Anatomy and Embryology最新文献

In vertebrates, BMPs (bone morphogenic proteins) play critical roles in establishing the basic embryonic body plan and are involved in the development of a large variety of organs and tissues. Here, we analyzed the expression pattern of various BMPs (2, 4, 5 and 7) by whole mount in situ hybridization during chick limb development. In limb, expression of BMPs suggests evolutionary conserved mechanisms of BMP-dependent differentiation between lower and higher vertebrates. During the early developmental stages, BMP-2 and BMP-7 are expressed in the posterior distal mesenchyme leaving a less prominent expression anteriorly. BMP-4 is initially expressed in the anterior mesenchyme and spreads later to the whole mesenchyme leaving a stronger expression at the anterior side. From HH-stage 25, expression of BMP-4 is observed in the anterior-posterior margins of the limb bud. The BMPs 2, 4 and 7 are expressed strongly in the AER, whereas BMP-5 is expressed as a weak signal in the distal mesoderm during the early stages of limb development. Later from HH-stage 25 onwards, BMP-5 is expressed in the dorsal and ventral muscular mass of the developing limb. As digits become identifiable, expression of BMPs are observed in the interdigital mesenchyme and can also be detected along the contours of the developing phalanges and at the distal tips of the digits. All these BMPs are found to be expressed in the developing feather buds from day 8 onwards.

A major subclass of hypaxial muscle groups is derived from long-range migrating precursor cells that delaminate from the dermomyotome. Migrating precursors are generated on particular axial levels only, i.e. occipitally, cervically, and on the levels of the fore and hind limbs. They express the homeobox gene Lbx1, which provides a useful marker for their visualization. In the mouse, migrating precursor cells give rise to muscles of the extremities, the hypoglossal chord, and the diaphragm. We discuss here the development of this migrating lineage, which critically depends on the correct specification of the precursors in the dermomyotome, their delamination and correct migration. Finally, proliferation at the targets is essential to ensure a correct size of the precursor pool and of the muscle that derives thereof.

Somites are sequentially formed, metameric units of the paraxial mesoderm of vertebrate embryos. They are the most obvious correlative of the segmental patterning along the cranio-caudal axis and transfer segmentation to other tissues such as the spinal nerves and dorsal aortic branches. Furthermore, somites are the source of numerous mesodermal cell types such as smooth and striated muscle, cartilage and tendon cells, and soft connective tissue. They also give rise to endothelial cells. Here we focus on the finding that two lineages of endothelial cells, blood vascular endothelial cells and lymphatic endothelial cells are derived from the somite. Their precursors, angioblasts, and lymphangioblasts, respectively, are born in the somite at different time points. Angioblasts are characterized by the expression of vascular endothelial growth factor receptor-2, whereas lymphangioblasts express the homeobox transcription factor Prox1. There seem to be two types of lymphangioblasts. Type 1 is derived from venous endothelium, while type 2 originates from mesenchymal precursor cells. The molecular networks of angioblast and lymphangioblast development and the relation between the two cell types and hematopoietic cells are discussed.

Somites are a common feature of the phylotypic stage of embryos of all higher chordates. In amniote species like mouse and chick, somite development has been the subject of intense research over many decades, giving insight into the morphological and molecular processes leading to somite compartmentalization and subsequent differentiation. In anamniotes, somite development is much less understood. Except for recent data from zebrafish, and morphological studies in Xenopus, very little is known about the formation of somite compartments and the differentiation of somite derivatives in anamniotes. Here, we give a brief overview on the development of myotome, sclerotome and dermomyotome in various anamniote organisms, and point out the different mechanisms of somite development between anamniotes and the established amniote model systems.

It is currently thought that the mechanism underlying somitogenesis is linked to a molecular oscillator, the segmentation clock, and to gradients of signaling molecules within the paraxial mesoderm. Here, we review the current picture of this segmentation clock and gradients, and use this knowledge to critically ask: What is the basis for periodicity and directionality of somitogenesis?

It is now over 30 years since Bodo Christ first demonstrated that the musculature of the limb originated from the somites and overturned the then prevailing view that limb muscle develops from a local source. Subsequently, using electron microscopy and histological procedures, Bodo Christ identified that cells of the somites undergo an epithelial to mesenchymal transition which enabled them to move from their paraxial point of origin to distal locations. These studies defined this translocation as one of the major mechanisms allowing myogenic cells to translocate around the body. The other means used to translocate muscle involves the movement of cells as a sheet. The deployment of one of these two mechanisms has been postulated to be involved in the formation of all the hypaxial musculature of the vertebrate body. In this paper we describe the formation of muscles both in the head and in the body, which use a translocatory mechanism during their development. We highlight recent data showing that muscle translocation is a far more complex process than first thought but which in itself can be used as a valuable tool to address questions regarding tissue patterning and development.

The segmental somites not only determine the vertebrate body plan, but also represent turntables of cell fates. The somite is initially naive in terms of its fate restriction as shown by grafting and rotation experiments whereby ectopically grafted or rotated tissue of newly formed somites yielded the same pattern of normal derivatives. Somitic derivatives are determined by local signalling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. The correct spatio-temporal specification of the deriving tissues, skeletal muscle, cartilage, endothelia and connective tissue is achieved by a sequence of morphogenetic changes of the paraxial mesoderm, eventually leading to the three transitory somitic compartments: dermomyotome, myotome and sclerotome. These structures are specified along a double gradient from dorsal to ventral and from medial to lateral. The establishment and controlled disruption of the epithelial state of the somitic compartments are crucial for development. In this article, we give a synopsis of some of the most important signalling events involved in somite patterning and cell fate decisions. Particular emphasis has been laid on the issue of epithelio-mesenchymal transition and different types of cell division in the somite.

The somite and its intermediate derivatives, sclerotome and dermomyotome (DM), are composed of distinct subdomains based on lineage analysis and gene expression patterns. This sets the grounds for elucidating the mechanisms underlying differential cell specification and morphogenesis. By examining the in vivo roles of N-cadherin on discrete domains of the somitic epithelium at various times, our recent studies highlight the existence of a regional and temporal heterogeneity in cellular responsiveness. As examples of this assortment, we document a coupling between asymmetric cell division and fate segregation in the DM sheet, sequential effects of N-cadherin-mediated adhesion on early myogenic specification compared to later myofiber patterning, and a differential behavior of pioneer myoblasts compared to later myogenic waves.