The development of the urinary system begins during

prenatal development Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal deve ...

, and relates to the development of the urogenital system – both the organs of the

urinary system The urinary system, also known as the urinary tract or renal system, consists of the kidneys, ureters, bladder, and the urethra. The purpose of the urinary system is to eliminate waste from the body, regulate blood volume and blood pressure, c ...

and the

sex organ A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...

s of the

reproductive system The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are ...

. The development continues as a part of

sexual differentiation Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the dev ...

. The urinary and reproductive organs are developed from the

intermediate mesoderm Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develop ...

. The permanent organs of the adult are preceded by a set of structures which are purely embryonic, and which with the exception of the ducts disappear almost entirely before birth. These embryonic structures are on either side; the pronephros, the mesonephros and the

metanephros Kidney development, or nephrogenesis, describes the embryologic origins of the kidney, a major organ in the urinary system. This article covers a 3 part developmental process that is observed in most reptiles, birds and mammals, including humans. ...

of the

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

, and the Wolffian and Müllerian ducts of the

. The pronephros disappears very early; the structural elements of the mesonephros mostly degenerate, but the

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sp ...

is developed in their place, with which the Wolffian duct remains as the duct in males, and the Müllerian as that of the female. Some of the tubules of the mesonephros form part of the permanent kidney.

Pronephros and Wolffian duct

Wolffian duct

In the outer part of the

, immediately under the ectoderm, in the region from the fifth cervical segment to the third

thoracic segment The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spinal ...

, a series of short

evagination Endodermic evagination relates to the inner germ layers of cells of the very early embryo, from which is formed the lining of the digestive tract, of other internal organs, and of certain glands, implies the extension of a layer of body tissue to ...

s from each segment grows

dorsally Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and extends caudally, fusing successively from before backward to form the

pronephric duct The pronephric duct is the predecessor of the mesonephric duct (Wolffian duct). Development The development of the pronephric duct is a part of the development of the urinary system, and the development of the reproductive system. In the outer ...

. This continues to grow caudally until it opens into the ventral part of the

cloaca In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, ...

; beyond the pronephros it is termed the

Wolffian duct The mesonephric duct (also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct) is a paired organ that forms during the embryonic development of humans and other mammals and gives rise to male reproductive organs. Stru ...

. Thus, the Wolffian duct is what remains of the pronephric duct after the atrophy of the pronephros.

Pronephros

The original evaginations form a series of transverse tubules each of which communicates by means of a funnel-shaped ciliated opening with the

abdominal cavity The abdominal cavity is a large body cavity in humans and many other animals that contains many organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is th ...

, and in the course of each duct a glomerulus also is developed. A secondary glomerulus is formed ventral to each of these, and the complete group constitutes the pronephros. In humans, the pronephros is just rudimentary, and undergoes rapid atrophy and disappears.

Wolffian duct

Mesonephros

On the medial side of the Wolffian duct, from the sixth cervical to the third lumbar segments, a series of tubules, the

Wolffian tubules The mesonephros ( el, middle kidney) is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included ...

, develops. They increase in number by outgrowths from the original tubules. They change from solid masses of cells to instead become hollowed in the center. One end grows toward and finally opens into the Wolffian duct, the other dilates and is invaginated by a tuft of capillary bloodvessels to form a glomerulus. The tubules collectively constitute the mesonephros. The mesonephros persists and form the permanent kidneys in fish and amphibians, but in reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the permanent kidney (

) develops beginning during the sixth or seventh week, so that by the beginning of the fifth month only the ducts and a few of the tubules of the mesonephros remain.

Development in male

In the male the

persists, and forms for example the

epididymis The epididymis (; plural: epididymides or ) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the ...

, the

ductus deferens The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube w ...

, the ejaculatory duct,

seminal vesicle The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some male mammals. They secrete fluid that partly composes the semen. The vesicles are 5� ...

and efferent ducts. In the female, on the other hand, the Wolffian bodies and ducts atrophy, leaving behind only remnants in the adult, involving e.g. the development of the suspensory ligament of the ovary.

Müllerian duct

Shortly after the formation of the Wolffian ducts a second pair of ducts is developed; these are the Müllerian ducts. Each arises on the lateral aspect of the corresponding Wolffian duct as a tubular invagination of the cells lining the abdominal cavity. The orifice of the invagination remains open, and undergoes enlargement and modification to form the abdominal ostium of the fallopian tube. The ducts pass backward lateral to the Wolffian ducts, but toward the posterior end of the embryo they cross to the medial side of these ducts, and thus come to lie side by side between and behind the latter—the four ducts forming what is termed the ''common genital cord'', to distinguish it from the ''genital cords of the germinal epithelium'' seen later in this article. The Müllerian ducts end in an epithelial elevation, the

Müllerian eminence Sinus tubercle (also known as sinual tubercle or Müllerian eminence) is the proliferation of endoderm induced by the paramesonephric ducts. It is located in the developing fetus between the orifices of the mesonephric ducts on the urogenital si ...

, on the ventral part of the cloaca between the orifices of the Wolffian ducts. At a later stage the eminence opens in the middle, connecting the Müllerian ducts with the cloaca.

