© 2016 Pearson Education, Inc.

Why This Matters Understanding of pregnancy and human development is critical so that you can counsel your pregnant patients about bodily changes they will experience and educate them on fetal development, as well as stages of the birthing process. © 2016 Pearson Education, Inc.

Video: Why This Matters
© 2016 Pearson Education, Inc.

Pregnancy and Development
Pregnancy: events that occur from fertilization until infant is 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 © 2016 Pearson Education, Inc.

Embryo Fertilization 1-week conceptus 3-week embryo (3 mm)
Figure 28.1 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) © 2016 Pearson Education, Inc.

28.1 Fertilization Before fertilization can occur, sperm must reach secondary oocyte Oocyte viable for 12 to 24 hours Sperm viable 24 to 48 hours after ejaculation For fertilization to occur, coitus must occur no more than 2 days before and at least 24 hours after ovulation Fertilization: sperm’s chromosomes combine with those of secondary oocyte to form fertilized egg, called a zygote © 2016 Pearson Education, Inc.

Sperm Transport and Capacitation
Most ejaculated sperm do not make the 12-cm (5-inch) trip to join with egg Some leak out of vagina immediately after deposition Some are destroyed by acidic vaginal environment Some fail to make it through cervix Some are dispersed in uterine cavity or destroyed by phagocytes Only a few thousand out of millions reach uterine tubes © 2016 Pearson Education, Inc.

Sperm Transport and Capacitation (cont.)
Sperm must be capacitated before they can penetrate oocyte Motility must be enhanced, and cell membranes must become fragile enough to release hydrolytic enzymes Secretions of female tract help to weaken and thin out acrosome membrane Sperm have olfactory receptors that can follow chemical trail released by by egg or surrounding cells Sperm “sniff” their way to oocyte © 2016 Pearson Education, Inc.

Acrosomal Reaction and Sperm Penetration
Sperm reaches oocyte by several steps: Approach: aided by enzymes, sperm weaves through corona radiata Hyaluronidase on cell surface of sperm acts to digest connection between granulosa cells, causing them to separate Sperm heads then bind to sperm-binding receptors in zona pellucida, causing sperm membrane calcium channels to open Ca2+ flows into each sperm, triggering acrosomal reaction © 2016 Pearson Education, Inc.

Sperm, delivered to the vagina and
Focus Figure Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. Blocks to polyspermy prevent further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized ovum. 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 Oocyte nucleus arrested in meiotic metaphase II Granulosa cells of corona radiata Polar body Zona pellucida Extracellular space Oocyte plasma membrane © 2016 Pearson Education, Inc.

Acrosomal Reaction and Sperm Penetration (cont.)
When triggered by calcium influx, enzymes from many sperm are released that digest holes in zona pellucida Enzymes include hyaluronidase, acrosin, proteases, and others Hundreds of acrosomes in region all release enzymes at same time to digest zona pellucida Many are needed to clear a path to oocyte membrane © 2016 Pearson Education, Inc.

2 5 3 4 Fusion. Sperm and oocyte plasma membranes fuse.
Focus Figure Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. Blocks to polyspermy prevent further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized ovum. Acrosomal reaction. Binding of the sperm to receptors in the zona pellucida causes 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 Blocks to polyspermy. Oocyte sperm-binding membrane receptors are shed. Ca2+ levels in the oocyte’s cytoplasm rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucida hardens and the zona pellucida’s sperm-binding receptors are clipped off. 5 Binding. The sperm’s membrane binds to the oocyte’s sperm-binding receptors. 3 4 Fusion. Sperm and oocyte plasma membranes fuse. Sperm contents enter the oocyte. Microtubules from sperm flagellum Oocyte sperm-binding membrane receptors Cortical granules Mitochondria Zona pellucida sperm-binding receptors Sperm nucleus © 2016 Pearson Education, Inc.

Binding After path has been cleared in zona pellucida, a single sperm forcibly swims towards oocyte membrane Acrosomal collar on rear portion of acrosomal membrane binds to oocyte plasma membrane sperm-binding receptor Binding causes: Oocyte to form microvilli that wrap around sperm head Trigger fusing of oocyte and sperm membranes © 2016 Pearson Education, Inc.

Fusion of membranes Oocyte and sperm membranes fuse Cytoplasmic contents of sperm enter oocyte Tail and other parts, such as sperm cell membrane and mitochondria, are left behind on oocyte cell membrane surface © 2016 Pearson Education, Inc.

