LYMPHOID ORGANS

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LYMPHOID ORGANS Lymphoid Tissue (Adenoid
Tissue) 2 tissue elements Reticular Tissue and
Cells (lymphocytes) fundamental building
material Can occur in various organs If alone
a lymphoid organ One of bodys largest organ
masses Reticular tissue component
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Reticular cells in typical arrangement Some are
primitive reticular cells Others are fixed
macrophages Argyrophil fibers ensheathed with
Reticular cell cytoplasm
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Free cells of lymphoid tissue Small
lymphocytes commonest cell Medium large
lymphocytes far less common Large forms divide
into medium lymphocytes Medium forms divide
into small lymphocytes Small lymphocytes can
grow into larger types Uncommon Free
macrophages in lymph gland sinuses splenic
pulp Plasma cells granulocytes
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LYMPHOID ELEMENTS Range from diffuse tissue to
large, complex organs Diffuse Lymphoid
Tissue Simplest form of lymphoid
tissue Infiltration into lamina propria of
mucous membranes Locations alimentary
repiratory tracts Intestinal location not
pure lymphoid tissue Collagenous elastic
fibers Other cells plasma, eosinophils No
special organization
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Lymph Nodules (Lymph Follicle / Primary
Nodule) Dense aggregations of lymphoid
tissue Masses up to 2 3 mm diamenter Most
common occurrence embedded in diffuse lymphoid
tissue (solitary nodules) 30,000 in intestinal
mucosa Other nodules components of lymphoid
organs (tonsil, lymph node, spleen) structural
unit TO UNDERSTAND THE LYMPH NODULE IS TO
UNDERSTAND HIGHER ORDERS OF ORGANIZATION!
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LYMPH NODULE General Characteristics Two
configurations homogenous with small
lymphocytes dominant Cortex (dark) Germinal
Center (lighter) Center cells paler,
larger Cortex reticulum denser concentric
arrangement Nodules not constant
(structurally in position)
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GERMINAL CENTER Absent in fetus old
age Inconstant features of nodules Sharply
separated from peripheral cortex Centers average
1mm diameter Cells larger, more cytoplasm, pale
nuclei (than cortex) Medium sized
lymphocytes Most actively dividing cells of
nodule Reticular cells (fewer) may also divide
in this area
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PROLIFERATIVE CYCLE OF GERMINAL CENTER Resting
nodules do not have centers Contain largely
small lymphocytes Periodically mitotic phase
begins Dominant dividing cells are medium
lymphocytes Some become large lymphocytes Primit
ive reticular cells give rise to some medium
lymphocytes also produce macrophages Dominant
cell of active phase is the small
lymphocyte Cell division creates peripherally
directed pressure (cortex is thus a compaction
region) Mitotic phase eventually ends ? begins
new inactive phase
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REACTION CENTER Inflammatory state ? central area
differs from Germinal Center Cells free
macrophages reticular cells New lymphocytes
not produced! Response to toxic agents and other
stimuli Function destroy toxic substance(s)
produce antibodies Commonest sites tonsils
appendix
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NODULE vessels and nerves Arteriole venule ?
peripheral plexus Second plexus forms when
germinal center exists Lymph capillaries
envelop exterior (do not invade) Nerve fibers
vasomotor
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AGGREGATE NODULES The next level small degree of
organization Grouped mass of lymph
nodules Visible bulge intestinal lining
Agminate nodules or Peyers patches Location
lamina propria (ileum) fewer in jejunum
duodenum Closely associated, pear shaped
nodules 25 30 patches gt25 sq. mm. size
variable project through muscularis muosa,
impinge on submucosa Lack definitive capsule
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Ileum, Nodular Group
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Ileum, Nodule
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TONSILS 3 tonsillar groups (Palatine, Lingual,
Pharyngeal) Structurally similar Add more
organization compared to aggregate
nodules Epithelial inpocketings Encapsulation
Coarse internal organization
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Palatine Tonsil Entrance to throat Ovoid, long
axis vertical Size 2.5 cm x 1.5 cm (1 x 0.5
inches) Medial surface exposed, bulges
outward Pits openings into crypts Dense mass
diffuse nodular lymphoid tissue Loosely
encapsulated, subdivided (septa) Free surface
epithelium continuous with that of
throat epithelium invaginates into crypts
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Capsule thin, poorly organized Compacted
fibrous C.T. Covers base sides only Septa
divide tonsil into 10 20 territories
(lobules) Cells in septa lymphocytes, mast
cells, plasma cells, neutrophils Lobules
center around crypts Reticular fibers permeate
parenchyma enter capsule, septa Epithelium
stratified squamous continuous with mouth
pharynx Basement membrane over loose C.T.
