Figure 207

Photomicrograph of the pars intermedia of an adult human pituitary gland. This photomicrograph of a toluidine blue-stained specimen shows the pars intermedia located between the pars distalis (on the left) and pars nervosa (on the right). In humans, this portion of the gland is somewhat rudimentary. However, a characteristic feature of the pars intermedia is the presence of different-sized follicles filled with colloid (CF) and small groups of cells consisting of chromophobes and basophils. x120.

developed Nissl bodies and, in this respect, resemble ventral horn and ganglion cells.

The posterior lobe of the pituitary gland is not an endocrine gland. Rather, it is a storage site for neurosecretions of the neurons of the supraoptic and paraventricular nuclei of the hypothalamus. The nonmyelinated axons convey neurosecretory products to the pars nervosa. Other neurons from the hypothalamic nuclei (described below) also release their secretory products into the fenestrated capillary network of the infundibulum, the first capillary bed of the hypothalamohypophyseal portal system.

Electron microscopy reveals three morphologically distinct neurosecretory vesicles in the nerve endings of the pars nervosa

Three sizes of membrane-bounded vesicles are present in the pars nervosa:

• Neurosecretory vesicles with diameters ranging between 10 and 30 nm accumulate in the axon terminals. They

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