood Chadwick’s sign: vagina develops purplish hue Breasts enlarge, and areolae darken Pigmentation of facial skin may increase (chloasma) © 2016 Pearson Education, Inc.

28.5 Effects of Pregnancy on Mother
Anatomical Changes Uterus expands, occupying most of abdominal cavity Ribs flare, and thorax widens Lordosis occurs with change in center of gravity Relaxin causes pelvic ligaments and pubic symphysis to relax to ease birth passage Weight gain of ~13 kg (28 lb) is usually seen © 2016 Pearson Education, Inc.

Anatomical Changes (cont.)
Good nutrition is vital 300 additional daily calories are required Multivitamins with folic acid reduce fetal risk of neurological problems, such as spina bifida, anencephaly, and spontaneous preterm birth © 2016 Pearson Education, Inc.

(Uterus the size of a fist and resides in the pelvis.) 4 months
Figure Relative size of the uterus before conception and during pregnancy. Before conception (Uterus the size of a fist and resides in the pelvis.) 4 months (Fundus of the uterus is halfway between the pubic symphysis and the umbilicus.) 7 months (Fundus is well above the umbilicus.) 9 months (Fundus reaches the xiphoid process.) © 2016 Pearson Education, Inc.

Metabolic Changes Placental hormone human placental lactogen (hPL), also called human chorionic somatomammotropin (hCS) Stimulates maturation of breasts, fetal growth, and glucose sparing in mother (reserving glucose for fetus; may cause gestational diabetes mellitus) Parathyroid hormone and vitamin D levels stay high throughout pregnancy to ensure adequate calcium for fetal bone mineralization © 2016 Pearson Education, Inc.

Physiological Changes
Gastrointestinal system Morning sickness is believed to be due to elevated levels of hCG, estrogen, and progesterone Heartburn and constipation are common Urinary system Increased urine production is due to increased maternal metabolism and fetal wastes Frequent, urgent urination and stress incontinence may occur as bladder is compressed © 2016 Pearson Education, Inc.

Physiological Changes (cont.)
Respiratory system Estrogens may cause nasal edema and congestion Tidal volume increases, and dyspnea (difficult breathing) may occur later in pregnancy © 2016 Pearson Education, Inc.

Physiological Changes (cont.)
Cardiovascular system Blood volume increases 25–40% to safeguard against blood loss during childbirth Cardiac output rises as much as 35-40% to propel greater volume around body Venous return from lower limbs may be impaired, resulting in varicose veins © 2016 Pearson Education, Inc.

Clinical – Homeostatic Imbalance 28.1
Teratogens: factors that may cause severe congenital abnormalities or even fetal death if potentially harmful substances cross placental barriers and enter fetal blood Pregnant women who drink alcohol cause fetus to also become inebriated, but fetal consequences may be much longer lasting and result in fetal alcohol syndrome (FAS) Typified by microcephaly (small head), intellectual disability, and abnormal growth © 2016 Pearson Education, Inc.

Nicotine hinders oxygen delivery to the fetus, impairing normal growth and development Many drugs (anticoagulants, antihypertensives, sedatives, and others), as well as maternal infections, such as rubella (German measles) can also cross placenta © 2016 Pearson Education, Inc.

Preeclampsia Dangerous complication of pregnancy that can result in deterioration of placenta and insufficient placental blood supply Can lead to fetus being starved of oxygen Woman becomes edematous and hypertensive, and proteinuria occurs May be due to immunological abnormalities Affects 1 in 10 pregnant women © 2016 Pearson Education, Inc.

Initiation of Labor Fetus determines own birth date
During last few weeks of pregnancy, estrogens reach their highest level in mother’s blood Fetal secretion of cortisol stimulates placenta to secrete more estrogen © 2016 Pearson Education, Inc.

Initiation of Labor (cont.)
Weeks before delivery, fetal lungs produce surfactant protein A (SP-A) May trigger inflammatory response in cervix, causing it to soften in preparation for labor Increased estrogen has three effects Causes production of oxytocin receptors by myometrium Causes formation of gap junctions between uterine smooth muscle cells Antagonizes calming effects of progesterone, leading to Braxton Hicks contractions in uterus Also called false labor © 2016 Pearson Education, Inc.

Fetal oxytocin causes placenta to produce prostaglandins Prostaglandins also stimulate synthesis of gap junctions in uterine smooth muscle Oxytocin and prostaglandins are powerful uterine muscle stimulants Prostaglandins initiate contractions, and oxytocin makes them more frequent and vigorous Prostaglandins also help thin and soften cervix © 2016 Pearson Education, Inc.

Increasing cervical distension: Activates hypothalamus, causing oxytocin release from posterior pituitary Positive feedback mechanism occurs Greater distension of cervix causes more oxytocin release, causing greater contractile force, leading to greater distension of cervix, causing release of more oxytocin © 2016 Pearson Education, Inc.

