Transcript Chapter 54
CHAPTER 54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization • In all sexually-reproducing animals, the first step is fertilization – union of male and female gametes • Fertilization itself consists of three events – Sperm penetration and membrane fusion – Egg activation – Fusion of nuclei 2 Fertilization • Sperm penetration and membrane fusion – Protective layers of egg include the jelly layer and vitelline envelope in sea urchins, and the zona pellucida in mammals – Sperm’s acrosome contains digestive enzymes that enable the sperm to tunnel its way through to the egg’s cell membrane – Membrane fusion permits sperm nucleus to enter directly into egg’s cytoplasm 3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sperm Jelly layer Plasma membrane Vitelline envelope Cytoplasm Cortical granules a. Nucleus of egg Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Granulosa cell Sperm Zona pellucida Oocyte Plasma membrane Granulosa cell First polar body Cortical granules c. 1.2 µm Cytoplasm b. d. c-d: © David M. Phillips/Visuals Unlimited 3.3 µm Fertilization 1. Sperm penetrates 2. Some of the zona 4. The sperm nucleus 3. Sperm and egg between granulosa pellucida is degraded dissociates and plasma membranes cells. by acrosomal enzymes. enters cytoplasm. fuse. Plasma membrane Granulosa cells Zona pellucida Cortical granules 6. Additional sperm can no longer penetrate the zona pellucida. 5. Cortical granules release enzymes that harden zona pellucida and strip it of sperm receptors. Hyalin attracts water by osmosis. 7. Sperm and egg pronuclei are enclosed in a nuclear envelope. 6 Fertilization • Membrane fusion – Egg activation • Dramatic increase in the levels of free intracellular Ca2+ ions in the egg shortly after the sperm makes contact with the egg’s plasma membrane • Act as second messengers to initiate changes – Block to polyspermy • Rapid transient change in membrane potential • Cortical granules remove sperm receptors • Vitelline envelope lifts off – fertilization envelope 7 Fertilization • Sperm penetration has three other effects – Triggers the egg to complete meiosis – Triggers a cytoplasmic rearrangement – Causes a sharp increase in protein synthesis and metabolic activity in general Primary Oocyte First Metaphase of Meiosis Second Metaphase of Meiosis Diploid nucleus Meiosis Complete Polar bodies Polar body Female pronucleus (haploid) • Roundworms (Ascaris) • Polychaete worms (Myzostoma) • Clam worms (Nereis) • Clams (Spisula) • Nemertean worms (Cerebratulus) • Polychaete worms (Chaetopterus) • Mollusks (Dentalium) • Many insects • Sea stars • Lancelets (Branchiostoma) • Amphibians • Mammals • Fish • Cnidarians • Sea urchins 8 Fertilization • Fusion of nuclei – 3rd and final stage of fertilization – Haploid sperm and haploid egg nuclei fuse to form diploid nucleus of the zygote 9 Cleavage • Rapid division of the zygote into a larger and larger number of smaller and smaller cells (blastomeres) • Not accompanied by an increase in the overall size of the embryo • Animal pole – Forms external tissues • Vegetal pole – Forms internal tissues 10 Cleavage • Blastula – Hollow ball of cells – Blastocoel – fluid-filled cavity • Cleavage patterns are quite diverse – Relative amount of nutritive yolk in the egg is the characteristic that most affects the cleavage pattern of an animal embryo – Vertebrates exhibit a variety of reproductive strategies involving different patterns of yolk utilization 11 Cleavage Patterns • Eggs with little or no yolk – Holoblastic cleavage – Invertebrates, amphibians, mammals • Eggs with large amounts of yolk – Meroblastic cleavage – Embryo forms thin cap on yolk Sea Urchin Frog Chicken Animal pole Nucleus Cytoplasm Cytoplasm Shell Nucleus Air bubble Nucleus Plasma membrane Albumen Yolk Yolk Vegetal pole a. b. Yolk 12 c. Holoblastic cleavage Meroblastic cleavage 13 Cleavage Patterns • Mammalian eggs contain very little yolk – Undergo holoblastic cleavage – Form a blastocyst composed of • Trophoblast – Outer layer of cells – Contributes to the placenta • Blastocoel – Central fluid-filled cavity • Inner cell mass – Located at one pole – Forms the developing embryo 14 Fate of Blastomeres • In many animals removal of committed cells results in embryos deficient in tissues that would have developed from those tissues • In mammals, early blastomeres do not appear to be committed to a particular fate – Cell removed for preimplantation genetic diagnosis – Split embryos form identical twins • In mammals, body form determined primarily by cell-to-cell interactions 15 Gastrulation • Process involving a complex series of cell shape changes and cell movements that occurs in the blastula • Establishes the basic body plan and creates the three primary germ layers – Ectoderm – Exterior • Epidermis of skin, nervous system, sense organs – Mesoderm – Middle • Skeleton, muscles, blood vessels, heart, blood, gonads, kidneys, dermis of skin – Endoderm – Inner • Lining of digestive and respiratory tracts, liver, pancreas, thymus, thyroid 16 Gastrulation • Cells move during gastrulation using a variety of cell shape changes – Cells that are tightly attached to each other via junctions will move as cell sheets – Invagination – Cell sheet dents inward – Involution – Cell sheet rolls inward – Delamination – Cell sheet splits in two – Ingression – Cells break away from cell sheet and migrate as individual cells 17 Gastrulation Patterns • Vary according to the amount of yolk • Gastrulation in sea urchins – Develop from relatively yolk-poor eggs – Form hollow symmetrical blastulas – Deuterostome – anus develops first and mouth second 18 Gastrulation Patterns • Gastrulation in mammals – Proceeds similarly to that in birds – Embryo develops from inner cell mass – Embryo gastrulates as if it was sitting in a ball of yolk • Embryo obtains nutrition from placenta 19 Extraembryonic Membranes • Adaptation to life on dry land – Reptiles, birds, and mammals – Amniotic species developed • Extraembryonic membranes – Amnion, chorion, yolk sac, and allantois • Nourish