A. Pars tuberalis:
- It is made up of columns of epithelial cells.
- They form clusters around the infundibulum.
- They do not give out any secretions.
B. Pars distalis:
- It is the largest part and forms about 75% of the gland.
- It consists of epithelioid cells arranged in clusters or cords.
- They are the main secretory cells and secrete tropic hormones.
- The clusters are seperated from one another by blood sinusoids.
- These clusters consist of two types of cells mainly the chromophils and chromophobes.
- Chromophils: They are large, ovoid and granular cells. Depending upon their staining properties, they are classified as:
Acidophils: They are stained with acidic dyes (red).
Basophils: They are stained with basic dyes (purple blue).
- Chromophobes: They do not contain any secretary granules, hence are poorly stained.
C. Pars intermedia (Intermediate lobe): In humans, it appears as a narrow (rudimentary) region between the Pars distalis and Pars nervosa. These cells do not have any secretory granules.
- It comprises of the Infundibulum and the Pars nervosa.
- The hypothalamus consists of two groups of cell bodies of neurons, i.e. Supra-optic nuclei and Para-ventricular nuclei.
- They are also called as neurosecretory cells because they release hormones.
- The axons of these neurons pass downwards through the infundibulum and terminate in the Pars nervosa.
- Many spindle-shaped Pituicytes (supportive cells) intermingle with these axonal-endings.
- Functions: It receives and stores Vasopressin (ADH) and Oxytocin secreted by the supra-optic nuclei and Para-ventricular nuclei respectively.