Book - The Pineal Organ (1940) 26
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The Pineal Organ - The Human Pineal Organ
Chapter 26 Structure of the Fully Developed Human Pineal Organ
As previously mentioned, the structure of the pineal body varies considerably at different ages and in different individuals, and like some other embryonic and infantile organs which develop up to a certain degree of perfection and then degenerate, e.g. the pronephros, the greater part of the mesonephros, and the thymus gland, the pineal body normally attains its maximum of development before the individual has reached maturity. This maximum, according to the general estimate, occurs somewhere between the ages of 5 and 7 years. Signs of degeneration are, however, frequently evident before this period, and the uniform alveolar appearance which is present in early infancy usually becomes less pronounced at the end of the second year, and signs of degeneration and replacement of parenchymal cells by fibroglial tissue are often seen in quite young children. The parenchymal tissue becomes surrounded by ingrowths of fibrovascular septa which are continuous externally with the capsule. These break up the parenchymal tissue into rounded lobes, as seen in Fig. 264, the pineal body of a child aged 5. This specimen may, however, be regarded as exceptional with regard to the age at which degenerative changes, accompanied by ingrowth of thick fibrovascular septa from the capsule, have taken place, and there are instances in which the uniform structure of the pineal organ present in the infant is retained even in advanced age. Speaking generally, however, it is commonly admitted that, although exceptions occur, there is an increase in the proportion that the fibroglial constituents have to the parenchyma from childhood onwards to old age. We shall, therefore, describe first the structure of the pineal organ as it appears in children between 3 and 6 years of age, in preference to commencing the description of the organ of adult individuals, in which both the parenchyma cells and the supporting tissue usually show signs of degeneration.
Besides variations in structure due to age changes there are differences in appearance which are brought out by different methods of preparation, and we propose, before dealing with the selective actions produced by the use of special methods of modern technique, to give a short description of the microscopic appearance of a section of the pineal gland of a child 5 years of age stained by the ordinary harmatoxylin and eosin method. We shall then endeavour to interpret the appearances seen at this stage of development and in the organ of adult individuals by a reference to the earlier stages of embryonic and foetal development and by the microscopic pictures brought out by the use of differential stains. The pineal organ at this stage is invested by a fibrovascular capsule, derived from the pia mater and lined internally by a glial stratum. This capsule sends trabecular containing blood-vessels into the substance of the gland. The trabecular pass inwards and partially surround the peripheral part of a
branched system of lobules which primarily originate from the ependymal epithelium of the embryonic pineal diverticulum. The epithelial tissue of the lobules is, however, penetrated throughout by fine trabecular of intralobular connective tissue containing capillary blood-vessels. The lobular areas between the larger trabeFig. 264. — Tangential Section of the Pineal Gland cu ]^ which grow inof a Child, showing the Fibrous Capsule, Lobes, , - *" , ,
and Interlobar Septa. (R. J. G.) ward from the capsule
Ca. : capsule. communicate with
I.L.S. : interlobular septum. eac h ot her i n the
I.Lr. S. : intralobular septum. , ,, ,
P. : parenchyma. central part of the
gland so that the lobules as a rule are not completely enclosed in separate compartments, but are continuous with a central mass of parenchymatous tissue which is more uniform in appearance than the lobulated peripheral zone. The lobules consist of a supporting glial tissue which has the character of a fibrillated sponge-work or reticulum enclosing clear intercommunicating spaces. Some of the spaces contain parenchyma cells with vesicular nuclei, while others appear empty. The reticulum is especially noticeable beneath the capsule and in relation with the larger trabecular. The cellelements of the reticulum have the appearance of being continuous with each other, no intercellular septa or intervals ever being visible with the ordinary methods of preparation. The network thus seems to be formed of a Plasmodium or spongioplasm within which the nuclei of the cellelements are imbedded. In many of the spaces of the network branched parenchyma cells containing pale vesicular nuclei are present. The processes of these cells appear to be :
(1) Continuous with processes of similar adjacent cells.
(2) Continuous with the matrix of the general neurospongium.
(3) Spread out on the perivascular sheaths of the vessels contained
in the trabecular or fibrous capsule.
