Lymphoid Organs-Histo

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thymus is present in the

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mediastinum

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fully development of thymus occurs in

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childhood w/ gradual involution following puberty

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What are the 3 embryonic sources for the thymus?

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ectoderm
endoderm
mesoderm

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Epithelial component of the cortex of the thymus is derived from

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ectodermal 3rd branchial cleft

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epithelial component of the medulla of the thymus is derived from

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endoderm of the 3rd & 4th pharyngeal pouches.

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Mesoderm gives rise to

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lymphocytes

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Thymus is surrounded by a capsule that subdivides the organ into lobules by way of

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penetrating septae.

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Each lobule consists of:

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Cortex
Medulla

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Cortex of the thymus is composed of an abundance of

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T lymphocytes
epithelial reticular cells
scattered macrophages

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T lymphocytes of the thymus are

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immature, many undergo apoptosis and subsequent degradation by macrophages

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Some t lymphocytes get to enter the medulla and gain access to

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venules, traveling through the bloodstream until they take up residence in lymphoid tissues as mature T cells

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Some t lymphocytes are also produced in

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bone marrow and thus migrate to the thymus for maturation

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epithelial reticular cells of the thymus cortex are _____in shape

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stellate in shape w/ light ovoid nucleus.

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Epithelial reticular cells of the thymus cortex are attached to neighboring reticular cells via

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desmosomes

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Cytokeratin intermediate filaments are found w/i the cytoplasm of

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epithelial reticular cells

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There are 3 subtypes of reticular cells which subdivide the cortex-isolating it form the rest of the body

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Type I cells
Type II cells
Type III cells

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Developing T cells can mature w/o contacting foreign antigens b/c of the

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3 subtypes of reticular cells

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Type I cells separate the

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cortex from the capsule

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Type II cells are found in the

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mid cortex & compartmentalize this region

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Type III cells are found in the

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lower cortex & at coticomedullary junction

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What do epithelial reticular cells do?

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secrete several hormones contributing to T cell maturation

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Type II and Type III cells play a major role along with antigen presenting cells of

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separting out developing T cells that recognize self epitopes or tha don't recognize MHC molecules - apoptosis-scavanged by macrophages

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In the medulla, which is located in the center of the lobule you may find:

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Hassall's corpuscles

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Hassall's corpuscles are

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concentrically arranged keratinized epithelial reticular cells w/ an unknown function.

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Same cells reside in the medulla, but in different

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proportions

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blood supplies of the thymus is due to

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arteries penetrating the capsule - run through septae.

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Arteries merge into arterioles, which course btwn the

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cortex & medulla

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Capillaries loop through the

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cortex & enter medulla, or directly enter the medulla

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All capillaries eventually drain into

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venules which return to the septae as veins

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There are no afferent lymphatics in the thymus: t/f

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true

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Occasional efferent lymphatics are found in the

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capsule & wall of large vessels in the thymus.

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function of thymus cortex

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This is where t lymphocytes rapidly divide & undergo terminal maturation into mature T cells. Most undergo apoptosis.

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Majority of remaining T cells of they thymus cortex are destined for the

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lymphnode paracorticol regions,
peyer's patches
spenic white pulp

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Thymus also secretes several growth factors which

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stimulate the division and differentiation of T cells

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Lymph nodes are

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encapsulated round or bean shaped organs

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Diameter of lymphnodes

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1-1.5 cm

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Lymph nodes act as

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filters against microorganisms and metastasizing tumor cells b/c of their serial arrangement

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All lymph passes through the filtration of lymph nodes prior to re entering the bloodstream t/f

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true

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indented portion of the lymph node is known as the

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hilium

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hilium functions as a

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vascular transit point

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lymph node is surrounded by a

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capsule

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lymph node capsule enters the parenchyma via

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trabeculae

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The lymph node has 3 parts:

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outer cortex
inner cortex
medulla

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outer cortex of the lymph node superficially bounded by

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supcapsular sinus

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supcapsular sinus subdivides into

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intermediate sinuses

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Intermediate sinuses merge with

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medullary sinuses

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Medullary sinuses run parallel to

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fibrous trabecular

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subcapsular sinus contains

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macrophages
reticular cells
reticular fibers

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a reticular fiber ntwk populated w/ B lymphocytes can be found in the

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outer cortex of lymph nodes

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Outer cortex is mainly

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ovoid lymphoid nodules w/ a lot of B cells undergoing proliferation

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immunocytes are the resulting cells of

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B cells undergoing proliferation in the outer cortex of the lymph node

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immunocytes are characterized by

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large nuclei w/ prominant nucleoli

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immunocytes eventually differentiate into

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memory or plasma cells

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What are germinal centers?

