Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. BIOLOGY A GUIDE TO THE NATURAL WORLD FOURTH EDITION DAVID KROGH An Amazingly Detailed Script: Animal Development

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings General Processes in Development

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Fertilization Development in animals begins with the formation of a zygote or fertilized egg, created when the male sperm fuses with the female egg. Once a zygote starts to divide, it is called an embryo.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Three Phases of Early Embryonic Development There are three phases of early embryonic development: –cleavage –gastrulation –organogenesis

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Cleavage The first phase is cleavage, which produces a ball of cells, called a blastula, with an interior, liquid-filled cavity.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Cleavage Figure 31.1 animal pole vegetal pole Zygote cross section of blastula Cleavage blastocoel Cell division begins Morula Blastula

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Gastrulation The second is gastrulation, in which the blastula’s cells rearrange themselves into three layers that give rise to specific organs and tissues.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Gastrulation Figure 31.2 Gastrulation animal pole vegetal pole gastrulablastula archenteron mesenchyme mouth anus digestive system blastocoel ectoderm mesoderm endoderm Start with fluid- filled blastula. Mesenchyme cells prompt vegetal pole to pinch inward toward animal pole. Archenteron elongates toward animal pole. Archenteron becomes digestive tube. Three tissue layers now formed.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Gastrulation Table 31.1

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Organogenesis The third is organogenesis, in which organs begin to form.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Organogenesis Figure 31.3 anterior posterior anterior dorsal ventral neural plate neural crest cells neural foldsneural tube notochord Organogenesis ectoderm mesoderm endoderm Start with embryo near end of gastrulation. Notochord induces development and infolding of neural plate. Neural folds deepen from anterior to posterior, giving rise to neural tube. Neural tube complete; will give rise to brain and spinal cord.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development The first product of cleavage is a tightly packed ball of cells called a morula.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development The cells in the morula rearrange themselves into a structure composed of one or more layers of cells surrounding a liquid filled cavity. This structure is the blastula. Its interior cavity is called the blastocoel.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development Gastrulation produces an embryo composed of three layers of cells: –the endoderm –the mesoderm –the ectoderm

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development The endoderm gives rise to interior tissues (the liver and bladder, for example). The mesoderm gives rise to tissues exterior to these (muscle and skeletal systems). The ectoderm gives rise to tissues that are more exterior yet (the skin and nervous system).

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development In early organogenesis in vertebrates, a rod- shaped support organ, called a notochord, develops near the dorsal surface. It induces the development of ectodermal tissue that lies above it. A neural plate develops from this tissue, folding over on itself to form a hollow neural tube that gives rise to both the brain and the spinal cord.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development Figure 31.4 neural crest cells neural tube epidermis notochord mesoderm gut migratory pathway site of future ganglion site of future adrenal glands sites of future pigment cells

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Early Embryonic Development Groups of neural crest cells break away from the top of the neural tube during its development and then migrate to different locations throughout the embryo. These groups go on to develop into different organs or tissues.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Themes in Development The early stages of development represent a transition from the general to the specific.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Themes in Development A small group of superficially similar cells gives rise to three different layers of cells, which give rise to tissues, which in turn give rise to specific organs.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Embryonic Development PLAY Animation 31.1: Embryonic Development

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings What Factors Underlie Development?

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Induction Induction is a process in which some embryonic cells direct the development of other embryonic cells.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Interaction of Genes and Proteins Development is fundamentally controlled by the interaction of genes and proteins. A key model organism in scientific understanding of these interactions is the fruit fly Drosophila melanogaster.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Positional Information A critical early step in development is the establishment of positional information–for example, the establishment of anterior and posterior in a growing embryo.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Positional Information Positional information in Drosophila is provided in part by a protein called bicoid that exists in a concentration gradient in the fly embryo. The greatest concentration is at the anterior end of the embryo, with a lesser concentration toward the center, and none at the posterior end.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Biocoid Bicoid helps bring about anterior structures, such as the head of the fly and center-portion structures as well.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Drosophilia Egg and Bicoid Gradient Figure 31.8 Bicoid mRNAs are deposited in unfertilized egg. Bicoid proteins diffuse through embryo. anterior posterior dorsal ventral bicoid mRNA High bicoid concentration Low to undetectable bicoid concentration nurse cells future head future thorax future abdomen

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Morphogens Bicoid is an example of a morphogen: a diffusable substance whose local concentration affects the course of local development in an organism.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Morphogens In general, morphogens diffuse from one set of cells to a second set of cells. Binding with these latter cells, morphogens prompt them to begin producing transcription factors: proteins that regulate DNA by binding with it, thus turning selected genes on or off.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Morphogens All morphogens work through the kind of concentration gradient seen with bicoid. Exposure to a greater concentration of a morphogen brings about one effect (a head in Drosophila), while exposure to a lesser concentration brings about another effect (center-portion structures in the Drosophila).

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Genetic Cascade Development works through a genetic cascade in which one set of genes regulates a subsequent set of genes. This cascade explains how development proceeds from the general to the specific.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Genetic Cascade A first level of genetic instruction specifies broad positional patterns in an embryo. For example, the head and center-section portions of Drosophila.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Genetic Cascade Subsequent levels of genetic instruction specify development within these body sections. Finally, control is exercised over the development of specific structures, such as organs.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Unity in Development: Homeobox Genes

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Homeobox Genes Within many of the developmental genes of animals as different as flies, mice, and human beings, there exists an identical sequence of about 180 DNA bases, called a homeobox.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Homeobox Genes The homeobox has been highly conserved—it has remained largely unchanged over hundreds of millions of years of evolution in all animal species studied.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Homeobox Genes As a result of this conservation, similar basic developmental processes are at work in all animals and possibly in fungi and plants as well.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Homeobox Genes Scientists are now looking at the differences in homeobox genes and their proteins as a means of learning more about both animal development and animal evolution.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Embryonic Development in Six Vertebrates Figure 31.6

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Developmental Tools: Sculpting the Body

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Developmental Tools Three cell capabilities help shape the animal body in development. The first is cell movement.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Developmental Tools The second is cell adhesion, in which cells produce proteins that protrude from their surface, which selectively adhere to other cells.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Developmental Tools The third is programmed cell death, or apoptosis, in which spaces in tissues are created by means of cells dying.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Developmental Tools Figure 31.9

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Development Through Life

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Development Through Life Development is sometimes thought of as ending at birth, but it continues through the life span of an organism. It is exemplified in human beings in such processes as puberty and menopause.