Mesenchymal Tissues

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The greatest amount of bulk of the body is composed of the cells forming tissues that are considered "soft" tissues or connective tissues. These are embryologically derived from the mesoderm. Hence, they are often called "mesenchymal" tissues.

The major cell types derived from mesoderm are:

  • Fibroblasts: form much of "connections" of connective tissues in the form of capsules, sheaths, fascial planes, tendons, and ligaments.

  • Mesothelium: forms the single layer cuboidal cell lining of body cavities and spaces, such as the peritoneal cavity or synovial cavity of the knee joint.

  • Endothelium: forms the inner surface of vessels (arteries, veins, capillaries, lymphatics) that contains the contents of the vascular system.

  • Adipocytes: forms the storage site for lipids

  • Myoblasts: form muscle (skeletal, cardiac, smooth)

  • Chondroblasts: form cartilage.

  • Osteoblasts: form bone.

Mesenchymal tissues can be part of many organs and help to give the organs shape and strength. The basic component of many soft tissues or supporting structures is the substance collagen. Collagen is a protein that is woven from fibrils that give it both strength and resilience (the ability to bend or bounce back). In addition to collagen, connective tissues include reticulin fibers (seen in many solid organs such as liver) and elastic fibers (which contain the proteins elastin and fibrillin and have even more resilience than collagen).

Connective tissues also contain a large amount of material that is extracellular. This material is called "ground substance". It provides much bulk to the tissues. Such ground substances include glycosaminoglycans such as hyaluronic acid, dermatan sulfate, and keratan sulfate. Intercellular materials can also include the fibronectin "glue" and supportive collagen fibers.

Soft tissues include:

  • Adipose tissue: this is "fat" that is composed of individual cells called steatocytes. Each steatocyte has a large amount of cytoplasm that is devoted to the storage of lipids. The nucleus is pushed to one side. An H&E section of adipose tissue looks like a "chicken wire" fence, since the lipids are removed in tissue processing.

    One additional type of fat appears in the fetus and infant as "brown fat" that consists of cells having multiple small lipid vacuoles. These cells can store lipid that provides a greater energy source for a small baby.

  • Tendons and Ligaments and Fascia: these tissues are mainly composed of collagen fibers that were made by fibroblasts. They are designed to have great tensile strength but be flexible. These tissues connect the bones and muscles. The fascia also provide support for adipose tissue and muscles.

  • Muscle: There are three types of muscle: skeletal muscle, cardiac muscle, and smooth muscle. The first two types are both forms of striated muscle.

    Skeletal muscle is called "voluntary" muscle because you make a conscious decision to move such muscles to perform some act, such as picking up a pencil or walking. A muscle is composed of individual muscle cells that contain many muscle filaments (myofibrils). The filaments compose a muscle fiber. A skeletal muscle fiber has multiple nuclei at its periphery. The size of a muscle is determined by the number of cells and the size of the fibers. A muscle that provides for fine movement, such as the muscles that move the eye, tends to be quite small. A muscle that provides strength, such as the biceps of the arm for lifting, tends to be large. The number of muscle cells is fixed at birth, and the size of the muscle is determined thereafter by the stresses placed upon it--exercise.

    Each myofibril is composed of many myofilaments that are the actual contractile actin and myosin proteins. Nuclei are shared and appear external to the long cylindrical myofibrils. Each myofibril has a surrounding sarcolemmal membrane which has a variety of stabilizing proteins, including dystrophin. In some disorders of muscle known as dystrophies, there are abnormal inherited dystrophin genes that lead to abnormal sarcolemmal membranes, degeneration of muscle fibers, and collagen deposition. One such disorder is Duchenne muscular dystrophy.

    By light microscopy skeletal muscle fibers have dark A bands with lighter I bands bisected by an anchoring Z disk. Z disks between myofibrils are anchored by the intermediate filaments desmin and vimentin. The contractile proteins in the fibril are thin actin and thick myosin filaments. Tropomyosin with troponin molecules (T, C, and I) adjacent to the actin change conformation when bound to calcium (troponin C) to allow actin and myosin binding. The sarcoplasmic reticulum surrounds the myofibril and forms the T-tubules flanked by cisternae that conduct impulses from neuromuscular junctions down to individual myofibrils that result in muscular contraction through voltage-gated calcium ion release channels. Each myofibril is surrounded by a thin collagenous extracellular matrix called an endomysium. A group of myofibrils forms a fasicle that is surrounded by a connective tissue perimysium. The fascicles that form an individual muscle are surrounded by an epimysium. The vascular supply and innervation of the muscle are supplied through these connective tissues.

    Muscle contraction is triggered by impulses traveling down nerve fibers to motor end plates of muscle fibers where there is a synapse at the myoneural junction. Depolarization with a nerve impulse opens voltage gated calcium channels to cause release of acetylcholine from synaptic vesicles that are taken up by acetylcholine receptors on the sarcolemma to generate an action potential extending into the T tubules. In the disease known as myasthenia gravis, the body's immune system makes antibodies to acetylcholine receptors (acetylcholine receptor antibody). The receptors are degraded. There is progressive muscular weakness, particularly with repetitive use and depletion of acetylcholine.

