BIOCHEMISTRY: A TEXTBOOK FOR UNIVERSITIES - E. S. Severin - 2004

SECTION 15. BIOCHEMISTRY OF THE EXTRACELLULAR MATRIX

V. Structural Organization of the Extracellular Matrix

As previously mentioned, the Extracellular matrix is a supramolecular complex formed by a complex network of interconnected macromolecules. In The Human Body, the extracellular matrix forms highly specialized structures such as Cartilage, tendons, and basement membranes, as well as bones and Teeth (via the secondary deposition of calcium phosphate).

These structures differ from one another both in their molecular composition and in the ways their main components (Proteins and Polysaccharides) are organized across Various Forms of the extracellular matrix.

A. Extracellular matrix of bone and dental tissue

Bone and dental Tissues are specialized types of Connective Tissue that perform several vital Functions in the human body:

✵ bones form the skeletal framework of the body;

✵ bones protect and support Internal Organs;

✵ bones serve as a storage depot for calcium and inorganic phosphate;

✵ Bone Marrow is a component of the Hematopoietic and Immune systems;

✵ teeth, as part of the masticatory apparatus, are part of the Digestive System;

✵ teeth are part of the human speech apparatus.

A remarkable property of bones is their combination of high tensile strength with very light weight. Bone and dental tissues feature a highly mineralized (or calcified) extracellular matrix, consisting by weight of approximately 50% Inorganic Compounds, 25% organic components, and 25% Water.

Inorganic Component

Bones contain 99% of the body's total calcium, 87% of its phosphorus, ~60% of its magnesium, and ~25% of its sodium. Calcium in bones is present in the form of the mineral hydroxyapatite, with the approximate composition Са₁₀(РО₄)₆(ОН)₂. Hydroxyapatite forms crystals typically measuring 20 x 5 x 1.5 nm. Bone tissue contains numerous Trace Elements, such as copper, strontium, barium, zinc, and fluorine, which play an essential role in the body's METABOLISM. The mineral fraction of bones also includes carbonates, hydroxides, and citrates.

The mineral composition of a tooth varies across its different parts. The hard Tissues of the tooth (enamel, dentin, and cementum) contain anywhere from 70% (cementum and dentin) to 96–97% (enamel) inorganic substances. The bulk of these substances consists of calcium phosphate, which forms part of the hydroxyapatite crystals (75%), along with calcium carbonate and calcium fluoride.

The soft tissues of the tooth (pulp and periodontium) are not classified as highly mineralized tissues. The pulp consists of loose Fibrous connective tissue (which is found in virtually all organs and forms their stroma or framework), whereas the periodontium is formed by Dense Fibrous Connective tissue, which is also a component of tendons and ligaments.

Organic Component

The organic substances of the bone matrix consist of proteins, Lipids, and small amounts of Proteoglycans.

The primary protein of bone tissue is type I Collagen (90–95%). In addition, the bone matrix contains proteins such as type V collagen, osteonectin, osteocalcin, bone morphogenetic proteins (BMPs), and Enzymes such as alkaline phosphatase (in osteoblasts) and acid phosphatase (in osteoclasts). Both of these enzymes serve as markers for their respective bone Cells. The carbohydrate portion of bone matrix proteoglycans is represented by dermatan sulfate and keratan sulfate.

The main organic component of dental tissue is type I collagen. Introduction/36.html">CARBOHYDRATES and lipids are present in small amounts. The organic matter content in the hard tissues of the tooth ranges from 2% (enamel) to 30% (dentin and cementum). The organic matter content in the soft tissues of the tooth is identical to that of corresponding types of connective tissue.