Fig. 265. — Section of an Adult Pineal Gland stained by van Gieson's Method and Eosin, showing the Apparent Continuity of the Reticulum in which the Parenchyma Cells are embedded ; and also the Relation of the Parenchyma Cells to the Spaces of the Reticulum and the Supporting Tissue or Neurospongium. An Interlobular Septum showing Capillary Blood Vessels and Nuclei of Fibrous Connective Tissue crosses the Upper Part of the Drawing obliquely. (R. J. G.)
Cp. : capillary vessel.
CT.C. : connective tissue cells.
Pa. C. : parenchyma cell.
S.D.N. : small darkly stained nucleus. Sp. : space.
The cells with the pale vesicular nuclei belong to the fully developed type of pineal or parenchymal cell ; many of the parenchyma cells, however, are embedded in the spongioplasm, and in this situation the nucleus is usually smaller and more deeply and uniformly stained than those of the cells just described. Between these two extreme types, in young as well as in adult specimens, there are numerous intermediate forms (Fig. 265), both with respect to the type of the nucleus and the position of the cell-element, namely, wholly contained within the spongioplasm, protruding from this into a space, or completely extruded into the space and connected to the surrounding structures merely by fine, tapering processes.
Fig. 266. — Pineal Body. 350.
Median vertical section, through the peduncle, of a human pineal body prepared by the Blair-Davis modification of Ranson's pyridin method. The larger, deeply stained fibres forming a network between the pineal cells are probably glial, the finer fibrillar in the centre of the photograph are indistinguishable from nerve-fibres, and possibly represent the axis-cylinders or nerve-fibres reaching the pineal body from the superior and posterior commissures.
Fig. 267. — Pineal Body. 350.
Section through the cortical zone of the same specimen as Fig. 266. The larger fibres forming a network surrounding the pineal cells are probably glial. The finer fibres between the cells and surrounding the capillary vessels are probably nerves of the sympathetic system.
In adult specimens, however, when specifically stained for neuroglia, the distinction between glial cells and the general plasmodium or syncytial reticulum in which they are imbedded is quite definite, and it is at once evident that the slender astrocytes and glial fibres in no sense form the principal constituent of the supporting tissue of the lobules.
The former appear as very sparsely scattered branched cells, chiefly of the astrocyte type (Fig. 272, p. 403), with fine delicate processes lying in a tissue which when specially stained by Hortega's silver impregnation method is seen to be principally composed of branched parenchyma cells which he believes to be separate and independent units.
By combining the knowledge gained by the different specific methods of silver impregnation with that obtained by the best nuclear and cytoplasmic stains we are able to distinguish in the parenchymatous tissue of the adult human pineal organ three principal elements, namely :
(1) A glial component, formed by the cell-bodies and slender branched
processes of a relatively small number of astrocytes.
(2) The parenchyma or pineal cells, distinguished by their large,
pale vesicular nuclei. These are much more numerous than the astrocytes and form the main bulk of the tissue.
(3) The mesodermal elements which consist of (a) the fibrous capsule and
trabecular, derived from the pia mater, and (b) minute profusely branched cells which are present beneath the pia mater, in relation with the perivascular sheaths, and distributed in the lobules. These are the microglia or mesoglia cells of Del Rio-Hortega, and are only clearly demonstrable by means of his silver carbonate process, though their nuclei are recognizable with other methods of staining, especially in glial plaques.
Fig. 268. — Pineal Body.
Section through the central region of the same specimen as Figs. 266, 267. Two types of fibres are visible, as in Fig. 267. The finer fibres appear to form a network on the walls of the capillary vessels.
(4) Cells and fibres, which resemble nerve-cells and nerve-fibres
but are in many cases only with difficulty distinguished from the parenchyma and glia cells and the processes of these cells (Figs. 266, 267, 268).
(5) Arterioles, capillaries, and venules.
