Bone and Muscle

Small Muscular Artery

As muscular arteries branch and decrease in size, the number of layers of smooth muscle cells in the tunica media decreases. Also, the internal and external elastic laminae become much less prominent.

Elastic Cartilage

Elastic cartilage is very cellular - note the abundant chondrocytes throughout this tissue. Its matrix has many elastic fibers in addition to type II collagen. This type of cartilage is typically found in their auricle of the ear and the epiglottis.

Intramembraneous Ossification

Intramembranous ossification is the direct conversion of embryological mesenchymal tissue to bone. The process begins when mesencyhmal cells differentiate into osteoblasts, which begin to synthesize osteoid that will eventually mineralize into bone.

Osteoclasts

Osteoclasts are large, multinucleated cells that are of the macrophage lineage. They resorb bone by secreting organic acids, which dissolve hydroxyapatite, and lysosomal enzymes, which break down the osteoid matrix. At the bone surface, osteoclasts lie in Howship's lacunae, surface depressions caused by the resorption of bone.

Smooth Muscle Cells

Smooth muscle fibers are long, spindle-shaped (fusiform) cells. Note the single and centrally placed nucleus in each smooth muscle cell. The absence of striation is also characteristic of the smooth muscle cells.

Fenestrated Capillary EM

This electron micrograph shows a capillary with a fenestrated endothelium. These capillaries are far more permeable than those with continuous endothelial linings. Identify the endothelial cells lining the capillary, and the fenestrae, or gaps, between the cells. Barely visible is an electron-dense line known as the diaphragm that functions as a filtration barrier. Fenestrated capillaries are prominent in the kidney, intestine and endocrine glands.

Venule

This image of a venule shows several of its characteristic features. Identify its endothelium and narrow layer of smooth muscle cells. The largest wall component of the venule is usually the adventitia composed of connective tissue.

Osteoblast EM

This EM shows an area of bone formation. Osteoblast cells are depositing osteoid, which is then calcified through calcium hydroxyapatite deposition into bone. Note the presence of collagen fibers next to the osteoblasts.

Hyaline Cartilage

- Most common type of cartilage, is composed of *type II collagen* - Often has a glassy appearance - Note the numerous chondrocytes in this image, each located within lacunae and surrounded by the cartilage they have produced. These cells have relatively small nuclei and often demonstrate lipid droplets in their cytoplasm. The spindle-shaped cells in the perichondrium can differentiate into chondroblasts that will eventually develop into chondrocytes. This type of cartilage is found in the nasal septum, at the ends of the ribs, and in the tracheal rings.

Chondrocytes

- Produce all of the structural components of cartilage, including *collagen, proteoglycans and glycosaminoglycans* - Can be found as individuals or in clusters called *isogenic groups* (represent recently divided cells) - Basophilic cytoplasm with lipid droplets

Elastic Artery (Aorta)

3 layers of the wall: 1. *Tunica intima:* Thin and composed of endothelial cells and their underlying supporting tissue 2. *Tunica media:* largest portion of the wall; composed of elastic fibers, smooth muscle and collagenous tissue 3. *Tunica Adventitia:* Outermost component; contains mostly connective tissue and a few small blood vessels called *vasa vasorum* that support the cells that make up the arterial wall.

Osteoclast EM

This EM shows an area of bone resorption. The osteoclast is a multinucleated cell that possesses a distinct membrane structure, known as the ruffled border. It is into this region that lysosomal enzymes and organic acids are secreted to break down the bone matrix.

Arteriole: Longitudinal Section

This arteriole contains a single layer of smooth muscle cells.

Capillary

Capillaries contain a single layer of endothelial cells and their basement membrane. The lumen of capillaries is so narrow that red blood cells have to pass in single file. The thin walls of capillaries facilitate exchange of gases and small molecules between the bloodstream and surrounding tissue.