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Crypts extensive 40 simple tubes to complex
branched tubes Surface area of crypts 12 times
that of pharynx Cheesy plugs lymphocytes,
plasma cells, desquamated epithelial cells,
microorganisms, cellular debris Lymphoid tissue
fairly dense 1 2 mm. thick follows
epithelium (surface crypts) Lymph nodules
abundant Germinal centers unusually large
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Palatine Tonsil
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Pharyngeal Tonsil
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Associated structures Mucous glands in
submucosa ducts open into pharynx on or near
tonsil, rarely into crypts Skeletal
muscle Under capsule mucous glands Islands
of cartilage or bone (embryonic branchial arch
remnants)
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Lingual tonsils On root of tongue vallate
papillae aggregation of epithelial
pits lymphoid tissue around pits 35 40
units each is a low mound with central
pit pit simple crypt pit lined with
stratified squamous epithelium Lymphoid tissue
(1 5 mm. diameter) ? mound single layer of
lymph nodules with germinal centers Thin, C.T.
capsule Duct of mucous glands open into pits
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Pharyngeal tonsils (Adenoids) Size 3 cm.
long Location median doral wall of
nasopharynx Series of longitudinal folds of
pharyngeal lining Look like crypts in
longitudinal section No pocket-like
crypts! Epithelium pseudostratified
columnar Islands of stratified epithelium
(adults) Lymphocytic infiltration Lymphoid
tissue layer 2 mm. thick Lymph nodules with
germinal centers
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Capsule thin, septa septa project outward
into fold Mixed sero-mucous glands under
capsule Ducts open into bottom of folds
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Tonsillar Vascular Supply Blood vessels in
capsule septa and supply lymphoid
tissue Lymphatic vessels for plexuses around
lymphoid tissue
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Involution of Tonsils Maximum development in
childhood Pharyngeal tonsil begins decline at
puberty Atrophic in adult Pseudostratified
epithelium replace with stratified squamous
epithelium Palatine tonsil childhood,
puberty, adulthood? Replaced by fibrous
tissue Lingual tonsil mature slowly, last to
involute
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Lymph Nodes 4 advances over Nodules, Aggregate
Nodules, and Tonsils Complete capsule
trabeculae Two regions cortex
medulla Lymphatic vessels enter and
leave Internal lymph sinuses Lymph nodes
only in mammals Occur in groups or chains
definite regions prevertebral, mesentery, large
joints, along large arteries
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Structure bean-shaped 1 25 cm long convex
contour, except at hilus Fibrous capsule
entire gland Internal trestlework
trabeculae Spongework finer reticular
fibers Parenchyma 2 regions Cortex (lymph
nodules) Medulla (medullary cords) Extensive
lymph sinuses Afferent lymphatic vessels open
into sinus system Efferent lymphatic vessels
drain system at hilus
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Lymph Node
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Lymph Node
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Lymph Node
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Hemal Nodes Poor understanding of these
structures Rich content red blood corpuscles in
sinuses Organs are red/dark in color Occur
along ventral side of vertebrae Distinct
entities ruminants (sheep, ox) Humans
doubtful occurance Size up to size of
pea General structure much like lymph
node Sinuses purely blood sinuses no lymphatic
supply Trabeculae nearly absent
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Lymph nodules absent No obvious connections
with blood vessels! How do RBCs get into
sinuses? Resemble spleen in structure Contents
of blood vessels and lymphatics mingle in
sinuses (in the hog)