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Pars intermedia
Median sagittal through the hypophysis of an adult monkey. (Pars intermedia labeled at bottom center.)
Details
Identifiers
Latin	pars intermedia adenohypophysis
TA98	A11.1.00.004 A09.4.02.017
TA2	3858
TH	H3.08.02.2.00007
FMA	74632
Anatomical terminology[edit on Wikidata]

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. It is a small region that is largely without blood supply. The cells in the pars intermedia are large and pale. They surround follicles that contain a colloidal matrix.

The pars intermedia secretes α-melanocyte-stimulating hormone (α-MSH), and corticotropin-like intermediate peptide. 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. In some animals including amphibians it mediates active camouflage, causing darkening of the skin when placed against a darker background.

Anatomy

Microanatomy

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.

Physiology

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.

Function

The pars intermedia is responsible for secreting α-melanocyte-stimulating hormone, and corticotropin-like intermediate peptide.

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.

Other animals

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. 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 as a means of active camouflage.

References

1. Ganapathy, MK; Tadi, P (January 2024). Anatomy, Head and Neck, Pituitary Gland. PMID 31855373.
2. Hall, John E.; Guyton, Arthur C. (2011). Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, Pa: Saunders/Elsevier. p. 895. ISBN 9781416045748.
3. Ilahi, Sadia; Ilahi, Tahir B. (3 October 2022). "Anatomy, Adenohypophysis (Pars Anterior, Anterior Pituitary)". Anatomy, Adenohypophysis (Pars Anterior, Anterior Pituitary). StatPearls Publishing. Retrieved 9 February 2023.
4. Hall, John E.; Guyton, Arthur C. (2011). Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, Pa: Saunders/Elsevier. p. 935. ISBN 9781416045748.
5. ^ Davies, Andrew; Blakeley, Asa G. H.; Kidd, Cecil, eds. (2001). Human Physiology. pp. 397–398. ISBN 978-0-443-04559-2.
6. Ilahi, Sadia; Ilahi, Tahir B. (3 October 2022). "Anatomy, Adenohypophysis (Pars Anterior, Anterior Pituitary)". Anatomy, Adenohypophysis (Pars Anterior, Anterior Pituitary). StatPearls Publishing. Retrieved 9 February 2023.

External links

• Histology image: 14001loa – Histology Learning System at Boston University
• Histology image: 14101loa – Histology Learning System at Boston University
• Histology image: 38_11 at the University of Oklahoma Health Sciences Center
• UIUC Histology Subject 991

• Endocrine system
• Human head and neck