The Pancreas
Alias: "stomach sweetbreads," so called by the ancient Greeks.
Location: in the abdominal cavity, near the start of the small intestine and below the kidneys.
Appearance: roughly triangular in shape and lumpy, with small lobes.
Structure: hormone-producing cell clusters called islets of Langerhans, which contain three different kinds of cells. The rest of the organ--the vast majority of it--serves digestive enzymes directly into the gastro-intestinal tract.
Hormones and actions: the primary players in the body's complex energy management scheme; micromanage mobilization, storage and cellular use of carbohydrates, proteins and fats.
- Insulin, the primary modulator of blood-sugar (glucose) concentration, acts swiftly after food intake to limit the liver's production of glucose and increase stored energy (glycogen) in that organ, to facilitate increased protein synthesis, to convert un-needed energy to fat for storage in body tissues and possibly to control appetite.
- Glucagon raises blood sugar levels by pulling reserves out of storage when circulating energy supplies begin to dwindle.
Diseases: Upsets of pancreatic endocrine function usually are secondary to other diseases or dietary problems.
- Diabetes mellitus, insulin deficiency, occurs when the pancreas does not produce enough of the hormone, excessive antagonistic hormones are present or target tissues have become sensitive to insulin. Cushing's syndrome, adrenal malfunctions, and tumors or inflammation of the pancreas itself may cause diabetes to develop in a horse.
- Obesity in ponies and late-term pregnancy may cause temporary abnormalities in insulin function.
The Gonads
Aliases: the testes/testicles (males); ovaries (females).
Location: scrotum/rear of the abdominal cavity not far below the spine.
Appearance:egg-shaped/bean-shaped.
Structures: seminiferous tubules for producing semen in each testicle, with attached duct (epididymis) to further concentrate and transport semen; follicles mature, erupt to produce one egg (ovum) per heat cycle, then regress; egg passes through the fallopian tubes to the uterus.
Hormones and actions: control reproduction, determine puberty, influence secondary sexual characteristics. Pituitary hormones give permission for reproduction, while local hormones look after the day-to-day details. Both sexes produce male and female hormones, but at different levels, depending on gender.
- Testosterone, the primary androgen, or male hormone, maintains sperm production, contributes to secondary sex characteristics and fires libido.
- Estrogen, produced by an active follicle, causes estrous behavior in mares in conjunction with ovulation and supports conception.
- Progestins (progesterone), produced by the erupted follicle, prepare the reproductive tract for pregnancy, suppress ovulation and estrous behavior for the first three months of pregnancy, and, if no conception occurs, diminish over the course of several weeks to allow a new estrous cycle to occur.
Diseases: Most reproductive problems in horses result from the breeding practices imposed by domestication. Overuse of anabolic steroids can disrupt normal reproductive cycles in mares. Malfunction of other endocrine glands can affect sexual maturation and normal cycling. Ovarian tumors may masculinize mares.
This article originally appeared in the June 1997 issue of EQUUS magazine.
