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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF
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Chapter 18 The Pineal System of the Class Pisces
This class includes the cartilaginous and bony fishes, the Holocephali and Dipnoi. Besides the living representatives of the class, there are many extinct forms (for example, Fig. 140) the consideration of which is of the greatest importance with respect to the history and nature of the pineal system as a whole.
The study of the pineal organs in the various types of living fishes
Fig. 140. — Dorsal View of the Head of Dipnorhynchus, a Primitive Fossil Fish from the Middle Devonian Rocks of New South Wales. (Reduced
b) A median spine subdivides the anterior border of the parietal foramen, there being a crescentic notch on each side of it. This is considered to be an indication of the bilateral origin of the parietal organ.
P.F. : parietal foramen. FR. : frontal.
PAR. : parietal. N. : nasal.
/. PAR. : interparietal. TAB. : tabular.
(After E. S. Hills.)
at first sight appears as if it would be somewhat barren. There is no well differentiated sense-organ such as that in Cyclostomes and certain reptiles ; nor is there any special development of the proximal part, as
in the pineal gland or epiphysis of birds and mammals. The general impression is that of a vestigial structure, any variations of which, such as are present in different types of living fish, being chiefly accountable on
Fig. 141. — The Parietal or Pineal Plate of Lacerta agilis and of Pleuracanthus, a Primitive Type of Fossil Fish belonging to the Carboniferous and Permian era. In Lacerta a Parietal Foramen and Parietal Sense-organ are still present. In the Extinct Pleuracanthus the Parietal Foramen had already disappeared.
Lacerta agilis. (After W. K. Parker.) ex. oc. : exoccipital. ext. nar. : external nares. /. magn. : foramen magnum. jr. : frontal. max. : maxilla. nas. : nasal. oc. cond. : occipital condyle.
par. : parietal. p. mx. : praemaxilla. p.f. : parietal foramen. pp. : pineal plate. pr.fr. : praefrontal. s. orb. : supraorbitals. supr. oc. : supraoccipital.
B : Pleuracanthus. Dermal bones of the head seen from above. (After Davies.) mx. : maxilla. pp. {Jr.): pineal plate, probably repre
nas. : nasal.
orb. c. : orbital cavity.
par. : parietal.
po. : postorbital.
senting fused frontals. sp. oc. : supra-occipital. sq. : squamosal. st. : supratemporal. tb. : tabular.
the hypothesis of there being a greater or less degree of retrogressive change from a more highly evolved system which existed in the extinct ancestors of the living species to the relics of that system which are left in the fishes of to-day.
A comparison of the pineal impressions present in fossil types of fish with the conditions found in living representatives of the class (Fig. 132, Chap. 17, p. 182, Fig. 231, Chap. 23, p. 330, and Fig. 49, Chap. 3, p. 73) reveals different stages in the devolution of a system in which paired and
Fig. 142. — Under Surface of the Head Shield of Cyathaspis (Jaekel), Upper
Silurian. P. : Impression which is believed to have been produced by median eye (epiphysis). Orb. : Orbital notch for lateral eye. Br. : Impressions supposed to be for branchial sacs. Au. : Impressions thought to have been caused by the auditory organs.
presumably sensory end-organs existed outside the skull immediately beneath the skin, to the vestigial condition which is found in many of the living adult forms of fish, in which all traces of the bilateral nature seem to have disappeared and nothing but a simple tubular stalk and the proximal part or base persist. There are, however, a few cases in which a double or partially subdivided parietal foramen has been recorded in living types of fish (Cattie), and indications of the bilateral nature of the pineal diverticulum have been recorded in the embryos of several species of fish belonging to different orders (Figs. 149, 150, 151, 152).
Some interesting observations were made by Locy on the earliest stages of the development of the pineal system in dog-fish embryos. He stated that in the neural plate of an embryo 3 mm. long of Squalus acanthias, before the closure of the plate to form the neural tube, there are three pairs of depressions : namely, an anterior, which gives rise to the optic vesicles of the lateral eyes, and two pairs of accessory depressions (Fig. 143), of which the anterior pair, placed immediately behind the optic grooves, gives rise to the walls of the thalamencephalon and the principal outgrowth from it, the epiphysis. When the neural tube is closed the two members of the pair unite to form the epiphysis which in his opinion is therefore double in origin, being formed from a united pair of accessory optic vesicles. He also considered it highly probable that the enlarged distal end of the epiphysis in Squalus is homologous with the pineal eye of those forms in which it is differentiated as a median eye.
With reference to these observations it would be interesting to obtain further observations on these early stages in other animals. The majority of writers describe the pineal outgrowth as being primarily single and first appearing after the neural tube has become closed, the subdivision of the primarily single diverticulum into parietal sense-organ (when this is developed) and epiphysis taking place later. As far as we are aware, the development of two separate diverticula, arising independently of each other in the median plane, has not been described, although such have sometimes been shown in a diagram, such as Fig. 136, p. 193, and Fig. 138, Chap. 17, p. 196, which is intended to represent in a simple, schematic form the relations which exist either in the embryonic condition or the adult animal in certain types, such as Petromyzon.
Fig. 143. A : Embryo of Squalus acanthias 3 mm. long ; three somites ; showing the first appearance of the optic vesicle, Op.
B : Slightly older embryo showing in addition to the primary pair of optic vesicles two pairs of accessory vesicles, Aop.', Aop."
