Introduction
The spinal cord develops out of the caudal portion of the neural tube. The cranial end of the spinal cord goes over into the rhombencephalon, which is always flanked by 4 occipital somites.
From the 6th week three layers or zones can be distinguished in the wall of the neural tube (ventricular zone, intermediate zone, and marginal zone).
From the 8th to the 10th weeks the spinal cord reaches its definitive form. It becomes surrounded by cerebral-spinal cord membranes (meninges) and lie in the vertebral canal. The spinal cord and vertebral canal develop in parallel up to the 4th month. From this age, the longitudinal growth of the neural tube diminishes while that of the vertebral column continues unabated.
- 4-7
- Somitomeres
- 1-5
- Somites 1 to 5 (the other 30 somites are formed in this stage)
- I-IV
- pharyngeal arches
- P
- Prosencephalon
- M
- Mesencephalon
- R
- Rhombencephalon
- NT
- Neural tube (spinal cord)
- ABCD
- Neuromere
The rostral part of the neural tube exhibits signs of a temporary segmental articulation (neuromeres A, B, C, D). The neuromeres in the rhombencephalon, known as rhombomeres, contain collections of neurons that do not intermingle with those other rhombomeres and are functionally determined already before their migration.
The neuromeres are here shown as being transparent (expecially the eight rhombomeres D) and the chorda is rendered in orange.
Metamere formation
Its externally recognizable articulation shows that the adult spinal cord is a segmented organ. From each segment the spinal nerves arise with their dorsal sensory and ventral motor roots in pairs. The roots are formed from the totality of all of the root fibers that belong to them and that sprout from the spinal cord over its whole length. Each segment stands in connection with a right and a left somite that have gone out from the metamere articulation of the paraxial mesoderm. Each somite can be subdivieded further into a sclerotome, a myotome and a dermatome
- 1a
- Dermatome
- 1b
- Myotome
- 1c
- Sclerotome
- 2
- Central canal of the neural tube
- 3
- Neural tube
- 4
- Epimere
- 5
- Hypomere
Left picture:
Human embryo in stage 12, ca. 30 days old.
Right diagram:
Transversal cut through the embryo at level A (of the left picture). The somites are separated from each other and form a dermatome and a myotome. The myotome lengthens in the dorsoventral direction whereby epimere and hypomere arise (right part of the diagram). In this stage, the aorta ist already unpaired.
The spinal cord thus maintains close connection to the surrounding spinal column (sclerotome), to the innervating striated muscles (myotome), and to the skin (dermatome), from which it receives sensory afferents. Admittedly, this metamere articulation becomes overlaid by the ascending and descending pathways of the connecting apparatus as well as by the intersegmental connections of the own apparatus of the spinal cord, but nevertheless remains clearly recognizable. This manifests especially in the lasting relationship between spinal cord segment, dorsal root on all sides of the respective spinal nerves and the dermatomes (skin field), which develop out of the associated somites. On the stem, the spreading of the dermatome is easily proven but is more difficult to see due to the contortions and overlapping of the appendages.
- Anterior root (radix anterior)
- Spinal ganglion
- Neural tube (medulla spinalis)
- Spinal nerve (spinal cord nerve)
- Dorsal branch (ramus posterior) to the epimere of the myotome
- Ventral branch (ramus ventralis) to the hypomere of the myotome
- Posterior root (radix posterior)
- Central canal (canalis centralis)
- Ectoderm
- Epimere
- Hypomere
- Ramus communicans albus
- Ganglion of the sympathetic nervous system
- Postganglion fibers
- Ramus communicans griseus
Every epimere is supplied by the ramus dorsalis of a spinal nerve and the hypomere by the ramus ventralis of the same spinal nerve. The present diagram also shows the connections to the sympathetic ganglia (ramus albus et griseus).