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E. T. Uluer et al.
elastic bers and contains numerous lymphocytes, as well as mucous and serous
glands. Lymphocytes tend to congregate around the openings of the Eustachian
tubes into the nasopharynx, forming the tonsilla tubalis. Similarly, on the posterior wall, they cluster to create the pharyngeal tonsil. Waldeyer’s lymphatic ring is
formed by the collective presence of the tonsilla lingualis and tonsilla palatina,
situated behind the tongue, along with other tonsils. The lamina propria connects
to the lateral walls of the pharynx through the submucosal layer and is directly
linked to the muscle tissue on the posterior wall. While the diameter of the nasopharynx might change, its passageway remains capable of both opening and fully
closing.
2.5 Larynx
The larynx is an elongated, slender tubular structure measuring 4–5cm in length,
characterized by its irregular morphology. It serves as a vital conduit connecting the
pharynx to the trachea. Beyond its role in phonation, the larynx also fulls the crucial function of averting the passage of liquid and solid substances into the trachea
during the process of swallowing. The laryngeal framework comprises cartilaginous
tissue, with both singular and paired cartilages constituting its architectural components. The solitary cartilages encompass the thyroid, cricoid, and epiglottis, while
the twofold cartilages encompass the corniculate, cuneiform, and arytenoid cartilages. Ligaments interconnect these cartilages, with intrinsic and extrinsic muscular
components orchestrating the intricate movements of the cartilaginous framework.
Intrinsic muscles govern the tension of the vocal cords, while extrinsic muscles
orchestrate the laryngeal movements required for swallowing.
Among the individual laryngeal cartilages, the thyroid and cricoid cartilages are
responsible for shaping the laryngeal structure. The epiglottis, located loosely on
the anterior surface of the larynx, functions in closing the laryngeal orice during
the act of swallowing. The laryngeal lumen encompasses two pairs of folds. The
upper pair, termed the vestibular fold (also known as the false vocal cord or false
plica vocalis), remains immobile. Comprising loose connective tissue, seromucous
glands, lymphoid tissue, and adipocytes, the vestibular fold resides in the superior
aspect of the false fold. The true vocal fold (also referred to as the true chord or true
plica vocalis) resides within the lower section of the false fold. This dynamic region
plays a central role in sound production, with its movements directed by the vocal
muscle, a type of skeletal muscle. A ventricle, characterized by a deep slit-shaped
recess, is positioned between these folds. The vocal cords’ free edges, composed of
elastic connective tissue, comprise the vocal ligament. Intricately controlled by the
intrinsic laryngeal muscles, the tension of the vocal cords governs the vibration of
air traversing the aperture between the cords (rima glottis) (Fig.2.1).
The laryngeal surface is enveloped by a lining of respiratory epithelium. The
orchestrated ciliary motion inherent to this respiratory epithelial layer facilitates the
propulsion of the overlaying mucus toward the oral cavity. Additionally, distinct
regions of the epiglottis facing the pharynx, the initial section of the laryngeal
2 Histological Characteristics of the Upper Respiratory Tract: Continuum with Lower…
Hyoid bone
23
Epiglottis
Pseudo-stratified columnar
Stratified squamous
epithelium
epithelium
False Vocal Cord
True Vocal Cord
Reinke’s Space
Vocal ligament
Seromucuous glands
Elastic cartilage
Thyroid cartilage
Ventricularis muscle
Laryngeal vntricle
Hyoid muscles
Vocalis muscle
Seromucuous
glands
Cricoid cartilage
Fig. 2.1 Diagram of a human larynx. The larynx, connecting the pharynx and trachea, comprises
singular and paired cartilages, controlled by muscles for voice and swallowing. Notably, the laryngeal surface features respiratory epithelium, but distinct areas, such as the epiglottis and the initial
part of the laryngeal surface and the epithelial layer covering the true vocal folds, exhibit stratied
squamous epithelium. The lamina propria contains elastic bers and various gland types. Extrinsic
and intrinsic muscles surround and regulate the laryngeal structure, while the epiglottis aids in
swallowing protection. The epiglottis consists of elastic cartilage with varying surface epithelium.
