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Cluj Vet J 2024, vol 29, issue 1 http://clujveterinaryjournal.ro 

 
Article 

Distribution and functional significance of muscle islands in 
the tunica media of the aorta in the goat (Capra hircus) 

Cosmin-Rareș Creț1, Melania Ioana Crișan1,*, Radu Constantinescu2, Călin Lațiu2 , Mircea Cipou3, Cristian Olimpiu Martonoș1,4 
and Aurel Damian1  

1. Faculty of Veterinary Medicine, University of Agricultural Sciences and Veterinary Medicine Cluj-

Napoca, 400372 Cluj-Napoca, Romania 

2. Faculty of Animal Science and Biotechnologies, University of Agricultural Sciences and Veterinary 

Medicine Cluj-Napoca, , 400372, Romania  

3.  Assisi Veterinary Clinic, Swords, Co. Dublin, Ireland  
4.  School of Veterinary Medicine, Ross University, Basseterre P.O. Box 334, Saint Kitts and Nevis 

 
* Correspondence: melania.crisan@gmail.com;  

 

Abstract: The presence of muscle islands in the tunica media of elastic arteries has been reported in both large and small ruminants. 
According to some authors, these islands can have a certain influence on the physical-mechanical properties and the pattern of dis-
eases in the aortic wall. The aim of the current study was to highlight the muscle islands and their distribution throughout the aortic 
segments in the goat and to understand the reason why they are present only in certain species of animals, including the goat. Gross 
anatomical dissection of the aorta was performed and samples from the following segments were harvested for histological investi-
gations: ascending aorta, aortic arch, descending thoracic aorta and descending abdominal aorta. The muscle islands in the wall of 
the aortic segments are formed by smooth muscle cells interconnected with connective tissue and are preferentially vascularized by 
vasa vasorum and innervated by nervi vasorum. The aorta in the goat presents polymorphic muscle islands, arranged in the outer 
two thirds of the tunica media in the ascending aorta, the outer half at the level of the aortic arch and the descending thoracic aorta, 
but they are absent in the descending abdominal aorta. Their presence completes the windkessel function of the aortic wall, thus 
constituting an additional pump necessary to propel the blood towards the abdominal viscera, which in goats, in addition to those 
present in monogastrics, also contain three large and very active organs, namely the rumen, the reticulum and the omasum. 

Keywords: goat, aorta, tunica media, muscle islands  
 

  

1. Introduction 

During ventricular systole, blood is forced into the aorta in the form of inter-
mittent jets, so it exerts a certain pressure on the vascular walls. Due to this pres-
sure, the elastic structures in the wall of the arteries expand and store part of the 
pressure exerted on them, causing the dilation of the arterial lumen, to take over 
the surplus blood that arrives here during the ventricular systole. During diastole, 
the elastic fibers passively contract and return to their original size, causing excess 
blood to propel to the next segments. In this way, the elastic fibers produce what 
is called the windkessel effect during ventricular systole and diastole [1]. The 
muscle cells existing between the elastic lamellae complement the action of the 
elastic fibers through active contraction, so that through the action of the elastic 

Received: 13 December 2023 

Accepted: 14 February 2024 

Published: 9 April 2024 

DOI:10.52331/z3nc1197 

 

 

 

Copyright: © 2024 by the authors. 

Submitted for possible open access 

publication under the terms and 

conditions of the Creative Commons 

Attribution (CC BY) license 

(http://creativecommons.org/licenses

/by/4.0/). 



Cluj Vet J 2024, vol 29, issue 1 10 of 37 
 

and muscular components, blood flow becomes continuous in the following vessels [2].  
In elastic arteries, the thickest tunic is the media containing the concentric elastic lamellae, 

between which there are circularly arranged smooth muscle fibers, collagen fibers, and reticular 
fibers [3]. These components have a particular arrangement among the elastic lamellae, which en-
sures their participation in completing the windkessel mechanism. In lower pressure arteries such 
as the abdominal descending aorta, the windkessel mechanism is complemented by a prominent 
internal elastic limiter [4]. 

In some species of mammals, the muscle component in the middle of the elastic arteries ap-
pears significantly better represented and with a particular disposition. Thus, [5] found that in the 
goat, the tunica media of the ascending aorta, the aortic arch and the descending thoracic aorta, 
present two areas: the luminal elastic area, which has a structure comparable to that of elastic ar-
teries from other species, and the adventitial musculo-elastic area, in which elastic lamellae are 
zonally interrupted by polymorphic muscle islands. The presence of muscle islands in the tunica 
media of elastic arteries has been reported in both large and small ruminants [6-9]. According to 
some authors, these islands can have a certain influence on the physical-mechanical properties and 
the pattern of diseases in the aortic wall [9]. The aim of the current study was to highlight the 
muscle islands and their distribution throughout the aortic segments in the goat and to understand 
the reason why they are present only in certain species of animals, including the goat. 
2. Materials and Methods 

The biological material used in this study was represented by 5 adult goats, of common 
breed, slaughtered for consumption in a CE approved slaughterhouse. The study was conducted 
according to the guidelines of the Declaration of Helsinki, and approved by the Bioethics Com-
mittee of the University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, nr. 403 
from 29.09.2023. The AVMA guidelines set criteria for euthanasia were followed, and the proce-
dure was in line with the recommendations of the permanent OIE (World Organisation for Ani-
mal Health) Animal Welfare Working Group for Humane slaughter of animals.   