C : Embryo with open neural groove placed in such a position as to give an external view of the vesicles on one side and an internal view on the opposite side of the neural folds. Locy believed that the accessory vesicles were homologous with the optic vesicles and that the pineal outgrowth was formed in Squalus acanthias by the union of the anterior pair of accessory optic vesicles.
(Redrawn from Locy's figures : Anat. Ariz., 1893, Bd. IX.)
The Pineal Organ of Elasmobranchs
It will be convenient to describe first the development of the parietal region of the brain of a typical cartilaginous fish, e.g. Squalus acanthias, the spiny dog-fish. The different stages of development of this fish were especially investigated by C. Sedgwick Minot in embryos ranging from ii«5 mm. to 86 mm. in length (Figs. 130, A, B, C, Chap. 17, p. 180). All the component parts of the roof of the diencephalon are present in Acanthias, namely the paraphysal arch, including the paraphysis ; the velum transversum ; the post-velar arch or dorsal sac ; the habenular commissure ; the epiphysis, pars intercalaris, and posterior commissure. Associated with the habenular commissure are, on each side, the habenular ganglia, which as compared with the corresponding ganglia in higher types are of relatively large size in Acanthias (Fig. 130, C).
In the early stages, e.g. a 15-mm. embryo, the paraphysal arch (Fig. 130, B) is a simple, rounded elevation projecting from the roof of the diencephalon immediately in front of the pineal diverticulum. In a sagittal section it might readily be mistaken for an anterior pineal diverticulum. Its walls are, however, thin, and at a later stage of development it gives rise to the paraphysis. The latter appears at the summit of the arch in a 34-mm. embryo, and at the 70-mm. stage is a simple but slightly folded sac which projects vertically upward immediately in front of the stalk of the pineal diverticulum. The velum transversum hangs downwards in the ventricular cavity, where it forms a transverse fold the lateral edges of which are continued forward as the choroid plexuses of the ventricle. The dorsal sac is inconspicuous, whereas the epiphysis in the 70-mm. embryo is a long, tubular process reaching from the interval between the habenular and posterior commissures to the level of the cranial vault, which in an 86-mm. specimen shows a definite gap, representing the parietal foramen. The distal end of the tube shows a clubshaped expansion, but there is no special differentiation of this part to form a retina and lens corresponding in structure to that of the parietal sense-organ of the lamprey. The growth in length of the epiphyseal tube is in a downward direction relative to the roof of the skull, for the slightly expanded distal end of the tube retains its primary close relation to the ectoderm, while the proximal part is gradually separated from the superficial structures by an increase in depth of the area between the cranial vault and the base of the pedicle — compare A, B, C, Fig. 130. This extension of the epiphyseal tube is largely due to the increase in size of the midbrain and hemispheres ; though in some types, e.g. Spinax (Fig. 49, Chap. 3, p. 73), there is a displacement forward, relative to the brain, of the parietal pit in the roof of the skull ; and there is a horizontal portion of the tube the lengthening of which is produced as a result of this displacement. In Spinax also there is a thickened and slightly expanded proximal part of the tube which lies between the pineal recess and the upper margin of the habenular ganglion. This corresponds in position to the epiphysis of mammals, but beyond a slight increase in the thickness and folding of its walls, there is no structural change in this segment and in most species the proximal part passes insensibly into
Fig. 144 . — Transverse Planes A, B
A' Sections of the 28-MM. Acanthias Embryo
WHICH ARE INDICATED IN FlG. 130, B, p. 180.
Ep. : epiphysis. /. ch. : lamina choroidea.
FB. : forebrain. MB. : midbrain.
he. : habenular commissure. v. : velum.
the stalk, without any abrupt change to mark the transition of the one into
the other. The relation of the habenular commissure to the habenular
ganglia and the walls of the midbrain is shown in the transverse sections
A and B, Fig. 144. The earliest stages of the development of the pineal
region of Acanthias were studied by Locy (1893-5). He described (see
p. 203) three paired hemispherical depressions arranged serially one behind
the other near the outer edge of the unclosed medullary plate. The first
pair give rise to the optic vesicles. The second and the third pair he
described as " accessory optic vesicles." With regard to the second pair of
primary depressions or anterior pair of " accessory optic vesicles," Locy
states : ' "I have been able to follow the anterior pair step by step
through a graded series of embryos without having once lost trace of them,
and to see that they enter the thalamencephalon, and give rise to the pineal
outgrowth. The posterior pair, which are smaller, are not to be followed
in this definite way ; they become fainter and I believe they fade away."
Later he states that " the bulging of the walls to form the midbrain vesicle
1 W. A. Locy, 1893, Anat. Anz., Bd. IX.
has come on insidiously, and has taken up a position behind the vesicles of the paired eyes, in apparently the same position previously occupied by the accessory vesicles. These transpositions are confusing, as they resemble the accessory optic vesicles grown larger." It will be unnecessary to enter into details of the discussion which followed the publication of Locy's work. This was chiefly concerned with the position of the accessory vesicles relative to the lateral optic vesicles and the supposed homology between the accessory optic vesicles and the median paired eyes of arthropods ; and if Locy's conception that " there are preserved on the cephalic plate of certain Elasmobranch embryos three pairs of optic vesicles " — the lateral and two accessory pairs — were true, a condition would be present in the embryo of these fishes which is well known in certain invertebrates, and it would provide a basis for the supposition that the ancestors of the vertebrates possessed two or more pairs of eyes arranged serially and that one pair, the lateral eyes, have evolved to form the highly differentiated and inverted lateral pair of eyes of living vertebrates ; whereas the other accessory pair (or pairs) have degenerated, or possibly have evolved along a different structural plan and functional direction, and have persisted as the pineal organ. Locy's conception of the ultimate fate of the depressions A. op.' ; A. op.", and their connection with the pineal diverticulum, has however not been generally accepted by later authors.
