Mammalian endocrine system

The gastrointestinal system of zebrafish presents clear differences from the human system. The zebrafish does not possess a stomach, the intestine is continuous with the pharynx through a short esophagus, and no sphincters are present [61]. However, zebrafish have most of the cell types observed in the small intestine -absorptive, endocrine, goblet, and interstitial cells of Cajal, although Paneth cells are absent. Gut contractions are under the control of the enteric nervous systems, which respond to different pharmaceuticals in similar way as the mammalian counterpart. For example, zebrafish embryos can be used as predictor of emetic response to pharmaceuticals, one of the most commonly reported clinical adverse effects to be considered in the development of new dmgs [61]. 

Figure 33.2. Endocrine feedback loops of the mammalian hypothalamic-pituitary-gonadal (HPG) axis. (Adapted from La Barbera A. R. Differentiation and function of the female reproductive system. In Boekelheide, K., Chapin, R. E., Hoyer, P. B., and Harris, C. (Eds.). Comprehensive Toxicology, Vol. 10, Reproductive and Endocrine Toxicology, Elsevier, New York, 1997, pp. 255-272 and Creasy, D. M., and Foster, P. M. D. Male reproductive system. In Haschek, W. M., Rousseaux, C. G. and Wallig, M. A. (Eds.). Handbook of Toxicologic Pathology, 2nd ed., Academic Press, San Diego, 2002,pp. 785-846. E2, estradiol T, testosterone, DHT, dihydrotestosterone, FSH, follicle stimulating hormone LH, luteinizing hormone.

The similarities between ecdysone synthesis in the insect prothoracic gland and the ovary are obvious and in each case synthesis is initiated in response to a hormone originating in the brain. Both bear a striking resemblance to the mammalian system where steroid hormone synthesis in the various endocrine tissues is initiated in response to the release of appropriate hormones from the anterior pituitary. 

Hormones are molecules organisms use to convey information between cells. When target cells are distant from the hormone-producing cell, such molecules are called endocrine hormones. To ensure proper control of metabolism, the synthesis and secretion of many mammalian hormones are regulated by a complex cascade mechanism ultimately controlled by the central nervous system. In addition, a negative feedback mechanism precisely controls various hormone syntheses. A variety of diseases are caused by either overproduction or underproduction of a specific hormone or by the insensitivity of target cells. 

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