BIOLOGY 03048, ANATOMY: WEEK 6: MORE BLOOD; LYMPHOID TISSUES 10/7/99
W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
Text: Cormack, Ch. 3
Possible web site: http://cellbio.utmb.edu/microanatomy/
(another med school's layout of histology slides, with a separate lymphoid tissues section. Short, to-the-point text)
The bone marrow is the primary site of hematopoesis in the adult, although yolk sac, liver, and spleen all participate at high rates during prenatal development.1,3 Bone marrow in the adult is active in the axillary skeleton and the proximal ends of the femur and humerus. Life span for blood cells is short--120 days for rbcs and hours-days for wbcs, so a constant level of activity is necessary for replenishment.
As seen in Fig. 3-16 of Cormack,1 there are two major lines of cell production from pluripotential stem cells: the myeloid and lymphoid lines (you can tell that an increasing understanding of growth factors is also occuring!). (Red) bone marrow: among the spicules of spongy (cancellous bone), with support from stromal cells (e.g., fibroblasts), reticular fibers, and sinusoids (for drainage).
hematopoetic stem cells:
lymphoid lineage: lymphocytes (T cells and B cells)
B cells will mature in the bone marrow, and are responsible for humoral immune response (antibodies).
T cells will need to mature in the thymus and are involved in cell-mediated immune response (e.g., antigens presented by macrophages).
myeloid lineage: separate lineages for erythrocytes (rbcs), megakaryocytes (will fragment into platelets), as well as for the different wbcs.
Myeloblasts and promyelocytes are precursors to mature granulocytes, while monocytes have similar stages of monoblasts and promonocytes. Blast cells in particular are large with prominent nuclei, and when seen in the periphery, are indicative of acute leukemia.
Lymphoid tissue contains lymphocytes, but with differing histologic features given their roles:
Lymph nodes (Fig. 3-23): are encapsulated with a clear subcapsular sinus, with a cortex, a medulla, and a hilum. Lymphatic nodules in the cortex often have germinal centers (mostly B cells), a paracortex with many T cells, and a medulla with draining sinusoids. Reactive lymph nodes enlarge from 1) increased fluid flow, 2) macrophage activation and increase, and 3) lymphocytic activation and increase.2
Spleen (Fig. 3-24) A dense capsule (ok, maybe not that dense!) and thick trabeculae surround and partition the spleen. Lymphatic nodules with germinal centers (B cells) and sheathed arteries (T cells) (the white pulp) are found throughout the spleen . Surrounding them is the red pulp consist of splenic cords of cells through which run venous sinuses (filtering). Given both lymphatic and filtering functions, it's no wonder why splenomegaly accompanies so many disease processes (think of a few?).
Thymus (Fig. 3-22) is more distinctively lobulated than spleen or lymph node. The lobules contain a dark-staining cortex and a lighter-staining medulla. The lymphocytes (T cells, right?) in the cortex are not nodulated. The medulla contains thymic corpuscles (keratinized reticular epithelial cells) that become larger with age. This epithelial trace should remind you of thymus's origins--off of the 3rd pharyngeal pouch?? As you see with dissection, adipose cells become more prevalent as well during aging and involution. Thymic hyperplasia is often associated with the autoimmune disease myasthenia gravis.2
Tonsil (check a website). As a final practice with lymphatic tissue, take a look at a section of a tonsil--lots of lymphatic nodules with deep invaginations or tonsillar crypts (think of the"wrinkling" of the tonsils you see inside the oral cavity). Tonsils are part of the MALT (mucosa-associated lymphatic tissue).1
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This page updated on October 25, 1999