RESPIRATORY SYSTEM
The complex of organs and tissue which are necessary to exchange blood carbon dioxide (CO2) with air oxygen (O2) is called the respiratory system. It consists of
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Nasal Cavity
The Nasal cavity is divided into three structurally and functionally different parts.
- The vestibules (the first ~1.5 cm of the conductive portion following the nostrils) are lined with a keratinised stratified squamous epithelium. Hairs, which filter large particulate matter out of the airstream, and sebaceous glands are also present.
- At the transition from the vestibule to the respiratory region of the nasal cavity the epithelium becomes first stratified squamous and then pseudostratified columnar and ciliated. This type of epithelium is characteristic for all conductive passages dedicated to the respiratory system and therefore also called respiratory epithelium. Mucus producing goblet cells are present in the epithelium.
The surface of the lateral parts of the nasal cavity is thrown into folds by bony projections called conchae. These folds increase the surface area of the nasal cavity and create turbulence in the stream of passing air, both of which facilitate the conditioning (warming, cooling and filtration) of the air. Mucous and serous glands in the connective tissue underlying the epithelium, the lamina propria, supplement the secretion of the goblet cells. Veins in the lamina propria form thin-walled, cavernous sinusoids, also called cavernous bodies.
- Tissues on the superior concha and the nasal septum form the olfactory region of the nasal cavity. Cilia in the epithelium of the olfactory region arise from olfactory cells. Although their internal structure resembles largely that of normal cilia they do not move, because they lack dynein arms which are necessary for cilial motility. The cell membrane covering the surface of the cilia contains olfactory receptors which respond to odour-producing substances, odorants, dissolved in the serous covering the epithelium. The axons of the olfactory cells collect into bundles in the lamina propria. The olfactory cells and their processes receive mechanical and metabolic support from supporting cells (or sustentacular cells). Basal cells can divide and differentiate into either olfactory or supporting cells.
The supporting cells and the secretion of the serous glands contain lipofuscin granules, which give a yellow-brown colour to the surface of the olfactory region.
Pharynx
The pharynx connects the nasal cavity with the larynx. Depending on the extent of abrasive forces on the epithelium, the pharynx is either lined with respiratory epithelium (nasopharynx or epipharynx) or with a stratified squamous epithelium (oropharynx or meso- and hypopharynx), which also covers the surfaces of the oral cavity and the oesophagus. Lymphocytes frequently accumulate beneath the epithelium of the pharynx.
Accumulations of lymphoid tissues surrounding the openings of the digestive and respiratory passages form the tonsils.
The nasal cavity and pharynx form the upper respiratory passages.
Inferior Concha, human - Alcian blue & van Gieson
Thin structures which consist of tissues with very different physical properties - like loose connective tissue and bone - may easily get damaged during tissue preparation. Find a spot where the layers from epithelium to bone appear intact. Identify goblet cells, basal cells and ciliated cells in the epithelium. Now have a look at the lamina propria. Depending on the exact location you may only see connective tissue between the epithelium and bone or large spaces (either empty or filled with red blood cells) which represent the cavernous sinusoids, or glandular tissue (mucous glands appear green and muco-serous glands appear brownish-green in this preparation), or combinations of these features. Lamellae and osteocytes in lacunae may be visible in the bone, while Haversian systems are rare or absent. Which type of bone is it?
Draw a survey of the tissue which includes bone, the lamina propria and structures within it, and epithelium.
Thin structures which consist of tissues with very different physical properties - like loose connective tissue and bone - may easily get damaged during tissue preparation. Find a spot where the layers from epithelium to bone appear intact. Identify goblet cells, basal cells and ciliated cells in the epithelium. Now have a look at the lamina propria. Depending on the exact location you may only see connective tissue between the epithelium and bone or large spaces (either empty or filled with red blood cells) which represent the cavernous sinusoids, or glandular tissue (mucous glands appear green and muco-serous glands appear brownish-green in this preparation), or combinations of these features. Lamellae and osteocytes in lacunae may be visible in the bone, while Haversian systems are rare or absent. Which type of bone is it?
Draw a survey of the tissue which includes bone, the lamina propria and structures within it, and epithelium.
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Nasal Cavity, Olfactory Region, rat - Alcian blue & van Gieson
In humans, olfactory epithelium lines the superior concha and parts of the nasal septum. The bony structures beneath the epithelium form an irregular surface, which increases turbulence in the air passing them and thereby the chances that odorants come into contact with the olfactory epithelium. In macrosmatic animals, like the rat, the olfactory epithelium also covers the middle conchae and the surface is considerably more irregular than in humans.
The olfactory epithelium is formed by olfactory cells, sustentacular cells and basal cells. Basal cells can be identified by their location. Sustentacular cells are preferentially located in the superficial cell tier of the epithelium but are difficult to distinguish from olfactory cells in this preparation. Cilia are not visible and goblet cells are absent from the olfactory epithelium. Lightly stained rounded areas in the lamina propria represent bundles of olfactory axons in the lamina propria. Small mucous glands, olfactory glands or Bowman's glands, in the lamina propria moisturise the epithelium.
