Histology of the Respiratory System

Respiratory System – two divisions with distinct epithelium: conducting and gas exchange systems

Nasal Cavity

Vestibule—anterior, epidermis lining continuous with nose (many hair follicles, sebaceous/sweat glands)

stratified squamous becomes pseudo stratified ciliated columnar with goblet cells

Respiratory portion—lamina propria contains numerous serous and mucous glands

pseudo stratified ciliated columnar with goblet cells (called respiratory epithelium)

Olfactory organ—specialized for scent lining most of the roof of each nasal cavity

olfactory receptor cells—modified bipolar neurons with an apical dendrite that extends to surface and an axon that projects to the olfactory bulb (ONLY mammalian cells that turn over)

sustentacular or support cells

basal cells—stem cells that give rise to new olfactory neurons

Bowman's glands—tubuloalveolar glands in lamina propria which produce a thin, watery secretion

Paranasal Air Sinuses

spaces in the bone, 4 associated with and communicating to each nasal cavity (sinuses infected if blocked)

Nasal Pharynx

pseudo stratified ciliated columnar with goblet cells (like nasal cavity)

exception: stratified squamous on roof (near esophagus) and lateral sides (near Eustachian tube)

glands of mixed mucous/serous type in lamina propria (where respiratory epithelium is present)

tonsil—large mass of lymphatic tissue on dorsal wall

Larynx—connects pharynx and trachea, cartilage in its wall

prevents anything by air from gaining access to lower respiratory passages, important for phonation

epiglottis—flap-like structure that projects from the top of the larynx, supported by plate of elastic cartilage

anterior—stratified squamous epithelium; posterior—pseudostratified ciliated columnar with goblet cells
mixed glands present in the lamina propria

false vocal cords—projections of two folds of laryngeal mucous membrane

true vocal cords—stratified squamous, dense connective tissue, no glands in lamina propria (vocal ligament)

Trachea—connects larynx and two primary bronchi

relatively thin walled tube with 18-20 U shaped cartilages that prevent collapse
pseudostratified ciliated with goblet cells
lamina propria—condensed to moderately distinct basement membrane, high proportion of elastic fibers
many mucous glands are interlacing bundles of smooth muscle fibers

Bronchi—two main, give rise to progressively smaller—histologically like trachea, except cartilage becomes plate

Bronchioles—branches that enter lung lobules—smaller than bronchi, no cartilage

ciliated columnar without mucous glands in the outer layers

Respiratory Bronchioles—final generation of bronchioles, identified by presence of alveolar ducts/alveoli

ciliated columnar becomes low cuboidal
walls of collagenous connective tissue interspersed with bundles of smooth muscle and elastic fibers

Alveolar Ducts, Atria, Alveolar Sacs and Alveoli—spaces that contain air continually being changed

alveolar wall—biological barrier through which gas exchange occurs

single capillary bed supported by elastic/collagenous fiber network which varies in thickness
continuous alveolar epithelium which lines the airspace contains:
endothelial cells
squamous alveolar epithelial cells (Type 1)—cover the greater part of the alveolar surface
great alveolar epithelial cells (Type II)—thought to secrete surfactant
phagocytes detached from alveolar wall, fibroblasts, plasma cells, rare smooth muscle cells, etc.

Upper Respiratory Tract—most important is epithelium and connective tissue and relationship to function

Epiglottis—between the pharynx and larynx, prevents fluids and food from entering larynx

Identify type of epithelium, location of cartilage; taste buds may be present in epithelium
lymphatic tissue protects against infection (characteristic of upper resp.)