The Parenchyma Cells
The form and full extent of the branches of the parenchyma or pineal cells is only fully revealed by the silver methods of Del Rio-Hortega, upon whose description the following account is largely based. The pineal cells of an adult human subject thus shown (Fig. 218, Chap. 22, p. 317) are characterized by an irregularly shaped body containing a clear vesicular nucleus. The amount of cytoplasm varies, in some cells being abundant, in others scanty. The cells thus differ greatly in size. The cell-body is usually branched, and the branches vary in number, size, and complexity. Thus the cell may be unipolar, bipolar, or multipolar. The main branches may give off a group of slender processes which may subdivide and end freely, or more commonly terminate in club-shaped swellings implanted on or in the sheath of a vessel wall. The free ends of the smaller branches are said by Del Rio-Hortega not to communicate with similar branches of neighbouring pineal cells. The processes of the cell may be polarized in one direction or they may be distributed irregularly in any direction. In the former case they are usually directed towards a vessel lying in the periphery of a lobule, and lie in what is termed the marginal zone, where they are radially disposed ; in other cases, more particularly in the centre of a lobule, the cells tend to be multipolar and stellate in form, the branches appearing to form a plexus ; the terminal branches of these cells may end in club-shaped swellings on the sheaths of endolobular vessels, or they may extend to the periphery of the lobule and become attached to the sheath of an interlobular vessel.
Typical branched cells with club-shaped endings are seldom seen in children below the age of eight years, and the complexity of the branching and the size of the club-shaped swellings appear to increase with the advance of age.
Although the general form and terminal processes of the pineal cells can only be clearly demonstrated by means of the silver methods of impregnation, very little of the structure of the cells and their content is seen in these preparations ; and in order to form a true estimate of the nature of these cells it is necessary to adopt special methods which will bring out particular characters, such as cytoplasmic granules, vacuoles, lipoid material, and other contents ; also mitochondria, blepharoplasts, the centrosome, and Golgi apparatus. Thus by staining frozen sections with Janus green, small rod-like mitochondria may be demonstrated in the pineal organ of such animals as the horse, ox, and sheep. The rods are most abundant around the centrosome and the part of the cell body which relative to the nucleus is the widest. Occasionally long rods or chondriocontes have been observed, and one or more thick rods close to the nucleus arranged in the form of crosses or bundles. Hortega states that in children the rods are short, while in adults and old subjects they have become elongated, their elongation coinciding with involutive changes.
The cytoplasm normally has a reticular structure, and frequently shows vacuoles which may be demonstrated by the use of neutral red. Lipoid material is also sometimes present. Granules of varying type are normally present. Some of these have been described as secretory (Dimitrowa, Rio-Hortega, Pastori, and others). There are also spherules which have been thought to correspond to the " gliosomes " found in the central nervous system. Some granules are apparently the result of degenerative changes and are seen abundantly in the pineal cells of aged subjects. Pigment granules, usually of small size, but sometimes large, are found in old subjects, and like the non-pigmented granules mentioned above are in some specimens due to involutive changes. These granules are of a yellow-brown colour, and differ considerably from the dark melanin granules which are also sometimes found in the pineal organ and may be of morphological interest (see pp. 61-63). These are chiefly found in the connective tissue elements, trabecular, capsule, and the surrounding pial tissue.
The nucleus of the parenchyma cells of adult subjects is typically spherical, of large size, and owing to its small chromatin content appears clear. A well-defined nucleolus is usually present and the nuclear membrane is conspicuous. The nucleus of the parenchyma cells is large, even in those in which the cytoplasm is scanty and the cell as a whole is small as compared with the average size of these cells ; and it is also large when compared with the small deeply stained nuclei of the fibroglial tissue. Under certain conditions spherular formations described by Dimitrowa as secretory in nature are present within the nucleus (Fig. 219, Chap. 22, p. 317). These spherules have been found in the human subject, the ox, lamb, and other animals. They appear as clear droplets which are stained red by Van Gieson's methods, a grey colour in Weigert preparations, and pink with saffranin. Dimitrowa regarded the existence and appearance of these spherules as undoubted evidence of secretory activity, and she believed that the nucleus produces a substance the chemical composition of which is unknown and which is periodically discharged into the cytoplasm. The spherules were stated to approach the nuclear membrane, pass through this into the cytoplasm, and afterwards disappear. The nuclear membrane was then said to be regenerated. Similar appearances have been described by other authors, e.g. Krabbe, who reported the presence of the nuclear spherules in the human subject commencing about the eighth year, reaching a maximum during the fourteenth year, and persisting in old age. Uemura and Weinberg have seen the spherules in the pineal body of a child of 4 years and under 3 years of age. The secretory nature of the nuclear spherules has, however, been doubted by many authors, more particularly by Achucarro and Sacristan, Biondi, Josephy, and Walter. Intranuclear granules and spherules, wrinkling, outpocketing, and inpocketing of the nuclear membrane are quite common occurrences in cells with large nuclei in many structures besides the pineal body. Moreover, the small recesses between the folds of a wrinkled nuclear membrane may enclose granules of protoplasm which stain with hematoxylin and the various stains mentioned above in a similar way to the spherules of the parenchyma cells. Nevertheless the abundant granules which were demonstrated by Hortega within the nucleus in the cell-body and around the parenchyma cells by means of his silver carbonate method, in the pineal body of the ox, sheep, and human subject, must be regarded as histological evidence of a granular deposit of some stainable substance differing in its chemical composition from that of the unmodified nucleus and cell-body. Whether the chemical changes indicated by the presence of these granules is evidence of an internal secretion seems as yet to be indecisive, and more particularly is this the case since, as far as we are aware, the presence of the granules has not been noted in the capillary vessels.
