Cell Tiss. Res. 152, 13--30 (1974) 9 by Springer-Verlag 1974

The Uhrastructure of the Human Fetal Pineal Gland I. Cell T y p e s a n d Blood Vessels Morten Moiler * Laboratory of Electron Microscopy, Anatomy Department A, University of Copenhagen, Denmark Received April 19, 1974 Summary. The ultrastructure of the pineal gland of 18 human fetuses (crown-rump lengths 30-178 mm) was investigated. The pineal gland exhibits a pyramidal shape and consists of an anterior and posterior lobe. Only one parenehymal cell type, the pinealocyte, was observed. Few neuroblasts were seen between the pinealoeytes and in the extended perivascular space. The pinealocytes possess all the organelles necessary for hormone synthesis. No specific secretory granule could be observed. The organ is abundantly vascularized and richly innervated. The morphology of the capillaries indicates the existence of a blood-brain barrier. The ultrastructure of the human fetal pineal gland suggests that the gland has a secretory function in early intrauterine life. Key words: Human fetal pineal gland - - Development - - Secretion - - Electron microscopy.

Introduction

Evidence accumulated during the past decade seems to assign secretory function to the mammalian pineal gland. Isolation of the hormone-like indole, melatonin (Lerner, 1959), and the accumulating evidence of an antagonistic function of the pineal gland in relation to the anterior pituitary has given rise to m a n y ultrastructural studies of the m a m m a l i a n pineal gland in various species, e.g. cattle and sheep (Anderson, 1965), dog (Sane and Mashimo, 1966), rat (Arstila, 1967), cat and m o n k e y (Wartenberg, 1967), and rabbit {gomijn, 1972). Only one ultrastructural s t u d y of the h u m a n fetal pineal gland has appeared (Hiilsemann, 1971) which mainly describes the development of its innervation. Previous histochemical and ultrastructural studies (Mollggrd, 1972; Mollgs et al., 1973) have shown the h u m a n fetal subcommissural organ to be highly active, and in a recent publication (Mollgs and Moller, 1973), we have demonstrated the presence of a nervous connection between the pineal gland and the subcommissural organ in h u m a n fetuses. The present s t u d y will give a description of the ultrastructure of the h u m a n fetal pineal gland at different ages. Special emphasis is laid on the different cell types and on the blood vessels in the gland. Subsequent publications will deal with the specific cellular junctions and the innervation of the gland. * Acknowledgements. The author is grateful to Mrs. Yael Balslev and Miss Inger ,~Egidius for their able technical assistance. This investigation was supported in part by The Carl and Ellen Hertz's foundation and the Johann and Hanne Weimann foundation.

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Material and Methods The material examined was obtained from legal abortions and consisted of pineal glands from 18 human fetuses of either sex. The crown-rump lengths (CRL) in mm of the unfixed material were: 30, 49, 54, 56, 71, 75, 77, 82, 83, 90, 94, 102, 115, 116, 119, 139, 152, 178. Fixation: The postmortem interval preceding fixation was less than i0 min. The whole brain was removed and immediately immersed in ice-cold 2.5% glutaraldehyde in 0.1 M Nacacodylate buffer (pH 7.4). Two extensive sagittal cuts were made in each hemisphere to ensure good penetration of the fixative into the ventricles (fixation time: 6 hours). After fixation the pineal gland was dissected out under a dissecting microscope. This was followed by washing in 0.1 M Na-cacodylate buffer for 12 hours. The specimens were then osmicated in 2% OsO4 in 0.1 M Na-eacodylate buffer for 2 hours, block-stained in 0.5% aqueous uranyl acetate for 1 hour, quickly dehydrated in increasing concentrations of ethanol and embedded in Epon. 1-micron thick survey sections were stained with toluidine blue. Silver to gray thin sections were cut on LKB ultratome. The thin sections were post-stained on grids with lead citrate and uranyl acetate. The sections were viewed and photographed with a Hitachi HS 8 electron microscope operated at 50 kV.

Results No difference b e t w e e n male a n d female pineal glands was o b s e r v e d in this study.

