Follicle-stimulating hormone , where

As a rule, a hypothalamic hormone should always control a hormone in the anterior pituitary. The pituitary hormones (e.g. follicle-stimulating hormone, prolactin, or thyrotropin) are then transported through the bloodstream to the secondary target organs, where, for example, they stimulate the production of corticosteroids in the adrenals or the formation of thyroxine in the thyroid. 

Leptin also has direct effects on a number of other factors influencing animal production. For example, leptin reduces fat deposition in adipose tissue, where it suppresses the expression of acetyl-CoA carboxylase, the rate-limiting enzyme involved in fat synthesis. Leptin also has direct effects on the reproductive system, where it induces the release of follicle-stimulating hormone and lutenising hormone from the anterior pituitary. Its effects on reproduction and embryonic development are broad and include factors such as inducing puberty, especially in thinner animals, and shortening the interval from parturition to oestrus. 

Figure 19.8 A brief summary of the pathways for formation and secretion of oestradiol and progesterone within the cells of the follicle. Cholesterol is taken up by thecal cells in a complex with low density lipoprotein. In the thecal cells, cholesterol is converted to testosterone which is released to be taken up by granulosa cells where it is converted into oestradiol. For synthesis of progesterone in the granulosa cells, cholesterol is synthesised de novo within the cells from acetyl-CoA. In the follicle the enzyme aromatase, which produces the aromab c ring in the female sex hormones, is restricted to the granulosa cells. The reacrions that are stimulated by LH and FSH increase synthesis and, therefore, secretion of testosterone and increased synthesis of oestrogens and progesterone.

In females, the target organs are the ovaries where it increases the number and size (maturation, development and ripening) of Graafian follicles and prepare them for ovulation. During its development, the ovarian follicles secrete its own hormone estrogen. In males, it stimulates spermatogenesis. Under the influence of this hormone, seminiferous tubules produce spermatozoa. 

The expression of the sodium iodide symporter is perhaps nowhere more important than in the thyroid gland. A complete review of the physiological importance of the thyroid is beyond the scope of this chapter. It is sufficient to say that the symporter provides the iodine needed for normal thyroid function. Once the symporter has been trafficked to the basolateral surface of the thyrocyte, it can transport iodine from the blood into the cell. Once inside the cells, iodine is transported to the apical membrane where it is organified through attachment to a tyrosine residue and incorporated into the thyroid hormone thyroglobulin. The thyroglobu-lin is then stored inside thyroid follicles as colloid, to be released into the bloodstream as thyroid hormones (thyroxine and triiodothyronine) via TSH stimulation. 

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