In the lamina propria are glands including serous and mucous glands. (This gure was created with
biorender.com)
surface, and the epithelial layer encompassing the true vocal folds exhibit a conguration characterized by the stratied squamous epithelium.
The lamina propria represents a matrix of loose connective tissue, interspersed with a substantial quantity of mast cells and lymphocytes distributed
throughout the connective framework. This lamina propria is notably abundant
in elastic bers, a constituent notably forming the vocal ligaments composed of
elastic bers.
Numerous serous and mucous glands are distributed within the lamina propria,
with mucous glands predominating in this milieu. Notably, the presence of seromucous glands and lymphatic vessels does not align with the level of the vocal cords.
Noteworthy also is the relatively inconspicuous manifestation of the submucosal
layer in the laryngeal conguration, characterized by a seamless transition from the
surrounding lamina propria. A notable clinical consideration is the absence of submucosal tissue behind the epiglottis and above the vocal ligaments, contributing to
their rm attachment to the underlying structures. This feature becomes pivotal in
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clinical contexts, particularly in the context of edema that tends to localize above
the level of the vocal cords without spreading further downward.
The laryngeal musculature encompasses both extrinsic and intrinsic components.
The extrinsic muscles envelop the cartilaginous structure externally, while the
intrinsic muscles are situated within the internal region. Striated muscle constitutes
the classication of the vocal cords themselves, while the encompassing musculature assumes a smooth muscle arrangement. The epiglottis, a planar entity that
extends upwards from the anterior laryngeal wall, is afxed to the laryngeal structure with a degree of laxity. Its active participation in the process of swallowing
engenders the coordinated elevation of the trachea, larynx, and pharynx, culminating in laryngeal closure. This dynamic motion functions as a protective measure,
precluding the ingress of ingested matter into the respiratory system.
The epiglottic scaffold is constituted by elastic cartilage, and its surface epithelium exhibits differing characteristics on its upper and lower aspects. The epithelium facing the pharynx mirrors the stratied squamous epithelium reminiscent of
the pharynx, while the laryngeal-facing surface is lined by the respiratory epithelium.
In the lamina propria, an assortment of tubuloalveolar glands, encompassing
serous, mucous, and mixed variants, can be identied. Positioned on the anterior
tongue surface, taste corpuscles serve as sensory entities [4, 5].
E. T. Uluer et al.
2.6 Trachea
The human trachea is about 2.5cm in diameter and 11–12cm in length. Extending
from the cricoid cartilage at the base of the larynx to the fourth thoracic vertebra, the
trachea bifurcates into the right and left primary bronchi. Hyaline cartilage, forming
a C-shaped structure, maintains tracheal lumen patency, while the connected smooth
muscle bundle controls lumen width, enabling efcient expulsion of foreign bodies
or mucus during rapid air passage, such as during coughing.
Histologically, the trachea comprises four layers: the innermost tunica mucosa,
covered with respiratory epithelium and an elastic ber-rich lamina propria; the
relatively dense connective tissue of the tunica submucosa; a C-shaped layer of
hyaline cartilage beneath the submucosa; and the outermost tunica adventitia. The
epithelium is characterized by typical respiratory epithelium, including pseudostratied columnar epithelium with ciliated and goblet cells, accompanied by a substantial basal lamina (Fig. 2.2). On an ultrastructural level, electron microscopy
distinguishes ve distinct cell types.
