This chapter begins with an analysis of the amphibian cleavage which is holoblastic and is unequal because of the presence of yolk in the vegetal hemisphere. The chapter shows how amphibian gastrulation begins with epiboly of the ectoderm followed by the invagination of the bottle cells and the coordinated involution of the mesoderm. It also reviews dorsal-ventral specification which begins with maternal messages and proteins stored in the vegetal cytoplasm. The chapter analyzes the Danio rerio, the zebrafish, which has a vertebrate model system that develops externally as a transparent embryo and completes embryogenesis in one day. It describes Kupffer's vesicle, which is a transient, fluid-filled organ in the most posterior position of the trunk.
Amphibians and Fish
Birds and Mammals
This chapter focuses on reptiles and birds which undergo discoidal meroblastic cleavage, wherein the early cell divisions do not cut through the yolk of the egg but form a blastoderm. It discusses the primitive streak which is derived from epiblast cells and the central cells of Koller's sickle. It also describes the prechordal plate which helps induce formation of the forebrain, wherein the chordamesoderm induces the formation of the midbrain, hindbrain, and spinal cord. The chapter examines the two signaling centers of mammalian gastrulation, one in the node and one in the anterior visceral endoderm. It reviews the homology of gene structure and the similarity of expression patterns between drosophila and mammalian Hox genes that suggest that this patterning mechanism is extremely ancient.
This chapter discusses the radial glial cells that serve as the neural stem cells in the embryonic and fetal brain. It examines the neurons of the brain and shows how they are organized into laminae (layers) and nuclei (clusters). It also details how new neurons are formed by the division of neural stem cells in the wall of the neural tube. They then migrate away from the ventricular zone and form a new layer, called the mantle zone. The chapter points out the difference between human brains and those of other primates, in terms of elements such as the retention of the fetal neuronal growth rate during early childhood, the altered transcriptional activity of certain genes, and the presence of human-specific alleles of developmental regulatory genes. It explores the number and complexity of gyri and sulci folds of the neocortex that are correlated with the level of intelligence.
Cell differentiation and stem cells
This chapter looks into cell differentiation and stem cells. It notes the link between gene expression and cell differentiation by addressing how referencing extracellular signals having have a key role in differentiation, as they by triggering intracellular signalling pathways that impact gene expression. Cell differentiation leads to distinguishable cell types, such as blood cells, nerve cells, and muscle cells. In addition, the chapter looks into the properties of mammalian embryonic stem cells. It mentions the degree of plasticity resulting after cell differentiation. The chapter then discusses how human stem cells cultured in vitro can also give rise to organoids that are used for drug testing and studying disease.
Mechanisms of Morphogenesis
This chapter focuses on cell-to-cell communication, which occurs between cells in direct contact with one another or across a distance through cells secreting proteins into the extracellular matrix. It discusses the extracellular matrix, which serves to modify how signals may be secreted across cells to influence differentiation and cell migration. It also describes cell migration that occurs through changes in the actin cytoskeleton, which can be directed by internal or external instructions. The chapter discusses hormone auxin, which is a major morphogen found in plants and plays a major role in the proper morphogenesis of the embryo that is conserved across plant species. It assesses the asymmetric positioning of PIN efflux transporters on select sides of plant cells that functions to control the directional flow of auxin throughout the plant.
Conceptualizing Early Development
An Overview of the Essential Processes
This chapter discusses embryonic cleavages in the earliest stages of animal development and shows how they rapidly produce large numbers of cells. It analyzes the pattern of embryonic cleavage which is determined by the amount and distribution of yolk protein within the cytoplasm. The chapter also looks at factors in the egg cytoplasm which influence the angle of the mitotic spindle and the timing of its formation. It also reviews many and varied patterns of animal gastrulation which are based on invagination, involution, ingression, delamination, epiboly, and convergent extension. The chapter looks at collinear Hox genes which determine cell identities along the anterior-posterior axis. It outlines the use of gastruloids which has helped define the intrinsically conserved patterning systems controlling axis determination.