Light Microscopy The h u m a n fetal pineal gland exhibits a p y r a m i d a l shape (Fig. 1). The a p e x is d i r e c t e d supero-caudally, t h e basis infero-rostrally.The g l a n d consists of a posterior lobe m a d e u p of a few cell layers surrounding the pineal recess, a n d a bigger a n t e r i o r lobe. A l a y e r of connective tissue s e p a r a t e s the a n t e r i o r a n d posterior lobe, e x c e p t for the more inferior region a d j a c e n t to t h e ventricles, where there is c o n t i n u i t y between t h e two lobes. The connective tissue a r o u n d the g l a n d is loosely condensed w i t h o u t forming a true capsule. A n a b u n d a n t v a s c u l a r i s a t i o n a n d m a n y nerve fibers are seen in t h e p a r e n c h y m a . Morphological changes a t t h e light microscopical level during d e v e l o p m e n t h a v e been e x t e n s i v e l y described elsewhere ( K r a b b e , 1915; Turkewitsch, 1933).

Ultrastructure Only one p a r e n c h y m a l cell t y p e , the pinealocyte, could be o b s e r v e d in the whole series. I m m a t u r e nerve cells were s c a t t e r e d between t h e pinealocytes. The u l t r a s t r u c t u r e of t h e p i n e a l o c y t e d i d n o t change v e r y m u c h during t h e age i n t e r v a l studied. All t h e organelles observed in the pinealocytes of the older fetuses were also seen in the younger, b u t t h e y increased in n u m b e r during growth.

The Pinealocytes The pinealocytes differ to some degree in size, nuclear shape a n d electron d e n s i t y (Figs. 2, 3, 8, 21, 22). T h e y consist of a r o u n d or e l o n g a t e d cell body, 8-12 microns in d i a m e t e r , from which several processes emerge. The processes often t e r m i n a t e in c l u b - s h a p e d endings (Figs. 3, 9). The nuclei arc often l o c a t e d eccentrically in the elongated cell bodies. Therefore, t h e opposite end will be called t h e anuclear end. Nucleus: The nucleus is p r e d o m i n a n t l y ovoid in shape (Figs. 2, 8). F e w are t r i a n g u l a r , a n d a v e r y few h a v e an irregular, n e a r l y S - s h a p e d a p p e a r a n c e . I n t h i n

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Fig. 1. Sagittal section through the pineal gland of a h u m a n fetus, CRL ~ 100 mm. Note the separation into a n anterior lobe (AL) a n d a posterior lobe (PL). H habenular area, SCO subcommissural organ, rp pineal recess. Toluidine blue, • 85 Fig. 2. Survey electron micrograph from the anterior lobe of the pineal gland. Several pinealocytes are seen. Note the difference in nuclear shape. A small extracellular space (arrowheads) is seen. • 3 000

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Fig. 3. Electron micrograph showing a slender pinealocyte process (P) between three pinealocytes. Three end-bulbs (t) of pinealocyte processes are also seen. Note their many filaments. N nuclei in pinealocytes. • 30000

sections most of the nuclei have one or two nucleoli (Fig. 4) made u p of spongelike dark masses a n d electron lucent areas in between. Condensed c h r o m a t i n is often seen to i n v a d e the nucleolus. C h r o m a t i n granules, evenly dispersed in the nueleoplasm give the nucleus a homogeneous electron lucent appearance. There is a slight c o n d e n s a t i o n of the c h r o m a t i n m a t e r i a l towards the nuclear envelope (Fig. 3). The nuclear envelope consists of a n i n n e r a n d outer m e m b r a n e . The outer m e m b r a n e often bulges outwards, so the space between m e m b r a n e s varies from 200-550 A. Nuclear pores have a d i a m e t e r of between 500-800 A, a n d a slight condensation of the cytoplasm a n d nueleoplasm on b o t h sides of the pore is seen (Fig. 5). Few ribosomes are associated with the outer nuclear m e m b r a n e

Fig. 4. Electron micrograph showing the nucleolus in a pinealocyte. A fragment of the perimembranous chromatin condensation (C) is seen invading the nucleolus (Nu). Arrow = nuclear membrane; arrowhead ~ nuclear pore. x36000 Fig. 5. Ribosomes (arrows) are attached to the nuclear membrane of a pinealocyte, x48000 Fig. 6. Typical elongated mitochondrium (M) in a pinealocyte. Arrowheads ~ mitochondrial cristae. • 57000 Fig. 7. Golgi apparatus (G) in a pinealocyte. Note that both coated vesicles (large arrowheads) and smooth vesicles (small arrowheads) bud off from the Golgi apparatus. Arrow = coated vesicles free in the cytoplasm. • 30000