Draw the olfactory epithelium and underlying lamina propria at high magnification. Label the features included in your drawing.
In humans, olfactory epithelium lines the superior concha and parts of the nasal septum. The bony structures beneath the epithelium form an irregular surface, which increases turbulence in the air passing them and thereby the chances that odorants come into contact with the olfactory epithelium. In macrosmatic animals, like the rat, the olfactory epithelium also covers the middle conchae and the surface is considerably more irregular than in humans.
The olfactory epithelium is formed by olfactory cells, sustentacular cells and basal cells. Basal cells can be identified by their location. Sustentacular cells are preferentially located in the superficial cell tier of the epithelium but are difficult to distinguish from olfactory cells in this preparation. Cilia are not visible and goblet cells are absent from the olfactory epithelium. Lightly stained rounded areas in the lamina propria represent bundles of olfactory axons in the lamina propria. Small mucous glands, olfactory glands or Bowman's glands, in the lamina propria moisturise the epithelium.
Draw the olfactory epithelium and underlying lamina propria at high magnification. Label the features included in your drawing.
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Larynx
The larynx connects the pharynx and trachea. The vocal folds of the larynx control airflow and allow the production of sound. The vocal folds are lined by stratified squamous epithelium and contain the muscle (striated, skeletal) and ligaments needed to control the tension of the vocal folds. The larynx is supported by a set of complexly shaped cartilages.
Trachea
The trachea is a fairly short tube (10-12 cm) with a diameter of ~2 cm.
- Epithelium, Mucosa and Submucosa
- The trachea is lined by respiratory epithelium. The number of goblet cells is variable and depends on physical or chemical irritation of the epithelium which increase goblet cell number. Prolonged intense irritation of the epithelium may lead to its transformation to a stratified squamous epithelium (squamous metaplasia). In addition to the staple of basal cells, ciliated cells and goblet cells
, the respiratory epithelium also contains brush cells, endocrine cells (or small granule cells, function not clear), surfactant-producing cells (or Clara cells), and serous cells.
Epithelium and underlying lamina propria are called the mucosa. The lamina propria consists of loose connective tissue with many elastic fibres, which condense at the deep border of the lamina propria to form an elastic membrane. This elastic membrane forms the border between the mucosa and the connective tissue below it, which is called the submucosa. Muco-serous glands in the submucosa (submucosal glands) supplement the secretions of cells in the epithelium. The submucosa ends with the perichondrium of the tracheal cartilages. - Tracheal cartilages
- The trachea is stabilised by 16-20 C-shaped cartilages (hyaline cartilage). The free dorsal ends of the cartilages are connected by bands of smooth muscle (trachealis muscle) and connective tissue fibres. Longitudinal collagenous and elastic connective tissue fibres (annular ligaments) link the individual cartilages and allow both the lengthening and shortening of the trachea for example during swallowing or movements of the neck. They are inseparable from the fibres of the perichondrium. The tracheal cartilages may ossify with age.Cartilages, annular ligaments and the trachealis muscle form the "skeleton" of the trachea which sometimes is referred to as tunica fibromusculocartilaginea. If you want to impress someone with this term make sure that you can pronounce and/or spell it.
The trachea bifurcates to give rise to the main bronchi. Their histological structure corresponds largely to that of the trachea.
 | Trachea, human - H&E In the trachea you should be able to identify the following structures:respiratory epithelium, basement membrane, submucosal glands (both serous and mucous parts), perichondrium, tracheal cartilage and trachealis muscle (smooth muscle). One can perceive different appearances of the connective tissue immediately below the epithelium and the connective tissue surrounding the submucosal glands, but the elastic lamina forming the border between the mucosa and submucosa is not visible in H&E stained slides. Accumulations of very dark small dots represent lymphocytes (not illustrated). If present, you are likely to see them close to the glandular tissue. Draw a composite, i.e. assemble all the main features of the trachea in one drawing, of a segment of the trachea. Label the main features. |
Conductive Structures in the Lung
Bronchi
In the lungs we find the last segments of the conductive portion of the respiratory system. The main bronchi divide into lobar bronchi which in turn give rise to segmental bronchi. The latter supply the bronchopulmonary segments of the lungs. Bronchial branches are accompanied by branches of the pulmonary artery, nerves and lymph vessels. These structures usually travel in intersegmental and interlobar sheets of connective tissue. Conductive structures of a size down to ~1 mm are termed bronchi. Smaller ones are called bronchioles. Aside from their different sizes, bronchi are characterized by the presence of glands and supporting cartilage. The cartilage supporting the bronchi is typically found in several small pieces.
The histological structure of the epithelium and the underlying connective tissue of the bronchi corresponds largely to that of the trachea and the main bronchi. In addition, bronchi are surrounded by a layer of smooth muscle, which is located between the cartilage and epithelium.