Development of nematodes and sea urchins
This chapter looks into the body plan development of invertebrates for nematodes and sea urchins. It notes the general plan of animal development: cleavage leads to a blastula, which then undergoes gastrulation with the emergence of a body plan. The chapter also cites the differences between regulative development and mosaic development. Nematodes are said to undergo mosaic development, while sea urchins undergo regulative development. Next, the chapter explains how specification on a cell-by-cell basis often utilize asymmetric cell division and the unequal distribution of cytoplasmic factors. It highlights the significance of cytoplasmic localization, cell lineage, and local intercellular interactions in terms of specifying the fate of a cell.
Development of the Drosophila body plan
This chapter looks into the development of the fruit fly, Drosophila melanogaster, in order to get a greater understanding of the morphological complexity of animals. It highlights how other animals’ developmental genes have been identified thanks to their homology with Drosophila melanogaster’s genes. The chapter discusses the development and life cycle of Drosophila melanogaster. It also looks into the process of oogenesis, fertilization, segmentation, and gastrulation in terms of cell and zygote development. In addition, the chapter notes the specification of segment identity such as with Hox genes. It also includes how segment and compartment boundaries are involved in patterning and the polarizing of segments of an organism.
Development of the nervous system
This chapter discusses the development of the nervous system. It recognizes the complexity of the nervous system in comparison to all the organ systems in the animal embryo. Moreover, the chapter mentions the divide between the central nervous system (CNS) and peripheral nervous system (PNS). It explores the different types and connections of neurons. In addition, the chapter explains how the nervous system is superficially similar to other developmental systems, except that it involves the acquisition of individual cell identities. It looks into how neural activity plays a major role in refining connections, such as those between the eye and the brain.
Development of the Tetrapod Limb
This chapter describes the positions from which limbs emerge from the body axis which depend on Hox gene expression. It also highlights the proximal-distal axis of the developing limb which is initiated by the induction of the ectoderm at the dorsal-ventral boundary by Fgf10 from the mesenchyme. It also explores the opposing gradients of FGFs and Wnts from the AER and retinoic acid from the flank which patterns the proximaldistal axis of the chick limb. The chapter investigates cell death in the limb which is mediated by bone morphogenetic proteins (BMPs) and is considered necessary for the formation of digits and joints. It points out the involvement of BMPs in inducing apoptosis and in differentiating the mesenchymal cells into cartilage.
Michael J. F. Barresi and Scott F. Gilbert
Development Biology presents exciting developments in this field. The first few chapters look at patterns and processes of becoming and provides a framework for understanding animal development. The text then turns to an examination of gametogenesis and fertilization. The next few chapters tackle early development: cleavage, gastrulation, and axis formation. There follow chapters about building with ectoderm and building with mesoderm and endoderm. The next few chapters cover postembryonic development. Finally, the last part looks at development in wider context including an examination of development in health and disease, and the environment, and evolution.
Developmental Mechanisms of Evolutionary Change
This chapter centers on evolution, which is the result of inherited changes in development and contributes to modifications of embryonic or larval development. It discusses the modularity of development that allows parts of the embryo to change without affecting other parts and is caused by the modularity of enhancers. It also considers the recruitment of existing genes and pathways for new functions as a fundamental mechanism for creating new phenotypes. The chapter details how new anatomical structures may result from duplicated genes whose regulation has diverged. It outlines the ways evolutionary change is affected through development at the level of gene expression: change in location (heterotopy), change in timing (heterochrony), change in amount (heterometry), and change in kind (heterotypy).
Differential Gene Expression
Mechanisms of Cell Differentiation
This chapter reviews the evidence from molecular biology, cell biology, and somatic cell nuclear cloning that has shown that each cell of the body carries the same nuclear genome. It analyzes differential gene expression from genetically identical nuclei that creates different cell types and occurs at the levels of gene transcription, pre-mRNA processing, mRNA translation, and protein modification. It also shows how enhancer sequences regulate a gene's transcription in time and space. The chapter talks about DNA methylation, which can block transcription by preventing the binding of certain transcription factors or by recruiting histone methyltransferases or histone deacetylases to the chromatin. It covers Class A, B, C, D, and E transcription factors that function as homeotic regulators of floral organ identity.
