At the end of this section, you will be able to:
- List differences in lung tissue between being in the body vs. preserved and mounted for presentation.
- Identify bronchioles and alveoli by gross anatomy and microscopically, being aware of the limitations of visualization.
Normal lung: gross anatomy
Lungs are normally encased in the thoracic (chest) cavity, within pleural membranes, under negative pressure (a vacuum). Thus, when we remove lungs to put in a specimen case, the lungs will appear more shrunken than its size when in the chest, thanks to the high amount of elastin within the lung. The high elastin content allows the lungs to ‘snap’ back into a smaller volume: this is akin to an elastic band which, when relaxed, snaps back to a smaller size. Due to this elasticity, exhalation is a passive process – i.e. no muscle contraction is necessary as all of the work is done by the elastin snapping back and squeezing the air.
Knowing that lungs will be smaller than in the chest, let’s look at the gross anatomy of the internal lung.
Gross Anatomy of Normal Lung with Blood (DHPLC specimen: A0001) by Jennifer Kong, licensed under All Rights Reserved
When the blood is washed out, it is easier to see the lung’s structure from a gross perspective. The obvious visible holes are the large- to medium size airways (bronchioles).
Looking at the gross anatomy of lung (Fig 5.8), what are the black specks?
Every breath one takes in their lifetime, we inhale pathogens, irritants, and debris. The lung’s defenses (e.g. ciliated epithelia, mucous, coughing, alveolar macrophages, etc.) can trap & remove them physically or immunologically. However, one does inhale irritants which the lung’s defenses can’t remove. As such, what can’t be removed stays within the lungs for one’s entire life. These black specks can be from everyday environmental pollution, forest fire smoke, inhaled occupational dust, etc. Thus, over the years, a certain amount (albeit small) of lung tissue will not be able to ventilate due to these accumulated foreign debris plugging up the airways.
To visualize the ‘holes’ better, lungs can be prepared using the Gough technique: a thin section of the lung lobe is dried flat, thus allowing for both visual and tactile examination. Even though this preparation still won’t allow us to view the microscopic alveoli, this method does allow for the homogeneity of the lung tissue and the size of intact small airways.
Gross Anatomy of Gough Section of Normal Lung by Jennifer Kong, licensed under All Rights Reserved
Histology of normal lung tissue
Recall that the bulk of the lung’s volumes are alveoli – thus giving the lungs an incredible amount of surface area for gas exchange. Thus, when viewing lung tissue at lower power magnification, the majority of the tissue seen are alveoli of similar shape and size, thus giving a pattern. Large spaces are bronchioles, with the amount of cartilage visible (stains purple with H&E) indicating the proximal airways to the primary bronchi and the amount of smooth muscle indicating the distal airways, closer to the alveoli. Although H&E stain protein in general, it can not differentiate between protein types (e.g. elastin vs collagen). However, normal lungs have a relatively large amount of elastin.
Let’s listen to histopathologist Dr. Jonathan Bush examine normal lung tissue, stained with H&E, at low- and high-power magnification.
Histology of Normal Lung on Low Power Magnification (DHPLC E-slide: Path 304 – 030a) by Jonathan Bush, licensed under All Rights Reserved
Histology of Normal Lung on High Power Magnification (DHPLC E-slide: Path 304 – 030a) by Jonathan Bush and Adrian Marcuzzi, licensed under All Rights Reserved
- Lungs are highly vascularized (via pulmonary blood vessels) and in the body, fully expanded, because of the negative pressure within the chest cavity. However, during preservation and mounting, the lungs have the blood either washed out or react with preservatives giving it a grey colour. Similarly, the lungs are smaller and collapsed in the presentation case as they are no longer under negative pressure.
- Large airways such as bronchi and bronchioles are visible to the naked eye. Microscopic examination on high power allows for visualization of alveoli and pulmonary blood vessels.
- Alveoli are identifiable by a large air space, surrounded by a very thin wall, one cell thick, with blood vessels in close proximity.
uniform in composition or character (i.e. color, shape, size, texture, etc.)