Blocks to Polyspermy Polyspermy does occur in some animals, but in humans, only monospermy, in which one sperm penetrates oocyte, is allowed One-sperm-per-oocyte condition Two mechanisms ensure monospermy Oocyte membrane block: when a sperm binds to sperm-binding receptor on oocyte, it causes oocyte to shed all other sperm-binding receptors Other sperm can no longer bind to oocyte plasma membrane © 2016 Pearson Education, Inc.

Blocks to Polyspermy (cont.)
Zona reaction Also called slow block to polyspermy Entry of sperm into oocyte triggers Ca2+ surge from oocyte ER that causes: Activation of oocyte to prepare for second meiotic division Cortical reaction: granules located just inside oocyte plasma membrane release zonal inhibiting proteins, or ZIP enzymes, into extracellular space below zona pellucida ZIPs destroy zona pellucida sperm-binding receptors; fragments bind water and swell, detaching any other sperm still around © 2016 Pearson Education, Inc.

Completion of Meiosis II and Fertilization
Events involved include: Ca2+ surge triggers completion of meiosis II in oocyte, resulting in ovum + second polar body Ovum nucleus swells to become female pronucleus As sperm nucleus moves toward oocyte nucleus, it also swells Forms male pronucleus © 2016 Pearson Education, Inc.

Figure 28.2a-1 Events of fertilization.
Sperm nucleus Extracellular space After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. 1 Corona radiata Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body © 2016 Pearson Education, Inc.

Completion of Meiosis II and Fertilization (cont.)
DNA in each pronucleus replicates, and as pronuclei get closer, a mitotic spindle forms between them Nuclear envelopes dissolve, releasing chromosomes together in vicinity of mitotic spindle Maternal and paternal chromosomes combine, forming diploid zygote Fertilization: moment chromosomes combine © 2016 Pearson Education, Inc.

Male pronucleus The DNA in each pronucleus replicates. The pronuclei approach each other and a mitotic spindle forms between them. 3 Mitotic spindle Centriole Female pronucleus © 2016 Pearson Education, Inc.

Figure 28.2 Events of fertilization.
Sperm nucleus Extracellular space After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. 1 Corona radiata Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body Male pronucleus Female pronucleus (swollen ovum nucleus) Sperm and ovum nuclei swell, forming pronuclei. 2 Polar bodies Male pronucleus The DNA in each pronucleus replicates. The pronuclei approach each other and a mitotic spindle forms between them. 3 Mitotic spindle Centriole Male and female pronuclei Female pronucleus Chromosomes of the pronuclei intermix. Fertilization is accomplished and the cell, now called a zygote, is ready for the first cleavage division. 4 Zygote Polar bodies © 2016 Pearson Education, Inc.

Figure 28.2b Events of fertilization.
Male and female pronuclei Polar bodies © 2016 Pearson Education, Inc.

28.2 Zygote to Blastocyst Implantation
Embryonic development continues as embryo travels through uterine tube to uterus, where it floats freely until it implants Significant events in this process: Cleavage Blastocyst Formation Implantation © 2016 Pearson Education, Inc.

Cleavage Occurs while zygote moves toward uterus
Rapid mitotic divisions of zygote occur Produces cells with high surface-to-volume ratio that enhances uptake of nutrients and oxygen and disposal of wastes First cleavage occurs after ~36 hours and produces two daughter cells called blastomeres, which continue to divide After 72 hours, cluster of cells contains16 or more cells and is referred to as a morula © 2016 Pearson Education, Inc.

Blastocyst Formation Around day 4 or 5, embryo, which consists of ~100 cells and is now referred to as a blastocyst, reaches uterus Blastocyst is fluid-filled hollow sphere composed of: Trophoblast cells Display immunosuppressive factors Participate in placenta formation © 2016 Pearson Education, Inc.

Blastocyst Formation (cont.)
Inner cell mass: cluster of 20–30 rounded cells Becomes embryonic disc, which will form embryo and three or four extraembryonic membranes Fourth extraembryonic membrane (chorion) is formed by trophoblast © 2016 Pearson Education, Inc.

Figure 28.3 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 © 2016 Pearson Education, Inc.

28.3 Implantation and Placentation
Blastocyst floats for about 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 become more permeable and leaky; inflammatory cells invade area © 2016 Pearson Education, Inc.

Figure 28.4a Implantation of the blastocyst.
Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity Lumen of uterus © 2016 Pearson Education, Inc.

Implantation (cont.) Trophoblast cells proliferate and form two distinct layers Cytotrophoblast (cellular trophoblast): inner layer of cells Syncytiotrophoblast (syncytial trophoblast): cells in outer layer lose plasma membranes, becoming multinuclear mass Send out long protrusions that invade and digest endometrium © 2016 Pearson Education, Inc.

