Book - The Pineal Organ (1940) 21
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Chapter 21 The Pineal System of Birds
In birds there is no parietal eye and no parietal foramen. Also there is usually no indication of a parietal spot. The pineal organ, moreover, is only represented by the proximal part, or epiphysis. The epiphysis, however, in many forms undergoes an important and characteristic transformation, which although foreshadowed in lower classes of vertebrates is not definitely evolved. This change consists in the outgrowth from the primary embryonic diverticulum of hollow epithelial buds the cavities of which are at first continuous with the lumen of the primary diverticulum (Fig. 196). At a later stage of development these often become cut off from the main stem and form independent vesicles lined by columnar epithelium. The vesicles or follicles are at first separated by a considerable amount of interfollicular connective tissue containing blood-vessels, and on section the organ has a glandular appearance resembling in some respects the thyroid gland. The epithelium lining the follicles in the pineal organ of birds is, however, ependymal in type and composed of tall, columnar cells as contrasted with the low, cubical cells of the thyroid gland ; and the content of the follicles of the pineal organ of adult animals seems to correspond to that of the ventricles of the brain, with a small amount of cell detritus, there being no colloid material. The whole organ is at first pear-shaped and the hollow epithelial buds at first communicate with the central lumen or one of its branches. Later, however, when the follicles are separated, the stalk becomes elongated and eventually its lumen disappears. The stalk may persist as a fibrovascular cord connecting the main organ with the roof of the brain or it may become ruptured and the organ freed from its primary connection with the diencephalon. Thus all escape of secretion into the cavity of the third ventricle is prevented. Up to this period the epithelium lining the follicles is in a sound, healthy condition, the cell outlines being well defined and the alignment of the cells being quite even (Fig. 197). Later, however, degenerative changes take place in the cytoplasm and nuclei, and cell detritus accumulates in the cavities of the follicles (Fig. 198). The follicles also diminish in size and come nearer together, the whole organ forming a dense mass of closely packed small vesicles in many of which the lumen has become greatly reduced in size, or has completely disappeared.
Fig. 196. — Upper : Medial longitudinal section of a chick embryo (8 days), showing the main pineal diverticulum and secondary diverticula, which have arisen as hollow outgrowths of the former ; also the relation of the superior and posterior commissures to the stalk of the main diverticulum, and the dorsal sac. Lozver : Oblique coronal section of pineal organ of a chick embryo, of the same age as A, showing the primary and secondary diverticula and some fibres of the posterior commissure. Ch.P. V .III. : choroid plexus of third P. Co. : posterior commissure.
ventricle. P.S. : pineal sac.
Ch.P. LV. : choroid plexus of lateral 5. C. : superior commissure.
ventricle. D.S. : dorsal sac. S.D. : secondary diverticula.
I.P.R. : infrapineal recess.
Fig. 197. — Transverse Section of the Pineal Organ of a Chick Embryo, incubated 8 days, showing hollow follicles, fol. ; lined by ependyma,
ep. ; AND THE SURROUNDING MESENCHYME, mes. (R. J. G.)
Fig. 198. — Section of Epiphysis of Adult Meleagris, showing Follicles lined by ependymal cells, and containing a coagulated material.
c. : coagulum.
ct. : interfollicular connective tissue. ep. : ependymal cells. /. : lumen of follicle. (After Mihalkowics — highly magnified.)
No nerve-fibres connecting the pineal organ with the brain are present either within or near the stalk ; the nerve-fibres of the habenular and the posterior commissures are, therefore, derived from sources entirely outside the pineal system, and in those cases in which the pedicle has been ruptured it is obvious that all nervous connections with the brain by means of it, should they have existed in embryonic life, will have disappeared in the adult.
