Muscles are bundles of muscle tissue that specialize in contraction. Muscle contraction, or shortening, generates force that is used to create bodily movements or change the shape of an internal organ.
Muscle cells are elongated and arranged in parallel bundles that correspond to the direction of contraction. Because they are long and narrow, they are often called muscle fibers, or myofibers. Myofibers are filled with myofilaments that run parallel to the length of the cell and that are responsible for the process of contraction. These myofilaments are composed of two cytoplasmic proteins, actin and myosin. Sarcoplasm, the cytoplasm of a muscle cell, contains high amounts of myoglobin, an oxygen-binding protein, and calcium ions, which regulate muscle contraction. Muscle cells are encased by a plasma membrane called a sarcolemma, which fuses with tendon fibers at each end of the muscle fiber to join the muscle to a bone. Attached to the outside of the sarcolemma and surrounding each muscle cell is a basal lamina, the endomysium, made of a network of thin collagen fibers. The endomysium is involved in the elasticity of the muscle – – the ability of the muscle to bounce back to the previous shape after muscle contraction is finished.
Muscle tissue has very little extracellular matrix. Groups of muscle fibers are bound by connective tissue to form fascicles. Within this connective tissue, called the perimysium, is a rich network of blood vessels, lymphatics, and nerves that are responsible for bringing nutrients, respiratory gases, and the electrical impulses that initiate muscle contractions. A connective tissue called the epimysium also wraps around entire muscles, protecting them from friction when contracting against other muscles or bones.
Based on the differences in the shape of muscle cells and the arrangement of myofilaments, muscles are classified as one of the three types: skeletal, cardiac, or smooth muscles. Skeletal and cardiac muscles have a striated – – a “lined,” or “striped” – – appearance due to the precise arrangement of their myofilaments. In smooth muscles, myofilaments are arranged with less regularity; therefore, these muscles are nonstriated. Skeletal muscles are voluntarily controlled by the brain, while cardiac and smooth muscles are involuntarily controlled by the autonomic nervous system. Skeletal muscles attach to bones, allowing for movement. Smooth muscles are in the walls of the hollow organs of the digestive, urinary, reproductive, and respiratory systems. When smooth muscles contract, they propel the contents of these organs forward. Smooth muscles also form the walls of blood vessels. Contraction of these smooth muscles decrease the volume of the circulatory system, increasing blood pressure – – an important aspect of homeostasis. This is not to be confused with cardiac muscle contraction in the heart, which pumps blood through the blood vessels.
Skeletal muscle fibers are large and cylindrical. They are very long because they always reach from tendon to tendon, and, for muscles like the quadriceps in the legs, that can mean a length of two feet, in tall individuals! Skeletal muscle fibers are formed by the fusion of hundreds of fetal stem cells called myoblasts. This fusion process results in muscle cells that are very long and have many – – sometimes hundreds – – of nuclei. Because the sarcoplasm is filled with myofilaments, the nuclei are pushed to the periphery of the cell, just under the sarcolemma, and seem flattened. In longitudinal sections, nuclei appear in rows alongside the cylindrical cells. This is even more apparent in transverse cross sections of the skeletal muscle where nuclei are visible as small purple dots located on the periphery of the dark pink-stained muscle fibers.
The most striking feature of skeletal muscle fibers is the presence of striations, or lines, visible in longitudinal sections of the muscle. The striations are the result of the precise arrangement of actin and myosin into repeating units called sarcomeres that create a characteristic pattern of transverse bands.
To maximize the force that human skeletal muscle can produce, the muscle cells are organized in fairly parallel bundles by connective tissue membranes. The first membrane, the endomysium, envelops every muscle fiber. The muscle fibers are then grouped into bundles, called fascicles, and wrapped in perimysium, a connective tissue membrane. Blood vessels and nerves run throughout the perimysium and can be seen under the microscope at high magnification. The epimysium envelops the entire muscle to maintain its integrity, and to prevent friction as the muscle contracts against bones and other muscles. Larger blood vessels and nerves run through this layer as well. Because the perimysium and epimysium are connective tissues, they do have some cells – – especially fibroblasts and adipocytes – – spread between the collagen fibers. The nuclei of fibroblasts are visible. These nuclei are smaller and darker than those of muscle cells and are clearly located within the connective tissue, rather than being directly pressed against the myofibers.
