On the basis of conventional hematoxylin-eosin-staining methods, cells of the normal pituitary gland were for many years classified as chromophobe, acidophil, and basophil, these types being present in a ratio of 5:4:1. Adenomas of the pituitary are most often composed of chromophobe cells (4 to 20 times as common as acidophil cell adenomas); the incidence of basophil cell adenomas is uncertain. Histologic study is now based on immunoperoxidase-staining techniques that define the nature of the hormones within the pituitary cells—both of the normal gland and of pituitary adenomas. These methods have shown that either a chromophobe or an acidophil cell may produce prolactin, growth hormone (GH), and thyroid-stimulating hormone (TSH), whereas the basophil cells produce adrenocorticotropic hormone (ACTH), β-lipotropin, luteinizing hormone (LH), and follicle- stimulating hormone (FSH).
The development of sensitive methods for the measurement of pituitary hormones in the serum has made possible the detection of adenomas at an early stage of their development and the designation of several types of pituitary adenomas on the basis of the endocrine disturbance. Hormonal tests for the detection of pituitary adenomas, preferably carried out in an endocrine clinic, are listed in Table 30-4. Between 60 and 70 percent of tumors in both men and women are prolactin secreting. About 10 to 15 percent secrete growth hormone, and a smaller number secrete ACTH. Tumors that secrete gonadotropins and TSH are quite rare. These tumors may be monohormonal or plurihormonal and approximately one-third are composed of nonfunctional (null) cells.
Pituitary tumors usually arise as discrete nodules in the anterior part of the gland (adenohypophysis). They are reddish gray, soft (almost gelatinous), and often partly cystic, with a rim of calcium in some instances. The adenomatous cells are arranged diffusely or in various patterns, with little stroma and few blood vessels; less frequently the architecture is sinusoidal or papillary in type. Variability of nuclear structure, hyperchromatism, cellular pleomorphism, and mitotic figures are interpreted as signs of malignancy, which is exceedingly rare. Tumors less than 1 cm in diameter are referred to as microadenomas and are at first confined to the sella. As the tumor grows, it first compresses the pituitary gland; then, as it extends upward and out of the sella, it compresses the optic chiasm; later, with continued growth, it may extend into the cavernous sinus, third ventricle, temporal lobes, or posterior fossa. Recognition of an adenoma when it is still confined to the sella is of considerable practical importance, since total removal of the tumor by transsphenoidal excision or some form of stereotactic radiosurgery is possible at this stage with prevention of further damage to normal glandular structure and the optic chiasm. Penetration of the diaphragm sellae by the tumor and invasion of the surrounding structures make treatment more difficult.