Ultrastructure of the age-involuted adult human thymus

Cell Tiss. Res. 186, 507-525 (1978) Cell and Tissue Research �9 by Springer-Verlag 1978

Ultrastructure of the Age-Involuted Adult Human Thymus*

Brita von Gaudecker**

Anatomisches Institut der Universit/it Kiel, Kiel, Bundesrepublik Deutschland

Summary. Age involuted thymus tissue from a middle aged (33 years) and an old (63 years) man have been examined by electron microscopy and compared with thymus tissue from children. Biopsies had been taken during surgical correction of congenital heart defects.

The fine structural architecture of cortex, medulla and connective tissue in the remaining lymphatic islands in the adult thymus investigated was not different to the thymus of children. We were surprised to find vigorous lymphocytopoiesis in the cortical regions and to recognize extended areas of medulla with a cellular composition which obviously provides the same microenvironment for T-cell maturat ion as the medulla of the non involuted thymus. Our findings are discussed in relation to the increasing arguments that the human thymus serves an immunological function throughout life.

Key words: Human thymus - Age involution - Cellular composition of cortex, medulla and connective tissue - Electron microscopy.

Introduction

Prior to puberty the thymus is composed of lobules incompletely separated by primary connective tissue septa. Each lobule comprises a darkly stained cortex and a lighter medulla. In adults the thymus is transformed into a mass of adipose tissue containing scattered islands of lymphatic tissue composed of thymic epithelial regions where entodermal epithelial cells form a network, and of reticular connective tissue with argyrophilic fibers. The epithelial regions of the thymus do not disappear completely, even in old age (Hammar , 1926).

Send offprint requests to: Priv.-Doz. Dr. Brita von Gaudecker, Anatomisches Institut der Universitfit, 2300 Kiel, Olshausenstr. 40 60, Federal Republic of Germany

* This investigation was supported by grants from the Deutsche Forschungsgemeinschaft ** ! express my thanks to Professor Dr. med. Alexander Bernhard (Kiel) for kindly providing the human thymus tissue. The author also appreciates the excellent technical assistance of Mrs. Knauer, Mrs. Parczany, Mrs. Siebke and Mrs. Waluk

0302-766X/78/018 6/0507/$3.80

508 B. von Gaudecker

The light microscopical aspect of the human adult thymus has been studied extensively (for review see Bargmann, 1943). Electron microscopical studies of the normal human thymus in advanced years are scanty: Bloodworth et al. (1975), Jones et al. (1975) and Kamameya et al. (1965) compare the ultrastructure of the normal thymus from children and adults with hyperplastic thymus and thymic tumors, thus only a very short and incomplete description of the normal age involuted thymus is available.

This investigation has the aim of elucidating the cellular composition of cortex, medulla and connective tissue spaces in the lymphatic islands of normal adult thymus of a middle aged (33 years) and a 63 years old man. For comparison normal thymus tissue of children has been processed with identical fixation and embedding.

We used normal thymus from male individuals because Simpson et al. (1974) revealed that the age involution takes a continuous linear course in men whereas in women the thymus involutes in a biphasic manner.

What does age involution of the thymus involve ? The question is whether there exists only a quantitative reduction of lymphatic tissue in this organ, or whether the remaining lymphatic islands additionally are changed qualitatively and by this way become incapable of serving an immunological function.

In recent years, increasing functional evidence suggests that the thymus is replacing the pool of T-cells in the periphery during the whole life of man and animals (Stutman et al., 1974) and that thymectomy of adult human patients leads to a significant T-lymphocyte deficiency and impairment of delayed skin reactivity against several antigens (Bj6rkholm et al., 1975). In relation to these new functional observations we considered it interesting to investigate the following question: In what way is the remaining lymphatic tissue in the adult thymus altered in contrast to the non-involuted thymic tissue of the child, and are there morphological equivalents of persistent function in the thymus after puberty?

Materials and Methods

Thymic tissue specimens have been obtained from five boys (4, 5, 6, 7, 11 years) underdoing surgery for congenital heart disease but who were otherwise normal, and from two adult patients (33 years old undergoing surgery for congenital septum defect an 63 years old undergoing mitral valve replacement).