Similar to skeletal muscle, cardiac muscle is also striated; however, cardiac muscle cells are smaller and shorter with a single, centrally located nucleus. Further, cardiac muscle cells are branched and are connected end-to-end by intercalated discs. Because cardiac muscle cells are not parallel to each other, but rather go in all directions to form the shape of the heart, in cross sections of cardiac muscle, cells seem irregularly shaped when compared to the cylindrical cells of skeletal muscle. Cardiac muscle cells are separated by plentiful endomysial connective tissue, which is rich in blood capillaries (identifiable by the leftover erythrocytes).
The centrally placed, ovoid nuclei of cardiac muscle cells are surrounded by a ring of lightly pink cytoplasm, making their central location even more noticeable, both on the longitudinal and transverse sections. Intercalated disks are darkly stained, thin lines across the cell, on the longitudinal sections of the muscle fibers.
Smooth muscle cells are spindle-shaped and much narrower than skeletal and cardiac muscle cells. They do not have visible striations as their actin and myosin fibers do not form repeated segments. The boundaries of individual fibers are indistinct. In longitudinal cross sections, cells are grouped into sheets and tend to overlap and stagger. Smooth muscle cells have centrally located, elongated nuclei that lie in the middle of the fiber length. In transverse cross sections, the fibers appear as circular disks with a centrally located nucleus.
Bone is a dense connective tissue that has calcified extracellular matrix, making bone a rigid and strong tissue with the ability to disperse tensile forces along the longitudinal axis of a bone, but not from the side. Side impacts quite often cause fractures.
There are four types of cells found within bone tissue. Osteoblasts, that deposit the bone, osteocytes that maintain it, and osteoclasts that resorb the bone to make space for new bone formation. Osteoprogenitor cells (also called osteogenic cells) are undifferentiated stem cells that develop into osteoblasts, when activated. Osteoblasts themselves become osteocytes, once surrounded by the bony matrix they secrete. Osteoclasts develop from monocytes and macrophages that arrive at the bone with the blood supply. Because they develop from a different stem cell line, osteoclasts differ in appearance from other bone cells.
As with all other connective tissues, bone tissue has cells, fibers, and ground substance. The cells account for about 2% of the bone mass. The rest of the bone consists of concentric layers of collagen fibers and the ground substance – – mostly hydroxyapatite crystals, a form of calcium phosphate – – which adheres to the collagen fibers. Both collagen and hydroxyapatite contribute to the resilience of bone: collagen gives bone flexibility and integrity, and hydroxyapatite provides strength. Low levels of collagen make for brittle bones; low levels of hydroxyapatite result in bones that are weak and easily bendable.
During bone deposition, osteoblasts produce collagen fibers, the organic portion of the bone matrix. This newly laid, not-yet-calcified bone tissue is called osteoid. Collagen fibers provide a surface onto which the hydroxyapatite crystals adhere. Hydroxyapatite crystals form when calcium phosphate and calcium carbonate combine with small quantities of other inorganic salts, including magnesium hydroxide, fluoride, and sulfate. During the process of deposition, osteoblasts get trapped within the matrix they are secreting. Trapped osteoblasts mature into osteocytes, slowing their metabolic processes and collagen production. Osteocytes do not add new bone, instead maintaining the mineral concentration of the surrounding matrix. Bone can only be added to the surface of a bone, or at the ends of the bone shaft, at the epiphyseal plates. At these sites, new osteoblasts are replenished via the differentiation of osteoprogenitor cells on the bone surface.
The functions and locations of the four types of bone cells are summarized in the table below.
|develop into osteoblasts||deep layers of the periosteum and the marrow|
|bone formation||growing portions of bone, by the periosteum and endosteum|
|maintain mineral concentration of matrix||entrapped in matrix|
|bone resorption||bone surfaces and at sites of old, injured, or unneeded bone|
table from BC OpenStax Anatomy and Physiology book, under the CC BY license
Compact and spongy bone
There are two types of bone tissue: compact bone, comprising the shafts of long bones, and spongy bone, which fills the ends of long bones. Most bones contain both compact and spongy bone tissue, but their concentration varies across different bones. Compact bone is dense and can withstand direct compression, while spongy bone has open spaces and can support weight, just as studs in the wall of a building can support sheetrock via the distribution of weight.