Figure 28.16 Hormonal induction of labor.
Start Estrogens Oxytocin (+) from placenta from fetus and mother’s posterior pituitary Induce oxytocin receptors on uterus Positive feedback Stimulates uterus to contract Stimulates placenta to release (+) Prostaglandins Stimulate more vigorous contractions of uterus © 2016 Pearson Education, Inc.

Stages of Labor Three stages Dilation stage
Lasts from labor’s onset to fully dilated cervix (10 cm in diameter) Longest stage of labor: 6–12 hours or more Initial weak contractions: 15–30 minutes apart, 10–30 seconds long Become more vigorous and rapid Cervix effaces and dilates fully to 10 cm Amnion ruptures, releasing amniotic fluid Engagement occurs: head enters true pelvis © 2016 Pearson Education, Inc.

Early dilation. Baby’s head engaged; widest dimension is
Figure Parturition. Early dilation. Baby’s head engaged; widest dimension is along left-right axis. 1a Umbilical cord Placenta Uterus Cervix Vagina © 2016 Pearson Education, Inc.

Late dilation. Baby’s head rotates so widest dimension is in
Figure Parturition. Late dilation. Baby’s head rotates so widest dimension is in anteroposterior axis (of pelvic outlet). Dilation nearly complete. 1b Pubic symphysis Sacrum © 2016 Pearson Education, Inc.

Stages of Labor (cont.) Expulsion stage
Lasts from full dilation to delivery of infant Strong contractions every 2–3 minutes, each about 1 minute long Urge to push increases (in absence of local anesthesia) Crowning occurs when largest dimension of head distends vulva Episiotomy, incision made to widen vaginal orifice, may be done to reduce tearing © 2016 Pearson Education, Inc.

Stages of Labor (cont.) Expulsion stage (cont.)
Baby’s neck extends as head exits perineum Once head has been delivered, rest of body is delivered more easily Vertex position: usual, head-first presentation Skull dilates cervix Early suctioning allows breathing prior to complete delivery Breech position: buttock-first Delivery is more difficult; often forceps or C-section (delivery through abdominal and uterine wall incision) is required © 2016 Pearson Education, Inc.

Expulsion. Baby’s head extends as it is delivered. Perineum 2
Figure Parturition. 2 Expulsion. Baby’s head extends as it is delivered. Perineum © 2016 Pearson Education, Inc.

Stages of Labor (cont.) Placental stage
Delivery of afterbirth (placenta and membranes) occurs within 30 minutes after birth Strong contractions continue, causing detachment of placenta and compression of uterine blood vessels Contractions limit bleeding and shear placental from uterine wall, causing detachment All placenta fragments must be removed to prevent postpartum bleeding © 2016 Pearson Education, Inc.

After baby is delivered, the placenta detaches and is removed.
Figure Parturition. Placental stage. After baby is delivered, the placenta detaches and is removed. 3 Uterus Placenta (detaching) Umbilical cord © 2016 Pearson Education, Inc.

Dystocia: if a woman has a deformed or narrow male-like pelvis, labor can be prolonged and difficult Can possibly result in maternal fatigue and/or fetal brain damage (example: cerebral palsy or epilepsy) Treatment: Cesarean (C-) section is performed © 2016 Pearson Education, Inc.

28.7 Extrauterine Life of Infant
Neonatal period: 4 week period immediately after birth Birth is a shock to baby, so physical status is assessed within 1–5 minutes after birth by Apgar score; 0–2 points each for: Heart rate Muscle tone Respiration Reflexes Color Score of 8–10 means healthy baby © 2016 Pearson Education, Inc.

Taking First Breath and Transition
Once CO2 is no longer removed by placenta, levels rise, causing central acidosis Stimulates respiratory control centers to trigger first inspiration Requires tremendous effort, as airways are tiny and lungs are collapsed Surfactant in alveolar fluid helps reduce surface tension Respiratory rate is ~45 per minute first 2 weeks, then declines © 2016 Pearson Education, Inc.

Taking First Breath and Transition (cont.)
Keeping lungs inflated is difficult for premature infants (< 2500 g, or 5.5 pounds, at birth) Surfactant production occurs in last months of prenatal life, so preemies usually require respiratory assistance until lungs mature © 2016 Pearson Education, Inc.

Taking First Breath and Transition (cont.)
Transitional period: unstable period lasting 6–8 hours after birth Alternating periods of activity and sleep; vital signs may be irregular during activity Baby gags frequently as it regurgitates mucus and debris Usually stabilizes, with waking periods occurring every 3–4 hours © 2016 Pearson Education, Inc.

Occlusion of Special Fetal Blood Vessels and Vascular Shunts
In newborn, umbilical arteries and vein constrict and become fibrosed Proximal umbilical arteries persist as superior vesical arteries to urinary bladder Distal umbilical arteries become medial umbilical ligaments Umbilical vein becomes round ligament of liver (ligamentum teres) © 2016 Pearson Education, Inc.

