The role of the notochord in amniote vertebral column segmentation.

The vertebral column is segmented, comprising an alternating series of vertebrae and intervertebral discs along the head-tail axis. The vertebrae and outer portion (annulus fibrosus) of the disc are derived from the sclerotome part of the somites, whereas the inner nucleus pulposus of the disc is derived from the notochord. Here we investigate the role of the notochord in vertebral patterning through a series of microsurgical experiments in chick embryos. Ablation of the notochord causes loss of segmentation of vertebral bodies and discs. However, the notochord cannot segment in the absence of the surrounding sclerotome. To test whether the notochord dictates sclerotome segmentation, we grafted an ectopic notochord. We find that the intrinsic segmentation of the sclerotome is dominant over any segmental information the notochord may possess, and no evidence that the chick notochord is intrinsically segmented. We propose that the segmental pattern of vertebral bodies and discs in chick is dictated by the sclerotome, which first signals to the notochord to ensure that the nucleus pulposus develops in register with the somite-derived annulus fibrosus. Later, the notochord is required for maintenance of sclerotome segmentation as the mature vertebral bodies and intervertebral discs form. These results highlight differences in vertebral development between amniotes and teleosts including zebrafish, where the notochord dictates the segmental pattern. The relative importance of the sclerotome and notochord in vertebral patterning has changed significantly during evolution.

Acute diffuse thyroid swelling: A rare complication of fine-needle aspiration.

We present a case illustrating the rare complication of acute generalized thyroid swelling shortly after sonographic-guided fine needle aspiration of a thyroid nodule. Ultrasound revealed the presence of characteristic linear hypoechoic avascular areas interspersed throughout the gland suggestive of edema. The patient was treated conservatively, with near complete normalization of the thyroid within 24 hours. Recognition of this potential complication is important, as the rapid onset of diffuse thyroid enlargement is often alarming but typically has a transient and self-limiting course. © 2017 Wiley Periodicals, Inc. J Clin Ultrasound 45:426-429, 2017.