Bone Mineralization

The mechanisms of bone mineralization are still not fully understood, although it is known that all major components of bone tissue play a crucial role in this process, including bone-forming cells (osteoblasts) and bone-resorbing cells (osteoclasts). A determining factor in mineralization is the staggered arrangement of tropocollagen molecules, shifted by 1/4 of their molecular length (see Fig. 15-3). The gaps between tropocollagen molecules are believed to act as centers of bone mineralization, where the deposition of calcium phosphate begins—initially in an amorphous form, followed by The formation of hydroxyapatite crystals.

Osteoblasts control mineralization by regulating The transport of calcium and phosphate ions across their Cell membranes. The alkaline phosphatase they contain releases inorganic phosphate from organic phosphorus-containing compounds. The liberated phosphoric acid reacts with calcium salts to form Са₃(РО₄)₂. The glycoprotein osteonectin has a high affinity for type I collagen and hydroxyapatite. It contains Ca²⁺-binding domains and facilitates the precipitation of Ca²⁺ and РО₄^3- in the presence of collagen. Bone-specific acidic Phosphoproteins also play a specific role in mineralization. They contain poly-Asp and poly-Glu sequences that bind calcium, which may act as a triggering mechanism in the mineralization process.

Despite its high degree of mineralization, bone tissue is in a dynamic state, constantly undergoing remodeling to renew its constituent substances and adapt to changing environmental conditions.

Regulation of Bone Tissue Metabolism

The Formation of the bone matrix is regulated by biomechanical, hormonal, and other factors. Osteoblasts, which serve as target cells for parathyroid hormone, respond to elevated Blood levels of this hormone by decreasing collagen synthesis and increasing collagenase activity. Like parathyroid hormone, calcitriol induces bone resorption indirectly via osteoblasts, given that osteoclasts lack receptors for it. This stimulation of osteoclasts presumably occurs through direct contact with osteoblasts or via osteoblast-derived osteoclast-activating factors.

Prostaglandins (A, B, E₁, E₂, and F) and certain cytokines (such as epidermal growth factor, tumor necrosis factor, and IL-1) stimulate bone resorption and remodeling by acting on osteoblasts, which in turn secrete an osteoclast-activating factor.

Glucocorticoids inhibit osteoblast proliferation by suppressing DNA, RNA, and Protein Synthesis within these cells.

Sex Hormones play a distinct role in regulating the functional state of bone tissue. It is well established that women gradually develop Osteoporosis during menopause, and that this can be prevented through estrogen replacement therapy, which presumably reduces bone resorption. Concurrently, evidence indicates that estrogens also inhibit Bone Formation; consequently, the total amount of collagen remains unchanged, while the overall turnover rate of bone tissue slows down.

Calcitonin acts directly on osteoclasts, which possess specific receptors for it.

B. Extracellular Matrix of Articular Cartilage

The principal Components of the extracellular cartilage matrix are type II collagen, aggrecan, hyaluronic acid, and water. In addition, the matrix contains minor proteoglycans, types VI, IX, and XI collagens, link protein, other non-collagenous proteins (such as Fibronectin, anchorin, cartilage oligomeric matrix protein, and chondroadherin), and various growth factors. The "endoskeleton" of the cartilage matrix is formed by a fibrillar network composed of types II, IX, and XI collagens, which provides structural strength to the cartilage (Fig. 15-22).

Class="center">Fig. 15-22. Organization OF THE extracellular matrix in articular cartilage. HA — hyaluronic acid; DS — dermatan sulfate; CS — chondroitin sulfate.

Type XI collagen is located within the fibrils formed by type II collagen and plays a specific role in their assembly. Type IX collagen binds in an antiparallel orientation to type II collagen fibrils. Its globular NC₄ domain is basic and unattached to type II collagen fibrils, thereby allowing it to bind matrix components such as hyaluronic acid. Microfibrils formed by type VI collagen tetramers associate with both type II collagen fibrils and hyaluronic acid. Furthermore, they can attach to cells, which is why type VI collagen is referred to as a "bridging" molecule between The Cell surface and extracellular collagen fibrils.