2.6.1 Cells ofTrachea Epithelium
2.6.1.1 Ciliated Columnar Cells
Ciliated columnar cells, the most numerous cells (30%) of the tracheal epithelium,
these elongated cells exhibit basal nuclei positioning. The apical surface of these
cells has numerous cilia, intricately connected to basal bodies within the apical
2 Histological Characteristics of the Upper Respiratory Tract: Continuum with Lower…
Trachealis muscle
Nerve
Blood vessel
Tunica submucosa
Hyalin cartilage
Seromucous glands
Tunica adventitia
Tunica mucosa
25
Ciliated columnar cell
Pseudostratified
columnar
epithelium
Goblet cell
Brush Cell
DNES cell
Basal Cell
Basement Membrane
Lamina Propria
Fig. 2.2 Diagram of a human trachea. This illustration showcases the human trachea, an essential
part of the respiratory system, featuring a cross-section to highlight its anatomy and histological
composition. The trachea, measuring about 2.5cm in diameter, begins at the cricoid cartilage and
extends to the fourth thoracic vertebra before dividing into the primary bronchi. Layers such as
tunica mucosa with specialized respiratory epithelium, tunica submucosa with dense connective
tissue, C-shaped hyaline cartilage, and the outer tunica adventitia are illustrated, emphasizing their
roles in maintaining the tracheal structure and functionality. Additionally, the presence of smooth
muscle bundles controlling lumen width and cellular components like ciliated and goblet cells is
depicted, enhancing understanding at a microscopic level. (This gure was created with bio-
render.com)
cytoplasm. The Golgi complex is small and located near the upper nucleus, accompanied by an abundance of mitochondria. These motile cilia have a continual, rhythmic beating motion of 1000–1500cycles/min, propelling the mucus layer at a rate of
5–20mm/min in narrower airways and 0.5–1mm/min in the trachea and principal
26
bronchi. This coordinated ciliary movement directs the mucus layer toward the nasopharynx, facilitating the clearance of particles to the oropharyngeal region.
E. T. Uluer et al.
2.6.1.2 Goblet Cells
Goblet cells are similar in appearance to the intestinal goblet cells and represent
another substantial portion (30%) of the cellular population. The cells extend
through the full thickness of the epithelium. The enlarged apical domains contain
numerous low-density mucinogen granules of varying sizes that combine with
water to generate the mucus layer. Sparse microvilli populate the apical surfaces,
while nuclei and organelles are predominantly located in the basal regions. Abundant
granular endoplasmic reticulum cisterns, well-developed Golgi complexes, and
numerous mitochondria characterize this basal portion.
2.6.1.3 Brush Cells
Brush cells are columnar cells and constitute up to 3% of the respiratory epithelium.
These cells bear blunt microvilli at their apical surface. A well-developed smooth
endoplasmic reticulum is evident within the cytoplasm, often accompanied by small
glycogen granules. Despite their precise function and interplay with neighboring epithelial cells remaining partially elucidated, their presence of synapses with intraepithelial nerve bers along their basal surfaces suggests potential sensory receptor
attributes. Some researchers propose a conceptual relationship to empty goblet cells.
2.6.1.4 Basal Cells
Basal cells have stem cell features and they maintain cell replacement in the epithelium. These cells are pyramidal in shape and their nuclei are situated in the basal
regions between the bases of columnar cells. Basal cells have a few organelles,
adopting an undifferentiated aspect. Their notable attributes encompass the capacity
for replication and transformation into other cellular constituents.
2.6.1.5 Enteroendocrine System Cells (Kulchitsky Cells or DNES Cells)
Enteroendocrine cells occur singly in the trachea and exhibit numerous membranebounded, dense-core granules. These granules, averaging 100–300nm in diameter,
primarily occupy basal cell locations. Within this category, a subset displays the staining prole like catecholamine-storing cells. Furthermore, certain types bear a resemblance to cells releasing enteropeptides (serotonin, calcitonin, and gastrin-releasing
peptide) in the epithelial context. Both subsets are conjectured to exert regulatory inuences over mucous and serous gland secretory functions within the lamina propria.
2.6.2 Lamina Propria
Beneath the tracheal epithelium lies the basement membrane, measuring 25–40μm
in thickness, exhibiting a uniform and luminous aspect under light microscopy.
Collagen bers present organized alignment. Chronic irritation from smoking
induces increased thickness.
2 Histological Characteristics of the Upper Respiratory Tract: Continuum with Lower…
The lamina propria appears as a typical loose connective tissue. Serous and
mucous glands are situated within this matrix, discharging their secretions into the
tracheal lumen. Connective tissue harbors lymphocytes and neutrophils, often forming lymphoid nodules inlocalized aggregations. Cellular, plasma cells, mast cells,
eosinophils, and broblasts are additionally noted, collectively constituting the
bronchial lymphatic tissue (BALT). A discernible elastic membrane, visible with
specialized tissue dyes, separates the deep lamina propria from the submucosa.
