Muscle Tissues

T lymphocyte (T cell) (tee LIM-fuh-syt): A type of lymphocyte that matures in the thymus; it is the primary mediator of cell-mediated immunity, responsible for recognizing and destroying infected or cancerous cells. B lymphocyte (B cell) (bee LIM-fuh-syt): A type of lymphocyte that matures in the bone marrow; its function is to become activated and differentiate into plasma cells that secrete antibodies (humoral immunity). Plasma cell (PLAZ-muh sell): A terminally differentiated B lymphocyte specialized for mass production and secretion of antibodies; histologically characterized by abundant, basophilic cytoplasm, an eccentric nucleus, and a prominent pale area (Golgi zone). Antibodies (Immunoglobulins) (AN-tih-bod-eez/IM-yoo-noh-GLOB-yoo-linz): Y-shaped proteins secreted by plasma cells; their function is to bind specifically to antigens (pathogens or foreign material) to neutralize them or mark them for destruction by macrophages or other immune cells. Golgi zone (GOHL-jee zohn): A clear or pale-staining area near the nucleus of a plasma cell; this represents the location of the extensive Golgi apparatus, which is actively engaged in packaging and modifying the vast amount of antibodies synthesized for secretion. Natural killer cell (NK cell) (NACH-er-ul KIL-er sell): A large, granular lymphocyte that is part of the innate immune system; its function is to recognize and directly kill virus-infected cells and tumor cells without prior activation. Granule (GRAN-yool): A small, membrane-bound vesicle in the cytoplasm of certain cells (e.g., mast cells, NK cells, granulocytes) that contains preformed substances (e.g., histamine, cytotoxic proteins); these contents are rapidly released upon activation. Macrophage (MAK-roh-fayj): A large phagocytic immune cell derived from blood monocytes, found in connective tissues throughout the body (including lymph nodes and the spleen); its function is to engulf cellular debris, pathogens, and foreign substances. Mast cell (mast sell): A connective tissue immune cell with a prominent cytoplasm packed with large granules containing inflammatory mediators (like histamine and heparin); it is central to initiating acute inflammatory and allergic reactions upon degranulation. Cortex (KOR-teks): In a lymphoid organ (e.g., lymph node, thymus), this refers to the dense, outer region located immediately beneath the capsule; it typically contains a high concentration of lymphocytes. Medulla (muh-DUL-uh): In a lymphoid organ (e.g., lymph node, thymus), this refers to the less dense, central region; it often contains lower concentrations of lymphocytes, a network of sinusoids, and specialized cells like thymic corpuscles or medullary cords. Thymic epithelial cells (THY-mik ep-uh-THEE-lee-ul sells): A network of non-lymphoid cells in the thymus that form a protective scaffolding; their function is to establish the microenvironment necessary for T lymphocyte maturation and selection (determining which T cells survive). Capsule (KAP-sool): A thick, dense layer of irregular connective tissue that completely surrounds and protects a lymphoid organ (e.g., lymph node, spleen, thymus); it gives off internal septa (trabeculae). Thymic corpuscle (Hassall's corpuscle) (THY-mik KOR-pus-ul/HAH-suhlz KOR-pus-ul): Concentrically arranged, whorled layers of degenerating, keratinized thymic epithelial cells found in the medulla of the thymus; their function is uncertain but they are a histological landmark. Atrophic (uh-TROH-fik): A histological term describing the reduction in size of a cell, tissue, or organ; the thymus becomes increasingly atrophic (involutes) after puberty, with lymphoid tissue being replaced by adipose tissue. White pulp (wyt pulp): The region of the spleen composed of lymphoid tissue, organized around a central arteriole; it is the site of immunological function, housing T lymphocytes and B lymphocytes organized into periarteriolar lymphoid sheaths (PALS) and splenic nodules (germinal centers). Germinal center (JER-mih-nul SEN-ter): A pale-staining, central region that develops in the lymphatic nodule (follicle) of the spleen or lymph node when B lymphocytes are actively proliferating and differentiating into plasma cells following an antigenic challenge. Red pulp (red pulp): The region of the spleen that specializes in blood filtration; it is composed of vast networks of vascular