Histology of the Respiratory System

Department of histology, cytology andembryology KhNMU

The respiratory system includes the lungs , system of passages (airways) and structures of ventilating mechanism, that link the site of gas exchange with the external environment. It is customary to divide the respiratory system into 3 principle regions: 1.a conducting portion, consisting of the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles. 2. a respiratory portion, consisting of respiratory bronchioles, alveolar ducts, atria, alveolar sacs and alveoli.3.ventilating mechanism , which creates pressure differences that move air. It includes the diaphragm, rib cage, intercostal muscles, abdominal muscles and elastic connective tissue in the lungs.

Development of the Respiratory System

Developmental Stages (Human)1.Embryonic phase (3-7 weeks)

Initial budding and branching of the lung buds from the primitive foregut.

Ends with the development of the presumptive broncho-pulmonary segements.

Presumptive - 1 : based on probability or presumption 2 : being an embryonic precursor with the potential for forming a particular structure or tissue in the normal course of development

Further branching of the duct system (up to 21 further orders) generates the presumptive conducting portion of the respiratory system up to the level of the terminal bronchioles. They are embedded within a rapidly proliferating mesenchyme. The structure has a glandular appearance.

2.Pseudoglandular phase (7-16) weeks

The onset of this phase is marked by extensive angiogenisis within the mesenchyme that surrounds a dense capillary network. The diameter of the airways increases with a consequent decrease in epithelial thickness to a more cuboidal structure. The terminal bronchioles branch to form several orders of respiratory bronchioles. The developing respiratory tree, giving rise to chondrocytes, fibroblasts and myoblasts.

3. Canalicular phase (16-24) weeks

Branching and growth of the terminal sacs or primitive alveolar ducts. Functional type I & type-II pneumonocytes differentiate via several intermediate stages from pluripotent epithelial cells in the prospective alveoli. These cells then flatten, increasing the epithelial surface area by dilation of the saccules, giving rise to immature alveoli. Maturation of the alveoli continues by further enlargement of the terminal sacs, deposition of elastin foci and development of vascularised septae around these foci.

4.Terminal sac phase (24-36) weeks

Maturation of the lung indicated by the appearance of fully mature alveoli begins at 36 weeks, though new alveoli will continue to form for approximately three years. A decrease in the relative proportion of parenchyma to total lung volume still contributes significantly to growth for 1 to 2 years after birth, thereafter all components grow proportionately until adulthood.

5.Alveolar phase (36 weeks - term/adult)

There are changes in the epithelial lining of the respiratory tree as one proceeds from the nasal cavity to the alveoli of the lungs: Pseudostratified Columnar Ciliated=> Simple Columnar =>Simple Cuboidal => Simple Squamous Epithelium.

Conducting Portion

System of ducts Conducts air to all parts of the lungs

Nose Nasopharynx Larynx Trachea Bronchi Bronchioles Preterminal bronchioles Terminal bronchioles Conditioning of the air

Warming, moistening and removal of particulate materials

Trachea 20 C-shaped cartilaginous rings Paries membranaceus

Connective tissue that fills gap between the two posterior ends

Neighbouring ringsConnected by regular dense connective

tissueContinuous with perichondrium of

cartilaginous rings

1. Mucosa

Pseudostratified columnar epithelium with cilia and goblet cells

2.Lamina propria,Elastic & Collagen fibers 3.Submucosa Mixed glands and lymph follicles

4. Fibrocartilagenous Layer Hyaline Cartilage & SMC

5.Adventitial layer

Trachea

Low Magnification of a Cross Section Through the Trachea

1. Lumen2. Pseudostratified ciliated columnar

epithelium3. Submucosa4. Hyalin cartilage5. Perichondrium6. Adventitia7. Mixed glands8. Secretory duct

Epithelium of the Trachea

1. Lumen 2. Cilia3. Columnar

epithelial cells4. Basal membrane5. Lamina propria6. Basal cell layer7. Goblet cell

Respiratory epithelial cell types

1.Ciliated columnar cells2.Mucous goblet cells3.Brush cells4.Basal cells5.Small granule cells

