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Chapter 27 Reproduction and Embryonic Development Figure 27.5A_2 2n Diploid cell Differentiation and onset of meiosis I 2n Primary spermatocyte (diploid; in prophase of meiosis I) Meiosis I completed n Secondary spermatocyte n (haploid) Meiosis II Developing sperm cells n Sperm cells (haploid) n Mature sperm released into center of seminiferous tubule n n n Differentiation n n n 27.5 The formation of sperm and egg cells requires meiosis Oogenesis and spermatogenesis are – alike in that both produce haploid gametes but – different in that – oogenesis produces only one mature egg and polar bodies that degenerate and – spermatogenesis produces four mature gametes. © 2012 Pearson Education, Inc. 27.6 Hormones synchronize cyclic changes in the ovary and uterus About every 28 days – the hypothalamus signals the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH), – which trigger the growth of a follicle and ovulation, the release of an egg. © 2012 Pearson Education, Inc. Table 27.6 Figure 27.6_2 0 5 10 Pituitary hormones in blood Days 14 15 20 LH peak triggers ovulation and corpus luteum formation LH FSH FSH stimulates follicle to grow LH surge triggers ovulation 25 28 Figure 27.6_3 0 10 5 Days 14 15 20 25 28 Ovarian cycle Growing follicle Mature follicle Ovulation Pre-ovulatory phase Estrogen secreted by growing follicle Corpus Degenerating corpus luteum luteum Post-ovulatory phase Progesterone and estrogen secreted by remnant of follicle Figure 27.6_4 0 5 10 Days 14 15 20 25 28 Peak causes LH surge Ovarian hormones in blood Estrogen Progesterone Low levels of estrogen trigger menstruation Progesterone and estrogen promote thickening of endometrium Figure 27.6_5 Menstrual cycle Endometrium 0 5 Menstruation 10 14 15 Days 20 25 28 PRINCIPLES OF EMBRYONIC DEVELOPMENT © 2012 Pearson Education, Inc. 27.9 Fertilization results in a zygote and triggers embryonic development Embryonic development begins with fertilization, – the union of sperm and egg, – to form a diploid zygote. © 2012 Pearson Education, Inc. 27.9 Fertilization results in a zygote and triggers embryonic development Sperm are adapted to reach and fertilize an egg. Sperm have – a streamlined shape, which moves easily through fluids, – many mitochondria, which provide ATP for tail movements, and – a head that contains a haploid nucleus and is tipped with an acrosome containing enzymes that help it penetrate the egg. © 2012 Pearson Education, Inc. Figure 27.9C_1 A sperm touches the egg’s jelly coat, and its acrosome releases enzyme molecules. Sperm The sperm’s acrosomal enzymes digest the egg’s jelly coat. Proteins on the sperm head bind to egg receptors. Acrosomal enzymes Plasma membrane Nucleus Acrosome Jelly coat Plasma membrane Vitelline layer Receptor protein molecules Figure 27.9C_2 The plasma membranes of sperm and egg fuse. The sperm nucleus enters the egg cytoplasm. The vitelline layer separates and becomes impenetrable. n Sperm nucleus Figure 27.9C_3 n Sperm nucleus n Egg nucleus n n The nuclei of sperm and egg fuse. 2n Zygote nucleus 27.10 Cleavage produces a ball of cells from the zygote Cleavage is a rapid series of cell divisions that produces – more cells, – smaller cells, and – a fluid-filled embryo called a blastula. Video: Sea Urchin Embryonic Development © 2012 Pearson Education, Inc. Figure 27.10_s4 Zygote 2 cells 4 cells 8 cells Many cells (solid ball) Blastocoel Blastula Cross section (hollow ball) of blastula 27.11 Gastrulation produces a three-layered embryo During gastrulation – cells migrate to new locations, – a rudimentary digestive cavity forms, and – the basic body plan of three layers is established with – ectoderm outside—becomes skin and nervous systems, – endoderm inside—becomes digestive tract, – mesoderm in the middle—becomes muscle and bone. © 2012 Pearson Education, Inc. Figure 27.11_s3 Blastula (end of cleavage) Animal pole Blastocoel Vegetal pole Gastrulation (cell migration) Blastocoel shrinking Gastrula (end of gastrulation) Simple digestive cavity Formation of a simple digestive cavity Blastopore Ectoderm Mesoderm Endoderm Table 27.11 27.12 Organs start to form after gastrulation Organs develop from the three embryonic layers. – The stiff notochord forms the main axis of the body and is later replaced by the vertebral column in most chordates. – The neural tube develops above the notochord and will become the – brain and – spinal cord. Video: Frog Embryo Development © 2012 Pearson Education, Inc. Figure 27.12A Neural Neural fold plate Notochord Ectoderm Mesoderm Endoderm Neural folds Figure 27.12B Neural fold Neural plate Outer layer of ectoderm Neural tube 27.12 Organs start to form after gastrulation As the embryo elongates, paired somites – form along the sides of