The pineal organ of birds, however, does not always conform to the type described above ; it is often a simple elongated tube, arising in the usual situation between the habenular and posterior commissures and extending upwards towards the vault of the skull, beneath which it ends in a slightly lobulated expansion (Fig. 199). This is, however, not regarded as an end vesicle. The tube is hollow and communicates in young specimens with the cavity of the third ventricle. It occupies the narrow interval between the cerebellum, which lies behind, and the two hemispheres, which are in front and lateral. Owing to the large size of the hemispheres and cerebellum in birds, the tube is of considerable length. It is invested by a fibrovascular sheath continuous with the pia mater at its attachment to the brain.
Fig. 199. — Parietal Region and Cerebellum of a Sparrow. (After Gaze.) Cbl. : cerebellum. Ep. : epiphysis.
Cp. : posterior commissure. Hem. : hemisphere.
The Development of the Pineal System of Birds
This has been studied chiefly in the chick, and since the early stages of development in the chick do not differ essentially from those of other types of bird, we shall confine our description to that of the chick, and only refer to special points in the development of the system in other birds when these have some bearing on the general problem we are considering.
The first indication of the development of the pineal organ in the chick is a thickening of the epithelium forming the roof of the third ventricle, which soon grows forward in the form of a hollow diverticulum. It rises a short distance in front of the posterior commissure and is approximately mesial in position. Rarely two epiphysial outgrowths have been observed. Thus in 1897 Saint Remy found on each side of the neural tube, before this had become closed, a small evagination of the epithelium in the pineal region. Hill in 1900 observed two small diverticula close to each other in the same region of a chick embryo in which the brain was already closed. Cameron in 1903 saw similar outgrowths in chick embryos, which he considered to be an indication of the primary bilateral origin of the pineal organ. It is possible that some of these cases may be explained on the assumption that they are simply due to an abnormal development, but their occurrence in birds, considered along with the evidence of similar indications of the bilateral development of the pineal organs in other classes of vertebrates, does lead one to think that the pineal, like other sense-organs, is a bilateral structure. Thus important evidence of its bilateral origin has been found in cyclostomes, Dendy ; in Selachian and Teleostean fishes (Locy, Hill) ; in Amia (Kingsbury) ; besides important embryological and geological evidence in Amphibia and reptiles.
The pineal diverticulum at the fourth day in the chick embryo is a conspicuous hollow, club-shaped organ (Fig. 200). Its wall consists of an, as yet, undifferentiated ependymal layer, which is continuous at its base with the ependyma forming the roof of the brain. Capillary vessels lie in the receding angle between it and the roof of the brain. The superior commissure has not yet appeared. The posterior commissure, on the other hand, is well-developed and forms a broad band some distance behind the pineal outgrowth. A low post- velar arch and velum are already recognisable. There is no evidence of any connection with the cutaneous ectoderm, a fairly thick stratum of loose mesenchyme lying between the diverticulum and the epidermis. Its distal end already shows a slight indication or lobulation, which increases as age advances and is most marked on its anterior surface and sides.
In an 8-day chick embryo, Fig. 196, A and B, the pineal organ, which is still directed forward over the superficial aspect of the dorsal sac, is seen to have budded out a number of hollow vesicles, lined by ependyma (Fig. 197), and separated from each other by a loose mesenchyme. The cavities of the vesicles are at first continuous with the central lumen or with branches which are given off from this, but already some are constricted off and form closed, independent vesicles which are surrounded on all sides by mesoderm. The mesoderm at this stage has condensed on the surface of the organ into a thin, fibrous capsule outside which are a number of dilated capillary vessels and small venous sinuses. A secondary stalk has developed at its base and its lumen has already become constricted, but still remains in communication with the third ventricle. At the eighth day the superior commissure has made its appearance and is connected on each side with the habenular ganglion. The posterior commissure has now become folded and forms a rounded cord immediately behind the pineal recess.
