Figure 11.

Scheme of the development of the extrinsic innervation in the thoraco‐abdominal body cavity. Panel A shows transverse views with increasing age of the embryo. Starting at 5.5 weeks, the left side of each scheme represents the level of the CA and the right side that of the IMA. At 5 weeks NCCs (blue) or SCPs (light blue) migrate through the mesenchyme or along the ventral roots of spinal nerves (yellow), respectively, to the para‐aortic region. Slightly later, nerve fibres extend medially as communicating branches (gold). At 5.5 weeks ganglionic cells either remain in the para‐aortic region to form the sympathetic trunk (ST) or migrate further ventrally. Concomitantly, nerve fibres (beige) extend ventrally to form the thoracic and lumbar splanchnic nerves (left and right part of panel, respectively). Note that the adrenal glands (Ad; brown) have formed at the level of the CA. At 6 weeks, most SCP‐derived ganglionic cells migrate towards the adrenal cortex to become the adrenal medulla or aggregate to form para‐aortic bodies at the level of the IMA. At the level of the CA, pre‐aortic nerve fibres extend further ventrally to connect to the intrinsic nerve fibres inside the stomach wall (light green diamonds). At 7.5 weeks, nerve fibres at the level of the IMA also connect to the intrinsic nerve fibres in the gut wall. Panel B shows the spatiotemporal distribution of the ganglionic cells and extrinsic nerve fibres along the ventral branches of the aorta. From 6 weeks onwards the opposing gradients of the cranially prominent adrenal medulla (blue) and the caudally prominent para‐aortic bodies (lighter blue) form. Preganglionic nerve fibres form the thoracic and lumbar splanchnic nerves (beige), while periarterial extrinsic nerve fibres extend and connect with the intrinsic nerve fibres (light green lines). Lumbar nerves extend caudally across the aortic bifurcation into the lesser pelvis (arrow). Nerve fibres surrounding the SMA and IMA extend through the mesentery (grey) to connect to the intrinsic fibres of the 1st secondary intestinal loop, caecum and hindgut at 8 weeks, and to those of the 2nd and 3rd secondary intestinal loops at 9.5 weeks. Note, however, that extrinsic nerves have not yet appeared in the mesentery of the colon at this stage (asterisks) [Colour figure can be viewed at wileyonlinelibrary.com]