Light and electron microscopy of the human fetal thymus

Dynamics of Thymus-Colonizing Cells during Human Development

Here, we identify fetal bone marrow (BM)-derived CD34hiCD45RAhiCD7+ hematopoietic progenitors as thymus-colonizing cells. This population, virtually absent from the fetal liver (FL), emerges in the BM by development weeks 8-9, where it accumulates throughout the second trimester, to finally decline around birth. Based on phenotypic, molecular, and functional criteria, we demonstrate that CD34hiCD45RAhiCD7+ cells represent the direct precursors of the most immature CD34hiCD1a- fetal thymocytes that follow a similar dynamics pattern during fetal and early postnatal development. Histological analysis of fetal thymuses further reveals that early immigrants predominantly localize in the perivascular areas of the cortex, where they form a lymphostromal complex with thymic epithelial cells (TECs) driving their rapid specification toward the T lineage. Finally, using an ex vivo xenogeneic thymus-colonization assay, we show that BM-derived CD34hiCD45RAhiCD7+ progenitors are selectively recruited into the thymus parenchyma in the absence of exogenous cytokines, where they adopt a definitive T cell fate.

Occurrence of adrenergic nerve fibers in human thymus during immune response

The adrenergic nerve fibers (ANF), the neuropeptide Y-like immunoreactive nerve fibers (NPY-NF) and the noradrenaline (NA) amount were studied in the human thymus in subjects previously treated or not treated with interferon therapy with the aim to identify the changes due to the interferon therapy. This therapy has been used in patients affected by multiple sclerosis (MS). Biochemical and morphological methods were used associated with quantitative analysis of images. The whole thymuses were removed during autopsies in young and adult patients not treated with interferon. Moreover, samples of thymus were removed from patients, either young or adult who had previously been treated with interferon therapy, and subjected, for diagnostic reasons, to thymic biopsy. All samples of thymus were weighed, measured and dissected. Thymic slices were stained with Eosin-orange for detection of the microanatomical details, or with Bodian's reaction for recognition of nervous structures. Histofluorescence microscopy was used for detection of ANF, and immunofluorescence microscopy for recognition of NPY-like immunoreactive structures. All morphological results were subjected to quantitative analysis of images. Noradrenaline contained in thymic structures was measured by biochemical methods. Our results only concerned the effects of the therapy and suggested that treatment with interferon therapy induces many changes in the thymic structures: (1) The protein content of thymus is significantly increased; (2) the NA content in the thymus is also significantly increased; (3) NPY-like immunoreactive structures in the thymus are significantly increased; (4) occurrence of NPY-like immunoreactivity is particularly and significantly increased both in thymic microenvironment and in structures resembling nerve fibers; (5) ANF are significantly increased in the same thymic structures in which NPY-like immunoreactivity is also increased (i.e. thymic microenvironment and structures resembling nerve fibers). The morphological and biochemical changes observed can also explain the immunological changes induced in the thymus after immunostimulating therapy.

Quantification of acetylcholinesterase-positive structures in human thymus during development and aging

Acetylcholinesterase (AChE) localization in the human thymus has been studied by biochemical and morphological methods during development and aging. The occurrence, the amount and the distribution of acetylcholinesterase and the changes with age were examined in 24 human thymuses. The whole human thymus was removed during autopsies in males of the following age-groups: prenatal of six months, new-born, infant, young, adult and elderly. The thymuses were weighed, measured and dissected: the microanatomical details were stained with Eosin-orange, nervous structures were identified by means of Bodian's method. Protein content was determined with biochemical methods. Histoenzymatical and biochemical demonstration of acetylcholinesterase was performed. The morphological results obtained were submitted to quantitative image analysis. Our results show that the thymic microenvironment changes with age; moreover, an increase of acetylcholinesterase-positive structures can be observed with age. Biochemical results are in agreement with morphological results and both are confirmed by the outcome of quantitative analysis of images. Acetylcholinesterase activity in human thymus may play a key role in thymic functions.