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2.6.3 Submucosa andAdventitia
The submucosal layer, characterized by loose connective tissue, hosts scattered
serous crescents alongside numerous mucous glands. These glands release glycoproteins via ducts lined with cuboidal epithelium. Abundant vascular structures,
encompassing blood and lymphatic vessels, are discernible. The submucosa is contiguous with cartilage perichondrium.
Arranged in a horseshoe or “C” conguration, the tracheal cartilage, numbering
between 16 and 20, resides beneath the submucosa. This hyaline cartilage array is
stacked in a rearward orientation, interspersed with broelastic membranes. Age-related
brous tissue augmentation occurs in these membranes. Thick bundles of smooth muscle interpose between the posterior cartilage ends, supported by collagen-elastic bers.
This architecture facilitates tracheal diameter modulation during respiration.
Continuity with adjacent connective tissues (esophagus and neck) is maintained
by the adventitia, forming the outermost layer of loose connective tissue. The inferior thyroid artery predominantly supplies blood to the trachea. Parasympathetic
bers originate from the recurrent branch of the vagus nerve, while sympathetic
bers arise from the truncus sympathicus [4, 5].
2.7 Lungs
2.7.1 Pleura
The pleura is a serous membrane that lines the inner surface of the thoracic cavity.
It consists of two layers: the parietal pleura, which covers the chest wall, and the
visceral pleura, which envelops the lung’s outer surface. These two layers converge
at the hilus region, forming a closed sac. Both the parietal and visceral pleura are
composed of a single layer of mesothelial cells covering an underlying connective
tissue rich in elastic bers. This connective tissue is continuous with the lung parenchyma’s elastic bers. The blood vessels supplying the visceral pleura originate
from the pulmonary and bronchial arteries, while nerves come from the vagus and
bronchial sympathetic nerves. Conversely, the parietal pleura receives its blood supply from the intercostal arteries and nerves. Between the visceral and parietal pleura
lies a potential space containing approximately 30–50cm3 of serous uid. This uid
is secreted by the mesothelial cells and serves to lubricate the pleura.
28
E. T. Uluer et al.
2.7.2 Bronchi
The bronchi divide into two main parts: extrapulmonary bronchi, which remain
outside the lungs, and intrapulmonary bronchi, which extend within the lung. The
trachea bifurcates into the right and left primary bronchi at the fourth thoracic vertebra, known as extrapulmonary bronchi. These primary bronchi enter the lungs
through the hilum region (intrapulmonary bronchi) and branch into secondary and
tertiary bronchi. Beyond the bronchi, the respiratory tree progresses into bronchioles, culminating in the terminal bronchiole. The respiratory bronchiole follows,
representing the rst site of gas exchange in this part. Approximately 20 branching
divisions occur from the trachea to the respiratory bronchioles. Subsequent divisions lead to the formation of alveolar ducts and alveolar sacs. Alveoli constitute the
primary structural and functional unit of the lung.
Throughout the course of the respiratory tree toward the alveoli, notable histological changes occur. The diameter decreases, glands and goblet cells decrease in
number, epithelial length shortens, while smooth muscle and elastic tissue increase.
The primary bronchi resemble the trachea structurally but are smaller in diameter
and have thinner walls. They enter the lungs alongside pulmonary arteries, veins,
and lymphatic vessels. The right bronchus is wider and more vertically oriented
than the left.
Secondary (lobar) bronchi form when primary bronchi enter the lung and divide,
resulting in three on the right and two on the left. Consequently, the right lung has
three lobes and the left lung has two. Tertiary (segmental) bronchi emerge from
secondary bronchi branching, leading to bronchopulmonary segments. These segments are separated by connective tissue and are signicant for lung surgical procedures. There are approximately ten bronchopulmonary segments in each lung.