Bronchioles
Bronchioles are the terminal segments of the conductive portion. At the transition from bronchi to bronchioles the epithelium changes to aciliated columnar epithelium, but most of the cell types found in the epithelium of other parts of the conductive portion are still present. Glands and cartilage are absent. The layer of smooth muscle is relatively thicker than in the bronchi.
Respiratory Structures in the Lung
Bronchioles divide into respiratory bronchioles, which are the first structures that belong to the respiratory portion of the respiratory system. Small outpouchings of the walls of the respiratory bronchioles form alveoli, the site of gas exchange. The number of alveoli increases as the respiratory bronchioles continue to divide. They terminate in alveolar ducts. The "walls" of alveolar ducts consists of entirely of alveoli.
Histological Structure of Alveoli
The wall of the alveoli is formed by a thin sheet (~2µm) of tissue separating two neighbouring alveoli. This sheet is formed by epithelial cells and intervening connective tissue. Collagenous (few and fine), reticular and elastic fibres are present. Between the connective tissue fibres we find a dense, anastomosing network of pulmonary capillaries. The wall of the capillaries are in direct contact with the epithelial lining of the alveoli. The basal laminae of the epi- and endothelium may actually fuse. Neighbouring alveoli may be connected to each other by small alveolar pores.
The epithelium of the alveoli is formed by two cell types:
- Alveolar type I cells (small alveolar cells or type I pneumocytes) are extremely flattened (the cell may be as thin as 0.05 µm) and form the bulk (95%) of the surface of the alveolar walls. The shape of the cells is very complex, and they may actually form part of the epithelium on both faces of the alveolar wall.
- Alveolar type II cells (large alveolar cells or type II pneumocytes) are irregularly (sometimes cuboidal) shaped. They form small bulges on the alveolar walls. Type II alveolar cells contain are large number of granules called cytosomes (or multilamellar bodies), which consist of precursors to pulmonary surfactant (the mixture of phospholipids which keep surface tension in the alveoli low)
Cilia are absent from the alveolar epithelium and cannot help to remove particulate matter which continuously enters the alveoli with the inspired air. Alveolar macrophages take care of this job. They migrate freely over the alveolar epithelium and ingest particulate matter. Towards the end of their life span, they migrate either towards the bronchioles, where they enter the mucus lining the epithelium to be finally discharged into the pharynx, or they enter the connective tissue septa of the lung.
 | Lung, human - H&E, elastin You should be able to find at least bronchioles, alveolar ducts and alveoli in the section. Respiratory bronchioles should be present but may require some time to find. Note that bronchioles are usually accompanied by a vessel, a branch of the pulmonary artery. Interlobular connective tissue is also present and may be identified by holding the slide against the light. It is represented by continuous and fairly straight streaks of tissue visible without a microscope. Have a look at the bronchioles and alveoli at high magnification. The bronchiolar epithelium is usually not well preserved, but the smooth muscle is clearly visible. Note the absence of cartilage and glands from the bronchioli. You should be able to identify both type I and II alveolar cells and capillaries in the alveolar walls. Note that red blood cells usually appear thicker than the entire wall of the alveoli. Sketch part of the tissue at low magnification. Include an alveolar duct and, if possible, a bronchiole and/or respiratory bronchiole in your drawing. |
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 | Lung, cat - reticulin Reticular and elastic fibres form the bulk of the connective tissue present in the walls of the alveoli. You have seen both types of fibres previously. Note that if you mentally superimpose the elastin and reticulin stains there is not much space for anything other than capillaries. Collagenous fibres are sparse and fine in the alveolar walls. Note also that the tissue stained for reticular fibres looks much denser than the other sections. This lung collapsed prior to fixation because of the recoil of the elastic fibres. Because of this artifact it may be a little easier to recognize alveolar ducts than in the other sections. |
Development of the Lungs
The formation of the lower respiratory passages begins in the fourth foetal week. An outpouching of the foregut gives rise to the laryngotracheal tube. The lining of this tube will eventually give rise to the epithelia covering the surfaces of the larynx, trachea, bronchi, bronchioles and alveoli. Most of the other tissues of the lower respiratory passages are derived from splanchnic mesoderm. The laryngotracheal tube divides distally to form two lung buds.
Dependent of the state of maturity of the lung, development is divided into three periods:
- The bronchi grow and branch during the glandular period, which last until approx. the 17th foetal week. Alveoli are not present at this time.
- Bronchi and bronchioles expand and branch during the canalicular period. The lung tissue is vascularised during the canalicular period. Bronchi and bronchioli begin to form terminal sacs (developing primitive alveoli), in which cuboidal and squamous cells become associated with vessels. Respiration becomes possible towards the end of this period around the 25th foetal week.
- The number of terminal sacs increases during the intial part of the alveolar period (sometimes also considered a separate period of lung development and called terminal sac period). The capillary network is developing between the terminal sacs. The late alveolar period is marked by the development of mature alveoli from the terminal sacs. The period begins shortly before birth, but the first mature alveoli appear only after birth. Alveolar sacs continue to be formed during early childhood (up to year 8) and mature into alveoli. Alveolar maturation and growth continue for another decade, but their numbers do not increase further.
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