pseudostratified epithelium

**Definition/Histology:** A simple (single-layer) epithelium in which every cell touches the basement membrane, but not every cell reaches the lumen. Cells vary in height, so nuclei sit at different levels and the epithelium looks multilayered ("pseudo" = false).

**Main locations (high-yield):** - **Respiratory tract:** nasal cavity, nasopharynx, larynx (parts), trachea, and bronchi. It is ciliated pseudostratified columnar epithelium with goblet cells. The cilia drive the mucociliary escalator, which moves mucus and trapped particles toward the pharynx. - **Epididymis and vas deferens (male reproductive tract):** non-ciliated, with **stereocilia** (long microvilli) that absorb fluid. - **Large excretory ducts:** for example, parts of the parotid duct and the male urethra (membranous and penile regions). Some sources also list the Eustachian tube and lacrimal sac. - **Olfactory epithelium:** a modified pseudostratified columnar epithelium (non-ciliated supporting cells plus olfactory neurons with modified cilia).

**Cell types in respiratory epithelium:** - **Ciliated columnar cells:** move mucus. - **Goblet cells:** secrete mucus. - **Basal cells:** stem/progenitor cells that give rise to the other cell types. - **Club (Clara) cells:** in bronchioles. They secrete surfactant-like components and detoxify via cytochrome P450, and they act as progenitors. - **Neuroendocrine (Kulchitsky) cells:** the origin of small cell lung carcinoma.

**Clinical correlations:** - **Primary ciliary dyskinesia (Kartagener syndrome):** an autosomal recessive dynein arm defect causes immotile cilia. It presents with situs inversus, chronic sinusitis, bronchiectasis, and infertility (male infertility from immotile sperm, and ectopic pregnancy risk in women). - **Cystic fibrosis:** a CFTR defect produces thick mucus and impaired mucociliary clearance, leading to recurrent infections and bronchiectasis. - **Smoking:** causes **squamous metaplasia** of the respiratory epithelium. Ciliated columnar cells are replaced by stratified squamous epithelium, which impairs mucociliary clearance and predisposes to squamous cell carcinoma. Vitamin A deficiency can also cause squamous metaplasia. - **Barrett esophagus:** the opposite pattern. Squamous epithelium becomes intestinal-type columnar epithelium with goblet cells. - **Chronic bronchitis:** goblet cell hyperplasia and Reid index >50% (mucus gland hypertrophy). - **Mnemonic:** all cells touch the basement membrane (one layer), but nuclei at different levels make it look stratified.