Plastische Rekonstruktion des bindegewebigen Ger�stes der Milz eines Neugeborenen

Fetal and early post-natal development of the human spleen: from primordial arterial B cell lobules to a non-segmented organ

Immunohistological analysis of 31 human spleens from the 11th week of gestation to the early postnatal period suggested that fetal organ development may be preliminarily divided into four stages. At stage 0 the organ anlage contained erythrocyte precursors, few macrophages and almost no lymphocytes. Fetal spleens of stage I exhibited arterial vascular lobules and lymphocytes just began colonizing the organ. At stage II, B and T lymphocytes formed periarteriolar clusters. B cell clusters predominated, because B cells aggregated around the more peripheral branches of splenic arterioles, while T cells occupied the more centrally located parts of the vessels. The vascular lobules of stage I and II consisted of central arterioles surrounded by B cells, capillaries and peripheral venules. The lobular architecture slowly dissolved at late stage II when sinuses grew out from the peripheral venules into the centre of the lobule. Interestingly, the B cell accumulations around peripheral arterioles did not represent the precursors of follicles, but apparently persisted as periarteriolar B cell clusters in the adult splenic red pulp, while follicles containing FDCs developed at late stage II from B cells in direct contact to T cell clusters around larger arterial vessels. At stage III before birth the lobular architecture was no longer recognized. The chemokine CXCL13 was already present in vascular smooth muscle and adjacent stromal cells at stage I before B cells immigrated. CCL21, on the contrary, was only demonstrated in fibroblast-like cells supporting T cell clusters from stage II onwards.

Splenic architecture reflected in the connective tissue structure of the human spleen

An analysis of the connective-tissue structure of the human spleen can give us information about the basic architecture of the organ. The most important part of the spleen is the lienic center around which the subcapsular zone forms an envelope, like a mantle. This zone has but little depth and develops superficially. The tangential radial beam net ('Tangentialbalkennetz') is formed partly by the radial trabeculae of the capsule and partly by the outer branches of the arbor trabecularis. This arbor divides into 5-6 branching orders. The branches of orders 1 to 3 surround the parenchyma of the spleen center's inner layer. The lienic lobuli which are found between the branches are relatively large and are connected very extensively with their parenchyma. The branches of orders 4, 5, and 6 enclose the lienic lobuli of the outer layer of the spleen center. The splenic lobuli are defined by the vascular course. Mostly they are provided with one or two arterial influxes and, as a rule, with only one venous drain. Their mutual delimitation is more of a functional than of a morphological nature. This led von Herrath to coin the term 'functional spleen lobuli'. The lienic envelope lies between the inside of the capsule and the outermost branchings of the arbor trabecularis. This arbor is subdivided, by the radial trabeculae, which never have any vessels, into elongated lobuli and serves first and foremost to regulate pressure. The lattice fibers are of high tensile strength and are extensions of the collagenous fibers seen at the microscopic level.