1. Introduction

Stem cells can be defined as undifferentiated cells that have the ability to self-renewal; proliferate into undifferentiated cells, and to differentiate into various mature specialized cells [1]. There are different types of stem cells that have been classified according to their potency. Cells are described as pluripotent that is, embryonic cells from the blastocyst (4–14 days after oocyte fertilization), they can differentiate into all cell types of the adult organism. If, in addition, they can form the extraembryonic tissues of the embryo, they are described as totipotent (1–3 days from oocyte fertilization) which can give rise to all the embryonic tissues and placenta. Multipotent stem cells that is, embryonic cells from the 14th day onward, have the ability to form all the differentiated cell types of a given tissue. The stem cells that maintain only one

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

lineage are described as unipotent [2]. In the trilaminar embryo, a middle mesodermal layer is formed between the ectodermal and endodermal cell layer. This mesodermal cell layer contains mesenchymal stem cells (MSCs), which develop into connective tissue (mesenchyme) and it maintains the progenitor stem cells that persist after birth [3].

differentiate into a multitude of mesodermal cell types [13–15]. Bone marrow MSCs represent a heterogeneous population derived from the nonblood-forming fraction of bone marrow, but have the ability to regulate hematopoietic cell development. In vitro, adult mesenchymal stem cells resident in this bone marrow fraction differentiate into bone, cartilage and fat [16]. Recently, a standardized nomenclature for MSCs has been proposed and the term "multipotent mesenchymal stromal cells" has been introduced [15] to refer to this population of fibroblast-like, plasticadherent cells [17]. Their asymmetric division produces one identical daughter stem cell and a second progenitor cell that becomes committed to a lineage-specific differentiation program [18].

Stromal Cell Ultrastructure

29

http://dx.doi.org/10.5772/intechopen.76870

MSCs produce many growth factors and essential cytokines needed for cell proliferation and differentiation [19]. They also support hematopoiesis in bone marrow and play an indirect role in supporting other cell types during tissue repair [20]. Adult stem cells could overcome many of the ethical and technical debate associated with ESC as they are isolated from adult tissues, including bone marrow stromal cells, adipose-derived stem cells and adult skin stromal cells [21]. However, because of their limited differentiation potential (multipotent), they are less

The exact location of these cells in vivo is not known, but recent work suggests that MSCs are located in the perivascular spaces as sub-endothelial cells surrounding the vascular sinusoids in the bone marrow [23]. Bone marrow contains three main cell types: endothelial cells, hematopoietic stem cells, and stromal cells. Bone marrow connective tissue network is called the stroma. The stroma consists of a heterogeneous population of cells that provide structural and physiological microenvironment to support hematopoietic cells and forms a complex extracellular matrix, which supports the hematopoietic process [23]. However, the frequency of MSCs in human BM has been estimated to be in the range of 0.001–0.01% of the total nucleated cells. Furthermore, the frequency of MSCs declines with age, from 1/104 nucleated marrow cells in a newborn to about 1/10<sup>6</sup> nucleated marrow cells in an 80-year-old person [24].

Interestingly, MSCs reside in diverse tissues throughout the adult organism [25]. Nowadays, MSC populations have been obtained from many tissues other than the bone marrow, [26]

Mesenchymal/stromal cells (MSCs) have the ability to differentiate into a variety of different cells/tissue lineages; osteoblasts, chondroblasts, adipoblasts and reticular stromal cells [29].

likely to form tumors, although some are thought to be related to certain tumors [22].

3.2. Importance and uses

4. Biology of stromal cells/MSCs

including the adipose tissue [27] and placenta [28].

4.1. Tissue distribution

4.2. Properties

3.3. Location
