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Chapter Opener 28 © 2013 Pearson Education, Inc.
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Embryo Fertilization 1-week conceptus 3-week embryo (3 mm) 5-week
Figure Diagrams showing the approximate size of a human conceptus from fertilization to the early fetal stage. Embryo Fertilization 1-week conceptus 3-week embryo (3 mm) 5-week embryo (10 mm) 8-week embryo (22 mm) 12-week fetus (90 mm) © 2013 Pearson Education, Inc.
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Figure Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical reaction prevents further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized ovum. Fusion. The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 4 5 Block of polyspermy. Entry of the sperm’s contents causes Ca2+ levels in the oocyte’s cytoplasm to rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucida hardens and the sperm receptors are clipped off (slow block to polyspermy). 2 Acrosomal reaction. Binding of the sperm to sperm-binding receptors in the zona pellucida causes the Ca2+ levels within the sperm to rise, triggering the acrosomal reaction. Acrosomal enzymes from many sperm digest holes through the zona pellucida, clearing a path to the oocyte membrane. 3 Binding. The sperm’s membrane binds to the oocyte’s Sperm-binding receptors. Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary oocyte. 1 Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulosa cells of the corona radiata. Extracellular space Sperm Sperm Zona pellucida Polar body Granulosa cells of corona radiata Oocyte nucleus arrested in meiotic metaphase II Oocyte sperm- binding membrane receptors Microtubules from sperm flagellum Cortical granules Zona pellucida Sperm-binding receptors Mitochondria Sperm nucleus Zona pellucida Extracellular space Oocyte plasma membrane © 2013 Pearson Education, Inc.
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Oocyte plasma membrane
Figure Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical reaction prevents further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized ovum. (1 of 2) Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulosa cells of the corona radiata. 1 Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary oocyte. Extracellular space Sperm Sperm Zona pellucida Polar body Granulosa cells of corona radiata Oocyte nucleus arrested in Meiotic metaphase II Zona pellucida Extracellular space Oocyte plasma membrane © 2013 Pearson Education, Inc.
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Oocyte sperm-binding membrane receptors
Figure Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical reaction prevents further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized ovum. (2 of 2) Fusion. The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 4 Block of polyspermy. Entry of the sperm’s contents causes Ca2+ levels in the oocyte’s cytoplasm to rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucida hardens and the sperm receptors are clipped off (slow block to polyspermy). 5 Acrosomal reaction. Binding of the sperm to sperm- binding receptors in the zona pellucida causes the Ca2+ levels within the sperm to rise, triggering the acrosomal reaction. Acrosomal enzymes from many sperm digest holes through the zona pellucida, clearing a path to the oocyte membrane. 2 Binding. The sperm’s membrane binds to the oocyte’s sperm-binding receptors. 3 Microtubules from sperm flagellum Oocyte sperm-binding membrane receptors Cortical granules Zona pellucida Sperm-binding receptors Mitochondria Sperm nucleus © 2013 Pearson Education, Inc.
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Figure 28.3 Events of fertilization.
Extracellular space Sperm nucleus Corona radiata After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. 1 Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body Male pro-nucleus Sperm and ovum nuclei swell, forming pronuclei. 2 Female pro-nucleus (swollen ovum nucleus) Polar bodies Male pronucleus 3 Pronuclei approach each other and mitotic spindle forms between them. Mitotic spindle Centriole Female pronucleus 4 Chromosomes of the pronuclei intermix. Fertilization is accomplished. Then, the DNA replicates in preparation for the first cleavage division. Zygote Male and female pronuclei Polar bodies © 2013 Pearson Education, Inc.
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Figure 28.3a Events of fertilization. (1 of 4)
Sperm nucleus Extracellular space 1 After the sperm penetrates the secondary oocyte, The oocyte Completes meiosis II, forming the ovum and second polar body. Corona radiata Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body © 2013 Pearson Education, Inc.
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Figure 28.3a Events of fertilization. (2 of 4)
Male pro- nucleus 2 Sperm and ovum nuclei swell, forming pronuclei. Female pro- nucleus (swollen ovum nucleus) Polar bodies © 2013 Pearson Education, Inc.
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Figure 28.3a Events of fertilization. (3 of 4)
Male pronucleus 3 Pronuclei approach each other and mitotic spindle forms between them. Mitotic spindle Centriole Female pronucleus © 2013 Pearson Education, Inc.