The Supporting Tissues of the Pineal Organ
In our general description we have already alluded to the fibrous connective tissue elements comprising the capsule, the trabecular, and the delicate intercellular connective tissue derived from the sheaths of the intralobular vessels. Besides the ordinary connective tissue, there are occasionally seen the branched microglial elements which have been described in connection with the central nervous system ; these are sometimes present in plaques of glial tissue, and more especially in certain pathological states. The microglial elements probably enter the tissue of the pineal body in the same way as they pass into the central nervous system, namely, as independent units which are first seen beneath the pia mater in the form of small rounded cells, with deeply stained nuclei, which afterwards assume amoeboid characters and migrate into the deeper central parts of the organ, and there may develop a phagocytic function. There remain to be considered certain non-cellular elements, namely, a supposed intercellular substance ; forming what has been termed a glial syncytium ; glial fibres ; and the tortuous thick fibrils which have been specially described and figured by Amprino (1935) (Figs. 269, 270, 271, 272).
Fig. 269. — Pineal Gland of a Woman aged 40. (After R. Amprino.)
Fig. 270. — Pineal Gland of a Man aged 76. (After R. Amprino.)
Fig. 271. — A— Pineal Gland of a Newly Born Lamb. B— Pineal Gland of a Sheep aged 4 years. (After R. Amprino.)
An intercellular substance is difficult to demonstrate with certainty, and possibly in the living subject simply exists as an albuminous semi-fluid material which everywhere fills the intercellular spaces. It is also probable that in the preparation of specimens for microscopic examination the coagulable material present in the interstitial tissue-fluid is concentrated on the cellular elements and fibres, so as to form on these a continuous membrane-like covering, such as was described by Held as the " grenz membran." On the other hand, it is quite possible that a similar process may occur normally during life and a more solid constituent be separated out from the more liquid intercellular tissue-fluid, the more solid material being deposited on the surface of the cellular elements and fibres and thus forming a continuous membrane-like covering or, when it completely fills the spaces, an intercellular ground substance. Such a conception is important in connection with the passage of nutritive material, or possibly secretory products, into or out of the cell-bodies and their processes — or in other words, furnishing the means by which osmotic processes can take place between the cells and the tissue-fluid.
Fig. 272. — Pineal Gland of a Man aged 40, showing Astrocyte Cells and Neuroglial Fibres. (After R. Amprino.)
The Nerves of the Parietal Organ and Pineal Body
The study of the nerve supply of the human pineal organ necessarily involves a preliminary consideration of the pineal nerves, commissural fibres, and the associated ganglia of lower types of animals in which the parietal sense-organs, pineal sac, and pineal stalk are more highly evolved than they are in mammals, and in which the nerve tracts have been definitely traced from their origin in the sensory-cells of the retina of the pineal eye or the wall of the pineal sac to their termination in the nerve tracts and ganglia of the brain.
