Editing Pulmonary alveolus (section)

===Type II cells===
Type II cells are cuboidal and much smaller than type I cells.<ref name="Knudsen"/> They are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous type I cells.<ref name=":0">{{Cite book |title=Gray's Anatomy: the anatomical basis of clinical practice |date=2021 |publisher=Elsevier |isbn=978-0-7020-7705-0 |editor-last=Gray |editor-first=Henry |edition=42nd |location=Amsterdam |pages=1035 |editor-last2=Standring |editor-first2=Susan |editor-last3=Anhand |editor-first3=Neel}}</ref> Type II cells (granulous pneumocytes) in the alveolar wall contain secretory [[organelle]]s known as [[lamellar bodies]] or lamellar granules, that fuse with the cell membranes and secrete [[pulmonary surfactant]]. This surfactant is a film of fatty substances, a group of [[phospholipid]]s that reduce alveolar [[surface tension]]. The phospholipids are stored in the lamellar bodies. Without this coating, the alveoli would collapse. The surfactant is continuously released by [[exocytosis]]. Reinflation of the alveoli following exhalation is made easier by the surfactant, which reduces surface tension in the thin [[Epithelial lining fluid|fluid lining of the alveoli]]. The fluid coating is produced by the body in order to facilitate the transfer of gases between blood and alveolar air, and the type II cells are typically found at the [[blood–air barrier]].<ref>{{cite book | title = Histology, A Text and Atlas | edition = Sixth | date = 2011 | last1 = Ross | first1 = Michael H | last2 = Pawlina | first2 = Wojciech | name-list-style = vanc}}</ref><ref name="pmid11686863">{{cite journal | vauthors = Fehrenbach H | title = Alveolar epithelial type II cell: defender of the alveolus revisited | journal = Respiratory Research | volume = 2 | issue = 1 | pages = 33–46 | date = 2001 | pmid = 11686863 | pmc = 59567 | doi = 10.1186/rr36 | doi-access = free}}</ref>

Type II cells start to develop at about 26 weeks of [[gestation]], secreting small amounts of surfactant. However, adequate amounts of surfactant are not secreted until about 35 weeks of gestation – this is the main reason for increased rates of [[infant respiratory distress syndrome]], which drastically reduces at ages above 35 weeks gestation.

Type II cells are also capable of cellular division, giving rise to more type I and II alveolar cells when the lung tissue is damaged.<ref>{{cite web|url=https://ntp.niehs.nih.gov/nnl/respiratory/lung/regen/index.htm|title=Lung – Regeneration – Nonneoplastic Lesion Atlas| work = National Toxicology Program | publisher =  National Institute of Environmental Health Sciences, National Institutes of Health, U.S. Department of Health and Human Services |access-date=2018-05-18}}</ref>

''[[MUC1]]'', a human [[gene]] associated with type II pneumocytes, has been identified as a marker in [[lung cancer]].<ref name="pmid9850098">{{cite journal | vauthors = Jarrard JA, Linnoila RI, Lee H, Steinberg SM, Witschi H, Szabo E | title = MUC1 is a novel marker for the type II pneumocyte lineage during lung carcinogenesis | journal = Cancer Research | volume = 58 | issue = 23 | pages = 5582–9 | date = December 1998 | pmid = 9850098 | url = http://cancerres.aacrjournals.org/cgi/pmidlookup?view=long&pmid=9850098}}</ref>

The importance of the type 2 lung alveolar cells in the development of severe respiratory symptoms of COVID-19 and potential mechanisms on how these cells are protected by the SSRIs fluvoxamine and fluoxetine was summarized in a review in April 2022.<ref>{{Cite journal |last1=Mahdi |first1=Mohamed |last2=Hermán |first2=Levente |last3=Réthelyi |first3=János M. |last4=Bálint |first4=Bálint László |date=January 2022 |title=Potential Role of the Antidepressants Fluoxetine and Fluvoxamine in the Treatment of COVID-19 |journal=International Journal of Molecular Sciences |language=en |volume=23 |issue=7 |page=3812 |doi=10.3390/ijms23073812 | pmid=35409171 |pmc=8998734 |issn=1422-0067|doi-access=free}}</ref>