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Pregnancy and Human Development: Part A
28 Pregnancy and Human Development: Part A
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Pregnancy - events that occur from fertilization until infant born
Conceptus - developing offspring Gestation period - time from last menstrual period until birth (~280 days) Embryo - conceptus from fertilization through week 8 Fetus - conceptus from week 9 through birth © 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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Oocyte viable for 12 to 24 hours
From Egg to Zygote Oocyte viable for 12 to 24 hours Sperm viable 24 to 48 hours after ejaculation © 2013 Pearson Education, Inc.
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For fertilization to occur, coitus must occur no more than
From Egg to Zygote For fertilization to occur, coitus must occur no more than Two days before ovulation 24 hours after ovulation Fertilization - sperm's chromosomes combine with those of secondary oocyte to form fertilized egg (zygote) © 2013 Pearson Education, Inc.
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Accomplishing Fertilization
Ejaculated sperm Leak out of vagina immediately after deposition Destroyed by acidic vaginal environment Fail to make it through cervix Dispersed in uterine cavity or destroyed by phagocytes Few (100 to a few thousand) reach uterine tubes © 2013 Pearson Education, Inc.
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Accomplishing Fertilization
Sperm must become motile Sperm must be capacitated before they can penetrate oocyte Motility must be enhanced; membranes must become fragile to release hydrolytic enzymes Secretions of female tract weaken acrosome membrane Sperm follow "olfactory trail" to reach oocyte © 2013 Pearson Education, Inc.
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Acrosomal Reaction and Sperm Penetration
Sperm must breach oocyte coverings Corona radiata and zona pellucida Sperm weaves through corona radiata, then binds to zona pellucida and undergoes acrosomal reaction Enzymes released to digest holes in zona pellucida Hundreds of acrosomes release enzymes to digest zona pellucida © 2013 Pearson Education, Inc.
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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 1 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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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 2 Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary oocyte. Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulosa cells of the corona radiata. 1 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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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 3 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 Zona pellucida Sperm-binding receptors © 2013 Pearson Education, Inc.
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Acrosomal Reaction and Sperm Penetration
Sperm head approaches oocyte Rear portion of acrosomal membrane binds to oocyte plasma membrane Oocyte and sperm membranes fuse Gametes fuse as sperm's cytoplasmic contents enter oocyte Only one sperm allowed to penetrate oocyte (monospermy) © 2013 Pearson Education, Inc.
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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 4 Binding. The sperm’s membrane binds to the oocyte’s Sperm-binding receptors. 3 Oocyte sperm-binding Membrane receptors © 2013 Pearson Education, Inc.
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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 5 Fusion. The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 4 Cortical granules © 2013 Pearson Education, Inc.
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Upon entry of sperm, Ca2+ surge from ER causes cortical reaction
Block to Polyspermy Upon entry of sperm, Ca2+ surge from ER causes cortical reaction Cortical granules release enzymes (zonal inhibiting proteins, or ZIPs) ZIPs destroy sperm receptors Spilled fluid binds water and swells, detaching other sperm (slow block to polyspermy) © 2013 Pearson Education, Inc.
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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 6 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 Microtubules from sperm flagellum Mitochondria Sperm nucleus © 2013 Pearson Education, Inc.
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Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 7 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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Completion of Meiosis II and Fertilization
As sperm nucleus moves toward oocyte nucleus it swells to form male pronucleus The Ca2+ surge triggers completion of meiosis II ovum + second polar body Ovum nucleus swells to become female pronucleus Fertilization – moment when membranes of two pronuclei rupture and chromosomes combine © 2013 Pearson Education, Inc.
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Figure 28.3a Events of fertilization.
Sperm nucleus Slide 1 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 Male pronucleus Female pro-nucleus (swollen ovum nucleus) 2 Sperm and ovum nuclei swell, forming pronuclei. Polar bodies Male pronucleus 3 Pronuclei approach each other and mitotic spindle forms between them. Mitotic spindle Centriole Female pronucleus 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 28.3a Events of fertilization.
Slide 2 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.
Slide 3 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.
Slide 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.
Slide 5 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 28.3a Events of fertilization.
Sperm nucleus Slide 6 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 Male pronucleus Female pro-nucleus (swollen ovum nucleus) 2 Sperm and ovum nuclei swell, forming pronuclei. Polar bodies Male pronucleus 3 Pronuclei approach each other and mitotic spindle forms between them. Mitotic spindle Centriole Female pronucleus 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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Events of Embryonic Development: Zygote to Blastocyst Implantation
Cleavage Occurs while zygote moves toward uterus Mitotic divisions of zygote First cleavage at 36 hours two daughter cells (blastomeres) At 72 hours morula (16 or more cells) At day 4 or 5, blastocyst (embryo of ~100 cells) reaches uterus © 2013 Pearson Education, Inc.