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lightly staining regions of memory/plasma cells.

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There are 2 types of lymphoid nodule:

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Primary nodule
secondary nodule

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Primary nodule is

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a round to ovoid zone of B lymphocytes entering or existing the node (temp resting phase)

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Secondary nodule is

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lymphoid nodule characterized by the presence of germinal centers

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Each germinal center of the secondary nodule has 3 zones:

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dark zone
basal light zone
apical light zone

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dark zone of secondary nodule germinal center contains

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numerous tightly packed proliferating B cells.

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B cells in the dark zone do not have receptors, so they are known as

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centroblasts

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Basal light zone contains

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centroblasts that have migrated from the dark zone.

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Where do B cells have receptors & exhibit class switching?

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Basal light zone

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In the basal light zone, centroblasts become

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centrocytes

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Follicular dendritic cells present antigen to B cells in the basal light zone, b cells that can't synthesize functioning B cell receptors undergo

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apoptosis

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Apical light zone contains

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fully functioning cells that have migrated from the basal light zone. B cell can leave the follicle after differentitating into memory or plasma cells

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The inner cortex of lymphnodes is also known as

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paracortex

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Paracortex consists mainly of

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T lymphocytes

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in the paracortex region, antigen presenting cells show the epitope-MHC II complex to TH cells - upon activation what happens?

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TH cells proliferate & expand the paracortex (inner cortex) of the lymphnode

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TH cells then enter_____ after expanding the paracortex region

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medullary sinuses & exit the node

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Medulla of lymphnode is comprised of

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medullary cords inhabited by numerous B lymphocytes & plasma cells.

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Series of vascular like structures are known as

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medullary lymphoid sinuses

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Medullary lymphoid sinuses present in the medulla of lymph nodes resemble

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subcapsular sinus in that they are partially lined by reticular cells & macrophages & its transmits lymph

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Blood & lymph supply of the lymph node consists of a

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series of afferent lymphatics which pierce the capsule & empty into the subcapsular space

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Lymph goes to

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intermediate sinuses, then medullary sinueses

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any foreign material is scavanged by

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marrow derived dentritic cells & macrophages

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Lymph then passes into the efferent lymphatics that exit the node at the

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hilium

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hilium is the point where

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arteries enter the node to form capillary plexueses around the lymphoid nodules & an exit point for veins

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Lymph node function:

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lymph filtration & antigen recognition

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when lymph enters the node, there is substanial

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slow down in velocity -allows for macrophages in the sinuses to engulf foreign matter & remove it from circulation

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Nearby antigen presenting cells migrate to the lymphnode to

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present their epitope MHC complex

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Resident follicular dendritic cells also acts as antigen scavengers so that

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any free antigen flowing through the node is entrapped & presented.

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A B cell that is receiving an antigen gets activated, it migrates to

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a primary nodule and undergoes rapid division resulting in a germinal center.

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Daughter cells differentiate into

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plasma cells & memory cells & enter the medulla to form medullary cords

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some plasma cells remain in medullary cords to

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continue the formation of antibodies & secrete them into sinuses.

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Most plasma cells migrate to the bone marrow and

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produce antibodies.

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the largest lymphoid organ in the body is the

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spleen

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spleen is located in the

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upper left quadrent of the abdomen, anterior to the superior portion of the kideny, posterior to the stomach.

