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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF

The Pineal Organ (1940): 1 Introduction | 2 Historical Sketch | 3 Types of Vertebrate and Invertebrate Eyes | Eyes of Invertebrates: 4 Coelenterates | 5 Flat worms | 6 Round worms | 7 Rotifers | 8 Molluscoida | 9 Echinoderms | 10 Annulata | 11 Arthropods | 12 Molluscs | 13 Eyes of Types which are intermediate between Vertebrates and Invertebrates | 14 Hemichorda | 15 Urochorda | 16 Cephalochorda | The Pineal System of Vertebrates: 17 Cyclostomes | 18 Fishes | 19 Amphibians | 20 Reptiles | 21 Birds | 22 Mammals | 23 Geological Evidence of Median Eyes in Vertebrates and Invertebrates | 24 Relation of the Median to the Lateral Eyes | The Human Pineal Organ : 25 Development and Histogenesis | 26 Structure of the Adult Organ | 27 Position and Anatomical Relations of the Adult Pineal Organ | 28 Function of the Pineal Body | 29 Pathology of Pineal Tumours | 30 Symptomatology and Diagnosis of Pineal Tumours | 31 Treatment, including the Surgical Approach to the Pineal Organ, and its Removal: Operative Technique | 32 Clinical Cases | 33 General Conclusions | Glossary | Bibliography

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Chapter 22 The Pineal Organ of Mammals

Only the basal segment of the organ, the epiphysis or conarium, is recognizable in man and mammals generally ; the terminal parts, namely, the parietal sense-organ, the end-vesicle or pineal sac, and the stalk, are seldom present in the adult animal, and in many cases it is doubtful whether any vestige of these parts, even in a rudimentary condition, is found in the embryo. It is possible, however, that the occasional occurrence of bifid pineal diverticula and accessory organs and the development in the foetus of an anterior lobe may be attributed to an inherited trait, which has not yet been completely exhausted or suppressed, and which serves as an indication of its primary dual origin. The sagittal or parietal fontanelle of the human skull and the parietal foramina which are formed at its lateral angles, each of which transmits a small vein and artery, may also be regarded as possible vestiges of the parietal foramen which in certain reptiles and fishes lodges the parietal sense-organ. Although in the adult human skull the roof is separated from the pineal organ by the whole depth of the falx cerebri and the splenium of the corpus callosum, in the human foetus at the 7th week, when the rudiment of the pineal is first recognizable, the roof of the diencephalon is quite near the condensation of mesenchyme which represents the future membranous capsule of the brain, and there is a special development of endothelial lined vascular spaces in this situation. Further, the occasional appearance of pigment in the skin of the head of the pineal region in certain swimming birds (Klinckowstroem), mentioned on p. 75, and the occasional occurrence of a parietal foramen in the skull of the goose (Mrazek), more especially in those examples which possess a tufted crest, indicate that remnants of a parietal fleck and a parietal foramen may persist in the higher classes of vertebrates long after all remnants of the parietal eye have disappeared ; and that, if this is true, the validity of the supposed morphological significance of the parietal foramen in the human subject is not so difficult to accept as was formerly thought. The hypothesis that the parietal fontanelle and the parietal foramina of the human skull are homologous with the parietal foramen of reptiles, amphibia, and fishes has, moreover, received a considerable amount of support from recent geological evidence of the foramen in fossil skulls of the Therapsids or mammal-like reptiles discovered in America and South Africa (Broom). Thus in the middle or upper Permian period a group of higher Synapsids * were evolved which are allied to the Theromorphs of America. These were the large Dinocephalia, some of which were 1 6 to 1 8 ft. in length ; also small rat-like animals and others with grotesquely shaped broad skulls. Changes in the skull of these types, and more especially in the parietal region, show transitions between reptilian and mammalian forms. They not only show transitional stages in the gradual obliteration of the parietal canal, but they also suggest how in certain orders of mammals the canal may have been retained, or if not quite obliterated may have reappeared in association with the widening of the skull which is correlated with an increase in the size of the brain.

Dicynodon (Fig. 205), although not directly in the line of mammalian descent, is very instructive with respect to reduction of width of the parietal bones, and the diminution in the transverse diameter of the parietal foramen, which becomes still further reduced in the Cynodont reptile Glochinodontoides gracilis (Haughton) (Fig. 206). In this the two parietals are fused and the antero-posterior ridge between the temporal fossae, mentioned on p. 268, is already prominent. In the skull of the small Ictidosaurian reptile (Fig. 228) found in beds of a later period (Rhoetic or Lower Jurassic) the development of this longitudinal ridge is still more pronounced, the parietal bones being fused throughout their whole length and the parietal canal having disappeared completely. In this animal, besides the keel-like interparietal crest, there are two occipital condyles and the articulation of the lower jaw with the base of the skull is still more mammalian in type than it is in the preceding forms, for although the joint between the articular and quadrate is still present, the dentary almost reaches the squamosal. For these and other reasons the Suborder Ictidosauria is regarded as forming one of the connecting links between the cynodonts (dog-toothed reptiles) and the Mammalia. A more human type of the parietal region of the skull-roof is met with in the Gorgonopsian reptile Scylacops capensis (Broom). This, although in other respects it is much more reptilian in character, serves as an example of the earlier type of fossil reptile-skull, in which the parietal foramen lies in the suture between two horizontally placed parietal bones, which are separated from the single temporal fossse merely by a backward prolongation of the postorbital. This bone is reduced in Glochinodontoides, and the parietal bone shares in the formation of the temporal fossa, as it does in the human subject and other living orders of mammals.

Fig. 205. — Skull of an Anomodont Reptile, Dicynodon Kolbei, Broom,

viewed from above. It shows an oval pineal foramen and narrow parietal bones, separated from the temporal fossa by a backward prolongation of the post-orbital bone. Although greatly specialized, Broom regards the skull as essentially similar to the mammalian type.

