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Thread: Pars Nervosa and Infindibulum picture - Endocrine Histology Atlas

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    Default Pars Nervosa and Infindibulum picture - Endocrine Histology Atlas

    Unlike the adenohypophysis, the neurohypophysis is not glandular and does not synthesize hormones. Instead, it is a site where axons project from neuronal cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus. These hypothalamic cell bodies produce hormones that undergo axonal transport through the pituitary stalk and into terminal axons within the neurohypophysis. The hormones are then stored and released directly into the systemic vasculature.

    The pituitary gland is enveloped by dura and sits within the sella turcica of the sphenoid bone. The sella turcica is a saddle-shaped depression that surrounds the inferior, anterior, and posterior aspects of the pituitary. The superior aspect of the pituitary is covered by the diaphragma sellae, which is a fold of dura mater that separates the cerebrospinal fluid–filled subarachnoid space from the pituitary. The infundibulum pierces the diaphragma sellae in order to connect the pituitary to the hypothalamus.

    The lateral aspects of the pituitary are adjacent to the cavernous sinuses (see the image below). From superior to inferior, the cavernous sinus contains cranial nerves III (oculomotor), IV (trochlear), VI (abducens), V1 (ophthalmic branch of trigeminal nerve), and V2 (maxillary branch of trigeminal nerve). The internal carotid artery also courses through the cavernous sinus, medial to these nerves.

    Microscopic Anatomy
    The pars distalis forms the majority of the adenohypophysis and resembles a typical endocrine gland. Cords and clusters of cuboidal secretory cells within the pars distalis contain hormones stored in cytoplasmic granules that are released via exocytosis and taken up by nearby sinusoidal capillaries. Histochemical staining of these granules with pH-dependent dyes allows categorization of the cells into acidophils, basophils, or chromophobes.

    In general, acidophilic cells contain polypeptide hormones, basophilic cells contain glycoprotein hormones, and chromophobes have minimal to no hormone content. The most common cell type is the acidophilic somatotrope, which is concentrated in the lateral regions of the adenohypophysis and secretes growth hormone (GH). Lactotropes are also acidophilic but are more scattered throughout the adenohypophysis and secrete prolactin (PRL).

    The basophilic cells include corticotropes, thyrotropes, and gonadotropes. Although corticotropes secrete nonglycosylated polypeptides such as adrenocorticotropic hormone (ACTH), these cells are basophilic as a result of the glycoprotein composition of the precursor hormone pro-opiomelanocortin (POMC). Thyrotropes are among the least prevalent secretory cells of the pars distalis; they release thyroid-stimulating hormone (TSH), whereas gonadotropes secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
    Pars Nervosa Infindibulum picture Endocrine attachment.php?s=83a752ba971552ff599c8a630ef59c4b&attachmentid=1323&d=1439064539

    There is growing evidence that the various hormones released by the pars distalis are not restricted to synthesis by a single secretory cell type, as classically described.In particular, thyrotropes have been shown to have a significantly mixed phenotype that contains several hormones, including a high percentage of LH and PRL.[9] In addition, pituitary secretory cells have been shown to be multiresponsive and thus capable of releasing hormones in response to a noncorresponding hypothalamic releasing hormone.[10]

    The pars intermedia of the adenohypophysis lies between the pars distalis and the pars nervosa of the neurohypophysis. In humans, this region is not well developed and has poor vascularization. Although secretory cells within the pars intermedia, like the corticotropes of the pars distalis, produce POMC, the principal hormones synthesized by the pars intermedia include melanocyte-stimulating hormone (MSH) and ß-endorphin.

    The pars tuberalis is a thin, highly vascularized component of the adenohypophysis that surrounds the infundibular stem. The principal secretory cell type within this tissue is the gonadotrope, which contains FSH and LH. In addition, melatonin receptors exist within the pars tuberalis that may play a role in rhythmic gene expression.
    The pars nervosa of the neurohypophysis contains unmyelinated axons that project from neuronal cell bodies in the hypothalamus. Oxytocin and antidiuretic hormone (ADH) synthesized in the cell bodies are transported via the axons and accumulate at the terminal ends within swellings called Herring bodies. A network of capillaries surrounds the axon terminals and facilitates the uptake of released hormones into the vasculature.

    Specialized glial cells known as pituicytes are also interspersed within the pars nervosa and have been hypothesized to actively participate in the modulation of hormone release.

    Pathophysiologic Variants
    Pituitary tumors are relatively common, accounting for about 15% of all primary brain tumors. The vast majority originate in the adenohypophysis and are typically nonsecretory benign adenomas. These adenomas frequently go undiagnosed, and meta-analyses of postmortem studies have demonstrated an 11-14% overall prevalence of silent pituitary adenomas in the general population Tumors of the neurohypophysis are rare and include metastasis, granular cell tumors, and potentially any primary tumor of the neuraxis.

    Pituitary adenomas are arbitrarily classified as microadenomas (< 1 cm) or macroadenomas (> 1 cm). Macroadenomas, when large, have a mass effect on adjacent structures, with clinical consequences. Compression of the pituitary gland itself may cause hypopituitarism, and compression of the optic chiasm results in bitemporal hemianopsia. Headache is also a common symptom of pituitary tumors.


    Last edited by Medical Photos; 08-08-2015 at 08:09 PM.

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