A feature of special interest in connection with the nerve supply of the pineal eye is the bearing that this has upon the question of the bilateral original of the pineal system, and the closely related problem of the homology of the pineal organ of birds and mammals with reference to the " parapineal organ " of cyclostomes and the pineal sac and pineal stalk of amphibia and reptiles. The nerve supply of the pineal eye and pineal sac in Sphenodon was specially studied with reference to this question by the late Professor A. Dendy (191 1, Phil. Trans. Roy. Soc, Ser. B., Vol. 201, pp. 228-339) (Figs. 183, 184, 185, A, Chap. 20, pp. 259, 260, 261). Unfortunately this important work appears to have escaped the attention of many modern writers on this subject, owing to the circumstance that only his earlier works have been quoted in many of the published references to the literature of the pineal system, and these authors have most probably been unaware of the existence of his later publications. Dendy, as is well known from his earlier communications, regarded the pineal eye of Sphenodon as the left pineal organ and the pineal sac (epiphysis) as the right pineal organ of a paired system which included the pineal eye and pineal sac, the pineal stalk, and the associated vessels and nerves. He also considered that the primarily right and left components of this system have, in the course of evolution, become displaced towards or into the median plane, so that the left organ has become anterior and the right posterior. The latter or pineal sac of Sphenodon has also become more degenerate than the left, which is separated off as the parietal organ and presumably has retained, to a much greater extent, the original structure of the ancestral pineal eye. In his later memoir, published in 191 1, Dendy clearly demonstrated that the wall of the pineal sac of Sphenodon has a nervous structure which is essentially similar to that of the pineal eye, and that although less highly differentiated it shows, as in the pineal eye, internal and external limiting membranes, radial supporting or glial fibres, which are comparable to Miiller's fibres in the retina of the lateral eyes of vertebrates, also neuro-epithelial cells which he regarded as sensory in nature, ganglion cells, nerve fibres, and at its distal extremity pigment cells. In one of his specimens this apical part was partially constricted off from the main diverticulum so as to form a thin- walled sac, containing pigment cells in the wall of the main sac. He regarded this sac as being comparable to the accessory parietal organs described by Leydig and Studnicka, and as supporting his view that " the structure of the pineal sac is fundamentally identical with that of the pineal eye."
In cyclostomes (Studnicka, Gaskell, Dendy) there is the same fundamental similarity in structure of the pineal eye and the parapineal organ as is met with in the pineal eye and the pineal sac or epiphysis of reptiles. Both in fishes and reptiles there are sometimes two outgrowths, one of which, Epiphysis I, is anterior, while the other, Epiphysis II, is posterior. In both fishes and reptiles the anterior epiphysis is usually to the left of the median plane. In fishes, however, the posterior organ is the more highly evolved and in cyclostomes it forms the pineal eye, whereas in reptiles the anterior organ is the more highly evolved and forms the pineal eye or parietal organ.
In Geotria Professor Dendy demonstrated non-medullated nervefibres which apparently arose from the ganglion cells of the retina of the right or posterior pineal eye ; these converged towards the optic stalk and then, forming a nerve bundle in the stalk, coursed backwards to end in the right habenular ganglion, the right bundle of Meynert, the ependymal groove or " subcommissural organ," and, he believed, also in the posterior commissure (Fig. 134, Chap. 17, p. 188). He likewise traced the connections of the nerve-fibres issuing from the parapineal organ (anterior or left pineal eye) to the left habenular ganglion, habenular tract, or superior commissure, and the left bundle of Meynert, which is much smaller than the corresponding bundle of the opposite side, which receives the larger pineal nerve coming from the more highly evolved right pineal eye. In his concluding remarks he states that " the connection of each of the two sense-organs with the corresponding member of the habenular ganglion-pair need no longer be questioned " ; and, further, " the marked asymmetry in point of size of the two habenular ganglia and of the two bundles of Meynert corresponds exactly to the unequal development of the two parietal sense-organs with which they are connected, and leaves no doubt as to the paired character of the whole system."
Fig. 273. -Section through the Vestigial Eye of a Frog Tadpole. (After Dendy.) (From a photograph).
ep. : epidermis. p.e. : pineal eye.
?-.s. : roof of skull.
v.n. : vestigial stalk and nerve.
Without entering further into this highly controversial question, we may conclude that these observations are highly suggestive of a system of nerve tracts with commissures passing from the parietal sense-organs to receptive centres in the brain, which in some respects is comparable to that of the paired lateral eyes — in other words, a system of afferent fibres of a sensory nature ; but, as might be expected from the vestigial condition of the receptive organs in these animals, the fibres are usually unmyelinated.