U l t r a s t r u c t u r e of t h e H u m a n F e t a l P i n e a l Gland. I

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(Fig. 5). No connection between the nuclear envelope and the endoplasmic reticulum or any other cytoplasmic organelle was observed. The nucleus is normally located eccentrically in the cell. The anuclcar part of the cell contains the majority of the organelles. Mitochondria: The mitochondria are located randomly in the cytoplasm, with the majority in the anuclear end (Fig. 8). The shape of the mitochondria varies. Most of them are elongated (Fig. 6), their size varying between 0.2-1.0 micron. Some are big, about 3 microns, and may have a kidney-shaped appearance. The inner membrane is folded and projects as cristae into the mitochondrial matrix, sometimes completely crossing from one side to the other. The electron density of the mitochondrial matrix is the same as that of the cytoplasm. Few electron dense granules are present in the mitochondrial matrix. In some cases a group of big mitochondria is located close to the Golgi apparatus. Endoplasmic Reticulum and Ribosomes: The moderate amount of granular endoplasmic reticulum is randomly distributed in the cytoplasm (Fig. 8). I t consists of elongated sacs of membranes with ribosomes attached to the outside (Figs. 10, 23). The longest diameter of the sacs varies between 0.2-3.0 microns, the shortest between 600-100/~. The ribosomes on the outside are located in groups of 2-5, with areas free of ribosomes in between. The interior of the sacs contains a matrix of the same electron density as the cytoplasm. The granular endoplasmic reticulum is never seen to be connected with the perinuclear space, and has no association with other cell organelles. Profiles of smooth endoplasmic reticulum are rare, but are observed in the terminal clubs of the pinealocytic processes. Many free ribosomes (140-150 A in diameter) are scattered throughout the cytoplasm (Figs. 8, 10, 21), and often form groups of polyribosomes up to 15 in each group. Golgi Complexes: When serial sections are made through the pinealocytes a Golgi complex is seen located in the anuclear end of the cell. The Golgi complex consists of a stack of 2-5 parallel membrane-bound sacs and has the classical concave-convex appearance (Fig. 7). Both ends of the sacs have dilatations and several both smooth and coated vesicles bud off from these dilatations. The Golgi complex has no relationship to other organelles. Granular Bodies: Granular bodies are located both in the cell body and the processes of the pinealocytes. The bodies are most often spherical or slightly elongated with a diameter from 1000-6000 A (Fig. 11). Few are very big, about 1 micron in length. They are sometimes nearly rod-shaped and the medial part of

Fig. 8. Electron micrograph of a pinealocyte. Located in the cytoplasm are mitochondria (m), glycogen (g), and granular endoplasmic reticulum (arrows). N nucleus. • 19000 Fig. 9. A pinealocyte process and a pinealocyte end-bulb are connected to each other by an "intermediate type" junction (large arrow). Many transversely-cut filaments are seen in the end-bulb together with smooth endoplasmic reticulum (small arrows) and granular bodies (arrowheads). • Fig. 10. A section of three pinealocytes which contains granular endoplasmic reticulum (e) and a lysosome (L). The lysosome has phagoeytised a mitochondrium. X 25000

Ultrastructure of the H u m a n Fetal Pineal Gland. I

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such a rod m a y be constricted. The granular b o d y is surrounded b y a m e m b r a n e 80 A-thick (Fig. 12). The majority of the bodies have a clear area just beneath the membrane. The interior of the granular b o d y contains material of varying density. The interior has never shown a n y crystalline or fibrillar structure in a n y section plane.

