Histological study of the fetus at 12 to 13 weeks of gestation (Fig. 1a, b)
Examining the sagittal sections obtained from the suboccipital region of 12-week gestation fetuses, the dura mater appeared to be thin and composed of multiple layers of thin and wavy fiber bundles. At this stage of development, there appeared to be primarily fibroblasts each containing a large nucleus as well as obvious nucleoli in the spinal dura mater (Fig.1ai). The connective tissue composing both the MDB and the posterior atlanto-axial membrane (PAAM) appeared to be relatively loose, and primarily composed of fibroblasts. We observed numerous short fiber bundles in the PAAM that appeared disordered with no obvious directionality (Fig.1aii). From 12 weeks onward, muscle tissue was readily observed. However, the muscle fibers appeared to be relatively less numerous and loose and were primarily in the form of myotubes or developing skeletal muscle fibers with a tubular appearance. Moreover, at this stage of development we observed that there were numerous nuclei and some myofibrils beginning to be arranged in parallel (Fig. 1aⅲ).
At 13 weeks, both the quantity of connective tissue in suboccipital region and the thickness of the dura mater increased. Additionally, at this stage of development, the fiber bundles appeared to be thicker, and the cells contained within them being mainly fibrocytes. These fiber bundles appeared to run in a wave-like form (Fig. 1bⅰ). Moreover, the nucleus appeared to be small and deeply stained, and the nucleoli were not obvious. In comparison to the 12 week specimens, the fibers of the MDB and PAAM at 13 weeks appeared to be denser. Although these cells were observed to be primarily fibroblasts, the number of wavy fiber bundles increased, and their arrangement became more orderly (Fig.1bⅱ). Within the suboccipital muscles, the myofibrils became more numerous and denser when compared to the 12 week specimens, and numerous nuclei were still observed (Fig. 1bⅲ).
Histological study of the fetus at 14 to 15 weeks of gestation (Fig. 1c, d)
From the 14th week onward, the spinal dura gradually separates from the posterior wall of the spinal canal, and an epidural space appears (Fig. 1c, d). The dura mater is now very dense, and includes many typical fibrocytes with small deeply stained nuclei. There also appeared inconspicuous nucleoli and varying thicknesses of the fiber bundles which were oriented in a wavy shape (Fig. 1cⅰ). At this stage, the MDB fibers are very thin and connected to the dura mater through the epidural space. The fibers of the MDB and PAAM have now lined up in parallel. Within the fibers, the fibroblasts still dominated (Fig. 1cⅱ). Considering the suboccipital muscles, no obvious change in the number of myofibrils was observed (Fig. 1cⅲ-dⅲ).
Histological study of the fetus at 19 weeks of gestation (Fig. 2a)
At 19 weeks, the thickness of the dura mater continued to increase (Fig. 2aⅰ). However, the fibers in the posterior atlanto-axial interspace now evidenced a unique shape. Although it is connected to the dura mater, the fibers of the PAAM do not extend from the muscle to the dura mater run longitudinally between C1 and C2. The cells in the MDB and PAAM are still dominated by fibroblasts (Fig. 2aⅱ). At this stage, the cells nuclei of the suboccipital muscles have now been deflected to one side, and the myotubes are filled with myofibrils. (Fig. 2aⅲ).
Histological study of the fetus from 21 to 30 weeks of gestation (Fig. 2b, c, d)
After 21 weeks, the morphological characteristics of the suboccipital region have stabilized. The fibers of the dura mater, MDB, and PAAM are have now all become denser. The cells within these structures have now become fibrocytes, and their fiber bundles are arranged in parallel. The suboccipital muscles have also become fuller (Fig. 2 b, c, d). Moreover, after observing the suboccipital muscles in different sections, it can be observed that the dense MDB fibers have the potential to pull the dura mater away from the spinal cord. In the section showing the To Be Named Ligament (TBNL), the posterior atlanto-axial interspace is now almost completely open with only a few fibers, and the dura mater is hardly stretched (Fig. 2c).
Histological study of the fetuses after 31 weeks of gestation (Fig. 3 a-d)
After 31 weeks, the development of the suboccipital region has stabilized and the morphology of the cells is mature. The cells of the dura mater and MDB have now differentiated into fibrocytes. As the fetus develops, the fibers become thicker and denser. The suboccipital muscles have matured, as well. Each muscle fiber has become fuller, and the intercellular space between muscle fibers has become smaller.