Histology of the Respiratory system Prof. Abdulameer Al-Nuaimi E-mail: a.al-nuaimi@sheffield.ac.uk E-mail: abdulameerh@yahoo.com

RESPIRATORY SYSTEM Contains a complex of organs and tissue which are necessary to exchange blood carbon dioxide (CO2) with air oxygen (O2) Respiratory system is composed of 1- structures, which function as ducts, and which together are called the conductive portion of the respiratory system 2- structures which form the respiratory portion of the respiratory system, in which the exchange of CO2 and O2 is occurring 3- the parts of the thoracic musculo-skeletal apparatus and specialisations of the lung which allow the movement of air through the respiratory system - the ventilating mechanism

The vestibules of the nose  Is the first ~1.5 cm of the conductive portion following the nostrils.  Vestibules are lined with a keratinised stratified squamous epithelium. Hairs, which filter large particulate matter out of the airstream, and sebaceous glands are also present keratinised stratified squamous epithelium Hair follicle

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 connective tissue underlying the epithelium is called the lamina propria, it contains Mucous and Serous glands. Veins in the lamina propria form thin-walled, cavernous sinusoids.

Sinusoid

olfactory region  Tissues on the superior concha and the nasal septum form the olfactory region of the nasal cavity. It is made up of the olfactory epithelium and the underlying lamina propria. The olfactory epithelium consists of a three basic cell types: 1- bipolar olfactory receptor neurons; 2- sustentacular cells (a type of supporting cell); and 3-basal cells (the stem cells) that continuously give rise to new olfactory receptor neurons and sustentacular cells. The lamina propria contains connective tissue containing fibroblasts, blood vessels, Bowman's glands (unknown function)and bundles of fine axons from the olfactory neurons. Cilia in the epithelium of the olfactory region arise from olfactory cells; the cilia do not move.

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 (sustentacular cells).  Basal cells can divide and differentiate into either olfactory or supporting cells. The supporting cells contain lipofuscin granules, which give a yellow-brown colour to the surface of the olfactory region

Olfactory mucosa

Pharynx The pharynx connects the nasal cavity with the larynx. The nasopharynx  is lined with respiratory epithelium  The oropharynx and laryngoharynx are lined with a stratified squamous epithelium which also covers the surfaces of the oral cavity and the oesophagus. Lamina propria that contains a thick layer of longitudinally oriented elastic fibers and mucous glands seen. Lymphocytes frequently accumulate beneath the epithelium of the pharynx.   The next layer is the muscularis externa is composed of somewhat irregularly arranged skeletal muscle, the longitudinal and constrictor muscles of the pharynx. Mucous glands might be seen in this muscular layer, they are extensions of those present in the lamina propria. The outer layer is a fibrous layer

Larynx The larynx and false vocal fold is lined by pseudostratified columnar epith with ciliae and goblet cells (respiratory epith) The true vocal folds are lined by stratified squamous epithelium and contain the muscle (striated) and ligaments. The larynx is supported by a set of complexly shaped cartilages Lamina Propia Contains mixed glands (mostly mucus) with excretory ducts from glands, open on surface of the epithelium. lymphatic nodules can be seen on the ventricular side Underneath the mucosa, cartilages can be seen

Trachea The trachea is lined by respiratory epithelium. the respiratory epithelium contains basal cells, ciliated cells and goblet cells,, endocrine cells (or small granule cells, function not clear), surfactant- producing cells  (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. The submucosa Contains muco-serous glands (submucosal glands), they supplement the secretions of cells in the epithelium. The submucosa ends with the perichondrium of the tracheal cartilages. elastic membrane

elastic membrane Hyaline cartilage of the trachea

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 longitudinal collagenous and elastic connective tissue fibres (annular ligaments)  These, 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 Tunica adventitia Is an outer layer of fibrous tissue surrounds the cartilages

Bronchi 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 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 within the lamina propria. The submucosa Contains muco-serous glands (submucosal glands), The Hyaline cartilage supporting the bronchi is typically found in several small pieces. The cartilage is surrounded by adventitia

Trachea

Bronchioles Bronchioles are the terminal segments of the conductive portion. At the transition from bronchi to bronchioles the epithelium changes to a ciliated columnar epithelium, but most of the cell types found in the epithelium of other parts of the conductive portion are still present.  As bronchioles get smaller, Goblet cells start to decrease in number and replaced by  Clara cells which are cuboidal-shaped cells and have no cilia, found in the small airways bronchioles; these cells may secrete glycosaminoglycans to protect the bronchiole lining; as well, they act as a stem cells. Glands and cartilage are absent. The layer of smooth muscle is relatively thicker than in the bronchi

Bronchus

Respiratory portion 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 there is 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 epithelial lining of the alveoli- and that of the 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: 1- 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. 2- 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, which consist of precursors to pulmonary surfactant (the mixture of phospholipids which keep surface tension in the alveoli low). There are just about as many type II cells as type I cells.

Cilia are absent from the alveolar epithelium and cannot help to remove particulate matter which continuously enters the alveoli with the inspired air.  3- 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 on the epithelium to be finally discharged into the pharynx, or they enter the connective tissue septa of the lung

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