Ultrastructural Correlates of Endocrine Control in the Pituitary of the CCL2 Transgenic Mouse

Abstract

CCL2 is an important chemokine that recruits macrophages and other immune cells to sites of injury and inflammation. High levels of CCL2 have been associated with certain CNS diseases and reproductive disorders. A transgenic mouse, CCL2-tg, has been engineered to continuously overexpress CCL2, primarily in astrocytes of the CNS. The CCL2-tg mouse is the subject of several studies at the UWSP, including CCL2-tg’s reproduction and reproductive behavior. It has been found in our laboratory that (1) the average day of vaginal opening in female CCL2-tg mice was delayed relative to non-transgenic littermates and (2) the pups of transgenic mothers gained weight faster in the first 2 weeks postpartum, indicating enhanced lactation. The processes of sexual maturity and lactation are under the hormonal control of the hypothalamic-pituitary-gonadal axis. In the present study, we focused on whether the higher levels of CCL2 in the CNS of the CCL2-tg mouse might alter the ultrastructure of its pituitary gland, whose hormones are involved in sexual maturation and lactation: the anterior pituitary secretes the protein gonadotrophins, LH and FSH, required for sexual maturity and prolactin, required for milk production in the mammary glands. The posterior pituitary, an extension of the hypothalamus, releases the peptide hormone, oxytocin, required for milk expression from the mammary ducts. This is the first ultrastructural study of the pituitary gland of the CCL2-tg mouse. CCL2-tg mice were perfusion-fixed with a glutaraldehyde-paraformaldehyde fixative. Tissues were post-fixed, then embedded in plastic and thin-sectioned for transmission electron microscopy (TEM). Overall, ultrastructural analyses indicated robust secretory function of both the anterior and posterior pituitary of the CCL2-tg female mouse. In the anterior pituitary, this included visualization of numerous membrane-bound secretory granules, abundant rough ER and Golgi, and evidence of exocytosis in the anterior pituitary. In the posterior pituitary, there was evidence of active accumulation of membrane-bound, hormone-filled vesicles in large dilatations of the unmyelinated axons originating in the hypothalamus, and numerous examples of axonal transport of individual hormone vesicles along microtubules. Both anterior and posterior pituitary contained fenestrated capillaries characterized by pores that allow diffusion of hormones. Future studies will use immunocytochemistry to identify specific hormone cell types and estimate their abundance relative to non-tg females.