Tissues were fixed by immersion in different solutions immediately after excision of the tissue:

Fixation Solutions

1. 3.5 ~o Glutaraldehyde in phosphate buffer (pH 7.8), postfixation in 2 ~ OsO4 in phosphate buffer. 2. 4~o Formaldehyde, freshly prepared from paraformaldehyde, in 0.I M phosphate buffer (pH 7.5)

(Geyer, 1973). 3. Bouin's solution (Romeis, 1968)

Methods for Light Microscopy

a) Staining reactions performed on paraffin sections (Romeis, 1968): 1. Haematoxylin-eosin 2. Giemsa's stain

Ultrastructure of Age-Involuted Adult Human Thymus 509

3. Azan 4. Silver impregnation for reticular fibers according to Gomori

b) Staining reaction performed on semithin sections after embedding in Araldite: 1. Azure-II-methylene blue (Richardson, 1960) 2. Silver impregnation for reticular fibers, connterstained with Richardson's stain (Movat, 1961)

Methods for Electron Microscopy

After immersion fixation in 3.5 % glutaraldehyde, followed by 2 % OsO4, the thymus tissue specimens were dehydrated in ethanol, passed through propylene oxide, and embedded in Araldite. Ultrathin sections were cut with a diamond knife on a Reichert Om U 2 microtome and stained with uranyl acetate and lead citrate. Electron microscope: Siemens Elmiscop 101.

Results

The Thymus of the Child

The structure of the non-involuted human thymus has been investigated extensively with both the light- and electron microscope (Bargmann, 1943; Metcalf, 1966; Hirokawa, 1969; Kaiserling et al., 1974b. The main architecture and cellular composition of cortex, medulla and perivascular spaces of the young thymus will be summarized here briefly in order to give a basis for the comparison with the adult thymus:

In the child each of the two thymic lobes is invested by a thin capsule of loose connective tissue and is subdivided by pr imary connective tissue septa into a number of pseudo-lobules. The epithelial region of such a pseudo-lobule is composed of a central medulla surrounded by a densely staining cortex. Secondary connective tissue septa with argyrophilic fibers extend inward from the surface of the cortex and reach as far as the cortico-medullary boundary where these septa frequently widen into broad perivascular spaces. They contain blood vessels and many free cells, mainly lymphocytes (Fig. 9). The cortex and medulla show a gradual transition into each other where no perivascular space separates the two regions.

The marginal region of the thymic cortex in general is known to be the location oflymphocytopoiesis. Therefore large lymphocytes, medium sized lymphocytes and mitoses are more abundant here than in the inner cortical zone (v. Gaudecker et al., 1965; v. Gaudecker, 1966; Hinrichsen, 1965; K6bberling, 1965). The same arrangement can be observed in the human thymus of the child. In the inner cortex smalllymphocytes are the most numerous cell type. The epithelial cells in the thymic cortex have a stellate shape. Their cytoplasm contains bundles of filaments (thickness approximately 90 A). Some of these filaments seem to insert on the desmosomes. The epithelial cells have elongated very flattened cytoplasmic projections which enclose the outer aspects of the lobule and all perivascular spaces. Towards the connective tissue they develop a basal lamina. In the cortex they form the frame work of a sponge enclosing spaces of variable size which are crowded with lymphocytes. Macrophages (by some authors called mesodermal reticulum cells or PAS-positive phagocytic reticulum cells) are scattered in the cortex. They have extended blunt projections and their cytoplasm contains lysosomal and phago-

510 B. yon Gaudecker

Fig. 1. Man, 63 years old, Araldite embedding, semithin section, Richardson's staining. The cortex (upper left) shows a prominent epithelial rim (arrows), several large and medium sized lymphocytes and two mitoses (arrow heads). It is separated by a perivascular space with three venules from the medulla which contains a small Hassall's corpuscle (HC). x 1120

somal structures. Several times degenerating small lymphocytes were seen ingulfed in the cytoplasm of the macrophages. Monocytes and mast cells are found in very low numbers inside the epithelial region of the cortex.

The medulla shows a less intense staining due to cells with more abundant branching cytoplasm and lightly staining nuclei. These cells are epithelial cells, interdigitating reticulum cells and some macrophages. Lymphocytes are not ar ranged in the medulla as tightly as in the cortex. Hassall 's corpuscles can always be recognized (Fig. 1). They have been described in detail in a previous publication (v. Gaudecker et al., 1974). The thymic epithelial cells in the medulla have a more


bulging cytoplasm than in the cortex. The interdigitating reticulum cells are characterized by electron lucent cytoplasm and long irregular and interdigitating cytoplasmic projections which extend between the epithelial cells and sometimes completely surround lymphatic cells. The Golgi complex is located in an indentation of the irregularly shaped nucleus. Small vesicles and frequently electron dense granules are assembled in the Golgi zone and sometimes single residual bodies and lysosomal structures are seen in the cytoplasm.

