**2. The mammary gland morphology of vizcachas**

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

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],

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

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

of each of the structural components of this organ.

8 Current Topics in Lactation

particularly on some reproductive tumors [5, 6].

nides.

Adult female vizcachas have two pairs of functional mammary glands located below the ventral skin and laterally on the thorax. The skin epidermis is formed by a stratified squamous and keratinized epithelium which rests on a layer of dense collagenous connective tissue that contains hair follicles, sweat glands and fibroblasts. From the opening of the nipple and into the mammary glands, the number of epithelial layers decreases until it reaches a two-layer epithelium which upholsters each branched tubulo-alveolar gland [15].

The mammary gland secretory parenchyma is divided into lobes and then lobules by connective tissue septa. Lobules are formed by intralobular ducts that connect to an interlobular duct which finally empty into the lactiferous duct. The lactiferous duct is the excretory duct of each lobe and connects to the opening nipple to allow the release of milk during lactation. Before reaching the opening nipple, the lactiferous duct lumen forms a lactiferous sinus that functions as a reservoir for milk during lactation.

The mammary gland epithelium that coats the ducts is composed by an inner layer of secretory cells and an outer layer of myoepithelial cells which lies on the basement membrane that separates parenchymal and stromal compartments. The surrounding stroma is mainly composed by connective tissue, endothelial vessels, fibroblasts and immune cells. Unlike to what have been described for mouse and rat, mammary glands of adult vizcachas have a poor fat content [15]. General morphology and a detailed description of each cellular component of adult mammary glands of *L. maximus* are depicted in **Figure 2**.

As adult females transit throughout pregnancy and lactation, the mammary gland develops a more elaborated structure as a result of proliferation, branching and differentiation of the ductal tree. The extent of the development of the ductal network is closely related to the female reproductive status and the hormonal milieu.

**Figure 2.** General morphology of the mammary gland of female *L. maximus*. Up: schematic draw depicts the general morphology of a post-pubertal mammary gland of *L. maximus*. Bottom: representative photomicrographs of each cellular component of the mammary glands. (A) Lactiferous duct epithelium is continuous with the stratified squamous and keratinized skin epidermis. (B) Underlying dermis is formed by a layer of dense collagenous connective tissue containing hair follicles, sweat glands and fibroblasts. (C) Secretory parenchyma composed by intralobular ducts forming lobules that join into lobes. Connective tissue septa surround each lobule. (D) Epithelial lining of the ductal network is made up of a luminal secretory cell layer and a basal myoepithelial cell layer which rests on a basal membrane that separates the epithelium from the surrounding stroma. a, secretory alveolus; bm, basal membrane; de, dermis; ep, epidermis; ex, exocrine gland; i, intralobular duct; I, interlobular duct; m, muscle; my, myoepithelial cell layer; n, nipple opening; s, secretory cell layer; sp, connective tissue septum; v, blood vessel. Filled arrowhead: lobes. Empty arrowhead: lobules. Scale bar is 150 μm for photos A and B; 100 μm for photo C and 25 μm for photo D.
