The pars intermedia is one of the three parts of the

anterior pituitary The anterior pituitary (also called the adenohypophysis or pars anterior) is a major Organ (anatomy), organ of the endocrine system. The anterior pituitary is the glandular, Anatomical terms of location#Usage in human anatomy, anterior lobe that t ...

. It is a section of tissue sometimes called a middle or intermediate lobe, between the pars distalis, and the

posterior pituitary The posterior pituitary (or neurohypophysis) is the posterior lobe of the pituitary gland which is part of the endocrine system. Unlike the anterior pituitary, the posterior pituitary is not glandular, but largely a collection of axonal projec ...

. 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 A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exten ...

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 The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...

. In the human

fetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...

, this area produces melanocyte stimulating hormone (MSH) which causes the release of

melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

produced in

melanocyte Melanocytes are melanin-producing neural-crest, neural crest-derived cell (biology), cells located in the bottom layer (the stratum basale) of the skin's epidermis (skin), epidermis, the middle layer of the eye (the uvea), the inner ear, vagina ...

s 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 Active camouflage or adaptive camouflage is camouflage that adapts, often rapidly, to the surroundings of an object such as an animal or military vehicle. In theory, active camouflage could provide perfect concealment from visual detection. Acti ...

, causing darkening of the skin when placed against a darker background.

Anatomy

Microanatomy

It contains

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...

-filled

cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubb ...

s and two types of cells - basophils and chromophobes. The cysts are the remainder of

Rathke's pouch In embryogenesis, Rathke's pouch is an evagination at the roof of the developing mouth in front of the buccopharyngeal membrane. It gives rise to the anterior pituitary (adenohypophysis), a part of the endocrine system. Development Rathke's pou ...

. 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

References

External links

* * * * {{Authority control Endocrine system Human head and neck