F. : frontal. Po. F. : post-frontal.

IP. : interparietal. Po. O. : post-orbital.

J. : jugular. P.P. : pre-parietal

L. : lacrimal. P. Mx. : pre-maxilla.

Mx. : maxilla. Pr. F. : pre-frontal.

N. : nasal. Q. : quadrate.

Par. : parietal. QJ. : quadratojugal.

P.F. : pineal foramen. Sq. : squamosal.

Tab. : tabular.

1 Synapsida : ay is, arch or recess — the type of cranial roof of Tetrapods in which there is a single temporal fossa.

Fig. 206. — View from above the skull of a primitive Cynodont Reptile, Glochinodontoides gracilis, Haughton. Natural size. (After Broom.)

The two parietal bones have fused to form a median longitudinal crest. The pineal foramen is still present and is oval in form. The parietal bones share in the formation of the temporal fossa.

Al. S. : ala of sphenoid.

E. Oc. : ex-occipital.

F. : frontal.

LP. : inter-parietal. J. : jugular. L. : lacrimal. Mx. : maxilla. N. : nasal.

Par. : parietal.

Po. O. : post-orbital & post-frontal.

Pr. F. : pre-frontal.

Pr. Mx. : pre-maxilla.

5. Oc. : supra-occipital.

Sq. : squamosal.

Tab. : tabular.

1 iktISes, weasel-like.

Development of the Pineal Organ of Mammals

Despite the wide differences in the general form and structure of adult types of mammalia, the early stages of development of the pineal organ appear to be very similar in all. We shall therefore give a short description, in the first place, of the development of the human pineal organ, of which we have a more complete knowledge than of other types, and will only refer to the development of the pineal system in lower mammals with respect to any corroborative evidence or peculiarity of structure which may be of general interest.

Fig. 207. — Median Linear Reconstruction of thf Pineal Region and Posterior Commissure of a 20-MM. Human Embryo. The Apex of the Pineal Diverticulum is directed Forward. (R. J. G.) Ant. D. : anterior diverticulum. N.Z. : nuclear zone.

Aq. C. : aqueductus cerebri. M.Z. : marginal zone.

Ep. : ependyma. P.C. : posterior commissure.

Epd. : epidermis. Post. D. : posterior diverticulum.

V. III. : third ventricle.

One of the first indications of the development of the pineal system in man is a thickening of the ependyma in the posterior part of the roof of the diencephalon and the adjacent parts of the alar laminae. This thickening has the form of a longitudinal band which is slightly raised above the general surface and is grooved on its under surface. It lies in front of the posterior commissure, which commissure has already appeared and extends backwards for a considerable distance above the ventricle of the midbrain, which becomes the future aqueduct of Sylvius. Soon, by a forward growth of the anterior end of the thickened band and a deepening of the groove, the rudiment assumes the typical form of the primary diverticulum in lower classes of vertebrates : namely, a rounded anterior segment with a small lumen which is separated by a slight constriction from the lumen of the basal segment, which opens freely into the cavity of the third ventricle (Figs. 207, 208, 209.)

Fig. 208. — Median Longitudinal Section of a Rabbit Embryo (16 days, 11 mm.). The Apex of the Pineal Body is directed Forward, and the Diverticulum shows a Central Constriction. (R. J. G.)

Aq. C. : aqueductus cerebri.

Cbl. : cerebellum.

C.N. IV. : Cranial nerve IV.

Inf. : infundibulum.

P.C. : posterior commissure.

P.O. : pineal organ. R.P. : pouch of Rathke. T. : tongue.

V. III. : third ventricle. V. IV. : fourth ventricle.

In a 20-mm. human embryo the wall of the diverticulum shows a division into the three typical zones of the neural tube, namely, an inner, thick ependymal layer (Ep.Z.), a thin middle layer or nuclear zone (N.Z.), and an outer reticular or marginal zone (M.Z.). The inner ends of the ependymal cells which immediately surround the lumen are clear and destitute of nuclei ; they show a radial arrangement and are bounded by an internal limiting membrane (Figs. 209, 210). The outer zone is limited by a less defined external membrane and pia mater, and it is in relation with large endothelium-lined spaces and capillary vessels which lie in the surrounding loose mesenchymal tissue. The extent and thickness of the posterior commissure at this stage of development are well seen in Figs. 207, 208, which represent the region in a human embryo and in a rabbit embryo at a corresponding stage of development (16 days, 11 mm.). Fig. 211, of a slightly older rabbit embryo (18 days), shows the direction of the fibres of the posterior commissure as seen in a paramedian sagittal section.

Fig. 209. — Pineal Evagination, 20-MM. Human Embyro. The Section passes transversely through the posterior part of the diverticulum., where its Lumen is continuous with the Cavity of the Third Ventricle. B.V. : blood-vessels. N.Z. : nuclear zone.

Ep. Z. : ependymal zone. M.Z. : marginal zone.

P.M. : pia mater.

Fig. 210. — Pineal Organ of the Same Embryo as Fig. 209. The Section passes transversely through the anterior part of the organ, and shows the Central Lumen surrounded by Radially Arranged Ependymal Cells. (R. J. G.)

B.V. : blood-vessel. Ep. : ependyma. End. S. : endothelium-lined space. hum. : lumen. Th. : thalamencephalon.

Fig. 211. — Paramedian Section through the Mid-brain of a Rabbit Embryo (18 days), showing the direction of the fibres of the posterior commissure and their connections with the pons varolii and interpedunCULAR Region. (R. J. G.)

CM. : corpus mammillaris. V.M. : ventriculus mesencephali.

P. Co. : posterior commissure. V.S. : venous sinuses.

P.V. : pons Varolii V.L. : ventriculus lateralis. 77? . : thalamus.