Heterogeneity of epithelial cells in the human thymus. An ultrastructural study

To evaluate interrelationships among epithelial cells, and between morphology and function in the microenvironment, we studied the ultrastructural morphology of epithelial cells in sections of human thymus from donors aged 2 months to 31 years. Six types of epithelial cells were observed: "subcapsular-perivascular" (type 1); "pale" (type 2); "intermediate" (type 3); "dark" (type 4); "undifferentiated" (type 5); and "large-medullary" (type 6). Cells of types 2, 3 and 4 were found throughout the organ. The type-2 to -4 epithelial cells may represent various stages in a differentiation process. In this, type-2 cells are very active and type-4 cells are possibly degenerating elements. Type-4 cells can also contribute to Hassall's corpuscles. Type-5 cells were located mainly in the cortico-medullary region and showed the morphological characteristics of undifferentiated elements. Type-6 cells were located exclusively in the medulla and displayed characteristics of cellular activity. Small Hassall's corpuscles consisted of type-6 epithelial cells; in larger corpuscles many nuclei of type-6 cells were found. Cells of types 2 and 6 contained tubular structures (diameter approximately 20 nm). Concerning the function of thymus epithelial cells, the features associated with protein synthesis observed in cellular types 2 and 6 make them likely candidates for humoral factor-producing and/or secreting elements. In addition, type-2 and -3 cells in the cortex appear to contribute to a special pattern of epithelium-lymphocyte interaction ("thymic nurse cells"), as demonstrated by the intracytoplasmic location of lymphocytes in the epithelial cells. The various steps in intrathymic T-cell maturation occur at locations in a microenvironment composed of morphologically distinct epithelial cells.

The liver hemopoietic environment: I. Developing hepatocytes and their role in fetal hemopoiesis

Mouse fetal liver was studied ultrastructurally to identify and characterize the developing hepatic parenchyma or prehepatocyte which may be responsible for producing the liver hemopoietic environment. It was observed that as the liver develops, there is close association of endodermal and mesenchymal cells in the region of the septum transversum. Numerous intercellular adhesions were observed between endodermal cells and mesenchymal cells. Twenty-four after endodermal and mesenchymal cells first intermingle, the liver extravascular space consisted of spherical hemopoietic cells dispersed among a heterogenous population of dark and light cells. The reticulum of prehepatocytes formed a three-dimensional cellular network which structurally supported the hemopoietic cells residing in the liver. By 12 days of gestation, prehepatocytes were a homogenous population of dark, stellate cells joined together by numerous intercellular adhesions. Broad areas of intercellular association were noted between processes and prehepatocytes and hemopoietic cells; however, no intercellular junctions between these two disparate cell populations were observed at this or any stage in development. Characteristics reflecting a cell population capable of synthesis and secretion of proteinaceous substances, namely, dilated Golgi apparati, increased numbers of polyribosomes and profiles of rough endoplasmic reticulum (RER), two types of vacuoles and/or vesicles, and intercellular microvillus-lined spaces, were observed in the prehepatocytes between 12 and 17 days gestation. By day 17 of gestation, glycogen accumulation, biliary channel development, appearance of a subendothelial microvillus surface, nuclear shape and chromatin pattern, and arrangement of cytoplasmic organelles reflected the maturation of prehepatocytes into hepatocytes, the adult liver parenchyma.

