32 - Hemivertebrae

*First read this question that appeared in the May 2006 AIIMS Medical Postgraduation Entrance Examination .


Q: Lumbar hemivertebra results due to the abnormal development of ?


a. dorsal sclerotome
b. intermediate cell mass
c. notochord
d. ventral sclerotome

answer d. ventral sclerotome

Now after you have seen the answer, u have to know what hemivertebra actually means .


The congenital vertebral anomalies are classified based on failure of formation, failure of segmentation and a combination of the two (mixed) (Fig. 1). The most common type of failure of formation anomaly is a hemivertebra. This is where a portion of the vertebra is missing resulting in a small, triangular shaped "half vertebra" or hemivertebra. Hemivertebrae can be subclassified based on their relationship to the adjacent spine (segmented, semisegmented, nonsegmented).

When several vertebral segments fail to separate bilaterally, a block vertebra results producing fused vertebral bones. Unilateral unsegmented vertebral bars are caused by the failure of segmentation only on the left or right side of the spine.

Fig. 1. Diagrammatic representation of classification system of congenital scoliosis. (A) Failures of formation; various types of hemivertebrae (semisegmented, fully segmented, wedge vertebra). (B) Failures of segmentation; block vertebra, unsegmented bar. (C) Mixed; combination of hemivertebra and unsegmented bar.




Now that you know what hemivertebra means lets see what sclerotome actually means . look at the figure below .

In the figure above ,somites are arranged in pairs alongside the neural tube(light orange) and notochord (blue). Each somite is composed of a dermatome (light purple), myotome (light brown), and sclerotome (dark yellow). The ectoderm lies above and the endoderm below. Now see the picture below which demonstrates the formation of the vertebrae. 

 The above figure shows that the sclerotomes present in the somites proceed dorsally over the neural tube to form dorsal somites and the sclerotomes which proceed ventrally below the neural tube form the ventral somites .

The dorsal somites form the dorsal part of the vertebrae and the ventral somites form the ventral part of the vertebrae . the neural tube forms the spinal cord .


AND A DEFECT IN THE FORMATION OF VENTRAL SOMITES LEADS TO THE FORMATION OF LUMBAR HEMIVERTEBRAE .