Erythropoiesis is the process of formation of RBC. Peptide hormone erythropoietin secreted from the juxtaglomerular cells of kidney stimulates erythropoiesis.

Pars intermedia of pituitary gland secretes only one hormone called Melanocyte Stimulating Hormone (MSH). This hormone causes dispersal of pigment granules in the pigment cells, thereby darkening the colour in certain animals like fishes and amphibians.

Calcitonin/thyrocalcitonin is a 32 amino acid, linear polypeptide hormone that is produced in humans primarily by the parafollicular cells of the thyroid gland. It checks excess $\mathrm{Ca}^{2+}$ and phosphate in plasma by decreasing mobilisation from bones. Deficiency of calcitonin results in osteoporesis or loss of bone density (due to dissolution of parathormone).

Thymosins play a major role in the development and differentiation of T-lymphocytes, which provide cell-mediated immunity. Thymosins also hastens attainment of sexual maturity.

The hormones that are derivatives of amino acids, polypeptides or proteins are called peptide hormones. These being insoluble in lipids can not enter the target cell.

These act at the surface of target cell as primary messengers and bind to the cell-surface receptor forming the hormone-receptor complex.

It involves following steps

(i) Hormone called first messenger attaches to the cell surface receptor protein on the outer surface of plasma membrane of the target cell, forming a hormone-receptor complex.

(ii) This complex activates the enzyme adenyl cyclase.

(iii) Adenyl cyclase catalyses the conversion of ATP to cyclic AMP on the inner surface of plasma membrane.

(iv) cAMP serves as the second messenger or intracelluar hormonal mediator delivering information inside the target cells. This activates appropriate cellular enzyme system by cascade effect. Which induces the cell machinery to perform its specialised function.

(v) cAMP has a very short existence. It is rapidly degraded by the cAMP phosphodiesterase. Water soluble hormones, such as amines, peptides, proteins and glycoproteins exert their control through the cyclic AMP. These are quick acting hormones and produce immediate effect.