BronchiExtrapulmonary bronchi

Two primary bronchi

Histologically similar to the trachea Intrapulmonary bronchi

Secondary bronchi (Lobar) - 5Bronchopulmonary segmental

bronchi –Tertiary (Lobular)- 20

Intrapulmonary BronchiMucosa

Pseudostratified columnar containing cilia and goblet cells

Lamina propria - elastica changes into longitudinally arranged elastic fibers

Smooth muscle Between mucosa and cartilage Mixed glands - between muscle layer and

cartilagous plates

Cartilage Irregular shaped cartilaginous plates

Bronchi Three-dimensional Representation of an Intrapulmonary Bronchus

cartilage plate

lamina propria

pseudostratified columnar epithelium with cilia and goblet cells

smooth muscle

BronchiolesBranches of bronchiPenetrate lung lobule at its apex

Preterminal bronchiole inside the lobule

Preterminal bronchioleBranches into terminal bronchiolesBranch further

BronchiolesMucosa

Epithelium in the larger bronchiolesSimple columnar epithelium with cilia Tall, non-ciliated secretory cells – cells of Clara

In smaller bronchiolesTall cuboidal epithelium

Goblet cells have disapeared higher up in the bronchioles

Lamina propriaThin elastic layer

Bronchioles

Muscle layerSame as that of the bronchi

Connective tissue

Attaches bronchioles to surrounding tissue

No glands or cartilage are present

Terminal bronchioles – end point of Conducting portion

Respiratory bronchioles – beginning of Respiratory Portion

Respiratory PortionConsists of smaller ducts and

sacsRespiration takes place

Respiratory bronchiolesAlveolar ductsAlveolar sacsAlveoli

Respiratory Bronchioles

Respiratory bronchioles Branch from terminal

bronchioles Alveoli

Thin bulging sacs in walls

Gas exchange takes place

Schematic of the Respiratory Portion of a Lung Lobule

Smooth musclerespiratory bronchiole

alveolar ductsalveoli

interalveolar septum

sacculus alveolaris

High Magnification of the Lung Demonstrating a Respiratory Bronchiole

1. High cuboidal epithelium

2. Alveolus3. Interalveolar

septum4. Lumen

Alveolar Ducts, Alveolar Sacs and Alveoli

Alveolar ducts Long passages into which respiratory bronchioles

open Alveolar ducts open directly into small spaces

Alveoli Interalveolar septa

Separate neighboring alveoli Alveolar sacs

Air spaces surrounded by clusters of alveoli Elastic and reticular fibers

Arranged around the capillaries

1. Alveolar sacs2. Alveoli3. Interalveolar septa

Blood – Air BarrierSeparation Between Air and

Bloodstream Cytoplasm of epithelium lining the

alveoliBasal lamina of the epitheliumBasal lamina of the capillary

endotheliumCytoplasm of capillary endothelium

Alveolar epithelium – PneumocytesSimple squamous alveolar epithelial cells – Type 1 cells -Thin cytoplasmLarge Secretory cells – Type 2 (secretory cells) - Secrete surfactant & Decrease surface tensionAlveolar macrophages - Type 3 (dust cells)

Pulmonary surfactant is a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells. It is 2-layer molecular film on the alveolar surface. The proteins and lipids that comprise the surfactant have both a hydrophilic region and a hydrophobic region. The main lipid component of surfactant, (DPPC)- dipalmitoylphosphatidylcholine.

. Functions of surfactant1.Decrease surface tension2.To increase pulmonary compliance3.To prevent atelectasis (collapse of the lung) at the end of expiration.4.To facilitate recruitment of collapsed airways.

Pulmonary Surfactant

Pores and Lambert-sinuses Alveolar pores

Direct contact between 2 alveoli Infections can spread from one lobe

to another via this route Lambert sinuses

Short cannelConnects terminal bronchioles with

alveoli

Acinus is a Structural and Functional Unit of the Lung

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Terminal Part of the Lungs