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).
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.
SHARK:
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.
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.
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).
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