Structure of the Pineal Organ of Selachia
The walls of the end-vesicle and stalk are throughout their whole extent built up on a foundation of ependymal cells which extend between the inner and outer limiting membranes. The oval nuclei of these cells are situated at varying distances from the internal limiting membrane. The ends of the cells where they are attached to the membrane are often expanded, as is the case in the supporting radial fibres of Miiller in the retina of the lateral eyes of vertebrates. These conical expansions are most noticeable at the outer ends of the cells. At the inner end some of the cells terminate flush with the membrane with which they are blended. Others send small club-shaped or irregularly tapering processes through the membrane, which project into the lumen of the tube. These processes are seen both in the terminal vesicle and in the stalk. No definite ciliated epithelium is, however, present. In many cases the lining membrane is quite smooth, though in others thread-like protoplasmic processes project inwards which end in a nucleated syncytial network, like the " vitreous " of the end-vesicles in Petromyzon. In some cases there is a coagulum resembling an inspissated secretion, such as is frequently seen in the ventricular cavities and central canal of the spinal cord. At later stages in development a certain degree of differentiation takes place, into neuroglia cells and cells which resemble ganglion cells. Neuroglial fibres and nerve-fibres also may be distinguished, the latter being most abundant near the base of the stalk, where they become collected together in bundles. A definite pineal tract is present in the posterior part of the stalk near its base, which according to Studnicka is traceable into the posterior commissure ; other fibres have occasionally been traced into the habenular commissure and ganglia, e.g. in Acipenser. The structure of the proximal part of the pineal organ is essentially the same as that of the stalk. In some specimens, however, e.g. Spinax, the wall may be considerably thickened and thrown into folds. Folding of the wall is also frequent in the end vesicle, but this is not so common as in the bony fishes, in some species of which it is so pronounced that the organ somewhat resembles a racemose tubular gland, more especially as the spaces between the folds are occupied by vascular connective tissue (Fig. 155, p. 220). In the cartilaginous fishes the wall is usually only slightly folded or is smooth. It is covered by a thin sheath of pia mater and accompanied by vessels which form a plexus around the terminal vesicle. Pigment is frequently present in the connective tissue around these blood-vessels, but not in the walls of the vesicle itself.
1 Cartilaginous fishes belonging to the sub-order of Elasmobranchii which includes the Sharks and Dog-fishes.
The General Form and Position of the End-vesicle of Cartilaginous Fishes
The end vesicle of these fishes, as compared with that of Cyclostomes and certain reptiles, is relatively small, and in some cases it is probable that it simply represents the slightly dilated distal extremity or blind end of the stalk — the true sensory vesicle not having been developed, or if developed in early embryonic life having been cut off from the stalk and subsequently degenerated, as appears to be the case in some amphibians. There are, however, certain points with regard to its shape, structure, and situation that seem to favour the view that it does in reality represent the sensory vesicle, in the majority of cases. In no instance, however, does it reach its full development, but remains small and imperfectly differentiated. With reference to its shape, certain cases occur in which the endorgan shows evidence of bilaterality : it may be heart-shaped, having a distinct notch on its anterior border, e.g. Raja clavata and Acanthias vulgaris (Cattie) ; or it may be mallet-shaped, forming with the distal extremity of the stalk a T-shaped junction which projects beyond the hemispheres, e.g. Centrophorus granulosus. Moreover, the stalk is frequently accompanied by two principaf arteries which spring from the right and left sides of the arterial anastomosis which surrounds its base. In many •cases, however, the end vesicle has been described as conical, club-shaped, round, oval, or disc-shaped, and the shape appears to vary within the same species. It is quite likely that some cases in which the end vesicle is in reality heart-shaped have been erroneously described as oval or club-shaped owing to the difficulty of forming a true estimate of its shape when it is seen only in sagittal sections. Unless a reconstruction of the organ is made from a complete series of such sections, an actually heart-shaped organ might easily be regarded as conical. The specimens described by Cattie were drawn from actual dissections of the region, and he gives detailed accounts of their size, relations to membranes, and vascular supply. He also describes in one specimen of Acanthias vulgaris two bilaterally placed parietal foramina separated by a median anteroposterior bridge. Bilateral parietal foramina or impressions are also occasionally found in ancient fossil fishes, e.g. Pholidosteus, Rhinosteus, and Titanichthys (Figs. 225, 226, Chap. 23, p. 325), a circumstance which considerably adds to the significance of their occurrence in living species — which although infrequent should, we think, be regarded as an indication of an inherited trait which is of distinct historical value.
The end vesicle in cartilaginous fishes is often flattened dorsoventrally and in some cases the proximal or under surface is thicker than the upper, as in the Cyclostomes. The reverse condition has, however, sometimes been observed, but the similarity in structure of the end vesicle of the fishes generally to that of Cyclostomes may be taken as an indication that in some cases the proximal segment of the end vesicle in fishes, corresponds to the proximal or retinal segment of the " pineal eyes " of the lamprey and the convex distal segment to the " pellucida " or lens.
