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Acta Scientiarum 


ISSN printed: 1679-9283 
ISSN on-line: 1807-863X 
Doi: 10.4025/actascibiolsci.v39i3.36463 


Acta Scientiarum. Biological Sciences 

Maringá, v. 39, n. 3, p. 335-338, July-Sept., 2017 

Mammary gland morphology in the northern Amazon red squirrel 

(Sciurus igniventris)  

Thamara Cozzi Gonçalves


, Érika Branco


, Rogério Antônio Ribeiro Rodrigues


, Elane 

Guerreiro Giese


, Juliana Teixeira dos Santos

and Ana Rita de Lima




Laboratório de Pesquisa Morfológica Animal, Faculdade de Medicina Veterinária, Universidade Federal Rural da Amazônia, Av. Presidente 

Tancredo Neves, 2501, Montese, 66077-530, Belém, Pará, Brazil. 


Mineração Paragominas S/A, Hidro Paragominas, Empresa Eco Florestal. 

*Author for correspondence. E-mail: vetlima@uol.com.br 

ABSTRACT. Sciurus igniventris is a squirrel with broad distribution in South America. We examined one 

female adult specimen which was donated after death by trampling. The aim of this work was describe the 
morphological characteristics of mammary gland of one specimen. The body was fixed using 10% aqueous 
formaldehyde. Tissues were processed and slides prepared following standard histological protocols. 
Morphological analysis of the mammary glands revealed similarities to those described in the literature for 
domestic and wild mammals. As special features in kind, macroscopically identified a pair of thoracic 
glands, followed by three pairs of abdominal glands. Microscopically the abundant presence of connective 
tissue dividing the alveolar-tubular glands. Similar to that described for domestic and wild mammals.


Keywords: Microscopy, breasts, rodent. 

Morfologia da glândula mamária em esquilo do Norte Amazônico (Sciurus igniventris

RESUMO.  Sciurus igniventris é um esquilo com distribuição na América do Sul. Nós examinamos uma 
fêmea adulta que foi doada após morte por atropelamento. O objetivo do trabalho foi descrever as 
características morfológicas da glândula mamária de um exemplar da espécie. O animal foi fixado utilizando 
solução aquosa de formoldeído a 10%. Os tecidos foram processados e lâminas foram preparadas seguindo 
protocolos histológicos padrões. A análise morfológica da glândula mamária revelou similaridades com o 
que temos descrito na literatura para animais domésticos e selvagens. Como particularidades na espécie, 
macroscopicamente foi identificado um par de glândulas torácicas, seguidas por três pares de glândulas 
abdominais. Microscopicamente ocorreu a presença abundante de tecido conectivo dividindo as glândulas 
túbuloalveolares. Semelhante ao descrito para mamíferos domésticos e selvagens. 

Palavras-chave: Microscopia, mamas, roedor. 


Sciurus igniventris (Mammalia: Rodentia) is a 

squirrel species in the family Sciuridae (Emmons, 

1990). This species builds nests in trees that are used 

as resting places and shelter from inclement 

weather, as well as for protection against predators 

and provision of offspring, which require parental 

care from 3 to 5 years (Yahner, 1980; Carey, Wilson, 

Maguirre, & Biswell, 1997). The gestation period 

varies from 21 to 40 days, being generated from 2 to 

5 puppies (Nowak, 1999). 

Breastfeeding is performed via the mammary 

glands, which in females develop during the 

embryonic period. Mammary glands constitute a 

system of ducts filled with connective and adipose 

tissues. Each mammary gland complex contains a 

glandular functional body and a mammary papilla, 

which is an excretory system (Gurtler, Kertz, Kolb, 

Shroder, & Seidel, 1987). There are few descriptions 

of macro and microscopic features of squirrel 
mammary glands, and such knowledge is critical for 

an understanding of the physiological process of 

lactation in these animals and the perpetuation of 

the species. Thus, we present here a description of 

Sciurus igniventris mammary gland morphology

Material and methods 

One adult, female Sciurus igniventris from the 

Municipality of Paragominas (PA - Bauxite Mine 

Area) was donated after death by trampling to the 

Animal Morphology Research Laboratory (LaPMA) 

at the Universidade Federal Rural da Amazonia 

(UFRA). under authorization SEMA-PA n° 


The corpse was fixed and preserved using 10% 

aqueous formaldehyde solution, after which thoracic 

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Gonçalves et al. 

