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mse263
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Nearly all oocytes in females are arrested in:
prophase of Meiosis I from a few months before birth until just before or during ovulation
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Lymphoid tissues
- masses of lymphocytes and associated cells required to mount an immune response
- provide an environment that promotes immune cell-antigen interaction
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What are the two ways lymphoid tissue may exist?
- 1. as discrete organs covered by either an epithelium or connective capsule
- 2. may exist as isolated masses of cells withIN
- various organs in close proximity to the outside world (lumen)
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These are the 4 lymphoid tissues we learned:
 diagnostic features: highly cellular, only a little CT, lots of cells per area, LARGE nucleus, small cytoplasm, mostly euchromatin (dark) present versus heterochromatin (light)
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primary lymphoid organs
- 1. bone marrow - development of immunocompetent B-cells

2. thymus - development of immunocompetent T-cells
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secondary lymphoid organs (3)
lymph nodes, spleen, mucosa associated lymphoid tissue
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thymus
- representative primary lymphoid organ where T-cells develop

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T lymphocyte
cells derived from the thymus and differentiate into several sub types (cytotoxic, helper, regulatory, etc)
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B lymphocyte
- type of lymphocyte is specialized to recognize foreign antigens and, once activated, to produce antibodies specific to those antigens
- activation of B-cells requires helper T lymphocytes
- in order to produce antibodies, activated B cells must move to connective tissues where they undergo terminal differentiation into plasma cells
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T-cells primary enact ________ immunity while B-cells primarily participate in _________ immunity
- T-cell: cell-mediated immunity
- B-cell: humoral immunity
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opsonization
when antigens are marked for an immune response by molecule that enhances phagocytosis
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thymic epithelial cells form a ____________ that becomes infiltrated by T-cell precursors called:
- the cytoreticulum (thymic epithelial cells) gets infiltrated by THYMOCYTES
- cytoreticulum acts as the sole physical support for the developing T-cells
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What is this?
- the thymus
- "starry sky" appearance
- CT capsule
- continuous brached medulla
- septa - CT that separates lobules
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thymic cortex contains epithelial cells that function to:
- mechanically support ‘nests’ of developing thymocytes
- produce thymic hormones that to promote T cell maturation
- stimulate ectopic expression of “self” proteins so thymocytes learn to distinguish self from non-self
- contribute to the formation of the blood-thymus barrier
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- the thymic cortex contains developing lymphocytes (dark pink) and macrophages (white gaps, "stars")
- *up to 95% of thymocytes fail to become immunocompetent and die by apoptosis
- that's why the medulla is lighter than the cortex, because fewer mature T-cells are allowed to enter there, where they can potentially be exposed to the rest of the body
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Is there connective tissue (from the CT capsule and running through the septa) in the thymic medulla?
- YES only connective tissue it has
- separated from thymus by epithelial layer
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What are the brown stains?
- epithelioreticular cells
- support the lymphocytes in the medulla & cortex
- in the medulla they can form Hassall’s corpuscles
- also function in the weeding out of T-cells that aren't immunocompetent
- (white = macrophages, dark purple dots = lymphocytes)
- in general stains: appear lighter than lymphocytes
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blood-thymus barrier
- prevents foreign antigens from getting into the compartment where thymocytes are developing and becoming immunocompetent
- composed of an inner endothelial layer and an outer epithelioreticular layer with a thick basement membrane in between that serves as a barrier for foreign material
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What might happen with a faulty blood-thymus barrier?
- without the barrier a foreign protein could enter the thymus, it will be treated as derived from 'self', and all T-cells will be killed off that bind to it
- if you later come in contact with this foreign antigen IT WON'T BE RECOGNIZED AS FOREIGN
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- thymic medulla
- has fewer cells (appears lighter), contains numerous blood vessels, many macrophages, epithelial cells that promote expression of self-peptides, Hassall’s corpuscles, & a cortico-medullary boundary characterized by post-capillary, high endothelial venules where immunocompetent T cells exit the thymus to enter the general circulation
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- Hassall's corpuscles
- concentric layers of epithelioreticular cells that secrete thymic stromal lymphopoietin (TSLP) which stimulates differentiation of regulatory T-cells
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What persists in an involuted thymus?
- Hassall’s or thymic corpuscles
- also has adipocytes present