Relations of the Pineal Organ to the Roof of the Skull in Selachians
The end vesicle may lie in a foramen in the anterior part of the cartilaginous roof of the skull and be situated in the median plane between the orbits. Although this foramen is often called the parietal foramen, it must be borne in mind that this region in fishes generally corresponds to the frontal or prefrontal region of the cranial vault (Fig. 132, Chap. 17, p. 182, and Fig. 141, B, p. 201). The term " parietal," signifying wall, is, strickly speaking, correct, but the opening is not necessarily in the region of the parietal bone or bones, as in amphibia and reptiles (Fig. 170, Chap. 19, p. 237, and Fig. 171, A, p. 237). The foramen may be closed superficially by a membrane which is continuous at the margins of the opening with the perichondrium and the dura-mater lining the canal. The end vesicle is usually attached to the walls of the canal by a loose connective tissue, containing vessels, as in Acanthias. In other cases the cartilaginous roof is not completely perforated, the canal being closed externally by cartilage and thus ending blindly, as in Spinax (Fig. 49, Chap. 3, p. 73) and the curious spoon-bill or paddle-fish {Polyodon) (Fig. 50, Chap. 3, and Fig. 229, Chap. 23, p. 329). The end vesicle may reach the surface in front of the cartilage at the prefrontal foramen, as in the thornback
Fig. 145. — Sagittal Section through the Skull of Callorhynchus Antarctica, a Primitive Form of Fish allied to Chimaira. (After Parker and Has well.)
The pineal organ has been withdrawn from the surface and lies wholly within the skull, its terminal vesicle lying at the apex of a recess — r.p. ; while the endolymphatic duct of the vestibule, e.l.d., perforates the skull.
Fig. 146. — Lateral View of the Brain of Callorhynchus Antarcticus> showing the Pineal Body and Membranes in Place.
Mm. : cerebellum.
ch. plx. 1 , ch. plx.' 1 : membranous roof
and choroid plexus of the third and
fourth ventricles. c. rest. : corpus restiforme. crb. h. : cerebral hemisphere. dien. : diencephalon. lb. inf. : lobus inferior.
me d. obi. : medulla oblongata.
Nv. olf. I. olf. p. opt. I. pn. b. pn. st. pty. :
optic nerve.
olfactory bulb. olfactory stalk. optic lobe. / pineal body.
pineal stalk, pituitary body.
skate, Raja clavata and the eagle-ray or devil-fish (Myliobatis). In the latter and in Ophidium barbatum (Fig. 153, p. 219) the pedicle of the pineal organ is exceptionally long, the end vesicle reaching to a point considerably in front of the hemispheres of the brain. It is enclosed in a connective tissue capsule which is loosely attached to the membrane which forms the roof of the skull in this situation, there being no groove or pit for the reception of the vesicle.
In the Holocephali, namely in Chimcera monstrosa and in Callorhynchus, there is a conical recess in the cartilaginous roof of the skull (Fig. 145). This is lined by dura mater which is continuous at the margins of the foramen with the perichondrium on the outer surface of the skull. The recess is filled by a tent-like prolongation of the roof of the diencephalon, which represents the dorsal sac of this region, and is called the " membranous pallium " (Fig. 146). The stalk of the pineal organ is attached to the brain in the usual situation between the habenular and the posterior commissures and ascends in front of the midbrain and on the posterior aspect of the dorsal sac to end in a minute vesicle at the apex of the recess in the cranial vault.
Structure of the Pineal Organ of a Fully Developed Sturgeon (Acipenser)
As compared with the simple ependymal structure of the pineal
diverticulum in the early stage of its development (Fig. 147) sections
through the fully developed organ show that a differentiation has taken
place into two principal types of cell, namely, supporting cells which
retain their ependymal character and cells of a sensory type. Two
other types of cells are also found, but are less frequent ; these are
large ganglion cells and neuroglia cells. Nerve-fibres and neuroglia
fibres are also present. The same structure is met with throughout
the whole extent of the organ, including the proximal part, the stalk,
and end vesicle. Fig. 148 represents a part of the walls of the proximal
part of the stalk, stained with iron-hEematoxylin and highly magnified.
The outer ends of the ependymal cells are attached by a slightly expanded
foot to the external limiting membrane ; the inner end of each cell is
slightly notched and lacks any special character (e.g. cilia). The cell
bodies are palely stained and the cytoplasm has a loose reticular structure ;
the nucleus also is feebly stained and has a fine nuclear reticulum. The
cells of the sensory type on the other hand are deeply stained with
hematoxylin, both as regards the denser cytoplasm and the nucleus ;
the outer end of the cell is often prolonged into a process which resembles
a nerve-fibre and runs in a direction parallel to the surface forming a right
angle with the long axis of the body of the cell. The inner ends of these
Fig. 147. — Longitudinal Median Section through the Pineal Diverticulum of an Embryo of a Sturgeon (Acipenser), showing the Continuity and Similarity of Structure of the Wall of the Diverticulum with the ependymal roof of the interbrain, and the position and extent of the Posterior and Habenular Commissures. (After Studnicka.)
c.h. : habenular commissure. Epi.
c.p. : posterior commissure. p.o. .
c.t. : connective tissue. r. pi. epen. : ependyma.
epidermis, pineal organ.
recessus pinealis.
