Pars intermedia
Template:Short description Template:Infobox anatomy
The pars intermedia is one of the three parts of the anterior pituitary. It is a section of tissue sometimes called a middle or intermediate lobe, between the pars distalis, and the posterior pituitary.<ref name="Ganapathy">Template:Cite book</ref> It is a small region that is largely without blood supply.<ref name="Hall2011">Template:Cite book</ref> The cells in the pars intermedia are large and pale. They surround follicles that contain a colloidal matrix.<ref>Template:Cite book</ref>
The pars intermedia secretes α-melanocyte-stimulating hormone (α-MSH), and corticotropin-like intermediate peptide.Template:Fact It appears to be tonically inhibited by the hypothalamus.
In the human fetus, this area produces melanocyte stimulating hormone (MSH) which causes the release of melanin produced in melanocytes that can give a darker skin pigmentation. In the adult the pars intermedia is either very small or entirely absent.
In less developed vertebrates the pars intermedia is much larger, and structurally and functionally more well defined.<ref name="Hall2011A">Template:Cite book</ref> In some animals including amphibians<ref name="Davies-2001">Template:Cite book</ref> it mediates active camouflage, causing darkening of the skin when placed against a darker background.
AnatomyEdit
MicroanatomyEdit
It contains colloid-filled cysts and two types of cells - basophils and chromophobes. The cysts are the remainder of Rathke's pouch. As technically part of the anterior pituitary, it separates the posterior pituitary and pars distalis. It is composed of large, pale cells that encompass the aforementioned colloid-filled follicles.<ref>Template:Cite book</ref>
PhysiologyEdit
The pars intermedia appears to be tonically inhibited by stimuli from the hypothalamus (either by dopaminergic innervation or by vascular mechanism) as experimental sectioning of the pars intermedia from the hypothalamus has been noted to result in hypertrophy of the pars intermedia in various animals.<ref name="Davies-2001" />
FunctionEdit
The pars intermedia is responsible for secreting α-melanocyte-stimulating hormone, and corticotropin-like intermediate peptide.<ref name="Davies-2001" />
It is prominent only during the fetal stage and is otherwise negligible. The characteristic pattern of skin hyperpigmentation seen during pregnancy may be a result of increased circulating maternal a-MSH (which may have originated from either the maternal or fetal pars intermedia), but a-MSH secretion does not seem to be involved in skin tanning in response to light exposure.<ref name="Davies-2001" />
Other animalsEdit
In lower vertebrates (fish, amphibians), MSH from the pars intermedia is responsible for darkening of the skin, often in response to changes in background color.Template:Citation needed This color change is due to MSH stimulating the dispersion of melanin pigment in the animal's skin melanocyte chromatophores. Some animals will thus increase a-MSH secretion when placed against a dark background<ref name="Davies-2001" /> as a means of active camouflage.
ReferencesEdit
External linksEdit
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