Fig 212 —Transverse Sections of Pineal Region of a 22-mm. Human Embryo 'and Median Aspect of the Right Half of a Model reconstructed from the Corresponding Series of Sections. (R. J. G.) A— Section through the posterior bifurcated end of the anterior ependymal diverticulum in the plane A of the model. B— Section through the anterior bilobed diverticulum and posterior median diverticulum in the plane B of the model. C— Section through the posterior median diverticulum and posterior commissure in the plane C of the model. :

D— Drawing of the median aspect of the right half of the model showing the relations of the superior and posterior commissures to the pineal diverticula. A. B. C, in D indicate the planes of the sections in A, B, and C. A.B.D. : anterior bilobed diverticulum. Epd. in D : epidermis. A.L. : epithelial buds forming rudiment of anterior lobe. Aq. C. : aqueductus cerebri. Ep. D. in A : ependymal diverticulum. I.P.R. : infrapineal recess. P.C. : posterior commissure. P.M.D. : posterior median diverticulum. S.C. : superior commissure. V. Ill : third ventricle.

Fig. 213. A — Transverse section through the pineal organ of a human embryo (6 cm.) showing its relations to the membranes, posterior commissure, and subcommissural organ.

B — A median linear reconstruction of the pineal region of a human embryo (6 cm.) showing the main diverticulum, infrapineal recess, anterior lobe, superior and posterior commissures, the dorsal sac, and an anterior diverticulum possibly representing a vestige of the paraphysis ; also the great cerebral vein and opening of the aqueductus cerebri. (R. J. G.) A.L. : anterior lobe. Aq. C. : aqueductus cerebri. C. Ep. : columnar epithelium C.H. : cerebral hemisphere. D.D. : dorsal diverticulum pineal recess. G.C.V. : great cerebral vein. I.P.R. : infrapineal recess. Pa. : ? paraphysis. P. Co. : posterior commissure P.D. : pineal diverticulum. P.M. : pia mater. P.O. : pineal organ. Post. D. : posterior diverticulum, or supra- P. St. : pineal stalk. Q.P. : quadrigeminal plate. S.C. : superior commissure. S.C.O. : subcommissural organ. I 7 . III. : third ventricle.

In a 22-mm. human embryo, Figs. 212, A, B, C, D, the simple primary diverticulum is subdivided by a transverse fold into a hollow, bilobed anterior segment, B, A.B.D., and a single median posterior segment, B, P.M.D. The superior or habenular commissure has also appeared and two diverticula from the dorsal sac have commenced to grow backward on each side of the main pineal diverticulum, A, Ep. D. There is also a slight indication of the development of the anterior lobe, which appears as two solid epithelial buds from the anterior wall of the pineal diverticulum, B, A.L. An infrapineal recess was also present in this specimen, D, I P.R., which probably represents a temporary fold and would have disappeared at a later stage of development, when the apex of the pineal organ becomes rotated backward and comes to lie on the dorsal aspect of the quadrigeminal plate.

Fig. 214. — Transverse Sections of the Pineal Region of a 6-cm. Human Embryo, and of a 4 J -month fcetus. (R. J. G.)

A — Section through the basal part of the main pineal diverticulum of a 6-cm. human embryo showing, in the upper part of the photograph, the solid anterior lobe. Below this is the main pineal diverticulum, the wall of which shows proliferating cords of ependymal cells growing outward, into the surrounding tissue. Below the pineal evagination is a section through the infrapineal recess, the epithelial lining of which is assuming a columnar type. Fibres of the posterior commissure are seen at the sides and below the pineal region.

B — Coronal section through the pineal region of a 4. \ -month human foetus showing the relation of the dorsal diverticulum (suprapineal recess) to the pineal gland. Clusters of elongated choroidal villi project into the lumen of the diverticulum. The pineal gland shows partial subdivision into an anterior and posterior lobe.

C — The pineal gland more highly magnified, showing the ingrowths of vascular processes of the pia mater between the outgrowing neuro-epithelial cords.

D — Peripheral portion of the gland X55 D., showing pale areas containing a central core of vascular pia mater, alternating with dark zones composed of neuro-epithelial cords.

[Continued at foot of next page

In a 6-cm. human embryo, the anterior lobe has considerably increased in size and a secondary pineal stalk has developed and forms rather more than one half of the total outgrowth, the distal end of which consists of the main pineal diverticulum (Fig. 213, A, P.O., and B, P.D.) and a smaller posterior diverticulum (Fig. 213, B, Post. ZX), which lies immediately above the posterior commissure. The anterior lobe consists of a number of irregularly branched processes of epithelium, which grow into the vascular mesenchyme in front of the main pineal diverticulum and above the habenular commissure. Similar proliferating outgrowths of ependymal cells from the sides of the main diverticulum are seen invading the mantle zone ; these, however, lie beneath the external limiting membrane Fig. 214, A, A.L., Ep. C, and have not, as yet, come into contact with the blood-vessels of the surrounding mesenchyme. Two diverticula were present in front of the pineal organ ; the more posterior of these probably represents the diverticulum from the dorsal sac, which will become the suprapineal recess (Fig. 213, B, D.D.) ; the more anterior is possibly a rudiment of the paraphysis {Pa.). The close relation of the great cerebral vein to the epiphysis is indicated in the lineal reconstruction (Fig. 213, B, G.C.V.) and also the position of the epidermis, which at this stage is raised a considerable distance above the pineal organ. The exact position relative to the membranes of the brain is shown in Fig. 213, A, P.O. The organ lies in a triangular space bounded below by the roof plate of the neural tube and the layer of pia mater which invests the brain stem, and laterally by right and left membranous lamina;, which are attached above to the lower border of the interhemispheric septum or primary falx cerebri. Along the line of junction of the lateral laminee with the interhemispheric septum is a membranous channel which encloses the great cerebral vein (Fig. 215, G.C.V. ). At a later stage of development, when the corpus callosum and fornix grow backward over the diencephalon and midbrain, the thin lower part of the interhemispheric septum disappears (Fig. 215, S.), while the thick upper part persists as the definitive falx cerebri, the inferior sagittal sinus (Fig. 215, I.S.S.) being developed in its lower border. The septum in the earlier stages of development is very much thicker than at the stage described, and is formed by a median condensation of the loose mesenchyme which occupies the space between the developing hemispheres. It is not formed by the union of the two layers of a fold, and the same remark applies to the formation of the tentorium and the falx cerebelli.