Fig. 148. — Section through the Proximal Part of the Stalk of the Pineal Organ of a Sturgeon (Acipenser sturio), highly magnified. (After Studnicka.)
e.l.m. : external limiting membrane.
ep. c. : clear ependymal cell without cilia.
jr. c. : free cell in lumen of stalk.
g.c. : ganglion cell.
gl. c. : neuroglial cell.
n.j. : nerve-fibres.
p. pr. : protoplasmic process. /. sc. : deeply stained cell resembling the sensory cells of the retina in the lamprey.
cells often protrude into the lumen of the tube between the inner ends of the ependymal cells ; here they form cylindrical or club-shaped swellings, with rounded free extremities and sometimes show a constriction at the level of the free ends of the ependymal cells. They frequently contain fine granules and threads, which are probably mitochondrial in nature. A few large ganglion cells are found, chiefly in the vicinity of the external limiting membrane ; they are feebly stained, the cell-body being clear or finely granular. The nucleus is also palely stained, but shows a welldefined nucleolus. The neuroglial cells are small, mostly spindle-shaped, with tapering processes ; the amount of cytoplasm relative to the size of the nucleus is very small, and the nucleus is deeply stained. Nervefibres are seen singly or in bundles, mostly lying parallel to the surface, and fine neuroglial fibres are also present.
The proximal part of the pineal organ of the sturgeon, as well as the stalk and end-organ, thus appears to have a structure, which is comparable with that of the parietal sense-organ of the Cyclostomes and reptiles rather than with that of a secreting gland.
With regard to the destination of the nerve-fibres ; these were studied by J. B. Johnston in Acipenser. He described a nerve-fibre layer which passes from the dorsal surface of the pineal sac to the roof of the brain, where the fibres passed across from one side to the other, dorsal to the habenular commissure, in what he called the " epiphyseal commissure." He was able to follow the fibres on into the region of the nucleus anterior, and he considered that these fibres at least in part had the value of neurites of the cells of the epiphyseal sac. Other fibres of the " decussatio epiphysis " were seen to end freely between and in contact with the cells forming the wall of the stalk. In the decussatio epiphysis a third type of cell was demonstrated, namely, cells, the fibres of which simply decussated and could be followed into the region of the nucleus anterior in front of the ganglion habenulas.
Other fibres were traced by Johnston from the proximal end of the stalk which turned laterally over the upper side of the epiphyseal sac to the upper border of the decussatio epiphysis ; some of these fibres then crossed forward and passed into the ganglia habenulae, about an equal number being distributed to each side.
This bundle of commissural fibres is probably the same as that described by Eycleshymer and Davis in the epiphyseal region of Amia calva (p. 213, Fig. 149, E).
The pineal organs of some special types of fish may now be alluded to with the view of drawing attention to certain points which are of value in the study of the pineal system as a whole. Among these is the primitive Ganoid fish Amia calva, which is commonly known as the American bow-fin. Its pineal organs were specially investigated by Hill (1894), Eycleshymer and Davis (1897), an d Kingsbury (1897). Hill found that in the early embryonic stages both parietal organs were present, there being
Fig. 149. — Drawings of Sections showing the Early Development of the Epiphyses in Amia. (After Eycleshymer and Davis.)
A — Sagittal section of primary vesicle of a larva 8-9 days old ; 5-6 mm. ; 160. B — Transverse section through the epiphysis of a larva 10 days old ; 7-8 mm.,
showing the bifid cavity. C — Transverse section through a larva at the same stage of development as B,
and Fig. 150, C ; showing the right and left pineal organs, lying side by
side. D — Transverse section through a larva, 14-15 days old; 9-10 mm. at the same
stage as Fig. 150, D, showing the right and left pineal organs. E — Sagittal section of epiphysial region of larva 12-13 mm., showing fibre
tract ft. lying in ventral portion of primary vesicle. F — Section snowing nerve-fibres passing to the posterior epiphysis from the
superior or habenular commissure.
a larger posterior organ — which probably corresponds to the posterior or right pineal organ of Petromyzon — and a smaller organ which lay in front and to the left, which appears to be homologous with the anterior or left parietal organ of Petromyzon or the " parapineal organ " of Studnicka. In 10-mm. embryos the larger posterior pineal organ has developed into an ovoid vesicle with a pointed anterior extremity. Its upper or distal wall is convex and consists of a single layer of cells, while the lower wall has three or four layers of nuclei. The vesicle is joined to the roof of the diencephalon by a tubular stalk, which arises in the usual situation between the habenular and posterior commissures. In older embryos the organ rises perpendicularly from the brain and comes in contact with the skin. The anterior parietal organ is situated close in front of and to the left of the posterior organ. Its distal end is directed forward and encloses a small cavity which communicates with the ventricle. In 15-mm. embryos this communication is found to be closed ; anteriorly it ends in a solid ovoid mass of cells which shows only a trace of a lumen. According to Eycleshymer (Figs. 149 and 150) a lumen appears first in 5 to 6-mm. embryos. He also describes nerve-fibres which, coming from the habenular commissure, penetrate the substance of the anterior organ ;
Fig. 150. from Serial Sections ; seen from Above. (After Eycleshymer and Davis.) Drawings of Models of the Epiphyses of Amia, Reconstructed
A — Larva 6-7 days old ; 4-5 mm. ; 100. B — Larva 9 days old ; 6-7 mm. ; 100. C — Larva 10 days old.; 7-8 mm. ; 100. D — Larva 14-15 days old; 9-10 mm. ; 100.
Ac. pn. : accessory epiphysis, or left pineal organ.
Ca. : caudal end. Pn. : pineal organ (Epiphysis
Ce. : cephalic end. posterior epiphysis).
/. : left side. r. : right side.
st. : stalk.
