Chondroblasts are precursor cells of chondrocytes and form the extracellular matrix of cartilage tissue. During the process, they find themselves isolated from their neighboring cells in a lacuna and at that moment become the cartilage cells chondrocytes. The best known disease related to cartilage tissue is degenerative osteoarthritis.
What is a chondroblast?
In Greek, “chondros” means “granule” or “cartilage.” The word “blastos” is literally translated as “germ” or “sprout.” Accordingly, the medical biological term chondroblast is a loanword from Greek, which is composed of the two words mentioned. Chondroblasts are precursor cells of the so-called chondrocytes, which are significantly involved in the formation of cartilage tissue in the human body. Chrondroblast and chrondrocyte are not synonymous terms. Chondrocytes develop from chrondroblasts, which are still capable of division at the stage of their development. Thus, medical science uses the term chondroblast to refer to a stage of chondrocyte development in which differentiation and specialization are not yet complete. Chondrocyte formation is summarized as chondrogenesis.
Anatomy and structure
Mesenchyme is formed during the period of embryonic development and corresponds to an important filling and supporting tissue with polypotency. This means that many different types of tissue can develop from mesenchyme via differentiation and division processes. Mesenchyme originates from the mesoderm, i.e. the middle germ layer. In addition to connective tissue, tendons and bones, cartilage tissue develops from the mesenchyme. The tissue consists of star-like branched cells connected by projections and the nexus, carrying loose intercellular substance in their interstices. So-called prechondrocytes are formed from the mesenchyme by mitotic processes on the way to cartilage tissue. These are precursor cells of the chondroblasts. Chondrocytes develop from these chondroblasts over time. There is a difference between early chondroblasts and late chondroblasts, which are characteristically columnar in structure.
Function and tasks
Chrondroblasts are the basis for chondrocytes. Although they are ultimately progenitor cells, they themselves already perform important tasks in the human body. These tasks correspond to the production and secretion of various components of cartilage ground substance. Essentially, chondroblasts are capable of producing all components of the cartilage matrix. In addition to type II collagen, these components include glycosaminoglycans, in particular chondroitin sulfates, keratan sulfates and hyaluronic acids. The cells release the extracellular matrix of collagenous cartilage into their environment. This secretion results in matrix accumulation around the cells. Because of the progressive formation and secretion of extrecellular matrix, the matrix itself undergoes appositional growth that separates the secreting cells from their environment. Substances such as fibroblast growth factor-18 (FGF-18) stimulate the cells to form cartilage matrix. As they grow, the chondroblasts find themselves in a lacuna. A lacuna is an enclosed cavity that separates a chondroblast from its neighboring cells. As long as the extracellular matrix is still subject to a certain flexibility, the chondroblast can still divide. As soon as a single chondroblast is firmly enclosed in the lacuna from all sides, it loses its ability to divide. Matrix formation also ceases from this point on. If a chondroblast in its lacuna does not divide any further, nor does it form any more matrix, it has reached the end of its differentiation phase. We are then no longer talking about a chondroblast, but chondrocytes. In this context, chondrocytes are cartilage cells resident in cartilage tissue that make up the main component of cartilage. With the formation of chondrocytes, chondrogenesis is complete. For example, cartilage is relevant in the context of bone formation and represents an intermediate stage of bone tissue.
Diseases
One of the most well-known diseases related to human cartilage and chondroblasts or chondrocytes is osteoarthritis. This degenerative disease causes damage to the joints that is independent of inflammation and causes severe pain.The extracellular matrix proteins of chondroblasts degrade by proteases. Meanwhile, the cartilage-stimulating effect of fibroblast growth factor-18 is well known. For this reason, medical research is currently focusing on intra-articular injection of growth factor to compensate for cartilage defects in patients of osteoarthritis. Recombinantly produced human FGF-18 is currently (as of 2016) undergoing clinical trials. Chondroblasts and their secretion processes play a role not only in the context of osteoarthritis. They are also relevant for the so-called achondroplasia. This pathological phenomenon is a relatively common mutation that affects the growth of the skeletal system. Patients suffer from dysproportionate dwarfism. They are endowed with a relatively long trunk and their middle limb region is more or less shortened. The limbs of the patients appear plump. The growth disorder is caused by a mutational quantitative disorder of chondral osteogenesis. The hereditary disease is associated with a reduced number of chondrocyte receptors for the growth-promoting fibroblast growth factor FGFR-3. As a consequence, the chondroblasts cannot lay down sufficient extracellular matrix and thus cannot develop into chondrocytes to a sufficient extent. Thus, in the growth plate of cartilage tissue, chondrocyte proliferation and differentiation is reduced. As a result, chondral bone formation is impaired. In this type of bone formation, bone is formed via the intermediate stage of cartilage material and eventually ossifies from the inside or outside. When this process is affected by disorders, fracture healing after a bone fracture is also disturbed.