channels (splenic sinusoids) and surrounding cellular tissue (splenic cords) that function to remove old or damaged red blood cells and recycle iron. Splenic sinusoid (Venous sinus) (SPLEN-ik sy-nuh-soyd): Wide, discontinuous, leaky capillaries with a "barrel-hoop" arrangement of reticular fibers, found in the red pulp of the spleen; this structure forces blood cells to squeeze through the endothelial slits, allowing macrophages to inspect them. Splenic cord (Cord of Billroth) (SPLEN-ik kord): A mass of reticular connective tissue and a high concentration of cells (macrophages, lymphocytes, plasma cells, and blood cells) that is located between the splenic sinusoids in the red pulp of the spleen. Venous sinus (VEE-nus SY-nus): The term used for the highly specialized, leaky vascular channels in the red pulp of the spleen (splenic sinusoid) and the open channels within the lymph node (sinus); they facilitate direct interaction between blood/lymph and immune cells. Central arteriole (of the white pulp of the spleen) (SEN-trul ar-TIR-ee-ohl): A branch of the splenic artery that travels eccentrically through the lymphoid tissue of the periarteriolar lymphoid sheath (PALS) in the white pulp; it supplies the immune cells before continuing on to the red pulp. Trabeculum (pl. Trabeculae) (truh-BEK-yuh-lum): Connective tissue septa that extend inward from the organ capsule of the lymph node and spleen; they carry blood vessels and provide structural support for the lymphoid parenchyma. Splenic nodule (SPLEN-ik NAH-jool): A spherical concentration of B lymphocytes within the white pulp of the spleen; it is often reactive, containing a germinal center. Lymphatic vessel (Efferent) (EF-uh-rent lim-FAT-ik VES-ul): A lymphatic vessel that carries lymph away from the medulla of a lymph node, eventually returning the filtered lymph to the bloodstream. Lymphatic nodule (Follicle) (lim-FAT-ik NAH-jool): A dense, spherical cluster of B lymphocytes found in the cortex of a lymph node or other lymphoid organs; may be primary (inactive) or secondary (containing a germinal center). Sinus (SY-nus): A vast, endothelium-lined channel (e.g., subcapsular, cortical, medullary) within a lymph node; these spaces allow lymph to percolate slowly through the node, facilitating monitoring by macrophages and lymphocytes. Lymphatic vessel (Afferent) (AF-uh-rent lim-FAT-ik VES-ul): A lymphatic vessel that penetrates the capsule of a lymph node to carry unfiltered lymph to the node, draining into the subcapsular sinus. Medullary cord (lymph node) (MED-uh-lair-ee kord): Interconnecting cord-like extensions of lymphoid tissue and plasma cells that branch off the cortex and extend into the medulla of a lymph node; they run parallel to the medullary sinuses. Mucous gland (MYOO-kus gland): Exocrine glands (mostly mucous acini) located in the connective tissue beneath the lymphoid follicles of the lingual tonsil; their ducts empty into the bottom of the tonsillar crypts to flush debris. Tonsillar crypts (TON-suh-ler kripts): Deep invaginations of the covering stratified squamous epithelium found in the tonsils (palatine, lingual); they increase the surface area and collect antigens, allowing immune cells to survey the luminal contents. Serosa (appendix) (suh-ROH-suh): The thin outermost layer of the appendix, composed of loose connective tissue covered by a mesothelium (simple squamous epithelium); it is the visceral peritoneum. Muscularis externa (appendix) (mus-kyoo-LAIR-is eks-TUR-nuh): The muscular wall of the appendix composed of inner circular and outer longitudinal layers of smooth muscle; its contraction causes peristalsis. Submucosa (appendix) (sub-myoo-KOH-suh): The layer of dense irregular connective tissue located between the muscularis mucosae and the muscularis externa of the appendix; it is particularly rich in large aggregates of lymphatic nodules (Peyer's patches). Crypt (intestinal gland) (appendix) (kript): Simple tubular glands (similar to intestinal crypts) found in the lamina propria of the appendix; they primarily house enteroendocrine cells and the stem cells necessary for renewing the epithelial lining. Lamina propria (appendix) (LAM-ih-nuh PROH-pree-uh): The connective tissue layer of the mucosa of the appendix; it is densely packed with diffuse lymphoid tissue and a high concentration of lymphatic nodules that characterize this organ's immune function.