High-molecular-weight aggregates consisting of aggrecan and hyaluronic acid function as polyanions due to their high content of acidic groups. This property promotes extensive Hydration of the cartilage matrix and ensures its Shock-absorbing function. The water content in articular cartilage is variable: under mechanical load, fluid is squeezed out until the Swelling pressure balances the external load. Once the load is removed, water flows back into the cartilage (Fig. 15-23). This phenomenon is vividly demonstrated in intervertebral discs. In the morning, following nocturnal rest, water accounts for approximately 75% of the disc's mass. Under daytime mechanical loading, the water content decreases by about 20%. As a result, a person's height is 1–2 cm shorter in the evening than in the morning. In astronauts under zero-gravity conditions, an increase in height of up to 5 cm has been observed.

Fig. 15-23. Changes occurring in articular cartilage upon compression and load removal. (+) — Na⁺, K⁺, Ca²⁺ ions.

Minor proteoglycans, such as decorin, bind to type II collagen fibrils and influence fibrillogenesis by restricting the diameter of these fibrils.

Fibronectin also plays a crucial role in organizing the cartilage extracellular matrix. The Biological Significance of these and other minor components of the cartilage matrix lies in their Participation in the assembly and organization of high-molecular-weight extracellular substances and in The regulation of chondrocyte function.

C. Extracellular Matrix of Skin Tissue

The primary organizing component of the skin extracellular matrix is type VII collagen. Bundles of fibrils formed by dimers of this collagen can attach via their C-termini to the lamina densa of the basement membrane (effectively "anchoring" into it) and form loops in the subepidermis. Such anchoring fibrils can connect the lamina densa of the basement membrane to anchoring plaques located in deeper subepithelial layers, which resemble basement membranes in composition (containing type IV collagen). Anchoring fibrils also entrap type I and type III collagen fibrils (Fig. 15-24).

Fig. 15-24. Organization of anchoring fibrils in subepithelial layers.

In this manner, type VII collagen anchoring fibrils ensure the secure attachment of the epidermis to the dermis.

D. Basement Membranes

Basement membranes are a specialized form of extracellular matrix synthesized by various cell types, including endothelial, epithelial, Muscle, nerve, and fat cells. Basement membranes appear as thin sheets that typically separate cells and cell layers from the surrounding connective tissue. For instance, they surround individual muscle fibers, adipocytes, and Schwann cells. In structures such as renal glomeruli and pulmonary alveoli, basement membranes are interposed between two distinct cell layers, functioning as a highly selective filtration barrier.

Electron Microscopy reveals a bilayered Structure of basement membranes: the lamina rara, located adjacent to The cell membrane, and the lamina densa, connected to the underlying connective tissue (Fig. 15-25). The primary components of basement membranes are type IV collagen, Laminin, and heparan sulfate proteoglycans (HSPGs).

Fig. 15-25. Structure of a typical basement membrane.

The insolubility and mechanical stability of basement membranes are provided by type IV collagen molecules, which organize into a specialized supporting network. This elastic three-dimensional network forms a structural scaffold to which other basement membrane components attach.

Laminin interacts with virtually all Structural components of basement membranes: type IV collagen, nidogen, and HSPGs.

Nidogen forms a non-covalently linked complex with laminin. In addition, nidogen contains a binding site for type IV collagen and can thus act as a bridging molecule between various components of the basement membrane.

Basement membrane HSPGs can form oligomers by joining the terminal domains of their protein core, as well as bind to laminin and type IV collagen.

Basement membranes perform diverse and complex functions. In the renal glomeruli, the basement membrane acts as a semipermeable filter that prevents macromolecules from passing from the plasma into the primary urine. A major role in this process is played by the high negative charge of proteoglycans, which impedes the passage of other negatively charged molecules (such as proteins) as well as negatively charged erythrocytes through the basement membrane. Furthermore, basement membranes play a crucial role in cell attachment and spatial orientation, embryonic development, and tissue regeneration.