The fine structure of the small lymphocytes exhibits striking differences between cortex and medulla as has been described in the mouse (Abe et al., 1970). We could identify the same differences in the thymus of the human, children and adults. As far as we know, this observation has not previously been noticed in the human thymus by other authors.

Small lymphocytes in the cortex (Fig. 2) have a rounded or ovoid nucleus, often slightly concave on one side. Clumps of chromatin material are disposed along the nuclear membrane and in the nucleoplasm. The nucleus is surrounded by a rim of scanty cytoplasm which appears relatively dark because of the dense concentration of free ribosomes. A consistent feature of cortical small lymphocytes is their polyhedral shape, due to mutual deformation, and they possess smooth surface plasma membranes which are closely applied to each other without interdigitations. A poorly developed Golgi zone, some mitochondria and some granules, probably of lysosomal nature are grouped around a pair of centrioles. This zone of organelles, however, is small and rarely transected by the plane of the section.

Small lymphocytes in the medulla (Figs. 3, 4) are irregular in cellular and nuclear outlines compared with those in the cortex. The nuclear surface exhibits infoldings of various depth. The cytoplasm is more abundant than that of cortical small lymphocytes and is frequently asymmetrically arranged around the nucleus. There are often interdigitations between the membranes of adjacent lymphocytes and the interdigitating reticulum cells. Organelles are more abundant than in the cytoplasm of cortical small lymphocytes. Beside free ribosomes and polysomes, there are single elements of smooth and rough endoplasmic reticulum, a pair of centrioles and a well developed Golgi apparatus with 3-4 laminated flattened saccules and numerous vesicles. Many of these medullary lymphocytes show clusters of small dark inclusion bodies (Fig. 4, arrow), probably of lysosomal nature. Sometimes these granular aggregations are associated with a fat droplet. Most mitochondria are arranged around the Golgi field but they also appear elsewhere in the cytoplasm. Fine filaments form more or less wavy bundles and are most abundant in cytoplasmic protrusions (Fig. 4 inset). The single filament has a diameter of about 70 A.

The Age-Involuted Thymus of the Adult

In paraffin- or semithin sections of age-involuted thymus one recognizes isolated islands of lymphatic tissue surrounded by vacuolated fat cells. The proportion between lymphatic tissue and adipose tissue in the adult man is extremely variable


Fig. 2. Man, 63 years old. Outer cortex active in lymphocytopoiesis, contains a large lymphocyte (IL) with a loosely structured nucleus and one mitosis (M). The chromatin of the small lymphocytes (sL) is much more condensed. A prominent nucleolus can be identified in the large nucleus of an epithelial cell (E). Desmosomes (arrows) connect slender epithelial processes, x 7800

and not strictly age dependen t ( H a m m a r , 1926). In the thymus o f the 63 years old m a n ra ther big cystic and calcif ied Hassa l l ' s corpuscles were s t r iking whereas the Hassa l l ' s corpuscles in the 33 years old person were smaller in the average. This also has to be cons idered as an indiv idual var ia t ion (see the tables in H a m m a r , 1926). In the lympha t i c is lands o f the adu l t thymus one recognizes cortex, medul la , and re t icular connect ive tissue. The medul la is no t comple te ly su r rounded by cortex. F requen t ly medu l l a ry regions have direct con tac t with the adipose tissue, and


Fig. 3. Man, 63 years old. Medulla with a cystic Hassall's corpuscle (HC) formed by thymic-epithelial cells (E); an interdigitating reticulum cell (IDC) with irregularly shaped nucleus and electron transparent cytoplasm. Projections of such IDCs are visible at several places among the lymphocytes, x 2800

cor tex and medu l l a in some areas seem to be comple te ly i sola ted f rom each o ther by ex tended re t icu lar connect ive tissue. The remain ing thymic cor tex seems to be active in lymphocy topo ies i s , for f requent ly mi toses are no ted (Figs. 1, 2). The ou te r cor tex conta ins large and m e d i u m sized lymphocy tes and the inner cor tex is c rowded with small lymphocytes . The cor tex in an adu l t person is f requent ly

514 B. yon Gaudecker

Fig. 4. Man, 63 years old. Medullary small lymphocyte. The cytoplasm is more abundant than that of cortical small lymphocytes and has irregular outlines. There are interdigitations with other lymphocytes and interdigitating reticulum cells. The Golgi field and the centriole are located near the indentation of the nucleus. Notice a group of small lysosomal structures (arrow). A profile of rough endoplasmic reticulum (ER) is located above the nucleus. Inset: Cytoplasm of a medullary small lymphocyte with a bundle of filaments (filament diameter about 70 A). x 16,000; inset x 62,000

surrounded by an epithelial margin which is clearly visible in the light microscope (Fig. 1, arrows). In the thymus of children, on the other hand, the epithelial border- lines which separate the thymic-epithelial region f rom the perivascular space are extremely flat and only recognizable with the electron microscope.