The nerve-fibres of the functional lateral eyes in the human subject are unmyelinated until a late period of foetal life, and do not become myelinated until shortly before birth (Lucas Keene and Hewer, Langworthy, O.R.). In the pineal eye of Geotria and Sphenodon the nerves remain unmyelinated even in the adult animals, a condition which is to be expected in organs which even in these species are degenerate and apparently have little or no function. In other types, for instance in many reptiles and amphibia, the pineal nerve or tract, though present in early embryos (Fig. 187, Chap. 20, p. 264, and Fig. 273), usually disappears later, when the terminal vesicle (parietal organ) becomes separated from the pineal sac and its peduncle (Beraneck, E., p. 246 ; Dendy, A., p. 261 ; Klinckowstrom, A. de, pp. 241, 243).
The Nerve-fibres and Nerve Cells of the Mammalian Pineal Organ
Both in the past and recently, and in addition to the work done on the nerve supply of the pineal system in fishes, amphibia, and reptiles, a large amount of work has been devoted to the study of the sensory cells, nerve cells, and tracts of nerve-fibres belonging to the pineal system in the human subject and in various types of mammals. This has been carried out largely with the object of demonstrating an anatomical basis by which it may be presumed the pineal organ or epiphysis is capable of being influenced by afferent impulses and can function : either by means of specific hormones secreted by the pineal cells and carried to distant organs in the circulating blood or by means of efferent nerves issuing from the gland and joining the habenular ganglia and other nerve centres of the brain or the intracranial sympathetic system — exerting through these systems a direct influence on other organs, e.g. the secretory cells of the choroid plexuses, or an indirect influence on these cells, by means of vasomotor nerves regulating the circulation of blood in the vessels of the organs supplied by them.
The anatomical demonstration of the distribution of the nerve-fibres has been greatly facilitated by the various methods of silver impregnation, and the definite results obtained by Retzius, Studnicka, Cajal, Pastori, and other workers have done much to establish the existence and connections of nerve-fibres, which are presumably afferent and efferent and may form the basis of a reflex mechanism by which it is possible for the pineal body to be influenced apart from the action of hormones reaching it through the circulating blood.
Theoretically one may postulate the existence of a pineal nerve supply consisting of a double central and a double sympathetic system, thus :
_ , fAfferent nerve-fibres to the pineal body.
Central nervous system < „„, n , r , , 1 j
[Efferent nerve-fibres from the pineal body.
Sympathetic system j Afferent nerve-fibres to the pineal body.
[Efferent nerve-fibres from the pineal body.
Also one might expect to find in connection with these fibres sensory or receptive cells and ganglion cells, the latter giving rise to efferent fibres which leave the pineal organ and pass to such ganglia as the habenular or optic thalami, or to the plexuses of sympathetic nerve-fibres on the surrounding blood-vessels. Moreover, one might look for two types of nerve-cells, a large ganglion-cell belonging to the central nervous system and a small type of nerve-cell having the characteristics of the sympathetic system.
Actually, it appears that if observations on the pineal system throughout the whole series of vertebrate animals are included, all these different types of sensory epithelial cells, nerve cells, and nerve-fibres have been seen and described by competent observers. The pineal system, especially that of the mammalia, is, however, vestigial in structure and has undergone marked modifications, and as a consequence the full complement of nerve cells and nerve-fibres is not found in any one species. Nerve cells, in particular, are rare, and when present are usually not fully developed. Such cells have been described as " neuronoid cells " or " amacrine nerve-cells." Moreover, the existence of typical nerve cells showing both Nissl granules and axis cylinder process as a normal constituent of the human pineal organ has been not only doubted but denied by some recent workers, who regard the occasional occurrence of such cells as anomalous.
There seem, however, to be transitional stages between nerve cells and typical parenchyma cells, and it is probable that in some cases branched pineal cells with bulbous extremities have been mistaken for fully developed nerve cells. True nerve cells, apparently belonging to the sympathetic system, are occasionally seen on or near the surface of the organ or in close relation with the vessels contained in the trabecular, and in our opinion a distinction should be made between these cells and the transitional or " neuronoid cells " seen in the parenchyma. It is possible that the latter indicate a stage in the differentiation of true nerve cells from the indifferent neuro-epithelial cells which form the primary elements of the developing organ, and which may give rise to neuroglial cells, parenchyma cells, or very occasionally to either imperfectly or fully developed nerve cells.
