Electron microscopic immunocytochemistry by the protein A-gold method of two types of rat anterior pituitary ACTH cells.

Immunocytochemical detection of ACTH using the protein A-gold method was performed in the adult male rat anterior pituitary gland after simple glutaraldehyde fixation and double fixation with glutaraldehyde and osmium tetroxide. Immunoreactive ACTH cells of the rat anterior pituitary can be classified into two groups: typical and atypical ACTH cells. The typical ACTH cells are small stellate cells extending slender processes, and contain secretory granules aligned in a single row along the cell surface. The atypical ACTH cells are rather fat, polygonal or oval cells with a few stout processes. Secretory granules are very numerous and fill the entire cytoplasm, and are not limited to the cell periphery. The latter cell's granules vary widely in electron density after glutaraldehyde and osmium. Among these variable secretory granules, the dense ones are strongly labeled with ACTH-reactive colloidal gold particles, but less dense ones are not. The typical ACTH cells reported here correspond to the Type II ACTH cells, while the atypical ones are the Type I ACTH cells previously reported. The results of protein A-gold immunocytochemistry of ACTH favor the view that two types of ACTH cells are present in the rat anterior pituitary. An intermediate type of ACTH cells was also observed, suggesting a transition between the above mentioned two types of ACTH producers.