Early Human Development
This chapter discusses the conceptus of an embryo on the first eight weeks after fertilization. Subsequently, it is then called a fetus after the eighth week, exploring the stage of growth of humans when cell specification and the beginnings of organ development occur. It details how a woman becomes pregnant when the embryo implants into her uterus at the beginning of the second week of development. It also covers gonadotropic hormones which promote oocyte growth by stimulating reactions in the granulosa cells. The chapter investigates in vitro fertilization, wherein the first successful assisted reproductive technology in humans was made possible when the necessity of sperm capacitation was discovered. It points out how chance plays a role in developmental outcomes as there is a large variation in the levels of gene transcription and translation, noting that cells make more or fewer developmentally important proteins at different stages.
Ectodermal Placodes and the Epidermis
This chapter focuses on ectodermal placodes, which are areas of columnar-shaped cells. It illustrates how cranial placodes in the head contribute to the sense organs forming the olfactory epithelium, the inner ear, and the lens of the eye, and to the cranial sensory ganglia. It also explains how the pre-placodal region separates into individual placodes, a process controlled by local signals from the neural tube and underlying mesoderm or endoderm. The chapter discusses eye development and shows how it starts with the specification of the eye field in the ventral diencephalon. The chapter also shows the major role Pax6 plays in eye formation. It mentions the enamel knot, which is the signaling center for tooth shape and development.
Tubes and Organs for Digestion and Respiration
This chapter discusses the Sox17 transcription factor, which is crucial for the specification of the endoderm that constructs the digestive (gut) tube and the respiratory tube. It analyzes the endoderm in the posterior which becomes a collection of midgut-hindgut precursor cells forming the intestines and the endoderm near the head that form the anterior foregut cells that generate the precursors of the lung and thyroid glands. It also mentions the pharyngeal pouches which become the endodermal lining of the eustachian tubes, the tonsils, the thymus, and the parathyroid glands. The chapter describes the muscular layers of the gut which are specified and placed in their particular locations by Shh signals from the intestinal endoderm. It reveals the three modes of branching morphogenesis in the developing lung: domain branching, planar bifurcation, and orthogonal bifurcation.
The Environmental and Symbiotic Regulation of Development
This chapter highlights agents of the environment, such as the temperature, diet, and the presence of predators, which play critical roles during normal development. It considers phenotypic reaction norms that quantitatively respond to environmental conditions, such as the phenotype reflecting small differences in environmental conditions. It also provides an overview on organisms that usually develop with symbiotic organisms and signals from the symbionts, which can be critical for normal development. The chapter includes organisms which have integrated life cycles, featuring marine organisms whose larvae require microbes in order to settle and undergo metamorphosis. It examines changes in the environment which can be detrimental to those organisms whose life cycles are predicated on specific conditions, such as temperature or prey species.
Evolution and development
The evolution of development The diversification of body plans The evolutionary modification of specialized characters Changes in the timing of developmental processes The evolution of multicellular organisms is fundamentally linked to embryonic development, for it is through development that genetic changes...
Beginning a New Organism
This chapter focuses on fertilization and relates it to sex and reproduction. It highlights the events of fertilization which include contact and recognition between sperm and egg, regulation of sperm entry into the egg, fusion of genetic material from the two gametes, and activation of egg metabolism to start development. It also describes eggs and their associated cells in plants or animals and shows how they secrete diffusible molecules to attract and activate the sperm. The chapter explores the two blocks to polyspermy: the fast block that is immediate, transient, and causes the egg membrane resting potential to rise and the slow block or cortical granule reaction that is physical, permanent, and is mediated by calcium ions. It shows the fusion of sperm and egg which causes re-initiation of the egg's cell cycle and subsequent mitotic division.
The Genetics of Axis Specification in Drosophila
This chapter considers drosophila cleavage as superficial as the nuclei divide thirteen times before being compartmentalized and reside in a syncytial blastoderm before cell formation. It details how the drosophila embryo undergoes a mid-blastula transition, wherein the cleavages become asynchronous and new mRNA is made. It also talks about gradients of morphogens which determine the specification of different cell types. The chapter looks at the gap gene proteins that activate and repress the pair-rule genes, which have modular enhancers that become activated in seven “stripes.” It analyzes how organs form at the intersection of dorsal-ventral and anterior-posterior regions of gene expression.