Watch the following video lessons of the gross anatomy and histology of a normal lung.

Normal gross anatomy of lung

^{Presented by Dr. Jennifer Kong using ‘Normal lung specimen” from DHPLC teaching set}

Normal Histology of the Lung

The next 4 videolessons look at the histology of lung tissue from the spots along the respiratory tract:  bronchi, bronchioles, and alveoli.  The make up of tissue changes as one progresses from the upper respiratory tract (i.e. bronchi) to the lower (i.e. alveoli)

Overview of Lung Histology

^{Created by Sarah Pinault under a CC BY NC ND license}

Bronchi

Bronchi are the largest airways (tubes) in the lung, with an average diameter in the centimeter scale. The defining features of a bronchus histologically are the cartilaginous plates that provide structural support for the large airway which can be found around the edge of the bronchus, similar to the trachea in the upper respiratory tract. Bronchi, like other airways in the lung, are arranged in layers of cells. These layers, starting from the most luminal cells, are made up of:

A. Pseudostratified columnar epithelium (“respiratory epithelium”) with 2 main cell types:

• Goblet cells – mucus-secreting, “balloon”-like. The mucus traps bacteria and debris and prevents it from reach the smaller airways
• Ciliated cells – columnar/”rectangular” with finger-like projections (cilia) facing the lumen of the airway. The cilia help to push the mucus with trapped substances upwards and towards the esophagus.

“Pseudostratified” refers to the fact that it appears like there are multiple layers of cells in the epithelium, however it is actually a single layer of densely packed cells that “stack up” on one another. All of these cells in this single layer are anchored to a thin layer of protein called the basement membrane

B. Lamina propria – wavy layer of loose connective tissue that lies beneath the epithelium

C. Smooth muscle layer – made up of distinctive wavy cells with stretched nuclei

D. Mucous glands – round arrangements of of secretory cells that secrete mucus and other products into a central lumen

E. Fat cells on the same layer as mucous glands

F. Cartilage plates (hyaline cartilage) – plates of thick, distinctive material that appear shiny under light microscopy and distinguish bronchi from bronchioles. The cartilage plates line the smooth muscle layer. Cells (called chondrocytes) can be found within “holes” (lacunae) within the cartilage.

^{Presented by Noah Stewart using a histology slide of H&E stained normal lung  (DHPLC e-slide: PATH 304-030a)}

Bronchioles

Bronchioles are another type of airway in the lung. They can be classified into different types depending on their position within the airway (between bronhi and alveoli). They are smaller than bronchi but larger than other airways in the lung, with an average luminal diameter of less than one millimetre. Their structure is very similar to that of the bronchi, with notable differences being their lack of cartilage or mucous glands. The respiratory epithelium also becomes more cuboidal (cube-like)/shorter and there are less goblet cells present in the epithelium as you move farther down the airways towards the alveoli. A bronchiole that opens up and begins to branch into the alveoli is called a terminal bronchiole (average diameter of around 0.1 millimeters).

Alveoli

Alveoli are the smallest airways (sack) in the lung with an average diameter of around 0.5 μm. They are the functional unit of the lung and are the structures responsible for the actual gas exchange that occurs. Under the microscope they appear as isolated sacs in the lung, and consist of two distinct parts: the very thin alveolar walls or septae (made up of pneumocytes and capillaries), and the lumen (where the air fills).

Unlike the larger airways, the alveoli are no longer lined by respiratory epithelium. They have completely distinct cell types present, which are:

• Type I pneumocytes – very thin cells that form the thin wall of the alveolus, which is necessary for gases to diffuse across. Nucleus appears elongated, and the rest of the cell body stretches far and thin to cover a lot of area

• Type II pneumocytes – a cuboidal cell also in the wall of the alveolus, that secretes surfactant into the lumen (and thus can appear “foamy”) to prevent collapse of the alveoli

• The capillaries, made up of endothelial cell(s) with red blood cells in the lumen – usually a single endothelial cell wraps around itself to form a very small lumen, where blood cells can pass through and carry out gas exchange across the very thin wall made by type I pneumocytes
• Alveolar macrophages – present in small quantities in the lumen, carry out immunological surveillance of the lung and can eat up debris and thus commonly appear darkly pigmented under the microscope

^{Presented by Noah Stewart using a histology slide of H&E stained normal lung  (DHPLC e-slide: PATH 304-030a)}

Section Summary

The lung contains many different tissue types, each with their own function.  The upper respiratory tract (i.e. trachea, primary bronchi) contain many cartilaginous rings and plates to allow for a physical opening of the airways, independent of nervous control.  As the airways get smaller, there is a reduction of cartilage and an increased presence of smooth muscle. Smooth muscle is neurally controlled and can allow for fine-tuned airway opening/closing dependent on the situation.

There are many cell types involved in the respiratory defense of the lungs. Goblet cells with ciliated epithelia work together the trap pathogens and debris in mucous made by goblet cells.  The cilia then brush that mucous up towards the pharynx for either swallowing or coughing.  Coughing is a neurologically controlled reflex that detects inhaled irritants and expel them with force.  Alveolar macrophages reside in the alveolar air space and roams around looking for irritants and pathogens that escaped the aforementioned defenses.

There are 3 major cell types in the alveoli: type I pneumocytes that make up the bulk of the alveolar wall; type II pneumocyte which makes surfactant to prevent the collapse of alveoli; and capillary endothelial cells which allow for quick gas exchange between blood and the Type I pneumocyte.

Review Question