Spleen and Thymus Flashcards

1
Q

what is the largest lymphoid organ?

A

spleen

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2
Q

function of spleen

A

filtering blood

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3
Q

describe spleen

A
  • surrounded by CT capsule (outer layer of mesothelium)
  • trabeculae from capsule to interior and anastamose (contains blood vessels)
  • splenic pulp: red and white
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4
Q

white pulp

A

surrounds and follows arteries - thickens to form splenic nodules

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5
Q

red pulp

A

occurs in irregular masses - splenic cords

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6
Q

where is the CT capsule heaviest in the spleen?

A

at the hilus where splenic blood vessels enter

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7
Q

what supports splenic pulp?

A

meshwork of reticular fibers

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8
Q

what types of lymphatics does the spleen have?

A
  • only efferent lymphatics

- in capsule, large trabeculae, and some white pulp

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9
Q

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

A

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

PALS = peri-arterial lymphatic sheath

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10
Q

how do splenic nodules form?

A

along a central artery

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11
Q

what is at the periphery of the white pulp?

A

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

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12
Q

what is splenic red pulp infiltrated with?

A

all elements of circulating blood - particularly macrophages

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13
Q

what supports red pulp?

A

reticular fibers and cells

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14
Q

function of macrophages in splenic red pulp

A

erythrophagia - removal of old RBCs

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15
Q

how do antigens enter spleen and how are they detected?

A

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

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16
Q

describe general spleen vascular supply

A

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

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17
Q

what are the three segments of pencillar arterioles?

A

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.)

18
Q

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

A

extensive processing and blood filtration

19
Q

cords of Billroth

A

splenic cords in red pulp - mixture of cells:

  • reticular cells (T3 collagen)
  • macrophages
  • other CT cells
20
Q

describe thymus

A
  • CT capsule w/ trabeculae dividing it into lobules

- each lobule = cortex and medulla

21
Q

thymus medulla

A

continuous from lobule to lobule via a common axial strand

22
Q

thymus cortex: what cells are present?

A
  • primarily T cells
  • epithelial reticular cells (large pale cells)
  • macrophages
  • large lymphocytes
23
Q

difference b/w cortical vs. medullary epithelial reticular cells

A

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

24
Q

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

A

T helper: CD4+, CD8-

T cytolytic: CD4-, CD8+

25
Q

do epithelial reticular cells (ERCs)make reticular fibers?

A

no

26
Q

what do cortical ERCs separate?

A
  • separate cortex from capsule

- surround blood vessels in cortex

27
Q

function of cortical ERCs

A

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

28
Q

which T cells progress from cortex to medulla?

A

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

29
Q

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

A

programmed for apoptosis

30
Q

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

A

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

31
Q

life span of thymic lymphocytes

A
  • most short (3-5 days)
  • undergo degeneration in thymus

-others leave to join circulating pool via high-endothelial post-capillary venules

32
Q

functions of thymic medullary ERCs

A
  • form ERC network
  • form thymic corpuscles (Hassal’s)
  • produce thymic hormones (thymosin, thymopoietin)
33
Q

what forms blood thymus barrier?

A
  • 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
34
Q

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

A

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

35
Q

what type of lymphatics does the thymus have?

A
  • only efferent lymphatics in interlobular CT

- NO afferent lymphatics, lymph sinuses, or lymphatic nodules

36
Q

what happens to thymus at puberty?

A

begins to involute, but still functions

37
Q

describe T cell selection: derivation, differentiation

A

-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
38
Q

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

A

sickle cells and spherocytes -> destroyed by macrophages

causes anemia, hyperbilirubinemia, splenomegaly

39
Q

splenomegaly

A

enlarged spleen

40
Q

what else can cause splenomegaly besides sickle cells or spherocytes?

A

portal hypertension - often associated w/ cirrhosis of liver

41
Q

DiGeorge syndrome

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