The twelfth thoracic vertebra (T12) is an atypical thoracic vertebra exhibiting some lumbar features.  Although T12 shares many similarities with the typical thoracic vertebrae, there are a number of differences.  It has a larger body than the thoracic vertebrae above and a small circular vertebral foramen.  On each side of the body, below the upper border, it has only one complete circular costal facet, for articulation with the head of the twelfth rib.

From the vertebral body pedicles project posteriorly and the short, thick laminae project postero-medially, overlapping the laminae of the vertebra below.  The triangular spinous process projects posteriorly from the junction of the laminae.  Anteriorly and infero-medially facing inferior articular facets are located on the inferior articular processes that project from the laminae.  The pediculo-laminar junctions give rise to thin, flat superior articular processes, which face posteriorly and supero-laterally and support the oval superior articular facets.  The inferior articular facets may resemble those of the lumbar vertebrae, if so the superior articular facets also resemble those of the lumbar vertebrae.  Also projecting postero-laterally from the pediculo-laminar junctions are small transverse processes without articular facets.  

Ossification
The centrum and each half of the vertebral arch ossify from single centers that appear in-utero in the seventh week and fourth month, respectively.  The arches unite during the first year and then join with the vertebral body by the fourth year.  The ring apophyses (or epiphyseal rings) form secondary ossification centers at approximately 12 years and fuse with the vertebral body between the ages of 14 and 25 years.  At 16 years centers also appear at the tips of the transverse and spinous processes and fuse with the rest of the vertebra by 25 years.

Secondary ossification centers may remain unfused at these sites and are known as accessory ossification centers.  They must be differentiated from fracture on plain-film radiographs.