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Embryonic Development
Blastocyst - fluid-filled hollow sphere composed of Trophoblast cells Display immunosuppressive factors Participate in placenta formation Inner cell mass Becomes embryonic disc ( embryo and three of embryonic membranes) © 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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Blastocyst floats for 2–3 days
Implantation Blastocyst floats for 2–3 days Nourished by uterine secretions Implantation begins 6–7 days after ovulation Trophoblast cells adhere to site with proper receptors and chemical signals Inflammatory-like response occurs in endometrium Uterine blood vessels more permeable and leaky; inflammatory cells invade area © 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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Trophoblasts proliferate and form two distinct layers
Implantation Trophoblasts proliferate and form two distinct layers Cytotrophoblast (cellular trophoblast) - inner layer of cells Syncytiotrophoblast (syncytial trophoblast) - cells in outer layer lose plasma membranes, invade and digest endometrium Blastocyst burrows into lining surrounded by pool of leaked blood; endometrial cells cover and seal off implanted blastocyst © 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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Implantation completed by twelfth day after ovulation
Menstruation must be prevented Corpus luteum maintained by hormone human chorionic gonadotropin (hCG) © 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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Hormonal Changes During Pregnancy
Human chorionic gonadotropin (hCG) Secreted by trophoblast cells; later chorion Prompts corpus luteum to continue secretion of progesterone and estrogen Promotes placental development via its autocrine growth factor activity hCG levels rise until end of second month, then decline as placenta begins to secrete progesterone and estrogen; low values at 4 months and rest of pregnancy © 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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Formation of placenta from embryonic and maternal tissues
Placentation Formation of placenta from embryonic and maternal tissues Temporary organ Embryonic tissues Mesoderm cells develop from inner cell mass; line trophoblast Together these form chorion and chorionic villi © 2013 Pearson Education, Inc.
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Villi lie in intervillous spaces, immersed in maternal blood
Placentation Cores of chorionic villi invaded by new blood vessels; extend to embryo as umbilical arteries and vein Erosion blood-filled lacunae (intervillous spaces) in stratum functionalis Villi lie in intervillous spaces, immersed in maternal blood © 2013 Pearson Education, Inc.
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Maternal portion of placenta
Placentation Maternal portion of placenta Decidua basalis (stratum functionalis between chorionic villi and stratum basalis of endometrium) Fetal portion of placenta Chorionic villi © 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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Placenta fully formed and functional by end of third month
Placentation Decidua capsularis - part of endometrium at uterine cavity face of implanted embryo Placenta fully formed and functional by end of third month Nutritive, respiratory, excretory, endocrine functions Placenta also secretes human placental lactogen, human chorionic thyrotropin, and relaxin © 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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If placental hormones inadequate, pregnancy aborted
Placentation If placental hormones inadequate, pregnancy aborted Throughout pregnancy blood levels of estrogens and progesterone increase Prepare mammary glands for lactation © 2013 Pearson Education, Inc.
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Maternal and embryonic blood supplies normally do not intermix
Placentation Maternal and embryonic blood supplies normally do not intermix Embryonic placental barriers include Membranes of chorionic villi Endothelium of embryonic capillaries © 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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Events of Embryonic Development: Gastrula to Fetus
Germ Layers During implantation, blastocyst begins conversion to gastrula Inner cell mass develops into embryonic disc (subdivides into epiblast and hypoblast) Three primary germ layers form; extraembryonic membranes develop © 2013 Pearson Education, Inc.
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Extraembryonic Membranes
Amnion - epiblast cells form transparent sac filled with amniotic fluid Provides buoyant environment that protects embryo Helps maintain constant homeostatic temperature Allows freedom of movement; prevents parts from fusing together Amniotic fluid comes from maternal blood, and later, fetal urine © 2013 Pearson Education, Inc.
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Extraembryonic Membranes
Yolk sac - sac that hangs from ventral surface of embryo Forms part of digestive tube Source of earliest blood cells and blood vessels © 2013 Pearson Education, Inc.
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Extraembryonic Membranes
Allantois - small outpocketing at caudal end of yolk sac Structural base for umbilical cord Becomes part of urinary bladder Chorion - helps form placenta Encloses embryonic body and all other membranes © 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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Embryonic disc three-layered embryo with primary germ layers present
Gastrulation Occurs in week 3 Embryonic disc three-layered embryo with primary germ layers present Ectoderm, mesoderm, and endoderm Begins with appearance of primitive streak, raised dorsal groove; establishes longitudinal axis of embryo © 2013 Pearson Education, Inc.
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Cells begin to migrate into groove
Gastrulation Cells begin to migrate into groove First cells form endoderm Cells that follow push laterally, forming mesoderm Notochord - rod of mesodermal cells that serves as axial support Cells that remain on embryo's dorsal surface form ectoderm © 2013 Pearson Education, Inc.
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Gastrulation Ectoderm, mesoderm, endoderm - primitive tissues from which all body organs derive Epithelia cells Ectoderm nervous system; skin epidermis Endoderm epithelial linings of digestive, respiratory, urogenital systems; associated glands Mesenchyme cells Mesoderm everything else © 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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