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Bone Microenvironment

Yingying Jing1 and Xiao Chen2

1Shanghai University, Institute of Translational Medicine, No. 333, Nan Chen Road, Shanghai, 200444, China

2Changhai Hospital, Department of Orthopedics, Shanghai, No. 168, Chang Hai Road, 200438, China

1.1 Introduction

1.1.1 Cell Types

There are many types of cells in bone microenvironment [1], including genuine bone cells (osteoblasts, osteocytes, osteoclasts, and their precursors), cells of the hematopoietic and immune systems, stromal cells, adipocytes, fibroblasts, and endothelial cells and so on [2]. A growing body of evidence, with the development of techniques such as single-cell sequencing, proposes a fluidity in the ability of bone marrow (BM) stem cells to differentiate toward distinct lineages [3]. In this section, the main cells in the bone microenvironment are presented below with origins, functions, and identifications.

1.1.1.1 Genuine Bone Cells

1.1.1.1.1 Bone Mesenchymal Stem Cells

As defined by the International Society for Cellular Therapy (ISCT), mesenchymal stem cells (MSCs) are capable of adhering to plastic and capable of differentiating toward adipogenic, osteogenic, and chondrogenic pathways and other specific phenotypes [4]. In the bone marrow, the percentage of MSCs is very low in terms of numbers, only 0.01% [5], but these cells play an important role, especially CAR cells (CXCL12-rich reticulocytes), CD146+ cells, and Nestin+ cells [6]. CAR cells are a subtype dispersed within the bone marrow that regulates the cell cycle and hematopoietic stem cell (HSC) self-renewal through high expression of CXCL12 and SCF [7, 8]. CD146+ cells are a subtype predominantly found in the human vascular ecology that interacts with s and endothelial cells through the expression of Tie-2 and CXCR4 [9]. Nestin+ cells are associated with the nerves of the sympathetic nervous system (SNS) [6, 10] in the perivascular area of bone marrow [11]. It supports the homing role of HSCs and also regulates homeostasis of HSCs by maintaining high expression of various genes such as CXCL12, SCF, and Ang1 [11]. Besides, skeletal stem cells (SSCs) have been identified as a lineage-restricted subset of bone marrow mesenchymal stem cells (BMSCs) with self-renewal and osteochondral properties [6, 12]. They are able to differentiate into osteo-lineage cells, bone, cartilage, and stroma [13, 14] (Figure 1.1).

Figure 1.1 Classification of BMSCs.

Source: Adapted from Gao et al. [15].

1.1.1.1.2 Osteo-Lineage Cells

Osteoblasts include osteogenic progenitor cells, preosteoclasts, and osteoblasts. It is now accepted that the whole process can be divided into three distinct stages of differentiation. In the first stage, the transition in osteoblasts to pre-osteoblasts is marked by the expression of RUNX2 in osteoblasts. In the second stage, WNT-ß-catenin signaling acts on pre-osteoblasts to induce Ostrix expression. In the third stage, the expression of both RUNX2 and Ostrix drives differentiation toward osteoblasts [16]. Osteoblasts are located between the bone matrix, and they are derived from a subpopulation of osteoblasts [17].

Osteoblasts secrete extracellular matrix proteins, such as type I collagen, periostin, osteocalcin, and alkaline phosphatase. Among them, type I collagen plays an important role in bone mineralization by depositing calcium together with the hydroxyapatite form. Moreover, the mechanism of mutual coupling between osteoblasts and osteoclasts maintains bone mass homeostasis. The process of bone maintenance is sensitive to mechanical forces, and in response to mechanical loading, osteoblasts lead to increased bone formation by activating multiple signaling pathways, mainly the WNT-ß-catenin signaling pathway [18]. There are other conditions such as radiation and diet that also have an impact on osteoblast function [19-21].

1.1.1.1.3 Bone Lining Cells

Bone lining cells are also differentiated from osteoblasts [22]. In general, bone lining cells are defined as elongated, flattened cells on the bone surface in areas where no bone remodeling activity occurs [23]. Bone lining cells, similar to osteoblasts, express some level of alkaline phosphatase. However, bone lining cells phenotypically express intercellular adhesion molecules, but not osteocalcin, which is the major difference between them and osteoblasts [24].

