respiratory system

HISTOLOGY Dr.Zainab Al-Shimmari

RESPIRATORY SYSTEM

It consists of two regions:

1-Conducting portion: it includes the nasal cavities, nasopharynx, larynx, trachea, and several generations of progressively smaller bronchi, followed by a larger number of different orders of bronchioles.

2- Respiratory portion: includes: respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli.

RESPIRATORY EPITHELIUM

Pseudo-stratified columnar ciliated epith., with goblet cells. It lines most of the
conducting part. It consists of five types of cells:
1- Ciliated columnar cells: represent the most common type. Each cell has
about 300 cilia on the apical surface. Beneath these cilia, there is a basal body, and small mitochondria, to supply ATP for ciliary beating. Ciliary movement transports a continuous layer of mucous to the pharynx. Foreign particles will be traped in this mucous, so this will protect lungs from any particulate matter and any bacteria.
*Immotile cilia syndrome, a disorder that causes infertility in men and chronic
respiratory tract infections in both sexes, is caused by immobility of cilia and flagella
2- Mucous goblet cells: they are global in shape, and their apical part contains mucous droplets composed of glycoproteins.
3- Brush cells: they have numerous microvilli on their apical surface. They are considered as sensory receptors due to the presence of afferent nerve endings on their basal surface.
4- Basal (short) cells: small, rounded cells, lie on the basal lamina, but do not extend to the luminal surface of the epithelium. They are believed to be the generative cells for other cell types.
5- Small granule cells: small, rounded cells, with numerous granules, 100-300 nm in diameter, with dense core. They are part of the diffuse neuro endocrine system (DNES) They produce calcitonin, somatostatin, serotonin, and bombesin.


From the nasal cavity through the larynx, portions of the epithelium are stratified squamous. This type of epithelium is evident in regions exposed to direct airflow or physical abrasion (eg, oropharynx, epiglottis, vocal folds); it provides more protection from attrition than does typical respiratory epithelium.
NASAL CAVITY
It is opened anteriorly into the nares, and posteriorly into the nasopharynx, at the choanae. Its wall is well supported by bone and cartilage.
Nasal cavity consists of two structures:
1- Vestibule: it is the most anterior and dilated part of the nasal cavity. The nares(nostrils) are the anterior openings and are covered by skin(stratified seq. keratinized ), which is rich in sebaceous and sweat glands, in addition to the thick short hair, or vibrissae. Inside the vestibule, the epith. will change into respiratory epith.
2- Nasal fossae: They are two cavernous chambers within the skull, separated by nasal septum. From each lateral wall projects three boney shelf-like projections; the chonchae, or turbinates. The middle and inferior ones are lined by respiratory epith. Superior choncha is covered by a specialized olfactory epith. It is composed of three types of cells:
a- Supporting cells: These cells act as glial cells, providing both metabolic and mechanical support.
b- Basal cells: small, spherical or cone shaped cells, rest on the basement membrane. Their cytoplasm contains few organelles, and they act as stem cells for other respiratory cells.
c- Olfactory cells: Bipolar neurons. The apices have elevated and dilated areas; the olfactory vesicle, from which arise 6-8 non motile long cilia,(although some research suggest some limited motility). The plasma membrane of cilia contains odorant-binding protein that act as receptors for smell sensation .
Lamina propria of the olfactory epith.contains the glands of Bowman, a branched tubuloalveolar serous glands, which secretes a fluid around the olfactory cilia to facilitate the dissolve of odoriferous substances to stimulate olfactory cells. The lamina propria of the concha contains large venous plexuses known as the swell bodies. Every 20-30 minutes the swell bodies on one side of the nasal fossae become engorged with blood, resulting in distention of the conchal mucosa, and decrease in the flow of air. These periodic intervals of occlusion reduce the air flow, allowing the respiratory epith. to recover from dryness. Allergic reactions and inflammation can cause abnormal engorgement of swell bodies in both fossae, severely restricting the air flow.

PARANASAL SINUSES

They are closed cavities in the frontal, maxillary, ethmoidal, and sphenoid bones.
They are lined by thin respiratory epith., with few goblet cells. Their lamina propria contains small glands. They communicate with nasal cavity through small openings. Mucous produced in the sinuses is swept into nasal cavity by ciliated cells.
Sinusitis is an inflammatory process of the sinuses.
NASOPHARYNX
It is the first part of the pharynx, lined by respiratory epith. It communicates with middle ear by Eustachian tube. Its wall is rich in diffuse and nodular lymphatic tissue.
LARYNX
An irregular tube that connects the pharynx to the trachea. The skeleton of the
larynx is made of cartilage within the lamina propria. The large cartilages are hyaline, while the small ones are elastic. These cartilages support the larynx to maintain an open air way, and prevent swallowed food from entering the trachea, also they participate in sound production (phonation).Larynx also counteract obstruction or irritation by coughing.
Epiglottis: is a leaf shaped small elastic cartilage that projects from the anterior wall of the larynx. It has both lingual and laryngeal surfaces. Lingual surface(anterior surface), and the apical part of epiglottis is covered by stratified seq.non keratinized epith. The lower part of the laryngeal surface (posterior surface)is covered by respiratory epith. Below the epith., the lamina propria contains both mucous and serous glands.
Below the epiglottis, the mucosa forms two pairs of folds that projects into the lumen of the larynx. The upper pair constitutes the false vocal folds, which are covered by respiratory epith. The lower pair represents the true vocal cords, and lined by stratified seq. epith. These folds are oriented in an antero-posterior direction, and define the lateral boundaries of the opening of the larynx; rima glottis. Within these folds, there is large bundles of parallel elastic fibers composing the vocal ligament. The tention and length of vocal ligament determines the kind of sounds produced. Parallel to this ligament, are bundles of skeletal muscle fibers; the vocalis muscle.
TRACHEA
Thin walled tube, about 10cm length, and 2.5 cm diameter, extends from the
larynx into the thorax where it bifurcates into two primary bronchi. The wall consists of four layers:
1- Mucosa: consists of respiratory epith. and lamina propria of loose connective
tissue, with diffuse lymphatic tissue, some times of nodular form.
2- Submucosa: loose connective tissue, rich in lymphatic tissue. It is separated
from lamina propria by an elastic membrane. . Muco-serous glands; tracheal
glands also present that produce mucous fluid.
3- Cartilage layer: There are 16-20 C- shaped hyaline cartilage to keep tracheal
lumen opened. A fibro-elastic ligament binds the open ends of the cartilage to prevent over distention of the lumen. Deep to this ligament, a band of smooth muscle; trachealis muscle regulates the lumen of trachea. The more flexible area
between the cartilages is supported by dense fibroelastic connective tissue
continuous with the perichondrium, to facilitate the extension of trachea if the
head is tilted back, or during respiration.
4- Adventitia: loose connective tissue rich in blood vessels and nerves.