The question of whether the parenchyma cells themselves are sensory in nature and capable of transmitting a sensory impulse from an afferent pineal nerve to an efferent pineal nerve is one of practical interest. Should they possess this function, their anatomical connections fully warrant the assumption that a reflex mechanism may exist within the pineal organ, which is capable of being influenced by impulses reaching it through its afferent nerve-fibres and transmitting such impulses by efferent fibres (e.g. sympathetic) to the organs or regions to which these nerves are distributed.
A general survey of the comparative anatomy of the pineal region, with detailed descriptions of the nerve cells and nerve-fibres of the pineal system in special types of animals, was published in 1905 by Studnicka Die Parietalorgane. Oppel. Teil V.), and recent accounts with references to the literature in such works as UEpiphyse, by J. Calvet, a special article on the pineal gland by del Rio Hortega in Cowdry's Special Cytology, Penfield (1928), and various articles such as those by Beraneck, Clarke, Darkschewitsch, Dendy, Dimitrowa, Herring, Pastori, and others. It will be realized on studying these contributions to the innervation of the pineal system that substantial agreement has been reached on the following points :
1. Tracts of nerve-fibres described as the nervus pinealis, nervus parietalis, tractus pinealis, and tractus habenularis have been traced from receptive sensory cells or ganglion cells in the retina of the parietal organs (namely, the pineal eye and end-vesicle of the parapineal organ) and found to terminate in or traverse the habenular ganglia, the superior and posterior commissures, and Meynert's bundles. These fibres have been observed in cyclostomes and other fishes, amphibia, and reptiles. They may be situated in the stalk of the vesicle, and thus resemble the optic nerve-fibres of the lateral eyes of vertebrates ; or they may course as an independent tract through the areolar connective tissue in the neighbourhood of the stalk ; or, after the disappearance of the stalk, they may lie in the region formerly occupied by the stalk. The nerve-fibres may be present only in the larval stages or they may persist in the adult animal.
2. Similar tracts of nerve-fibres may arise from sensory cells in the wall of the pineal sac or the epiphysis in elasmobranch and teleostean fishes, amphibia, reptiles (saurians and snakes), and in mammals. These fibres terminate for the most part in the posterior commissure, but connections are established in some species also with the internal capsule, stria; medullares thalami, Meynert's bundles, habenular commissure and ganglia, and the optic tracts (Darkschewitsch).
There is, however, a considerable amount of variation in different species of mammals, e.g. Herring states that occasional nerve-fibres may enter the pineal body from the habenular commissure in the cat, monkeys, and man, but have probable no functional significance ; whereas in the rat the pineal body is anatomically widely separated from the habenular commissure, and no nervous connection persists between them. In the adult rat the pineal body is an isolated organ which lies on the surface of the brain between the cerebral hemispheres and cerebellum. Its only apparent functional connection with the organ is vascular, and its nerve supply reaches it only in the form of non-medullated fibres accompanying the blood-vessels (Cajal).
The direction in which nerve impulses travel in the fibres connecting the epiphysis with the habenular ganglia, optic thalami, and the superior and posterior commissures is difficult to determine in mammalia, owing to the absence of experimental evidence. It seems, however, to be generally assumed that impulses travelling from the central fibres coming from the posterior commissure through the " tractus intercalaris " not only enter the stalk of the epiphysis, but are also distributed in the parenchyma of the epiphysis.
Pastori states that in some species of mammals (e.g. man and dog) the nerve-fibres coming from the optic thalami and habenular ganglia partly decussate in the inter-habenular commissure ; while in other species of mammals (e.g. the cat) the corresponding nerve-fibres remain homolateral and travel directly from the optic thalamus and habenular ganglion into the parenchyma of the epiphysis, that is to say, without decussating.
On the other hand, the primary direction in which the nerve impulses travel in the lower classes of vertebrates is apparently from the sensory cells of the pineal eye, pineal sac, or epiphysis to the central ganglia — vide Beraneck, Dendy, Gaskell, Klinckowstrom, Studnicka, and others. Moreover, some writers have supposed that the parenchyma cells of the human epiphysis may be sensory, or receptor, cells ; and it has also been suggested that they may be specially sensitive to pressure, and, further, that they may function in regulating the pressure of the cerebrospinal fluid, either through the direct action of the sympathetic system on the choroidal epithelium or by an indirect action on the epithelium through the choroidal blood-vessels.
