The urinary system plays a critical role in the body's functioning, performing a number of important functions that include removal of metabolic waste products such as uric acid, urea and creatinine, controlling electrolyte, water, and pH balance, and regulation of blood pressure, blood volume, erythropoiesis, and vitamin D production. Development of the urinary system is closely related to the development of the reproductive system, with both systems developing from the same origin in the early stages. The urinary system, however, develops ahead of the reproductive system.
The urinary system consists of four primary organs - the kidneys, ureters, bladder, and urethra - which originate from a region of intermediate mesoderm known as the urogenital ridge. In this article, we will explore the embryology of the urinary system and its clinical correlations.
The development of the kidneys is comprised of three sequential systems - the pronephros, mesonephros, and metanephros - which all arise from the urogenital ridge.
The pronephros appears in the fourth week of development and is found in the cervical region of the embryo. Intermediate mesoderm forms segmented tubules, known as nephrotomes, from which 6-10 pairs are formed. These tubules join together to form the pronephric duct, a single duct that extends from the cervical region to the distal end of the embryo (cloaca). This early system is not functional and fully regresses by the end of the fourth week.
Development of the mesonephros occurs caudally (inferiorly) to the pronephros. It begins with the presence of the pronephric duct, which induces nearby intermediate mesoderm in the thoracolumbar region to form mesonephric tubules. These tubules receive a tuft of capillaries from the dorsal aorta, allowing for the filtration of blood, and they drain into the mesonephric duct (a continuation of the pronephric duct). The mesonephric duct also sprouts a ureteric bud caudally, which is necessary for the development of the definitive kidney. This primitive excretory system is functional in the embryo, but most of the tubules regress by the end of the second month.
The metanephros forms the definitive kidney, appearing in the fifth week of development and becoming functional around the twelfth week. The ureteric bud from the mesonephric duct meets with a caudal region of intermediate mesoderm, known as the metanephric blastema, to form the metanephric system. This system is composed of two components - the collecting system and the excretory system. The collecting system, derived from the ureteric bud, comprises the ureter, renal pelvis, major and minor calyces, and collecting tubules, which all terminate at the distal convoluted tubule. If the uretic bud splits too early, this can result in two ureters or two renal pelvices connecting to a single ureter. The excretory system, derived from the metanephric blastema, comprises the excretory tubules which, along with the developing glomeruli, form the kidney's functional units - the nephron. The proximal end of the excretory tubule forms the Bowman's capsule around a glomerulus, which then elongates to form the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
The definitive kidney initially develops in the pelvic region before ascending into the abdomen. It is supplied with blood from a pelvic branch of the abdominal aorta in the pelvis, which is replaced by new arteries as the kidney ascends. The pelvic vessels usually regress, but can persist as accessory renal arteries.
In the embryo, the kidneys develop from three overlapping sequential systems; the pronephros, the mesonephros, and the metanephros. They are all derived from the urogenital ridge.
The pronephros appears in the 4th week of development. Its development begins in the cervical region of the embryo, with segmented divisions of intermediate mesoderm forming tubules known as nephrotomes. In total, 6-10 pairs of nephrotomes are formed. These tubules join into a single channel, the pronephric duct, which is a duct that extends from the cervical region to the cloaca (distal end) of the embryo. This early system is non-functional and completely regresses by the end of week 4.
The mesonephros develops caudally (inferiorly) to the pronephros. Initially, the presence of the pronephric duct induces nearby intermediate mesoderm in the thoracolumbar region to form mesonephric tubules. These tubules receive a tuft of capillaries from the dorsal aorta, allowing for the filtration of blood, and they drain into the mesonephric duct (a continuation of the pronephric duct). They act as a primitive excretory system in the embryo, with most tubules regressing by the end of the 2nd month. Additionally, the mesonephric duct sprouts the ureteric bud caudally, which induces the development of the definitive kidney.