Gross anatomical dissection of the aorta was performed and samples from the following seg-
ments were harvested for histological investigations: ascending aorta, aortic arch, descending tho-
racic aorta and descending abdominal aorta. The samples were fixed in 10% formalin solution for 
7 days, then dehydrated in three successive baths of ethyl alcohol (70%, 96%, and absolute), clari-
fied with three baths of 1-Butanol and embedded in paraffin. Sections with a thickness of 5 μm 
were made, which were stained by the Trichrome Goldner method. The obtained preparations 
were examined under an Olympus BX41 microscope, equipped with a digital camera for capturing 
images, model Olympus E-330.  
3. Results 

The first segment of the arterial system at the exit from the heart is the ascending aorta, which 
in the goat has a very peculiar wall structure, differing in some respects from that of most mam-
mals. These particularities are found in the media, while the intima and adventitia are comparable 
to the existing situation in other mammals. The particularity consists in the fact that the ratio be-
tween the elastic and the muscular component is different here and the disposition of the muscular 
component has some particularities. In the media of the ascending aorta of the goat, two areas can 
be distinguished, an internal one that occupies approximately 40% of the wall thickness that has 
the appearance found in the elastic arteries of other mammalian species, and an external one that 
occupies approximately 60% of the wall thickness in which the muscular component it is much 
better represented compared to the intima.  Here the muscle component forms islands arranged 



Cluj Vet J 2024, vol 29, issue 1 11 of 37 
 

at a certain distance from each other, without respecting a rigorous distribution. Islets are poly-
morphic in both shape and size and even muscle cell orientation (Fig. 1). 

 
Figure 1. Ascending aorta of the domestic goat (ob. 4X) 

Black arrow: muscle islands in tunica media; red arrow – tunica adventitia; blue arrow - tunica 
intima 

The situation is largely the same in the middle of the aortic arch, both in terms of the presence, 
distribution, shape, density or appearance of the muscle islands in the tunica media. (Fig. 2). 

 

 
Fig. 2 Aortic arch (ob 4X) 

Black arrow: muscle islands in tunica media; red arrow – tunica adventitia; blue arrow - tunica 
intima 



Cluj Vet J 2024, vol 29, issue 1 12 of 37 
 

In the descending thoracic aorta, the situation is similar to that existing at the level of the aortic arch, 
which is otherwise continued by this arterial segment. Muscle islands are numerous, with the most and larger 
islands being found in the outer third of the media versus the middle where the islands are somewhat scarce 
and the muscle cells less dense (Fig. 3). 

 

 
Fig. 3 Descending aorta – thoracic segment 221 (ob 4X) 

Black arrow: muscle islands in tunica media; red arrow – tunica adventitia; blue arrow - tunica 
intima 

The situation changes radically at the level of the descending abdominal aorta where muscle islands 
are no longer found as in the previous segments, but the general appearance is that of a typical elastic artery 
in which the main component is the elastic one represented by relatively rigorously arranged wavy elastic 
lamellae and between them cells muscle and collagen fibers. It should be noted that the adventitia of the 
descending abdominal aorta is thicker than that at the level of the ascending aorta, the aortic arch and the 
descending thoracic aorta (Fig. 4). 

 
Fig. 4 Descending aorta – abdominal segment 345 (ob. 4) 

Black arrow: tunica intima; blue arrow -  tunica intima; red arrow – tunica adventitia   
 



Cluj Vet J 2024, vol 29, issue 1 13 of 37 
 

4. Discussion 
In our study, we found the presence of polymorphic muscle islands in the middle of the 

ascending aorta, aortic arch, and descending thoracic aorta, but not in the descending abdominal 
aorta. These muscular islands zonally interrupt the concentric elastic lamellae characteristic of 
elastic arteries. In terms of number and location, the muscle islands were the most and largest in 
the ascending aorta, for their number and size to decrease progressively so that at the level of the 
abdominal aorta they disappeared completely. As an extension they are present in the outer 2/3 of 
the media in the ascending aorta and in the outer half in the case of the aortic arch and the 
descending thoracic aorta. Muscle islands present in the external half of the media were also 
identified by [10] in ascending aorta, aortic arch and goat thoracic aorta up to T9. From T10 and 
T11, the islands visibly decreased in both size and arrangement, being present here only in the 
outer third of the media. At the level of the abdominal descending aorta, the muscle islands were 
no longer found. The presence of muscle islands only in the proximal segments of the aorta 
suggests that they are related to the blood pressure aspects of each segment. At least two functions 
have been attributed to these muscle islands: strengthening the aortic wall and supplementing the 
windkessel function [8,9].  

