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Respiratory System & Bone
In this lab you will be looking at the respiratory system, from the olfactory epithelium down through the bronchopulmonary tree and into the alveoli. You will also be looking in more detail at bone and cartilage.
The key objects to concentrate on during this lab are listed below. You need to learn 1) how to recognize each object, 2) understand it's primary functions, and 3) understand how structure is related to function.
Cartilage and Bone:
In man, the olfactory epithelium is restricted to a small area in the roof of the nasal cavity. It is a specialized form of respiratory epithelium: a tall, pseudostratified columnar epithelium containing three cell types. Click link  and identify the olfactory mucosa and the bone. The space between the bone and the mucosa is an artefact. Continue with [1,1]. Using the drop-down menu identify the epithelium resting on its basal lamina and the myelinated (most likely trigeminal) and unmyelinated (olfactory) nerve bundles within the submucosa.
There are three types of cells in the olfactory epithelium: the olfactory receptor cells, supporting (sustentacular) cells, and basal cells. Click [2,1]. The olfactory cells are true bipolar neurons whose round, dark nuclei occupy the middle area of the olfactory epithelium. Each sensory cell gives rise to an olfactory knob at the apical surface. Highlight these structures. This knob is the dendritic process of the neuron. Although it is not clearly seen at this magnification, the olfactory knob gives rise to long modified cilia, which form a tangled mat. These cilia are thought to have the receptors for the substances that we smell. Zoom out twice and click [1,1] and find the matted cilia. Also notice the lymphocyte. It is probably a gd T cell that is permanently located in the epithelium.
Zoom out twice and click link [2,1]. The supporting cells are elongated cells with tapered bases that rest on the basal lamina. Their light lavender staining, round nuclei occupy the upper area of the epithelium. The supporting cells are probably secretory.
Zoom out once and click . The basal cells rest on the basal lamina and are thought to be stem cells for the other cells in this epithelium. Use the drop down menu to identify the basal cells.
Zoom out once. The submucosa contains, in addition to the nerve fibers, numerous serous glands called Bowman's glands. The Bowman's glands produce the watery substance in which the odorants are dissolved. Note the ducts of these glands piercing the epithelium - use the drop down menu. Zoom out once and click link  for more glands and nerve fibers.
Use this unlabeled virtual microscope slide to practice finding the features described above w81 olfactory epithelium. Can you identify the different cell types of the olfactory epithelium? How about the Bowman's glands and the unmyelinated nerve fibers in the submucosa?
First orient yourself with this 12x magnification using the drop-down menu.
Follow [6,1]. The epiglottis in this region is surrounded by stratified squamous epithelium. Most of the epiglottis is composed of elastic cartilage surrounded by a perichondrium. Find these structures. Continue with link [1,3,1]. The chondrocytes of elastic cartilage are irregular in shape with large dark nuclei. The matrix that they produce has many elastic fibers interposed with type II collagen fibers (collagen fibers only visible in EM). The elastic fibers do not stain well and are white or pink in appearance. Chondrocytes do divide (interstitial growth), note the post-mitotic chondrocyte. Note that there are no blood vessels in the cartilage.
Click the thumbnail and click link . This view shows the ventricular fold with numerous glands and adipocytes. While going through the rest of this slide you should pay attention to blood vessels, which are everywhere except in the cartilages. Continue with [1,1] to see a higher magnification view of the mucous glands and duct. Click [1,1] to see the cuboidal epithelium of the gland duct. The spindle-shaped cells outside the duct are fibroblasts.
Zoom out three times and click link . Remind yourself of what stratified squamous epithelium looks like. Click link  and find the mitotic figures. Note that the human larynx, except for part of the epiglottis and the vocal cords, is lined by pseudostratified ciliated columnar epithelium (so-called respiratory epithelium).
Click the thumbnail and then . Using the drop-down menu identify the vocal cord, vocal fold and the thyroarytenoid muscle. This muscle is skeletal muscle (cross-sectioned in this specimen so striations are not evident) and is voluntary. Follow [2,1,1]. The vocal cords have a connective tissue core, the stroma, covered by a thin layer of stratified squamous epithelium (in humans this epithelium is much thicker - Dr. Kokko-Cunningham notes that monkeys probably don't talk as much humans).