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Figure 28.3a Events of fertilization. (4 of 4)
Chromoomes of the pronuclei intermix. Fertilization is accomplished. Then, the DNA replicates in preparation for the first cleavage division. Zygote © 2013 Pearson Education, Inc.
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Figure 28.3b Events of fertilization.
Male and female pronuclei Polar bodies © 2013 Pearson Education, Inc.
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Figure 28.4 Cleavage: From zygote to blastocyst.
(fertilized egg) 4-cell stage 2 days Morula (a solid ball of blastomeres). 3 days Early blastocyst (Morula hollows out, fills with fluid, and “hatches” from the zona pellucida). 4 days Zona pellucida Degenerating zona pellucida Implanting blastocyst (Consists of a sphere of trophoblast cells and an eccentric cell cluster called the inner cell mass). 7 days Sperm Blastocyst cavity Uterine tube Fertilization (sperm meets and enters egg) Ovary Oocyte (egg) Trophoblast Uterus Blastocyst cavity Ovulation Endometrium Inner cell mass Cavity of uterus © 2013 Pearson Education, Inc.
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Figure 28.4a Cleavage: From zygote to blastocyst.
Zygote (fertilized egg) Zona pellucida © 2013 Pearson Education, Inc.
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Figure 28.4b Cleavage: From zygote to blastocyst.
4-cell stage 2 days © 2013 Pearson Education, Inc.
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Figure 28.4c Cleavage: From zygote to blastocyst.
Morula (a solid ball of blastomeres). 3 days © 2013 Pearson Education, Inc.
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Figure 28.4d Cleavage: From zygote to blastocyst.
Early blastocyst (Morula hollows out, fills with fluid, and “hatches” from the zona pellucida). 4 days Degenerating zona pellucida Blastocyst cavity © 2013 Pearson Education, Inc.
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Figure 28.4e Cleavage: From zygote to blastocyst.
Implanting blastocyst (Consists of a sphere of tropho- blast cells and an eccentric cell cluster called the inner cell mass). 7 days Inner cell mass Trophoblast Blastocyst cavity © 2013 Pearson Education, Inc.
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Figure 28.5 Implantation of the blastocyst.
Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity Lumen of uterus Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Blastocyst cavity Lumen of uterus © 2013 Pearson Education, Inc.
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Figure 28.5a Implantation of the blastocyst.
Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity Lumen of uterus © 2013 Pearson Education, Inc.
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Figure 28.5b Implantation of the blastocyst.
Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast © 2013 Pearson Education, Inc.
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Figure 28.5c Implantation of the blastocyst.
Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Blastocyst cavity Lumen of uterus © 2013 Pearson Education, Inc.
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Figure 28.5d Implantation of the blastocyst.
Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Lumen of uterus © 2013 Pearson Education, Inc.
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Figure 28.6 Hormonal changes during pregnancy.
Human chorionic gonadotropin Estrogens Relative blood levels Progesterone 4 8 12 16 20 24 28 32 36 Gestation (weeks) Ovulation and fertilization Birth © 2013 Pearson Education, Inc.
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Figure Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Amniotic cavity Lacuna (intervillous space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Maternal blood vessels Chorion • Mesoderm Proliferating syncytiotrophoblast Amnion • Endoderm Cytotrophoblast Forming umbilical cord Amniotic cavity Yolk sac Allantois Bilayered embryonic disc • Epiblast • Hypoblast Extraembryonic mesoderm Endometrial epithelium Lumen of uterus Chorion being formed Extraembryonic coelom Implanting 71/2 -day blastocyst. The syncytiotrophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac, and an allantois, which forms the basis of the umbilical cord. Placenta Decidua basalis Decidua basalis Maternal blood Chorionic villi Chorionic villus Yolk sac Umbilical blood vessels in umbilical cord Amnion Amniotic cavity Amnion Amniotic cavity Yolk sac Extraembryonic coelom Umbilical cord Chorion Lumen of uterus Uterus Decidua capsularis Decidua capsularis Lumen of uterus Extraembryonic coelom 41/2 -week embryo. The decidua capsularis, decidua basalis, amnion, and yolk sac are well formed. The chorionic villi lie in blood-filled intervillous spaces within the endometrium. The embryo is nourished via the umbilical vessels that connect it (through the umbilical cord) to the placenta. 13-week fetus. © 2013 Pearson Education, Inc.
