Histological organization
The main structural element of the intermediate lobe is follicles. In educational and scientific literature, the terms "pseudofollicles" or "follicle-like cysts" are also used to designate these structures.
The process of their formation occurs as follows:
- The glandular cells of the intermediate lobe secrete a specific fluid — colloid.
- This secretion gradually accumulates in the intercellular space.
- As fluid accumulates, the cells push apart, forming closed cavity structures.
The wall of each such pseudofollicle is formed by just a single layer of cuboidal secretory cells.
Extensive connective tissue layers run between the follicles, which is an important diagnostic sign of the intermediate lobe in histological preparations. Large blood vessels and a dense secondary capillary network belonging to the hypophyseal portal system are located in these connective tissue septa (layers).
Colloid features
The cavity of the follicle-like cysts is filled with fluid, which can be proteinaceous or mucous in its chemical composition. It is this chemical composition that determines the tinctorial properties of the colloid (i.e., its ability to take up stains). In micropreparations, the color of the pseudofollicle contents can vary over a wide range — from pale blue to rich orange.
Important clinical and morphological note: despite the fact that colloid is a secretion product of glandular cells, the presence of active hormones within it has not been reliably established to date.
Hormones and their effects
The intermediate lobe is associated with the production of substances that regulate pigment metabolism, lipid metabolism, and pain sensitivity.
Main hormones:
- Melanocyte-stimulating hormone (MSH, melanotropin). Its main target is pigment cells. MSH stimulates the synthesis of the pigment melanin. In this process, the hormone exclusively enhances the pigment formation process, but does not cause proliferation (i.e., division and formation of new ones) of melanocytes.
- Lipotropin. Acts on adipose tissue. Its main function is to stimulate lipolysis — the process of fat breakdown with the subsequent release of fatty acids into the bloodstream.
- Endorphins. These are special peptides that have a pronounced morphine-like analgesic effect.
Biosynthesis features
According to modern concepts, the synthesis of intermediate lobe hormones proceeds in several stages and begins outside of it.
There is a hypothesis that a large protein — a common polypeptide precursor — is formed in the neurons of the central nervous system (CNS). In the pseudofollicle cells of the middle lobe itself, only its maturation (processing) takes place. During the cleavage of this massive precursor into smaller fragments, the final active molecules are formed: MSH, lipotropin, and endorphins.
Briefly: answers to topical questions
What is the name of the common precursor protein for intermediate pituitary lobe hormones?
The common precursor protein for the hormones of the intermediate (middle) lobe of the pituitary gland is called pro-opiomelanocortin (POMC). This is a polypeptide chain of 265 amino acid residues, which is the product of a common gene.
During the cleavage (processing) of this single precursor within the cells, separate peptides are formed:
- Melanocyte-stimulating hormone (MSH) — stimulates melanin synthesis in pigment cells.
- Lipotropin — stimulates the release of fatty acids (lipolysis).
- Endorphins — peptides with a morphine-like analgesic effect.
From which embryonic germ layer does the intermediate lobe of the pituitary gland develop?
The intermediate (middle) lobe of the pituitary gland develops from an ectodermal rudiment — the epithelium of the roof of the primary mouth (oral cavity).
In weeks 4–5 of embryogenesis, an epithelial outgrowth of the oral bay ectoderm forms, creating the hypophyseal pouch (Rathke's pouch). In the course of further development, the pouch detaches from the oral ectoderm and becomes a hollow islet of epithelium. The intermediate lobe forms directly from the posterior wall of Rathke's pouch, while the anterior wall gives rise to the anterior lobe of the adenohypophysis.
For quick memorization of the cleavage products of the common precursor in the middle lobe, use the acronym MEL: MSH, Endorphins, Lipotropin.
Class and exam questions
Is melanocyte-stimulating hormone (MSH) capable of increasing the number of pigment cells?
No, this hormone exclusively stimulates the intracellular synthesis of the pigment melanin, but does not induce proliferation (multiplication) of the melanocytes themselves.
Where are intermediate lobe hormones initially synthesized?
It is hypothesized that their common polypeptide precursor is formed in CNS neurons, while the cuboidal cells of the pseudofollicles carry out only processing (cleaving the precursor into active hormones).
What is inside the follicles of the intermediate lobe?
The cavities are filled with proteinaceous or mucous colloid. However, there is no reliable evidence that finished hormones are stored in this colloid.
What else to study on this topic
- Mechanisms of intracellular processing of the common polypeptide precursor
- The connection of the secondary capillary network of the portal system with the function of pseudofollicles
- Tinctorial properties of colloid: histochemical bases of staining
- The mechanism of action of lipotropin on adipocytes (the lipolysis process)
- Neurophysiological effects of endorphins as morphine-like peptides
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