Click the thumbnail and then link [1,1]. The hyaline cartilage matrix stains a distinctive deep, basophilic purple with widely spaced rounded chondrocytes embedded in the matrix. The perichondrium contains chondrogenic cells for appositional growth of the cartilage. Continue with  for a higher magnification view of the chondrocytes.
Practice with this unlabeled virtual microscope slide: w52 larynx, Rhesus monkey. Can you find the epiglottis? What kind of epithelium covers it? Do you recognize the elastic cartilage in the epiglottis, and can you distinguish it from nearby examples hyaline cartilage? Can you find perichondrium? Do you recognize the vocal chord and the striated muscle that controls it? Can you find the mucous glands and ducts in the submucosa?
At this magnification identify the plates of cartilage that surround the bronchus. Check the drop-down menu. We believe that the vessel to the right is the pulmonary artery and the one to the left is the pulmonary vein. In the lung the arteries are not as thick walled as the same size arteries in the systemic circulation, reflecting the much lower blood pressure in the pulmonary circulation. Pulmonary veins, on the other hand, are thicker walled than the corresponding sized veins in the peripheral circulation. When you reach the smaller caliber vessels in the lung it becomes even more difficult to distinguish between arteries and veins.
Follow [3,3,1,1]. This is the best example of respiratory epithelium in our collection. Identify the epithelial type, the goblet cells, the ciliated cells, and the lamina propria. Zoom out twice and then click [2,1]. The chondrocytes are surrounded by a deeply basophilic rim, reflecting the relative abundance of ground substance over collagen fibers near the chondrocytes.
Click the thumbnail and then . This is an otherwise well-fixed specimen, but the alveoli are partially collapsed. Find the parenchyma using the drop-down menu. The parenchyma consists of a lacy network of thin walled tissue and a lot of open space. Using the drop-down menu identify the structures in this view. The wall of this small bronchus is much thicker than that of the nearby pulmonary vessels, and is thrown into folds. Click . Notice that there are bundles of smooth muscle in the wall. Click . There are many mucous glands and goblet cells present. The mucus traps particles that are then propelled by the ciliated cells toward the trachea.
Zoom out twice and click link . Highlight the respiratory bronchiole. The terminal bronchioles give rise to respiratory bronchioles. Click link . The respiratory bronchioles are lined by cuboidal epithelium without goblet cells. The transition between these epithelia is gradual. Zoom out once. The walls of the respiratory bronchioles are interrupted by out-pocketings of alveoli. The respiratory brochioles are the first area where gas exchange can take place. The respiratory bronchioles give rise to the alveolar ducts. Highlight an alveolar duct. It is extremely difficult to visualize the three dimensional architecture of the lung, but you might picture in your mind a bunch of grapes. To fully appreciate the architecture, you need to study many sections, or look at SEM views of lung casts.
Zoom out once and click [4,1]. This is a fairly decent picture of several alveoli. To review an idealized respiratory unit you may want to look at the lecture slide that shows a schematic diagram showing of the respiratory subdivisions. Follow this link to open the web lecture slides in a new browser window. When you're done reviewing the lecture slides, just close the window.Each alveolus is only 200 microns in diameter, but the total number approximates 300 million in both lungs. This gives an estimated total surface area of 140 m2 for gas exchange.
Click link . Between any two alveoli is the interalveolar wall or septum. It contains an extensive capillary bed that is supplied by the pulmonary artery and drained by the pulmonary vein. This is where blood is oxygenated and carbon dioxide is released. Find the capillary lumina within the interalveolar wall. Although not seen in this view, the septum may contain interstitial fibroblasts, macrophages, mast cells, and a few lymphocytes.
There are three layers that make up the air-blood barrier: the type I pneumonocyte, the capillary endothelial cell, and their fused basal lamina. Highlight the air-blood barrier. The individual layers of the air-blood barrier can only be seen in the TEM. The type I pneumonocytes make up approximately 95% of the alveolar surface area, although only 10% of the cell population. Using the drop-down menu find the type I pneumonocyte nucleus. Locate an example of an endothelial cell nucleus.
The alveolar pores (of Kohn), up to 15 microns in diameter, are present in some of the interalveolar walls. Use the drop-down menu to locate a pore in this section. These pores connect adjacent alveoli and thereby equalize the pressure between them. Unfortunately, bacteria also use them to travel between alveoli.
