Paper - Early development of the cervical vertebrae and the base of the occipital bone in man
|Embryology - 31 Oct 2020 Expand to Translate|
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Early Development of the Cervical Vertebrae and the Base of the Occipital Bone in Man
Charles Russell Bardeen
University of Wisconsin.
With 3 Figures.
During the earlier stages of development the cervical vertebrss resemble those of the thoracic region. The two regions soon become differentiated from one another by the much greater development of the costal processes of the thoracic region. The seventh cervical vertebra alone;, as a rule has a large costal process and this does not extend far beyond the transverse process of the neural arch (Fig, 1), In the cervical as in the thoracic vertebrae the development of a region of loose tissue in the base of the primitive ventral process serves to separate the costal element from the transverse process. In this loose tissue an anastomosing artery extends from the intersegmental artery on the posterior to that on the anterior side. The anastomosing artery between the costal element and transverse process of the seventh cervical vertebra remains smalls but the more anterior anastomosing arteries give rise to a large continuous vessel^ the vertebral artery^ which extends anteriorly between the costal processes and transverse processes of the root of the vertebral artery In the costal processes of the seventh cervical vertebra centers of chondrification are found at the period when similar centers appear in the ribs. Centers of chondrification in the costal processes of the rest of the cervical vertebra? appear much later^ usually not until the embryo has reached the length of from 16-18 mm.
As in the thoracic vertebate there are two bilaterally placed centers of chondrification for each of the vertebral bodies. These soon fuse with one another ventral and dorsal to the chorda dors alls. In the first two vertebra? the ventral fusion takes place before the dorsal fusion.
- The early development of the thoracic, sacral and coccygeal vertebra has previously been described in The Amebican Journal of Anatomy, Vol. IV Ambhican Journal of Anatomy Vol. Till, No, 2,
There are separate centers of chondrification for the neural arches. In the more distal cervical vertebrse these centers are similar to those of the thoracic vertebrae. In the more proximal cervical vertebrae the centers of chondrification appear as basal plates lateral to the anterior end of the bodies of the vertebrae. With these they soon fuse. From the plate-like base chondrification extends rapidly into the main part of the arch. From the neural arches, laminar, articular and transverse processes are developed. Tlie costal elements have separate centers of chondrification which soon fuse proximally with the bodies of the vertebra: and distally with the tips of the transverse processes of the vertebrae- The dorsal growth of the laminar processes and the formation of the spinous processes of the cervical vertebrae take place in the main like that of the thoracic. A^Tien fully formed^ however^ the cartilaginous cervical vertebrae have essentially the shape of the adult osseous cervical vertebrae. Even before the end of the second month of development distinct cervical characters may be distinguished (Figs, 1 and 2).
Some investigators hold that the neural arches of the mammalian vertebrae contain elements of both the ventral and dorsal arches found in the lower vertebrates (see Schauinsland Hertwig's Handbuch, 1903). There are theoretical grounds for believing that the ribs primitively belong to the ventral arches. In the higher mammals and man, however, the presence of the ventral arch elements is manifest merely in the caudal region where temporary h^mal processes are developed and in the upper cervical region where in the membranous stage, there are differentiated from the ventral margins of the primitive discs bands of tissue which connect the bases of the neural processes. These bands of tissue have been called by Froriep the hypochordal braces (Spangen). In reptiles and birds the hypochordal brace becomes converted into cartilage and connects the cartilaginous arches of each side with one another. It finally becomes fused with the ventral portion of the proximal end of the vertebral body. In the primitive type of development the chondrification takes place from two bilaterally placed centers, each of which, according to Schauinsland, represents a ventral heuiiareh. In other instances chondrification takes place from a single center situated in the median line. According to Froriep, in the cow a median center of chondrification appears in all the hypocliordal braces, but except in the first two vertebra? the brace disappears before cartilage is actually formed. In the liypochordal brace of the second vertebra tlie cartilagiBoiis anlage is very transitory, in the first vertebrse it forms the anterior portion of the arch of the atlas. Weiss describes two bilaterally placed centers of cliondrifieation in the hypochordal brace of the atlas in the white rat. No cartilage is fonnd in the more distal hypochordal braces. In man a hypochordal brace becomes well developed "merely in connection with the atlas. It becomes cartilaginous later than the neural arch. There are indications of two bilaterally placed centers of chondrification^ but fusion with one another and with the neural arches takes place as soon as chondrification is well under
Fig. 1. Lateral view of a model of the occipital cartilage, the cervical vertebras the first thoracic vertebra aud the proximal end of the first rib in an embryo 20 mm. long. The costal elements of the cervical vertebrae are cartilaginous and are connected by ligamentous tissue with the transverse processes of the neural arches and with the vertebral bodies. The intervertebral discs are shown except between the fourth and fifth, and the fifth and sixth vertebnie. Dense interarticular tissue is shown between the articular processes from the second cervical to the first thoracic vertebra and, posterior to this, interlamiuar connectiye tissue membranes and a ligamentous band running from the neural process of the atlas to the thoracic region.
