Development of the Musculoskeletal System: Bones of the vertebral column

The sclerotome migrates medially to surround the notochord. A segment of the mesenchyme becomes more dense at the middle of the segment. To see this better, check out the figure in the notes. The more dense area will become the annulus fibrosa of the IV disc. The divisions in between the sclerotomes disappear and will eventually form the vertebral body. There are two consequences of this:

1. The myotome will span two vertebrae and hold them together.
2. The intersegmental arteries “move” to the middle of the vertebral body.

Mesenchyme will surround the neural tube, and eventually become the neural arch. The transverse processes and ribs come from the lateral portion of the sclerotome. In the cervical region, the ribs fuse with the transverse processes to form the transverse foramina.

*The only remnant of the notochord is the nucleus pulposus in the IV disc.

Chondrification: There are four centers in the vertebral body, two in the center of the vertebral body and one in each pedicle. This process starts before mesenchyme is fully formed and the neural arch has closed.

Ossification: It starts at 9 wks, and it heralds the beginning of the fetal stage. Ossification of the vertebrae begins in the upper lumbar vertebrae and goes cranially, while the arches ossify cranially to caudally. In the cervical region, the cranial arches ossify before the body. Do not ossify vertebral column until 25yo. Secondary ossification sites begin to finish the job during the 16^th year. It isn’t until you’re 25 that your ass is fused – that is the sacrum and coccyx.

The ribs begin to ossify at the angle and go until the end closest to the vertebrae. This area – the neck and head of the rib – are finished by secondary ossification. In the sacrum, the “ribs” become the pars lateralis.

Sternum: Who could forget the sternum? Longitudinal bands of mesenchyme form the ventral body wall. When they fuse with the ribs, they unite in the middle.

Category: Anatomy Notes