**1. Introduction**

Lactation has evolved as a vital part of the mammalian reproduction strategy [1]. During this process, ovarian, hypophyseal and hypothalamic hormones together with a myriad of factors synchronize actions for the growing and remodeling of the mammary glands. Over the past years, our understanding on how this complex hormone-driven process coordinate mecha-

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nisms to guide mammary glands throughout growth-lactation-regression cycles has greatly improved. Nonetheless, there is still much to be learned about their roles in the development of each of the structural components of this organ.

The vast majority of mammary glands investigations have been performed in mice and rats. However, many aspects still remain unfulfilled covered by these conventional animal models since they differ considerably in mammary glands development and types of breast cancer from women [2, 3]. On the other hand, studies performed on unconventional rodents such as guinea pigs and hamsters that share with humans some endocrine and reproductive biology aspects have contributed to a better understanding of human physiology and disease [4], particularly on some reproductive tumors [5, 6].

**Figure 1.** Adult female plains vizcacha (*Lagostomus maximus*) nursing a pup. Credit: J. Halperin, Universidad Maimónides.

The South American plains vizcacha, *Lagostomus maximus*, is a hystricomorph rodent closely related to guinea pig (**Figure 1**) [7]. This species has attracted significant attention in the reproductive research field since female ovaries exhibit exceptional and unique characteristics among rodents. Females display natural massive poly-ovulation that can go up to 800 oocytes per cycle, the highest ovulatory rate so far recorded for a mammal, as a result of an unusual constitutive suppression of apoptosis that greatly decreases intra-ovarian oocyte dismissal caused by follicular atresia [8–12]. In addition, gestation lasts 154 ± 6 days [8], an unusually long period for a rodent and one of the longest recorded among hystricomorphs. Moreover, pregnant females exhibit an ovulatory event at mid-pregnancy that leads to a considerable number of secondary corpora lutea with oocyte retention (i.e., pseudo-ovulation) and to an important rise of the progesterone levels [10, 11, 13]. This boost up in the circulating progesterone may contribute to an accurate maintenance of the uterus and embryo development up to the end of pregnancy [11, 14]. Given that ovarian hormones modulate growth and development of post-pubertal mammary glands, the reproductive peculiarities of the ovaries of *L. maximus* make of this species an interesting model to examine the mammary glands morphology according to its reproductive status.

The purpose of this chapter is to give a brief representation of the morphological changes that occur in the mammary glands between pregnancies under the action of ovarian, hypophyseal and hypothalamic hormones of adult plains vizcachas.
