HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 20 LYMPHOID ORGANS

A INTRODUCTION

l Lymphoid cells mediating the immune response - lymphocytes, plasma cells, dendritic cells (APCs) and macrophages - occur:

(a) pathologically, at any site of chronic infection;
(b) regularly in the lamina propria of tracts exposed to antigens - respiratory, GI and genito-urinary, and the ocular conjunctiva; lymphocytes also migrate into the epithelia;
(c) in dedicated secondary lymphoid organs - spleen and lymph nodes.

2 Lymphocyte and APC traffic Lymphocytes circulate between tissue sites and blood and lymph flows; and APCs, such as the Langherhans cell, travel to nodes as the lymph-borne dendritic antigen-presenting cell - the veiled cell. Lymphocytes travel locally in a lamina propria of a mucosa.

3 The lymphoid cells are densely packed in rounded nodules/follicles in parts of the spleen and nodes. Aggregates of nodules occur in the tonsils, appendix and ileal Peyer's patches of the GI tract; whereas solitary nodules may exist anywhere in the mucosae of all 'open' tracts.
Wherever nodules may be found, close by are lymphoid cells dispersed more diffusely.

4 Most nodules have paler central regions - germinal centres, but these are not essential for cell proliferation. Germinal centres recruit virgin B cells, and follicular dendritic cells then present them with antigen. The B cells progressively refine their response to the antigen, in terms of Ig class, affinity, cell numbers, and whether to be plasma cells or memory cells.

5 The primary lymphoid organs - thymus and fetal bone marrow - store, release and confer competence on the lymphocytes that populate the secondary organs and CTs, but do not participate directly in defence. Primary-secondary Powerpoint.

6 Lymphocytes migrate in the blood and lymphatic flows for:
.. (a) the initial colonization of spleen, etc;
.. (b) a constant vigilant patrol by recirculation around the body, as memory or naïve cells;
.. (c) the propagation of an active immune response, as activated cells.

7 The secondary lymphoid organs provide:

(a) APCs/reticular cells and macrophages to activate lymphocytes;
(b) many lymphocytes to respond to a major antigenic challenge coming via the blood (spleen) or lymph (nodes);
(c) lymphocytes to propagate the immune response further, say, to recruit other nodes;
(d) a cleansing action by macrophages to remove undesirable materials from blood and lymph undesirable.

B MUCOSO-LYMPHOID ORGANS

1 Aggregates of nodules occur in the tonsils, appendix and ileal Peyer's patches of the GI tract; whereas solitary nodules may exist anywhere in the mucosae of all tubular systems open to the outside.
2 Wherever nodules may be found, close by are lymphoid cells dispersed more diffusely.
3 The gut- and bronchus-associated diffuse lymphoid tissues (GALT, BALT) are notable. MALT (mucosa-associated lymphoid tissue) usually refers to the unorganized lymphoid tissue of the gut.
4 Having an epithelium between the microörganisms and the connective tissue, where most of the lymphoid cells reside, poses problems:

(i) Over the nodules, special low columnar epithelial cells - M cells - develop in order to pass antigens to the underlying antigen-presenting cells in the lamina propria. The APC and lymphocytes sometimes lie in a pocket in the M cell. ('M' for microfolds on the M cell surface.)
(ii) The antibodies subsequently made by the plasma cells are immunoglobulins of a kind that the typical epithelial cells can take up basally, and secrete apically into the lumen needing protection.
(iii) It is also necessary for certain types of lymphocyte to enter the epithelium.

C LYMPH NODES

l Situation
Nodes are small bodies placed at intervals along the lymphatic vessels, and structured so that the lymph has to pass through them. Afferent lymphatics bring lymph from a drainage area. The node is responsible for combating intruders and confining infection to that area, by sending out antibodies and cells via efferent lymphatics.

