Histology of Pituitary gland or Hypophysis cerebri

Hypophysis cerebri

The gross structure of this gland includes two lobes – an anterior and a posterior. Histologically the anterior lobe includes pars distalis or pars anterior and the posterior lobe includes pars nervosa and pars intermedia.

Pars tuberalis surrounds the upper part of pars nervosa and is continuous with pars intermedia. Between the pars intermedia and pars anterior there is a cleft.

Pars anterior

• Contains a fine network of connective tissue, which supports tortuous and anastomosing cell cords. Sinusoids between cell cords are numerous. The cells of these cords are of two main types;
  • The chromophobes are characterized by pale staining non-granular cytoplasm,
  • The choromophils are granular and lager.
• The chromophils are classified as acidophils and basophils on the basis of the staining reactions of their cytoplasmic granules. Of these the acidophil cells take eosin stain in H and E preparations.
• Chromophobes: These appear in small groups. The nuclei are surrounded by a small amount of diffuse light staining non-granular cytoplasm. The cell boundaries are usually not distinguishable in ordinary preparations. These are considered to be reserve or undifferentiated cells, which give rise to granular varieties.
• Chromophils
  • Acidophils: These are larger than chromophobes and their cytoplasm contains granules, which take acid dyes. Two types of acidophils showing preference to azocarmine (epsilon acidophils). Orangeophils or alpha acidophils showing preference to Orange G-stain.
    • Carminophil (epsilon acidophil) – Prolactin or Luteotrophic hormone.
    • Orangeophil or alpha acidophil – Somato trophin or Growth stimulating hormone.
  • Basophils: The cytoplasm of these contain granules of irregular size and considerable variation has been found in their staining properties. The morphological characters vary form large angular cells to small rounded ones.
    • A number of staining techniques have been used to differentiate the different types among the basophils. (Periodic Acid Schiff reaction, Aldehyde-fuchsin, aldehydethionin) etc.
    • Based on the staining reaction to these procedures and on their morphological characters, the basophils have been classified into Beta-1 and Beta-2 and delta-1 delta-2 types.
    • Pars anterior is known to secrete a number of hormones and a number of attempts have been made to connect the secretion of one hormone with one particular cell type. The cell types have not been identified uniformly in all animals nor the correlation between secretion and cell type has been conclusively demonstrated in many animals. The present evidence indicates the following association between secretion and cell type.
    • Beta-1 cell – Adreno-corticotrophin
    • Beta-2 cell – Thyroid stimulating hormone.
    • Delta cell – Luteininzing or Interstitial cell stimulating hormone
    • Delta-2-cell – Follicle stimulating hormone.

Pars intermedia

• Consists of a connective tissue framework containing (basophilic) cells which with form dense masses.
• Colloidal vesicles are often seen. The cells in pars in intermedia produce the melanocyte stimulating hormone.

Par tuberalis

• Contains numerous sinusoids and has cuboidal cells with faintly basophilic cytoplasm.
• The cell form vesicles, which contain colloid. Functional significance of this part is not known.

Pars nervosa

• Has a vascular connective tissue framework housing neuroglia like cells.
• The cells pituicytes show granules in their cytoplasm.
• The nuclei of these cells are round or oval, with a fine chromatin network.
• The cytoplasm is drawn into a number of fine processes which often end either on the walls of blood vessels or on the connective tissue septa.
• In routine preparations the cytoplasm and the processes cannot be made out. Between the cells, there is a fine mesh work of interweaving processes, which stain lightly with eosin.
• Contrary to earlier beliefs, it is now known that pars nervosa by itself does not secrete any hormone. The nerve cells located in the supraoptic and paraventricular nuclei of the hypothalamus are the actual secretory cells. Their secretions are conveyed by their axons to the pars nervosa, where they are released into capillaries, adjoining axon terminations.
• Accumulations of neuro secretion appear as homogenus stainable masses in pars nervosa and are known as Herrings bodies.
• Two hormones have been isolated form pars nervosa.
  • Oxytocin causes contraction of uterine musculature at the end of pregnancy
  • Vasopressin or Antidiuretic hormone – increases blood pressure and water resorption in kidney tubules.