Also present in the alveolar wall are type II pneumonocytes, which are more numerous than the type I, but occupy only 5% of the alveolar surface. These cuboidal cells normally secrete the surfactant that allows the alveoli to remain patent during expiration. The clear vacuoles are the spaces that once contained surfactant.
Zoom out once. Using the drop-down menu highlight the capillary branching from the arteriole. This specimen was fixed by rinsing the alveoli with the fixative. Therefore the alveolar macrophages, normally seen roaming the alveoli, were removed.
Practice with this unlabeled virtual microscope slide: w51a bronchus, pulmonary artery and vein, dog. Can you find the plates of cartilage surronding the bronchus? Do you recognize the respiratory epithelium lining the bronchus? Can you find goblet cells and recongize cilia? Can you find the small bronchus and identify the smooth muscle cells in the smooth muscle cells in the lamina propria? Can you identify alveoli in the lung parenchyma? How about an alveolar duct or a respiratory bronchiole? Look at the alveoli in high magnification. Can you identify the nucleus of a type I pneumocyte? How about a type II pneumocyte? Can you find a pore Kohn?
Follow [2,2] and using the mouse-over highlight the bronchiole and the pulmonary vessel. The bronchiole has considerable amounts of smooth muscle in its wall. Click link  and highlight the cuboidal epithelium and smooth muscle. Cartilage is no longer present in the bronchiolar wall.
Click the thumbnail and then [1,1]. Highlight the alveolar duct. Off the duct are four or more alveoli. Click link . The smooth muscle in the duct wall is visible at this magnification.
Click the thumbnail and then [2,1]. The thoracic cavity is invested with a serous lining called the parietal pleura. At the hilum, it is continuous with the visceral pleura that covers the lungs. Highlight the visceral pleura. Click link . The pleura consists of a layer of squamous epithelial cells under which lies a fibroconnective tissue layer containing capillaries, lymphatics and a few nerves.
There are also a few labelled type II pneumonocytes here. They can be recognized by the clear vacuoles that once contained surfactant.
Practice finding all of these structures on this comparable virtual microscope slide: w51b lung.
There are two main types of bone: compact (cortical) bone and cancellous (spongy or trabecular) bone. Compact bone forms the dense outer layer of all bones, constituting up to 80% of adult skeletal mass. Spongy bone, which provides the rest of the skeletal mass, forms a meshwork of trabeculae that are interdigitated with red (or yellow) bone marrow. In a ground bone section, such as this, the cells have died and fallen off; we see only the spaces where they once were.
The bone in this slide is compact bone and the best example in our collection of osteons and osteocyte canaliculi. Follow . The functional unit of compact bone is the Haversian system (or osteon). Find this using the drop-down menu. The maximum diameter of an osteon is about 300 microns, and the maximum length 3 - 5 mm. In the middle of the Haversian system is the Haversian canal that houses the blood vessels and nerves. Zoom out once and go to [3,1]. Surrounding the canal are concentric rings or lamellae of bone. Find these. Zoom out twice and click [1,1]. The osteocytes are contained in small cavities (lacunae) located between the lamellae. Find the labelled osteocytes. Osteocytes are bone cells that maintain the bone matrix. The osteocytes send out radial projections through the lamellae that communicate with other osteocytes and the Haversian canal. There are many Haversian systems within a single bone, and the long axes of the canals are parallel to the long axis of the bone. Bone is constantly being remodeled, even in the adult. Between the osteons are remnants of previous osteons, called interstitial lamellae. Zoom out twice and click . Highlight the interstitial lamellae.
Practice on this similar virtual microscope slide: b11 ground bone, cross section. Do you recognize osteons? Can you see the lacunae that used to house the osteocytes when this was living tissue? How about canaliculi and Haversion canals? Do you remember what each contained when this was intact, living bone? Can you pick out interstitial lamellae?
Follow . Using the drop-down menu highlight the Haversian canal. This section shows an almost longitudinal section of the canal. The lamellae, which were apparent in the above cross-section, are not evident in this oblique section. Zoom out once and follow [3,1]. The lacunae are labeled. The radial extensions of the osteocytes occupy small channels in the bone matrix called canaliculi.
Can you spot the lacunae and canaliculi in the comparable virtual microscope slide: b12 ground bone, longitudinal section.