Lysosomes: Lysosomes are very rarely seen. W h e n found t h e y show up as dense bodies surrounded by a membrane. I n contrast to the granular bodies, t h e y have no electron lucent area just beneath the membrane. Sometimes lysosomes, which have phagocytised an organelle, are observed (Fig. 10). Filaments: The filaments are a very prominent feature of the pinealocyte. T h e y are of indeterminate length with a diameter between 50-80 A. A p a r t from a perinuclear bundle, the cell processes contain the majority of filaments (Figs. 14, 21), and there is a tendency for the filaments to concentrate at the center of the processes. The n u m b e r of filaments varies from pinealocyte to pinealocyte, but no difference was observed between the cells located in the center, at the periphery, or perivascularly in the gland. Microtubules: A few microtubules with a diameter between 230-270 A and a wall thickness of 80 A are r a n d o m l y distributed in the cytoplasm, both in the cell b o d y and the processes. Centrioles: I n favourable sections a pair of centrioles can be seen in the cytoplasm close to the nucleus (Fig. 13). E a c h of these is made up of nine triplets of tubules and the pair of centrioles is oriented at an angle of 90 ~ to each other. Cilia : A moderate a m o u n t of cilia are seen in the pineal gland protuding from some of the pinealocytes (Figs. 15, 18). There is no specific location of these pinealocytes in the gland. The cilia are often seen invaginating the plasma m e m b r a n e of a neighbouring pinealocyte. I n transverse sections (Fig. 16) the cilia contain 9 pairs of microtubules but do not show a pair at the center. No basal plate or basal root are seen in relation to the basal corpuscle (Figs. 15, 17). Inclusions: Lipid droplets are rarely seen in the cytoplasm of the pinealocyte. Glycogen particles are very often seen in the cytoplasm (Fig. 8) especially in the processes. Sometimes these particles are concentrated in clusters.

Coated Vesicles : Coated vesicles are c o m m o n (Fig. 7). The majority of t h e m are small, 500-700 A in diameter, few are bigger with an average diameter about

Fig. 11. Electron micrograph showing different types of granular bodies in a pinealocyte. The majority of the granular bodies are small and circular (small arrows). One rod shaped granular body (large arrow) is thin at the midpiece. G large, elongated granular body. • 57000 Fig. 12. Electron micrograph of an elongated granular body (G) in a pinealocyte. • 72000 Fig. 13. Two ceutrioles (c) located in a pinealocyte. • 48000 Fig. 14. Electron micrograph showing a pinealocytic process (Pr) filled with filaments emerging from a cell body. m mitochondria, N nuclei. • 36000

Ultrastructure of the H u m a n Fetal Pineal Gland. I

Figs. 11--14

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Fig. 15. Longitudinally-cut cilium (c). • 36000 Fig. 16- Transversely-cut cilium (c). Note the absence of a central pair of microtubules. • 57 000 Fig. 17. Longitudinally-cut basal corpuscle of a pinealocyte cilium. • 57 000 :Fig. 18. The bulbous end (arrow) of a pinealocyte cilium is seen, Triangleextracellular space. • 36000 Fig. 19- Electron micrograph showing a coated vesicle (J) in a pinealocyte opening into the extracellular space (arrow). x 57 000 ]fig. 20. Melanin (M) located in a pinealocyte process. • 13000

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1500 A. The small ones are often seen to bud off from the Golgi apparatus. Omega figures are often seen on the cell membrane (Fig. 19). Melanin: Melanin granules, in the form of large masses, are rarely observed in the pinealocyte processes (Fig. 20). Junctions: Different types of cell junctions are observed between the pinealocytes, such as gap junctions, "intermediate t y p e " junctions (Fig. 9), and desmosomes. ~To special junctions seem to seal off the pinealocytes from the subarachnoid space (Fig. 23). The ependymal cells in the pineal recess are connected to each other by tight junctions. A more detailed description of the junctions will be given subsequently. The Neuroblasts

In addition to the pinealocyte another cell type (Fig. 22) is observed in the human fetal pineal gland. This cell is slightly larger than the pinealocyte. The chromatin pattern in the nucleus is more inhomogeneous owing to a higher degree of chromatin condensation along the nuclear membrane, and the cytoplasm is more electron lucent. The cells are located either singly, in small groups between the pinealocytes, or in the extended perivascular space. They contain only a moderate number of mitochondria and lysosomes, and very little granular endoplasmic reticulum. A characteristic feature of these cells is their content of free ribosomes, usually polyribosomes. These cells thus resemble the ncuroblast of the human fetal Pastori ganglion situated at the apex of the pineal gland. I t has not been possible so far, even in the older fetuses, to observe synapses on the perikaryon of these neuroblasts. Nerve Fibers

Many small groups of unmyelinated nerve fibers are seen in the parenchyma of the pineal gland and in the perivascular spaces. A more detailed description will be given subsequently. Blood Vessels