Like the medulla in the thymus of children this region in adults also shows a less intense staining, due to cells with large lightly staining nuclei or with more voluminous cytoplasm.

The ultrastructure o f the epithelial cells in the cortex is not altered compared to those in young thymus: They separate the epithelial region of the thymus f rom the connective tissue space by forming a closed border-line o f cytoplasmic projections and developing a basal lamina (Fig. 8) and form a loose network enmeshing large medium sized and small lymphocytes.


Large lymphocytes are found predominantly at the periphery of the outer cortex immediately beneath the epithelial boundary which separates the thymic epithelial region from the connective tissue. They have a large, round, pale nucleus with a diameter of approximately 7-9 gm. The chromatin is evenly distributed without forming clumps except along the inner side of the nuclear membrane and adjacent to nucleoli where heterochromatin material is condensed to form a very narrow rim (Fig. 2). The nucleus contains one or two prominent nucleoli which often adhere to the nuclear membrane.

Medium sized lymphocytes have a nucleus measuring 5-7 gm in diameter. The chromatin is usually condensed to form small clumps which are irregularly distributed within the nucleus and along the nuclear membrane. Medium sized lymphocytes are found predominantly in the outer cortex, but some are also scattered between the small lymphocytes in the inner cortex and in the medulla.

Cortical small lymphocytes, as has been mentioned above, have the same' appearance in children and adults, and some macrophages, as well as very few monocytes and mast cells are scattered among the small lymphocytes. Summarizing, it can be stated that the ultrastructure and cellular composition of the remaining cortical areas in the lymphatic islands of a normal age involuted thymus even in a 63 years old person remain unchanged compared to the cortex of a normal thymus before puberty.

The thymic epithelial cells in the medulla have more bulging cytoplasm than in the cortex. Some of them contain aggregations of vacuoles, sometimes with a fine flocculent substance (Fig. 5 a). Numerous Hassall's corpuscles are formed by epithelial cells (Fig. 3). In adult persons cystic Hassall's corpuscles are more abundant than in children. Some medullary areas contain irregular regions exclusively composed of tightly attached epithelial cells (Fig. 5 c). In such regions one finds many desmosomes, and the cytoplasm of the epithelial cells contains small dark irregularly shaped inclusions. An increasing proportion of medullary regions occupied exclusively by epithelial cells, though with great individual variations, has been reported as a phenomenon occurring in the aging human thymus (Hammar, 1926; Bargmann, 1943). In our material these epithelial regions seem not to be very extensive.

Interdigitating reticulum cells and their manifold cytoplasmic projections can be noticed in the medulla of adults in the same way as in the thymus of children (Fig. 3). They can be identified by the irregularly shaped nucleus with a characteristic small rim of heterochromatin along the inner surface of the nuclear envelope. Frequently small dark granules are assembled in the Golgi field adjacent to the nucleus (Fig. 5b).

Several other cell types are located in small numbers in the medullary epithelial region of adults: Such are monocytes, macrophages, and mast cells.

Plasma cells have been identified very rarely in the epithelial region of the thymus. In the medulla of children and adults we found some cells with the appearance of the "lymphoblast-like plasma cells" described by Kaiserling et al. (1974b), Lennert et al. (1975) and Mfiller-Hermelink et al. (1973) with a rather electron lucent nucleus and relatively few ergastoplasmic profiles. Other plasma cells have well developed parallel cisternae of rough endoplasmic reticulum and a nucleus containing large blocks of heterochromatin around the periphery.