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What is the difference between an encapsulated secondary lymphoid tissue and an unencapsulated secondary lymphoid tissue?
- encapsulated: covered by CT
- unencapsulated: covered by epithelium, associated w/ mucosa
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ALL secondary lymph organs contain BOTH nodular and diffuse lymphoid tissue
- nodulues (follicles): B-cells & in the center a germinal center
- diffuse: T-cells found between nodules and in the connective tissue underlying most epithelia (NO germinal centers)
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Why aren't all B cells daughter cells the same if they divide by mitosis?
- affinity maturation
- cells with the strongest affinity for antigens due to mutations will be selected over those that bind less strongly
- selected cells must migrate to a connective tissue to fully differentiate as antibody producing plasma cells
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Will plasma cells (differentiated B cells) appear in nodules?
NEVER: B cells terminally differentiate in plasma cells only in CT
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examples of encapsulated secondary lymphoid tissue
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- Lymphoid Nodules (lymph nodes)
- has a CT capsule (encapsulated)
- filter particulate matter & promote the interaction between antigens and cells of the immune system
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MALT (mucosa Associated Lymphoid Tissue)
- these encompass the UNENCAPSULATED secondary lymphoid tissue
- 1. Peyer's Patches (gastrointestinal)
- 2. Appendix (gastrointestinal)
- 3. Tonsils (gastrointestinal & respiratory tract)
- (other unencapsulated types of secondary lymph tissue = Bronchus ALT, GutALT, SALT)
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What 3 layers constitute a mucosa?
- epithelial lining
- lamina propria
- muscularis mucosa
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- masses of nodular & diffuse lymphoid tissue lying underneath the oral or pharyngeal epithelium
- epithelium frequently has CRYPTS (deep invaginations) which increase surface area and maximize the antigen-lymphocyte interactions
- lymphocytes will often migrate into the epithelium and sample antigen at the surface
 - have mast cells, plasma cells and neutrophils are usually present as well as lymphatics to drain the tissue.
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What are the three types of tonsils?
- 1) palatine (paired), covered by stratified squamous epithelium

- 2) pharyngeal (single), covered by pseudostratified ciliated columnar epithelium
- 3) lingual (multiple), covered by stratified squamous epithelium
- (MALTs can be distinguished by their epithelial covering)
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- lingual tonsil
- stratified squamous non-keratinizing epithelium (SSNKE)
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has an undulating basement membrane
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- pharyngeal tonsil - pharynx is digestive & respiratory
- it has a pseudostratified, ciliated, columnar outer epithelial layer
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- things sticking up = cilia
- pseudostratified
- respiratory epithelia overlying lymphocytes --> has to be pharyngeal tonsil
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- Peyer's patch
 - nodules (with intervening diffuse lymphoid tissue) in the intestinal mucosa and/or submucosa
- frequently found in the ileum
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Microfold cells (M cells)
- cells that sample antigen; have a deep basal invagination that forms a pocket of immune cells
- found in the epithelium overlying the Peyer's patch
- nodules
- (allows immune cells to get close to gut lumen)
 - weird shaped cells are M cells
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- rich in both nodular and diffuse lymphoid tissue
- Appendix functions in
- immune surveillance (abundant M-cells)
- endocrine organ
- reserve of gut flora (following loss due to diarrhea, toxic compounds)
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- MALT
- lymphoid tissue that isn't necessarily an organ can be anywhere pathogens can access the body
- just a clump of lymphocytes near a lumen should be a BALT or MALT indicator
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