    Cardiac muscle resembles skeletal muscle in form, but it is involuntary, and must continue to constantly work your whole life long to pump blood. The cardiac muscle myofilaments do not form distinct myofibrils, but form a syncytium of fibers. In addition, there are intercalated discs between muscle cells. The nuclei are centrally located in the fibers. The T tubules form diads and are larger than in skeletal muscle and calcium must be actively transported into cardiac muscle cells. Contraction is not as rapid as with skeletal muscle. Compared to skeletal muscle, cardiac muscle has increased numbers of mitochondria and increased amounts of myoglobin because of the increased need for energy and oxygen.

    Smooth muscle is known as "involuntary" muscle because it moves in response to nervous system or hormonal control that is not predicated by direct conscious mental activity. Smooth muscle is mainly found as part of the gastrointestinal tract and urinary system, providing movement of fluids. Smooth muscle is associated with vascular channels to control blood flow and regulate pressure. Smooth muscle cells form loose bundles arranged either longitudinally or circularly within organs to produce peristaltic waves of movement. Smooth muscle cells are fusiform in shape and each cell has a central nucleus. The contractile proteins are not regularly arranged to form striations.

    Smooth muscle cells do not have a T tubule system and do not have troponin binding, so contraction proceeds by release of calcium ion from vesicles beneath the cell membrrane known as caveolae. The calcium ions bind to calmodulin, which then activates a myosin light-chain kinase. Smooth muscle can be innervated by parasympathetic (norepinephrine) or sympathetic (epinephrine) endings. In some locations requiring fine control, as in the iris of the eye, each smooth muscle cell is innervated. In visceral organs such as the GI tract, some cells are innervated and impulses are passed through gap junctions to adjacent smooth muscle cells. Smooth muscle cells are labile cells that can divide and regrow if injured.

  • Bone: The real support for the body is provided by the skeleton. The skeleton is composed of bone and cartilage. Bones that form the skull are arranged in the configuration of flat plates. Most of the remaining skeleton has bones that have a tubular structure, giving them great strength but light weight.

    Bone has three major cell types:

    • Osteoblasts: these cells can divide and form new cells. They produce the bone matrix and sit on the surface of the bone that is formed. The matrix is known as osteoid before it becomes calcified. Once calcified, the matrix consists mainly of hydroxyapatite crystal. Osteoblasts can produce new bone in response to an injury such as a fracture.

    • Osteocytes: once an osteoblast becomes surrounded by matrix, sitting in a lacuna, it transforms into an osteocyte. These cells appear to be relatively inert and inactive, but they help to regulate remodelling of the skeleton, which is constantly taking place, albeit slowly.

    • Osteoclasts: these cells are derived from bone marrow stem cells and have a macrophage-like function. They are responsible for diminishing the amount of bone in the remodelling process. They sit within a Howship's lacuna on the surface of the bone. Parathyroid hormone stimulates osteoblasts to secrete osteoclast stimulating factor to cause osteoclasts to resorb bone (by the action of carbonic anhydrase and collagenase) and increase the serum calcium.

    Bone is formed in two ways:

    • Intramembranous ossification forms the flat bones of the skull. The osteoblasts differentiate directly from mesenchymal cells in a vascularized area that will become the bone. The osteoblasts produce osteoid that becomes calcified.

    • Endochondral ossification forms most of the bones of the body from a cartilagenous template. Perichondrium surrounding the template becomes a periosteum from which osteoblasts are derived that produced bone matrix and increase the diameter of the growing bone. Osteoclasts resorb bone centrally to form a marrow cavity. At the ends of the bone, epiphyseal regions are formed which have proliferating cartilage replaced by bone, so that the bone grows in length. Thus, endochondral bones have several regions: the central shaft is called the diaphysis, the eiphyses are at the ends of the bone, and the metaphysis is the junction between these two regions.

  • Cartilage: Cartilage is mainly composed of collagen. It is derived from chondroblasts that give rise to chondrocytes in a cartilagenous matrix. It very dense and strong but also resilient. In mature cartilage that has formed the cells called chondrocytes are not numerous and are present within an abundant matrix. The "territorial matrix" is found immediately adjacent to chondrocytes and is rich in chondroitin sulfate ground substance but sparse with collagen. The interterritorial matrix comprises most of the matrix and has abundant collagen. There are several types of cartilage based upon the composition of the matrix:

    • Hyaline cartilage: the ends of the bones that form movable joints have hyaline cartilage, with type 2 collagen, which is smooth and resilient. A tissue lining such a joint is synovium. Synovial cells secrete fluid that acts as a lubricant for the joint. The most common form of arthritis-degenerative arthritis, or "osteothritis"--results from excessive wear and tear on the cartilage of joints.

    • Fibrocartilage: this is a very stiff, durable cartilage that connects bones that do not have a wide range of motion--the vertebrae of the vertebral column and the symphysis pubis are examples. There is dense type 1 collagen with minimal proteoglycans. There is a fibrous appearing collagenous matrix between the scattered chondrocytes.

    • Elastic cartilage: this tissue has type 2 collagen and elastic fibers interwoven. This provides great resilience for structures such as the external ear, the nose, and the epiglottis, all of which can bend and resume their shape.

  • Joints provide articulation between bones. Synarthrodal joints, such as those between skull bones or the symphysis pubis, have almost no movement. Diarthrodal joints, such as those in extremities that move, have bone surfaces covered by hyaline (articular) cartilage and surrounded by a joint cavity lined by synovium and filled with a small amount of synovial fluid.