Acta Scientiarum. Biological Sciences 

Maringá, v. 39, n. 3, p. 335-338, July-Sept., 2017 

mammary glands and abdominal glands were 
removed by incision. The glands were dehydrated 

and soaked in paraffin using standard histological 

protocols, after which tissues were sliced using a 

microtome (LEICA 2165) to 5 μm thickness and 

subsequently stained with hematoxylin-eosin (HE). 

The histological slides were analyzed and 

photographed using a microscope with camera 

attachment (LEICA E - 400).  

Tissues were prepared for Scanning Electron 

Microscopy (SEM) by washing in distilled water, 

post-fixation in osmium tetroxide 1% (OsO


) for 2 

hours, and dehydration using ethanol solutions with 

increasing concentrations (from 50%) until reaching 

the critical point (QUORUM/K850). Mammary 

gland tissues were then mounted on aluminum base 

and metalized with Gold + Paladium 

(QUORUM/SC7620) for analysis (SEM model 


Results and discussion


Macroscopically, the distribution and quantity of 

S. igniventris mammary glands match the location 

and pair organization, i.e., on the thorax and 

abdomen.  We  found  a  pair  of  thoracic  glands 

followed by three pairs of abdominal glands (cranial, 

caudal and inguinal) (Figure 1). In general, domestic 

carnivores have 10 mammary  glands  arranged  in  5 

pairs, which are then arranged in 2 series. The two 

series are divided into 2 pairs in the thoracic region 

(2 cranial thoracic and 2 caudal thoracic) and 3 pairs 

in the abdominal region (2 cranial abdominal, 2 

caudal abdominal, 2 inguinal) (Ellenport, 1981). 

Casals et al. (2013) in a study with mammary glands 

of coati (Nasua nasua) and Bellatine et al. (2010) in a 

study with the "crab-eating racoon" (Procyon 

cancrivorus) both reported abdominal region 

glandular quantity and organization similar to that of 

S. igniventris

Microscopic evaluation revealed layering of 

mammary glands, including the epidermis, dermis, 
muscle tissue, and more internally, a layer of 
connective tissue (Banks, 1992; Samoto et al., 2006; 
Lima et al., 2008). The same cell composition was 
found in domestic specimens (Banks, 1992). The 
epidermis had typical characteristics, being 
composed of keratinized, stratified, squamous 
epithelial tissue, covered by a layer of loose 
connective tissue with some visible hair follicles. 
Just below the epithelial tissue is a thinner layer of 
muscle tissue that gradually increases in thickness, 
giving rise to a layer of connective tissue septa 
between the cells of the mammary gland complex 
(Figure 2A), as described by Banks (1992). The same 
pattern was observed by Casals et al., (2013) in a 
study of Nasua nasua. The presence of sebaceous 
glands in the dermis at the level of the tegmentum is 
generally common in mammals (e.g., coati, mocó, 
and raccoons) (Lima et al., 2008; Bellatine et al., 
2010; Casals et al., 2013). In contrast, we did not 
find these glands in S. igniventris. These glands were 
also absent in the rodent Kerodon rupestres (Lima et 
al., 2008). In the nipple region, the outermost layer 
of epithelial tissue was squamous, stratified, thicker 
and not pilous. Internally, microscopy revealed the 
presence of alveolar-tubular glands, also chambered 
by connective tissue and near the ducts (Figure 2B). 



Figure 1. Photomacrograph of Sciurus igniventris mammary glands in situ. Thoracic region mammary glands (pair) (T). Abdominal region 
(A) with a pair of cranial (Cr), caudal (Cd) and inguinal (In) mammary glands. Scale Bar: 3cm. 


