multiple layers of flat cells near the end of the urethra, which is the same as the external skin around it. There are small mucus-secreting urethral glands, as well as bulbo-urethral glands of Cowper, that secret mucous acting to lubricate the urethra. The urethra consists of three coats: muscular, erectile, and mucous, the muscular layer being a continuation of that of the bladder. Blood and nerve supply and lymphatics Somatic (conscious) innervation of the external urethral sphincter is supplied by the pudendal nerve. Development In the developing embryo, at the hind end lies a cloaca. This, over the fourth to the seventh week, divides into a urogenital sinus and the beginnings of the anal canal, with a wall forming between these two inpouchings called the urorectal septum. The urogenital sinus divides into three parts, with the middle part forming the urethra; the upper part is largest and becomes the urinary bladder, and the lower part then changes depending on the biological sex of the embryo. The cells lining the urethra (the epithelium) come from endoderm, whereas the connective tissue and smooth muscle parts are derived from mesoderm. After the third month, urethra also contributes to the development of associated structures depending on the biological sex of the embryo. In the male, the epithelium multiples to form the prostate. In the female, the upper part of the urethra forms the urethra and paraurethral glands. Function Urination The urethra is the vessel through which urine passes after leaving the bladder. During urination, the smooth muscle lining the urethra relaxes in concert with bladder contraction(s) to forcefully expel the urine in a pressurized stream. Following this, the urethra re-establishes muscle tone by contracting the smooth muscle layer, and the bladder returns to a relaxed, quiescent state. Urethral smooth muscle cells are mechanically coupled to each other to coordinate mechanical force and electrical signaling in an organized, unitary fashion. Ejaculation The male urethra is the conduit for semen during sexual intercourse. Urine is removed before ejaculation by pre-ejaculate fluid – called Cowper's fluid – from the bulbourethral gland. Clinical significance Infection of the urethra is urethritis, which often causes purulent urethral discharge. It is most often due to a sexually transmitted infection such as gonorrhoea or chlamydia, and less commonly due to other bacteria such as ureaplasma or mycoplasma; trichomonas vaginalis; or the viruses herpes simplex virus and adenovirus. Investigations such as a gram