The pineal gland contains an abundant vascular network (Fig. 1). Perivascular spaces of varying size surround the capillaries, especially in the younger fetuses. These spaces decrease with increasing fetal age, first in the anterior lobe, later in the posterior lobe. However, even in the oldest fetus some vessels are still surrounded by perivascular spaces, especially in the posterior lobe. The himina of the capillaries are invested by continuous endothelial ceils (Fig. 25) connected to each other by junctions (Fig. 26) which look tight. No fenestrae are seen in the endothelial cytoplasm. The endothelial cells have an elongated nucleus and a cytoplasm rich in organelles, such as rough-surfaced endoplasmic reticulum, free ribosomes, Golgi apparatus and mitochondria. Lipid droplets are often seen in the cytoplasm. These droplets can be seen in evaginations of the endothelial cells into the capillary lumen (Fig. 26). Granular bodies are observed less frequently. No difference is found between these granular bodies and the ones observed in the pinealocytes. Coated vesicles are sometimes seen to open into the capillary lumen. Pericytes are not a constant feature. The endothelial cells are wrapped in an inner basal lamina, which also enclose the pericyte when present. An outer basal lamina separates the pinealocytes from the extended perivascular space.

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Fig. 21. Electron micrograph of a pinealocyte at the periphery of the pineal gland in a human fetus. Note the cytoplasm around the nucleus (N) without any filaments, contrasting with the many filaments in tbe process (P). The surface of the gland facing the subarachnoid space is covered by a basal lamina (arrow). ~'<12000 Fig. 22. Electron micrograph of a neurob]ast (Nb) among pinealocytes (Pi). Note the coarse chromatin pattern in the neuroblast nucleus and the numerous ribosomes (arrows) in the cytoplasm. Granular endoplasmic reticulum (L) is only sparse, m mitochondria ; large arrow = Golgi apparatus. X 9000

A x o n s a n d single n e r v e cells are o f t e n seen in t h e p e r i v a s c u l a r space e v e n in t h e y o u n g e r fetuses. I n t h e o l d e r f e t u s e s t h e a x o n s p e n e t r a t e t h e o u t e r b a s a l lamina and enter the pineal parenchyma.

Fig. 23. Electron micrograph showing the subarachnoid surface of the pineal gland covered by a basal lamina (thin arrow). Note the extended extracellular space (thick arrow) between the pinealocytes just beneath the basal lamina; no junction connects the pinealocytes here. Arrow with square = "intermediate t y p e " junction; triangle = extracellular space. X25000 Fig. 24. Small tracts of unmyelinated nerve fibers (F) in the pineal parenchyma. The fibers are surrounded by pinealocytes (Pi). Arrow = axodendritic synapse, x 25000

Fig. 25. Electron micrograph of a part of a capillary in the pineal gland. A very narrow perivascular space (arrow) surrounds this capillary. Typical pinealocytes (Pi) border the external basal lamina. L U capillary lumen, Pc perieyte, e endothelial cell. X 13000 Fig. 26. Lipid droplet protruding into the capillary lumen, but still inside the endothelial cell. x 30000. Inset A: Coated vesicle opening into the lumen of the same capillary, x 57000

Ultrastructure of the Human Fetal Pineal Gland. I

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Discussion This study confirms the observation by Krabbe (1915), and the recent report by Hfilsemann (1971) that the human fetal pineal gland consists of an anterior and posterior lobe. The dominant parenchymal cell type is the pinealocyte. These cells possess, in our material, all the organelles necessary for a hormone synthesis. The granular bodies in the pinealocytes described here are ultrastructurally like the neurosecretory granules described by Bargmann and Gaudecker (1969), although they contained no crystalline or fibrillar structures. These granular bodies also fit the description of the "grumose" or "vermiculatc" bodies described in the rat by Milofsky (1957), Wolfe (1965), and Arstila (1967), and the membranebound dense inclusions in the light pinealocytes in the rabbit described by Romijn (1972). Wolfe believes them to be secretory granules, but Arstila points out that they may be lysosomal in nature, because he could demonstrate acid phosphatase and aryl sulfatase activity inside these bodies. Phosphohydrolases have, however, been demonstrated in the secretory granules, especially the ones newly released from the Golgi apparatus (Smith, 1969). No relationship between these granular bodies and the Golgi apparatus was observed in the present study and exocytosis of the granular material was never seen. The same type of granular bodies was observed in the endothelial cells, but was never seen in the perivascular space or in the lumen of the blood vessels. Light microscopical observations using hematoxylin-phloxin and paraldehyde fuchsin failed to demonstrate granular staining (unpublished observations). Because of the paucity of these granular bodies in the pinealocytes and the failure to demonstrate exocytosis, the author does not believe this organelle to be a secretory granule. The storage and release of the different indoles known to be synthesized and released by the pineal gland has at present not been demonstrated by electron microscopy. The coated vesicles are commonly observed in the pinealocyte and in the endothelial cells. The omega-figures opening both to the extracellular space and the capillary lumen indicate that these vesicles are very likely transporting vesicles. A transport function for the coated vesicle has previously been suggested (Friend and Farquhar, 1967). Tracer studies will determine whether the coated vesicles transport from the pinealocytes to the extracellular space or vice versa. A great many lipid droplets were found in the endothelial cells. The usual location of these droplets in the cytoplasmic evaginations into the capillary lumen suggests that they may be secreted into the blood. Lipid droplets were rarely seen in the pinealocytes, therefore lipid synthesis may occur in the endothelial cells. The pineal lipids in the rat may take part in endocrine secretion (Quay, 1957). Continuous illumination which decreases various pineal enzyme activities, decreases the lipid content of the rat pineal (Quay, 1961). An increase was found after ovariectomy (Zweens, 1963). Lipid droplets in human pinealocytes were shown by Quay (1957) who demonstrated later that they concentrate next to the capillary walls (Quay, 1965), but lipid in endothelial cells has not been studied. Cilia with a 9 ~- 0 microtubular pattern are often associated with photoreceptor or other sensory cells. The pinealocytes in the pineal organ of the lizard, Lacerta sicula, possess a bulbous cilium of the 9 ~-0 pattern (Oksche, 1971). This organ

