Specification of digit number and identity is central to digit pattern in vertebrate limbs. The classical talpid(3) chicken mutant has many unpatterned digits together with defects in other regions, depending on hedgehog (Hh) signalling, and exhibits embryonic lethality. The talpid(3) chicken has a mutation in KIAA0586, which encodes a centrosomal protein required for the formation of primary cilia, which are sites of vertebrate Hh signalling. The highly conserved exons 11 and 12 of KIAA0586 are essential to rescue cilia in talpid(3) chicken mutants. We constitutively deleted these two exons to make a talpid3(-/-) mouse. Mutant mouse embryos lack primary cilia and, like talpid(3) chicken embryos, have face and neural tube defects but also defects in left/right asymmetry. Conditional deletion in mouse limb mesenchyme results in polydactyly and in brachydactyly and a failure of subperisoteal bone formation, defects that are attributable to abnormal sonic hedgehog and Indian hedgehog signalling, respectively. Like talpid(3) chicken limbs, the mutant mouse limbs are syndactylous with uneven digit spacing as reflected in altered Raldh2 expression, which is normally associated with interdigital mesenchyme. Both mouse and chicken mutant limb buds are broad and short. talpid3(-/-) mouse cells migrate more slowly than wild-type mouse cells, a change in cell behaviour that possibly contributes to altered limb bud morphogenesis. This genetic mouse model will facilitate further conditional approaches, epistatic experiments and open up investigation into the function of the novel talpid3 gene using the many resources available for mice.
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.
We present here a draft genome sequence of the red jungle fowl, Gallus gallus. Because the chicken is a modern descendant of the dinosaurs and the first non-mammalian amniote to have its genome sequenced, the draft sequence of its genome--composed of approximately one billion base pairs of sequence and an estimated 20,000-23,000 genes--provides a new perspective on vertebrate genome evolution, while also improving the annotation of mammalian genomes. For example, the evolutionary distance between chicken and human provides high specificity in detecting functional elements, both non-coding and coding. Notably, many conserved non-coding sequences are far from genes and cannot be assigned to defined functional classes. In coding regions the evolutionary dynamics of protein domains and orthologous groups illustrate processes that distinguish the lineages leading to birds and mammals. The distinctive properties of avian microchromosomes, together with the inferred patterns of conserved synteny, provide additional insights into vertebrate chromosome architecture.
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.
This chapter explores some features of vertebrate development involved in completing the body plan, using the chick and the mouse as model organisms. It focuses on the development of the spinal cord, somites, antero-posterior axis of the body, neural crest cells, and left-right asymmetry in the body. The chapter acknowledges the striking differences seen between birds, mammals, and amphibians, in terms of early development. It notes the significance of nourishment in relation to the early development of the chick and mouse. Moreover, the chapter mentions the similarities in how germ layers are specific and patterned among all vertebrates despite the different topologies of embryos.
Abstract The products of Hox-4 genes appear to encode position in developing vertebrate limbs. In chick embryos, a number of different signalling regions when grafted to wing buds lead to duplicated digit patterns. We grafted tissue from the equivalent regions in mouse embryos to chick wing buds and assayed expression of Hox-4 genes in both the mouse cells in the grafts and in the chick cells in the responding limb bud using species specific probes. Tissue from the mouse limb polarizing region and anterior primitive streak respecify anterior chick limb bud cells to give posterior structures and lead to activation of all the genes in the complex. Mouse neural tube and genital tubercle grafts, which give much less extensive changes in pattern, do not activate 5’-located Hox-4 genes. Analysis of expression of Hox-4 genes in mouse cells in the grafted signalling regions reveals no relationship between expression of these genes and strength of their signalling activity. Endogenous signals in the chick limb bud activate Hox-4 genes in grafts of mouse anterior limb cells when placed posteriorly and in grafts of mouse anterior primitive streak tissue. The activation of the same gene network by different signalling regions points to a similarity in patterning mechanisms along the axes of the vertebrate body.
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