**Author details**

may be dependent on the species of animals. In the case of sheep, seasonal breeders, the mechanism of ghrelin activity is complicated. As revealed by the studies in lactating sheep,

**Figure 7.** Mean concentrations of prolactin in control and ghrelin-treated pituitary explant cultures during long day.

In seasonal animals, the process of triggering and maintaining lactation requires numerous hormones. The interaction of growth factors and other hormones is necessary in processes such as mammogenesis, lactogenesis and galactopoiesis. Due to the proper synchronization of pregnancy and changes in the area of the mammary gland, the gland is ready for the production of milk at the moment the offspring is born. Mammogenesis is a phenomenon that requires the participation of a number of hormones, including prolactin (PRL), growth hormone (GH), estrogens, progesterone, oxytocin, placental lactogen (PL) and insulin-like growth factor (somatomedin, e.g., IGF1). The coparticipation of IGF and GH is necessary in coordinating the differentiation and proliferation of epithelial cells. The manner in which the growth factors stimulate or inhibit the growth of cells or their influence on the cell cycle is not fully understood. The role of IGF in particular stages of functioning of the mammary gland (mammogenesis, lactogenesis, galactopoiesis and desiccation), particularly in the case of ruminants, is highly complicated. Recently, attention has been given to the metabolic hormones, particularly the role of leptin, orexin and ghrelin in mammogenesis, lactogenesis and galactopoiesis, respectively. Due to the recently increased interest in sheep's milk products, an understanding of the endocrine mechanisms facilitating the maintenance of lactation during autumn and winter

may contribute to the improved profitability and usefulness of sheep's milk.

See text for statistical comparisons.

**4. Summary**

36 Current Topics in Lactation

administration of exogenous ghrelin modulates the secretion of prolactin.

Edyta Molik\* and Dorota Zięba-Przybylska

\*Address all correspondence to: rzmolik@cyf-kr.edu.pl

Department of Animal Biotechnology, Agricultural University in Krakow, Kraków, Poland
