1. KIDNEYS
Nick Milne School of Anatomy and Human Biology UWA
Ontogeny recapitulates
Phylogeny
Early development and
evolution of the kidney
Intermediate mesoderm
Pronephros
Mesonephros
Metanephros
Ascent and abnormalities
Readings
Larson “Human
Embryology”
Carlson “Embryology and
developmental biology”
McLachlan “Medical
Embryology”
Sadler “Langman’s
Medical embryology”
Basmajian “Grant’s
Method of Anatomy

2. Ontogeny Recapitulates
Phylogeny Ernst Haeckel 1860?
Ontogeny is the development of the individual
Phylogeny is the evolution of the species
So this is the idea that during development an
organism (or an organ) goes through the same
stages as during their evolution.
Consider the Frog
It development retraces the evolution of vertebrates
from fish to reptiles
From the tadpole stage
Water breathing with tail and no limbs (like a fish)
Rudimentary limbs
Reduction of tail
Development of lungs – breathes air
Fully developed limbs, loss of tail moves onto land
To the fully developed frog
This idea should be
considered as a “Parable”
It is not necessarily true
but it is a useful idea.

3. INTERMEDIATE MESODERM
Mesoderm appears in the 3
week embryo
Paraxial = somites
Intermediate
Lateral plate
Two layers separated by the
Coelome
Intermediate mesoderm
gives rise to “Paired
Glands” (Kidneys, Adrenals
and Gonads)

4. Pronephros
the first kidney
A cavity, like the coelom
develops inside the
intermediate mesoderm
Balls of bloods vessels from
the aorta bulge into the space
– The Glomerulus.
The glomerulus allows excess
water to leave the blood while
salts and macromolecules are
retained.
This kind of kidney is found
in primitive fish (eg Lampreys)
and in the embryos of most
vertebrates
Initially the water filters into the
coelom
Later, part of the cavity inside the
intermediate mesoderm links up
with similar parts in adjacent
segments to form a duct.

5. Pronephros – in humans
Appears at Day 21
In segmented
intermediate
mesoderm in the
cervical region
It degenerates by
day 24
It is never functional
in humans

6. Mesonephros – middle kidney
When some primitive fish returned
to the sea they had the reverse
osmotic problem, water tended to
diffuse out of their blood.
They developed the mesonephric
kidney located in the thoracic
region.
The mesonephros had a smaller
glomerulus but a larger system of
tubules so that the filtration part
was reduced and the reabsorbing
part was increased.
This was an important preadaption
to life on the land where drying out
is a constant problem.
Many bony fish and some amphibians
have a mesonephric kidney.
The tubules all connect with the
mesonephric (Wolfian) duct.

7. Mesonephros - in humans
The Mesonephros first appears
early in week 4
In thoracic and lumbar
segments of intermediate
mesoderm.
Urine is produced and drains
along the mesonephric
(Wolfian) duct to the
cloaca/bladder
In week 5 the thoracic
segments regress but the
mesonephric kidney continues
functioning until week 10

8. Fate of the Mesonephric (Wolfian) ducts
The development of the metanephric
kidney accompanied the changes in
the reproductive system.
The gonads of primitive vertebrates
release their eggs and sperm into the
coelom, from there they pass via small
pores into the cloaca.
In higher vertebrates the eggs are still
released into the coelom, but the
cloacal pores have become specialised
tubes which open adjacent to the
ovary.
In the embryo, this egg collecting tube
(paramesonephric duct) lies parallel to
the mesonephric duct.

9. Male and Female genital ducts
Initially both sexes have both mesonephric and paramesonephric
ducts
In females
Eggs are still released into the coelom (peritoneal cavity) but are gathered up in
the uterine tubes.
The two paramesonephric ducts become the uterine tubes.
Distally the paramesonephric ducts fuse together to form the uterus and vagina.
The mesonephric duct degenerates completely.
In higher vertebrates sperm is never released into the coelom but
reaches the outside by passing through some derivative of the urinary
system. In birds, reptiles and mammals the testis develops a
connection with the mesonephric duct (at the time that the mesonephros is degenerating).
In males
The mesonephric duct becomes the ductus deferens, seminal vesicle and parts
of the prostate gland.
The paramesonephric duct degenerates completely.