Compact Bone

Compact bone consists of outer and inner sheets of lamellar bone (not seen here) and Haversian systems, shown here, that run parallel to the long axis of bones. Begin by identifying the concentric rings of lamellar bone that surround a Haversian canal. Osteocytes can be seen embedded in concentric rings in the bone matrix. Haversian canals contain nerves, blood vessels, and lymphatic vessels. Connecting adjacent Haversian canals at right angles are Volkmann's canals.

Artery and Venule

Compare the structure of the venule to that of the small artery. The lumens of the vessels are similar in size but the artery has a thicker medial layer with more smooth muscle. Veins, with thinner walls, are more compliant and capable of holding more blood. Consequently, arteries tend to maintain their shape better than veins in histological sections.

Continuous Capillary EM

Endothelial cells in continuous capillaries completely enclose the lumen of the blood vessel. The only gaps are the junctions between adjacent endothelial cells where small molecules can diffuse between the bloodstream and surrounding tissue. Continuous capillaries are prominent in adipose and muscle tissue and in the brain. Which protein primarily determines the rate of paracellular diffusion in endothelial cells? Which protein mediates adhesion between endothelial cells? Tight junctions regulate paracellular diffusion and claudins primarily determine the types of molecules that freely diffuse through the junctions. Vascular-endothlial cadherin (VE-cadherin) mediates attachments between endothelial cells.

Fibrocartilage

Fibrocartilage has a dense arrangement of cartilage fibers that are arranged in an orderly manner. Numerous chondrocytes are located within their lacunae and are spaced between the fibers. Fibrocartilage is primarily composed of type I collagen, and is located in areas like the intervertebral discs and the pubic symphysis. Note that the chondrocytes are surrounded by a matrix which helps differentiate fibrocartilage from dense connective tissue.

Skeletal Muscle: Cross-Section

Focusing on the individual muscle fibers, note the peripherally placed nuclei, which are characteristic of skeletal muscle, and the fibrillar texture of the sarcoplasm. Each muscle cell is surrounded by endomysium. Note the perimysium in between two adjacent fasciculi, and the small blood vessels within this supportive tissue.

Arteriole

In arterioles, the tunica media contains only one or two layers of smooth muscle cells. Contraction of the smooth muscle cells constricts the lumen of the arteriole, increasing vascular resistance and reducing the flow of blood.

Endochondrial Ossification

In endochondrial ossification, bone is synthesized over a cartilage template. This image shows a growing tibia, with staining dependent on when each component was laid down. Begin by identifying the purple growth plate composed of cartilage with embedded chondrocytes. Similar areas of purple within the trabeculae are regions of calcified cartilage (note that this stain does not distinguish calcified from uncalcified components). Woven bone, laid down over the calcified cartilage, appears light blue in this slide. This preliminary bone will eventually be replaced by more organized lamellar bone. Recently formed bone, appearing red, can be seen laid down over the primary trabeculae of woven bone with a cartilage core.

Osteoblasts

Lining the trabecular bone are osteoblasts that are synthesizing bone. The initial step in bone synthesis is the formation of osteoid that consists of unmineralized collagen fibers which are produced by osteoblasts. Osteoblasts also secrete proteins and enzymes that catalyze the formation of calcium-phosphate crystals. Over time, these crystals will coat the surface of the collagen fibers to generate fully mineralized bone.

Lymphatic Vessel

Lymphatic vessels are responsible for draining interstitial fluid and returning it to the bloodstream. These vessels typically have thin walls and delicate valves that prevent backflow. Lymphatic vessels notably lack red blood cells, which help distinguish them from veins. The lymphatic system also plays an important role in generating immune responses.

Muscular Artery

Muscular arteries continue from elastic arteries and control the distribution of blood throughout the body. The tunica media contains 3 or more layers of smooth muscle cells along with collagen fibers and a few elastic fibers.

Muscular Artery Wall

The tunica media of muscular arteries contains fewer elastic fibers and more smooth muscle cells than elastic arteries. Note the prominent internal elastic lamina, a layer of elastic tissue in the tunica intima. The external elastic lamina separates the tunica media from tunica adventitia.