A gradual decrease was also noted in the case of elastic lamellae, an aspect that seems to be 
directly related to the stress given by blood pressure [11,12]. Blood propelled from the heart exerts 
the greatest tension on the ascending aorta, then the aortic arch and the proximal portion of the 
thoracic descending aorta. The structure of the walls of these aortic segments provides an elastic 
recoil that causes blood flow to become continuous and under significantly reduced pressure in 
the abdominal descending aorta [11]. In this context, the descending abdominal aorta has a lower 
number of elastic lamellae than the cranial segments, an aspect that has also been reported in 
humans [13]. Due to this similarity, we believe that among the goat aortic segments, the only one 
that lends itself as a model for studying aortic diseases in humans is the abdominal descending 
aorta. Being adapted to lower blood pressure than the cranial aortic segments, the goat abdominal 
descending aorta is structurally weaker than the thoracic aorta. This aspect appears to make it 
more vulnerable (prone) to atherosclerosis and aneurysm formation compared to the thoracic 
lineage [10, 14]. But there are also conditions such as penetrating atherosclerotic ulcers, which 
occur more frequently in the thoracic than in the abdominal lineage [15,16]. 
  The muscle islands in the wall of the aortic segments are formed by smooth muscle cells 
interconnected with connective tissue and are preferentially vascularized by vasa vasorum and 
innervated by nervi vasorum. During ventricular systole, the blood is forced as a jet into the aorta, 
the elastic lamellae stretch and due to the fact that they are interconnected with the muscle cells, 
they pull the muscle islands, which also stretch to some extent. During diastole, by the rebound of 
the elastic lamellae, the excess blood is pushed to the next segments, and the muscle cells contract 
and complete the action of the elastic lamellae. These muscle islands thus represent an auxiliary 
pump that helps propel the blood by amplifying the elastic recoil during diastole but also ensures 
the increase of the mechanical resistance of the aortic wall [9]. Some authors claim that the muscle 
islands are probably involved in the regulation of blood flow to certain anatomical regions, more 
requested in a certain period. An argument in support of this statement is the fact that a similar 
arrangement allows for drastic circulatory changes in the posterior part of the bird's body during 
flight [17]. Another example is the regulation of blood flow for the brachiocephalic trunk and 



Cluj Vet J 2024, vol 29, issue 1 14 of 37 
 

common carotids in the giraffe [18]. These examples suggest the continuous adaptation of animals 
regarding feeding and locomotion with and against gravity [1]. These examples suggest that 
muscle islands constitute an auxiliary pump in vessels that conduct blood in a cranial direction, 
while the aortic segments taken in our study conduct blood in a caudal direction. The presence of 
muscle islands in the wall of very well represented goat aortic segments suggests the need for an 
auxiliary pump to propel blood caudally. In other words, the goat needs a larger amount of blood 
sent to the abdominal viscera than other species of comparable size. The explanation is the fact that 
the goat's digestive system contains three organs in addition to monogastric animals, namely the 
rumen, the reticulum and the omasum. These organs, in addition to being large, are also very 
actively involved in the complex digestion process characteristic of ruminants. The existence of 
these gastric compartments makes the need for blood sent through the aorta greater in a 
polygastric animal than in a monogastric of comparable size. This would explain why, in the goat, 
the aortic media requires this additional blood propulsion pump to provide the right amount of 
blood and pressure to supply the viscera. 
 
5. Conclusions 

The aorta in the goat presents polymorphic muscle islands, arranged in the outer two thirds 
of the tunica media in the ascending aorta, the outer half at the level of the aortic arch and the 
descending thoracic aorta, but they are absent in the descending abdominal aorta. Their presence 
completes the windkessel function of the aortic wall, thus constituting an additional pump 
necessary to propel the blood towards the abdominal viscera, which in goats, in addition to those 
present in monogastrics, also contain three large and very active organs, namely the rumen, the 
reticulum and the omasum. 
 
 

Author Contributions:  
Conceptualization: Rareș Cosmin Creț, Melania I. Crișan, and Aurel Damian; methodology: Cristian Olimpiu Mar-
tonos, Mircea Cipou and Aurel Damian;  software : Radu Constantinescu and Călin Lațiu; writing—original draft 
preparation: Melania Ioana Crișan; writing—review and editing: Melania I. Crișan, supervision - Aurel Damian. All 
authors have read and agreed to the published version of the manuscript. 
Funding: This research received no external funding  
Institutional Review Board Statement:  The study was conducted according to the guidelines of the Declaration of 
Helsinki, and approved by the Bioethics Committee of the University of Agricultural Sciences and Veterinary Medicine 
Cluj-Napoca, nr. 403 from 29.09.2023. 
Acknowledgments: In this section, you can acknowledge any support given which is not covered by the author contri-
bution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials 
used for experiments). 
Conflicts of Interest: The authors declare no conflict of interest. 

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