2 Lymph-node structure
l A CT capsule, with some smooth muscle cells, sends in thin CT trabeculae, supporting a network of reticular fibres, and reticular cells of fibroblastic and the accessory dendritic kinds.
2 A denser outer cortex and a looser, inner medulla are present.
3 Efferent lymphatics leave at a hilus: the point of entry for blood vessels, serving a mostly cortical microvasculature.
4 Afferent lymphatics open through the capsule at several places to feed a system of 'sinus' channels running so:
subcapsular/marginal sinus --> cortical/intermediate sinuses --> medullary sinuses --> efferent lymphatics.
Sinuses are lined by reticular cells, accompanied by macrophages.
5 Denser masses of lymphoid tissue, extensive and follicular/nodular in the cortex, and continuing into the medulla as widely spaced medullary cords, have packed cells: lymphocytes, lymphoblasts and antigen-trapping dendritic reticular cells with processes. Lymphoblasts/centroblasts occur in the paler germinal centres of the cortical follicles.
The follicular zone contains B lymphocytes separated by follicular dendritic cells (FDCs).
6 The deeper lying paracortical region has mostly T lymphocytes, and dendritic APCs wrapping so intimately around lymphocytes that they received the name interdigitating reticular cells (IPCs).

3 Lymph-node functions

Mechanical filtration of lymph in the sinuses, trapping, for instance, soot carbon particles. Tumour cells are not so easily stopped.
Phagocytosis of materials in lymph by macrophages along the sinuses or lodged across them. The materials taken up include antigens, e.g., on bacteria. APCs and MØs process antigens for lymphocyte activation.
Proliferation of sensitized lymphocytes to become lymphoblasts, large, with little GER, but many ribosomes, stainable with pyronin. These lymphoblasts are the source of:
.. (a) plasma cells, and hence humoral antibodies, or
.. (b) cytolytic lymphocytes, which set out for their distant target - antigen in the drainage area - in the sinus lymph.
Recirculation of mature lymphocytes from venule blood to sinus lymph by migration through the cuboidal endothelium of the venules (high-endothelial venules - HEVs).

D SPLEEN

l Situation
Lies in the upper left of the abdomen, but there may also be small accessory spleens. It receives blood from the splenic artery for a treatment similar to that given the lymph by the node.

2 Splenic structure
l Thick fibro-elastic CT capsule has some myofibroblasts and a covering mesothelium.
2 Internally, thick CT trabeculae bear branches of the splenic artery and veins, entering and leaving at the hilum.
3 To the naked eye, most of the freshly cut organ is red pulp with white spots - white pulp.
4 Red pulp consists of a loose reticular tissue infiltrated with blood cells, and arranged in the so-called cords of Billroth around sinusoidal channels/sinuses - a Swiss-cheese situation of red-pulp cheese and sinusoidal holes.
The outermost white pulp, abutting the red pulp, is a boundary zone - the marginal zone, not to be confused with the mantle zone of densely packed mature lymphocytes around germinal centres.
(A mantle zone is not usually symmetrical; it is concentrated to one side of its germinal centre.)
5 Cord tissue has dendritic and fibroblastic reticular cells, and collagen fibrils supporting macrophages, and white and red blood cells.
6 Sinusoids/sinuses are lined by non-phagocytic endothelial/littoral cells, separated by slits and oriented longitudinally on a fenestrated BL. Blood cells thus can pass from sinusoid to cord and back, and cordal macrophages can extend pseudopodia into the sinusoidal lumen.
7 White pulp is a dense lymphoid tissue ensheathing branches of the arteries, once these have left the trabeculae. The sheath (PALS) dilates into follicles/nodules, some with germinal centres.
8 Lymphocytes are predominantly B in the nodules, and T in the periarterial lymphoid sheath (PALS). To match, reticular antigen-presenting cells are follicular/dendritic in the B-zone, interdigitating (IDCs) in the T-zone.
However, PALS and nodules/follicles work together, in that, the outer PALS is where B lymphocytes are initially selected for population-expansion in the nodules.

3 Splenic blood flow

Fed by the splenic artery, a trabecular artery branches out away from the CT as a
central artery (arteriole) of the white-pulp lymphoid sheath, which it supplies by small branches. The artery is not central in the nodules.
The arteriolar branches of the central artery turn towards the red pulp, as several very straight branches - penicilli/pulp arterioles.
The vessels become smaller, and some have discontinuities in the BL, and gain a sheath of macrophages - sheathed capillaries - before
the terminal capillaries open into a cord (Open Circulation Theory) or a sinusoid (Closed/Fast Circulation). Probably both kinds of termination exist.
Sinusoids and cords both contain blood.
Pulp venules collect the blood and carry it to
trabecular veins for return to the hilum,
and exit via the splenic vein.

Note that the spleen displays substantial species differences: the dog spleen has very muscular trabeculae; rodent spleens have a significant marginal sinus, along the white-red border; MØ-sheathed capillaries are not prominent in man, and lie in a perifollicular zone of red pulp, special to man.

