Thoracic duct

Microorganisms that escape phagocytosis in a local lesion may now be transported to the regional lymph nodes via the lymphatic vessels. If massive invasion occurs with which the resident macrophages are unable to cope, microorganisms may be transported through the thoracic duct into the bloodstream. The appearance of viable microorganisms in the bloodstream is termed bacteraemia and is indicative of an invasive infection and failure of the primary defences. 

Chylomicrons leave the absorptive cell by way of exocytosis. Because they are unable to cross the basement membrane of the blood capillaries, the chylomicrons enter the lacteals, which are part of the lymphatic system. The vessels of the lymphatic system converge to form the thoracic duct that drains into the venous system near the heart. Therefore, unlike products of carbohydrate and protein digestion that are transported directly to the liver by way of the hepatic portal vein, absorbed lipids are diluted in the blood... 

Ramaswamy, K., Goodman, R.E. and Bell, R.G. (1994) Cytokine profile of protective anti-Trichinella spiralis CD4 0x22- and non-protective CD4+ 0x22+ thoracic duct cells in rats secretion of IL-4 alone does not determine protective capacity. Parasite Immunology 16, 435-445. 

Brucellosis is caused by Brucella melitensis, B. suis, B. abortus, or B. canis bacteria. After entering the human body, the organisms travel from their entrance point to the lymph channels and nodes, eventually reaching the thoracic duct and bloodstream. Carried through the bloodstream, they are deposited eventually in... 

M. Matsuo, K. Koizumi, S. Yamada, M. Tomi, R. Takahashi, M. Ueda, T. Terasaki, M. Obinata, K. Hosoya, O. Ohtani, and I. Saiki. Establishment and characterization of conditionally immortalized endothelial cell lines from the thoracic duct and inferior vena cava of tsA58/EGFP double-transgenic rats. Cell Tissue Res. 326 749-758 (2006). 

Chylomicroiis leave the lymph and enter the peripheral blood where the thoracic duct joins the left subdavian vein, thus initially bypassing the hver. After a high-fat meal, chylomicrons cause serum to become turbid or milky. While in the blood, chylomicrons acquire apoC-II and apoE from HDL particles. 

Testosterone (T.) derivatives for clinical use. T. esters for im. depot injection are T. propionate and T. heptanoate (or enanthate). These are given in oily solution by deep intramuscular injection. Upon diffusion of the ester from the depot, esterases quickly split off the acyl residue, to yield free T. With increasing lipophilicity, esters will tend to remain in the depot, and the duration of action therefore lengthens. A T. ester for oral use is the undecanoate. Owing to the fatty acid nature of undecanoic acid, this ester is absorbed into the lymph, enabling it to bypass the liver and enter, via the thoracic duct, the general circulation. 17-0 Methyltestosterone is effective by the oral route due to its increased metabolic stability, but because of the hepatotoxicity of Cl 7-alkylated androgens (cholestasis, tumors) its use should be avoided. Orally active mesterolone is 1 a-methyl-dihydrotestosterone. Trans-dermal delivery systems for T. are also available. 

The food components resorbed by the epithelial cells of the intestinal wall in the region of the jejunum and ileum are transported directly to the liver via the portal vein. Fats, cholesterol, and lipid-soluble vitamins are exceptions. These are first released by the enterocytes in the form of chylomicrons (see p. 278) into the lymph system, and only reach the blood via the thoracic duct. 

In the mucosal cells, long-chain fatty acids are resynthesized by an ATP-dependent ligase [5] to form acyl-CoA and then triacylglycerols (fats see p. 170). The fats are released into the lymph in the form of chylomicrons (see p. 278) and, bypassing the liver, are deposited in the thoracic duct—i. e., the blood system. Cholesterol also follows this route. 

Traber, M. G., Kayden, H. ]., Green, J. B., and Green, M. H. (1986). Absorption of water-misdble forms of vitamin E in a patient with cholestasis and in thoracic duct-cannula ted rats. Am.. Clin. Nutr. 44, 914—923. 

Chylomicrons are discharged from the enterocytes by exocytosis into lacteals, which are lymphatic vessels that originate in the intestinal villi, drain into the cisternae chyli, and follow a course through the thoracic ducts to enter the bloodstream through the left subclavian vein. 

Chylomicrons are formed in the intestine and carry triglycerides of dietary origin, unesterified cholesterol, and cholesteryl esters. They transit the thoracic duct to the bloodstream. 

The lymph is a system of circulating materials which exists alongside the blood supply. As the metabolites and other species in blood are drained into the tissues, the used fluids must pass through and out of the tissues. The exudate goes into the lymph system which eventually drains through the thoracic duct in the upper portion of the chest back into the blood supply. It is important to remember this connectivity of the blood and lymph and its relation to the tissues in order to appreciate the complexities of the immune system. 

The antilymphocyte globulin is obtained through the immunization of horses with human lymphoid cells or with fetal thymus cells. The antilymphocyte antibody destroys the T cells and impairs delayed hypersensitivity and cellular immunity without altering humoral antibody formation. The pattern of immunosuppression obtained with antilymphocyte globulin is identical to that brought about following thoracic duct drainage that depletes the numbers of small lymphocytes. 

There is immunochemical evidence that in man apoA-II is produced in intestinal epithelial cells (B33, S16, S20). Anderson et al. estimated that 28-82% of total body apoA-II synthesis takes place in the intestine (A23). Most lymph apoA-II (90 11%) is associated with HDL particles. As with apoA-I, a high proportion (perhaps more than 70%) of thoracic duct apoA-II is calculated to be derived originally from plasma and to be recirculating back into the plasma (A23). 

K16. Klein, R. L., and Rudel, L. L., Effect of dietary cholesterol level on the composition of thoracic duct lymph lipoproteins isolated from nonhuman primates. ]. Lipid Res. 24, 357-367 (1983). 

The newly synthesized triacylglycerol becomes organized into chylomicrons (a type of lipoprotein see next section), which are secreted by the intestinal epithelial cell into the lacteals, small lymph vessels in the villi of the small intestine. Then from the lymphatics, the chylomicrons pass into the thoracic duct, from which they enter the blood and thus contribute to the transport of lipid fuel to various tissues. A feature of chylomicron metabolism is their ability to deliver lipid fuels to extrahepatic tissues. 

Critical to vitamin D3 action is its further metabolic conversion to more active compounds (Figure 1.3). Via its transport by DBP, vitamin D3 accumulates in the liver [48]. In rats, as much as 60-80% of an injected or oral dose of vitamin D3 locates to the liver [49-51], Intestinal absorption of vitamin D3 is in association with the chylomicron fraction via the lymphatic system. Vitamin D3 is delivered to the liver in blood from the thoracic duct only a few hours post ingestion [44], A specific portion of hepatic vitamin D3 in the rat is converted to 25-OH-D3 by a 25-hydroxylase system in the endoplasmic reticulum of hepatocytes [52, 53]. This enzyme (Km 10"8 M) is regulated to an extent by 25-OH-D3 and its metabolites. Higher concentrations of vitamin D3 are handled by a second 25-hydroxylase located in liver mitochondria [54], This enzyme, also known as CYP27, 27-hydroxylates cholesterol and thus appears less discriminating than the microsomal 25-OHase which does not use cholesterol as substrate [55, 56]. In humans, however,... 

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