(C 29-35) Slide 145: human spleen (Groat’s stain)

In this slide note the capsule (29, c) and trabeculae (t), as well as the red (r) and white (w) pulp. In one or more of the larger trabeculae, you can identify a trabecular artery (C30, ta) and a trabecular vein (C30, tv). In many examples of the white pulp, you will be able to identify the central artery (C31, ca), even though it is seldom centrally located. Fine capillaries from the central artery course through the white pulp (C31, arrow) to terminate in the marginal zone. The white pulp contains a region around the central artery where lymphocytes are densely packed called the PeriArteriolar Lymphatic Sheath (C32 PALS). This is the zone of T cells. Outside of this region is a zone where lymphocytes are less densely packed - the Peripheral White Pulp (C32, PWP). This is the zone of B cells. At the edge of the PWP is the marginal zone (C32, mz), which contains many erythrocytes. Note that the boundary between the peripheral white pulp and the marginal zone is sometimes quite distinct and sometimes indistinct.

When the central arteries leave the white pulp and enter the red pulp, they are then called red pulp arteries or penicillar arteries (pa in C31 and C33). These "arteries" are really arterioles or metarterioles. After these vessels diminish to capillary size they acquire a sheath of macrophages and become sheathed arteries (C34, sa). These again are not true arteries but are actually modified capillaries. Try looking for them, but if you don’t find one, don’t be dissapointed, we will not hold you responsible for identifying them. The sheathed arteries are drained by a true capillary, which you need not look for. Such capillaries may open freely into the red pulp cords (open circulation) or directly enter into sinuses (closed circulation). Neither one of these features will be obvious on your slides. Since this spleen is human, open circulation predominates. The sinuses (C35, s) are the open spaces in the red pulp, whereas the red pulp cords are the compact areas between the sinuses. Sinuses are drained by venules, which you need not identify. These are drained by pulp veins (C35, v).