The development of the superior commissure in birds was specially studied by Cameron in 1904. Owing to its late appearance, the commissure was thought by authors writing previous to this date to have been absent in Aves. Cameron, however, demonstrated its presence in a 9-day chick embryo. He states that " the nerve-fibres are found to emerge from the ganglion habenulee — from the cells of which they take origin. They pass backwards for a considerable distance (histologically speaking) in the lateral wall of the thalamencephalon, and then cross the mesial plane immediately in front of the epiphysial opening, and pass to the ganglion habenulae of the opposite side, in which they appear to end. Therefore, although both the ganglia and nerve-fibres appear early (by the 5th day), it takes the latter some time to grow backwards in order to cross the median plane. It is thus obvious that they will appear last of all at the latter position. This explains why it is not until the 8th or 9th day that they are seen in vertical mesial sections of the same region."
Fig. 200. — Median Longitudinal Section of the Pineal Diverticulum of a Chick Embryo (fourth day). The Apex is directed forward. (R. J. G.)
P. Co. : posterior commissure. P.V.A. : post-velar arch.
P.D. : pineal diverticulum. V.F. : velar fold.
P.I.P. : posterior intercalary plate. V . III. : third ventricle.
Cameron was unable to find distinct evidence of fibres passing from the superior commissure to the epiphysis in chick embryos, and concluded that if they do exist they must be very scanty, but in the blenny, Zoarces vivipara, he described and figured a bundle of nerve-fibres passing from the upper part of the superior commissure some of which ended in the epiphysial segment of the same side, while others, decussating, crossed to the epiphysial segment of the opposite side (Fig. 201). He also published photographs of sagittal and horizontal sections through the superior commissure in the adult human subject, prepared by the Weigert-Pal method, and showing the same division of the habenular commissure into an upper and lower segment, the former sending decussating fibres into the pineal body, the lower and posterior segment being composed of commissural fibres between the two habenular ganglia and of fibres joining the striae medullares.
Fig. 201. — Transverse Section through the Ganglia Habenulai and Epiphyseal Elements of an Embryo Blenny (Zoarces vivipara) showing Decussation of the Upper Fibres of the Superior Commissure. (After Cameron, 1904.) C. Hab. : commissura habenularis. G. Hb. D. : right ganglion habenuke. Ep. D. : right epiphyseal element. G. Hb. S. : left ganglion habenulae.
Ep. S. : left epiphyseal element.
Structure and Contents of the Adult Pineal Organ of Birds
Three different types of pineal organ were described by Studnicka :
1 . Simple tubular.
3. Solid lobular.
1 . The simple tubular type is further subdivided into :
(a) Epiphyses with thin walls.
(b) Epiphyses with thick walls.
The first (Fig. 199) is exemplified by the pineal organ of the sparrow, which has an elongated tubular epiphysis ending in a small lobulated vesicle. It lies immediately in front of the cerebellum, between the hemispheres ; and in young birds the cavity, which is lined by columnar ependymal cells, communicates with the third ventricle. No nervefibres are present either in the stalk or external to it.
The second type of tubular epiphysis with thick walls (Fig. 202) occurs in the grosbeak, which is allied to the finches. The organ is a thick- walled tube ending blindly beneath the roof of the skull. It is somewhat lobulated, and in the adult animal is frequently separated from its original connection with the roof of the brain, its narrow stalk having been ruptured. Its lumen is lined by ependymal epithelium and sends tubular diverticula into the substance of the wail, but there is no true formation of follicles.
Fig. 202. — Sagittal Section through the Cerebellum and Epiphysis of a
Grosbeak, Coccothraustis vulgaris. (After Reichert.)
Cbl. : cerebellum. Ep. : epiphysis.
2. The follicular type of epiphysis is the most common. It consists of a mass of completely separated hollow follicles and short tubes which have originated as outgrowths from the original embryonic diverticulum from which they have subsequently been cut off as independent vesicles. Even in the adult some follicles may still retain their primary connection with the central lumen or one of its branches. A vascular connective tissue is interposed between the follicles, which is continuous externally with a thin fibrous capsule. There is a rich vascular supply, but, as in the other types, no nerves connecting the epiphysis with the central nervous system are present.