Segmental branches further divide into subsegmental branches, eventually forming segments into lobules. The branching pattern in the bronchial tree is dichotomous, resulting in 9–12 branch divisions. In intrapulmonary bronchi, the histological
structure is similar to primary bronchi but characterized by shorter epithelium,
fewer goblet cells, and irregularly shaped cartilage that entirely encircles the bronchi to maintain their open position (Fig.2.3). Two layers of smooth muscle bers
are present between the broelastic bers of the lamina propria and the cartilage
tissue. The submucosa consists of dense connective tissue with numerous serous
and mucous glands. Lymphocytes are abundant in the lamina propria among the
epithelial cells, and lymph nodes are primarily found at the bifurcation regions of
the bronchial tree.
2.7.3 Bronchioles
Bronchioles, the terminal branches of bronchi, enter lung lobules and further divide
into 5–7 terminal bronchioles with a 1mm diameter. Large bronchioles have a single-layer columnar epithelium with goblet cells, while small bronchioles have a
cuboidal epithelium with Club cells. Cartilage and glands are absent.
2 Histological Characteristics of the Upper Respiratory Tract: Continuum with Lower…
29
Bronchi
Pseudostratified
Ciliated Columnar
Epithelium
Ciliated
Cell
DNES
Cell
Goblet
Cell
Basal
Cell
Smooth muscle
Cartilage
Bronchioles
Simple Ciliated
Columnar
Epithelium
Ciliated
Cell
Club
Cell
Basal
Cell
Smooth muscle
Terminary Bronchioles
Club
Ciliated
Cell
Simple Ciliated
Cuboidal Epithelium
Cell
Smooth muscle
Respiratory Broncholes to Alveoil
Alveolar Duct
Type I
Alveolar
Cells
Type II
Alveolar
Cells
Alveolar
Macrophage
Alveolar Saccus
Alveoli
Endothelial
Cell
Blood-Air Barrier
Dual Basal
Lamina
Type I
Alveolar
Cell
Fig. 2.3 Diagram of a human bronchiole tree. Respiratory epithelium in bronchi is composed of
pseudostratied columnar cells rich in ciliated and goblet cells. The hyaline cartilage forms the
structural support, maintaining bronchial lumen patency. The lamina propria beneath comprises
elastic ber-rich tissue. The bronchioles are absent of cartilage but have spiral smooth muscle in
the lamina propria and varying epithelial cell types: single-layered columnar or cuboidal cells,
including goblet cells and Club cells. Terminal bronchioles have delicate connective tissue rich in
elastic bers within the lamina propria, and predominantly cuboidal epithelial cells like Club cells.
The respiratory bronchioles to alveoli image highlights the transition, showcasing the thin-walled
structures, smooth muscle, and the critical role of Type I alveolar cells for gas exchange. It further
includes the alveolar canals and saccules, which are tiny air-lled sacs that facilitate gas exchange.
The lamina propria in alveoli is rich in collagen and elastic bers. (This gure was created with
biorender.com)
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E. T. Uluer et al.
Club cells (Clara cells), or nonciliated bronchial epithelial cells, lack cilia and
possess secretion-producing organelles. They release glycoprotein-rich secretion,
similar in function to alveolar surfactant, preventing airway adhesion and facilitating
watery secretion by removing Cl− ions. CC16 protein from Club cells indicates lung
health, while KL-6 indicates advanced bronchopulmonary dysplasia [6]. Club cells
also have anti-inammatory and immunomodulatory functions [7]. Bronchioles lack
glands and feature spiral smooth muscle bers with surrounding elastic bers for
expansion and airway maintenance. Cartilage is absent (Fig.2.3). Fibroblasts, lymphocytes, mast cells, and rarely eosinophilic leukocytes populate the lamina propria.
The autonomic nervous system controls bronchi and bronchioles, with the parasympathetic vagus nerve causing constriction and sympathetic nerves counteracting it.
Bronchioles can connect directly to alveoli through rarely observed Lambert’s
ducts, potentially facilitating collateral ventilation with adjacent alveoli.
2.7.3.1 Terminal Bronchioles
Terminal bronchioles, with a diameter of approximately 0.5mm, represent the ultimate segment of the conducting airways. Predominantly comprising Club cells, the
epithelium of terminal bronchioles also includes ciliated cubic epithelial cells.
