Terminal sacs are thin-walled dilations that develop at the ends of the respiratory bronchioles. During alveolization, the sacs are subdivided by the ingrowth of ridges or crests known as secondary septae. Both myofibroblast progenitor cells and endothelial cells migrate into these crests, and a scaffold of matrix proteins is deposited, enriched in elastin at the tip. Sacs develop a close relationship with the capillaries, and their appearance marks the point at which limited respiration becomes possible.
The terminal sacs appear in human at 36 weeks of gestation and in mice between E17.5 and postnatal day (P)5.
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