General Histology Lecture 13: Lymphoid System 1 PDF

General Histology Lecture 13 Lymphoid system 1 Ahmed Abdul-Aziz Muhammad. PhD Lymphoid System Outline Primary lymphoid organs Bone marrow Thymus Secondary lymphoid organs Lymph nodes Spleen MALT  Functions of the Lymphoid System The main purpose of t...

General Histology Lecture 13 Lymphoid system 1 Ahmed Abdul-Aziz Muhammad. PhD Lymphoid System Outline Primary lymphoid organs Bone marrow Thymus Secondary lymphoid organs Lymph nodes Spleen MALT  Functions of the Lymphoid System The main purpose of the lymphoid system is to detect and get rid of unwanted substances, such as: Invading microorganisms Foreign materials (like pollen) Cancer cells The lymphoid system also carries waste products away from tissues and dumps them into the blood for disposal.  Lymph (Lymphatic Fluid) Lymph consists of interstitial (“between cells”) fluid, lymphocytes, cellular waste, and sometimes other stuff (bacteria, metastatic tumor cells). Lymph travels through the body in lymphatic vessels. Lymphatic vessels begin as tiny open-ended vessels in tissues. Eventually they merge together, pass through lymph nodes, and empty into the blood circulation via the thoracic duct and right lymphatic duct.  Right lymphatic duct Thoracic duct  Primary (Central) Lymphoid Organs Primary lymphoid organs are the places where lymphocytes arise and mature. There are two primary lymphoid organs: Bone marrow All lymphocytes (B, T, and NK cells) arise here. B cells and NK cells mature here. Thymus T cells mature here. Normal bone marrow aspirate  Thymus Located in mediastinum. Super rigorous ‫ صارم‬where T cells mature. Sterile! No afferent lymphatics; tight blood-thymus barrier. Consists of lobes which are incompletely divided by fibrous septae into numerous lobules. Thymus  Cortex Contains T cells and unique stromal cells called epithelial reticular cells. Immature T cells from bone marrow enter the outer cortex and undergo proliferation. In the deeper cortex, T cells undergo rigorous “selection.” Only about 3% of T cells survive; the rest are killed. Surviving T cells leave the thymus via post-capillary venules located at cortico-medullary junction and in medulla.  Wait, epithelial reticular cells? Yes! Epithelial cells! Weird! Stroma of all other lymphoid organs is composed of reticular (type III collagen) fibers. Stroma of the thymus is composed of epithelial cells with long, thin processes which connect to each other via desmosomes, forming a network (reticulum!). These cells also surround blood vessels to help form the blood- thymus barrier (keeps antigens from getting in and messing up the selection process). Cortex: tons of T cells  Medulla Contains mature T cells, epithelial reticular cells, and Hassall corpuscles. Hassall corpuscles are formed of concentric rings of degenerating epithelial reticular cells with keratin at the center. Found only in the medulla; function unknown. Medulla stains lighter than cortex because it contains Hassall corpuscles and fewer T cells than the cortex.  Keratin Degenerating epithelial reticular cells Cute little Hassall corpuscle  Age-related changes The thymus is well-developed before birth. The thymus is he aviest at puberty. After puberty, it involutes ‫ مطوي‬but never completely disappears. In old age, the thymus is composed mainly of connective tissue and fat, but it is still capable of producing lymphocytes. The thymus in elderly people consists mostly of fat and connective tissue  Lymphoid System Lecture questions Briefly describe the functions of the lymphoid system. Describe the composition and circulation of lymph. Describe the following features of lymphoid organs: Classification as primary or secondary Function of each organ Histologic regions and structures that are characteristic of each organ Location of B and T lymphocytes within each organ Composition of the stroma