The metanephros forms the definitive kidney. It appears in the 5th week of development and becomes functional around the 12th week. The ureteric bud from the mesonephric duct makes contact with a caudal region of intermediate mesoderm - the metanephric blastema - to form the metanephric system. This system has two components: the collecting system and the excretory system.
The definitive kidney initially develops in the pelvic region before ascending into the abdomen. In the pelvis, the kidney receives its blood supply from a pelvic branch of the abdominal aorta and as it ascends, new arteries from the abdominal aorta supply the kidney. The pelvic vessels usually regress, but can persist as accessory renal arteries.
The bladder and urethra of the urinary system are ultimately derived from the cloaca - a hindgut structure that is a common chamber for gastrointestinal and urinary waste. In the 4th-7th weeks of development, the cloaca is divided into two parts by the uro-rectal septum: the urogenital sinus (anterior) and the anal canal (posterior).
The urinary bladder is initially drained by the allantois. However, this is obliterated during fetal development and becomes a fibrous cord - the urachus. A remnant of the urachus can be found in adults; the median umbilical ligament, which connects the apex of the bladder to the umbilicus.
As the bladder develops from the urogenital sinus, it absorbs the caudal parts of the mesonephric ducts (also known as the Wolffian ducts), becoming the trigone of the bladder. The ureters, which have formed as outgrowths of the mesonephric ducts, enter the bladder at the base of the trigone.
The final structure of the bladder and urethra vary between sexes:
The development of the kidneys, bladder, and urethra in the embryo are highly complex processes that involve numerous layers of induction and morphogenesis. Through these processes, the metanephric system is formed, allowing for the formation of the definitive kidney which is able to perform its vital functions of filtration, reabsorption, and excretion.
As the kidneys ascend into the abdomen, the ureteric openings move cranially. This anatomical event is accompanied by the degeneration of the mesonephric ducts due to a lack of testicular androgens.
The male urethra is formed from three parts of the urogenital sinus: the pre-prostatic, prostatic and membranous sections. The spongy urethra is derived from the phallic part of the urogenital sinus.
In the female body, the urethra is formed from the pelvic part of the urogenital sinus. It passes through the external genitalia, and this anatomical structure is much shorter than the male urethra.
The female urethra is located within the anterior wall of the vagina, with an opening located just above the vagina. It is responsible for expelling urine from the bladder, and it is lined with mucous membrane which secretes a small amount of lubricant to facilitate the passing of urine.
The urethra is composed of a series of contours that form a pathway allowing urine to flow from the bladder to the exterior. These contours are known as the internal urethral sphincter and the external urethral sphincter. The internal urethral sphincter is formed from muscle, while the external urethral sphincter is composed of a network of connective tissue.
The urethra begins in the lower urinary tract, and like the male urethra, the female urethra is subject to infection by urinary tract infections. As such, it is important to practice good genital hygiene so as to avoid such infections.
The female urethra also serves as an important component of the reproductive system, as it helps to transport sperm from the vagina to the uterus during sexual intercourse. The urethra also produces a lubricant fluid that helps sperm travel up the reproductive tract.
The female urethra is comprised of three sections: the spongy urethra, the proximal urethra, and the distal urethra. The spongy urethra is comprised of tissue that is highly sensitive to stimulation, and it is located adjacent to the clitoris. The proximal urethra is the longest section of the urethra, and it is located between the bladder and the vagina. The distal urethra is the shortest section, and it is located between the bladder and the external urethral orifice.
There are a number of conditions that can affect the female urethra. One of the most common is urinary tract infections, which can cause inflammation, burning, and pain upon urination. Urethritis, cystitis, and other conditions can also affect the female urethra and should be evaluated by a doctor if any symptoms arise.
In addition to its role in urination and reproduction, the female urethra also plays a role in sexual arousal and orgasm. The female urethra contains a number of nerve endings that are stimulated by sexual activity, and this can lead to a heightened sense of pleasure.
The female urethra is an important part of the female anatomy, and it is essential for proper urinary and reproductive functioning. It is also important to practice good genital hygiene to avoid health conditions that can affect the urethra.
