Spleen and Thymus

Question 1

what is the largest lymphoid organ?

Answer 1

spleen

Question 2

function of spleen

Answer 2

filtering blood

Question 3

describe spleen

Answer 3

• surrounded by CT capsule (outer layer of mesothelium)
• trabeculae from capsule to interior and anastamose (contains blood vessels)
• splenic pulp: red and white

Question 4

white pulp

Answer 4

surrounds and follows arteries - thickens to form splenic nodules

Question 5

red pulp

Answer 5

occurs in irregular masses - splenic cords

Question 6

where is the CT capsule heaviest in the spleen?

Answer 6

at the hilus where splenic blood vessels enter

Question 7

what supports splenic pulp?

Answer 7

meshwork of reticular fibers

Question 8

what types of lymphatics does the spleen have?

Answer 8

• only efferent lymphatics

- in capsule, large trabeculae, and some white pulp

Question 9

describe blood vessels as they leave the trabeculae in the white pulp

Answer 9

ensheathed in lympathic tissue (PALS) - w/ concentrations of T cells

PALS = peri-arterial lymphatic sheath

Question 10

how do splenic nodules form?

Answer 10

along a central artery

Question 11

what is at the periphery of the white pulp?

Answer 11

marginal zone w/ marginal sinuses where white pulp transitions to red pulp
-important for immune function

Question 12

what is splenic red pulp infiltrated with?

Answer 12

all elements of circulating blood - particularly macrophages

Question 13

what supports red pulp?

Answer 13

reticular fibers and cells

Question 14

function of macrophages in splenic red pulp

Answer 14

erythrophagia - removal of old RBCs

Question 15

how do antigens enter spleen and how are they detected?

Answer 15

enter via blood and are detected by antigen-presenting cells in splenic nodules - especially in marginal sinuses

Question 16

describe general spleen vascular supply

Answer 16

splenic a. -> course in trabeculae -> leave and get PALS, now are central a. -> formation of splenic nodules around these -> central a. branches w/i nodules to follicular/radial arterioles -> empty into marginal sinuses -> into pencillar arterioles (red pulp) -> empties into sinuses (closed circulation) or into red pulp (open circulation) -> venous or splenic sinuses -> splenic cords -> venous circulation

Question 17

what are the three segments of pencillar arterioles?

Answer 17

1st: pulp arteriole (retains thin smooth muscle sheath)
2nd: sheathed arteriole (acquires sheath of macrophages, reticular cells/fibers)
3rd: terminal capillary (empties into closed or open circ.)

Question 18

what is the region where the capillaries end in spleen a site of?

Answer 18

extensive processing and blood filtration

Question 19

cords of Billroth

Answer 19

splenic cords in red pulp - mixture of cells:

• reticular cells (T3 collagen)
• macrophages
• other CT cells

Question 20

describe thymus

Answer 20

• CT capsule w/ trabeculae dividing it into lobules

- each lobule = cortex and medulla

Question 21

thymus medulla

Answer 21

continuous from lobule to lobule via a common axial strand

Question 22

thymus cortex: what cells are present?

Answer 22

• primarily T cells
• epithelial reticular cells (large pale cells)
• macrophages
• large lymphocytes

Question 23

difference b/w cortical vs. medullary epithelial reticular cells

Answer 23

cortical: ectodermal - program which T cells will die and which ones live and progress to thymus medulla + add to blood-thymus barrier
medulla: endodermal - govern differentiation into T helper and T cytolytic cells

Question 24

T helper vs. T cytolytic cells: CD4 and CD8?

Answer 24

T helper: CD4+, CD8-

T cytolytic: CD4-, CD8+

Question 25

do epithelial reticular cells (ERCs)make reticular fibers?

Answer 25

no

Question 26

what do cortical ERCs separate?

Answer 26

- separate cortex from capsule | - surround blood vessels in cortex

Question 27

function of cortical ERCs

Answer 27

-present MHC I and MHC II complex to developing T cells

Question 28

which T cells progress from cortex to medulla?

Answer 28

T cells that recognize self-MHC but not self-antigen

Question 29

what happens to T cells that can't recognize MHC?

Answer 29

programmed for apoptosis

Question 30

what happens to T cells that recognize both self-MHC and self-antigen?

Answer 30

eliminated by negative selection by macrophages and dendritic cells in cortico-medullary jxn

Question 31

life span of thymic lymphocytes

Answer 31

- most short (3-5 days) - undergo degeneration in thymus -others leave to join circulating pool via high-endothelial post-capillary venules

Question 32

functions of thymic medullary ERCs

Answer 32

- form ERC network - form thymic corpuscles (Hassal's) - produce thymic hormones (thymosin, thymopoietin)

Question 33

what forms blood thymus barrier?

Answer 33

- continuous endothelium of capillaries - thick basement membrane of capillaries - sheath of ERCs around capillaries of thymus - antigens will penetrate the extravascular space of thymic cortex, but antigenic response is suppressed

Question 34

describe endothelium of post-cap venules in cortico-medullary jxn of thymus

Answer 34

thickened endothelium similar to high endothelial venules - lymphocytes migrate through here

Question 35

what type of lymphatics does the thymus have?

Answer 35

- only efferent lymphatics in interlobular CT | - NO afferent lymphatics, lymph sinuses, or lymphatic nodules

Question 36

what happens to thymus at puberty?

Answer 36

begins to involute, but still functions

Question 37

describe T cell selection: derivation, differentiation

Answer 37

-derived from bone marrow lymphoid progenitor cells - differentiation in thymus from subcortical -> cortical -> medullary thymocyte -> enter circulation as either Th or Tc cells - differentiate more after interaction w/ antigen-presenting cells

Question 38

what type of RBCs become trapped in splenic sinuses and what does this lead to?

Answer 38

sickle cells and spherocytes -> destroyed by macrophages causes anemia, hyperbilirubinemia, splenomegaly

Question 39

splenomegaly

Answer 39

enlarged spleen

Question 40

what else can cause splenomegaly besides sickle cells or spherocytes?

Answer 40

portal hypertension - often associated w/ cirrhosis of liver

Question 41

DiGeorge syndrome

Answer 41

- inherited immunodeficiency - lack of developed cortical epithalial cells - no T cell development (similar to nude mouse)