ARACHNOIDAL GRANULATIONS [granulationes arachnoideales (PNA), granulationes arachnoidales (Pacchioni) (BNA), granula meningica (JNA); synonym: villi arachnoideales, glandulae conglobatae, pakhionova of granulation] — the outgrowths of a soft cover of a brain located on its outer surface. For the first time And. as independent educations described Pakhioni (A. Pacchioni, 1665 — 1726) who considered that granulations are glands of a firm cover of a brain. Granulations of a soft cover of a brain are similar to outgrowths of other internal covers: to vorsina and arcades of serous covers, synovial vorsina of joints and tendinous vaginas, nipples and plaques of an amniotic cover, etc.
In a form A., in particular subdural (fig. 1), remind a flask with an expanded distal part and the pedicle attached to a soft cover of a brain. In mature And. a distal part branches. Being derivatives of a soft cover of a brain, And. are formed by two of its components: an arachnoidal cover and subarachnoidal (leptomeningeal) fabric (see. Meninx ). Arachnoidal cover And. includes three layers: outside endothelial (arakhnoidendotelialny), reduced fibrous and internal endothelial (arakhnoidendotelialny).
It is histochemical established that the outside endothelial layer differs in high activity of an acid and alkaline phosphatase. Due to the proliferation of this layer near top And. «multinuclear caps» form. Subarachnoidal fabric of granulations is presented by dense network of trabeculas from collagenic and argyrophil fibers. Subarachnoid (leptomeningeal) space And. it is formed by a set of the small cracks located between trabeculas. It is filled with cerebrospinal liquid and is freely reported with cells and channels of a subarachnoid space of a soft cover of a brain. V A. there are blood vessels, nerve fibrils and their terminations in the form of balls and eyelets (V. K. Beletsky, 1946, 1948; S.E. Chait, 1967). Depending on the provision of a distal part distinguish subdural, intradural, intralakunarny (fig. 2), intrasinus-ny, intravenous, epidural, intracranial and ekstrakranialny And. During the opening of venous receptacles of a firm cover, in particular side lacunas, the set is found And., to-rymi the bottom of lacunas «is as if paved». Between the blood circulating in these receptacles and a subarachnoid space And. the membrane 40 — 50 microns thick dividing them is located.
At newborns And. are absent and cells of an outside endothelial layer of an arachnoidal cover are distributed evenly. To 3 — to 4-year age in this layer local condensations of elements appear. The last are located with concentric belts in the form of sockets. It is an initial stage of development And. — «cellular spots» (maculae cellulares). As a result of proliferation they will be transformed to three-dimensional «hillocks» (colliculi cellulares), and then, increasing in sizes and branching, hillocks turn into mature granulations.
And. in development are exposed to fibrosis, hyalinization and calcification with formation of psammous little bodies. The perishing forms are succeeded by again formed. Therefore all stages of development at the same time occur at the adult And. and their involutional transformations. As approaching the upper edges of big cerebral hemispheres number and the sizes A. sharply increase. Their ground mass is concentrated in the most sublime part of a brain — in the place of crossing of its largest vertical diameter with the upper edges of big hemispheres. At the person this place corresponds to frontal corners of parietal bones (N. V. Kolesnikov, 1938).
