Introduction to The ThymusThe ancient Greek word uumos has two forms. The

form with an accent on the u, describes a wartyexcrescence (Simpson and Weiner, 1989). But if theaccent is on the o it is the name of a plant, thyme, or thesoul, the mood. The last form of the word is frequentlyemployed in medicine (lipothymia, cyclothymia). Thethymus, as a warty excrescence, was rediscovered byAmbroise Pare in 1560 (according to Dauzat et al.,1971). It remained an enigmatic organ until J.F.A.P.Miller showed, in 1961, that thymectomy of newbornmice resulted in animals that were immunodeficient(Miller, 1961). It was regarded as a DNA container(Dubreuil and Baudrimont, 1950) in the 1950s andDNA was initially called thymonucleic acid. Studentswere happy to be questioned on this organ, since therewas almost nothing to say about its function.Now, in the 1990s, it takes a long time for teachers to

explain the function of the thymus as a primary sourceof T lymphocytes, just as the bone marrow or the bursaof Fabricius is for B cells. This diarchy governs all thesecondary lymphoid tissues, including the spleen, lymphnodes, andmucosa-associated lymphoid tissues. Histolo-gists have an additional difficulty: they must find insitu the traces of the brilliant debates of modernthymology. There is a positive selection of thymocytes,with expansion of cells which can respond to foreignantigens in association with their ownMajor Histocom-patibility Complex (M.H.C.) molecules. But which cellsare responsible for that selection?The same questions can be asked about negative

selection. How are autoreactive thymocyte clones elimi-nated? But here there is, as in television games, asubsidiary question. What happens to themany deletedthymocytes, if they die in the thymus?Where, and at which step in the development, are the

T-cell receptors (TCR) and coreceptors CD41 or CD81

formed at the cell surface, when the thymocytes mi-grate from the subcortex to the medulla? Experimentalmodels have used in vivo antibodies, cyclosporin, lym-phoid irradiation, transgenic mice, mice with severecombined immunodeficiency (S.C.I.D.) that disruptsboth B and T cell development, or knock-out mice,which completely lack functional B and T cells. Theyhave shown that the maturation of thymocytes requiresan intact thymic stroma and that the integrity of thestroma depends on the presence of developing thymo-cytes (the cross-talk of Van Ewijk et al., 1994; thedialogue of Ritter and Boyd, 1993).Do any histological modifications take place during

the reciprocal exchanges between thymocytes and stro-mal cells, or are there only cell-to-cell contacts orsecretion of soluble factors? In the midst of the stromalcells, there are, above all, the epithelial reticular cells,peculiar among lymphoid organs with long processessurrounding thymocytes, desmosomes, tonofilamentsin the cytoplasm, and a characteristic staining withkeratin-specific antibodies.Do their ultrastructural features and immunopheno-

types vary according to their location and their rela-tion-

ships with thymocytes? If they are heterogeneous,would the contribution of the ectoderm in the hypoth-esis of a dual origin (endoderm plus ectoderm of thethird pharyngeal pouch) explain the heterogeneity?What about the morphological substrates of the endo-crine secretion of the epithelial reticular cells, in particu-lar storage granules? I have always had doubts aboutthe presence of these substrates, since my paper of thesixties (Izard, 1966).Since neurological events can affect immunological

functions, can we positively state that there is neuroen-docrine control of thymic activity?There are many other histological problems.Some were deliberately set aside in this topical issue.

Among them the Hassall’ corpuscles, since their struc-ture is apparently well known (whorled structures ofkeratinized cells seen in the medulla) and their func-tion (graveyard of reticular cells) does not raise toomany questions (until such time as it changes). I havealso avoided coverage of lymphoid progenitor cells, age,and seasonal changes, drug or hormonal modifications,and the thymus in pregnancy or disease.The authors of the papers of this topical issue have

