**3. Types of stem cells**

Depending on their potential, stem cells can be classified into three types: totipotent (cells have the ability to originate all types of cells from the three germ layers endoderm, mesoderm, and ectoderm and extra-embryonic tissues); pluripotent (cells can originate cells from the three germ layers but not cells from extra-embryonic tissues), and multipotent (cells can originate cells from several types of tissues but only from one of the germ layers) [3]. Furthermore, stem cells can be collected from two major types of tissues, namely embryonic tissues and adult tissues. Their collection using biotechnology is a complex and costly process [11].

Embryonic stem cells (ESC) can be obtained from the inner cell mass of an early embryo. When removed, these cells can be cultured *in vitro* and have immortal characteristics. In addition, ESC may be induced to originate various cell/tissue types. For these reasons, ESCs are studied to better understand the mechanisms of organ formation and healing. Despite this, these cells can promote the formation of teratomas and can be rejected, when implanted into a patient, and there is some ethics controversy about the use of embryos in science [12].


#### **Table 1.**

*Summary of Adult Stem cells and Embryonic Stem cells and general characteristics.*

Adult stem cells (ASC) can be found in almost every tissue, including adipose tissue, skin, bone marrow, muscle, among others. Some ASC, especially MSCs, can produce growth factors and can differentiate into many lineages. As opposed to ESC, ASCs do not lead to the formation of teratomas, unless there has been some damage prior to its implantation. These different characteristics between ESC and ASCs are compared in **Table 1**. Neonatal stem cells from the amnion, placenta, and umbilical cord are commonly considered as ASCs [12].

Among all these sources, bone marrow-derived MSCs (BM-MSCs) and adiposederived MSCs (ADSCs) are the most studied and used in veterinary medicine due to the ease of obtaining, abundance of tissue of origin, and lack of moral restrictions [13].
