pseudostratified columnar epithelium

**Histology:** A single layer of cells of varying heights, all attached to the basement membrane, though not all reach the lumen. Nuclei lie at multiple levels, which creates the false "stratified" look (hence "pseudo").

**Locations (high-yield):** - **Respiratory epithelium (ciliated pseudostratified columnar with goblet cells):** nasal cavity, nasopharynx, larynx (parts), trachea, and bronchi. The nasal vestibule, oropharynx, and true vocal cords are not included. The oropharynx and true vocal cords have stratified squamous epithelium. - **Male reproductive tract:** epididymis and vas deferens (non-ciliated, with stereocilia). Stereocilia are long microvilli that aid absorption. - Also: part of the male urethra (membranous and penile portions), the Eustachian tube, and the lacrimal sac.

**Cell types in respiratory epithelium:** - **Ciliated columnar cells:** move mucus toward the pharynx (the mucociliary escalator). - **Goblet cells:** secrete mucus. - **Basal cells:** stem/progenitor cells that regenerate the epithelium. - **Club (Clara) cells:** in bronchioles; secrete surfactant-like components and detoxify via cytochrome P450. They also act as progenitors. - **Neuroendocrine (Kulchitsky) cells:** the origin of small cell lung carcinoma.

**Clinical correlations:** - **Primary ciliary dyskinesia / Kartagener syndrome:** dynein arm defect (autosomal recessive) leads to immotile cilia. It causes recurrent sinusitis and bronchiectasis, situs inversus, and infertility (immotile sperm). Women have an increased risk of ectopic pregnancy. - **Smoking:** causes **squamous metaplasia**, in which ciliated pseudostratified columnar epithelium is replaced by stratified squamous epithelium. This impairs mucociliary clearance and predisposes to infection. It is reversible, but persistent injury may progress to dysplasia and squamous cell carcinoma. - **Vitamin A deficiency:** also causes squamous metaplasia. - **Chronic bronchitis:** goblet cell hyperplasia and mucous gland hypertrophy (Reid index >50%). - **Cystic fibrosis:** CFTR defect produces thick mucus and impaired clearance. - **Barrett esophagus** is a different metaplasia (squamous to non-ciliated columnar with goblet cells) and is not pseudostratified.

**Memory aid:** Every cell touches the basement membrane, but not every cell reaches the lumen. Remember "Respiratory tract + Epididymis/vas deferens."