Structural Biochemistry/Cell Signaling Pathways/Paracrine Signaling
General Information 
Like "para" which means near, paracrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell. There is difference between paracrine and autocrine signaling. Both affect neighboring cells, but autocrine signaling is within same type of cells, while paracrine signaling affects different types of (adjacent) cells.
Paracrine is the secretion of a hormone by an organ other than an endocrine gland.
Below is a figure of the Oncogenic Ras of the paracrine signaling that promotes the growth of tumor.
File:Paracrine signaling.gif Oncogenic Ras promotes the transcription of cytokine genes (the double helix), leading to elevated levels of secreted cytokines (the blue spheres) that act to modulate the immune system (yellow cells), promoting angiogenesis (purple capillaries), activating tumor stroma, the brown cells.
Other Uses 
Paracrine signaling is used in the signaling through the epithelial estrogen receptor which allows for proliferation and morphogenesis in the mammary gland. Estradiol is a major regulator of postnatal mammary gland development. It uses paracrine signaling to exert its effects through the estrogen receptor and is expressed in the mammary gland stroma and epithelium. Estradiol stimulates the secretion of prolacti by the pituitary gland and suppresses the secretion of other hormones called gonadotropins. These facts help understand that impairment of mammary gland development may be due to the mammary glands inability to react to the estradiol itself.. The signaling failure is usually a result of inactivation of the ER alpha (estrogen receptor alpha).
More conclusive tests will be needed in order to more clearly understand the mechanism behind this process, but it is clear that the ER alpha expression in the epithelium is a heterogenous process that involves using ductal cells which are triggered by estradiol processes. The ER alpha cells proliferate and act on a subtype of cells which releases paracrine signals that permit other epithelial cells to participate in growth.
Two paracrine signaling agents are growth factor and clotting factors. Growth factor signalling affects the development of tissues. An example is in insects where Allotostatin controls growth through paracrine action on the corpora allata.
In mature organisms, paracrine signaling is involved with allergen responses, tissue repair, formation of scar tissue, and blood clotting.
Paracrine Signaling with the ER Alpha-Sonia Mallepell, Sndree Krust, Pierre Chambon, Cathrin Brisken
Estrogen Target and SourceEndocr. Rev. 2006 27:677-706-U. Ohnemus, M. Uenalan, J. Inzunza, JA Gustafsson, R. Paus