and protect the developing embryo 20 Extraembryonic Membranes • Amnion – Encloses amniotic fluid • Chorion – Located near eggshell in birds – Contributes to the placenta in mammals • Yolk sac – Food source in bird embryos – Found in mammals, but it is not nutritive • Allantois – Unites with chorion in birds, forming a structure used for gas exchange – In mammals, it contributes blood vessels to the developing umbilical cord 21 Extraembryonic Membranes Chick Embryo Mammal Embryo Chorion Amnion Chorion Yolk sac Amnion Umbilical blood vessels Yolk sac Villus of chorion frondosum Allantois Maternal blood a. b. 22 Organogenesis • Formation of organs in their proper locations • Occurs by interaction of cells within and between the three germ layers • Thus, it follows rapidly on the heels of gastrulation – In many animals it begins before gastrulation is complete 23 Organogenesis • To a large degree, a cell’s location in the developing embryo determines its fate • At some stage, every cell’s ultimate fate becomes fixed – cell determination • A cell’s fate can be established by – Inheritance of cytoplasmic determinants – Interactions with neighboring cells • Induction 24 Organogenesis in Vertebrates • Begins with the formation of two structures unique to chordates – Notochord – Dorsal nerve cord – neurulation 25 Development of Neural Tube • Notochord – Forms from mesoderm – Region of dorsal ectodermal cells situated above notochord thickens to form the neural plate – Cells of the neural plate fold together to form a long hollow cylinder, the neural tube – Will become brain and spinal cord 26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neural plate Amniotic cavity Ectoderm Mesoderm Notochord Endoderm Yolk sac a. Neural groove Neural fold Ectoderm Notochord Mesoderm Endoderm b. Neural tube Ectoderm Neural crest Mesoderm Endoderm Somite 27 c. Human Development • Human development from fertilization to birth takes an average of 266 days, or about 9 months – Commonly divided into three periods called trimesters 28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First Trimester • First month a. © Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell Publishing Company – Zygote undergoes its first cleavage about 30 hr after fertilization – By the time the embryo reaches the uterus, 6–7 days after fertilization, it has differentiated into a blastocyst – Trophoblast cells digest their way into the endometrium in the process known as implantation 29 First Trimester • First month – During the second week, the developing chorion and mother’s endometrium engage to form the placenta • Mom and baby’s blood come into close proximity, but do not mix – gases are exchanged • One hormone released by the placenta is human chorionic gonadotropin (hCG) – – – – Gastrulation occurs in the second week Neurulation occurs in the third week Organogenesis begins in the fourth week Embryo is 5 mm in length 30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chorion Amnion Yolk sac Umbilical cord Chorionic frondosum (fetal) Decidua basalis (maternal) Placenta Umbilical artery Umbilical vein Uterine wall a. 31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First Trimester • Second month – Organogenesis continues – Miniature limbs assume adult shape – All major organs in the body established – Embryo grows to about 25 mm in length – Weighs about 1 g, and looks distinctly human – 9th week marks the transition from embryo to fetus b. © Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell Publishing Company 32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First Trimester • Third month – Nervous system develops c. – Limbs start to move – Secretion of hCG by the placenta declines, and so corpus luteum degenerates – Placenta takes over hormone secretion © Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell Publishing Company 33 Increasing Hormone Concentration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. hCG Estrogen Progesterone 0 1 2 3 4 5 6 Months of Pregnancy 7 8 9 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Second Trimester d. © Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell Publishing Company • The basic body plan develops further • Bones actively enlarge in fourth month • Rapid fetal heartbeat can be heard by a stethoscope • By the end of the sixth month, fetus is over 300 mm long, and weighs 600 g 35 Third Trimester • A period of growth and organ maturation • Weight of the fetus doubles several times • Most of the major nerve tracts in the brain are formed • Brain continues to develop and produce neurons for months after birth 36 Birth • Estrogen stimulates mother’s uterus to release prostaglandins, and produce more oxytocin receptors – Prostaglandins begin uterine contractions – Sensory feedback from uterus stimulates oxytocin release from posterior pituitary • Oxytocin and prostaglandins further stimulate uterine contractions 37 Birth • Strong contractions, aided by the mother’s voluntary pushing, expel the fetus • Now called a newborn baby, or neonate • After birth, continuing uterine contractions expel the placenta and associated membranes – Collectively called the afterbirth 38 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intestine Placenta Umbilical cord Wall of uterus Cervix Vagina 39 Nursing • Milk production (lactation) occurs in alveoli of mammary glands when stimulated by the anterior pituitary hormone prolactin • During pregnancy, the mammary glands are prepared for, but prevented from, lactating 40 Nursing • After birth – Prolactin – stimulates the mammary alveoli to produce milk – Suckling triggers posterior pituitary to release oxytocin • Stimulates contraction of smooth muscles surrounding alveolar ducts • Milk is ejected (milk let-down reflex) • The first milk produced after birth, colostrum, is rich in nutrients and maternal antibodies 41 Postnatal Development • Growth of the infant continues rapidly after birth • Babies typically double their birth weight within 2 months • Different components grow at different rates – Allometric growth 42