the notochord, – hollow out to form a coelom, and – eventually contribute to muscles, bone, and other connective tissues. Other systems develop at the same time. © 2012 Pearson Education, Inc. Figure 27.12C Neural tube Notochord Somite Coelom Somites Digestive cavity Eye Tail bud 27.13 Multiple processes give form to the developing animal Through induction, adjacent cells and cell layers – influence each other’s differentiation – via chemical signals. © 2012 Pearson Education, Inc. 27.14 EVOLUTION CONNECTION: Pattern formation during embryonic development is controlled by ancient genes Pattern formation, – the emergence of the parts of a structure in their correct relative positions, – involves the response of genes to spatial variations of chemicals in the embryo, and – results in tissues and organs developing in their proper positions at the correct times. © 2012 Pearson Education, Inc. HUMAN DEVELOPMENT © 2012 Pearson Education, Inc. 27.15 The embryo and placenta take shape during the first month of pregnancy Pregnancy, or gestation, is the carrying of developing young within the female reproductive tract. Human pregnancy – averages 266 days (38 weeks) from fertilization or – 40 weeks (9 months) from the start of the last menstrual period. © 2012 Pearson Education, Inc. 27.15 The embryo and placenta take shape during the first month of pregnancy Human development begins with fertilization in the oviduct. Cleavage produces a blastocyst whose – inner cell mass becomes the embryo and the – trophoblast, the outer cell layer, which – attaches to the uterine wall and – forms part of the placenta. Gastrulation occurs and organs develop from the three embryonic layers. © 2012 Pearson Education, Inc. Figure 27.15A–B Cleavage starts Fertilization of mature egg Blastocyst Trophoblast Uterine cavity Cavity Oviduct Ovary Inner cell mass Blastocyst (implanted) Secondary oocyte Ovulation Endometrium Uterus Uterine cavity Figure 27.15C Endometrium Uterine cavity Multiplying cells of trophoblast (contribute to future placenta) Trophoblast Embryo Future yolk sac Blood vessel (maternal) 27.15 The embryo and placenta take shape during the first month of pregnancy Four extraembryonic membranes develop. 1. The amnion – surrounds the embryo and – forms a fluid-filled amniotic cavity that protects the embryo. 2. The yolk sac, – in reptiles, stores yolk, – in humans, does not store yolk but is a source of the first germ cells and blood cells. © 2012 Pearson Education, Inc. 27.15 The embryo and placenta take shape during the first month of pregnancy 3. The allantois – contributes to the umbilical cord, – forms part of the urinary bladder, and – in reptiles, stores embryonic waste. 4. The chorion – contributes to the placenta and – secretes human chorionic gonadotropin (HCG), which prevents menstruation in mammals. © 2012 Pearson Education, Inc. Figure 27.15D Yolk sac Chorion Amnion Amniotic cavity Mesoderm cells Figure 27.15E Embryo: Endoderm Mesoderm Ectoderm Chorionic villi Chorion Amnion Allantois Yolk sac Figure 27.15F Placenta Amnion Amniotic cavity Embryo Mother’s blood Allantois vessels Yolk sac Chorion Chorionic villi Figure 27.17A Estrogen from ovaries Oxytocin from fetus and mother’s pituitary Induces oxytocin receptors on uterus Stimulates placenta to make Prostaglandins Stimulate more contractions of uterus Positive feedback Stimulates uterus to contract 27.17 Childbirth is induced by hormones and other chemical signals Labor occurs in three stages: 1. dilation of the cervix, 2. expulsion, delivery of the infant, 3. delivery of the placenta. © 2012 Pearson Education, Inc. 27.18 CONNECTION: Reproductive technologies increase our reproductive options New techniques can help many infertile couples. – About 15% of couples wanting children are infertile. – Drug therapies can help address problems of impotence (erectile dysfunction) and induce ovulation. – Assisted reproductive technologies (ART) require eggs to be harvested from the ovaries, fertilized, and returned to a woman’s body. – In vitro fertilization (IVF) is the most common assisted reproductive technology. Fertilization occurs in a culture dish and an early embryo is implanted in the uterus. © 2012 Pearson Education, Inc. Figure 27.18 Implantation Zygote Collected egg In vitro fertilization Collected sperm 8-cell embryo Figure 27.UN01 Oogenesis 2n Once per month Spermatogenesis Primary oocyte Primary spermatocyte Continuously n Polar body n Secondary oocyte 2n Secondary spermatocyte Developing n sperm cells n n n n Sperm n Fertilization Polar body n Mature egg n 2n Zygote n Figure 27.UN02 Cleavage Gastrulation Ectoderm Mesoderm Endoderm Zygote 2-cell embryo Many-celled Blastula (cross solid ball section) Gastrula (cross section) https://www.youtube.com/watch ?v=BFrVmDgh4v4