Animal Development PDF

42 Animal Development © Oxford University Press Chapter 42 Animal Development Key Concepts 42.1 Fertilization Activates Development 42.2 Mitosis Divides Up the Early Embryo 42.3 Gastrulation Generates Multiple Tissue Layers 42.4 Or...

42 Animal Development © Oxford University Press Chapter 42 Animal Development Key Concepts 42.1 Fertilization Activates Development 42.2 Mitosis Divides Up the Early Embryo 42.3 Gastrulation Generates Multiple Tissue Layers 42.4 Organs Develop from the Three Germ Layers 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos © Oxford University Press Concept 42.1 Fertilization Activates Development (1) In addition to producing a diploid zygote, fertilization: Triggers ion fluxes across egg membrane Creates blocks to entry of additional sperm Changes the pH of the egg cytoplasm Increases egg metabolism and stimulates protein synthesis Initiates cell divisions © Oxford University Press Concept 42.1 Fertilization Activates Development (2) Eggs are large and well stocked with organelles, nutrients, and cytoplasmic determinants such as transcription factors and mRNAs. The sperm contributes only DNA and a centriole to the zygote. The centriole becomes the centrosome, which organizes mitotic spindles for cell divisions; also the origin of primary cilia. © Oxford University Press Concept 42.1 Fertilization Activates Development (3) Cytoplasmic determinants in the egg play important roles in setting up the signaling cascades that orchestrate the four major events of development: Determination Differentiation Morphogenesis Growth © Oxford University Press Concept 42.1 Fertilization Activates Development (4) Amphibian eggs make good models to show how rearrangements of egg cytoplasm influence determination. Sperm entry establishes polarity of the zygote and organizes the informational molecules that guide development. When cell division begins, informational molecules are not divided evenly among daughter cells. © Oxford University Press Concept 42.1 Fertilization Activates Development (5) In an unfertilized frog egg: Vegetal hemisphere: lower half of the egg, where nutrients are concentrated. Animal hemisphere: opposite side of the egg; has pigments and contains the nucleus. © Oxford University Press Concept 42.1 Fertilization Activates Development (6) Sperm enters the animal hemisphere and the cortical (outer) cytoplasm rotates toward site of entry. The gray crescent is a band of pigmented cytoplasm opposite the site of sperm entry; involved in specifying body axes and other events. © Oxford University Press Concept 42.1 Fertilization Activates Development (7) The centriole from the sperm initiates cytoplasmic reorganization. It causes microtubules in the vegetal hemisphere to form a parallel array to guide movement of cytoplasm and organelles. Cytoplasmic movement changes the distributions of critical developmental signals. © Oxford University Press Concept 42.1 Fertilization Activates Development (8) Interaction of the protein kinase GSK-3, an inhibitor of GSK-3, and the protein β-catenin results in higher concentration of b- catenin on the dorsal side, which specifies the dorsal–ventral axis of the embryo. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (1) Cleavage: early cell divisions with no cell growth. Embryo becomes a solid ball of small cells. Blastocoel: a central fluid-filled cavity that forms in the ball. The embryo becomes a blastula and its cells are called blastomeres. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (2) The pattern of cleavage varies with species. Complete cleavage occurs in eggs with little yolk; the blastomeres are similar in size. In frogs the vegetal pole has more yolk; division is unequal and daughter cells in animal pole are smaller. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (3) Incomplete cleavage occurs in eggs with a lot of yolk when cleavage furrows don’t penetrate the egg completely. In discoidal cleavage the embryo forms as a blastodisc that sits on top of the yolk. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (4) Superficial cleavage is a variation of incomplete cleavage in insects. The plasma membrane grows inward around nuclei and forms cells. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (5) Mitotic spindles of successive cell divisions determine planes of cleavage. Positions are defined by cytoplasmic determinants. Radial cleavage: mitotic spindles form parallel or perpendicular to the animal–vegetal axis. Spiral cleavage: cell layers form at oblique angles to the animal–vegetal axis. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (6) Mammals have rotational cleavage: First cell division is parallel to the animal– vegetal axis and yields two blastomeres. In the second division, the blastomeres divide at right angles to each other—one is parallel to the axis and the other is perpendicular to it. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (7) Mammalian cleavage is slow and asynchronous. When the zygote reaches the 8-cell stage, the blastomeres change shape and maximize contact with one another to form a tight ball. