Respiratory

Respiratory Region of the Nasal Cavity

General

  • Forms most of the nasal cavity volume

  • Lined by:

    • Respiratory mucosa

Epithelium

  • Ciliated pseudostratified columnar epithelium

Lamina Propria

  • Firmly attached to:

    • Periosteum (bone)

    • Perichondrium (cartilage)

Structural Features

Medial Wall

  • Nasal septum

  • Surface:

    • Smooth

Lateral Wall

  • Contains:

    • Conchae (turbinates) → shelf-like bony projections

Functions of Conchae

  • Divide nasal cavity into:

    • Separate air chambers

  • Increase:

    • Surface area

  • Cause:

    • Air turbulence

  • Function:

    • Improve conditioning of inspired air

Respiratory Epithelium — Cell Types (5 types)

1. Ciliated Cells

  • Tall columnar cells

  • Have:

    • Cilia projecting into mucus layer

2. Goblet Cells

  • Function:

    • Synthesize and secrete mucus

3. Brush Cells

  • Have:

    • Short, blunt microvilli

4. Small Granule Cells (Kulchitsky Cells)

  • Resemble:

    • Basal cells

  • Contain:

    • Secretory granules

  • Type:

    • Endocrine cells

  • Belong to:

    • Diffuse neuroendocrine system (DNES)

5. Basal Cells

  • Function:

    • Stem cells

  • Give rise to:

    • Other epithelial cell types

Additional Point

  • Respiratory epithelium here is:

    • Same as epithelium in conducting system (e.g., trachea)

High-Yield Points

  • Ciliated pseudostratified columnar epithelium

  • 5 cell types (must know all)

  • Kulchitsky cells = DNES endocrine cells

  • Basal cells = stem cells

  • Conchae → turbulence + ↑ surface area

  • Lamina propria attached to periosteum/perichondrium

Respiratory Region of the Nasal Cavity

General

  • Forms most of the nasal cavity volume

  • Lined by:

    • Respiratory mucosa

Epithelium

  • Ciliated pseudostratified columnar epithelium

Lamina Propria

  • Firmly attached to:

    • Periosteum (bone)

    • Perichondrium (cartilage)

Structural Features

Medial Wall

  • Nasal septum

  • Surface:

    • Smooth

Lateral Wall

  • Contains:

    • Conchae (turbinates) → shelf-like bony projections

Functions of Conchae

  • Divide nasal cavity into:

    • Separate air chambers

  • Increase:

    • Surface area

  • Cause:

    • Air turbulence

  • Function:

    • Improve conditioning of inspired air

Respiratory Epithelium — Cell Types (5 types)

1. Ciliated Cells

  • Tall columnar cells

  • Have:

    • Cilia projecting into mucus layer

2. Goblet Cells

  • Function:

    • Synthesize and secrete mucus

3. Brush Cells

  • Have:

    • Short, blunt microvilli

4. Small Granule Cells (Kulchitsky Cells)

  • Resemble:

    • Basal cells

  • Contain:

    • Secretory granules

  • Type:

    • Endocrine cells

  • Belong to:

    • Diffuse neuroendocrine system (DNES)

5. Basal Cells

  • Function:

    • Stem cells

  • Give rise to:

    • Other epithelial cell types

Additional Point

  • Respiratory epithelium here is:

    • Same as epithelium in conducting system (e.g., trachea)

High-Yield Points

  • Ciliated pseudostratified columnar epithelium

  • 5 cell types (must know all)

  • Kulchitsky cells = DNES endocrine cells

  • Basal cells = stem cells

  • Conchae → turbulence + ↑ surface area

  • Lamina propria attached to periosteum/perichondrium

Lumen of trachea.

  • Remains open due to:

    • Series of cartilaginous rings

Layers of Tracheal Wall (4)

1. Mucosa

  • Epithelium:

    • Ciliated pseudostratified epithelium

  • Lamina propria:

    • Elastic fiber–rich

2. Submucosa

  • Denser connective tissue than lamina propria

3. Cartilaginous Layer

  • C-shaped hyaline cartilage rings

4. Adventitia

  • Connective tissue

  • Function:

    • Binds trachea to adjacent structures

Cartilage — Key Features

  • C-shaped hyaline cartilages

  • Arranged:

    • Stacked series (one above another)

Functions

  • Provide:

    • Support (skeletal framework)

  • Prevent:

    • Collapse of tracheal lumen (especially during expiration)

Bridging Structure

  • Gap bridged by:

    • Fibroelastic tissue

    • Smooth muscle (trachealis muscle)

High-Yield Points

  • C-shaped hyaline cartilage rings

  • Posterior gap + trachealis muscle

  • 4 layers (mucosa, submucosa, cartilage, adventitia)