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Mammary gland morphology in red squirrel 


Acta Scientiarum. Biological Sciences 

Maringá, v. 39, n. 3, p. 335-338, July-Sept., 2017 


Upon deeper examination, we discovered muscle 

fibers in the periphery of the mammary gland 

showed variation in size, initially thinner and then 

thicker. According to Franzo (2010), these constitute 

anatomical barriers against invading 

microorganisms, and act in the physical protection 

of the gland itself (Banks, 1992; Samoto et al., 2006; 

Lima et al., 2008). 

In  Kerodon rupestres, the connective tissue is 

intermingled with the gland, forming trabeculae in 

the cells (Lima et al., 2008). The interior of the 

mammary gland in S. igniventris is divided by 

connective tissue septa, and this feature is more 

pronounced than in Kerodon rupestres. This pattern of 

septation reported in squirrels is also found in wild 

animals such as the raccoon (Bellatine et al., 2010), 

and some domestic animals (Banks, 1992). 

However, this septation was not reported by Franzo 

(2010) in a study of mammary glands in ruminants 

and coati. Further, there have been reports in the 

literature that the presence or absence of this 

structure could influence physiology. 



Figure 2. Photomicrographs of Sciurus igniventris mammary glands. A: keratinized stratified squamous epithelium (1). Layer of thick 
striated muscle (2). Layer of thin striated muscle (2.1). Connective tissue dividing the glands (3). The alveolar duct (4). Hair follicles (red 
arrows). Scale Bar: 500 μm. B: Non-pilous, stratified squamous epithelium (1). The alveolar duct (2). Ducts (red arrow). Scale Bar: 500 
μm. C: A small quantity of connective tissue dividing the glands (1). Cells with a large quantity of parenchyma along the gland (red 
arrows). Scale Bar: 100 μm. D: Large quantity of connective tissue dividing the gland (1). Cells with large quantity of stroma (yellow 
arrows). Scale Bar: 200 μm. Color: Hematoxylin-eosin. E and F: Electromicrographs of S. igniventris mammary gland. E: Pilous tegument 
lining the outside of the gland (1). Connective tissue (2). The glandular region (3). Continuity of connective tissue dividing the glandular 
lobes (white arrows). Scale Bar: 500 μm. F: Connective tissue around the glandular lobules (1). Glandular lobules with acini present along 
the entire structure (white arrows). Scale Bar: 100 μm. 

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Gonçalves et al. 

Acta Scientiarum. Biological Sciences 

Maringá, v. 39, n. 3, p. 335-338, July-Sept., 2017 


Histological analysis of thoracic mammary 

glands indicated that for females in the lactation 

period, cells constituents present abundant 

parenchyma and reduced connective tissue (Figure 

2C). Cellular patterns of mammary glands in the 

abdominal region indicated a period of late lactation, 

in which cells had a higher amount of stroma 

compared to parenchyma, in addition to abundant 

connective tissue (Figure 2D); we have not yet 

found similar reports in the literature. 

SEM showed epithelial follicles lining the tissue 

around the nipple region, followed by connective 

tissue that continues into the adjacent layers within a 

connective septum, intermingling with the glandular 

lobules. The longitudinal exposed in the center of 

the region in which are located the glandular 

lobules, which consist of a set of acini connective 

tissue covering (Figure 2E and F). Although 

myoepithelial cells have been described and clearly 

identified in SEMs of female rat (Rattus norvegicus

mammary glands (Wigley, 2011), we did not find 

these cells in S. igniventris


Mammary glands morphology in this specimen 

was similar to that described for domestic and wild 

mammals in in terms of the numbers of layers and 

the general structural composition of cells. 

However, we highlight the distinguishing 

macroscopic characteristic of only one pair of glands 

in the thoracic region, and the presence of abundant 

connective tissue at the microscopic level. 


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Received on March 28, 2017. 
Accepted on April 26, 2017. 



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