Fig. 5a. Man, 63 years old. Thymic-epithelial cell in the medulla containing vacuoles with a fine flocculent substance, x 12,000. h Man, 33 years old. An interdigitating reticulum cell contains a Golgi complex, located in the concavity of the irregularly shaped nucleus (N). Small vesicles and electron dense granules are assembled in the Golgi field, x 24,000. e Man, 63 years old. Irregular area purely composed of tightly attached epithelial cells. Note many desmosomes and dark irregularly shaped inclusions. x 21,000


Multinucleated giant cells (Fig. 6) were found rarely in several tissue blocks of the thymus from the 33 years old man and one each in the thymus of the 63 years old man and a 7 years old boy. They were surrounded by small lymphocytes and epithelial cells. These giant cells have a rather dense cytoplasm packed with many mitochondria and they contain several nuclei of irregular shape with a small rim of condensed chromatin along the inner nuclear membrane and up to three prominent nucleoli. The outer surface of the giant cells is covered by numerous irregular slender microvilli. Manifold interdigitations and infoldings of the plasma membrane can be noticed at some places from which coated vesicles are budding off. Frequently double walled vacuoles of different sizes and shapes with a flocculent content are aggregated into clusters. Besides the many mitochondria there are several Golgi zones, single irregular cysternae of rough endoplasmic reticulum, granules, some fat droplets, autophagosomes, and bizarre large inclusion bodies.

Multinucleated giant cells have not previously been described in normal human thymus but they regularly form in cell cultures of human thymic tissue from different ages (Matagne-Dhoossche, 1972). These authors suggest that multinucleated giant cells are formed by a confluence of macrophages. From the ultrastructural aspect we define them as histiocytic giant cells with phagocytic capacity. Their function in the normal thymus is obscure.

Myoid cells are located exclusively inside the thymic epithelial region. They have been found in the human thymus of all ages. The myoid cells can be elongated but normally they are rounded with a nucleus having a bizarre outline (Fig. 7). Thick and thin filaments, probably myosin and actin, are arranged circularly around the nucleus in the central cytoplasmic region. The marginal cytoplasm is occupied by mitochondria, some vacuoles and other organelles. The microfibrils occasionally show densifications. In cross section one recognizes that, similar to the arrangement in striated muscle, thick myosin filaments are surrounded regularly by thin actin filaments (Fig. 7, inset).

The cellular composition of the remaining medullary areas in the adult thymus is not altered as compared to the medulla of the child.

The reticular connective tissue in the lymphatic islands of adults can be recognized after silver impregnation due to argyrophilic fibers. Narrow perivascular spaces surrounding just one capillary are more frequent in the cortex than in the medulla.

In most cross sections of these small perivascular spaces one discerns a closed boundary of thymic epithelial cells with their basal lamina (Fig. 8 a). In the inner cortex and sometimes also in the thymic medulla, however, this basal lamina cannot always be recognized around the whole circumference of the perivascular space. Occasionally the epithelial border line is discontinuous and lymphocytes directly contact the perivascular space. Diapedesis of lymphatic cells into the vascular lumen, however, has not been found in our material.

Apart from narrow perivascular spaces surrounding small capillaries there are others which are filled only with reticular fibers and some small projections (Fig. 8 b). They all are enclosed by thymic-epithelial cells with their basal lamina. Thus, argyrophilic fibers never are located naked inside the thymic epithelial region as might appear by light microscopy. They always are surrounded by a sheath of epithelial cells.


Fig. 6a and b. Man, 33 years old. Thymus-epithelial region at the cortico-medullary junction, a A multinucleated giant cell is located between the lymphocytes. It contains several irregularly shaped nuclei with prominent nucleoli and has rather electron dense cytoplasm. The surface is covered by microvilli. • 2800. b At higher magnification it becomes obvious that the cytoplasm of the giant cell contains many mitochondria, Golgi fields (G), single irregular cysternae of rough endoplasmic reticulum, some granules, some fat droplets (F) and bizarre shaped, large inclusion bodies (arrow). x 36,000


Fig. 7. Man, 63 years old. Myoid cell in the thymus-epithelial region. The nucleus (N) has a bizarre outline. It is surrounded by concentric myofilaments. The marginal cytoplasm contains mitochondria, vacuoles and other organelles. At high magnification (inset) thick myosin filaments and thin actin filaments become obvious. Notice the regular arrangement of these filaments in cross section (arrow). • 15,000, inset 60,000,

The more ex tended re t icu lar connect ive tissue areas in the l ympha t i c is lands o f the age involu ted thymus con ta in many lymphocytes and o ther wander ing cells: Two kinds o f small lymphocy tes can be recognized. One k ind resembles the small cor t ica l lymphocy tes with a r o u n d o r ovoid nucleus and scanty cy toplasm. The o ther k ind has an i r regular ly shaped nucleus and more a b u n d a n t cy top lasm, thus


Fig. 8 a and b. Man, 33 years old. Medulla. a A small perivascular space containing a capillary (Cap) is completely enclosed by thymus-epithelial cells (E) and their basal lamina. Reticular fibers are seen in the perivascular space, x 24,000. bA small perivascular space containing a cell projection and some reticular fibrils is completely surrounded by thymus-epithelial cells with their basal lamina. • 24,000