A.L. : anterior lobe. P.C. 1 , P.C. 2 : posterior commissure. B.V. : blood-vessel. P.L. : posterior lobe. C.V. : choroidal villi. P.M. : pia mater. D.D. : dorsal diverticulum. Post. D. : posterior diverticulum. Ep. C. epithelial cords. V.C.T. : vascular connective tissue.

Fig. 215. — Transverse Section through the Posterior Thalamic Region of a 6-cm. Human Embryo in Front of the Pineal Organ, showing the Relation of the Cerebral Membranes to the Great Cerebral Vein and Neural Tube at the Junction of the Lateral Plate with the Dorsal Lamina, and the Extreme Thinness of the Membrane (Lower Part of Interhemispheric Septum) which joins the Sheath of the Great Cerebral Vein to the Lower Margin of the Falx Cerebri. (R. J. G.)

C.H. : cerebral hemisphere. Ch. P. : choroid plexus. C.N., III., IV., V. : cranial nerves. I.S.S. : inferior sagittal sinus. G.C.V. : great cerebral vein. M.B. : bundle of Meynert.

O.T. : optic thalamus.

P.M., P.M. 1 , P.M. 2 : pia mater.

5. : septum interhemisphericum (falx cerebri). T. Cbl. : tentorium cerebelli.

Fig. 216. — Median Linear Reconstruction of the Pineal Region of a 4 1 -month Human Fcetus showing Anterior Opening of Aqueductus Cerebri ; the Columnar Epithelium of the Subcommissural Organ between this and the Posterior Commissure, above which is the Pineal Organ, consisting of a Main Posterior Lobe which in the greater part of its extent is solid and a Small Anterior Lobe. Above and in front of the Pineal Recess is the Superior Commissure and above this the Bifid Dorsal Diverticulum. (R. J. G.)

A.L. : anterior lobe. Aq. C. : aqueductus cerebri. C. Ep. : columnar epithelium. D.D.', D.D." : dorsal diverticulum. P.C. : posterior commissure. P.L. : posterior lobe. P.R. : pineal recess. Q.P. : quadrigeminal plate. S.C. : superior commissure. S.C.O. : subcommissural organ.

The recess (Fig. 213, B, Post. D.) which projects backward from the posterior wall of the main diverticulum probably corresponds to the posterior pineal diverticulum described in the preceding specimen (22-mm. human embryo). This pocket, which has been described as the infrapineal recess, disappears at a later stage of development, probably by opening out into the cavity of the third ventricle ; and it seems probable that the cavity of the secondary pineal stalk (Fig. 213, B, P. St.) also becomes absorbed into the ventricle in the same way, leaving only its apical part, which persists in the adult as the definitive pineal recess. The central cavity of the body of the main pineal diverticulum, which according to Krabbe becomes closed by constriction at the neck, persists for a variable time ; thus the cavity has usually disappeared at birth, but the pineal body may be solid, with the exception of the pineal recess at its base, at a much earlier stage, e.g. in a 40 -month human foetus, which we shall describe next (Fig. 216, and Fig. 214, B and C).

The mid-foetal stage of development of the pineal organ in the human subject is very instructive. The anterior lobe, which is medial in position, is seen in a coronal section to be partially separated from the main part of the organ by two fibro-vascular septa, which pass obliquely downwards and medially towards the centre of the organ, each traversing about onethird of its total width, the remaining median third corresponds to a zone where the anterior lobe is continuous with the substance of the main posterior lobe. The structure of the two lobes is similar, but that of the anterior lobe, especially its central part, is more homogenous. Each lobe consists of a lobulated mass of small epithelial cells with deeply stained oval nuclei. The most active growth is at the periphery, where irregularly branched epithelial processes are growing out into the surrounding vascular connective tissue. The epithelial cells appear to be mostly directly derived from the inner ependymal zone of the diverticulum, but there are also a certain number of cells which have the character of the nuclear or mantle zone, which may be distinguished by their position and by their nuclei being vesicular and pale in colour, as contrasted with the deeply stained nuclei of the undifferentiated ependymal cells. At the surface branched finger-shaped or club-shaped processes interdigitate with vascular processes, which appear to grow inward between the epithelial cords (Fig. 214, B, C, D). Further, if cross-sections of the interdigitating processes are observed, it will be seen that the vascular processes of pial tissue appear paler than the surrounding zones of densely packed small epithelial cells. The vascular mesenchymal areas are at first separated from contact with the neural epithelium by the external limiting membrane, between which and the deeply stained ependymal cells are a few sparsely scattered cells, with pale vesicular nuclei belonging to the mantle zone. The general arrangement of the epithelial tubes or cords and their relation to the ingrowing vascular processes, are seen with diagrammatic clearness in Fig. 217, B and D, photographed from a section of the wall of the pineal sac of an adult Sphenodon. 1 In the illustration, Fig. 217, it will be observed that the wall of the pineal sac is folded, and that between the hollow epithelial outward projections of the wall there are ingrowths of vascular connective tissue. Such an ingrowth is seen in the centre of Fig. 217, D, and it will be noted that the vascular core, B. V., which lies in the middle of the lobule is surrounded on all sides by neuroepithelium. Should a cross-section of such an apparent lobule be examined, it will be seen to consist of a core of sinusoidal bloodvessels surrounded by a perivascular sheath of pial tissue, which is in contact peripherally with the external limiting membrane of outgrowing neuroepithelial processes. Proceeding farther outward from the central pale area formed by the vessels and their loose mesenchymatous sheath, there will be found beyond the limiting membrane the reticular and nuclear zones, the latter containing pale cells with large vesicular nuclei. Finally, there is a peripheral zone of deeply stained ependymal cells and the internal limiting membrane.