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (8) At the 32-cell stage, cells separate into two groups: Inner cell mass—becomes the embryo. Cells are pluripotent embryonic stem cells. Trophoblast—sac that forms from outer cells; secretes fluid to create the blastocoel cavity. Embryo is now called a blastocyst. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (9) In mammals, fertilization occurs in the oviduct; cleavage occurs as the zygote travels down the oviduct to the uterus. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (9) Implantation occurs when the trophoblast adheres to the endometrium, or uterine lining. Implantation that occurs in the oviduct is an ectopic, or tubal, pregnancy. The zona pellucida normally prevents this. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (10) Specific blastomeres rearrange during development and form specific tissues and organs. Fate maps are produced by labeling blastomeres to identify the tissues and organs they generate. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (11) Blastomeres become determined— committed to specific fates—at different times in different species. In mosaic development each blastomere contributes certain aspects to the adult animal (autonomous specification). If one blastomere is removed, a portion of the embryo will not form. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (12) Regulated specification: in most vertebrates, blastomere fate is determined by information from the environment and neighboring cells. Results in regulative development—other cells compensate for any lost cells. In humans, one blastomere can be removed for genetic analysis without harming the embryo. © Oxford University Press Concept 42.2 Mitosis Divides Up the Early Embryo (13) In species with regulative development, if blastomeres separate into two groups, each can become an embryo. Monozygotic twins come from the same zygote and are identical. Non-identical twins develop from two eggs fertilized by two sperm. © Oxford University Press Concept 42.3 Gastrulation Generates Multiple Tissue Layers (1) Gastrulation: massive movements of cells transform the blastula into an embryo with multiple tissue layers and distinct body axes. In triplobastic animals, three germ layers (tissue layers) form. © Oxford University Press Concept 42.3 Gastrulation Generates Multiple Tissue Layers (2) Endoderm layer: becomes lining of digestive and respiratory tracts, pancreas, thyroid, and liver. Ectoderm!"outer layer; becomes the nervous system, eyes, ears, and skin. Mesoderm!"#iddle layer; contributes tissues to many organs, including heart, blood vessels, muscles, and bone. © Oxford University Press Concept 42.3 Gastrulation Generates Multiple Tissue Layers (4) Archenteron cells extend, flatten, interdigitate, and migrate over one another to form a long thin tube (convergent extension). The mouth forms where the archenteron meets the ectoderm. The blastopore is the opening formed by invagination of the vegetal pole, and becomes the anus. © Oxford University Press Concept 42.3 Gastrulation Generates Multiple Tissue Layers (5) Amphibian gastrulation Bottle cells in the gray crescent bulge inward and form the dorsal lip. Cells move over the lip and into the blastocoel— involution. Convergent extension occurs when cells elongate as they move, and intercalate (move in between each other). © Oxford University Press Figure 42.8 Gastrulation in the Frog Embryo Concept 42.3 Gastrulation Generates Multiple Tissue Layers (7) Hans Spemann experimented with bisecting fertilized salamander eggs. Results differed depending on how the eggs were bisected. He found that cytoplasmic factors, such as those in the gray crescent, are necessary for normal development. © Oxford University Press Concept 42.3 Gastrulation Generates Multiple Tissue Layers (10) Activity of the organizer (dorsal lip) involves multiple transcription factors. Presence of b-catenin creates the organizer, which then induces the beginnings of the body plan. This involves a complex series of interactions between transcription factors and growth factors that control gene expression. © Oxford University Press Concept 42.3 Gastrulation Generates Multiple Tissue Layers (11) The organizer changes its activity in order to induce different structures. The first organizer cells to enter the embryo become head endoderm and head mesoderm. Later cells induce structures of the trunk; the last cells induce tail structures. Organizer cells express appropriate growth factor antagonists at the right times to achieve different patterns of differentiation on the anterior– posterior axis. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (1) Gastrulation produces an embryo with three germ layers that influence one another through inductive tissue interactions. Organogenesis: organs and organ systems develop. Neurulation: initiation of the nervous system— occurs early in organogenesis. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (2) One group of cells that passes over the dorsal lip on the midline becomes chordamesoderm. It produces a rod of mesoderm called the notochord, which provides support for the embryo. The chordamesoderm has organizer functions and induces the ectoderm to form the nervous system. