THE HISTOLOGICAL APPEARANCES OF THE MAMMALIAN PITUITARY BODY
Authors: Herring P. T. 
Published: December 2014

Three types of mammalian pituitary body are recognised. In one, e.g., the cat, the posterior lobe is hollow and its cavity is in free communication with the third ventricle of the brain, while the epithelium of the anterior lobe affords an almost complete investment for the posterior lobe; in the second type, e.g., the dog, the body of the posterior lobe is solid, but the neck is hollow, and communicates with the third ventricle: the posterior lobe is here again almost completely surrounded with epithelium; in the third type, e.g., man, monkey, ox, pig, and rabbit, the body and neck of the posterior lobe are solid, although traces of a cavity are occasionally found in the neck; in this type the epithelium does not invest the posterior lobe so completely, but is aggregated around the neck and spreads over and into the adjacent surface of the brain.

The epithelial portion of the pituitary body is differentiated into two distinct parts: an anterior lobe proper, consisting of solid columns of cells, between which run wide and thin‐walled blood‐channels; and an intermediate portion, which lies between the anterior lobe and the nervous tissue of the pituitary, forming a closely‐fitting investment of the latter.

The anterior lobe contains cells which are clear or hold in their protoplasm varying amounts of deeply‐staining granules. They are probably different functional stages of one and the same kind of cell, and the granules give rise to a secretion which is absorbed by the blood‐vessels.

The intermediate portion consists of finely granular cells arranged in layers of varying thickness closely applied to the body and neck of the posterior lobe and to the under surface of adjacent parts of the brain. The part of it which is separated from the anterior lobe by the cleft is almost devoid of blood‐vessels. In the cat the portion lying in front of the anterior lobe has a tubular appearance and is very vascular. Colloid material occurs between the cells of the pars intermedia, and in most situations appears to pass into the adjacent nervous substance, to be absorbed by blood‐vessels or lymphatics.

The nervous portion of the pituitary body is made up of neuroglia cells and fibres. Ependyma cells line the central cavity in the cat and send long fibres forwards and upwards towards the brain, most of which terminate in the outer part of the neck. There are no true nerve cells and the nerves supplying the pituitary probably reach it through sympathetic fibres accompanying the blood‐vessels (Berkley). The nervous portion is invaded to a large extent by the epithelial cells of the pars intermedia. Columns of epithelial cells grow into it, especially in the region of the neck, and islets of these cells are frequently found throughout the posterior lobe; in the pituitary of the cat epithelial cells may even grow into its central cavity.

A substance histologically resembling the colloid of the thyroid gland, but probably of a different nature, occurs in large quantities in the nervous portion of the posterior lobe. It appears to be a product of the epithelial cells, and, in the cat at any rate, to be carried by lymphatics into the central cavity, and so into the third ventricle of the brain. In this respect the posterior lobe of the pituitary is an infundibular gland. Whether this substance is modified by its passage through the nervous substance or not is unsettled. Its distribution corresponds with the site of the tissue, the extracts of which have active physiological results when injected into the blood.

The anterior lobe of the pituitary is extremely vascular and its circulation sinusoidal. The posterior lobe is supplied for the most part by a central artery which enters it at its postero‐superior angle and runs forward giving off branches; the veins begin immediately below the epithelial investment and run backwards in this situation, to emerge near the entry of the artery. The veins of both lobes enter large blood sinuses lying close to the sides of the pituitary body.

Histological evidence is against the statement of Bela Haller that the anterior lobe is a tubular gland which pours its secretion directly into the subdural space.