Implantation (cont.) As endometrium is eroded, blastocyst burrows into lining, surrounded by pool of leaked blood Endometrial cells then cover and seal off implanted blastocyst © 2016 Pearson Education, Inc.

Figure 28.4b Implantation of the blastocyst.
Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Blastocyst cavity Lumen of uterus © 2016 Pearson Education, Inc.

Figure 28.4c Implantation of the blastocyst.
Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Lumen of uterus © 2016 Pearson Education, Inc.

Implantation (cont.) Implantation is usually completed by day 12 after ovulation (day 26 of menstrual cycle); about same time menstruation would occur Corpus luteum is maintained by hCG to prevent menstruation © 2016 Pearson Education, Inc.

Implantation (cont.) Hormone human chorionic gonadotropin (hCG):
Secreted by trophoblast cells and 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 month 2 Decline as placenta begins to secrete progesterone and estrogen Low values occur at 4 months and continue for rest of pregnancy © 2016 Pearson Education, Inc.

Implantation (cont.) In cases where implantation fails to occur, uterus becomes nonreceptive again About two-thirds of all zygotes formed fail to implant by end of first week or spontaneously abort An estimated 30% of implanted embryos later miscarry because of genetic defects of embryo, uterine malformation, or unknown problems © 2016 Pearson Education, Inc.

Figure 28.5 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 © 2016 Pearson Education, Inc.

Placentation Formation of placenta, temporary organ that originates from both embryonic and maternal tissues Embryonic portion of placenta includes: Inner cell mass, which gives rise to layer of extraembryonic mesoderm that lines inner surface of trophoblast Together these structures form chorion that then develops fingerlike projections called chorionic villi © 2016 Pearson Education, Inc.

Placentation (cont.) Chorionic villi are then invaded by new blood vessels, which extend to embryo as umbilical arteries and vein Continuing erosion of endometrium produces large, blood-filled lacunae (intervillous spaces) in stratum functionalis Villi lie in intervillous spaces totally immersed in maternal blood © 2016 Pearson Education, Inc.

Implanting 7½-day blastocyst. The syncytio-
Figure 28.6a Events of placentation, early embryonic development, and extraembryonic membrane formation. Maternal blood vessels Proliferating syncytiotrophoblast Cytotrophoblast Amniotic cavity Bilayered embryonic disc • Epiblast • Hypoblast Endometrial epithelium Implanting 7½-day blastocyst. The syncytiotrophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. © 2016 Pearson Education, Inc.

Endometrium Maternal blood vessels Proliferating Amnion
Figure 28.6b Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Maternal blood vessels Proliferating syncytiotrophoblast Amnion Cytotrophoblast Amniotic cavity Yolk sac Bilayered embryonic disc • Epiblast • Hypoblast Extraembryonic mesoderm Lumen of uterus Chorion being formed 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. © 2016 Pearson Education, Inc.

16-day embryo. Cytotrophoblast and associated mesoderm have
Figure 28.6c Events of placentation, early embryonic development, and extraembryonic membrane formation. Amniotic cavity Lacuna (intervillous space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Chorion • Mesoderm Amnion • Endoderm Forming umbilical cord Yolk sac Allantois Extraembryonic mesoderm Lumen of uterus Extraembryonic coelom 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. © 2016 Pearson Education, Inc.

Placentation (cont.) Maternal portion of placenta includes:
Decidua basalis: stratum functionalis of endometrium located between chorionic villi and stratum basalis of endometrium Decidua capsularis: part of endometrium at uterine cavity face of implanted embryo Portion of placenta that expands to accommodate growing fetus Villi in decidua capsularis degenerate as fetus grows, while villi in decidua basalis increase in number and branches Together chorionic villi and decidua capsularis make up placenta © 2016 Pearson Education, Inc.

4½-week embryo. The decidua capsularis, decidua basalis, amnion, and
Figure 28.6d 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 4½-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. © 2016 Pearson Education, Inc.

Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
Figure 28.6e 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. © 2016 Pearson Education, Inc.

Placentation (cont.) Placenta is fully formed and functional by end of month 3 Provides nutritive, respiratory, excretory, and endocrine functions Maternal and embryonic blood supplies normally do not intermix Embryonic placental barriers include: Membranes of chorionic villi Endothelium of embryonic capillaries © 2016 Pearson Education, Inc.