Underlying this epithelial layer, a thin muscular coat, typically consisting of 1–2
layers, surrounds a delicate connective tissue framework. The outer elastic bers
establish connections with other components of the bronchial tree.
2.7.3.2 Respiratory Bronchioles
Terminal bronchioles split into two or more respiratory bronchioles. These structures
look similar to terminal bronchioles but have thin walls where small air sacs (alveoli)
poke through, giving the bronchioles an intermittent appearance. The epithelium lining respiratory bronchioles consists of a single layer of cuboidal cells, including ciliated cells intermingled with Club cells. A distinctive feature of respiratory bronchioles
is the presence of prominent smooth muscle bers beneath the epithelium, situated
between the alveoli, along with elastic bers within the alveolar walls.
2.7.4 Ductus Alveolaris
Respiratory bronchioles transform into ductus alveolaris, with 2–11 branches.
These ducts are lined with alveoli. In narrower regions away from the alveoli, they
contain ciliated epithelial cells and elastic-rich connective tissue. Single alveolar
sacs or atria connect to the ductus alveolaris, forming a complex network of elastic
and reticular bers at their openings, allowing for alveolar expansion during inhalation and contraction during exhalation.
2 Histological Characteristics of the Upper Respiratory Tract: Continuum with Lower…
31
2.7.5 Alveoli
The alveoli, constituting the terminal segment of the respiratory tree, serve as the
anatomical and functional keystones within the pulmonary system. Their remarkably
thin walls facilitate the crucial exchange of carbon dioxide (CO2) and oxygen (O2).
Although each individual alveolus boasts a modest volume of approximately 200μm3,
their collective abundance, numbering between 150 and 250 million per lung, results
in an extensive gas exchange interface, totaling roughly 75m2. This perfusion of alveoli signicantly contributes to the lung’s characteristic spongy architecture.
For the essential task of gas exchange between the inhaled air and the bloodstream, the alveolar walls possess a specialized conguration. A rich network of
capillaries envelops the alveoli, affording a substantial surface area, albeit somewhat smaller at approximately 60m2. The intervening tissue that separates the capillaries from the alveoli bears the designation “interalveolar septum.” Within these
regions, the alveolar epithelium and capillary endothelium are in close proximity,
facilitating the intricate process of gas exchange.
Blood arriving from the pulmonary artery, laden with carbon dioxide, undergoes
oxygenation within the alveoli, catalyzing the exchange of oxygen for carbon dioxide. The oxygenated blood subsequently returns to the heart via the pulmonary
veins and circulates throughout the entire body. The structural integrity of the septum is reinforced by Type III collagen bers, which provide crucial support.
In regions contiguous to the alveoli, the interstitial tissue, primarily composed of
connective tissue, is notably diminished. Additionally, certain regions feature pores
ranging from 8 to 60μm in diameter, referred to as Kohn’s intervals. These intervals, while generally not implicated in normal ventilation, facilitate airow in
instances of atelectasis (lung collapse) and obstructions [8].
When scrutinizing the alveolar structure, two primary cell types, namely Type I
and Type II cells, predominate (Fig.2.3).
2.7.5.1 Type IAlveolar Cell (Squamous Alveolar Cell, Small
Alveolar Cell)
Type I alveolar cells, making up 40% of alveolar cells, form a continuous, squamous
monolayer covering a substantial 95% of the alveolar surface. Their unique ultrastructure includes a centrally located nucleus, minimal cytoplasmic thickness, and abundant pinocytotic vesicles. Tight junctions and desmosomes connect these cells, which
are intimately associated with a network of capillaries in the interalveolar septum.
2.7.5.2 Type II Alveolar Cell (Septal Cell, Large Alveolar Cell)
Type II alveolar cells, occupying approximately 60% of the alveolar cell population,
cover 5% of the alveolar surface. These cuboidal cells are typically found in clusters
of two or three at the alveolar corners. Type II cells exhibit a vesicular cytoplasm
when observed under a light microscope and are often referred to as “granular