1. Introduction

When an animal is born, it is necessary that several changes in the fetal circulation appear such as fast closure of some anatomical communication channels by coordinated mechanisms which arrows to autonomic life. The umbilical orifice permits the passage of the umbilical cord, being this characteristic a natural defect [1].

The majority of mammals have an umbilical cord and eventually an umbilicus, but sea shellfish, the whole animals, looks like an umbilicus [1].

The umbilical cord was originated from the embryonic stem which connects the bladder of both yolk sac and amniotic. This is discarded after birth in all species of mammals [2]. The umbilical cord forms a connection between placenta and the fetus. This structure is responsible for exchange of nutrients during the gestation. The main characteristic that umbilical cord shows is a specific gross morphology of vein and arteries surrounded of mucous connective tissue. It is known that fetus-mother nutrients exchange is very delicate and difficult to maintain; however, many morphological and functional alterations may produce changes in this mechanism of exchange easily.

In relation to the hematopoietic cells, these have been widely studied functionally, molecularly and structurally, but there are few studies about ultrastructural characterization [3]. The use of stem cells obtained from umbilical cord have originated a lot of expectative for use it in cell therapy and regeneration of organs [4]. Furthermore, it is known that the umbilical cord cells have been studied only in preclinical approaches [5].

The scarcity of bibliographic information about studies of optical microscopy of the umbilical cord in mammals and lack researches about these topics in mammalian species have motivated to execution of this type of reviews that reveal main characteristics in relation to the histology of umbilical cord and comparative aspects between different species of mammals of interest.

depend on their oxygen supply making them more vulnerable to changes originated by

Figure 1. Photograph of umbilical cord of alpaca (Vicugna pacos) showing two umbilical veins (V) and umbilical arteries (a). Next to the umbilical arteries is located allantoid duct (AD). (adapted from barrios-Arpi et al. 2017. Histological

Histology of Umbilical Cord in Mammals http://dx.doi.org/10.5772/intechopen.80766 49

This structure is coated by amniotic epithelium (simple squamous epithelium) and the conjunctive layer is adhered closely to the fundamental substance of the umbilical cord majority known as fetal mesenchyme, which is constituted of mesenchymal connective tissue with stellate cells and amorphous ground substance which contains abundant glycogen. This gelatinous composition also called Wharton's jelly or mucous connective tissue has been of great interest and potential impact due to research about tissue repair and differentiation [10]. However, many of the investigations have not been able to continue due to the lack of an animal model that can be used in the preclinical studies [11]. This mucous connective tissue (Wharton's jelly) is an active metabolically tissue involved in fluid exchange between umbilical

In the majority of domestic species, two arteries and two veins wound spirally being immersed in a mucous connective tissue appears [13, 14]; however, it is known that in some species, appear the formation of anastomosis of arteries in middle third and proximal to the maternal

There are important changes evidenced in the structure of umbilical cord in different animal species. It is indicated that from the beginning, vessels of umbilical cord are represented by two umbilical veins and arteries in species such as bovines and small ruminants [16], zebucrossed bovines [17], buffaloes [18–20], pigs [21], African lions and gazelles [22], and Bactrian camels and dromedaries [23, 24] (Figure 2); however, it is known that in some species the disintegration of right umbilical vein appears without have a strong explanation about this feature [25] (Figure 3). Among these species are carnivores, horses, guinea pigs, nutrias,

Finally, as the stages of gestation, the tunica adventitia is becoming thicker in relationship to

hemodynamic disorders and similar conditions [10].

characterization of umbilical cord in alpaca (Vicugna pacos).

vessels and amniotic fluid [12].

part of the placenta appears [15].

chinchillas, cavies and rock cavies [26–32].

the tunica intima in both arteries and veins [23].