The deeply stained cells surrounding the clear vascular areas have the appearance of epithelial cords cut in various directions. When the section is transverse (Fig. 217, F) the cords appear as rings of cells, " rosettes," with deeply stained oval nuclei arranged radially round a small palely stained central zone, which is formed by the inner ends of the cells coming into contact in the central axis of the cord. If the section of the cord is longitudinal, two parallel rows of nuclei are seen, which are separated by a palely stained axial zone, where the inner, palely stained ends of the cells come into contact in the situation of a virtual lumen. These cords of proliferating ependymal cells are usually grouped in lobules (Fig. 214, D), which grow outward between the vascular ingrowths of the pia mater.

In the later stages of development neither the internal nor external limiting membrane is visible, and it appears that the lumen of the outgrowing epithelial lobules, seen in Fig. 217, A and B, is replaced by a virtual lumen, which forms the central axis of the solid cords which are seen in cross-section in Fig. 214, D, and Fig. 217, F. The external limiting membrane also disappears, as it has been shown to do in birds, and the ependymal tissue mingles with the connective tissue, as was clearly demonstrated by Studnicka to be the case in Strix (Fig. 204, p. 297).

1 This specimen is from one of a series of microscopical sections illustrating the development and the structure of the adult pineal region of Sphenodon and Geotria which were prepared by the late Professor Dendy of the University of London, King's College, and we take this opportunity of thanking Professor D. Mackinnon for permission to make use of this most valuable collection in our recent investigation.

Fig. 217. A — Transverse section through the distal end of the pineal evagination of a 6-cm. human embryo. It shows folding of the wall of the diverticulum and outgrowth of the proliferating ependymal cells into the mantle zone.

B — Section through the pineal region of an adult Sphenodon (Dendy collection), showing in the upper part of the photograph the pineal evagination, the wall of which is folded in a manner similar to the human, but the extension of

C — Transverse section through the basal part of the pineal evagination of the 4i-month human foetus, seen in Fig. 214, B, C, and D, showing outgrowth of neuro-epithelial cords, more especially from the anterior aspect and sides of the tube ; on the posterior aspect (below in photograph) the epithelium is differentiating into the columnar type characteristic of the subcommissural organ.

D — Detail of B, 139 D., showing in the centre a pseudo-lobule, with its central core of vascular pia mater, between two hollow neuro-epithelial outgrowths.

E — Section through the subcapsular part of a pineal gland of an infant (1 year 4 months), showing the fibrous capsule and the penetration of a blood-vessel inro the substance of the gland. The vessel is surrounded by a perivascular sheath of fibrous connective tissue, outside which is a glial sheath the cellular and fibrous components of which are continuous with the neuro-spongium which forms the supporting tissue of the lobules and contains the parenchyma cells.

F — Portion of the same specimen 261 D., showing the predominance at this stage of the cells with small dark nuclei and the arrangement of the cells in cords, which when cut in cross-section are seen to be disposed radially round a central core which is destitute of nuclei, giving the acinar appearance sometimes described as " rosettes." (R. J. G.)

B.V. : blood-vessel. M.Z. : marginal zone.

C.E. : columnar epithelium. P. Sh. : pial sheath.

E.L.M. : external limiting membrane. Pr. Ep. : epithelial processes.

Ep. : ependyma. Ros. : " rosette."

F.C. : vessels in fibrous capsule. S.E. : secondary evagination.

G. Sh. : glial sheath. V.C.T. : vascular connective tissue.

There is thus an appearance produced, by cross-sections of the cords, of groups of deeply stained epithelial cells arranged in ring-like zones or " rosettes." These surround the vascular ingrowths, which appear pale. This arrangement of alternating series of branched epithelial and vascular cords is the key to the mosaic appearance which is described by Globus and Silbert as characteristic of the later stages of foetal and early postnatal life. The " streams" of deeply stained epithelial cells described by these authors are longitudinal sections of the epithelial cords. The central parts of the large clear areas of the mosaic pattern are transverse or oblique sections through the ingrowing vascular processes surrounded by pale cells with vesicular nuclei. Owing to the radial disposition of the epithelial cells around a central axis, which is destitute of nuclei, the general appearance of a group of epithelial cords cut transversely is similar to that of an acinar gland, but with the important difference that in the human subject the acini usually have no lumen and no ducts are present.

The later foetal and early post-natal stages of development of the human pineal organ have been specially studied by Globus and Silbert, 1 93 1, and Krabbe, 1915 ; the former have published an excellent series of photographs of the pineal body, illustrating the structure of the organ the lumen of the diverticulum into the outgrowing processes is much more pronounced. Below and to the right of the photograph are seen sections of portions of the paraphysis and dorsal sac.

in human foetuses from 5! months to the time of birth, and of 18 infants in the first month of post-natal life, during which critical period considerable changes in structure occur which have been described by these authors and also by Krabbe. Globus and Silbert have also described the pineal organ of children varying in age from 2 months to 5 -J- years and of older persons up to the age of 72 years. They distinguish two principal types of cell-elements, namely, small cells with deeply stained oval nuclei and larger cells with more abundant protoplasm and pale vesicular nuclei. The latter, more especially during the first two months of postnatal life, occupy the central zones of lobular areas, which are bounded externally by the small dark cells. These rounded or polygonal areas appear to form structural units of a system which on section has a mosaiclike appearance. About the beginning of the 2nd month of post-natal life the mosaic appearance becomes less pronounced, and from this time up to the 10th month these authors believe that a transformation takes place of the small dark cells into fibroblasts. Krabbe, on the other hand, holds the view that the small, darkly staining cells, which he calls " proparenchyma cells," undergo a metamorphosis during the first year of post-natal life, which is usually completed by the end of the first year. The small proparenchymatous cells give rise to large cells with clear vesicular nuclei ; these are the " parenchymatous cells," and he believes that the fibrous elements are derived entirely from the connective tissue. Most authors agree with Krabbe in assigning the origin of the fibrous connective tissue to the ingrowth of mesoderm, which accompanies the penetration of vessels between the outgrowing buds of epithelium, and they believe that the parenchyma cells originate by transformation of the small darkly stained cells of which the epithelial outgrowths are primarily composed ; but it seems probable that the transformation of the indifferent epithelial cells into the large round cells with pale nuclei commences at a much earlier period than the post-natal, namely, about the middle of foetal life, and that it only becomes a pronounced feature during the critical period of early infancy (1st to 2nd month). It is also generally believed that the small round cells give rise to a certain number of glial cells in addition to the parenchymal cells which in young subjects form the bulk of the tissues composing the pineal organ ; but they do not give origin to fibroblasts, these being wholly derived from the ingrowth of mesodermal tissue.

