1. Epithelial component of cortex derived from the ectodermal 3rd branch cleft

2. Ep. components of medulla derived from ectoderm of 3rd and 4th pharyngeal pouches

3. Mesoderm gives rise to the lymphocytes

-T lymphocytes-numerous but immature

-Epithelial reticular cells

-Macrophages

Numerous but immature.

Some degraded by macrophages.

Others enter medulla-> venules, bloodstream -> find lymphoid tissues and mature.

Some t cells come from bone marrow to mature in the thymus

-Stellate shaped, light oviod nucleus

-Attached to neighbors by desmosomes

-Cytoplasm contains cytokeratin interm. filaments

-3 subtypes that subdivide cortex (t cells can mature without contacting antigens)

-Secrete hormones for t cell maturation

1. Type I - Separate cortex from capsule

2. Type II - found in mid-cortex, divide region

3. Types III - in lower cortex and corticomedullary junction

Separate out develping T cells that recognize self epitopes or don;t recognize MHC molecules.

Bad ones are killed, good ones enter medulla

-Center of lobule

-Contains Hassall's corpuscles - concentrically arranged keratinized pink blobby things (dead reticular cells)

-Arteries penetrate capsule, run through septae

-Merge into arterioles that course between medulla and cortex

-Capillaries loop through cortex, enter medulla

-Drain into venules that return through septae

NO afferent lymphatics in thymus, possible efferents in capsule

-Place for T cells to mature (secrete some growth factors)

-Mature cells mostly destined for:

1. lymph node paracortical regions

2. Peyer's patches

3. Splenic white pulp

-Round or bean shaped organs

-Situated along lymphatic routes in body

-Act as filters against microorganisms and metastasizing tumor cells

-All lymph passes through before back to blood

-Bounded by subcapsular sinus, subdivides into intermediate sinuses, merge wih medullary sinuses.

-Subcap. sinus contain macrophages, reticular cells and reticular fibers

-Reticular fiber network filled with B cells

-Large prtion of network filled with lymphiod nodules (contain proliferating B cells called immunocytes)

- Immunocyes have large nucleol, turn into memory + plasma cells

Collection of nodules known as Germinal Centers

1. Primary - round/oviod zone of B cells entering/exiting node

2. Secondary - nodule with presence of germinal centers

1. Dark - tightly packed proliferating B cells, no receptors -> Centroblasts

2. Basal light - centroblast migrating from dark zone. Express receptors, switch IG classes -> Centrocytes

Checked if good by presented antigens (if bad, then killed)

3. Apical light - fully functioning B cells. Leave follicle after differentiating into memory and plasma cells

-Mainly T cells

-Anitgen presenting cells show epitope-MHC II comlex to T helper cells.

-Once activated, T cells proliferate and expand region

-Cells enter medullary sinuses and exit node

-Comprised of medullary cords inhabited by B and plasma cells

-Medullary lymphiod sinuses in this region resemble subcapsular sinus (lined with reticular cells and macrophages and transmit lymph)

-Afferent lymphatics pierce capsule, empty into subcapular space

-Lymph ->intermediate sinuses-> medullary sinuses ->out via efferent lymphaticsat at hilium

-Foreign material eaten by macrophages and dendritic cells

- Arteries enter at hilium, form capillary plexuses, exit at hilum

-Lymph filtration and antigen recognition

-Lymph slows down for macrophages to eat stuff

-site where antigen presenting cells migrate to shoe epitope-MHC complex

-migrates to a primary nodule, undergoes rapid division -> germinal center

-Daughter cells become plasma and memory cells, enter medulla to form medullary cords

-Some plasma cells remain and secrete antibodies in the sinuses

-Largest lymphoid organ in body

-Upper left abdomen

-Surrounded by dense irregular CT capsule which extends into parenchyma via trabeculae

- Hilum provied entrance and exit for nerves/vessels

-NO lymphatics in parenchyma, ONLY in trabeculae

-Contains abundance of lymphocytes, macrophages, antigen presenting cells and reticular fiber network

1. Red pulp

2. White pulp

-Arteries -> trabecular arteries after enting hilium

 -When enters paranchyma, gets surrounded by T cells -> Forms periarterial lymphatic sheath (PALS)

-Now termed CENTRAL arteries

-Some branch off and leave white pulp, called PENICILLAR arteries (terminus surrounded by reticular cells, lymphocytes, macrophages)

1. Closed - blood moves from capillaries directly into sinusoids without exposure to splenic cords

2. Open - blood enters parenchyma by passing through splenic cords, then enters sinusoids

FAVORED THEORY

-lymphiod tissue surrounding the central arteries and lymphiod nodules associated with PALS

-lymph tissue adjacent to central arteries is T cells

-B cells is majority at periphery of PALS and nodules

-Separated from red by Marginal Zone

-Vascular spaces are Marginal Sinuses

-Introduction site of spleen to circulating cells, antigens anf foreign matter to parenchyma

-Contains plasma cells, T and B cells, macrophages and dendritic cells

-Separated White pulp from Red pulp

-Antigen presenting cells scavange circulating antigens

-Microorganisms are attacked by macrophages

-B and T cells leave circulation and enter/exit white pulp

-Lymphocytes communcate with dendritic cells ->immune response is epitope-MHC complex is recognized

-Discontinous capillary sinusoids and reticular fiber network called Splenic Cords (Billiroth's cords)

-Macrophages, T and B cells, red blood cells, granulocytes, platelets and plasma cells

1. Lymphocyte production - produced in WP -> RP -> circulation

2. Erythocyte destruction - determined by flexability (spectrin)

3. Microbiologic defense - macrophages engulf bacteria/viruses, trigger immune response with B, T and AP cells

4. Extramedullay hematopoiesis - outside bone marrow

5. Blood reservoir

-Posterior to palatoglossal arch and anterior to palatopharangeal arch on the Lateral Oropharnyx

-Lymph tissue with germinal centers

-Oral mucosa invaginates (Strat SE) to create crypts

-Dense irregular CT capsule

-Solitary tonsil in posterior-superior nasopharynx

-covered by strat SE and respiratory Ep.

-NO crypts, thin capsule

-Multiple, small, at tongue base

-Each possesses a single crypt

-Covered in Strat SE