Pituitary hormones.

Cells in the anterior lobe make and secrete trophic hormones that regulate growth (growth hormone-GH) lactation and reproductive functions (prolactin-PRL, luteinising hormone-LH, follicle-stimulating hormone-FSH) or control the release of other hormones from the thyroid gland (thyroid-stimulating hormone-TSH) or adrenal gland (adrenocorticotrophic hormone-ACTH). Therefore the pituitary regulates many body processes and the endocrine system. Nerves and blood vessels from the brain communicate with the pituitary, allowing central control of this system. Nerve endings from the brain terminate in the posterior pituitary and release substances such as vasopressin and oxytocin (neurosecretion). The intermediate lobe in most animals produces melanocyte-stimulating hormone only, which controls the action of cells that produce skin pigment.

Cells can be identified using a technique called immunocytochemistry, using specific antibodies to bind to individual proteins that are produced in the cells. The antibodies are labelled with agents that can be visualised using coloured dyes, so that the labelled cells can be viewed using a microscope. The following images show different types of pituitary cells, each of which has a characteristic shape and distribution:

Apart from the hormone producing cells of the anterior pituitary, there are folliculo-stellate cells, identified using S-100 protein staining. Although they do not produce classical hormones they are in close contact with anterior pituitary endocrine cells and may produce other substances that regulate hormone secretion.

Regulatory Peptides.

Biologically active peptides act as regulatory factors in many cells, some are neuropeptides and are released by neurones, others are endocrine, paracrine or autocrine regulators. These peptides have many actions in different organs and tissues, often found in specific regions of the brain and in other organs such as the gut. Investigations have shown that most of these regulatory peptides are found in cells of the pituitary gland and much of my early work was concerned with localising these peptides to particular pituitary cell types. Below are some examples, with references of the original papers.

These studies were performed by cutting pairs of very thin (2 micron) sections of pituitary tissue and staining each section from the pair with a different antibody, for the peptide or for each of the pituitary hormones. By this method, the identity of cells that were positive for peptides could be determined. The arrows indicate cells that are stained in both sections.

NPY, a neuropeptide expressed in nerves in the brain, is found in endocrine cells of the rat anterior pituitary.
This image shows NPY on the left and TSH-immunoreactive cells on the right (ref 1).

Galanin is expressed in growth hormone (GH) or thyroid-stimulating hormone (TSH) cells in male rats (left).
In female rats, under the influence of oestrogen, most cells containing galanin are identified as prolactin (PRL) cells (right) (see ref 2).

Another peptide, VIP is also found in PRL cells (ref 2).

References:

• 1. O'Halloran D.J, Jones P.M, Steel J.H, Ghatei M.A, Polak J.M, Bloom S.R. Neuropeptide Y and the anterior pituitary.
  Annals of the New York Academy of Sciences 611: 329-335, 1990.
  
• 2. Steel J.H, Gon G, O'Halloran D.J, Jones P.M, Yanaihara N, Ishikawa H, Bloom S.R, Polak J.M.
  Galanin and vasoactive intestinal polypeptide are colocalised with classical pituitary hormones and show plasticity of expression.
  Histochemistry 93: 183-189, 1989.

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