Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 156

MyoD

Paraxis

Myf5

D

Myogenesis

Wnt-1 Pax3

BMP4

Shh Noggin

V

Chondrogenesis

Pax1

Figure 3.28 Schematic presentation of signaling molecules affecting somitic myogenesis.
The relationships between signaling molecules and important transcription factors that
participate in compartmentalization and differentiation of the somite. The dotted black line
indicates that Bmp4 presumably creates the ability to express noggin. The dashed gray line
indicates a possible Shh signal upregulating early myotomal markers. Abbreviations :
D, dorsal; V, ventral.
Source : From Piran et al. (2009) .

hindlimbs, RALDH-2 precedes the arrival of motoneurons ( Wang and Scott, 2008 ).
This, and the fact that denervation performed before the nerves enter the limb
impairs muscle growth, may suggest that the innervation is necessary for survival
and proper growth of muscles, rather than for initial muscle differentiation ( Butler
et al., 1982 ).
Firing patterns of motor neurons that innervate chick muscles determine mus-
cle fiber developmental type by differentially regulating the expression of genes for
myosin-heavy chains in fast and slow muscle fibers, which differ in morophology
and contractility ( Chin et al., 1998 ; Jordan et al., 2004 ).

Development of the Heart

The neural tube negatively controls heart development. Despite the presence of
heart-inducing BMP in the mesoderm, the anti-BMP signals (Wnt-3 and Wnt-8)
released by the neural tube inhibit the formation of the heart Anlage in the meso-
derm in the vicinity of the neural tube ( Tzahor and Lassar, 2001 ; see Figure 3.29 ).
The inhibitory action of the neural tube in heart development is overcome by the
secretion of the anti-Wnt signals (dkk-1 and crescent) from the Spemann organizer
and the ectopic expression of dkk- 1 or crescent induces the expression of cardiogenic
genes in the noncardiogenic mesoderm ( Schneider and Mercola, 2001 ).