III./1.7.: Development of the meninges (ontogenezis)





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III./1.7.: Development of the meninges (ontogenezis)

Relevant phases of the development of the brain and spinal cord conclude during the first two month of the pregnancy, corresponding to the development of the major parts of the central nervous system and cartilage-connective tissue of the skull and vertebrae.

At the beginning the mesoderm propagates around the neural tube. It is ticker and segmented around the caudal portion which represent the spinal cord according to the sclerotome of the vertebrae (somite) and unsegmented around the brain vesicles, where it is called the head mesoderm. Henceforth the differentiation proceeds from ventral to lateral then to dorsal.

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Several theories - only partially overlapping - exist about the process of development. According to one theory the outer, stronger layer of the thickened mesoderm - the developing spinal cord - which lies together with the developing neurocranium is called ectomeninx and the inner, softer layer connected with the neural tube is called entomeninx. The ectomeningeal layer tightly coalesce to the inner surface of the developing spinal cord and neurocranium, then the connation between its inner and outer layers weakens to form the ectomeningeal space (spatium ectomeningicum). In case of the spinal cord the epidural space arises from the beforementioned space, while network of the extradural veins coursing along later forms the internal vertebral venous plexus.

The outer border surface forms the endorachis of the spinal cord and the inner surface forms the spinal dura mater. Similar veins in the neurocranium are found just in limited places, forming the dural venous sinuses (sinus durae matris), therefore no significant veins are found in the intracranial epidural space, hemorrhages of the region usually originate from the meningeal arteries. Consequently the anatomical and clinical meaning of the term "epidural" differs according to its cranial or spinal location.

The layers of ectomeninx fuse - except where they form the dural sinuses - forming the solid dura mater secundaria. By all means the morphological phenomenon, that the dura, even under surgical condition, is separable into two tougher layers by careful dissection, supports the existence of ectomeningeal space. According to the same theory the leptomeninges originate from the ectomeninx, and only later - within the ectomeninx - during cavernisation evolves the leptomeningeal space (spatium leptomeningicum), viz. the latter subarachnoideal space (spatium subarachnoideale).

According to another theory the expanded connective tissue is first slackened through gelatinization, then its inner surface comes into close contact with the neural tube forming the pia mater, which therefore is first of the meninges to evolve. The outer, tougher part of the gelatinized connective tissue remained forms a joint dura-arachnoid layer. This layer does not coalesce with the inner surface of the spinal canal, but between itself and the latter endorachis beside graduated fattening and vessel formation evolve the internal vertebral venous plexus and the epidural space. At the same time this dura-arachnoid layer coalesce with the skull and therein forms the dural sinuses.

The arachnoid evolves later by separating from the dura mater. This theory without doubt supports the existence of the delicate fibrous dura-arachnoid connections demonstrated in further studies, labeled as dura arachnoid interface. The primitive subarachnoid cavity evolves through the cavernisation of the gelatinized tissue between the pial and the common dura-arachnoid layers, then with the atrophy and disappearance of the initially numerously attendant arachnoid trabecules the eventual subarachnoid cavity and cistern system develops. Interesting, that by the time the primitive subarachnoid cavity appear the crown-to-rump length of the fetus is nearly transcends the 2 (!) centimeters. contrary the subdural cavity evolves only in the second part of the pregnancy.

The theories essentially cover the same process, however because of practical reason the second theory is favored, which accents the common origin of the dura mater and arachnoid.

Last modified: Monday, 10 March 2014, 6:27 PM