  • Respiratory epithelium = ciliated pseudostratified

  • Prevents collapse during expiration

Tracheal Epithelium

General

  • Same as:

    • Respiratory epithelium of conducting airway

  • Type:

    • Ciliated pseudostratified columnar epithelium

Cell Types

Major Cells

  • Ciliated columnar cells

  • Mucous (goblet) cells

  • Basal cells

Minor Cells

  • Brush cells

  • Small granule cells (Kulchitsky cells)

1. Ciliated Cells

  • Most numerous

  • Extend through:

    • Full thickness of epithelium

  • Each cell:

    • ~250 cilia

Structure

  • Cilia appear:

    • Short, hair-like projections

  • Below cilia:

    • Dark line = basal bodies

Function

  • Coordinated movement of mucus:

    • Toward pharynx

  • Mechanism:

    • Mucociliary escalator

2. Mucous (Goblet) Cells

  • Similar to:

    • Intestinal goblet cells

  • Location:

    • Interspersed among ciliated cells

  • Extend through:

    • Full thickness of epithelium

Identification (H&E)

  • Clear cytoplasm

  • No apical cilia

Function

  • Secrete:

    • Mucus

Clinical Point

  • Increase in chronic irritation

3. Brush Cells

  • Columnar cells

  • Have:

    • Blunt microvilli

Function

  • Receptor cells

  • Basal surface:

    • Synaptic contact with afferent nerve endings

    • (epitheliodendritic synapse)

4. Small Granule Cells (Kulchitsky Cells)

Type

  • Enteroendocrine cells (DNES)

Features

  • Usually:

    • Single and sparsely distributed

  • Difficult to distinguish without:

    • Special stains (e.g., silver staining)

Ultrastructure (TEM)

  • Membrane-bound dense-core granules

  • May have:

    • Cytoplasmic process toward lumen

Secretions

  • Catecholamines

  • Peptide hormones:

    • Serotonin

    • Calcitonin

    • Gastrin-releasing peptide (bombesin)

Organization

  • May form:

    • Neuroepithelial bodies (with nerve fibers)

Function

  • Involved in:

    • Reflexes regulating airway or vascular caliber

5. Basal Cells

  • Stem (reserve) cells

Function

  • Cell replacement

Features

  • Located near:

    • Basement membrane

  • Prominent nuclei:

    • Form a row near basal lamina

High-Yield Points

  • Mucociliary escalator = cilia + mucus movement

  • ~250 cilia per cell

  • Goblet cells ↑ in chronic irritation

  • Kulchitsky cells = endocrine (serotonin, calcitonin, GRP)

  • Brush cells = receptor cells (nerve synapse)

  • Basal cells = stem cells

Bronchi — Overview

Formation

  • Trachea divides into:

    • Main (primary) bronchi

      • Right main bronchus

      • Left main bronchus

Right vs Left Bronchus

  • Right bronchus:

    • Wider

    • Shorter

  • Left bronchus:

    • Narrower

    • Longer

Branching Pattern

Main → Lobar (Secondary) Bronchi

  • Right lung:

    • 3 lobar bronchi

  • Left lung:

    • 2 lobar bronchi

Lobar → Segmental (Tertiary) Bronchi

  • Supply:

    • Bronchopulmonary segments

Segments

  • Right lung:

    • 10 segments → 10 segmental bronchi

  • Left lung:

    • 8 segments → 8 segmental bronchi

Bronchopulmonary Segment (High-yield)

  • Defined by:

    • Segmental bronchus + supplied lung parenchyma

  • Features:

    • Own blood supply

    • Connective tissue septa

  • Importance:

    • Surgical resection unit

Structural Changes (Trachea → Bronchi → Bronchioles)

Cartilage

  • Trachea:

    • C-shaped rings

  • Bronchi:

    • Irregular cartilage plates

    • Arranged circumferentially

  • With decreasing size:

    • Plates smaller & fewer

    • Eventually disappear → bronchioles

Shape

  • Bronchi:

    • Circular / cylindrical lumen

  • Trachea:

    • Flattened posterior wall

Bronchiole Definition

  • When airway diameter ≈ 1 mm

  • No cartilage

Key Identification Features of Bronchi

  • Cartilage plates

  • Circular layer of smooth muscle

Wall Layers of Bronchi (5)

1. Mucosa

  • Epithelium:

    • Pseudostratified (like trachea)

  • Changes:

    • Cell height decreases with size

  • Basement membrane:

    • Prominent in primary bronchi

    • Less distinct in smaller bronchi

  • Lamina propria:

    • Similar to trachea but reduced

2. Muscularis

  • Continuous smooth muscle layer

  • Arrangement:

    • Spiral / interlacing bundles

  • In smaller bronchi:

    • May appear discontinuous

  • Function:

