Endometrial cups form during pregnancy in

mare A mare is an adult female horse or other equidae, equine. In most cases, a mare is a female horse over the age of three, and a filly is a female horse three and younger. In Thoroughbred horse racing, a mare is defined as a female horse more th ...

s and are the source of

equine chorionic gonadotropin Equinae is a subfamily of the family Equidae, known from the Hemingfordian stage of the Early Miocene (16 million years ago) onwards. They originated in North America, before dispersing to every continent except Australia and Antarctica. They ...

(eCG) and a placenta-associated structure, which is derived from the

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 ...

. Their purpose is to increase the immunological tolerance of the mare in order to protect the developing

foal A foal is an equine up to one year old; this term is used mainly for horses, but can be used for donkeys. More specific terms are colt (horse), colt for a male foal and filly for a female foal, and are used until the horse is three or four. Whe ...

Function

Endometrial cups are unique to animals in the horse family, and so named because of their concave shape. They are a placenta-associated structure, found in the uterine wall of a mare from about 38 to 150 days into a pregnancy. After about 70 days, they begin to regress, and are eventually destroyed by the

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

. They begin to develop at approximately 25 days of pregnancy, deriving from the chorionic girdle. At approximately 36–38 days of pregnancy, the cells that will become the endometrial cup begin to burrow into the endometrial tissue, through the

basement membrane The basement membrane, also known as base membrane, is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between epithelial tis ...

, and into the uterine stroma. Their invasion of the uterine stroma begins the cells' maturation process, which takes 2–3 days. Endometrial cups can be circular, U-shaped, or ribbonlike and are pale compared to the rest of the endometrial tissue. They can range in size from 1 cm to 10 cm in diameter at the widest point. They resemble

ulcer An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caused by sloughin ...

s in form, and when examined under a microscope have large epithelioid decidual-like cells and large nucleoli. They produce high concentrations of equine chorionic gonadotropin (eCG), also called pregnant mare's serum gonadotropin, in the bloodstream of pregnant mares. eCG is actually an equine

luteinizing hormone Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (G ...

. Endometrial cups behave somewhat like cells from

metastatic tumor Metastasis is a pathogenic agent's spreading from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, ...

s, in that they leave the placenta and migrate into the uterus. Their purpose appears to be to work with other placental cells to control the expression of histocompatibility genes so that the developing fetus is not destroyed by the mare's immune system. Similar types of cells that invade the placenta have been described in humans. The purpose of these cells In both humans and horses is believed to be to interact with the mother's immune system and increase maternal immunological tolerance of the developing fetus.

Histology

Mature endometrial cup cells appear rounded and secrete eCG; all but a few cells have two nuclei. As the mare's immune system begins to react to the presence of the pregnancy, histological changes are visible. First, as the endometrial cup grows and develops, maternal white blood cells, including

T cell T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell ...

B cell B cells, also known as B lymphocytes, are a type of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasm ...

s, and

macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...

s, mass in the uterine stroma. They begin to destroy the cup around days 70-80 of pregnancy, and it is completely destroyed by 100–140 days, at which point it naturally separates from the endometrium.

References

{{reflist Anatomy Horse anatomy

Function

Histology

See also

References