Later, a still further development takes place of the larger round cells, with clear vesicular nuclei, namely, the outgrowth of the characteristic processes of the parenchyma cells (see Fig. 218) ; and according to Dimitrowa and others the appearance of what they believe to be " secretory " granules in the nucleus and in the cytoplasm of these cells (Fig. 219).

Fig. 218. — Pineal Body of a Young Boy, showing Branched Parenchyma Cells, Peripheral Processes of which end in Club-shaped Enlargements in the Interlobular Connective Tissue or in the Sheaths of Vessels. (After del Rio-Hortega.)

A : parenchymatous cells. C : interlobular tissue. B : marginal claviform processes. D : vessel with club-shaped processes in its adventitia.

Fig. 219. — Cells with Granular Protoplasm from the Epiphysis of Bos TAURUS. (Weigert's method. After Dimitrowa, 1901.)

Some (a) have their whole body filled with granulations, others (b) have only a thin peripheral layer showing granules ; (c) cell with a vacuole.

The processes of the parenchyma cells are especially well seen in the marginal plexus, at the periphery of the pineal lobules (Fig. 220). According to the prevailing view held by the more recent authors, e.g. del Rio

Fig. 220. - Diagram representing Stages in the Development and Differentiation of the Pineal Organ.

A — Early stage, showing part of the epithelial wall of a lobule of the primary pineal diverticulum, with a vascular strand of connective tissue separating it from an adjacent lobule on the right.

B — Later stage, the external limiting membrane has disappeared, and the vessels with their connective tissue sheaths have penetrated the epithelial wall. A differentiation of the primary ependymal cells has now taken place, a middle zone of pale cells with vesicular nuclei now being present between the small darkly staining cells and the reticular zone.

C and D — The differentiated neuro-epithelial cells of the adult organ — ependymal, C ; glial and parenchymatous, D.

Ar. : arteriole.

C. : capillary vessel.

C.T. : connective tissue.

E.L.M. : external limiting membrane.

E.Z. : ependymal zone.

G.l. : glial cell. I.L.M. : internal limiting membrane.

M.Z. : mantle zone (large pale cells).

P.C. : parenchyma cells.

P.E.Z. : primary ependymal zone.

R.Z. : reticular zone.

V.C.T. : vascular connective tissue.

Hortega, the majority of the processes end in club-shaped swellings which are attached to the walls of the blood-vessels running in the trabecular of connective tissue, while others join with each other in the formation of the central and marginal plexuses. It is not at all clear, however, what is the exact relation between the connective tissue elements and the processes of the parenchyma cells. According to the description by Studnicka of the pineal organ in birds, and more especially in Strix flammea (see p. 297, Fig. 204), after the disappearance of the external limiting membrane at an early stage of development the connective tissue elements become inextricably blended with the branched processes of the ependymal cells which in birds line the follicles of the epithelial buds, and thus presumably correspond to the parenchyma cells of the mammalian pineal body, since the latter arise by transformation of the indifferent primary ependymal cells. In the adult both collagenous and glial fibres are found in addition to cellular elements having the distinctive characters of connective tissue cells and glial cells (astrocytes). Also, large areas, or " plaques," of degenerated glial tissue, containing few or no parenchyma cells are found in old subjects and sometimes even in children and infants only 4 months old. These areas are conspicuous owing to their being composed of a network of feebly stained, fine glial fibres, and by the sparseness and small size of the nuclei (Fig. 221, A, B, C, Gli.). The existence of these degenerate areas in the pineal bodies of young subjects, quite apart from any diseased condition of the central nervous system generally, appears to afford very strong evidence of the retrogressive nature of the pineal organ in the human subject. The regressive character of the pineal organ is also plainly indicated by the frequent formation of calcareous deposits, which occur in the parenchyma in the glial plaques, in the connective tissue capsule, and in the trabecular (Fig. 317, B, p. 465, Ca., and Fig. 318, p. 466). The deposits are found in the pineal body and the surrounding vascular pia mater from early infancy to old age, and although they are not always present, their existence in or around the gland is sufficiently frequent to have led some authors to describe the condition as being normal in the adult (Fig. 222). The frequent onset of fibrosis and gliosis in early life and the great variability in the general appearance of the gland which is associated with these states are further evidences of involution. Although the maximum degree of differentiation is usually attained by about the seventh year, in many cases, judging from the microscopic appearances of the cells and of the supporting tissues, the development of the organ appears to have been arrested in early childhood or even in infancy.

