The following CBME core competencies are covered in this chapter. EN1.1: Describe the anatomy of nose and paranasal sinus.

Embryology of Nose and Paranasal Sinuses

Embryology of the nose and paranasal sinuses explains many clinical conditions, surgical landmarks, and anatomical variations. Understanding this development helps students interpret congenital anomalies, read CT scans, and plan endoscopic sinus surgery.

Development of the Face

Facial development begins in the 4th week of gestation. Five primordial structures, known as swellings, appear around the stomodeum (primitive mouth), which lies below the developing brain. These swellings form the basic framework of the face.

The Five Facial Primordia are:

1. Frontonasal Process: Forms the forehead and the bridge of the nose.
2. Right & Left Maxillary Processes: Form the cheeks and upper lip.
3. Right & Left Mandibular Processes: Form the lower jaw.

All nasal structures develop through interaction between the frontonasal and maxillary processes.

Development of the External Nose and Nasal Cavities

1. Stage 1: Formation of Nasal Placodes (Week 4–5). Ectoderm on the frontonasal process thickens to form nasal placodes. These placodes lie superior and lateral to the stomodeum. These oval placodes develop into the nose. They represent the earliest sign of nasal development.
2. Stage 2: Formation of Nasal Pits (Week 5–6). By the 5th week, mesenchyme around each placode proliferates and grows. This growth pushes the placodes inward and forms depressions, known as nasal pits.
3. Stage 3: Formation of Nasal Sacs and Primitive Choanae (Week 6). The nasal pits deepen to form nasal sacs, which grow toward the oral cavity. A thin buccanasal membrane separates the nasal and oral cavities at this stage. This membrane later ruptures to form the primitive choanae. Clinical correlation: Failure of this membrane to rupture leads to choanal atresia.
4. Stage 4: Formation of Medial and Lateral Nasal Processes. Mesoderm around the nasal sacs differentiates into medial and lateral nasal processes. These processes shape the nostrils (nares). The medial nasal process forms the septum, philtrum, and primary palate. The lateral nasal process forms the alae of the nose.
5. Stage 5: Fusion with Maxillary Process (Week 6–7). By the end of 6th weeks, maxillary processes grow medially and fuse with the nasal processes and frontonasal process This fusion forms the separate nasal cavities. At the junction of the lateral nasal and maxillary processes, ectoderm thickens to form epithelial cords that later canalize into the nasolacrimal duct and lacrimal sac. The lateral nasal prominence forms the nasal bones, upper lateral cartilages, and the lateral crus of the lower lateral cartilage.

Flowchart

Frontonasal Process  →  Nasal Placodes (Week 4)  →  Nasal Pits  →  Nasal Sacs  →  (Membrane rupture) Primitive Choanae  →  Fusion with Maxillary Process  →  Definitive Nasal Cavity + Palate

Development of the Upper Lip and Palate

Formation of the Upper Lip

Between the 7th and 8th weeks, the two medial nasal processes fuse and the maxillary processes. Consequently, this fusion forms the central part of the upper lip (philtrum) and the primary palate. However, if this fusion fails, a cleft lip occurs. Therefore, the mesenchyme must penetrate these junctions to ensure a solid union of the upper jaw.

Formation of the Palate: Primary vs. Secondary

The palate originates from two distinct embryological sources:

• Primary Palate: Formed by the fusion of the nasomedial processes. It eventually becomes the premaxilla, which holds the upper incisor teeth.
• Secondary Palate: Formed by the palatal shelves (outgrowths of the maxillary processes). Initially, these shelves grow vertically alongside the tongue. However, as the mandible grows and the tongue drops, the shelves flip horizontally and fuse in the midline.

Clinical Note: The incisive foramen serves as the landmark dividing the primary and secondary palate and their partial fusion leads to submucous cleft palate and non-union leads to cleft palate.

Development of the Paranasal Sinuses (PNS)

The sinuses develop as diverticula of the nasal mucous membrane. While some are present at birth, others appear much later.

Table : Summary of Sinus Development

Sinus	Appearance (Gestation/Age)	Radiologically Visible	Adult Size Reached
Maxillary	7–10 Weeks (Gestation)	At birth	17–18 Years
Ethmoid	9–10 Weeks (Gestation)	At birth	12 Years
Sphenoid	12 Weeks (Gestation)	3 Years	18 Years
Frontal	16 Weeks (Gestation)	8 Years	18 Years

 Maxillary Sinus

This is the first sinus to appear. It begins as a shallow groove in the ethmoidal infundibulum. Although it is small at birth, it undergoes rapid expansion during the first seven years of life.

Ethmoid Sinus

The lateral nasal wall develops folds called ethmoturbinals. Consequently, these folds give rise to specific structures:

• 1st Ethmoturbinal: Agger nasi and Uncinate process.
• 2nd Ethmoturbinal: Bulla ethmoidalis.
• 3rd Ethmoturbinal: Basal lamella of the middle turbinate (Crucial for FESS surgery).
• 4th & 5th Ethmoturbinals: Superior and supreme turbinates.

Sphenoid Sinus

The sphenoid sinus originates from the sphenoethmoidal recess. Furthermore, clinicians categorize its pneumatization into three types relative to the sella turcica:

1. Sellar (90%): Most common; pneumatization extends behind the sella.
2. Pre-sellar (9%): Pneumatization reaches the anterior wall of the sella.
3. Conchal (1%): Minimal pneumatization; makes transsphenoidal surgery difficult.

Frontal Sinus

The frontal sinus is the most variable. Because it originates from the anterior ethmoid cells, it only becomes a distinct “frontal” cavity after birth. It is clinically “silent” on X-rays until age 8.

Table: Important Clinical Correlations

Developmental Error	Clinical Condition
Bucconasal membrane persistence	Choanal atresia
Maxillary–medial nasal fusion failure	Cleft lip
Palatal shelf fusion failure	Cleft palate
Ethmoid overdevelopment	Concha bullosa
Abnormal sphenoid pneumatization	Pituitary surgery risk

———— End of the chapter ————

High-Yield Points for Quick Revision

• Nose develops from frontonasal process.
• Choanal atresia = failure of bucconasal membrane rupture.
• Maxillary sinus is first to develop.
• Ethmoid sinus present at birth.
• Frontal sinus absent in neonates.
• Sellar pneumatization most common (90%).
• Incisive foramen marks primary-secondary palate junction.
• Nasolacrimal duct forms from ectodermal cord between maxillary and lateral nasal processes.

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Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Cummings, Otolaryngology-Head and Neck Surgery.
• Stell and Maran’s, Textbook of Head and Neck Surgery and Oncology.
• Ballenger’s, Otorhinolaryngology Head And Neck Surgery
• Susan Standring, Gray’s Anatomy.
• Frank H. Netter, Atlas of Human Anatomy.
• B.D. Chaurasiya, Human Anatomy.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery.
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.
• Ganong’s Review of Medical Physiology.
• Guyton & Hall Textbook of Medical Physiology.

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• Please read. Anatomy of External Ear. https://www.entlecture.com/anatomy-of-ear/
• Please read. Anatomy of Temporal Bone. https://www.entlecture.com/anatomy-of-temporal-bone/
• Please read. Stenger’s, Chimani Moos, Teal test. https://www.entlecture.com/special-tuning-fork-tests/

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