Atrophy in males

In the male the Müllerian ducts atrophy, but traces of their anterior ends are represented by the

appendices testis The appendix testis (or hydatid of Morgagni) is a vestigial remnant of the Müllerian duct, present on the upper pole of the testis and attached to the tunica vaginalis. It is present about 90% of the time. Clinical significance Torsion The appen ...

(

hydatids of Morgagni The hydatid of Morgagni can refer to one of two closely related bodily structures: * Appendix of testis (in the male) * Paraovarian cyst Paraovarian cysts or paratubal cysts are epithelium-lined fluid-filled cysts in the adnexa adjacent to the f ...

of the male), while their terminal fused portions form the

utriculus The utricle and saccule are the two otolith organs in the vertebrate inner ear. They are part of the balancing system (membranous labyrinth) in the vestibule of the bony labyrinth (small oval chamber). They use small stones and a viscous fluid to ...

in the floor of the

prostatic urethra The prostatic urethra, the widest and most dilatable part of the urethra canal, is about 3 cm long. It runs almost vertically through the prostate from its base to its apex, lying nearer its anterior than its posterior surface; the form of ...

Development in females

In the female the Müllerian ducts persist and undergo further development. The portions which lie in the ''genital cord'' fuse to form the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...

and

vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...

. This fusion of the Müllerian ducts begins in the third month, and the septum formed by their fused medial walls disappears from below upward. The parts outside this cord remain separate, and each forms the corresponding

Fallopian tube The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In o ...

. The ostium of the fallopian tube remains from the anterior extremity of the original tubular invagination from the abdominal cavity. About the fifth month a ring-like constriction marks the position of the

cervix The cervix or cervix uteri (Latin, 'neck of the uterus') is the lower part of the uterus (womb) in the human female reproductive system. The cervix is usually 2 to 3 cm long (~1 inch) and roughly cylindrical in shape, which changes during ...

of the uterus, and after the sixth month the walls of the uterus begin to thicken. For a time the vagina is represented by a solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the future vaginal fornix. At about the fifth or sixth month the lumen of the vagina is produced by the breaking down of the central cells of the epithelium. The

hymen The hymen is a thin piece of mucosal tissue that surrounds or partially covers the external vaginal opening. It forms part of the vulva, or external genitalia, and is similar in structure to the vagina. In children, a common appearance of the ...

represents the remains of the Müllerian eminence.

Metanephros and definitive kidney

The metanephros is the definite, permanent, but yet immature kidney. It arises from two directions. On one hand, the precursor of the ureter buds from the Wolffian duct, while on the other hand, the precursor of the renal tubules develop from the metanephrogenic blastema. The ureteric bud subsequently grows into the latter mass, forming the parts of the nephron. Other changes include e.g. the translocation of the ureteric opening directly into the cloaca.

Ureteric bud

The rudiments of the permanent

s make their appearance about the end of the first or the beginning of the second month. Each kidney originates as a ureteric bud from the caudal end of the Wolffian duct, which, in turn, originates from

. The ureteric bud starts close to where the Wolffian duct opens into the cloaca, and grows dorsalward and

rostralward Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

along the posterior abdominal wall, where its blind extremity expands and subsequently divides into several buds, which form the rudiments of the

renal pelvis The renal pelvis or pelvis of the kidney is the funnel-like dilated part of the ureter in the kidney. It is formed by the covnvergence of the major calyces, acting as a funnel for urine flowing from the major calyces to the ureter. It has a muco ...

and renal calyces; by continued growth and subdivision it gives rise to the

collecting duct system The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct system is the last part of nephron and participates in el ...

of the kidney. The other, more superficial, portion of the diverticulum, on the other hand, becomes the ureter.

Metanephrogenic blastema

The renal corpuscles and renal tubules, in contrast, are developed from the metanephrogenic blastema instead of from the ureteric bud. The metanephrogenic blastema is moulded over the growing end of the latter, and becomes a part of the metanephros in this way. The renal tubules of the metanephros, unlike those of the pronephros and mesonephros, do not open into the Wolffian duct. Instead, the tubules rapidly elongate to form the parts of the

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ...

: the

proximal tubule The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. It can be further classified into the proximal convoluted tubule (PCT) and the proximal strai ...

s, the loops of Henle and the

distal convoluted tubule The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule. Physiology It is partly responsible for the regulation of potassium, sodium, calcium, and pH. On its apical surface (lumen ...

s. These last join and establish communications with the collecting duct system derived from the ultimate ramifications of the ureteric bud. In the other end, the renal tubules give rise to

Bowman's capsule Bowman's capsule (or the Bowman capsule, capsula glomeruli, or glomerular capsule) is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form u ...

s and glomeruli.

Other changes

The mesoderm around the tubules becomes condensed to form the connective tissue of the kidney. The ureter opens at first into the hind-end of the Wolffian duct; after the sixth week it separates from the Wolffian duct, and opens independently into the part of the cloaca which ultimately becomes the

urinary bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine ente ...