About physiological value A. there is a number of hypotheses. Attribute them the most various functions: 1) the device of outflow of cerebrospinal liquid in venous beds of a firm cover [H. Quincke, 1872; Kay and Rettsius (E. A. H. Key, M. G. Retzius), 1875]; 2) the mechanism regulating pressure in venous sine of a firm cover [P. Trolard, 1892] and in a subarachnoid space [Le Gro Clark (W. E. Le Gros Clark), 1921]; 3) the device suspending a brain in a head cavity and protecting his thin-walled veins from stretching (H. N. Burdenko, 1930); 4) the device of a delay and processing of toxic products of the exchange interfering penetration of these substances into cerebrospinal liquid [Faith and Wagner (Veith, Wagner), 1955], or absorption of protein from cerebrospinal liquid [L. Turner, 1961]; 5) the difficult baroreceptors perceiving pressure of cerebrospinal liquid and blood in venous sine [Kish and Zattler (F. Kiss, J. Sattler), 1956]; 6) hemadens — «organon arachnoideale» (Kish and Zattler, 1956, etc.). The look is most widespread on And. as on the device promoting outflow of cerebrospinal liquid in a venous bed of a firm cover. Outflow of cerebrospinal liquid through And. — private expression of the general pattern — its outflow through all arachnoidal cover. Emergence washed by blood A., extremely strongly developed at the adult, creates the shortest outflow tract of cerebrospinal liquid directly in venous sine of a firm cover, passing a bypass way through a subdural space. At small children and small mammals at whom is not present And., release of liquor is carried out through an arachnoid membrane in a subdural space.
Subarachnoidal cracks intrasinusny And., the thinnest, easily falling down «tubules» from 4 to 12 microns representing in the diameter, are the valve mechanism which is opening at build-up of pressure of cerebrospinal liquid in a big subarachnoid space and closed at build-up of pressure in a sine. This valve mechanism provides unilateral advance of cerebrospinal liquid in sine and, according to experimental data, opens with a pressure of 20 — 50 mm w.g. in a big subarachnoid space [Welsh and Friedman (K. Welch, V. Friedman), I960]. At addition to perfusate of particles of various diameter passing through tubules of particles to dia is revealed. to 6 — 7,5 microns [Welsh and Polly (K. Welch, M. Pollay), 1961].
The question of the nature of communication of a subarachnoid space of granulations with a gleam of venous sine did not receive the final decision and at elektronnomikroskopichesky researches yet. One authors consider that there is a free message of the tubules of subarachnoidal fabric of granulations covered by an endothelium with a gleam of venous sine, and prove it to those that on tubules of granulations the particles of thorium dioxide entered into cerebrospinal liquid move, in particular, [A. Jayatilaka, 1965]. Other authors believe that tubules are an artifact. Between cerebrospinal liquid of a subarachnoid space of granulations and blood of venous sine the barrier — a continuous endothelial membrane is located, through to-ruyu there is filtering of cerebrospinal liquid [Shabo and Maxwell (A. Shabo, D. Maxwell), 1968; Alksne and Lovings (J. F. Alksne, E. T. Lovings), 1972]. Patholologically overdevelopment And. it is observed at developments of stagnation of various origin in a brain (V. K. Beletsky). An opposite picture — a hypoplasia and an atrophy And. — it is noted at patients with hydrocephaly.
Bibliography: Alov I. A. Outflow tracts of cerebrospinal fluid, Vopr. neyrokhir., t. 17, No. 4, page 19, 1953; Baron M. A. Reactive structures of internal covers, L., 1949, bibliogr.; Chait S. E. Blood vessels in granulations of an arachnoid membrane of a brain of the person, in book: Vopr. it is normal of a morphogenesis and regeneration and patol., under the editorship of P. V. Dunayev, century 2, page 29, Tyumen, 1967; And 1 to-sne J. F. and. Lovings E. T. The role of the arachnoid villus in the removal of red blood cells from the subarachnoid space, J. Neurosurg., v. 36, p. 192, 1972; Jayatilaka A. D. An electron microscopic study of sheep arachnoid 0ranulations, J. Anat., v. 99, p. 635, 1965; Kiss F. u. Sattler J. Struktur und Funktion der Pacchionischen Granulationen, Anat. Anz., Bd 103, S. 273, 1956, Bibliogr.; Shabo A. L. a. Maxwell D.S. The morphology of the arachnoid villi, J. Neurosurg., v. 29, p. 451, 1968, bibliogr.; Welch K. FriedmanV. The cerebrospinal fluid valves, Brain, v. 83, p. 454, 1960, bibliogr.; Welch K. Pollay M. Perfusion of particles through arachnoid villi of the monkey, Amer. J. Physiol., v. 201, p. 651, 1961, bibliogr.
M. A. Baron.