Histology 2: Lymphoid Tissues Flashcards
What are primary and secondary lymphoid follicles?
Primary:
- Bone marrow: site of B cell maturation
- Thymus: site of T cell maturation
Secondary:
- MALT
- Tonsils
- Lymph nodes
- Spleen
- *GI tract to a lesser extent than above organs via GALT
- *MALT subdivided into GALT and bronchiolar-associated LT (BALT)
Describe the structure of dense lymphoid tissue or lymphoid nodules, composition, cells present, zones.
Compact, spherical nodule
Found in CT throughout the body
Composed of reticular CT
Contain cell types:
- fibroblasts
- lymphocytes
- antigen-presenting dendritic cells
- macrophages
- follicular dendritic cells
Zones:
B-cell zone = dense spherical aggregate
T-cell zone = less defined surrounding zone
Describe the B cell zone, primary and secondary follicles, germinal center and mantel
Are the primary composition of lymphoid follicles, contain mostly B cell structures but also has some TH cells.
Primary follicles:
- uniform aggregates of B cells
- storage site of memory B cells
Secondary follicles:
- in response to antigenic stimulation activated B cells migrate to lymphoid follicle and proliferate, creating a germinal center and converting the primary follicle to a secondary.
- Germinal Center: center portion of secondary, less densely packed.
- Mantel: surrounds germinal center, more densely packed, displaced cells from original primary follicle and maturing cells from germinal center.
Purpose of follicular dendritic cells in lymphoid follicle:
Express receptors for soluble immunoglobulinsl binds them and in turn binds antigens.
Does not endocytose the immunoglobulin receptor, store receptor and antigens bound to them on their surface for longer periods of time.
Cells are important for maintenance of memory B cells and in the selection of B cells with very high affinity antibodies.
Describe the T cell zone of lymphoid follicles, function.
Adjacent to lymphoid follicle proper and rich in T cells.
Activation pathways occur in this region
Contains the high endothelial venues (HEVs) associated with lymphoid follicles.
Tonsils; palentine vs. pharyngeal tonsils, structure, location
Located in wall of pharynx
Principal portion is PALATINE tonsil; located in posterior soft palate. PHARYNGEAL tonsil is located on posterior aspect of nasopharynx.
Smaller lingual (tongue) and auditory (tubal) tonsils also present.
Tonsils have their own CT capsule which separates them from surrounding tissue.
Tonsils are covered by epithelium lining of the pharynx (non-keratinized stratified squamous = palentine & lingual / ciliated pseudostratified columnar for pharyngeal & tubal)
Peyer’s Patches; location, shape
Located in ileum, consist of many aggregated lymph nodules containing T and B cells.
Pear shaped structure w/ apex just below intestinal epithelium.
Veriform Appendix; location, characteristics
Arises from Cecum
Lamina propria is infiltrated with lymphocytes and has many lymphatic nodules
Describe the embryological origin and adult location of the THYMUS.
Develops from epithelial endodermal lining of 3rd pharyngeal pouch and lymphoblasts that originate from bone marrow.
Beginning around 10wk. gestation the gland is invaded by lymphocytes and APCs, and this causes differentiation into cortex and medulla of thymus.
Reaches maximal size in childhood and gradually involutes after puberty.
Bilobed organ in adults located in mediastinum.
Describe the structure of the adult THYMUS.
Divided into lobules w/o nodules, is a paired glad straddling the midline of the body.
Describe the Capsule of the THYMUS.
CT covering of thymus.
Does invaginate the thymus but forms incomplete lobules w/ septa.
Each lobule has an outer-dark-staining cortex and inner-light-staining medulla.
Describe the Cortex of the THYMUS, what types of cells are present there, why does it stain dark on slides, are cells mitotically active here?
Extensive population of T-cells, macrophages in stroma of epithelial reticular cells.
Cortex stains darkly due to dense packing of lymphocytes and also because these are the lest differentiated T cells.
Cortex adjacent to the capsule (subcapsular region) contains actively dividing cells.
Understand the two-step lymphocyte selection process in the THYMUS, which step happens in the cortex or the medulla, which is positive or negative.
Step 1: Positive selection in Cortex.
-cells whose TCR recognizes and binds to self MHC (w/o bound antigen) are allowed to survive, provided the binding isn’t too tight.
Step 2: Negative selection in Medulla
-Lymphocytes are presented with self-antigens bound to MHC, cells that recognize and bind to these self antigens are induced to undergo apoptosis.
What are Thymic/Hassall’s corpuscles, where are they found in the thymus?
Found in Medulla.
Are structures formed from a type of epithelial reticular cell and consist of a concentric ball of cells; inner cells may be keratinaized, necrotic or calcified.
Helpful in identification of thymic tissue in slide prep.
Which vessels, afferent or efferent lymphatic, enter on the convex or concave surface?
Afferent: enter lymph node on convex surface
Efferent: enter on concave surface at hilum
