Melanocytes origin

The principal pigment of human skin, hair, and eyes is melanin, which is synthesized in specialized cells, the melanocytes. They lie between the epidermis (outer layer) and the dermis (inner layer) as shown in Box 8-F. Melanocytes originate from embryonic nervous tissue and migrate into the skin by the third month of fetal life. They retain the highly branched morphology of neurons. Persons of different races all have the same numbers of melanocytes but the numbers and sizes of the pigmented melanosomes (Box 8-F) vary as does the content and chemical composition of the melanin.a d Melanosomes not only are found in the dendrites of the melanocytes but are transferred from them into adjacent epithelial cells.6 1... 

Cangul IT, van Garderen E, Van der Poel HJ, et al. Tyrosinase gene expression in clear cell sarcoma indicates a melanocytic origin Insight from the first reported canine case. APMIS. 1999 107 982-988. 

Melanins are prodnced in mammals in two types of cells of different developmental origin (1) the melanocytes of the skin, hair, choroids and iris and (2) the retinal pigment epithelium (RPE). Specialized organelles of the melanocytes, the melano-somes, synthesize and store eumelanins and phaeomelanins. 

Mintz, B. (1967). Gene control of mammalian pigmentary differentiation. I. Clonal origin of melanocytes. Proc. Natl. Acad. Sci. USA 58 344-351. 

There are differences between the receptors on nerves (presynaptic receptors) and those on effector cells (postsynaptic receptors). Furthermore, some a-agonists and antagonists exhibit selectivity for one of these receptor types. Terminology classifies receptors as either i or tt2. i-Receptors are those whose stimulation has traditionally been associated with the postsynaptic a-receptors of smooth muscle, while a2-receptors are those originally associated with the presynaptic a-re-ceptors of peripheral nerves. However, the designation of receptors as either or 2 cannot be categorized strictly by anatomical location (i.e., presynaptic or postsynaptic), since evidence now indicates that az-recep-tors occupy, in addition to peripheral nerves, a variety of sites including smooth muscle, adrenal medullary cells, the brain, and melanocytes. 

A great number of tissue-specific promoters has been isolated and characterized, and some of these have been used in experimental cancer gene therapy (see Table 1). Obviously, tissue-specific promoters are also active in the normal tissue from which the tumor originated. Unless the loss of these normal cells is acceptable (as in the case of melanocytes), additional specificity mechanisms need to be considered to make the resulting vector useful for a systemic delivery. This can be achieved, for example, by introducing an additional level of specificity, such as a specificity for proliferating cells or a selectivity for other conditions that are characteristic of tumor cells (see later). 

Proenkephalin-A contains six copies of Met-enkephalin and one of the Leu-analog. Some of the probable cleavage products (e.g., Met-enkephalin-Arg-Gly-Leu) occur as normal constituents of adrenal chromaffin cells. /3-Endorphin originates from a prohormone which is the common precursor of ACTH, a-, and /3-melanocyte-stimulating hormone and /3-endorphin (the prohormone also includes the /3-lipotropin sequence)/227 ... 

Table V summarizes experiments on the preferential cleavage of aspartyl peptide bonds by the action of 4.5% acetic acid. Such a method is desirable for the rapid assay of species differences in simple peptide hormones, such as the melanocyte-stimulating hormones (/3-MSH) of porcine, bovine, or human origin. There the species difference is located after the...

Natural melanins are generally differentiated by their origin, e.g., bovine eye, melanoma, sepia melanin. They usually occur in the form of granular particles, the melanosomes, and are secretory products of pigment-producing cells, the melanocytes. Synthetic melanins are named after the compound from which they were prepared via chemical or enzymatic oxidation (e.g., (/./-dopa, 5,6-dihydroxyindole, catechol melanin). 

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