Fig. 9. Boy, 11 years old. Widened perivascular space at the cortico-medullary junction containing all mobile cells of defense as found in reticular connective tissue at other places of the body. L Lymphocytes; PI Plasma cells; Ma Macrophage with a big dense body; eL eosinophilic leucocyte. • 7800


looking much like the small lymphocytes of the medulla. This morphological aspect may permit the conclusion that lymphocytes from both the medulla and the cortex enter into the connective tissue space and from here may reach the periphery. Also some medium sized lymphocytes are scattered in between.

Frequently there are accumulations of plasma cells (Fig. 9). Mast cells are more abundant in the connective tissue than in the thymic-epithelial regions. One also meets myelocytes, granulocytes, monocytes and macrophages.

Discussion

The existence of large as well as medium sized lymphocytes and of numerous mitoses in the outer cortex of the adult thymus indicates lymphocytopoiesis. Towards the inner cortex these blast cells differentiate into small lymphocytes. Cortical small lymphocytes do not yet seem to have achieved full T-cell maturity (tested in the mouse by Abe et al., 1970; Matter, 1975). These precursor cells can achieve immuno-competence either in the thymic medulla or in the periphery (Miller, 1975). The number of degenerating small lymphocytes and macrophages does not seem to be raised, indicating that acute accidental involution with a high death rate of cortical small lymphocytes is not just going on in the adult thymus tissue investigated here.

Beyond the irregular regions composed exclusively by epithelial cells the cellular composition of the remaining adult thymus medulla is not altered as compared to the medulla of the child. Medullary small lymphocytes like those in the fully functional thymus tissue of the young mouse (Abe et al., 1970; Droege, 1974 a, b; Matter, 1975) and of children (own observations) show more advanced differentiation towards the mature T-cell morphology. Some of them seem to have achieved full maturity (compare Stutte et al., 1976; Watanabe et al., 1974). Also the interdigitating reticulum cells which were thought to be of mesodermal origin and to be important for the maturation of immunocompetent T-cells (Friess, 1976; Heusermann et al., 1974; Kaiserling et al., 1974a, b) seem to exist in the adult thymus medulla in undiminished number and the vacuolate inclusions in some epithelial cells may possibly indicate the production of a humoral thymus factor. These subcellular findings in our material lead us to conclude that the remaining medulla of the adult thymus provides the same microinvironment for T-cell maturation as does the medulla of non-involuted thymus.

In the thymus of the child secondary connective tissue septa extend inward from the surface of the cortex and widen into broad perivascular spaces at the corticomedullary junction. Hammar (1926) calls these areas "circum-medullary connective tissue" and reports its increase in volume with increasing age. We assume the quite extended reticular connective tissue space in the lymphatic islands of the age-involuted thymus to be coincident with this circum-medullary connective tissue. The widened perivascular spaces at the cortico-medullary junction in the thymus of the child are crowded with all kinds of mobile cells of the immune defense. We found the same cells in the reticular connective tissue of the adult thymus. In all human ages lymphocytes are the most numerous free cells in this connective tissue areas. Diapedesis of lymphatic cells into the vascular lumen has not been found in our material. This agrees with observations in human thymus tissue by Blanc et al. (1973) and Kameya et al. (1965), whereas diapedesis of


lymphocytes into venules has been demonstrated in the thymus of the mouse, the rat, and the guinea pig (Clark, 1963; Sainte-Marie et al., 1971; T6ro et al., 1967). There seem to exist different pathways by which thymic lymphocytes can reach the periphery. The thymus has also a lymphatic drainage system and some lymphocytes appear to leave the organ by this route (Blanc et al., 1973; Kotani et al., 1966). The reticular connective tissue space contains cortical and medullary small lymphocytes which obviously leave the organ from here. Cortical small lymphocytes are supposed to be less mature T-precursor cells than medullary small lymphocytes (Dr6ge et al., 1974 a, b; Matter, 1975; Miller, 1975).

From experimental work it is obvious that thymus-derived lymphocytes reach the periphery in different states ofmaturatity and that immature T-cells can achieve immunocompetence in the thymus dependent areas of lymph node and spleen (for review see Stutman et al., 1974). The same population of the reticular connective tissue space with different kinds of mobile wandering cells in the thymus of the adult as compared to the thymus of the child indicates an unaltered function of these areas in the lymphatic islands of the age-involuted thymus.