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dimensions of the spleen

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12 cm long and 7 cm wide

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Spleen is covered by

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a dense irregular fibrous ct capsule that contains occasional smooth muscle cells which give rise to fibrous trabeculae, extend to parenchyma

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There are no lymphatics in the splenic parenchyma but lymphatics are found only in

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fibrous trabeculae

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Spleen has an abundance of

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lymphocytes
macrophages
antigen presenting cells
ntwk of reticular fibers

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2 major regions of spenic parenchyma:

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red pulp
white pulp

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blood supply of spleen: splenic arteries subdivde into

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trabecular arteries of varying size after entering the hilium

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Upon entering the parenchyma, the arteries b/c surrounded by

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Tcells forming the periarterial lymphatic sheath (PALS)

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arteries = central arteries when surrounded by

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T cells

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Some branches of the central arteries leave the white pulp forming small calibre straight

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penicillar arteries

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At the terminus of penicillar cells, there are

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reticular cells
lymphocytes
macrophages surrounding the terminus of penicillar cells

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2 theories of splenic circulation:

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1. closed
2. open

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in closed circulation the blood moves from

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capillaries directly into sinusoids without exposure to the splenic cords

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in open circulation the blood

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enters the parencyma of the spleen by passing through the splenic cords and then entering the sinusoids

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open circulation is

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favored

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White pulp of the spleen is comprised of

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lymphoid tissue surrounding the central arteries & lymphoid nodules associated w/ PALS.

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lymphoid tissue adjacent to the central arteries is largely

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T cells

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B cells account for the majority of the cells at the periphery of the

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PALs and in nodules.

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White pulp is separated from the red pulp by a

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marginal zone containing plasma cells, T & B cells, macrophages & interdigitating dendritic cells

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vascular spaces in white pulp are known as

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marginal sinuses

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Marginal sinuses account for the site

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of first introduction of circulating cells, antigens, & foreign matter to the splenic parenchyma.

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What occurs at marginal sinuses?

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antigen presenting cells scavage circulating antigens
microorganisms are attacked by macrophages
B & T cells leave the circulation & enter/exit white pulp
lymphocytes communicate w/ dendritic cells

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lymphocytes communicating w/ dendritic cells result in an

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immune response forming in the white pulp if the epitope MHC complex is recognized.

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Red pulp of the spleen consists of

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sinusoids & meshwork of reticular fibers & reticular cells = splenic chords

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Splenic chords/Billroth's cords w/ admixed macrophages,lymphocytes (B&T), erythrocytes, granulocytes,platelets, & plasma cells are found in

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red pulp

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Sinusoids are composed of

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discontinous capillaries w/ large gaps btwn endothelial cells
no basal lamina
surrounding ntwk of reticular fibers

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Functions performed by the spleen

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lymphocyte production
erythrocyte destruction
microbiologic defense
extramedullary hematopoiesis
blood resevoir

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lymphocyte production in the spleen occurs in the

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white pulp, migration to red pulp,then enter circulation via sinusoids

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erythrocyte destruction occurs in the spleen - determining factor is in the changes

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of character & flexibility of erythrocyte cell membranes.

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erythrocytes are scavenged by

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macrophages, broken down & some constituents are recycled into the blood stream

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In regards to microbiologic defense, bacteria & viruses passing through the spleen & not vital foreign material are engulfed by

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macrophages which then carry out an immune response in conjunction w/ B & T lymphocytes & antigen presenting cells

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What is MALT?

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Mucosa associated lymphoid tissue - aggregates of lymphoid tissue w/i the mucosa & submucosa of GI, respiratory & GU tracts.

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Scattered immune cells are present on the skin but do not constitute

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MALT

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Tonsils are

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large aggregates of partially encapsulated lymphoid tissue

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Tonsils are w/i the

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oropharynx & nasopharynx

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3 types of tonsils:

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palatine tonsils
pharyngeal tonsil
lingual tonsil

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Palatine tonsils are located

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posterior to the palatoglassol arch & anteriort o the palatopharyngeal arch on the lateral oropharynx

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Palatine tonsils are comprised of a mass of

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lymphoid tissue w/ muiltiple lymphoid nodules showing germinal centers

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Numerous invaginations of oral mucosa (covered by stratified sq. epithelium) form

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prominent crypts

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A "capsule" is formed subadjacent to the lymphoid tissue which is made up of

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a zone of dense irregular fibrous ct

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Pharyngeal tonsil is a

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solitary tonsil residing in the posterior superior nasopharynx

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mucosa of pharyngeal tonsil is surfaced by areas of

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stratified squamous & respiratory epithelium

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Do pharyngeal tonsils have crypts?

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no

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Capsule is thinner in pharyngeal tonsils than in

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palatine tonsils

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Lingual tonsils are

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multiple small tonsils on the tongue base

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Each tonsil of the lingual tonsil possesses a single

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crypt & is covered by stratified squamous epithelium.