Larynx and Vocal Cords—true and false vocal cords should first be identified macroscopically

false vocal cord—covered by both stratified squamous and pseudostratified columnar epithelium

amount of stratified squamous epithelium increases with age or abrasion (smoking or coughing)
many serous and mucous glands underlie the false vocal fold

true vocal cord— elastic connective tissue (vocal ligament) covered by stratified squamous epithelium

vocal ligament vibrated by the expulsion of air from the lung during vocalization
striated muscle is associated with the true vocal fold
no glands are present in this region
passing the lower larynx, pseudostratified ciliated epithelium and submucosal glands appear

Trachea—respiratory epithelium (pseudostratified structure with secretory cells and ciliated cells)

contains series of C-shaped cartilages—band of smooth muscle is inserted into the open ends of cartilages
cartilage surrounded by a perichondrium (dense fibrous tissue)—merges with submucosa and adventitia
thick basal lamina and lamina propria (loose CT), bordered by elastin fibers (may be condensed into dense elastic lamina)
glands in submucosa—connected to the lumen by ducts

mixed, i.e., mucous (foamy apical cytoplasm) in the proximal portions

serous in the terminal portions

aggregations of lymphatic tissue may be present in the submucosa
plasma and mast cells may be present in the lamina propria.

Lower Respiratory Tract

Lung—pleural epithelium covers the surface of the lung, vascularization well defined

terminal bronchioles, respiratory bronchioles, alveolar ducts and alveoli are visible on this slide

alveolar wall is filled with capillaries which can be identified by the presence of red blood cells

alveolar phagocytes (macrophages, also called dust cells) in CT—contain aggregates of carbon particles

alveolar epithelial cells (squamous cells or Type I pneumocytes) and Great Alveolar Cells (Type II pneumocytes)

The delicate structure of the thin alveolar septum can be seen in this slide

alveolar walls are extremely flattened, very little cytoplasm between the air in the alveolus and blood

Vasculature

pulmonary arteries—follow the bronchial tree and ramify into smaller and smaller branches along lung

pulmonary veins—collect oxygenated blood from the capillary bed and convey it back to the heart

found in the interlobular connective tissue and are not associated with the conducting system

bronchiolar arteries—small, supply bronchi and large bronchioles with oxygenated blood, drain into pulm.v.

can be found in association with the large elements of the conducting system

Respiratory Tract – with branching, the diameter decreases and the type of epithelium/CT changes

Trachea

epithelium—tall, pseudostratified, ciliated with many goblet cells
musculature—smooth muscle connecting ends of cartilage rings
cartilage—18-20 C-shaped rings
elastic tissue—mostly in the form of a membrane
glands—serous and mucous; mostly mucous
other remarks—mixed glands in the submucosa and penetrating into the smoothe muscle layer

Bronchi (main and secondary)

epithelium—tall, pseudostratified and ciliated; with branching—shorter height, fewer goblet cells
musculature—relative increase forming a myoelastic membrane, smooth muscle cells arranged spirally
cartilage—like trachea in the main branches; as branching continues, becomes incomplete rings and plates
elastic tissue—relative increase
glands—mostly mucous but some serous (fewer serous than in the trachea)
other remarks—right and left bronchi divide into secondary bronchi; several orders of latter occur

Bronchioles

epithelium—to ciliated columnar with decreasing diameter;no goblet cells in small; Clara cells in smallest
musculature—spirally arranged smooth muscle cells interlace in a myoelastic lattice
cartilage—present but disappear at a bronchiolar diameter of about 1 mm.
elastic tissue—present
glands—decrease in number and disappear in small diameter branches
other remarks—just when a bronchus becomes a bronchiole can be a matter of termiology

Terminal Bronchiole

epithelium—simple columnar or cuboidal; ciliated, no goblet cells, Clara cells present
musculature—relative increase
cartilage—absent
elastic tissue—present
glands—absent
other remarks—note relative increase in musculature

Respiratory Bronchioles

epithelium—cuboidal except in alveoli; may be ciliated; no goblet cells, some Clara cells
musculature—present; strips as seen in longitudinal section
cartilage—absent
elastic tissue—present
other remarks—each terminal b. divides into two or more respiratory b.; cilia disappear along resp. b.

Alveolar Ducts

epithelium—thin, no cilia or goblet cells
musculature—present as buttons seen in longitudinal section
cartilage—absent
elastic tissue—present
glands—absent
other remarks—there are 2 to 11 alveolar ducts from each respiratory bronchiole

Alveolar Sacs

epithelium—groups of alveoli, no cilia or goblet cells
musculature—none
cartilage—absent
elastic tissue—present
glands—absent
other remarks—thin epithelium is a "continuous layer"