Osteons are the microscopic structural units of compact bone. An osteon is a long, cylindrical structure with a central channel called a Haversian canal. Bone is a highly vascularized tissue and Haversian canals contain blood vessels, lymphatics, and nerves. Deposition of bone occurs in concentric circles around the Haversian canal and is limited by the availability of nutrients. New layers of bone are added by the osteoblasts located on the external surface of the cylinder. Once osteoblasts are trapped by their own secretions, they mature into osteocytes, and the next generation of osteoblasts starts depositing the next layer.
Osteocytes are located in lacunae and fully surrounded by bone matrix. Osteocytes have a characteristic shape, with many processes branching from their cell body and reaching toward neighboring cells. During bone formation, the matrix calcifies around these extensions and forms tiny canals between lacunae called canaliculi. In canaliculi, the extensions of the osteocytes form cell junctions with their neighbors and, through gap junctions, are able to pass hormonal signals arriving in the blood to deeper layers of the bone. A transverse cross section of a long bone shows a typical arrangement of lacunae in concentric circles around a central Haversian canal.
Spongy bone, also known as cancellous bone or trabecular bone, looks like a sponge under the microscope. Spongy bone also contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae are found in the loosely arranged bone spikes called trabeculae. The trabeculae form along lines where load is exerted on the bone. Spongy bone, which is lighter than compact bone, helps balance the strength of the bone against the weight of the skeleton, thus reducing the energy needed to move it.
Gross anatomy of the bone
adapted from the BC OpenStax Anatomy and Physiology book under the CC BY licence
A long bone has three parts; the diaphysis and two epiphyses, one at each end. The diaphysis is the cylinder in the middle of the bone. The hollow inside of the diaphysis is called the medullary cavity, and is filled with fat or yellow bone marrow. The walls of the diaphysis are composed of compact bone.
The wider section at each end of the bone is called the epiphysis, which is made from a thin layer of compact bone and filled with spongy bone. Red marrow fills the spaces in spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing bone. When the bone stops growing in early adulthood, the cartilage is replaced by bone tissue and the epiphyseal plate becomes an epiphyseal line.
The outer surface of the bone is covered with a fibrous membrane called the periosteum. The periosteum contains blood vessels, nerves and lymphatic vessels that nourish compact bone. On the side of medullary cavity, the bone is covered by a delicate membrane called the endosteum where bone growth, repair and remodeling occur.
The periosteum covers the entire outer surface of the bone except where the epiphyses touch other bones to form joints. In this region, the epiphyses are covered with articular cartilage (Figure 8), a thin layer of hyaline cartilage that reduces friction and acts as a shock absorber.
Epiphyseal plate and longitudinal bone growth
Bones grow in length only at the zone between the epiphysis (head of the bone) and diaphysis (bone shaft). This zone, called the epiphyseal plate, is made of proliferating cartilage. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length. The epiphyseal plate is composed of the following four zones of cells and activity:
- the reserve zone
- the proliferative zone
- zone of maturation and hypertrophy
- zone of calcified matrix
The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the bone tissue of the epiphysis.
The proliferative zone is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes. It makes new chondrocytes to replace those that die at the diaphyseal end of the plate. Chondrocytes in the next layer, the zone of maturation and hypertrophy, are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. The longitudinal growth of bone is a result of cellular division in the proliferative zone and the maturation of cells in the zone of maturation and hypertrophy.
Most of the chondrocytes in the zone of calcified matrix, the zone closest to the diaphysis, are dead because the matrix around them has calcified. Capillaries and osteoblasts from the diaphysis penetrate this zone, and the osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus, the zone of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows in length when bone tissue is added to the diaphysis.
Bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones, which will be discussed later. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line.
Vocabulary list (partial)
compact bone (cortical bone)
osteoprogenitor cell (osteogenic cell)
spongy bone (cancellous bone, trabecular bone)
Scanning the vocabulary list, the prefixes “myo-” and “sarco-” are used repeatedly in words referring to muscles, just as “osteo-” refers to bone. Thinking back to other chapters, what prefixes are important when considering vocabulary and histological concepts?
How could you differentiate between a nuclei from a myofiber and one from the epimysium?
Construct a table describing the major histological differences and similarities between the three types of muscle tissue.
What is the difference between a myofiber and a myofilament? an osteoblast and an osteoprogenitor cell? an osteoblast and an osteocyte? an osteoblast and an osteoclast?
Why is the mandatory fluoridation of the municipal water supply an important government program?