Occlusion of Special Fetal Blood Vessels and Vascular Shunts (cont.)
Ductus venosus is converted to ligamentum venosum Pressure changes from infant breathing cause pulmonary shunts to close Foramen ovale becomes fossa ovalis up to a year after birth Ductus arteriosus becomes ligamentum arteriosum © 2016 Pearson Education, Inc.

Occlusion of Special Fetal Blood Vessels and Vascular Shunts (cont.)
Except for foramen ovale, which can take up to a year to close, all shunts are usually closed within 30 minutes after birth Failure of ductus arteriosus or foramen ovale to close leads to congenital heart defects © 2016 Pearson Education, Inc.

28.8 Lactation Production of milk by mammary glands
Toward end of pregnancy, hypothalamus is stimulated to release prolactin-releasing factors (PRFs) by: Placental estrogens Progesterone Human placental lactogen PRFs stimulate anterior pituitary to release prolactin 2–3 days later, true milk production begins © 2016 Pearson Education, Inc.

28.8 Lactation During 2–3 day delay until true milk comes in, mammaries secrete colostrum Yellowish fluid that has less lactose but more protein, vitamin A, and minerals than true milk, as well as almost no fat Also rich in IgA antibodies IgA is resistant to digestion, so may protect infant against bacterial infection Some may be absorbed into bloodstream to provide broader immunity © 2016 Pearson Education, Inc.

28.8 Lactation Prolactin release wanes after birth, so lactation is sustained by stimulation of nipples during suckling Suckling stimulates mechanoreceptors in nipple, causing afferent impulses to be sent to hypothalamus, triggering release of PRFs Anterior pituitary increases release of prolactin that stimulates milk production for next feeding © 2016 Pearson Education, Inc.

28.8 Lactation Hypothalamus is also prompted by suckling to trigger release of oxytocin from posterior pituitary Oxytocin causes let-down reflex, which is actual ejection of milk from alveoli of mammary gland © 2016 Pearson Education, Inc.

Hypothalamus releases prolactin- releasing factors (PRFs)
Figure Milk production and the positive feedback mechanism of the milk let-down reflex. Hypothalamus releases prolactin- releasing factors (PRFs) to portal circulation. Start Stimulation of mechanoreceptors in nipples by suckling infant sends afferent impulses to the hypothalamus. Hypothalamus sends efferent impulses to the posterior pituitary where oxytocin is stored. Anterior pituitary secretes prolactin to blood. Oxytocin is released from the posterior pituitary and stimulates myoepithelial cells of breasts to contract. Prolactin targets mammary glands of breasts. Positive feedback Milk production Let-down reflex. Milk is ejected through ducts of nipples. © 2016 Pearson Education, Inc.

28.8 Lactation Advantages of breast milk for infant
Fats and iron are better absorbed and breast milk’s amino acids are more easily metabolized compared with cow’s milk Has a host of beneficial chemicals, including: IgA, complement, lysozyme, interferon, and lactoperoxidase (protect infant from infections) Interleukins and prostaglandins (prevent overzealous inflammatory responses) Glycoprotein, which deters ulcer-causing bacterium (Helicobacter pylori) from attaching to stomach mucosa © 2016 Pearson Education, Inc.

28.8 Lactation Other advantages of breast milk
Encourages bacterial colonization of infant’s gut Has natural laxative effect that helps cleanse infant’s bowels of meconium, a tarry green- black paste containing sloughed off cells, bile, and other substances Eliminating meconium as soon as possible helps prevent physiological jaundice Women nursing >6 months can lose bone calcium; replaced after weaning if diet is healthy Women may ovulate when nursing despite inhibition of GnRH and gonadotropins © 2016 Pearson Education, Inc.

28.9 Assisted Reproductive Technology
Various technologies and processes may help an infertile couple’s ability to have offspring Hormone therapies can increase sperm and egg production Surgery can open blocked uterine tubes © 2016 Pearson Education, Inc.

Assisted reproductive technology (ART) procedures involve: Surgical removal of oocytes following hormone stimulation, fertilizing oocytes, then returning fertilized oocytes to woman’s body Disadvantages of ART procedures: costly, emotionally draining, and painful for oocyte donor © 2016 Pearson Education, Inc.

Examples of ART In vitro fertilization (IVF): most common ART Oocytes and sperm are incubated in culture dishes for several days Embryos at 2-cell to blastocyst stage are then transferred to uterus in hopes of implantation Zygote intrafallopian transfer (ZIFT) In vitro fertilized oocytes are transferred to uterine tubes, so natural implantation can occur Gamete intrafallopian transfer (GIFT) Does not use in vitro fertilization; sperm and oocytes are transferred together into uterine tubes so natural fertilization to take place © 2016 Pearson Education, Inc.

Cloning Entails insertion of somatic cell nucleus into an oocyte from which nucleus is removed, followed by incubation period for dedifferentiation to occur Legal, moral, ethical, political roadblocks Has proved more successful in creating stem cells for therapeutic use than for reproductive cloning © 2016 Pearson Education, Inc.

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