Recent studies have shown that bone lining cells play an important role in bone remodeling. They communicate with osteoblasts deep in the bone matrix through gap junctions and regulate the transformation of HSCs between the undifferentiated state and osteoblasts under different conditions.

In addition, before bone-forming activity, bone lining cells first remove osteoclast remnants of matrix-by-matrix metalloproteinases [25], such as demineralized collagen [26]. Afterwards, osteoblasts can then enter the site to deposit new bone [27].

1.1.1.1.4 Osteoclasts

Osteoclasts are special cells from the monocyte/macrophage hematopoietic lineage, and morphologically, they are multinucleated cells. Their main hallmark is the expression of high levels of tartaric acid-resistant acid phosphatase and cathepsin K [28]. Osteoclasts play an important role in the coupling of bone formation to bone resorption through the RANK signaling pathway [29].

1.1.1.2 Chondral-Lineage Cells

Chondrocytes are cells that produce and maintain the cartilage matrix and characteristically express the SOX gene [30]. Prechondrocytes develop from MSCs, which later differentiate into chondrocytes. Growing chondrocytes continue to undergo cell division as they grow, and the divided daughter cells usually form cell clusters distributed in the cartilage matrix. In contrast, older chondrocytes have a basophilic cytoplasmic nature due to an increase in the rough endoplasmic reticulum [31]. Chondrocytes release extracellular matrix and collagen fibers to form elastic and collagen fibers [32].

1.1.1.3 Adipocytes

Adipocytes are abundant and occupy a large amount of space in bone marrow. The types of adipocytes include preadipocytes and mature adipocytes [31]. Adipose precursor cells are a specialized class of cells that do not contain lipid droplets but express adipocyte markers. They are usually present in large numbers in the perivascular area, especially in the intraosseous veins, and are not proliferative. They can maintain vascular function and inhibit bone formation by occupying space [33]. In addition, it is noteworthy that adipocytes have been found to be associated with many pathophysiological mechanisms [34]. For example, preadipocytes and mature adipocytes can recruit multiple myeloma cells via monocyte chemotactic protein-1 and stromal cell-derived factor-1a and produce many secreted factors that support multiple myeloma cells in the bone marrow [35].

1.1.1.4 Cells of the Hematopoietic Systems

1.1.1.4.1 Hemopoietic Stem Cells

HSCs produce billions of new blood cells every day and are responsible for the continuous renewal of blood. It is generally acknowledged that HSCs can further differentiate into two main types: common lymphoid cells and common myeloid cells [36]. HSCs can be obtained from umbilical cord blood, bone marrow, and adult peripheral blood. The most primitive human HSCs were identified as CD34+CD90+Lin- [37]. Depleted expression of CD45RA has also been used in combination with the above markers to identify primary HSCs [14]. Most HSCs are in a resting state and are activated upon external stimulation [38].

1.1.1.4.2 Lymphoid Cells

Common lymphoid progenitor cells are differentiated from HSCs stimulated by IL-7 [39]. Further, stimulated by cytokines such as IL-3 and IL-4, lymphoid progenitor cells differentiate into B lymphocytes [40]. Once maturation, B cells enter the circulatory system and eventually localize in the lymphoid follicles of peripheral lymphoid organs [41]. B cells are one of the major specific immune cells, accounting for 20% of peripheral lymphocytes [42]. In addition, lymphoid progenitor cells differentiate into natural killer (NK) cells in response to IL-15 stimulation [43].

1.1.1.4.3 Myeloid Cells

Common myeloid progenitor cells are differentiated from HSCs in response to stimulation by IL-3, GM-CSF, and M-CSF [44]. Myeloid progenitor cells can differentiate in two directions, toward granulocyte-macrophage progenitors and megakaryocyte-erythroid progenitors, depending on the stimulating factors in the bone...