¿y The equatorial section shows the array of organelles. Particularly prominent ^ are the many a-granules H as well as the sections through the open tubular " systems [2. The latter consist of many tubules and vacuoles, which intercon-
c nect with each other and the extracellular space. This system influences the release of secretory products from stored secretory granules and is instrumental in the activation of thrombocytes. The peripheral layer also attracts attention. It contains a bundle of microtubules and somewhat thicker, electron-dense tubules [3, which are involved in the regulation of the Ca++ concentration and the prostaglandin metabolism.
2 Open system of canaliculi
3 Peripheral bundles of microtubules and electron-dense tubules ^ 4 Mitochondrion
Lo 5 Golgi apparatus
¡2 6 Pseudopodium
Electron microscopy; magnification: x 27 600
The thymus is derived from the pharyngeal pouch. In relationship with other lymphatic organs, it takes a central, superordinate place. In a histological cross-section, the infantile thymus is seemingly built of lobes. Each lobule consists of a cortex [1 (stained blue-violet), which is rich in lymphocytes, and the medulla (stained lighter and reddish) with lower cell density [2 .Vascularized connective tissue 0, HI fills the spaces between lobules. It also pushes into the organ, up to the border between cortex and medulla, forming septa (trabeculae). Meshworks of reticulum cells (epithelial reticulum cells, lym-phoepithelial organ), not fibers, form the supportive structures for cortex and medulla. The reticular cells have arisen from entodermal epithelium. Small lymphocytes (T-lymphocytes) predominate underneath the cortex. The medulla contains mostly lymphoblasts, lymphocytes and epithelial reticulum cells. The thymus also contains mast cells, macrophages and interdigitating dendritic cells. The literature often describes six different types of epithelial cells. Lymph follicles with germinal centers do not exist in the thymus.
1 Cortex p 2 Medulla
§ 3 Blood vessels
4 Connective tissue
^^ Stain: alum hematoxylin-eosin; magnification: x 10
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