One cannot of course, draw a general conclusion from this investigation of two cases of age-involuted thymus. If, however, we compare our material with the many cases of normal thymus tissue studied with the light microscope by Hammar (1926) we may state that the material presented has the appearance of normal age- involuted thymus with a rather well preserved portion of lymphatic tissue.

Our morphological results indicate that the age-involuted human thymus, though quantitatively reduced to a great extent, contains a remnant of lymphatic tissue, which is unchanged in its cellular composition and which presumably is able to serve its function in cell mediated immunological responses. The quantitative reduction of lymphatic tissue in the thymus after puberty is followed by a decrease in volume of the T-cell regions in the lymphnode (Gheshlaghi, 1977) and by a decrease in number of T-cells in the blood stream with increasing age (Alexopoulos et al., 1976).

The remaining thymus epithelial regions, however, provide new immunocompetent T-cells even in a rather old person. This is in agreement with functional data which indicate that significant numbers of aged individuals are able to develop vigorous responses to new antigens (Stutman et al., 1974). Furthermore, adult thymectomy leads to a reduction of this ability (Bj6rkholm et al., 1975). Adult thymectomy also seems to reduce the natural resistance against cancer: Souadjian et al. (1968), for instance, report that 31 out of 146 patients thymectomized for thymoma developed extra-thymic malignancies 10-15 years later.

In this context we consider our ultrastructural analysis of the parenchyma remaining in the normal age-involuted thymus of some value as a contribution to the understanding of thymic function in older persons.

References

Abe, K., Ito, T.: Fine structure of small lymphocytes in the thymus of the mouse: qualitative and quantitative analysis by electron microscopy. Z. Zellforsch. 110, 321-335 (1970)

Alexopoulos, C., Babitis, P.: Age dependence of T-lymphocytes. Lancet 1976 I, 426 Bargmann, W.: Der Thymus. In: Handbuch der mikroskopischen Anatomie des Menschen. Herausg.

W.v.M611endorff, Bd. VI/4, S. 100. Berlin: Springer 1943


Bj6rkholm, M., Holm, G., Johansson, B., Hhkan, M.: T-lymphocyte deficiency following adult thymectomy in man. Scand. J. Haematol. 14, 210-215 (1975)

Blanc, F., Fabre, M., Koritke, J.G.: Le thymus du nouveau-n~ humain et ses espaces p~rivasculaires- 6rude ultrastructurale. Arch. Anat. Hist. Embr. norm. et exp. 56, 223-242 (1973)

Bloodworth, J.M., Jr., Hiratsuka, H., Hickey, R.C., Wu, J.: Ultrastructure of the human thymus, thymic tumors and myasthenia gravis. Pathol. Annu. 10, 329-391 (1975)

Clark, S.L.: The thymus in mice of the strain 129/J studied with the electron microscope. Amer. J. Anat. 112, 1-34 (1963)

Droege, W., Zucker, R., Hannig, K.: Developmental changes in the cellular composition of the chicken thymus. Cell Immunol. 12, 186-193 (1974a)

Droege, W., Zucker, R., Jauker, U.: Cellular composition of the mouse thymus: developmental changes and the effect of hydrocortisone. Cell. Immunol. 12, 173-185 (1974b)

FrieB, A.: Interdigitating reticulum cells in the popliteal lymph node of the rat. An ultrastructural and cytochemical study. Cell Tiss. Res. 170, 43-60 (1976)

Gaudecker, B. v.: Elektronenmikroskopische Autoradiographie mit H3-Thymidin an der Thymusrinde der Maus. Z. Zellforsch. 72, 281-294 (1966)

Gaudecker, B. v., Hinrichsen, K.: Elektronenmikroskopische Untersuchungen zur Cytologic von Thymusrinde und Keimzentrum. Z. ZeUforsch. 65, 139-162 (1965)

Gaudecker, B. v., Schmale, E.M.: Similarities between Hassalr s corpuscles of the human thymus and the epidermis. An Investigation by electron microscopy and histochemistry. Cell Tiss. Res. 151,347-368 (1974)

Geyer, G.: Ultrahistochemie. Stuttgart: Gustav Fischer 1973 Gheshlaghi, M.E.: Altersver~inderungen der B- und T-Regionen im menschlichen Lymphknoten.