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shows an electrical response to light stimulation (Hamasaki and Dolt, 1969). Cilia of this type are also associated with secretory cells, e.g. the baso- and acidophil cells of the pituitary (Barnes, 1961 ; Andersen et al., 1970). In the secretory mammalian pineal gland cilia have been observed associated with the chief and interstitial cells in the rat (Arstila, 1967). Arstila does not mention if the observed cilia are of the 9 ~ 0 type. Whether these cilia are a phylogenetic recapitulation during ontogony is not known because there are no ultrastructural studies of the postnatal human pineal organ. The second cell type in the human fetal pineal gland is the neuroblast. The identification of this cell type was possible because of the concomitant study of the Pastori ganglion at the apex of the pineal gland (Moller and Mollgs unpublished). The dominating organelle is the free ribosome. The granular endoplasmic reticulum is poorly developed. No synapses were seen on the cell body or processes. Synapses in the Pastori ganglion were observed, but they were extremely rare. This is in agreement with Pick et al. (1964), who studied the sympathetic ganglia in a 15 and 17 week old human fetus, and at t h a t stage had not observed any synapses. The adult human pineal gland contains astrocytes of the fibrillar type (P. del Rio Hortega, 1932; Scharenberg and Liss, 1965). In human fetuses Krabbe (1915) could not identify any glial cells. Hfilsemann, on the contrary, describes in his study filamentous astrocytes located around the blood vessels and at the periphery of the pineal gland of a three-months-old human fetus (82 m m CI~L). His observations are, however, not supported by the present study. Even in the oldest fetuses it was not possible to identify glial cells with certainty. In the present material all the pinealocytes contained a considerable amount of filaments, most of them located in the processes. The number of filaments varied from cell to cell but, by tracing processes rich in filaments back to the cell bodies, no differences could be observed between the nuclei or cytoplasm of these cells, compared to other pinealocytes. Ultrastructural observations (unpublished) of the cerebral cortex of the present material have shown, that identification of glial cells in the present age group is not possible by morphological criteria but only on the basis of their perivascular location. The pineal gland is traditionally considered to lack a blood-brain barrier (Wislocki and Leduc, 1952). We have investigated the blood-brain barrier in different mammals in our laboratory with different dyes (unpublished results). These investigations show the blood-brain barrier to be absent in the pineal gland of rats and rabbits, but to be present in the cat. Both rats and rabbits have fenestrae in the endothelial cells in the capillaries of the pineal gland, in contrast to cats where the endothelial ceils have no fenestrae. This study has shown the endothelial cells in the capillaries of the human fetal pineal to be without fenestrae, which argues for the presence of a blood-brain barrier in the gland. The "classical" neuroendocrinc areas (e.g. eminentia mediana, area postrema) are without a blood-brain barrier, so the secreted molecules m a y easily enter the blood stream. The presence of a barrier in the pineal gland does not necessarily prevent endocrine secretion into the blood because the most important hormone synthesized here, melatonin, has a lipophilic, uncharged molecular structure, which enables it to penetrate the barrier (Wurtman and Axelrod, 1965). Peripherally-released care-

