Progenitor Cells in Vertebrate Segmentation

Benjamin Martin

CONTENTS

  • 5.1 Segmentation i n Vertebrates...........................................................................99
  • 5.2 The Clock and Wavefront Model of Somitogenesis.....................................101
  • 5.3 Neuromesodermal Progenitors As the Cellular Source of

Vertebrate Segments.....................................................................................102

  • 5.4 Do All Somites along the Anterior-Posterior Axis Come from NMPs?.........103
  • 5.5 Are NMPs Stem Cells?.................................................................................104
  • 5.6 The Transcription Factor Brachyury Links NMP Maintenance

with Segmentation........................................................................................105

5.7 Signaling Pathways Coordinating Mesoderm Induction

and Segmentation..........................................................................................107

  • 5.8 Induction of Paraxial/Somite Fate in NMP-Derived Mesoderm..................109
  • 5.9 Termination of Somitogenesis......................................................................110
  • 5.10 Progenitor Cell Behaviors That Influence the Synchronization of

Cycling Gene Expression..............................................................................Ill

5.11 NMPs As a Development Module Affecting Evolutionary Change in

Segment Number and Body Length.............................................................113

  • 5.12 Do NMPs Exist Outside the Vertebrate Clade?............................................115
  • 5.13 Conclusions...................................................................................................116

References..............................................................................................................116

Segmentation in Vertebrates

Segmentation of the vertebrate body is most noticeable from the clade’s eponymous vertebrae that surround the spinal cord, as well as the ribs and skeletal muscles of the trunk. These features arise from transient embryonic structures called somites (Figure 5.1). Somites originate from the paraxial mesoderm, which forms in bilateral stripes on either side of the embryonic midline (Hubaud and Pourquie, 2014). Pairs of somites bud off from the presomitic paraxial mesoderm in a progressive fashion, with the first pair forming close to the head and the final pair at the posterior end of the body. The rate at which somites form, and the final number generated, is specific to each species (Maroto et ah, 2012). In addition to ribs and vertebrae, somites also give rise to skeletal muscle, dermis, tendons, cartilage, and endothelial cells. This chapter will focus on the formation of the somites, which represent the origin of the

Segmentation in humans

FIGURE 5.1 Segmentation in humans. (A) A dorsal view of a Carnegie stage 10 human embryo. Forming somites (red) are present on either side of the spinal cord. (B) The segmental somites later give rise to segmental structures in the adult, such as the vertebrae of the spinal column and the ribs (red).

bulk of the segmented tissue in the vertebrate body and also play a direct role in patterning adjacent segmental structures.

Other tissues in the vertebrate embryo undergo a segmental mode of development, including the nervous system, vasculature, notochord, and specific head structures. Alongside the spinal cord, sensory, motor, and autonomic nerves of the peripheral nervous system are present in a segmental organization. This pattern is organized by the somites, which express a repulsive migratory molecule exclusively in the posterior half of the somite, causing lateral migration of the nerves along the anterior half somite (Keynes, 2018). Similarly, a repulsive cue expressed throughout the somite causes intersomitic blood vessels to develop in a segmental pattern between somites (Gu et al., 2005). Recent work in zebrafish showed that the outer layer of cells in the notochord, called the sheath cells, display segmental gene expression (Lleras Forero et ah, 2018). These cells later become the chordacentra of the vertebrae. The segmental pattern of the notochord sheath is influenced by the signals from the segmented somites, but also can occur in the absence of proper somite segmentation, suggesting that the axial mesoderm undergoes an autonomous segmentation program (Lleras Forero et ah, 2018). Nevertheless, the developmental life history of the axial mesoderm progenitors share many similarities with the somite progenitor cells (Row et ah, 2016), and thus the segmental underpinnings of both tissues may be influenced by a common molecular network while they are in the progenitor state.

There are other segmental structures in the embryo for which the segmentation mechanism is not directly influenced by the segmental somites. These include the rhombomeres of the hindbrain and the pharyngeal arches, which give rise to the majority of the musculoskeletal and neurovascular tissues of the head. Although there are some shared molecular mechanisms involved in these segmentation processes, they occur in a largely unique fashion that has been described in recent reviews (Frank and Sela-Donenfeld, 2019; Frisdal and Trainor, 2014; Wilkinson, 2018).

 
Source
< Prev   CONTENTS   Source   Next >