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Figure 28.7a–c Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Amniotic cavity Lacuna (intervillous space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Maternal blood vessels Chorion • Mesoderm Proliferating syncytiotrophoblast Amnion • Endoderm Forming umbilical cord Cytotrophoblast Amniotic cavity Yolk sac Allantois Bilayered embryonic disc • Epiblast • Hypoblast Extraembryonic mesoderm Endometrial epithelium Lumen of uterus Chorion being formed Extraembryonic coelom Implanting 71/2 -day blastocyst. The syncytiotrophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac, and an allantois, which forms the basis of the umbilical cord. © 2013 Pearson Education, Inc.
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vessels that connect it (through the umbilical cord) to the placenta.
Figure 28.7d Events of placentation, early embryonic development, and extraembryonic membrane formation. Decidua basalis Maternal blood Chorionic villus Umbilical blood vessels in umbilical cord Amnion Amniotic cavity Yolk sac Extraembryonic coelom Chorion Lumen of uterus Decidua capsularis 41/2 -week embryo. The decidua capsularis, decidua basalis, amnion, and yolk sac are well formed. The chorionic villi lie in blood-filled intervillous spaces within the endometrium. The embryo is nourished via the umbilical vessels that connect it (through the umbilical cord) to the placenta. © 2013 Pearson Education, Inc.
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Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
Figure 28.7e Events of placentation, early embryonic development, and extraembryonic membrane formation. Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic cavity Umbilical cord Uterus Decidua capsularis Lumen of uterus Extraembryonic coelom 13-week fetus. © 2013 Pearson Education, Inc.
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Placenta Decidua Maternal Maternal basalis arteries veins Chorionic
Figure Detailed anatomy of the vascular relationships in the mature decidua basalis. Placenta Decidua basalis Maternal arteries Maternal veins Chorionic villi Umbilical cord Myometrium Stratum basalis of endometrium Uterus Lumen of uterus Maternal portion of placenta (decidua basalis) Decidua capsularis Chorionic villus containing fetal capillaries Maternal blood in lacuna (intervillous space) Fetal portion of placenta (chorion) Fetal arteriole Umbilical arteries Fetal venule Umbilical vein Amnion Connection to yolk sac Umbilical cord © 2013 Pearson Education, Inc.
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Figure 28.9 Formation of the three primary germ layers.
Amnion Bilayered embryonic disc Head end of bilayered embryonic disc Yolk sac Frontal section 3-D view Section view in (e) Primitive streak Head end Epiblast Cut edge of amnion Yolk sac (cut edge) 14-15 days Hypoblast Right Endoderm Left Ectoderm Primitive streak Tail end Bilayered embryonic disc, superior view 16 days Mesoderm Endoderm © 2013 Pearson Education, Inc.
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Figure 28.10 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 © 2013 Pearson Education, Inc.
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Figure 28.10a Folding of the embryonic body, lateral views.
Tail Head Amnion Yolk sac Ectoderm Mesoderm Trilaminar embryonic disc Endoderm © 2013 Pearson Education, Inc.
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Figure 28.10b Folding of the embryonic body, lateral views.
Future gut (digestive tube) Lateral fold © 2013 Pearson Education, Inc.
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Figure 28.10c Folding of the embryonic body, lateral views.
Somites (seen through ectoderm) Tail fold Head fold Yolk sac © 2013 Pearson Education, Inc.
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Figure 28.10d Folding of the embryonic body, lateral views.
Neural tube Notochord Primitive gut Hindgut Foregut Yolk sac © 2013 Pearson Education, Inc.
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Figure 28.11 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 © 2013 Pearson Education, Inc.
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Figure 28.12 Neurulation and early mesodermal differentiation.
Head Amnion Amniotic cavity Left Right Neural plate Cut edge of amnion Primitive streak Ectoderm 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Tail Mesoderm Notochord Endoderm Yolk sac Neural groove Somite Neural crest Neural fold Intermediate mesoderm 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). Lateral plate mesoderm Coelom Surface ectoderm Neural crest 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. Neural tube Somite Notochord Neural tube (ectoderm) Dermatome Myotome Sclerotome Somite Epidermis (ectoderm) Gut lining (endoderm) 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. Kidney and gonads (intermediate mesoderm) Lateral plate mesoderm Limb bud Smooth muscle of gut Visceral serosa Peritoneal cavity (coelom) Parietal serosa Dermis © 2013 Pearson Education, Inc.
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Figure 28.12a Neurulation and early mesodermal differentiation.
Head Amnion Amniotic cavity Left Right Neural plate Cut edge of amnion Primitive streak Ectoderm 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Tail Mesoderm Notochord Endoderm Yolk sac © 2013 Pearson Education, Inc.