Fig. 2. Postero-medial view of the model represented in Fig 1. The anterior extremity of the base of the occipital and the posterior part of the body of the sphenoid are also shown. Between the posterior part of the base of the occipital and the epistropheus the intervening dense tissue has in part been removed so as to reveal the ventral arch of the atlas. The intervertebral discs are omitted between the fourth and fifth and sixth cervical, and between the seventh cervical and first thoracic vertebrae.
Specific mention must be made of the mode of development of the epistropheus, of the atlas and in coimectinn with the latter of the base of the occipital.
The general mode of development of the epistropheus is like that of the other cervical vertebrae. Its marked distinction comes from its union with the body of the first cervical vertebra. This union takes place through the transformation of tlie intcrverteliral disc into cartilage^ first lateral to the mid-sagittal plane and later in this plane.
Fig. 3. Sagittal section through the lateral part of tbe cervical region of the spinal column of an embryo 14 mm long.
The base of each neural hemi-arch of the atlas becomes temporarily fused with the body (14 mm. embryo), but this fusion is incomplete and soon is followed by the development of dense fibrous tissue between the arch and the body (Fig. 3) At the same time the hypocliordal brace becomes cartilaginous and unites the arches of the atlas in front of the Body- Each costal process becomes fused medially to the basal process, laterally to the transverse process of the corresponding hemi-arch.
The articulation between the lateral mass of the atlas and the sxiperior articular surface of the epistropheus seems to be formed rather in the interventral than as in the other inter Yertebral diathroses^ in the interdorsal membranes. This is also true of the atlanto-occipital diathrosis For a brief period (14 mm. embryo) the bases of the neural arches of the atlas and epistropheus^ together with the tissue intervening between the bases of arches of the atlas and the occipital^ become fused into a nearly continuous mass of precartilage (Fig, 3).
Hagen Ilis^ Arehiv 1900 gives a somewhat different account of the development of the atlas and epistropheus in man. He concludes (1) that the dens epistrophei arises from the region of the body of the epistropheus and a portion of the body of the atlas; (2) that the massae lat. of the definite atlas arise from the rest of the primary anlage of the body of the atlas^ and (3) that the short piece which unites them in front arises from the fusion of both neighboring septa.
Opposite the last occipital myotome the axial mesenchyme is differentiated like that of the spinal sclerotomes into a light anterior half and a dense posterior half or scleromere. In the spinal region each scleromere joins with the light half of the sclerotome next posterior in giving rise to the body and arch processes of a spinal vertebra- In man the occipital scleromere is not thus associated with the light half of the first spinal sclerotome. On the contrary, it becomes associated with the lighter tissue of its own segment and with the tissue into which this is continued anteriorly. Laterally the tissue differentiated at the side of the anterior half of the first spinal sclerotome-, the interventral membrane becomes temporarily converted into a precartilaginous membrane (Fig. 3).
Chondrifiication of the base of the occipital begins in two bilaterally situated centers in the posterior portion of the occipital anlage. The union of these centers takes place posteriorly ventral to the notochord and anteriorly dorsal to the notochord (Fig. 2), The neural processes of the posterior part of the occipital anlage seems to have separate centers of chondrification but these centers fuse almost immediately with the centers of chondrification of the body.
From the tissue derived from the first and second sclerotomes and not utilized in the formation of the atlas nnd the epistropheus are derived the various ligaments which unite these bones. The details of the formation of these ligaments are too complex for description here.
Cite this page: Hill, M.A. (2020, October 31) Embryology Paper - Early development of the cervical vertebrae and the base of the occipital bone in man. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Early_development_of_the_cervical_vertebrae_and_the_base_of_the_occipital_bone_in_man
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