attempted to answer some questions of thymologyusing histological techniques. The most widely usedtechniques are based on light or electron microscopeimmunocytochemistry. Monoclonal antibodies wereraised against keratin, clusters of differentiating leuko-cytes (CD), clusters of differentiating thymic epithelialcells (CTES), MHC class I and class II molecules,thymic hormones, hypophyseal hormones, and neuro-peptides. Some authors use enzyme histochemistry,e.g., for the thymic vessels or in situ hybridization tomonitor the local production of hypophyseal hormonesby the thymus. Histology also embraces other tech-niques, such as the enzymatic dissociation of tissues(for thymic nurse cells), the culture of thymic epithelialcells or dendritic cells, the isolation of cells by densitygradient separation and flow cytometry. It would be agood idea in another topical issue to cover the applica-tion of other modern techniques to the thymus. Thesemight include confocal microscopy, which is seldomused in this area, and lectin histochemistry, as we havedone in our study on Kurloff cells at the cortico-medullary junction of the thymus in the guinea-pigs(Oulhaj et al., 1993).The first paper by Bockman reviews the development

of the thymus especially the importance of mesenchy-mal derivatives from the neural crest and the stilldebatable contribution from the ectoderm. The secondpaper by Schuurman et al. is a light microscopy surveyof the major cell population, epithelial cells, with theirconsiderable morphologic and immunophenotypic het-erogeneity, and thymocytes (their migration from thesubcortex to the medulla, positive and negative selec-tion, death). In the third paper, De Waal and Rademak-ers demonstrate the ultrastructural heterogeneity ofthe thymic epithelium in the rat thymus using theusual techniques of fixation-staining. They found 4

MICROSCOPY RESEARCH AND TECHNIQUE 38:207–208 (1997)

r 1997 WILEY-LISS, INC.

subtypes in the cortex and 2 additional subtypes in themedulla. The next study by Von Gaudecker et al. usesgold immunoelectron microscopy to identify some phe-notypes of epithelial cells (C.T.E.S. II, III, V) anddemonstrate the presence of cytokeratins, thymulin,and M.H.C. molecules. Brelinska and Warchol thenexamine the thymic nurse cells, isolating the cells,describing their morphology and function, and discussthe problem of their presence in situ in the thymus.Lafontaine et al. in the sixth paper explore the humanthymic dendritic cells (interdigitating cells), bone-marrow derived cells, expressing high levels of M.H.C.class II molecules and acting as antigen-presentingcells. In the seventh paper Ropke provides new informa-tion, obtained by culturing thymic epithelial cells, onthe surface determinants for interactions with T cellprecursors and the repertoire of cytokines. This isfollowed by the paper of Kato on the post-capillaryvenules, the perivascular spaces and the lymphaticvessels, in particular at the ‘‘strategic’’ cortico-medul-lary-junction. Finally, Moll reviews the intricate inter-actions between the neuroendocrine and the immunesystem with special attention to the morphologicalsubstrates of the neuropeptides in the thymus and theinnervation of the thymus.

ACKNOWLEDGMENTSI thank all the authors, the reviewers, and Annick

Deshayes who handled all our massive correspondence(mail and fax) in a foreign language.

JACQUES IZARDCentre hospitalo-universitaire

14032 CAEN (France)

REFERENCESDauzat,A., Dubois, J., andMitterand, H. (1971) Nouveau DictionnaireEtymologique et Historique. Larousse, Paris.

Dubreuil, G., and Baudrimont, A. (1950) Manuel Theorique et Pra-tique d’Histologie. Vigot, Paris.

Izard, J. (1966) Ultrastructure of the thymic reticulum in guinea pig.Cytological aspects of the problem of the thymic secretion. Anat.Rec., 155:117–132.

Miller, J.F.A.P. (1961) Immunological function of the thymus. Lancet,2:748–749.

Oulhaj, N., Letaief, S.E., Landemore, G., and Izard, J. (1993) Glycocon-jugates with Neu5Ac(a2,6)Gal(b1,4)GlcNAc sequences: A selectivelectin-histochemical property of Kurloff cells in guinea pig thymus.Cell Tissue Res. 271:69–75.

Ritter, M.A., and Boyd, R.L. (1993) Development in the thymus: Ittakes two to tango. Immunol. Today, 14:462–469.

Simpson, J.A., and Weiner, E.S.C. (1989) The Oxford English Dictio-nary, 2nd edition. Clarendon Press, Oxford.

Van Ewijk, W., Shores, E.W., and Singer, A. (1994) Crosstalk in themouse thymus. Immunol. Today, 15:214–217.

208