3. The solid lobular type (Fig. 203). The organ consists of a conglomeration of solid lobules held together by thin fibrous septa. The cavities of the lobules are either partially or completely obliterated, and it is possible that in some cases the lobules may have arisen as solid outgrowths. In the adult the main body of the organ is often separated from the roof of the brain by rupture of the thinned-out stalk. It lies in the narrow triangular space between the cerebellum and the two hemispheres. This relation in the adult bird contrasts markedly with the embryonic condition in which there is a wide interval between the cerebellum and the pineal outgrowth, which space is occupied by the prominent optic lobes. The space is reduced in the later stages of embryonic development by the straightening out of the cephalic flexure of the brain and the relatively greater growth of the hemispheres and cerebellum as compared with the optic lobes.
Fig. 203. — The Parietal Region of the Thalamencephalon and the Epiphysis of an Adult Fowl (Gallus domesticus), showing the Almost Solid Condition of the Follicles and Ingrowth of Connective Tissue. (After Studnicka, 1905.)
Cbl. : cerebellum. Ep. : epiphysis.
C. hab. : habenular commissure. Hem. : hemisphere.
C. post. : posterior commissure. St. : stalk.
Ds. : dorsal sac.
The shape of the epiphysis in the adult is usually either spindleshaped or clavate, and it is directed almost vertically upward towards the roof of the skull. The solid type of epiphysis may be seen in the adult fowl, but different grades exist between the embryonic follicular form (Figs. 196, 197) and the almost solid form sometimes seen in adult examples.
Between these three principal types of epiphysis there are all degrees of intermediate forms, and different parts of the organ in a single individual may conform to the character of any of the three types. In some examples there are evident signs of degeneration, such as the disappearance of the external limiting membrane and mingling of the outer ends of the ependymal cells with the fibres of the surrounding vascular connective tissue, as is shown in Fig. 204, of a part of the wall of one of the follicles in the epiphysis of an owl. This observation, which we owe to Studnicka, is of great importance in the comparative study of the structure of the pineal body in man and mammals, as it represents a stage in the late foetal development which is paralleled in the development of the mammalian organ, and explains certain difficult points in the interpretation of the appearances seen in the late foetal and post-natal stages of its growth in the human subject.
Fig. 204. — Section through a Part of the Wall of the Epiphysis of an Owl, Strix flammea, showing Absence of an External limiting Membrane, and Apparent Continuity of the Tapering Processes of the Outer Ends of the Ependymal Cells, with the Fibres of the Supporting Tissue. (After Studnicka.)
bv. : blood-vessel. ct. : connective tissue.
ep. : ependyma.
The inner ends of the ependymal cells in this specimen, Fig. 204, show the same rounded projections of protoplasmic material and threadlike processes that are seen in the epiphyses of some fishes, Figs. 148, 154 ; amphibia, p. 230 ; and reptiles, Fig. 178, p. 249 ; and there is the same difference of opinion as to whether these projections are evidence of a definite secretion or are simply due to degenerative changes in the cytoplasm and extrusion of effete material from the cell into the lumen of the follicle. The contents of the follicles are very similar to those found in the cavity of the epiphysis in fishes and reptiles, namely, a few detached groups of cells either fused in a syncytium or separate, and thin coagula containing cell-detritus such as are sometimes seen in the ventricles of the brain.
The structure of the adult epiphysis of birds, in spite of the resemblance which the developing organ has to a compound racemose gland with a secretory duct opening into the ventricular system, does not bear out the expectation which might be anticipated from its embryonic history. Degenerative changes seem to occur soon after the obliteration of the lumen of the duct, and it thus neither resembles an ordinary racemose gland nor does it truly resemble a ductless gland such as the thyroid, which gland continues to develop and shows signs of secretory activity after the follicles have been cut off from the parent stem.
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