Chondrocyte Growth Sequence

This image shows the different stages of chondrocyte development during endochondrial ossification. In the initial stage, the chondrocytes are resting and the cartilage is not being converted to bone matrix. They then go through a stage of proliferation, where each lacuna contains numerous chondrocytes. This is followed by maturation/hypertrophy phases in which the chondrocytes and their lacunae appear much larger than when they were resting. The cartilage is eventually calcified. Finally, the calcified cartilage is adsorbed by osteoclasts, woven bone is laid down and after ~70 days is converted to lamellar bone.

Vena Cava

This image shows the wall of the vena cava, which is the largest vein in the body. Note the relatively thin media compared to the aorta.

Macroscopic Organization of Bone

This is a cross-section of a long bone. At the outer regions of the section, you can see a dense, thick layer of compact bone. The inner portion of the bone is composed of trabecular bone and the intervening bone marrow. Muscle attachments are visible along the outer surface.

Cardiac Muscle Cells

This is a high power view of cardiac muscle cells. Like smooth muscle, each cardiac muscle cell has a single (sometimes two) centrally located nucleus. Like skeletal muscle, cardiac muscle cells are striated due to a similar arrangement of contractile proteins. Unique to the cardiac muscle are a branching morphology and the presence of intercalated discs found between muscle fibers. The intercalated discs stain darkly and are oriented at right angles to the muscle fibers. They are often seen as zigzagging bands cutting across the muscle fibers. In the intercellular spaces, note the supporting tissue with an extensive network of capillaries. The abundance of capillaries ensures adequate delivery of oxygen and nutrients to meet the high metabolic demands of cardiac cells.

Skeletal Muscle: Longitudinal Section

This is a longitudinal section of skeletal muscle fibers. The skeletal muscle fibers are long and multinucleated. Again, note the peripherally placed nuclei in each cell and capillaries within the perimysium. Observe the striated pattern of skeletal muscle in longitudinal sections: the dark A-bands and the light I-bands.

Skeletal Muscle EM

This is an electron micrograph of a longitudinal section of skeletal muscle. First, focus on the components in the different bands. The I-Band contains actin filaments and is bisected by the Z-disk. The A-Band contains myosin and actin filaments. The M-line is a disc-like zone where myosin filaments are crosslinked. Where are the plus and minus ends of actin filaments located? The conducting system of skeletal muscle can also be observed in this slide. Near the junction of A-and I-bands, note the T-tubules in contact with the terminal cisternae of the sarcoplasmic reticulum. Each T-tubule is in contact with two terminal cisternae at the junction, and together they form a triad.

Haversian System

This magnified view of a Haversian system shows a prominent Haversian canal surrounded by concentric rings of lamellar bone in which osteocytes are embedded in their lacunae. Note the numerous canaliculi around the osteocytes. These are small canals in the bone matrix through which the osteocytes send thin cytoplasmic extensions. They allow the osteocytes to access the blood supply of the bone and to communicate with one another

Long Bone

This slide shows the regions of the end of the growing tibia. The epiphysis is the end of the bone, and is the portion that participates in the joint. The growth plate is located just below the epiphysis and is the portion of the bone in which cartilage proliferates and is mineralized. This is followed by the metaphysis, which separates the growth plate from the diaphysis, or shaft, of the bone. Observe regions of trabecular bone and cortical bone in this specimen. The outer surface of compact bone is covered with a fibrous material called periosteum to which muscles attach. The inner surface is called endosteum. A layer of hyaline cartilage called the articular cartilage covers the articulating surface of the bone. This sample was stained by Masson's Trichrome which stains cartilage violet-dark blue, mineralized bone blue-green and osteoid red.

Trabecular Bone

This slide shows trabecular, or spongy bone. Begin by identifying the trabeculae, the spongy network of bone tissue in the center of the bone. The surface of trabeculae are covered by a thin layer of inactive cells called endosteum. Between the trabeculae is the bone marrow.