The rat skull is thin intramembranous bone composed of bone marrow sandwiched between two layers of lamellated compact bone. Follow link  and highlight the bone, bone marrow and periosteum (connective tissue). Follow  for a higher power view. The periosteum, called dura when it covers the inside of the skull as in this view, is a dense fibrous covering that contains osteoprogenitor cells and many blood vessels. Zoom out once and go to link . Highlight the bone and the periosteum on this side of the skull, noticing the artery within the periosteum. Follow  for a higher magnification view of the bone marrow. Bone marrow contains the stem cells for the bone cells (osteoblasts and osteoclasts) as well as stem cells for all blood cells. The osteoprogenitor cells migrate from the bone marrow stroma and, with the proper stimulus, can become osteoblasts. As they produce bone matrix they become trapped into the bone and become osteocytes. Zoom out once and follow . Use the drop-down menu to locate labelled osteocytes. Click  for a higher magnification view of a single osteocyte.
Zoom out once and identify the internal circular lamina. Now click the thumbnail and follow [1,3,1] to see the external circular lamina. These laminae are also found in the long bones.
Now practice on this unlabeled virtual microscope slide: w11 bone, rat skull. Can you find marrow, periosteum and bone? How about an osteocyte within the bone matrix?
The spongy bone is arranged in anastomosing trabeculae. The interconnecting spaces are filled with bone marrow. Follow [2,1] and highlight the bony trabeculae and bone marrow. The bone in this slide is endochondral bone from the cartilage end of the rib. Bone is constantly turned over by the closely coupled actions of osteoclasts, which resorb old or damaged bone, and osteoblasts, which fill in with new bone. Osteoclasts derive from mononuclear hematopoetic stem cells in the marrow and are specialized for resorbing live bone. Macrophages eat dead bone only, even though they are also derived from the mononuclear lineage. Follow link . Using the drop-down menu highlight the osteoblasts (the bone forming cells). They resemble a cuboidal epithelium lining the bone in places. They have a remarkably basophilic cytoplasm. Highlight the osteoclast. These are multinucleated, irregularly shaped large cells with cytoplasm that often appears foamy or vacuolated. They are often seen adhering to the adjacent bone. Zoom out once and click [3,1] for a high magnification of an osteoclast.
Click the thumbnail and go to link [1,1,1,2]. This view demonstrates endochondral ossification in action: Use the drop-down menu to locate the cartilage core of the trabecula. Notice the osteoblasts that have produced the bone matrix. One osteoblast has been transformed into an osteocyte after being enclosed by the bone matrix. The large osteoclast is poised to start remodeling the bone.
Zoom out once and try to identify more osteocytes,osteoclasts, and osteoblasts.
Now pactice on a similar virtual microscope slide: w10 cancellous bone. Can you identify osteoblasts and osteoclasts?
Mesenchymal cells specialize and produce a hyaline cartilage model for bone. Highlight one of the models using the drop-down menu. The perichondrium in the middle of the shaft of the model transforms into osteoprogenitor cells and forms a bone collar by intramembranous ossification. Follow link 1 and identify the collar. Then follow link 3 for higher magnification. Blood vessels (not visible in this picture) penetrate the bone collar and bring more osteoprogenitor cells and osteoclast precursors to the inside of the cartilage model (the so called "primary ossification center"). Concomitantly, the cartilage cells hypertrophy and die. Zoom out once, follow [1,3]. Identify the calcifying cartilage cells. Osteoclasts form tunnels in the cartilage, and bone is laid down by osteoblasts on the remnant cartilage spicules. Bone grows in length by endochondral ossification at the epiphyseal (growth) plates. In this slide one can see: 1) the interstitially growing cartilage (near the joint cavity), zoom out twice, follow link . 2) The hypertrophying chondrocytes in rows, zoom out once, follow link . 3) Calcifying cartilage, follow link . 4) Endochondral bone formation around a cartilage spicule core, zoom out once, follow link . The bone grows in thickness by intramembranous ossification. The growing bone is constantly modeled by the closely coupled action of osteoclasts and osteoblasts to achieve the final adult shape. Remodeling takes place throughout life.
The virtual slide we have for practice is a dog digit: Digit, paraffin section. Can you visualize the process of endochondral bone formation? Can you find the cartilage model? The zone where cartilage cells hypertrophy and then die? The region where the cartilage matrix becomes calcified? How about where the new bone has formed?
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