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THE URINARY SYSTEM
     There are three types of kidneys found in craniates: pronephros, mesonephros, and metanephros.  Metanephric kidneys are the most advanced and represent the kidneys found in human adults and in all amniotes after birth.  Functioning pronephric kidneys are found in hagfish.  In the image of the hagfish below, note that the kidney enters the region of the pharynx (where the gills are located).
HAGFISH

THE HUMAN PRONEPHROS

    In the fourth week of development, intermediate mesoderm in the cervical region forms nephrotomes.  Nephric tubules develop which open into the intra-embryonic coelom.  Branches of the dorsal aorta enter these nephrotomes to form internal and external glomeruli.  The tubules and glomeruli form nephrons and longitudinal ducts form to connect the nephrotomes.  This urinary apparatus is never functional and it forms the vestigial pronephros (Sadler, p.261).

PRONEPHROS PRONEPHROS

Lampreys, the most primitive vertebrates, develop both mesonephric and metanephric kidney tissue (together referred to as the opistonephros) which run much of the length of the body.  This system is typical of the lower vertebrates.

LAMPREY

SHARK:

SHARK

LUNGFISH

LUNGFISH

THE HUMAN EMBRYONIC MESONEPHROS

     In the thoracic, lumbar, and sacral segments, the mesonephros forms.  Although it is composed of intermediate mesoderm, as is the pronephros, it differs from the pronephros in that its urinary structures are not connected to the coelom, it loses its segmentation, and there are typically multiple tubules per segment.  The mesonephros may function during part of development (Sadler, p. 261). In elephants, there are developed nephrostomes in the mesonephros during a much longer part of gestation which may be a result of the gestation time being so prolonged (Gaeth, 1999).

The pronephros, mesonephros, and metanephros of the human embryo are depicted below.

EMBRYONIC KDINEYS

 

METANEPHROS

     The metanephros differs from the mesonephros in the developmental pattern of its tubules.  In all amniotes, this is the only kidney which will function after birth.  The following images are of the kidneys of the fetal cat.

FETAL CAT FETAL CAT

THE POSITION OF THE METANEPHRIC KIDNEY

     The metanephric kidney develops in the pelvic region of the embryo but its position in adults is superior to this.  As a result, the metanephric kidney must rise cranially during fetal development.  As it ascends, new branches of the aorta must form new renal arteries.  In some people, embryonic renal arteries persist as supernumerary renal arteries. Accessory urinary arteries may exist in 25% of people and may number between 2 and 4 (shell 311-2).

KIDNEY ABNORMALITIES
If a kidney does not ascend, it forms a pelvic kidney. 

PELVIC KIDNEY

If the two ascending kidneys become too close to one another, they may fuse to form a horseshoe kidney which occurs in about 1 in every 600 people (Sadler, p. 267-8).

HORSESHOE KIDNEY

Two kidneys may also fuse to form a duplex kidney.

DUPLEX KIDNEY

URETER ABNORMALITIES

URETER ABNORMALITIES

     Some infants possess an ectopic ureter which can open into the neck of the bladder, the urethra, the vagina, or uterus. Extra ureters may form from mesonephric ducts.  As the distance between the two increases, there is an increased probability that the ureter will end blindly or fuse to another embryonic ducts.   In some people there are double ureters, or the ureter may empty into the vagina or urethra.  Some ureters are bifid, although one branch may end blindly.  Instances of uterovaginal duplication are also known (Acien, 1990; Babcock, 1977; Peterson, 1975; Sadler, p.266).

 

The following images are of the embryonic kidneys of chicks and pigs.  Note the size of the mesonephros which will degenerate before birth.

Chick

CHICK EMBRYONIC KIDNEYS CHICK EMBRYONIC KIDNEYS
CHICK EMBRYONIC KIDNEYS CHICK EMBRYONIC KIDNEYS
ARCHINEPHRIC DUCT CHICK EMBRYONIC KIDNEYS
CHICK EMBRYONIC KIDNEYS
EMBRYONIC KIDNEYS IN PIG
GLOMERULUS EMBRYONIC KIDNEYS IN PIG
EMBRYONIC KIDNEYS IN PIG
EMBRYONIC KIDNEYS IN PIG
EMBRYONIC KIDNEYS IN PIG
EMBRYONIC KIDNEYS IN PIG
EMBRYONIC KIDNEYS IN PIG
EMBRYONIC KIDNEYS IN PIG