Figure 28.7 Detailed anatomy of the vascular relationships in the mature decidua basalis.
Chorionic villi Decidua capsularis Decidua basalis Chorion Amnion Amniotic fluid Yolk sac Placenta Fetal portion of placenta (chorion) Maternal portion of placenta (decidua basalis) Stratum basalis of endometrium Umbilical cord Myometrium Lumen of uterus Uterus Umbilical arteries Mucous plug Umbilical vein Amnion Umbilical cord Maternal veins Connection to yolk sac Fetal venule Fetal arteriole Maternal arteries Maternal blood in lacuna (intervillous space) Chorionic villus containing fetal capillaries © 2016 Pearson Education, Inc.

Placentation (cont.) If placental hormones are inadequate for any reason, pregnancy is aborted Throughout pregnancy, blood levels of estrogens and progesterone increase Prepare mammary glands for lactation Placenta also secretes human placental lactogen, human chorionic thyrotropin, and relaxin © 2016 Pearson Education, Inc.

28.4 Embryonic Development
During process of implantation, blastocyst begins being converted into gastrula Three germ layers, as well as extraembryonic membranes, develop from gastrula Inner cell mass divides into two layers: epiblast and hypoblast Subdivided inner cell mass is now called embryonic disc © 2016 Pearson Education, Inc.

Extraembryonic Membranes
Extraembryonic membranes form during first 2–3 weeks of development and include: Amnion: epiblast cells form transparent sac filled with amniotic fluid that envelopes embryo Also called “bag of waters,” it provides buoyant environment that protects embryo Helps maintain constant homeostatic temperature Allows freedom of movement; prevents parts from fusing together Initially, amniotic fluid comes from maternal blood; later, fetal urine contributes to volume © 2016 Pearson Education, Inc.

Extraembryonic Membranes (cont.)
Yolk sac: sac that hangs from ventral surface of embryo Forms part of digestive tube Source of earliest blood cells and blood vessels 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 © 2016 Pearson Education, Inc.

Implanting 7½-day blastocyst. The syncytio-
Figure 28.6a Events of placentation, early embryonic development, and extraembryonic membrane formation. Maternal blood vessels Proliferating syncytiotrophoblast Cytotrophoblast Amniotic cavity Bilayered embryonic disc • Epiblast • Hypoblast Endometrial epithelium Implanting 7½-day blastocyst. The syncytiotrophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. © 2016 Pearson Education, Inc.

Endometrium Maternal blood vessels Proliferating Amnion
Figure 28.6b Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Maternal blood vessels Proliferating syncytiotrophoblast Amnion Cytotrophoblast Amniotic cavity Yolk sac Bilayered embryonic disc • Epiblast • Hypoblast Extraembryonic mesoderm Lumen of uterus Chorion being formed 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. © 2016 Pearson Education, Inc.

16-day embryo. Cytotrophoblast and associated mesoderm have
Figure 28.6c Events of placentation, early embryonic development, and extraembryonic membrane formation. Amniotic cavity Lacuna (intervillous space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Chorion • Mesoderm Amnion • Endoderm Forming umbilical cord Yolk sac Allantois Extraembryonic mesoderm Lumen of uterus Extraembryonic coelom 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. © 2016 Pearson Education, Inc.

4½-week embryo. The decidua capsularis, decidua basalis, amnion, and
Figure 28.6d 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 4½-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. © 2016 Pearson Education, Inc.

Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
Figure 28.6e 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. © 2016 Pearson Education, Inc.

Gastrulation: Germ Layer Formation
Gastrulation occurs during week 3, when embryonic disc transforms into three-layered embryo with three primary germ layers present: Ectoderm, mesoderm, and endoderm Begins with appearance of primitive streak, a raised dorsal groove that establishes longitudinal axis of embryo © 2016 Pearson Education, Inc.

Gastrulation: Germ Layer Formation (cont.)
Cells begin to migrate into groove First cells that enter displace hypoblast of yolk sac and form endoderm Cells that follow push laterally, forming mesoderm Notochord: rod of mesodermal cells that serves as first axial support of embryo Formed from aggregation of mesoderm cells Cells that remain on embryo’s dorsal surface form ectoderm (formerly epiblast layer) © 2016 Pearson Education, Inc.

Gastrulation: Germ Layer Formation (cont.)
Ectoderm, mesoderm, and endoderm are primitive tissues from which all body organs are derived Epithelia cells found in: Ectoderm—become nervous system and skin epidermis Endoderm—become epithelial linings of digestive, respiratory, and urogenital systems and associated glands Mesenchyme cells are found in mesoderm Mesoderm—becomes everything else © 2016 Pearson Education, Inc.

Figure 28.8 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) Hypoblast 14-15 days Endoderm Right Left Ectoderm Primitive streak Tail end Bilayered embryonic disc, superior view 16 days Mesoderm Endoderm © 2016 Pearson Education, Inc.

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