II or
and, in an earlier stage — 12 to 13 -mm. embryos — similar nerve-fibres which enter the posterior organ (Fig. 149, F). Later the anterior organ was found by Hill to have disappeared ; but Kingsbury has described an adult specimen of Amia calva (Fig. 151) in which the left pineal organ was still present although less developed than the right organ. It consisted of a hollow vesicle which was connected by a thick nerve-cord with the left habenular ganglion. The left habenular ganglion, in correspondence with the small size of the left pineal organ as compared with the right pineal organ, was smaller than the right habenular ganglion (Fig. 151). The two organs were enclosed in a membranous septum formed by two opposed layers of the dorsal sac. This sac is very large in Amia, and is prolonged backwards in the pineal region where in the section shown in the figure, four posterior diverticula have been cut across. From the above descriptions it will be seen that the pineal system of Amia shows definite evidence of bilaterality, and also indications of degenerative or devolutionary changes in both organs, but more especially of the left, which in some cases exists only in the early embryonic stages of development.
Eycleshymer and Davis, commenting on the question of the bilateral origin of the pineal organ, remark that : " The embryologist who contemplates the study of the pineal organ recalls the syllogistic statement : ' The pineal organ is probably a sense-organ. The sense-organs are paired structures ; ergo, the pineal organ should be a paired structure.' " They also quote from an article by Dr. Ayers (J. Morph., 4, 1890, p. 228) : " After a careful study of the Amphioxus eye-spot and related structures, I have become convinced that the animal presents us with the earliest stage in the phylogenetic development of the vertebrate eye." Although this pigment presents a variety of forms the author finds " the most usual form that of a slightly bilobed mass, the lobes being placed to the right and left of the median line, so as to cover the roots of the first pair of cranial nerves. . . . For greater functional power, the central median portion of the pigment spot has grown upwards (dorsal) and carrying with it a portion of the ventricular wall, has produced the median eye. . . . The parietal pineal eye of the Cyclostomata and other vertebrates has been developed from a median portion of the pigmented eye o£ Amphioxus. . . . The rudiments of the eye were developed from (segmental) sense-organs, but the eye itself is never developed from two right and left halves, in so far as the closure of the medullary folds would necessitate this."
Fig. 151. — Transverse Section through the Diencephalon of Amia calva, showing the stalk of the right plneal organ, and the left plneal Organ in a Membranous Septum between Two Diverticula of the Dorsal Sac (After Kingsbury.)
The left pineal organ is connected by nerve-fibres with the habenular commissure ; the left habenular ganglion is smaller than the right habenular ganglion. ch. : chiasma. /. po. : left pineal organ.
ds. : dorsal sac. r. hg. : right habenular ganglion.
/. hg. : left habenular ganglion. st. : stalk of right pineal organ.
We have quoted these extracts in full because they illustrate, besides the important observation with regard to the bilateral disposition of the pigment of the eye-spot in Amphioxus, several errors of interpretation which were common at the time, more especially with regard to the position of Amphioxus in the vertebrate phylum, namely — not as an offshoot from a branch of the main stem, but as a living representative of the parent stock from which the various higher types of vertebrates, including the Cyclostomes, have descended ; secondly, the assumed visual function of the pigment spot of Amphioxus which has been disproved (see pp. 178, 179) ; thirdly, the denial of the possibility of two laterally situated sense-organs, right and left, becoming united in a single organ : this we now know occurs normally in the formation of the triple median eye of arthropods which is produced by the fusion of the right and left members of one of two pairs of ocelli, the other pair coming into contact but not being completely fused ; fourthly, the failure to recognize the degenerate condition of the anterior end of Amphioxus, the whole animal being regarded as having reached only a very simple stage of evolution, whereas it is in many respects highly evolved, and it is probable that the head-region was in the less remote ancestors of Amphioxus much more highly developed than it is in the living representatives of the species. It seems more probable that the pigment-spot of Amphioxus instead of " representing the earliest stage in the phylogenetic development of the vertebrate eye " is merely a vestige of a more highly developed organ, or possibly a pair of light-percipient senseorgans which were present in the head of the remote ancestors of the present stock, before the latter took to burrowing head downwards in the sand at the bottom of the sea.
Polypterus Bichir
Another example of enormous development of the dorsal sac and special modification of the pineal organ is found in the primitive Crossopterygian ganoid fish Polypterus, found in the Congo and Upper Nile. In this fish the dorsal sac arises by a relatively narrow stalk and expands as it extends upwards towards the cranial vault. It thus appears triangular in a sagittal median section, the apex of the V being directed downwards. The posterior wall of the sac is folded by vascular ingrowths, but elsewhere it is smooth and membranous. The pineal organ consists of a hollow, tubular stalk which arises immediately behind the habenular commissure. It extends upwards and backwards, in close relation with the posterior wall of the dorsal sac, to its posterior angle, where it bends sharply forwards and becomes continuous, without any abrupt change in diameter, with an elongated tubular end vesicle. This extends forwards, in a slight groove on the superficial aspect of the dorsal sac, as far as the level of the posterior ends of the hemispheres, where it terminates in a pointed extremity. In the posterior part of this remarkable end vesicle, the wall is slightly folded, while in the anterior part the walls are smooth and the lumen somewhat larger. The wall of the vesicle shows the usual ependymal structure, containing certain cells which resemble sensorycells. The lumen contains a few free cells, but no special syncytial formation. No parietal impression or foramen was found in the roof of the skull.
1 J. Waldschmidt, 1887, "Beitrag zur Anatomie des Zentralnervensystems und des Geruchsorganes von Polypterus bichir," Anat. Anzeiger., Jahrg. 11.
