**2. Overview of chondrogenesis**

Cartilage is a specialized type of supporting connective tissue that contains cartilage cells, or chondrocytes, and a specialized extracellular matrix. The type of matrix is specific to the type of cartilage. Depending on the type of cartilage, there may be a perichondrium present, which is a connective tissue layer that contains mesenchymal cells or fibroblasts, as well as chon‐ droblasts. Cartilage develops from mesenchyme cells that commit to become chondrocytes. Cartilage is first seen in the developing embryo in the areas that serve as templates for endochondral ossification, as well as in Meckel's cartilage in the head area [1]. Differentiated chondrocytes are surrounded by matrix and can appear to have space around them and they retain their proliferative capacity within their matrix. Development of cartilage involves several different transcription factors, such as Sox 9, that control this process, as well as growth factors that interact with cellular receptors to facilitate various cellular functions [2]. Interac‐ tion with the extracellular matrix has been implicated in development as well. Various epigenetic mechanisms and several different microRNAs also are important in the develop‐ ment of cartilage [3,4]. Chondrocytes may remain as cartilage in specific regions of the body or may be used as a template for the formation of many of the bones in the body during endochondral ossification. Cartilage remains in the adult in several specific places including the bridge of the nose, the trachea and bronchi, the ear, the intervertebral discs of the spinal column, and on the ends of bones as articular cartilage [5]. Chondrocytes are usually consid‐ ered quiescent in the adult, although this can change during pathological processes such as osteoarthritis. Cartilage has very little capacity to regenerate, even though progenitor cells are present in the adult [6]. This lack of repair ability causes a number of disease issues as humans age, including the most prevalent form of degenerative joint disease, osteoarthritis [7].

#### **2.1. Types of cartilage**

Three kinds of cartilage have been identified: hyaline, elastic, and fibrous based on differences in matrix and cellular arrangement and function [5,8]. Hyaline is present in the bronchi, parts of the trachea, and also as articular cartilage. It also forms the model for bone formation during endochondral ossification. Another elastic cartilage is present in the oropharynx and ear, among other regions of the body. Both of these types of cartilage are typically surrounded by a connective tissue layer and a chondrogenic layer of the perichondrium. Cells from this layer can undergo proliferation and can differentiate into chondrocytes for growth or renewal of cartilage, a type of growth that is appositional in that the newly formed chondrocytes come from outside the cartilage matrix. In addition, cells within the cartilage can proliferate, forming isogenous groups of similar chondrocytes, which is termed interstitial growth. By contrast, articular cartilage, at the surface, is not covered by a perichondrium. This lack of a perichon‐ drium results in the proliferation of articular cartilage occurring only from within the matrix itself. An unfortunate consequence of this lack of a perichondrium is that the articular cartilage, continually exposed to stress from the movement of the joint, is less able to repair itself since no proliferation can occur from the perichondrium. Aging also leads to decreased healing ability and thus damage to cartilage may not repair sufficiently to prevent disease [9]. Damage to the articular cartilage leads to the most common [10] affliction of the aging population, osteoarthritis.

The third type of cartilage, fibrocartilage, is present as a transitional tissue generally connecting two other structures or tissues. Fibrocartilage does not have a perichondrium and thus only grows interstitially. Due to stress on this cartilage, the chondrocytes are typically arranged in a linear fashion [5]. One prominent place where fibrocartilage is present is in the annulus fibrosus of intervertebral discs. It is present in other areas such as part of the knee joint as well. Damage to this tissue may lead to herniation of the intervertebral disc, another major health issue as this may cause back pain [11].

Chondrocytes are surrounded by a thin pericellular matrix containing specific proteins, termed matricellular proteins, which have significantly different properties than the majority of the cartilage matrix [12]. This matrix and the associated chondrocyte are termed a chondron [13]. The matricellular proteins may confer the ability to withstand degradation by matrix metal‐ loproteinases (MMPs), and this may be due to the localization of type VI collagen in this region [14,15]. This collagen is not broken down by many matrix metalloproteinases. Surrounding the pericellular matrix is the matrix termed histologically the territorial matrix and in between groups of chondrocytes is the interterritorial matrix [5].