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (3) Steps in neurulation: Ectoderm lying over the notochord thickens and forms the neural plate. Edges of the neural plate fold and a deep groove forms. The folds fuse, forming the neural tube and a layer of ectoderm. © Oxford University Press Figure 42.12 Neurulation in a Vertebrate (Part 3) Figure 42.12 Neurulation in a Vertebrate (Part 4) Concept 42.4 Organs Develop from the Three Germ Layers (4) Neural crest cells dissociate from the neural tube and migrate outward. They lead development of connections between the brain and spinal cord and the rest of the body. They generate many diverse structures, including jaws, skull, face, pigment cells, glands, smooth muscle, and peripheral nerves. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (5) The anterior end of the neural tube becomes the brain; the rest becomes the spinal cord. Failure of the neural tube to fuse in a posterior region results in spina bifida. Anencephaly: failure of the neural tube to close at the anterior end; no forebrain develops. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (6) Body segmentation develops during neurulation. Somites form from mesoderm on either side of the neural tube. They produce cells that become vertebrae, ribs, muscles, and lower skin layer. Somites guide peripheral nerve development. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (7) Homeotic genes control body segmentation; all have a DNA sequence called the homeobox. In vertebrates, Hox genes are homeotic genes that control differentiation along the anterior– posterior body axis. © Oxford University Press Concept 42.4 Organs Develop from the Three Germ Layers (8) Hox genes are expressed along the anterior– posterior axis of the embryo in the same order as their arrangement on the chromosome. Different segments of the embryo receive different combinations of Hox gene products, which act as transcription factors. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (1) Extraembryonic membranes surround the vertebrate embryo. They function in nutrition, gas exchange, and waste removal. In mammals, they interact with the mother’s tissues to form the placenta. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (2) In the chick embryo, four membranes form: Yolk sac: encloses yolk within the egg. Yolk is digested by cells of the yolk sac and the nutrients are transported to the embryo. Allantoic membrane: forms the allantois, a sac for waste storage. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (3) Amnion: surrounds the embryo, forming the fluid-filled amniotic cavity that protects the embryo. Chorion: forms a continuous membrane just under the eggshell; reduces water loss and exchanges gases. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (4) In placental mammals, the trophoblast is the first extraembryonic membrane. Trophoblast cells interact with the endometrium; adhesion molecules attach them to the uterine wall. The trophoblast burrows into the uterine wall, and sends out villi to increase contact with maternal blood. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (5) Hypoblast cells form the chorion. The placenta develops from the chorion and uterine tissues. The placenta exchanges nutrients, gases, and wastes between mother and embryo. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (6) The epiblast produces the amnion, which surrounds the embryo and is filled with amniotic fluid. Rupturing of the amnion and chorion and loss of the amniotic fluid (the “water breaks”) herald the onset of labor in humans. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (7) Allantoic tissues contribute to formation of the umbilical cord, by which the embryo is attached to the chorionic placenta. Blood vessels in the umbilical cord carry nutrients and oxygen from the mother to the developing fetus, and carry away wastes, including CO2 and urea. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (8) Gestation, or pregnancy, in humans is about 266 days (9 months) and is divided into trimesters. In the first trimester, the embryo becomes a fetus: Heart begins to beat by week four Limbs form by week eight © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (9) The first trimester is a time of rapid cell division and tissue differentiation. This is when the fetus is most susceptible to damage from radiation, drugs, chemicals, and pathogens that cause birth defects. Embryos can be damaged before the mother even realizes she is pregnant. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (10) Second trimester: limbs elongate; fingers, toes, and facial features form; nervous system develops. Third trimester: internal organs mature. Birth occurs when the lungs are mature. © Oxford University Press Concept 42.5 Extraembryonic Membranes Nurture Avian and Mammalian Embryos (11) The potential for serious effects from exposure to environmental factors exists throughout pregnancy. Factors such as protein malnutrition and exposure to alcohol and cigarette smoke can result in low birth weight, mental retardation, and other developmental complications. © Oxford University Press