Teleostean Fishes
In adult bony fishes, as a rule, only one pineal organ has been found in the adult animal, although definite indications of two organs have been described by Hill in early embryos of several types of fish belonging to this class, namely : Salmo (Fig. 138, C, Chap. 17, p. 196) ; Catostomus teres (sucking-carp) ; Corregonus albus (Fig. 152) ; Stizostedion (perch) ; and Lepomis. Of the two organs, one, the smaller, is placed in front and to the left ; the other, the larger, is behind and to the right. These observations are of considerable value, as they tend to confirm the view held by Locy with respect to the bilateral origin of the pineal organ, which was based on his observations of paired hemispherical evaginations near the outer edge of the unclosed neural-plate of dog-fish embryos, and also the observations recorded by Eycleshymer and Davis on the early developmental stages of Amia, and that of Kingsbury on an adult example of Amia calva.
The pineal organ of the teleostean fishes is much more variable, both in structure and in form, than it is in the Selachii and ganoids. It may consist of a minute end vesicle at the end of a thin, elongated stalk, as in Ophidium (Fig. 153), or a small solid nodule without a definite stalk. In other cases the end vesicle is expanded and has the shape of a flat, toad-stool fungus ; its structure is also complicated by the intergrowth of vascular folds of the surrounding mesoderm and outgrowing buds or folds of the neuro-epithelial wall. A good example of such is found in the pineal organ of the gar-pike {Belone acus) (Fig. 155). In bony fishes there is, in general, a tendency for the stalk of the pineal organ to be short and the end vesicle large, as compared with the cartilaginous fishes and ganoids, in which the stalk is usually long and the end vesicle small.
152. — The Pineal Organs of Corregonus albus. (After Charles Hill.)
Dorsal view of embryo of Corregonus albus, showing the position of the rudimentary epiphysis to the left of the median plane.
Longitudinal section through the anterior and posterior epiphysis of a 7-mm. embryo of C. albus. C. : Transverse section through the posterior epiphysis of an embryo of C. albus ; the arrow marks the median plane.
Ant. Ep. 1 : Epiphysis I. Ol. PI. : olfactory placode.
Cbl. : cerebellum. Op. V. : optic vesicle.
Ep. 1 : anterior epiphysis. Post. Ep. 2 : Epiphysis II.
Ep. 2 : posterior epiphysis. St. O. : static organ. L. : lens.
Hill considered that the posterior epiphyseal vesicle of teleosts and Amia is homologous with the " epiphysis " of Lacertilia, and that the anterior vesicle is homologous with the parietal eye of Lacertilia.
The general structure of the pineal organ of the bony fishes is very similar to that of the cartilaginous fishes, namely, a neuro-epithelium of ependymal character, containing a few glial cells and cells resembling nerve-cells, enclosed between two limiting membranes (Fig. 154). Cytoplasmic processes frequently project through the internal limiting membrane into the lumen of the organ. The vessels surrounding the pineal organ form a rich anastomotic plexus, but they do not appear to enter the epithelial walls. The end vesicle is never differentiated into an eye-like structure, such as that of Petromyzon, although cells resembling sensory-cells, unipolar nerve-cells, and ganglion-cells ending in nerve fibres have been described ; also in some cases there is a definite nervetract ending in the posterior commissure, and in embryos of Clupea Holt has demonstrated nerve-fibres entering the habenular commissure.
According to Galeotti some of the cells in the pineal organ of the socalled white fish Leuciscus show indications of a secretory function, some of the nuclei containing special fuchsinophile granules and secretory products being found by him in the lumen of the organ. According to Holt, however, the lumen of the organ contains only plasmatic strands connecting the opposite sides of the walls, or a delicate syncytium (Clupea).
Fig. 153. — The Pineal Organ of Ophidium barbatum, allied to the Blind Deep-sea Fish Typhlonus. It is remarkable for the Great Length of the Stalk and Insignificant Size of the End Vesicle. (After Studnicka.)
Hem. : hemisphere. MB. : midbrain. olf. : olfactory nerve. po. : pineal organ. r. sk. : roof of skull. st. : stalk.
Fig. 154. — Longitudinal Section through a Single Tube of the Pineal Organ of a Gar-pike (Belone acus), showing Knob-shaped Projections into the Lumen of the Tube. These resemble the Refractile-knobs which are present in the sensory vesicle of petromyzon, and were considered by studnicka not to be secretory. the cells are ependymal in Origin, and the Large Multipolar Cells beneath the External Limiting Membrane resemble Ganglion Cells. (After Studnicka.)
bl. corp. : blood corpuscles ; bl. ves. : blood vessel ; lum. : lumen.
Relation of the Pineal Organ to the Roof of the Skull in Teleostean Fishes
The tip of the end vesicle of the pineal organ may either simply come in contact with the roof of the skull, or the whole superficial area of an expanded vesicle may be in relation with the roof as in the garpike (Fig. 155). Sometimes the stalk is so short that the end vesicle does not reach the vault. It is only occasionally that the under surface
Fig. 155. — The Parietal Region of the Brain and Pineal Organ of a Garpike (Belone acus). Longitudinal Section. Adult Specimen, Enlarged. (After Reichert.)
c. cr. cartilaginous roof of cranium.
MB. midbrain.
ch. : habenular commissure.
par. paraphysis.
cp. : posterior commissure.
po. : pineal organ.
d.s. : dorsal sac.
r.p. • recessus pinealis
Hem. hemisphere.
r. sk. roof of skull.