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Figure 28.12b Neurulation and early mesodermal differentiation.
Neural groove Somite Neural fold Neural crest Intermediate mesoderm 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). Lateral plate mesoderm Coelom © 2013 Pearson Education, Inc.
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Figure 28.12c Neurulation and early mesodermal differentiation.
Surface ectoderm Neural crest 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. Neural tube Somite Notochord © 2013 Pearson Education, Inc.
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Figure 28.12d Neurulation and early mesodermal differentiation.
Neural tube (ectoderm) Dermatome Myotome Sclerotome Epidermis (ectoderm) Somite Gut lining (endoderm) 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. Kidney and gonads (intermediate mesoderm) Lateral plate mesoderm Limb bud Smooth muscle of gut Visceral serosa Peritoneal cavity (coelom) Parietal serosa Dermis © 2013 Pearson Education, Inc.
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Figure 28.13 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 • Connective tissues of limbs (bones, joints, and ligaments) • 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) • Visceral serosa • Myotome: trunk and limb musculature • Heart • Blood vessels © 2013 Pearson Education, Inc.
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Figure 28.14 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 © 2013 Pearson Education, Inc.
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Figure 28.14a 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 © 2013 Pearson Education, Inc. Very low oxygenation
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Figure 28.14b 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 High oxygenation Moderate oxygenation Low oxygenation Very low oxygenation © 2013 Pearson Education, Inc.
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Figure 28.15 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. Fetus in month 3, about 6 cm long. Fetus late in month 5, about 19 cm long. © 2013 Pearson Education, Inc.
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Figure 28.15a 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. © 2013 Pearson Education, Inc.
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Figure 28.15b Photographs of a developing fetus.
Fetus in month 3, about 6 cm long. © 2013 Pearson Education, Inc.
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Figure 28.15c Photographs of a developing fetus.
Fetus late in month 5, about 19 cm long. © 2013 Pearson Education, Inc.
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(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.) © 2013 Pearson Education, Inc.
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Table 28.1 Developmental Events of the Fetal Period (1 of 3)
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Table 28.1 Developmental Events of the Fetal Period (2 of 3)
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Table 28.1 Developmental Events of the Fetal Period (3 of 3)
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Figure 28.17 Hormonal induction of labor.
Start Estrogen Oxytocin (+) from placenta from fetus and mother's posterior pituitary Induces oxytocin receptors on uterus Positive feedback Stimulates uterus to contract Stimulates placenta to release (+) Prostaglandins Stimulate more vigorous contractions of uterus © 2013 Pearson Education, Inc.
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Umbilical cord Placenta Uterus Cervix Vagina Sacrum Perineum Uterus
Figure Parturition. Umbilical cord 1a Placenta Early dilation. Baby’s head engaged; widest dimension Is along left-right axis. Uterus Cervix Vagina 1b Late dilation. Baby’s head rotates so widest dimension is in anteroposterior axis (of pelvic outlet). Dilation nearly complete Pubic symphysis Sacrum 2 Expulsion. Baby’s head extends as it is delivered Perineum 3 Placental stage. After baby is delivered, the placenta detaches and is removed. Uterus Placenta (detaching) Umbilical cord © 2013 Pearson Education, Inc.
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Figure 28.18 Parturition. (1 of 4)
Early dilation. Baby’s head engaged; widest dimension Is along left-right axis. Umbilical cord Placenta Uterus Cervix Vagina © 2013 Pearson Education, Inc.
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Figure 28.18 Parturition. (2 of 4)
Late dilation. Baby’s head rotates so widest dimension is in anteroposterior axis (of pelvic outlet). Dilation nearly complete 1b Pubic symphysis Sacrum © 2013 Pearson Education, Inc.
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Figure 28.18 Parturition. (3 of 4)
Expulsion. Baby’s head extends as it is delivered 2 Perineum © 2013 Pearson Education, Inc.
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Figure 28.18 Parturition. (4 of 4)
Uterus Placental stage. After baby is delivered, the placenta detaches and is removed 3 Placenta (detaching) Umbilical cord © 2013 Pearson Education, Inc.
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Hypothalamus releases prolactin releasing factors (PRF)
Figure Milk production and the positive feedback mechanism of the milk let-down reflex. Hypothalamus releases prolactin releasing factors (PRF) 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. © 2013 Pearson Education, Inc.
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Closer Look 28.1 © 2013 Pearson Education, Inc.
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