Histology of Respiratory System of Animal
Hope you are doing well. If you want to learn about the histology of respiratory system organs, then you are in the right place now. Today, I will discuss the histology of the respiratory system organs of animals (identification of respiratory system organs under a microscope). I hope you have a basic idea on LINING EPITHELIUM, CONNECTIVE TISSUE, and GENERAL STRUCTURAL PATTERN OF A TUBULAR ORGAN. If you don’t have basic knowledge about those topics, you may go through those links.
Histology of the respiratory system
We know the respiratory system provides gas exchange between the environment and blood. It consists of two major portions – a system of the tube or conducting portion and lung or respiratory portion.
Conducting portion includes –
Nasal cavity
Nasopharynx
Larynx and
Trachea
And respiratory or gas exchange portion includes –
Bronchus (primary, secondary, and tertiary)
Bronchioles (primary, secondary, and tertiary)
Respiratory bronchiole
Alveoli
Alveolar ducts
Alveolar sac
Here, we will identify the following organs from the histology of the respiratory system of animals –
Histology of larynx
Histological structure of the trachea
Histological features of lung
“You should know the details about the nasal cavity, nasopharynx and other information as I am not going to discuss on those topics.”
Okay, so first, we should know about the general structural pattern of a tubular organ. If you have a better knowledge about it, then you may continue.
You should know the following exceptions in the general structural pattern in the conducting portion of the respiratory system –
There is no muscularis mucosa. So, the connective tissue of lamina propria is continued with the submucosa layer.
In the larynx (cartilage of larynx), trachea and bronchus, you will find cartilage in the tunica muscularis layer. They prevent the collapse of the lumen of those tubular organs and provide rigidity to those structures.
Important for respiratory organs
You should remember those points during the study of the histology of respiratory organs –
Lining epithelium gradually decreases in thickness as it is traced distally
Glands, goblet cells gradually decrease and disappear completely at the distal parts of respiratory organs
Cartilaginous support decrease while the number of elastic fibers increases at the distal parts of respiratory organs
Okay, now start to identify the respiratory system organs under a microscope
Histology of larynx
In the larynx, we will identify the following structures –
Lining epithelium
Lamina propria
Submucosal layer
Tunica muscularis with a type of cartilage
Tunica adventitia layer
Okay, let’s start to identify –
“I am going to represent epiglottis from larynx.”
Presence of non-keratinized stratified squamous epithelium. You will find pseudostratified ciliated columnar epithelium with goblet cells in most parts of the larynx; except vocal cord, epiglottis)
Presence of dense fibro-elastic connective tissue in lamina propria
Presence of dense connective tissue with blood vessels, lymph vessels, and submucosal glands
Presence of elastic cartilage (in the epiglottis, cuneiform, and corniculate) in the inner part of tunica muscularis.
Presence of skeletal muscle layer in the outer part of tunica muscularis
Presence of thick connective tissue adventitia
Histological structure of the trachea
We will identify the following structures of the trachea under the microscope –
Respiratory epithelium
Propria submucosa of trachea
A layer of tunica muscularis of the trachea
Adventitia of trachea
“Respiratory epithelium includes – pseudostratified ciliated columnar epithelium with goblet cells, basal cells, brush cells, small granule cells and clara cells”.
Okay, let’s start to identify those structures –
The mucosa of the trachea is lined by pseudostratified ciliated columnar epithelium with goblet cells
Presence of thin layer of loose fibro-elastic connective tissue with diffuse lymphatic tissue in lamina propria
Presence of dense to moderately dense fibro-elastic connective tissue with tubule-acinar glands in submucosa layer
Presence of C – shaped (tracheal ring) hyaline cartilage ring, open dorsally. Smooth muscles and fibro-elastic ligaments connect this open portion.
Presence of loose connective tissue adventitia
“In extrapulmonary bronchus, you will also find the same histological structure in tunica muscularis – C –shaped hyaline cartilaginous ring; But in intrapulmonary bronchus, you will find isolated or irregular hyaline cartilage plate along with smooth muscle; we will learn it later”.
Histological features of lung
We will complete the following structures from the histology features of the lung –
The external covering of serous membrane of visceral pleura (mesothelium lining)
Alveoli of lung parenchyma
Alveolar ducts
Alveolar sac
Histological features of the bronchus (primary bronchus or extrapulmonary bronchus; secondary and tertiary bronchus or intrapulmonary bronchus)
Histological features of a bronchiole (Primary, terminal and respiratory bronchioles)
“Hope, you have a better idea on bronchiole tree”.
Okay, let’s start to identify those structures –
The lung contains the loose fibroelastic connective tissue stroma.
Several connective tissue septa separate the lung parenchyma into different lobes and lobules. The most available portion of the lung is filled with a clump of alveoli.
Characteristics of alveoli histology
It is a sac-like evagination of the respiratory bronchiole, alveolar duct, and alveolar sac, which is responsible for the spong structure of the lung.
The mucosa of the alveoli is lined by simple squamous epithelium (having three major cells – type I pneumocytes, Type II pneumocytes, and pulmonary or alveolar macrophage or dust cell; Type II pneumocytes produces pulmonary surfactant)
Presence of very thin loose fibro-elastic connective tissue in lamina propria
Absence of submucosa and muscularis mucosa
Characteristics of the alveolar duct and alveolar sac
You will find the following characteristics –
The mucosa of the alveolar duct and the alveolar sac is lined by simple cuboidal epithelium.
Presence of very thin fibro-elastic connective tissue in lamina propria of the alveolar duct and alveolar sac
Absence of submucosa and muscularis mucosa in alveolar ducts and alveolar sac
Histological structure of bronchus
We should differentiate the histology of intrapulmonary bronchus from the histology of extrapulmonary bronchus.
In the histology of intrapulmonary bronchus, you will find isolated or irregular hyaline cartilage in the tunica muscularis layer. Smooth muscle occur complete circular pattern between cartilage plate and lamina propria
But in the extrapulmonary bronchus, you will find C – shaped hyaline cartilaginous ring which opens dorsally, and this ring is connected by smooth muscle with fibro-elastic ligament (Same as in the histological structure of the trachea)
The other characteristics from different layers (lining epithelium, lamina propria, submucosa, and adventitia) will be the same as described in the histological structure of the trachea.
Histological structure of bronchioles
We should differentiate the histology structure of three types of a bronchiole (primary, terminal and respiratory bronchioles). The common characteristics are given below –
Presence of pseudostratified ciliated columnar epithelium in primary bronchioles, ciliated simple columnar epithelium in terminal bronchioles, and simple low cuboidal epithelium in respiratory bronchioles.
Presence of loose fibro-elastic connective tissue in lamina propria
Presence of spirally arranged smooth muscles in tunica muscularis of primary and terminal bronchioles, but there is no tunica muscularis layer in respiratory bronchioles.
Conclusion
I hope you have got an idea of the histology of the respiratory system, and you will be able to identify the structures of respiratory organs under a microscope. You may also learn the basic histology of the organs from the DIFFERENT SYSTEM OF ANIMAL BODY. If you want to know more about the histology of the respiratory system, you should follow class lectures or go through the BOOKS. (Get the histology learning guides or book here).
If possible, I will update or enrich the information on this topic in the future.
If there is any mistake in the above information or if you have any suggestions for me, please, let me know in the comment box. Thank you so much.