These considerations suggest that relays of nerve-fibres which originally carried impulses from the receptive organs of the pineal system to ganglia of the central nervous system have been either wholly or partially supplanted by nerves which are afferent to the epiphysis, and also that impulses arising by stimulation of the parenchyma cells of the epiphysis may be transferred to fibres of the sympathetic system.
An anatomical basis which affords support for the latter hypothesis is furnished by Pastori's recent work on the nervous connections of the epiphysis. He has demonstrated in the human subject and in the dog the constant presence of a sympathetic ganglion situated in the membranes just behind the posterior pole of the epiphysis. This ganglion is connected by a large number of very fine nerve-fibres with the epiphysis, and also by less numerous but coarser nerve-fibres, which form a definite bundle which joins the plexus of nerve-fibres on the great cerebral vein and its tributaries. The bundle is the nervus conari of Kolmer. Pastori describes both the fine and the coarse nerve-fibres as arising from small sympathetic nerve-cells situated in the ganglion. The fine fibres enter the epiphysis with the vessels contained in the trabecular.
It is thus possible that some of the fibres may be efferent nerves from the ganglion to the gland, and others afferent from the gland to the plexus of nerve-fibres on the neighbouring vessels, and that these furnish a means by which the epiphysis may be influenced by or act upon the sympathetic system.
The Vascular Supply of the Pineal Organ
The arteries of the pineal body are derived from the posterior choroidal branches of the two posterior cerebral arteries. The posterior choroidal artery on each side enters the transverse fissure of the brain between the two layers of the tela choroidea, and gives off small branches near its origin to the pia mater investing the pineal body ; from these branches numerous arterioles enter the capsule and trabecular of the organ, and ultimately give off capillary vessels for the supply of the parenchyma. The capillary net is drained by venules which passing through the trabecular and capsule unite to form a vessel which joins the great cerebral vein of Galen. This terminates in the anterior part of the straight sinus.
Since a tumour of the pineal organ may by pressure obstruct the great cerebral vein, it is important to know the exact course of this vessel. It will be remembered that the internal cerebral vein on each side is formed in the region just behind the interventricular foramen of Monro by the union of the anterior choroidal vein with the terminal or striate vein, and that the two internal cerebral veins course backwards below the fornix and between the two layers of the tela choroidea or velum interpositum. Here they receive tributaries from the choroid plexus of the third ventricle and optic thalami. They unite near the base of the pineal body to form the great cerebral vein of Galen, which curves upwards in the cisterna vense magnar cerebri around the splenium of the corpus callosum (Fig. 274). Here after receiving the right and left basal veins and the internal occipital veins, it opens into the anterior end of the straight sinus, the latter vessel commencing as a continuation of the inferior sagittal sinus. It is important to remember also that some of the superior cerebellar veins run inwards to terminate in the straight sinus or in the internal cerebral veins.
The opening of the right and left basal veins into the great cerebral vein of Galen has a practical bearing in connection with occlusion of the great vein, for unless the pressure on the great cerebral vein involves its terminal part and the openings into it of these two vessels, any resulting congestion of the choroidal veins which might result from an obstruction at the commencement of the vein would be relieved by the anastomoses between the tributaries of the basal veins and the choroidal veins in the inferior horns of the lateral ventricles. For detailed description of the
Fig. 274. — Diagram showing the Principal Tributaries and Relations of the Great Vein of Galen, and the position occupied by a Pineal Tumour.
anatomy of these veins in connection with the production of hydrocephalus the reader should consult articles by Dandy and Blackfan (1914), Stopford (1926, 1928), and Bedford (1934).
Since no lymphatic vessels are present in the central nervous system of which the pineal organ is a part, it is probable that secretory or waste products contained in the tissue-fluids of the pineal body would, like the cerebrospinal fluid, be absorbed directly into the venous system through perivascular channels which are in communication with the tissue spaces.
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Cite this page: Hill, M.A. (2021, May 7) Embryology Book - The Pineal Organ (1940) 26. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_26
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