The changes that take place in the normal development of the human pineal body in some respects are similar to those which take place in the development of the central nervous system generally, but in the pineal organ they do not reach the high degree of differentiation which occurs in the central nervous system, and in the pineal there is an outgrowth of epithelial cords into the surrounding vascular connective tissue and an intimate blending of the epithelial and mesodermal elements. The resemblance lies chiefly in the mode of differentiation of the definitive cellular elements from the primary ependymal cells of the developing neural tube. It will be remembered that in the development of the central nervous system the medullary plate at first consists of a single layer of columnar cells, between which on the primarily superficial surface (Fig. 223, A), there are scattered here and there cells which are undergoing mitosis. The medullary or neural plate is soon converted into the neural groove, which afterwards by fusion of its margins along the mid-dorsal line becomes the neural tube. The primarily superficial surface of the plate is now the internal or ventricular surface ; and the primarily deep or under surface is external and in relation with the vascular mesenchyme which will become the pia mater. The wall of the neural tube rapidly increases in thickness and the cell-elements lengthen out into protoplasmic strands containing small, oval, nuclei (Fig. 223, B and C) ; an external and an internal limiting membrane is developed and the mesenchyme lying outside the external limiting membrane condenses to form the pia mater. The radiating protoplasmic strands become joined by the union of lateral processes and thus give rise to a continuous network of fibres enclosing spaces. Imbedded in the radiating strands are deeply stained oval nuclei, which occupy the inner and middle zones ; whereas the large cells showing mitotic figures are found next the internal limiting membrane. There is at first no definite limit demarcating the ependymal zone from the middle or mantle zone. The outer or marginal zone is, however, easily recognized (Fig. 223 B, RZ) by the absence of nuclei and the clear visibility of the glial fibres, which form the bed in which

A — Section through a glial plaque of an adult human pineal gland showing sparsely scattered oval nuclei, imbedded in a reticulum of glial fibres, x 194 D. Gli. : glial tissue.

B — Section through another part of the same gland as A, less highly magnified. The parenchymatous tissue, Par., contains closely packed cells with large, deeply stained, round nuclei. The glial tissue, Gli., on the left is devoid of parenchyma cells.

C — Section of pineal tumour showing lobules of tumour tissue, T. Tis., on the right of the photograph and strands of degenerated glial tissue on the left, Gli.

Fig. 222. — Section through a Perivascular Space in an Adult Human Pineal Organ showing Minute Droplets of a Finely Granular Material probably of a colloid nature and indicating an early stage in the Formation of a Corpus Arenaceum. (R. J. G.)

Cap. : capillary containing red blood corpuscles. Pv. Sp. : perivascular space.

Fig. 223. — Differentiation of Cells in Wall of Neural Tube.

A — Section of medullary plate of rabbit embryo before closure of neural tube. B — 7-mm. pig embryo 690, after Hardesty.

C — 10-mm. human embryo showing ependymal, mantle and reticular zones. D. — Ependymal cells from a part of the wall of the neural-groove of a first-day chick embryo ; Golgi preparation (Cajal).

C.C. : columnar cell or central canal.

D.S. : deep surface

E.C. : primary undifferentiated ependymal cells which will give rise to definitive ependymal cells, supporting glial cells (spongioblasts) and nerve cells (neuroblasts)

E.L.M. : external limiting membrane

E.Z. : ependymal zone.

G.C. : germinal cell

I.L.M. : internal limiting membrane.

M. : mitosis.

M.Z. : middle or mantle zone containing neuroblasts.

N. : nuclei of ependymal cells.

P.M. : pia mater.

R.Z. : reticular or marginal zone.

S.S. : superficial surface later becoming the internal or ventricular surface.

the white medullated fibres are afterwards developed. In a 10-mm. human embryo (Fig. 223, D) the three zones are distinct and consist of : (1) an inner primary ependymal layer, formed of undifferentiated cellelements containing small, darkly stained nuclei, which are arranged in a radial manner round the central canal and are closely packed ; the nuclei are about five or six deep and are almost uniform in size ; (2) the middle or mantle zone — this is the forerunner of the grey matter and contains large, rounded cells with pale vesicular nuclei. These are the neuroblasts and are comparable with the large pale cells with vesicular nuclei of the pineal organ in late foetal life and early infancy, both as regards their origin from the primary ependymal cells of the inner zone and with respect to their further differentiation, namely, the neuroblasts of the central nervous system into nerve cells ; the large pale cells with vesicular nuclei of the pineal organ into parenchyma cells with processes resembling the processes of nerve-cells, but lacking an axis cylinder or myelinated nerve-fibre. A differentiation also takes place of the primary ependymal cells into the definitive ependyma, consisting of cubical or columnar cells lining the ventricular cavities and central canal of the spinal cord, and the spongioblasts which develop into the supporting or neuroglial cells. The primary ependymal cells of the central nervous system thus give rise to three types of cells : (1) the definitive ependyma, (2) the neuroglial cells, and (3) neuroblasts which differentiate into nerve cells. In the pineal organ a similar differentiation takes place of three types of cell from the primary ependymal zone, namely : (1) the definitive ependyma, lining the pineal recess ; (2) spongioblasts giving rise to glial cells ; and (3) large pale cells with vesicular nuclei which give origin to the branched parenchymatous cells of the adult organ.

As a rule the lumen of the epiphysis in mammals and the cylindrical ependymal epithelium which lines it disappear entirely during the later periods of foetal life ; remnants of the lumen may, however, persist in the adult animal, in the form of minute, often microscopic, cysts, of which an example is shown in the section of an epiphysis of an ox (Fig. 304 (Dimitrowa), Chap. 32, p. 452).

We do not propose to do more than allude here to some of the general relations of the pineal organ to neighbouring structures and to note some points of interest with regard to the relative size and form of the pineal organ in some types of the Mammalia, since the variations in microscropical structure can be most conveniently dealt with in the chapter on the structure of the human pineal organ.

Relations of the Pineal Organ in Mammals

The pineal body has been found to be present in nearly all species of mammalia, from the Prototheria or Monotremes, including the duckbill or Ornithorhynchus (Fig. 224) and the spiny ant-eater or Echidna, up to man. Its attachment to the roof of the thalamencephalon between the anterior and the posterior commissures is always the same, but there are differences with regard to its general form. As a rule it is conical, with the rounded base of the cone, corresponding to the root of attachment, lying just above the posterior commissure and the apex directed backwards. In some cases, however, as in the rabbit, it may consist of a terminal pyriform expansion which is attached by a long, narrow stalk to the roof of the third ventricle in the usual situation (Fig. 225) ; while in the rat the terminal vesicle becomes separated by rupture of the stalk, and in the adult, as pointed out by Herring, all communication with the habenular ganglia is cut off and the only connections of the organ with the body generally are by means of the vascular and sympathetic systems.