Inauguraldissertation Med. Fak., Kiel Univ. 1977 Hammar, J.A.: Die Menschenthymus in Gesundheit und Krankheit I. Das normale Organ. Z. mikr.-

anat. Forsch. (Erg/inzungsband) 6, 1-570 (1926) Heusermann, U., Stutte, H.J., Mfiller-Hermelink, H.K.: Interdigitating cells in the white pulp of the

human spleen (Short Communication). Cell Tiss. Res. 153, 415-417 (1974) Hinrichsen, K.: Zellteilungen und Zellwanderungen im Thymus der erwachsenen Maus. Z. Zellforsch.

68, 427-444 (1965) Hirokawa, K.: Electron microscopic observation of the human thymus of the fetus and the newborn.

Acta path. jap. 19, 1 13 (1969) Jones, D.L., Thomas, K., Williams W.J.: A fine structure study of human thymus. Beitr. Path. 156, 387-

400 (1975) Kameya, T., Watanabe, Y.: Electron microscopic observations on human thymus and thymoma. Acta

path. jap. 15, 223-246 (1965) Kaiserling, E., Lennert, K.: Die iuterdigitierende Retikulumzelle im menschlichen Lymphknoten. Eine

spezifische Zelle der thymusabh/ingigen Region. Virchows Arch. Abt. B 16, 51-61 (1974 a) Kaiserling, E., Stein, H., Mfiller-Hermelink, H.K.: Interdigitating reticulum cells in the human thymus.

Cell Tiss. Res. 155, 47-55 (1974b) K6bberling, G.: Autoradiographische Untersuchungen fiber Zellursprung und Zellwanderung in

lymphatischen Organen fetaler und neugeborener M~iuse. Z. Zellforsch. 68, 631-659 (1965) Kotani, M., Seiki, K., Yamashita, A., Horit, J.: Lymphatic drainage of thymocytes to the circulation in

the Guinea pig. Blood 27, 511-520 (1966) Lennert, K., Kaiserling, E., Mfiller-Hermelink, H.K.: T-associated plasma-cells. Lancet 1975, 1031-

1032 Matagne-Dhoossche, F.: Culture in vitro de fragments de thymus humain. Description des diffrrents

types cellulaires et 6tude de la myogrn+se. C.R. Acad. Sc. (Paris) (D) 275, 1529-1530 (1972) Matter, A.: Morphological definition ofthymocyte subpopulations. Cell Tiss. Res. 158, 319 332(1975) Metcalf, D.: The thymus. In: Recent results in cancer research. Berlin-Heidelberg-New York: Springer

1966 Miller, J.F.A.P.: T-cell regulation of immune responsiveness Ann. N.Y. Acad. Sci. 249, 9-26 (1975) Movat, H.Z.: Silver impregnation methods for electron microscopy. Amer. J. clin. Path. 35, 528-537

(1961) Miiller-Hermelink, H.K., Kaiserling, E., Lennert, K.: Pseudofollikul~ire Nester von Plasmazellen (eines

besonderen Types ?) in der paracorticalen Pulpa menschlicher Lymphknoten. Virchows Arch. Abt. B 14, 47-56 (1973)


Richardson, K.C., Jarett, L., Finke, E.H.: Embedding in epoxy resins for ultrathin sectioning in electron microscopy. Stain Technol. 35, 313-323 (1960)

Romeis, B.: Mikroskopische Technik. Miinchen-Wien: R. Oldenbourg 1968 Sainte-Marie, G., Peng, F.-S. : Emigration of thymocytes from the thymus: a review and study of the

problem. Rev. canad. Biol. 30, 51-78 (1971) Simpson, J.G., Gray, E.S., Beck, J.S.: Age involution in the normal human adult thymus. Clin. exp.

Immunol. 19, 261-265 (1975) Souadjian, J.V., Silverstein, M.N., Titus, J.L.: Thymoma and cancer. Cancer (Philad.)22, 1221-25

(1968) Stutman, O., Good, R.A.: Duration of thymic function. Ser. Haematol. 7, 505-523 (1974) Stutte, HJ., Mtiller-Hermelink, H.K.: Lysosomen in Blutzellen als diagnostischer Parameter. Verh.

dtsch. Ges. Path. 60, 155-175 (1976) T6r6, I., Olfih, I. : Penetration of thymocytes into the blood circulation. J. Ultrastruct. Res. 17, 439-451

(1967) Watanabe, Y., Tamaoki, N., Habu, S., Tashiro, Y., Akatsuka, S., Enomoto, Y.: Fine structural study on

human T- and B-lymphocytes. Acta haemat, jap. 37, 655 (1974)

Accepted May 13, 1977

Documents

Ultrastructure of the age-involuted adult human thymus