The thymus as haematopoietic tissue of non-lymphoid cells

Non-lymphoid haematopoiesis in the thymus was studied in 33 cases ranging from 14 weeks gestation up to 15 months postnatally. All thymuses displayed focally granulopoiesis in the connective tissue septa and in the lymphoid tissue, where it was localized in the peripheral cortex and at the cortico-medullary junction. Within the lymphoid tissue the granulocytic series showed many precursor cells with large rounded nuclei. Their origin could only be identified by the naphtol AS D chloroacetate esterase stain. In cases with thymic atrophy these precursor cells were present in relatively large numbers. Within the connective tissue septa many granulocytes with nuclear segmentation were intermingled with precursor cells. The erythroid series were detected with an immunohistochemical method for demonstration of haemoglobin. The positively stained nucleated cells occurred mainly in nests mostly localized in the peripheral cortex. These precursor cells were usually found in different places from the granulopoietic cells. No megakaryocytes were found. With appropriate techniques non-lymphoid haematopoiesis can be demonstrated in the developing thymus and must be considered as an expression of normal growth.

Ontogeny and organization of the stationary non-lymphoid cells in the human thymus

Ontogenetic differentiation of the human thymus was investigated in 50 embryos by means of light and electron microscopic methods in an attempt to clarify the morphogenesis of the complicated microecology of thymic tissue. At the 8th gestational week (g.w.), the primordium of the thymus contains almost exclusively undifferentiated epithelial cells. At the 10th g.w., the epithelial cells in the central part are spindle-shaped. During the subsequent weeks the cortical region of the thymus becomes separated into lobes by mesenchymal septa containing hemopoietic precursor cells and large electronlucent cells with irregularly shaped nuclei. The latter cells are also found in the deeper presumptive medullary regions of the thymus; they differentiate into interdigitating reticulum cells (IDC). The permeation of the medulla of the thymus by non-epithelial IDC occurs concurrently with the formation of cortical and medullary epithelial cells. Between the 12th and 14th g.w. the cortical and medullary differentiation is completed. At this time-stage cortical small lymphocytes differ in morphological shape from medullary lymphocytes, the latter acquiring the appearance of immunocompetent T cells and establishing intimate contact with the IDC. These findings indicate that the thymic cortex and medulla contain different epithelial cells. In addition, the thymic medulla displays cells characterized by the morphology of typical interdigitating reticulum cells of peripheral lymphoid tissue. The structural pattern of the thymus is correlated to morphologically differing lymphoid cell populations in the cortical and medullary regions.

The ultrastructure of the normal human thymus: a study of 36 cases

Electron microscopy of the normal human thymus demonstrates a supporting framework of epithelial-reticular cells with long branchticular cell processes lie lymphocytes, macrophages, and rare myoid cells. Both small and large lymphocytes are evident. No desmosomes are observed between the lymphocytes and the epithelial-reticular cells. Macrophages are most numerous in the cort(x were they often contain phagocytized nuclear debris. The possible functional significance of the above-described fine structural features is discussed.

Multinucleate cells in thymus of C3H mice

Multinucleate epithelial cells occur in the thymus of C3H mice. They are poorly differentiated and scarce, but are more numerous in the medulla than in the cortex. Their increase in number with age is particularly significant between the first and the third months especially for cells with a large number of nuclei, and may be related to thymic involution. Viral particles of type C, similar to those described in murine leukemias, are found in mono- and multinucleate medullary epithelial cells.

The normal human thymic vasculature: an ultrastructural study

Electron microscopy of the normal human thymus demonstrates a characteristic vascular-parenchymal relationship. The vascular lumen is always separated from the thymic parenchyma by: the endothelial cell cytoplasm, a muscular coat in arterioles and veins, the vascular basal lamina, a perivascular space containing collagen fibers and cells, the epithelial-reticular cell basal lamina and the epithelial-reticular cell cytoplasm. The width of this perivascular space is proportional to the size of the vessel it surrounds; it is wide around the vessels in the septa and at the cortical-medullary junction, but narrow around capillaries. While many cells are present in this space around larger vessels, only collagen is observed around the capillaries. Lymphocytes are the predominant cell type in the space; however, plasma cells, eosinophils, histiocytes, polymorphonuclear leukocytes, mast cells and unidentified granulated cells are also seen. The vascular complex described above may function as a blood-thymus barrier, as the initial site of exposure of the lymphocytes to circulating antigen and as the route of lymphocytes from the thymus.