10. Metanephros – definitive kidney
The metanephros or definitive
kidney of higher vertebrates,
begins when the metanephric
ducts (ureteric buds) sprout
from the distal end of the
mesonephric duct at about 5
weeks.
The ureteric buds induce
intermediate mesoderm in the
sacral region to form a
metanephric blastema which
forms the glomeruli and
tubules of the nephrons.

11. Evolution of the Kidneys: Metanephros
The ureteric buds bifurcate again and again to
form the calyces and collecting duct system of
the definitive kidney.
The kidneys begin producing urine by week 12,
and it adds to the volume of the amniotic fluid.
The fetus drinks this fluid in utero.
The fetal kidneys are not responsible for
excretion as the placenta serves this function

12. Ascent of the
Kidneys
In the 6th
week the kidneys begin to ascend from the sacral region to
their position in the upper abdomen.
The metanephric ducts elongate and become the ureters.
As the kidney ascends it receives new segmental arteries from the aorta
and loses those vessels below (“climbing a ladder”). Thus sometimes
there is more than one renal artery.
Sometimes one kidney fails to ascend => pelvic kidney
Sometimes the left and right kidneys become attached in the pelvis then
the horseshoe kidney can’t ascend above the inferior mesenteric artery

13. Position of kidneys
Kidneys lie on the psoas muscle beside
the vertebral bodies.
The diaphragm and 11th
and 12th
ribs lie
behind the upper half of each kidney.
Therefore they move with breathing
Left is higher than right (liver)
Upper poles T12
Hilum is at L1/2
Lower poles at L3
Upper poles are more medial (psoas).
In the hilum:
Renal vein is the most anterior.
Followed by renal artery & pelvis/ureter
Note that the left renal vein is longer .
It crosses the aorta
Is crossed by the SMA
Receives left gonadal vein

14. Anterior relations
Right
Adrenal
Liver
bare area
Hepatorenal pouch
Duodenum
Pancreas
Right colic
flexure
Jenunum
Left
Adrenal
Stomach
Spleen
Pancreas
Descending
Colon
Jenunum

15. Lecture summary
Early development and
evolution
Pronephros
Mesonephros
Metanephros
Ascent and
abnormalities
Position and relations
Lobes and segments

16. Orientation and surroundings
The medial border of each kidney is anterior to the lateral border
(psoas). Thus the coronal plane of the kidney is at 30 degrees to
the coronal plane of the body.
Layers surrounding the
kidney
Outside the renal
capsule is perirenal fat
Then is the renal fascia
which also surrounds the
adrenals
This is embedded in
extraperitoneal fat
(pararenal fat)

17. Renal Lobes and their papillae/calyces
There are 14 lobes originally
Each lobe corresponds with a renal pyramid
(plus its cap of cortex), and empties via its
papilla, into a minor calyx
Lobes fuse so that there are between 14 and 6,
usually 8 (26%)
Most fusion happens at the upper and lower
poles so that those papillae are compound.
Simple papillae have valve-like slits where the
ducts open – Non-refluxing
Compound papillae allow reflux
Vescicouretal reflux occurs in children and if
the urine can enter the parenchyma of the
kidney it causes infection and damage

18. 5 Segments of the Kidney
Each segment is the area supplied one branch of the renal artery. (end arteries)
The front of the kidneys has four segments: apical, upper anterior, middle
anterior, lower
The back of the kidney has the posterior segment and the posterior aspects
of the apical and lower.
Easy way to remember: like your hand holding a glass four fingers in front
and the thumb behind.
The posterior branch of the
artery branches off first and
passes over the renal pelvis to
reach the posterior aspect. The
four anterior branches continue in
the plane between the vein and
renal pelvis.