Maxillary Sinus Histology

Overview

1. An epithelial layer resting on a basement membrane
2. A subepithelial connective tissue layer (lamina propria)

1. Epithelial Layer

Cell types present:

Key distinction: compared to the nasal cavity, the maxillary sinus epithelium has fewer ciliated cells, fewer goblet cells, and fewer seromucous cells - making it less robust at clearance.

2. Basement Membrane

3. Subepithelial Connective Tissue (Lamina Propria)

• Composed of collagen bundles (predominantly type I), few elastic fibers, connective tissue cells (fibroblasts, macrophages, mast cells, lymphocytes, plasma cells), and intercellular ground substance
• Moderately vascular - thin-walled capillaries and venules supply nutrients and contribute to mucosal immune defense
• Contains subepithelial antral glands (also called seromucous glands): mixed glands with serous and mucous acini, plus myoepithelial cells; their excretory ducts open directly onto the epithelial surface
  • These glands are concentrated around the ostium (the drainage opening)

4. Periosteum (Mucoperiosteum)

5. Mucus Composition and Ciliary Function

• Mucus (produced by goblet cells + antral glands) is ~96% water; the rest consists of glycoproteins, immunoglobulins (especially IgA), lysozyme, lactoferrin, histamine, and prostaglandins
• Ciliary beat produces a spiral mucus flow pattern - from the floor of the sinus, sweeping upward and medially toward the natural ostium (located supero-posteriorly on the medial wall, draining into the infundibulum of the middle meatus)
• This transport works against gravity - a reason why the maxillary sinus is especially prone to infection when mucociliary clearance fails

Summary Diagram

Lumen
  |
  |── Ciliated pseudostratified columnar epithelium
  |      (ciliated cells > goblet cells > basal cells)
  |── Basement membrane
  |── Lamina propria (collagen, elastic fibers, vessels,
  |      antral seromucous glands, immune cells)
  |── Periosteum (fused = mucoperiosteum)
  |── Maxillary bone (thin cortical bone)

Clinical Relevance of Histology

• The thinness of the mucoperiosteum over the sinus floor means odontogenic infections can breach it, causing odontogenic sinusitis
• Reduced goblet cell and gland density = reduced mucociliary clearance capacity → prone to chronic sinusitis
• The ostium is positioned near the roof of the medial wall, not the floor, so dependent drainage is impossible - ciliary action is the only drainage mechanism; when cilia are damaged (e.g., by viral infection, smoking, Kartagener syndrome), secretions pool
• Schneiderian papillomas and carcinomas arise from this respiratory epithelium

Rapid Fire Viva - Maxillary Sinus Histology

Ciliated pseudostratified columnar epithelium (respiratory epithelium) - the Schneiderian membrane.

It is thinner, with fewer ciliated cells, fewer goblet cells, and fewer seromucous glands.

Columnar ciliated cells (most numerous), goblet cells, basal cells, and columnar non-ciliated cells.

9+2 arrangement - 9 peripheral doublets and 2 central singlets (dynein arms connect the doublets and generate movement).

They synthesize and secrete mucus. Morphologically they resemble an inverted wine glass - narrow basal end with nucleus, swollen apical end with mucin granules. They secrete by apocrine mechanism (rupture and regeneration of apical membrane).

~96% water; remainder is glycoproteins, IgA, lysozyme, lactoferrin, histamine, prostaglandins. Functions to trap debris and provide immunological defense.

Mixed seromucous glands in the lamina propria - contain serous acini, mucous acini, and myoepithelial cells. Concentrated around the natural ostium.

The maxillary sinus has NO distinct submucosa. The lamina propria is fused directly to the underlying periosteum of the maxillary bone, forming a single layer called the mucoperiosteum. Total thickness is approximately 0.3-0.8 mm.

Osteoblasts, at the periosteal-bone interface.

Mucus flows in a spiral pattern from the sinus floor upward and medially toward the natural ostium, which is located supero-posteriorly on the medial wall. This transport works against gravity - the ostium is near the roof, not the floor. If cilia fail, drainage is impossible and secretions pool.

Three reasons from histology: (1) fewer ciliated cells, (2) fewer goblet cells, (3) fewer seromucous glands - together this means reduced mucociliary clearance. Plus, the high-placed ostium means gravity does not assist drainage.

From the olfactory epithelium of the lateral wall of the middle nasal meatus - it invaginates into the maxillary mesenchyme from around 12 weeks gestation.

Collagen bundles (mainly type I), few elastic fibers, fibroblasts, moderate vascularity (capillaries and venules), and immune cells - macrophages, mast cells, lymphocytes, and plasma cells.

Schneiderian (sinonasal) papillomas - exophytic, inverted, and oncocytic types. They arise from the respiratory (Schneiderian) epithelium lining the sinus. The inverted type (most common) shows endophytic growth with inversion of epithelium into the lamina propria and carries risk of malignant transformation.

The thin mucoperiosteum between maxillary teeth roots and sinus becomes a pathway for odontogenic infection to enter the sinus (odontogenic sinusitis), and tooth roots or implants may protrude into the sinus cavity with progressive pneumatization.

• 13-15 correct = Excellent (distinction level)
• 10-12 = Good (pass with confidence)
• 7-9 = Fair (revise lamina propria and ciliary details)
• Below 7 = Revise the full topic

Location of the Maxillary Sinus

Boundaries / Relations

Key Positional Facts (Viva-ready)

• Located below the orbit, above the upper teeth, lateral to the nasal cavity
• The natural ostium opens on the supero-posterior part of the medial wall (near the roof - NOT the floor), draining into the ethmoidal infundibulum under the middle turbinate
• Because the ostium is near the roof, gravity does not help drainage - mucociliary clearance is the only mechanism
• The infraorbital nerve runs in a canal along the roof
• Roots of upper 1st and 2nd premolars, 1st and 2nd molars are the most closely related teeth (1st molar root most commonly projects into the sinus)

Simple Memory Aid

"Cheekbone cavity" - under the eye, above the teeth, beside the nose, draining upward against gravity.

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1
• Cummings Otolaryngology Head and Neck Surgery
• Kanski's Clinical Ophthalmology

Why is it Called "Pseudostratified" Columnar Epithelium?

The Key Reason

Why It Looks Stratified (But Isn't)

Simple Analogy

• All their feet touch the same floor (= all cells touch the basement membrane)
• But their heads are at different levels (= nuclei at different heights)
• From a distance it looks like multiple rows - but it is still just one row of people

Why "Columnar"?

Summary in One Line

"Pseudostratified" = single layer of cells, all touching the basement membrane, but with nuclei at different heights giving a false appearance of multiple layers. "Columnar" = the predominant cells are taller than wide.

Viva Follow-up Questions on This

Special stains or electron microscopy showing all cells attached to the basement membrane - no floating cells above.

In stratified squamous, only the basal layer touches the basement membrane; upper cells do NOT. In pseudostratified, every cell touches the basement membrane.

Having tall ciliated cells at the surface maximizes ciliary coverage and mucus transport, while short basal cells act as stem cells for regeneration - all in a single thin, economical layer that allows efficient gas/fluid exchange with the underlying lamina propria.