The stroma of the thymus of the rat: morphology and antigen diffusion, a reconsideration

The lymph node revisited: development, morphology, functioning, and role in triggering primary immune responses

Scenarios have been proposed to explain how lymphoid components of a lymph node favor the encounter of a drained antigen with a circulating competent naïve lymphocyte to trigger a primary immune response. However, these scenarios rest on incorrect concepts about the organ. This situation resulted from a loss of interest for studies on in vivo lymphoid organs due to a widespread switch, decades ago, to work on suspended lymphoid cells. However, an in vivo holistic study of the organ continued in our laboratory. The present review synthesizes resulting knowledge on lymph node morphology and global functioning. We show that the opening of an afferent lymphatic vessel into the subcapsular sinus is the focal point from which the related portion of a lymph node-a node compartment-is developed. As to the formation of a compartment's lymphoid components, it is neonatally orchestrated by the dichotomic nature and distribution of antigens in this subcapsular sinus, which determines a dichotomic recruitment of circulating cells and the compartment's architectural complexity. The transport process of an antigen from a given tissue territory into restricted sites of the draining compartment further defines its local morphological features and activities, while providing the possibility to reduce the wandering of a short-lived naïve cell through innumerable target-devoid sites. We also explain that the nodal lymphoid components are not implicated in the triggering of primary responses, but are rather products of such responses. Scenarios for the triggering of primary responses, consistent with real node morphology and functioning, are proposed.

Ultrastructural study of thymic microenvironment involution in aging mice

Aging involves morphological alterations of the thymus and deregulation of various immune response parameters. Altogether, these phenomena have been termed thymic involution. Using electron microscopy, we studied the morphological ultrastructure of the thymic microenvironment in aged mice. We observed cellular damages which progressively affected all the thymic stroma. At later stages (i.e., about 18-20 months old), a disappearance of the organ architecture with a drastic decrease in lymphocyte number was observed. The loss of cellular integrity of the microenvironment with lysis of cellular membranes and formation of a large and clear cytoplasmic layer engulfing a few remaining lymphocytes was noted. Extensive lipidic invasion surrounding the remaining epithelial cells grouped in nest formations and/or bordering cytics cavities was also present in these thymus from aged mice. Because the thymic microenvironment plays an important role in the "education" and functional maintenance of T cells and because the alteration of this cellular entity precedes a decline in certain immune functions, it can be suggested that membrane alterations, lack of cellular microenvironment integrity, and T cell dysfunction are correlated.

Thymic vascular system of the European common frog, Rana temporaria: a scanning electron-microscopic study of vascular casts

Vascular corrosion casts of the thymus of adult individuals of the European common frog, Rana temporaria, were analysed by scanning electron microscopy. The main arterial vessel, which is derived either from the temporal artery or from the auricular ramus, approaches the central territory of the gland and branches into "twigs" that, on penetrating the parenchyma, give rise to capillaries. Most of these capillaries run vertically towards the surface of the gland; they either join the superficial capillary plexus or follow this plexus for a variable distance and then run back towards the medulla, forming capillary loops. The former capillaries link with the extensive venous plexus composed of irregular meshes, whereas the latter capillaries join the venules at the cortico-medullary boundary and finally escape into collecting veins on the gland surface. The venous twigs, which join together near the gland, form the main thymic vein, which empties into the external jugular vein. The details of the thymic vasculature of the anuran amphibian, R. temporaria, are compared with those described in mammalian species, viz. the mouse, rat and guinea pig.

Thymic reticulum of mice. III. The connective compartment (innervation, vascularisation, fibrous tissues and myoid cells)

T lymphocytes interact at various levels of differentiation, with cells of the thymic reticulum, forming a peculiar and complex microenvironment. Following earlier descriptions by electron microscopy of three types of epithelial cells and two types of non-epithelial cells (macrophages and interdigitated cells) forming the thymic microenvironment, we report a study on a third compartment, the connective tissue, whose elements occur throughout the organ. The components of the capsule and trabeculae, the vascularisation and the innervation of the thymus and the presence of a few myoid cells are described. This is very rarely studied in ultrastructure. All these cells are completely imbricated and form a network trapping the lymphocytes, playing an essential role in the differentiation, maturation and selection of T cells.