Is. : lamina supraneuroporica.
st. : stalk.
of the skull shows any depression for the reception of the end vesicle, or a part of this ; such has, however, been observed by Cattie in the common pike (Esox lucius) and in the Salmon (S. salar) ; also by Rabl Riickhard (S. fario) and Hill {S. purpuratus). Cases in which a parietal foramen closed superficially by fibrous membrane, is present either in the cartilaginous or bony skull are exceptional. Handrick has, however, published figures of such in Argyropelecus hemigymnus, a species of fish allied to the salmonoids and the herrings ; and Klinckostroem has shown a true parietal foramen in the bony vault of the skull in Callichthys asper, belonging to the cat-fish family. It is noteworthy, however, that in this case, although a parietal foramen is present, the pineal organ is not well developed.
The Pineal Region of the Dipnoi
The primitive nature of this sub-class, the three genera of which are collectively known as lung-fishes, might lead one to expect a specially instructive condition of the pineal system. This anticipation is, however, hardly realized, for although from many standpoints these fishes — Ceratodus, Protopterus, and Lepidosiren — are of prime importance in the study of comparative anatomy, the parts of the brain included in the pineal system do not show a high grade of organization. No pineal eye is present. The pineal organ shows signs of degeneration and all trace of a parietal foramen has disappeared. Incidentally, it may be mentioned that the lateral eyes of Lepidosiren are relatively small and show a peculiarity in development which was described by Graham Kerr (1919), namely, instead of the optic vesicle being formed as a hollow outgrowth it is developed as a solid bud from the forebrain, which at the time when the optic rudiment first appears is itself solid ; later a cavity appears as a secondary formation in the primarily solid optic outgrowth, and the subsequent development of the eye follows the usual course. Another peculiarity which the lateral eyes of the Dipnoi share with Elasmobranchs and Cyclostomes is the absence of a processus falciformis. The origin of the lateral eyes as solid buds is probably a devolutionary rather than an evolutionary change. Moreover, with regard to the structure of the end vesicle of the pineal organ of Polypterus, Studnicka found the walls markedly folded, the lumen thus being almost obliterated. He also found that some of the epithelial cells contained a brown pigment, both of which conditions indicate a degenerate state of an organ previously more highly developed. It would seem, therefore, that we are not only dealing with a class of animals which is primitive in the evolutionary sense, the Dipnoi having persisted up to the present age from the early Mesozoic period without having undergone much further developmental organization — that is to say, have remained in a more or less stationary condition — but that during or even before this period retrogressive changes have taken place both in the development of the lateral eyes and in the pineal organ.
Ceratodus Fosteri
Huxley (1876) described the pineal organ as consisting of a cylindrical stalk ending anteriorly in a heart-shaped expansion, the latter lying in a depression in the cartilaginous roof of the skull. He also gave an admirable illustration of the bones on the dorsal surface of this remarkable skull (Fig. 156), which closely resembles Krefft's figure. In neither of these is there any indication of a parietal foramen on the superficial aspect of the skull. A more detailed description of the pineal region of Ceratodus is given by Studnicka (1905), more especially with regard to the relation of the pineal organ to the posterior wall of the dorsal sac, and of the end vesicle which extends forward to and overlaps the paraphysis as in Protopterus.
Fig. 156. — Dorsal View of Skull of Ceratodus Fosteri. (After Huxley.) Investing bones, lightly stippled. Cartilage, dark stipple.
am. : anterior median bone.
art. : articular surface for 2nd fin ray.
cr. : cartilaginous cranium.
i.l. : inner lateral bone.
lab. : labial cartilages.
lam. : process covering lungs.
ol. : outer lateral bone.
op. : operculum.
p.m. : posterior median bone.
pr. orb. : preorbital.
sb. orb. : suborbital.
Fig. 157. — Sagittal Sections through Brain of Lepidosiren. Left : Early Stage. Right : Detail of Later Stage of Development. (After J. Graham Kerr.)
ax. : anterior commissure. par. : paraphysis.
ch. pi. : choroid plexus of lateral p.c. : posterior commissure.
ventricle. pin. : pineal diverticulum.
h.c. : habenular commissure. op. ch. : optic chiasma.
h.g. : habenular ganglion. op. I. : optic lobe. n. ch. : notochord.
Lepidosiren
The development of the South American lung- or mud-fish was specially studied by Graham Kerr (1919). The pineal organ arises in the usual situation between the habenular and posterior commissures and extends forwards on the roof of the diencephalon as a tubular, " carrotshaped " diverticulum, which reaches a point just behind the hemispheres and well above the paraphysis (Fig. 157). On either side of it are the welldeveloped habenular ganglia, which as they enlarge become connected by a transverse bridge of nerve fibres, the habenular commissure. The roof of the mesencephalon, which lies close behind the pineal organ, becomes slightly thickened on each side of the mesial plane, forming the arched tectum opticum, but " correlated with the small size of the eyes in Lepidosiren the thickening never becomes so great as to produce projecting optic lobes such as are formed in most vertebrates."
Protopterus
The pineal organ of the South African lung-fish was described by Burckhardt in 1892 and another specimen was described by Studnicka in 1905. In both these specimens the organ was divisible into three parts, a club-shaped end vesicle which was subdivided into several compartments by folds of its wall ; an elongated stalk which in Studnicka's case was hollow throughout, whereas in Burckhardt's specimen the central part was solid, only the third or proximal part having a lumen. The lumen, however, was closed at its base, there being no communication with the ventricle. This proximal part probably corresponds to the " pineal gland " or epiphysis of mammals ; the stalk and end vesicle may fail to develop as in the higher vertebrates, or the latter may be cut off by the growth of the skull from the proximal part, the end vesicle persisting as Stieda's organ, e.g. in the frog, while the proximal part differentiates as the epiphysis.
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