Estrogen-induced microvilli and microvillar channels and entrapment of surfactant-lipids by alveolar type I cells of bovine lung.

The ATI cells are simple, flat squamous epithelial cells, which are evolved to function as a component of the alveolar-capillary membrane, ideally designed for gaseous exchange. They inherently lack an active metabolic machinery and lead a precarious existence in the face of hostile environment. On the other hand, the ATI cells of the lung of ruminating animals are endowed with structure-functional properties which enable them to exert a selective barrier function against a wide range of osmotic pressure gradients at their luminal surface. Such gradients are created by a complex gaseous homeostasis due to expectoration of several gases and volatile fatty acids originating from the complex stomach of the ruminants. The purpose of this study is to examine the effect of estradiol propionate on the ultrastructure of the ATI cells and their interaction with the surfactant lipids. The lungs of estrogen and dexamethasone treated male calves were harvested for electromicroscopic examination. The evidence is presented that estradiol induced the formation of microvilli and microvillar channels at the luminal surface. At these regional modifications, intense interactions with the surfactant lipids and their entrapment into the pathways of endocytosis, took place in the squamous part of the ATI cells. Concurrently, large basal protrusions ended up as long lamellipods deep into the alveolar interstitium. The filamentous cytoskeletal network and microtubules intermixed with the translocated organelles such as Golgi apparatus and associated coated and uncoated vesicles. The results of this study support the hypothesis that estrogen regulate the selective barrier-function of the ATI cells. The entrapment of surfactant lipids under the influence of estrogen by ATI cells is a significant change perhaps in response to extracellular stimuli and expression of transmembrane receptors. It implies that these epithelial cells are specially evolved to adapt to a complex gaseous homeostasis in the lung of the ruminating ungulates.