Histological Organization
The main structural element of the pars intermedia is the follicle. In academic and scientific literature, these structures are also referred to as pseudofollicles or follicle-like cysts.
The process of their formation occurs as follows:
- Glandular cells of the intermediate lobe secrete a specialized fluid known as colloid.
- This secretion gradually accumulates in the intercellular space.
- As fluid accumulates, the cells are pushed apart, forming closed cystic structures.
The wall of each pseudofollicle is formed by a single layer of cuboidal secretory cells.
Extensive connective tissue layers run between the follicles, serving as an important diagnostic feature of the pars intermedia on histological slides. Large blood vessels and a dense secondary capillary network belonging to the hypophyseal portal system are located within these connective tissue septa.
Characteristics of the Colloid
The lumen of the follicle-like cysts is filled with fluid, which can be proteinaceous or mucous in chemical composition. This chemical composition determines the tinctorial properties of the colloid (its ability to bind stains). On microscopic slides, the color of the pseudofollicle contents can vary widely from pale blue to deep orange.
Important clinical and morphological note: Although the colloid is a secretory product of glandular cells, the presence of active hormones within it has not been reliably established to date.
Hormones and Their Effects
The pars intermedia is associated with the production of substances regulating pigment metabolism, lipid metabolism, and pain perception.
Main hormones:
- Melanocyte-stimulating hormone (MSH, melanotropin). Its main targets are pigment cells. MSH stimulates the synthesis of the pigment melanin. The hormone exclusively enhances the process of pigment production but does not cause proliferation (i.e., division and formation of new melanocytes).
- Lipotropin. Acts on adipose tissue. Its primary function is to stimulate lipolysis, the process of fat breakdown followed by the release of fatty acids into the bloodstream.
- Endorphins. These are specialized peptides that possess pronounced morphine-like analgesic effects.
Biosynthesis Features
According to current understanding, the synthesis of intermediate lobe hormones occurs in several stages and begins outside the gland.
It is hypothesized that a large protein—a common polypeptide precursor—is synthesized in central nervous system (CNS) neurons. The pseudofollicle cells of the pars intermedia itself only carry out its maturation (processing). During the cleavage of this massive precursor into smaller fragments, the final active molecules are formed: MSH, lipotropin, and endorphins.