The urethra differs in structure in the two sexes. In the male the prostatic portion is lined with epithelium resembling that swollen after maceration in the altered urine of the bladder. Further forward, it gradually passes into cylindrical epithelium, at first more than one layer thick; but in the cavernous portion of the urethra it consists of but a single layer. The stratified epithelium covering the glans extends for a short distance from the meatus into the urethra. The epithelial lining rests upon fibrous tissue containing a number of elastic fibers, and this is bounded externally by a muscular coat. In the prostatic portion the muscular coat consists of an inner longitudinal and an outer circular layer of fibers, which become less well marked as the course of the urethra is followed, the circular coat disappearing in the bulbous portion and the longitudinal fibers becoming scattered toward the anterior part of the cavernous portion. The mucous membrane contains little tubular glands, "Littre's glands," some of which are simple, while others are compounded. In the collapsed condition the urethral mucous membrane is thrown into one or more longitudinal folds.
In the female the epithelial lining of the urethra is either stratified or composed of a single layer of columnar cells. The glands are more sparsely distributed than in the male, except for a group situated near the meatus. On the other hand, the muscular coat is thicker and consists throughout the course of the urethra of a well-defined internal longitudinal and external circular layer of fibers.
From the pelvis of the kidney to the stratified epithelium of the meatus the mucous membranes are capable of secreting mucus, which is much increased in amount under the influence of irritating sub- stances, such as concentrated urine or the various causes of inflammation. The blood vessels are most numerous and of largest size in the areolar tissue beneath the epithelium, and are accompanied by the lymphatics. The nerves are distributed chiefly to the muscular coats, but also extend into the fibrous tissue, up to and into the epithelium. The cells of the latter are connected by little protoplasmic bridges, as in the case of the epidermis, leaving minute channels between the cells for the passage of nutrient fluids.
From “Normal Histology” (1905) by Edward K. DUNHAM (1860-1922).