BRONCHEAL TREE
The trachea divides into two main bronchi, each enters the lung through the hilum, where arteries, veins, and lymphatics enter and leave the lungs, where they are invested by dense connective tissue.
Each primary bronchus course downwards dividing into three bronchi in the right
lung, and two in the left lung, each supply a pulmonary lobe. These lobar bronchi divide repeatedly into bronchioles, then 5-7 terminal bronchioles.
Bronchus:
Primary bronchus has the same histological structure as trachea, except that the
cartilage is a complete ring. At the level of secondary bronchus, the cartilage become as an isolated plates.
Lamina propria is rich in elastic fibers, mucous and serous glands, lymphocytes, and lymphatic nodules. Well-developed smooth muscle fibers also present.
Bronchioles:
They are an intralobular air ways with a diameter of 1-5mm or less. They are lined by respiratory epith., with few goblet cells. No glands and no cartilage present in their wall.
Terminal bronchioles:
They are lined by simple columnar or simple cuboidal cells ciliated with Clara cells. Clara cells are non ciliated, and have an apical secretary granules that secrete
glycosaminoglycans, which has a protective function against oxidative agents and
inflammation, and has a detoxifying effect on noxious inhaled particulate matters. Goblet cells disappear above the level where ciliated cells disappear.
Lamina propria has smooth muscle fibers and elastic fibers arranged in a helical
crisscrossing pattern.
Respiratory bronchioles:
Each terminal bronchiole gives two or three respiratory bronchioles. These are lined by simple ciliated cuboidal cells, with non-ciliated Clara cells.
Lamina propria is rich in smooth muscle fibers and elastic fibers. The wall of
respiratory bronchioles is interrupted by the alveolar sacs and alveolar ducts, where the epith. changes into simple seq. epith.
Alveolar duct:
It is a tubular structure that is connected to the respiratory bronchioles. It is lined by simple seq. epith.
Lamina propria has smooth muscles which disappear distally, and replaced by
elastic and reticular fibers.
Alveolar sac:
It is a space where a group of alveoli open at each other. It is lined by simple seq.
epith., and invested by elastic and reticular fibers. The elastic fibers are for the expansion and contraction, while reticular fibers prevent over distention.
Alveoli:
Sac like evaginations of respiratory bronchioles, alveolar ducts, and sacs. They are about 200μm in diameter, and are specialized for O2 and CO2 exchange between air and blood.
Each alveolus is lined by simple seq. epith., and is separated from adjacent alveolus by the inter-alveolar septum. Within this septum, we have the Blood- Air Barrier which separates the air in the alveolus from blood in the capillaries.
Blood-Air barrier consists of the following structures:


1- Cytoplasm of the alveolar cell wall.
2- Fused basal laminae of alveolar and capillary endothelial cells.
3- Cytoplasm of endothelial cells.

There are four types of cells in the inter-alveolar septum:

1- Endothelial cells: extremely thin, It is of the continuous type, with no fenestrae.
2- Type Ι pneumocytes (sequamus alveolar cell): extremely thin cells, and constitute about 97% of the alveolar surface.
3- Type ΙΙ pneumocytes (great alveolar cells) : they form only 3% of the alveolar surface, found in between type Ι cells, They are cuboidal in shape, and found in groups at the angles of alveolar wall. they produce the pulmonary surfactant, which spread over the alveolar surface as a coating to lower their surface tension, so, less inspiratory force is needed to inflate the alveoli, and also to prevent alveolar collapse during expiration.
4- Macrophage: They are called dust cells, and seen on the surface of alveoli, also in the connective tissue around major blood vessels or in the pleura. These cells phagocytize debries that passed from alveolar lumen.

Alveolar pores: These are pores of 10-15μm diameter, found at the inter-alveolar septum. They equalize the pressure between alveoli and act as collateral of air if a bronchiole is obstructed.

Pleura

Serous membrane that covers the lungs. It consists of two layers; parietal and
visceral. Pleura consists of mesothelial cells that rest on fine connective tissue layer of collagen and elastic fibers. There is a cavity between parietal and visceral pleurae which is lined by mesothelial cells, called pleural cavity. It contains a thin film of liquid act as a lubricant for smooth sliding during respiration.