Ultrastructure of the Human Fetal Pineal Gland. I

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cholamines are, on t h e o t h e r h a n d , p r e v e n t e d from reaching the p i n e a l o c y t e s because of the i m p e r m e a b i l i t y of the b a r r i e r to these substances (Axelrod et al., 1959). I t is concluded t h a t t h e h u m a n fetal p i n e a l o c y t e in t h e p r e s e n t m a t e r i a l contains all t h e organelles necessary for h o r m o n e synthesis. E v e n t h o u g h no s e c r e t o r y granules could be d e m o n s t r a t e d w i t h c e r t a i n t y , t h e rich v a s c u l a r i z a t i o n a n d i n n e r v a t i o n of t h e g l a n d c o m b i n e d w i t h t h e existence of t i g h t j u n c t i o n s bet w e e n t h e e p e n d y m a l cells in t h e pineal recess argue in f a v o u r of a secretory role of t h e g l a n d in e a r l y i n t r a u t e r i n e life.

References )mdersen, H., Billow, F. A. yon, Mollgs K.: The histochemical and ultrastructural basis of the cellular function of the human foetal adenohypophysis. Progr. Histochem. Cytochem. 1, 153-184 (1970) Anderson, E.: The anatomy of bovine and ovine pineals. J. Ultrastruct. Res., Suppl. 8, 1-80 (1965) Arstila, A. U.: Electron microscopic studies on the structure and histochemistry of the pineal gland of the rat. Neuroendocrinol. 2, Suppl. 6, 1-101 (1967) Axelrod, J., Weil-Malherbe, H., Tomchick, 1~.: The physiological disposition of 3H-epinephrine and its metabolite metanephrine. J. Pharmacol. exp. Ther. 127, 251-256 (1959) Bargmann, W., Gaudecker, v. Br.: ~ber die Ultrastruktur neurosekretoriseher Elementargranula. Z. Zellforsch. 96, 495-504 (1969) Barnes, B. G.: Ciliated secretory cells in the pars distalis of the mouse hypophysis. J. Ultrastruct. Res. 5, 453-467 (1961) Friend, D. S., Farquhar, M. G.: Function of coated vesicles during protein absorption in the rat vas deferens. J. Cell Biol. 85, 357-376 (1967) Hamasaki, D. I., Dodt, E.: Light sensitivity of the lizard's epiphysis cerebri. Pflilgers Arch. 313, 19-29 (1969) Hortega, P. del Rio: Pineal gland: In cytology and cellular pathology of the nervous system, vol. 2, ed. W. Penfield. New York: Hoeber 1932 ttillsemann, M.: Development of the innervation in the human pineal organ. Z. Zellforsch. 116, 396-415 (1971) Krabbe, K . H . : Histologiske undersogelser over corpus pineale. Thesis, Julius Giellerup Copenhagen (1915) Lerner, A. B., Case, J. D., Heinzelman, 1~. V.: Structure of melatonin. J. Amer. chem. Soc. 81, 6084-6085 (1959) Milofsky, A.: The fine structure of the pineal in the rat, with special reference to parenchyma. Anat. Rec. 27, 435-436 (1957) Mollgs K.: Histochemical investigations on the human foetal subcommissural organ, 1. Histochemie 32, 31-48 (1972) Mollgs K., Moller, M.: On the innervation of the human fetal pineal gland. Brain Res. 62, 428-432 (1973) Mollgs K., Moller, M., Kimble, J.: Histochemical investigations on the human fetal subcommissural organ, 2. Histochemie 87, 61-74 (1973) Oksche, A.: Sensory and glandular elements of the pineal organ: In: The pineal gland. Ciba Foundation Symposium, ed. G.E.W. Wolstenholmc and J. Knight, p. 127-146. London: Churchill Livingstone 1971 Pick, J., Gerdin, C., Delemos, C.: An electron microscopical study of the developing sympathetic neurons in man. Z. Zellforsch. 62, 402-415 (1964) Quay, W. B.: Cytochemistry of the pineal lipids in rat and man. J. Histochem. Cytochem. 6, 145-153 (1957) Quay, W.: Reduction of mammalian pineal weight and lipids during continuous light. Gen. comp. Endocr. 1, 211-217 (1961)

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