4 Splenic functions
l Until birth, the spleen takes part in myelopoiesis, as do lymph nodes.
2 White pulp serves for:
.. (a) recirculation of lymphocytes;
.. (b) formation of new lymphocytes and plasma cells for immune responses to blood-borne antigens, met first at the marginal zone.
3 Red pulp provides:
.. (a) blood cleansing by the sequestration and phagocytic destruction by macrophages of unfit blood cells and platelets, and bacteria;
.. (b) metabolic breakdown of RBCs so that their iron can be reused;
.. (c) a place to accumulate platelets;
.. (d) sites by the marginal zone for plasma cells after antigenic stimulation, analogous to the cords and medulla of the active lymph node.

E THYMUS

l Situation and basic structure
l Lies in the upper midline of the thorax.
2 Markedly lobulated, with thin partitioning septa of fibrous CT, and adipose tissue which increases greatly with age.
3 In each lobule, a cortex surrounds a more palely staining medulla.
4 However, the medullary tissue is continuous from lobule to lobule as an axial cord.

2 Thymic finer structure
l Cells are:

(a) packed lymphocytes (thymocytes), less densely packed in the medulla, making it paler, supported by
(b) stellate epithelio-reticular cells of pharyngeal-pouch endodermal origin, not phagocytic, and with their processes attached by desmosomes (note that the main thymic stromal cell is thus an epithelial cell);
(c) pale interdigitating dendritic/reticulum cells in the medulla;
(d) a few macrophages in cortex and medulla;
(e) some myoid cells, resembling dystrophic skeletal muscle fibres;

2 Absent are afferent lymphatics, germinal centres, and significant numbers of reticular fibres.
3 Epithelio-reticular cells form concentrically lamellated, rounded, keratinizing, eosinophilic bodies - thymic/Hassall's corpuscles - in the older medulla.
4 Blood capillaries have intact basal laminae, few fenestrations in the endothelium, and an outside sheath of epithelio-reticular cells: all comprising the basis for a barrier hindering cells, e.g., B cells, and perhaps blood-borne antigens, from reaching the thymic cortical lymphocytes.

3 Thymic function
l Neonatal removal of the thymus causes the secondary lymphoid organs - nodes, spleen, tonsils, etc - to develop only partially and be unable to respond to many antigens.
2 Before birth, the thymus - a primary lymphoid organ - receives stem cells from the marrow that proliferate and undergo selection and maturation (by interacting with epithelial-reticular cells and APC reticular cells), before seeding out via the blood to populate the secondary organs with T or thymus-dependent immunologically competent lymphocytes.
Self-reactive lymphocytes are selected against, die, and are phagocytosed, while the surviving T lymphocytes migrate from subcapsular cortex towards the medulla.
3 At puberty the thymus starts a slow involution and replacement by adipose tissue, accelerated by severe stresses.
4 Despite the involution, the adult thymus maintains a low level of T-cell development from immature precursors that have not yet rearranged their TCR genes.
5 The thymus was assigned the status of an endocrine organ because the epithelio-reticular cells produce thymosin. It turns out that members of this family are not made solely by the thymus; and they act intracellularly as actin-binders and locally as cytokines.

4 More details of T-lymphocyte development based on the mouse
1 T progenitor cells arrive in the subcapsular region, where they multiply and express each its own pre-T cell receptor type.
2 This expression is used to select thymocytes to become cells that are double positive (CD4⁺8⁺) and expressing low levels of TCR-alpha/beta.
3 From encounters with peptide-MHC on the membranes of cortical thymic epithelial cells, some double-positive thymocytes - those with the appropriate TCR - are positively selected to become active and to downregulate either CD4 or CD8 expression to become mature CD8⁺ or CD4⁺ thymocytes.
4 Meanwhile, in both inner cortex and medulla, dendritic antigen-presenting cells negatively select, by the MHC-complexed presentation of self peptides, those thymocytes with TCRs for self antigens. Autoreactive thymocytes undergo apoptosis and removal by macrophages.
5 Thus, the thymocytes leaving the medulla as T lymphocytes have experienced positive selection and survived negative selection, and now await any further peripheral instruction on tolerance - how not to react with one's own tissues.
6 The story is similar, but more complicated for lymphocytes with gamma/delta TCRs.

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159