. The renal tubules become arranged into renal pyramids, and the lobulated condition of the kidneys exists for some time after birth, while traces of it may be found even in the adult. The kidney of the ox and many other animals, on the other hand, remains lobulated throughout life.

Urinary bladder

The

is formed partly from the endodermal

and partly from the ends of the Wolffian ducts. In other words, the

allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...

takes no share in its formation. After the separation of the rectum from the dorsal part of the cloaca, the ventral part becomes the primary urogenital sinus. The urogenital sinus, in turn, divides into the superficial definitive urogenital sinus and the deeper anterior vesico-urethral portion.

Definitive urogenital sinus

The definitive urogenital sinus consists of a caudal phallic portion and an intermediate narrow channel, the pelvic portion.

Vesico-urethral portion

The vesico-urethral portion is the deepest, continuous with the allantois. It absorbs the ends of the Wolffian ducts and the associated ends of the renal diverticula, and these give rise to the

trigone of urinary bladder The trigone (a.k.a. vesical trigone) is a smooth triangular region of the internal urinary bladder formed by the two ureteric orifices and the internal urethral orifice. The area is very sensitive to expansion and once stretched to a certain ...

and part of the

. The remainder of the vesico-urethral portion forms the body of the bladder and part of the prostatic urethra; its apex is prolonged to the umbilicus as a narrow canal, the urachus, which later is obliterated and becomes the median umbilical ligament of the adult.

Sex organs

Until about the ninth week of gestational age,The University of North Carolina at Chapel Hill - Embryo images nr 024
/ref> the

sex organs A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, ...

of males and females look the same, and follow a common development. This includes the development of a genital tubercle and a membrane dorsally to it, covering the developing urogenital opening, and the development of labioscrotal folds. Even after differentiation can be seen between the sexes, some stages are common, e.g. the disappearing of the membrane. On the other hand, sex-dependent development include further protrusion of the genital tubercle in the male to form the penis. Furthermore, the labioscrotal folds evolve into the scrotum in males, while they evolve into labia in females.

Diagram of internal differentiation

A.—Diagram of the primitive urogenital organs in the embryo previous to sexual distinction. * 3. Ureter. * 4. Urinary bladder. * 5. Urachus. * cl. Cloaca. * cp. Elevation which becomes clitoris or penis. * i. Lower part of the intestine. * ls. Fold of integument from which the labia majora or scrotum are formed. * m, m. Right and left Müllerian ducts uniting together and running with the Wolffian ducts in gc, the genital cord. * ot. The genital ridge from which either the ovary or testis is formed. * ug. Sinus urogenitalis. * W. Left Wolffian body. * w, w. Right and left Wolffian ducts. B.—Diagram of the female type of sexual organs. * C. Greater vestibular gland, and immediately above it the urethra. * cc. Corpus cavernosum clitoridis. * dG. Remains of the left Wolffian duct, such as give rise to the duct of Gärtner, represented by dotted lines; that of the right side is marked w. * f. The abdominal opening of the left uterine tube. * g. Round ligament, corresponding to gubernaculum. * h. Situation of the hymen. * i. Lower part of the intestine. * l. Labium majus. * n. Labium minus. * o. The left ovary. * po. Epoophoron. * sc. Corpus cavernosum urethrae. * u. Uterus. The uterine tube of the right side is marked m. * v. Vulva. * va. Vagina. * W. Scattered remains of Wolffian tubes near it (paroöphoron of Waldeyer). C.—Diagram of the male type of sexual organs. * C. Bulbo-urethral gland of one side. * cp. Corpora cavernosa penis cut short. * e. Caput epididymis. * g. The gubernaculum. * i. Lower part of the intestine. * m. Müllerian duct, the upper part of which remains as the hydatid of Morgagni; the lower part, represented by a dotted line descending to the prostatic utricle, constitutes the occasionally existing cornu and tube of the uterus masculinus. * pr. The prostate. * s. Scrotum. * sp. Corpus cavernosum urethrae. * t. Testis in the place of its original formation. * t’, together with the dotted lines above, indicates the direction in which the testis and epididymis descend from the abdomen into the scrotum. * vd. Ductus deferens. * vh. Ductus aberrans. * vs. The vesicula seminalis. * W. Scattered remains of the Wolffian body, constituting the organ of Giraldès, or the paradidymis of Waldeyer.

References

In-line

General

Henry Gray (1821–1865). Anatomy of the Human Body. 1918. 3. The Urogenital ApparatusUNSW Embryology - Development of the Kidney and Reproduction SystemsHow the Body Works / Sex Development / Sexual Differentiation / Duct Differentiation
- The Hospital for Sick Children (GTA - Toronto, Ontario, Canada) {{DEFAULTSORT:Development Of The Urinary And Reproductive Organs Embryology of urogenital system

Pronephros and Wolffian duct

Wolffian duct

Pronephros

Wolffian duct

Mesonephros

Development in male

Müllerian duct

Atrophy in males

Development in females

Metanephros and definitive kidney

Ureteric bud

Metanephrogenic blastema

Other changes

Urinary bladder

Definitive urogenital sinus

Vesico-urethral portion

Sex organs

Diagram of internal differentiation

See also

References

In-line

General