Fig. 224. — Diagram of a Median Section through the Forepart of a Brain of a fcetal ornithorynchus, showing the plneal body and its relation TO the Habenular and Posterior Commissures. (After G. Elliot Smith.) a.c. : commissura anterior. p.a. : precommissural area.

b.o. : bulbus olfactorius. par. : paraphysis.

ch. 3 : choroid plexus of third ventricle, p.c. : posterior commissure. ep. : epiphysis cerebri. pit. : pituitary gland.

f.M. : Foramen of Monro. r. inf. : recessus infundibuli.

hip. : rudiment of the hippocampus. r.o. : recessus opticus.

hyp. : hypophysis cerebri. s.c. : superior commissure.

/. inf. : lamina infraneuroporica. vel. : velum.

opt. n. : optic nerve.

The pineal organ of the rabbit and of the rat thus differ markedly from those of the sheep, ox, or horse, in which the organ has the usual conical or oval form and is connected by well-defined superior and inferior peduncles with the habenular and posterior commissures (Figs. 226 and 227).

Complete separation of the pineal organ from the central nervous system in the adult animal, such as occurs in the rat, precludes any possibility of any secretory function being under the direct control of the central nervous system ; and if such a secretion exists, it can only be regulated by the blood circulating through its vessels ; thus it may be supposed that the amount of blood passing through the vessels could be influenced through the sympathetic system, and that the secretory activity of the cell-elements might be stimulated by the action of hormones circulating in the blood, or by efferent inpulses reaching the organ by means of sympathetic nerve-fibres terminating in direct relation with the

Fig. 225. — Median Vertical Section through Brain of a Rabbit, Lepus cuniculus, showing plneal organ with long narrow stalk, situated behind the splenium of the corpus callosum. (after parker.)

ca. : anterior commissure. Mm. : cerebellum. c.c. : Corpus callosum. ch. : habenular commissure.

cp. : posterior commissure. m. int. : massa intermedia. olf. I. : olfactory lobe. p.o. : pineal organ.

parenchyma cells. The epiphysis in these rodents is, however, small and imperfectly developed with respect to its microscopical structure ; and it appears to be very doubtful whether true secretory nerve-fibres are actually present in direct association with the cell-elements. The condition of the separated end-organ of the adult rat is very similar to that of Stieda's organ of the frog, namely, a degenerate vestige, which has lost the characteristic features of the parietal eye, and does not show the special characters of an actively functioning endocrine gland. The basal portion of the stalk or epiphysis in the frog, as was described in Chap. 19, p. 228, Figs. 161 and 166, undergoes a certain degree of development and differentiation ; in the rat, however, this segment remains small and insignificant. It may be noted here that most of the experimental work that has been carried out on rats has been by way of the injection of extracts of the epiphysis of the horse, or ox, or grafts of the epiphysis ; Kolmer and Lowy have, however, cauterized the pineal region in young rats, and Lehmann removed the gland in rats and mice without any positive results.

, hob ,, „ R.V.111 ^Hip.Com/

,M.lnb. V X^<>/C.PT.

W$$&fewm. >S> C - c c .

Fig. 226.

-Median Sagittal Section (After

A.C. : anterior commissure.

C.C. : corpus callosum.

Ch. : optic chiasma.

CM. : corpus mammillare.

C. Op. : colliculus opticus.

C.P. : posterior commissure.

C.P.T. : corpus paraterminalis.

F.M. : foramen of Monro.

G. IP. : ganglion interpedunculare.

"Pit

THROUGH THE PlNEAL REGION OF A SHEEP. J. WlLKIE.)

Hab. : habenular ganglion.

Hip. Com. : hippocampal commissure.

Inf. : infundibulum.

L.T. : lamina terminalis.

M. Int. : massa intermedia.

P. : pons.

Pin. : pineal

R.V. III. : roof of third ventricle.

5. Pin. R. : suprapineal recess.

Fig. 227. — Pineal Organ and Habenula of a Sheep's Brain seen from above.

(After J. Wilkie.)

A. Col. : anterior colliculus. Pin. : pineal organ.

CN. iv. : fourth cranial nerve. Pulv. : pulvinar.

Hab. : habenula. Str. Th : stria thalami.

Par. T.B. : paraterminal body. Taen.Th. : taenia thalami. P. Col. : posterior colliculus.

The Pineal Organ (1940): 1 Introduction | 2 Historical Sketch | 3 Types of Vertebrate and Invertebrate Eyes | Eyes of Invertebrates: 4 Coelenterates | 5 Flat worms | 6 Round worms | 7 Rotifers | 8 Molluscoida | 9 Echinoderms | 10 Annulata | 11 Arthropods | 12 Molluscs | 13 Eyes of Types which are intermediate between Vertebrates and Invertebrates | 14 Hemichorda | 15 Urochorda | 16 Cephalochorda | The Pineal System of Vertebrates: 17 Cyclostomes | 18 Fishes | 19 Amphibians | 20 Reptiles | 21 Birds | 22 Mammals | 23 Geological Evidence of Median Eyes in Vertebrates and Invertebrates | 24 Relation of the Median to the Lateral Eyes | The Human Pineal Organ : 25 Development and Histogenesis | 26 Structure of the Adult Organ | 27 Position and Anatomical Relations of the Adult Pineal Organ | 28 Function of the Pineal Body | 29 Pathology of Pineal Tumours | 30 Symptomatology and Diagnosis of Pineal Tumours | 31 Treatment, including the Surgical Approach to the Pineal Organ, and its Removal: Operative Technique | 32 Clinical Cases | 33 General Conclusions | Glossary | Bibliography

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