Boomer's a 7-year-old intact male Bassett hound you've been caring for since his owners moved to the area 2 years before. They've brough him in for his annual checkup and shots. Taking the history, you ask the owner asked about any recent problems. Well, he replied, it seems like recently he wants to go out to pee every hour and somehow it never seems like he never gets it all out.
You do a rectal palpation, and find that Boomer's prostate gland is significantly enlarged. Blood samples taken as part of the exam showed normal levels of testosterone. Urinalysis showed normal BUN and creatine levels, no evidence of hematuria, and no presence of epithelial cells or casts. Ultrasound confirms the size of the prostate and the presence of cysts.
This is an easy one: Boomer has benign prostatic hyperplasia, a common age-related change in intact male dogs. Dogs, like humans, have a "body" type prostate that surrounds the urethra. Boomer's increased desire to urinate and the failure to empty the bladder are the result of mild cystitis and urethritis from the pressure of the enlarged organ. Most dogs are asymptomatic, however, and this condition is usually detected only by physical examination.
Hyperplasia is an increase in the number of cells in an organ or tissue; this in turn leads to an increased volume of the organ or tissue. Hyperplasia can be physiologic, compensatory or pathologic. Hyperplasia can take place only on those cellular populations that are capable of mitotic division.
Some labile cells continue to multiply throughout life: surface epithelia, lining mucosae, and hematopoietic tissues, are places where constant mitosis is essential to tissue function. Some cells are normally quiescent but remain capable of multiplying: these include parenchymal cells of all glandular organs, fibroblasts, vascular endothelium. They can proliferate in response to some appropriate signal.
As Boomer has reached his senior years, his balance of estrogen and testosterone production has changed, the epithelial cells of the gland have taken this to be a signal to increase in number, and the gland has enlarged. Concurrently there's been an increase in vascularity and development of cysts, which are typical in dogs over 5 years of age.
A section through the prostate of a dog is shown at left: this is a normal organ. You can see the urethra in the center.
At right in a slightly hiher-magnification view, the glandular regions of the organ and the ducts leading its secretions to the urethra are visible. Boomer's prostate look like the one shown below and at right: it has developed large fluid-filled cysts and the cells making up the glandular region are enlarged. If this process continues unabated, Boomer's going to have even more problems urinating: worse, he runs a real risk of prostate cancer.
Hyperplasia is a controlled process. If the promoting stimulus subsides, hyperplasia disappears. A malignancy, however, is not controlled, by definition. It is the responsiveness to normal regulatory control mechanisms that differentiates hyperplasia from cancer. Nevertheless, hyperplasia constitutes a fertile soil in which cancerous, uncontrolled proliferation may eventually arise, and it can't be regarded lightly.
Castration to remove the source of testosterone completely prevents benign prostatic hyperplasia from occuring at all, since prostate tissue requires the presence of this hormone to undergo hyperplasia. Though an option little used in humans, it's a common means of preventing or curing prostate enlargement in dogs. Boomer is about to undergo The Big Chop, which will reduce his prostate and his chances of having the "benign" condition progress to a malignant tumor.
Hypertrophy and hyperplasia often have the same causes. When an organ or a tissue is called upon to increase its size, it can do so by increasing the volume of its individual cells (hypertrophy), their number (hyperplasia), or often both. Knowing the capacity of each cell type for multiplication it's easy to predict the type of response to the need for increased function (work). Pure hypertrophy without any hyperplasia occurs only in organs such as heart and the skeletal muscle, whose cells cannot multiply and have no option but to become larger.
More often than not, hyperplasia is combined with hypertrophy. In this case we have hyperplasia of the cells, and hypertrophy to a certain extent; and the organ as a whole has increased in size, i.e., become hypertrophic.