    • Regulates airway diameter

3. Submucosa

  • Loose connective tissue

  • Contains:

    • Glands

    • Adipose tissue (in larger bronchi)

4. Cartilage Layer

  • Discontinuous cartilage plates

  • Decrease in:

    • Size

    • Number (with decreasing diameter)

5. Adventitia

  • Moderately dense connective tissue

  • Continuous with:

    • Pulmonary vessels

    • Lung parenchyma

High-Yield Points

  • Right bronchus = wider + shorter

  • 3 lobar (right) vs 2 (left)

  • 10 segments (right) vs 8 (left)

  • Cartilage: rings → plates → none

  • Smooth muscle becomes prominent

  • Bronchiole = no cartilage (~1 mm)

  • Bronchi = cartilage plates + smooth muscle

Respiratory cells !!

Supporting Cells (Sustentacular Cells)

Main Function

Provide:

  • Mechanical support

  • Metabolic support
    to olfactory receptor cells

Comparable to:

Neuroglial cells

Histologic Features

Shape

  • Tall columnar cells

  • Narrow basal ends

Nuclei

  • Located more apically than other cells

Apical Surface

Contains:

Numerous microvilli

Cytoplasmic Organelles

Contain:

  • Numerous mitochondria

  • Smooth ER (sER)

  • Some rough ER (rER)

  • Lipofuscin granules

Junctions

Adhering junctions present between:

  • Supporting cells

  • Olfactory receptor cells

Absent:

  • Tight junctions

  • Gap junctions

Lipofuscin

  • Yellow-brown pigment

  • Gives olfactory mucosa its yellow-brown color

Odorant-Binding Proteins (OBPs)

Supporting cells:

Synthesize and secrete OBPs

Function of OBPs:

  • Bind odorants in mucus

  • Transport odorants to olfactory receptors

Brush Cells

General Features

  • Present in small numbers

  • Same type as brush cells in respiratory epithelium

Structure

  • Columnar cells

  • Large blunt microvilli on apical surface

Innervation

Basal surface contacts nerve fibers from:

Trigeminal nerve (CN V)

Function

  • General sensory transduction

  • Possibly:

    • Absorptive

    • Secretory

Ultrastructural Features

Contain:

  • Vesicles near apical membrane

  • Well-developed Golgi apparatus

Basal Cells

Features

  • Small, round cells

  • Located near basal lamina

  • Nuclei lie below nuclei of olfactory receptor cells

Function

Stem/progenitor cells

Differentiate into:

  • Olfactory receptor cells

  • Supporting cells

Cytoplasm

  • Contains few organelles

  • Stem-cell appearance

Additional Feature

Some basal cells partially surround:

  • Initial part of olfactory receptor cell axons

Olfactory Glands (Bowman’s Glands)

Important Identification Feature

Characteristic feature of:

Olfactory mucosa

Structure

  • Branched tubuloalveolar serous glands

  • Located in lamina propria

Ducts

  • Short ducts lined by cuboidal cells

  • Pass through basal lamina

  • Open onto epithelial surface

Secretions

Serous secretions:

  • Dissolve odoriferous substances

  • Trap odorants

  • Wash away old odor molecules

Allows:

Continuous detection of new odors

Histologic Identification of Olfactory Mucosa

Key identifying features:

  1. Olfactory nerves

  2. Bowman glands in lamina propria

Olfactory Nerves

Important Features

  • Contain unmyelinated fibers

  • Fibers relatively large in diameter

  • Easily visible histologically

Structure

Key Point

Supporting cells

Most numerous cells

Supporting cells

Produce OBPs

Supporting cells

Contain lipofuscin

Brush cells

Associated with CN V

Basal cells

Stem cells

Bowman glands

Serous tubuloalveolar glands

Olfactory nerves

Unmyelinated fibers

Olfactory mucosa ID

Nerves + Bowman glands

TRACHEA — HIGH-YIELD HISTOLOGY NOTES

Layers of the Tracheal Wall

1. Mucosa

Made of:

  • Ciliated pseudostratified epithelium

  • Elastic fiber-rich lamina propria

2. Submucosa

  • Denser connective tissue
    than lamina propria

3. Cartilaginous Layer

Made of:

C-shaped hyaline cartilage

Function:

  • Prevents collapse of trachea

4. Adventitia

Made of:

Connective tissue

Function:

  • Binds trachea to surrounding structures

Trachealis Muscle

Location

  • Between free ends of C-shaped cartilage posteriorly

Composition

  • Smooth muscle

  • Fibroelastic tissue

Relation

  • Adjacent to esophagus

Tracheal Epithelium

Type

Respiratory epithelium

=

Ciliated pseudostratified columnar epithelium

Main Cell Types in Trachea

  1. Ciliated cells

  2. Mucous (goblet) cells

  3. Basal cells

  4. Brush cells

  5. Small granule (Kulchitsky) cells

1. Ciliated Cells

Most numerous cells

Structure

  • Extend through full thickness of epithelium

  • About:

    250 cilia per cell

Histology

  • Cilia appear as hair-like projections

  • Basal bodies form:

    dark apical line

Function

Mucociliary escalator

Moves mucus toward:

Pharynx

Purpose:

  • Removes inhaled particles

2. Mucous Cells (Goblet Cells)

Features

  • Similar to intestinal goblet cells

  • Interspersed among ciliated cells

  • Extend full epithelial thickness

Histologic Appearance

In H&E:

  • Mucin washed out

  • Cytoplasm appears pale/clear

  • No apical cilia

Function

Mucus secretion

Mucus:

  • Traps particles

  • Moistens air

Clinical Correlation

Number increases in:

Chronic airway irritation

3. Brush Cells

Features

  • Columnar cells

  • Have blunt microvilli

Basal Surface

Forms synaptic contact with:

Afferent nerve endings

(epitheliodendritic synapse)

Function

Receptor/sensory cells

4. Small Granule Cells (Kulchitsky Cells)

Type

Neuroendocrine / enteroendocrine-like cells

Location

  • Usually single cells

  • Scattered among epithelial cells

Structure

  • Nucleus near basement membrane

  • More cytoplasm than basal cells

  • May have apical cytoplasmic process

Membrane-bound dense-core granules

Secretions

Can secrete:

  • Catecholamines

  • Serotonin

  • Calcitonin

  • Gastrin-releasing peptide (bombesin)

Neuroepithelial Bodies

Some cluster with nerve fibers forming:

Neuroepithelial bodies (NEBs)

Function:

  • Reflex regulation of airway and vascular caliber

5. Basal Cells

Features

  • Reserve/stem cells

  • Near basal lamina

  • Nuclei form row near basement membrane

Function

Cell replacement in epithelium

ALVEOLI — HIGH-YIELD HISTOLOGY NOTES

General Features

Alveoli

  • Terminal air spaces of respiratory system

  • Main site of:

    Gas exchange

Surface Area

  • About:

    150–250 million alveoli/lung

  • Combined surface area:

    ~75 m²

Shape and Size

  • Thin-walled polyhedral chambers

  • Diameter:

    ~0.2 mm

Blood Supply

Each alveolus surrounded by:

Dense capillary network

Function:

  • Brings blood close to inhaled air

  • Enables gas exchange

Alveolar Ducts

Features

  • Elongated airways

  • Almost no walls

  • Alveoli form peripheral boundaries

Contain:

  • Smooth muscle rings at alveolar openings

Alveolar Sacs

Definition

Spaces surrounded by clusters of alveoli

Location

Usually at end of alveolar ducts

Alveolar Septum (Septal Wall)

Definition

Thin connective tissue wall between adjacent alveoli

Contains:

  • Capillaries

  • Connective tissue

Function:

Structural basis for gas exchange

Alveolar Epithelium

Composed of:

  1. Type I alveolar cells

  2. Type II alveolar cells

  3. Occasional brush cells

1. Type I Alveolar Cells

Other Name

Type I pneumocytes

Percentage

  • About:

    40% of alveolar lining cells

But cover:

~95% of alveolar surface

Structure

  • Extremely thin squamous cells

  • Joined by:

    Occluding junctions (tight junctions)

Function

  • Form blood-air barrier

  • Allow gas exchange

Important Point

Cannot divide

2. Type II Alveolar Cells

Other Names

  • Type II pneumocytes

  • Septal cells

Structure

  • Cuboidal secretory cells

  • Usually located at septal junctions

  • Bulge into alveolar space

Percentage

  • About:

    60% of alveolar lining cells

But cover only:

~5% of alveolar surface

Cytoplasm

Contains:

Lamellar bodies

Lamellar bodies:

  • Membrane-bound granule

Function

Produce:

Pulmonary surfactant

Surfactant

Composition

Contains:

  • Phospholipids

  • Neutral lipids

  • Proteins

Function

Reduces surface tension

Prevents:

  • Alveolar collapse

Regenerative Function

Type II cells:

Progenitor cells for Type I cells

Increase in number during:

  • Alveolar injury

  • Repair

Clinical Correlation

Type II cell hyperplasia

=
marker of:

  • Alveolar injury

  • Repair

3. Brush Cells

Features

  • Few in number

  • Present in alveolar wall

Function

May act as:

Receptors monitoring air quality

Blood-Air Barrier (Respiratory Membrane)

Main components:

  1. Type I alveolar cell

  2. Fused basal laminae

  3. Capillary endothelial cell

Function:

Gas diffusion