A structural and immunological study of chorionic gonadotrophin production by equine trophoblast girdle and cup cells.

This research article examines how embryonic horses produce a hormone called equine chorionic gonadotropin (eCG) during pregnancy. The researchers analyzed the structural and biological processes behind its formation and secretion.

Process of eCG Production

• The research shows that eCG production starts around the 27th day of horse pregnancy, from an equatorial band of cells called the trophectodermal cells found on the surface of the conception sphere. This band forms the chorionic girdle.
• The girdle initially consists of a flat trophectodermal epithelium that corrugates into folds as the cell proliferation progresses.
• These folds are then pressed against the uterine epithelium as the conceptus grows. Some cells become binucleate, having two nuclei, before they start to penetrate the endometrium around days 35-37 of the pregnancy.

Early eCG Synthesis and Migration

• Immunogold labelling, a technique used to visualize the location of proteins or other molecules in cells or tissues, shows that these cells begin to synthesize eCG as early as day 32, before they invade the maternal epithelium.
• Clusters of binucleate cells from the girdle penetrate deep into the endometrium where they mature further.

Appearance of Mature Cup Cells

• The cytoplasm of mature cup cells, which are involved in eCG production, is filled with mitochondria, rough endoplasmic reticulum cisternae, and a large Golgi apparatus.
• Using immunocytochemistry, or the use of antibodies to visualize proteins, the research identifies strong immunoreactivity for glucose transporter 1 isoform on its plasmalemma, or the plasma membrane.
• The researchers also found that eCG is localized in the Golgi cisternae, and small dense granules, similar to those found in the migrating cells.

Release of eCG Hormone

• The research suggests the release of eCG presumably happens via exocytosis, the usual mechanism for other protein hormones.
• Markers suggest cup cells continuously secrete eCG with no requirement for secretagogues, in contrast to pituitary cells which utilize a regulated pathway capable of massive secretion under appropriate stimulation.
• This constant secretion of eCG is necessary for the establishment and maintenance of a normal equine pregnancy.

Distinct Hormonal Characteristics

• The study notes a marked contrast in the small size and absence of significant accumulation of eCG-containing granules compared to the larger granules containing a hormone called luteinizing hormone (eLH) in the equine pituitary gonadotroph.
• Interestingly, despite this difference in granule accumulation and their distinct roles in the body, both hormones share complete identity at the amino acid sequence level.