Embryology pdf - د. ملاك

Urinary System

Prof. Dr. Malak A. Al-yawer

Learning Objectives

At the end of this lecture, the medical student will be able to:
 Explain the following statement “ the genital and urinary systems

are intimately interwoven embryologically and anatomically”

 Identify the three stages of kidney development and give

characteristics of each of the stages

 List the adult derivatives of the embryonic structures of each of

the early kidney stages

 Describe the ascent of the kidneys including changes to arterial

blood supply

 List the three components of the uro-genital sinus and describe

their adult derivatives

 Explain the formation of the urinary bladder proper
 List some clinical correlates

Urogenital System

can be divided into two

entirely different
components:

Urinary system

2. Genital system

Embryologically and anatomically, they are

intimately interwoven

Both systems
(a)

develop from

intermediate mesoderm (uro

genital ridge): a common mesodermal ridge along
the posterior wall of the abdominal cavity

(b) initially the excretory ducts of both systems

enter a common cavity, the cloaca.

Kidney Systems

 Three slightly overlapping kidney systems are formed in a

cranial to caudal sequence during intrauterine life in humans:

(a) pronephros: rudimentary and nonfunctional
(b) mesonephros : may function for a short time during the early
fetal period
(c) metanephros: forms the permanent kidney.

Pronephros

• At the beginning of the 4

week, the

pronephros is represented by 7 to 10

solid cell groups in the cervical region,

nephrotomes, that regress before

more caudal ones are formed.

• The pronephric ducts run caudally and

open into the cloaca

• By the end of the 4

week, the

pronephroi soon degenerate; however,

most of the length of the pronephric

ducts persists and is used by the next

set of kidneys.

Mesonephros

• The mesonephros and

mesonephric ducts are derived
from intermediate mesoderm
from upper thoracic to upper
lumbar (L3) segments.

Mesonephros –Urogenital ridge

• In the middle of the second month

the mesonephros forms a large
ovoid organ on each side of the
midline.

• Since the developing gonad is on

its medial side, the ridge formed
by both organs is known as the
Urogenital ridge.

Mesonephros

• During regression of the

pronephric system, the first
excretory tubules of the
mesonephros appear. They
lengthen rapidly, form an S-
shaped loop, and acquire a tuft
of capillaries that will form a
glomerulus at their medial
extremity.

• Around the glomerulus the

tubules form Bowman's capsule,
and together these structures
constitute a renal corpuscle.

Mesonephrons

• Laterally, the tubule enters the

longitudinal collecting duct
known as the mesonephric or
wolffian duct.

• By the end of the second month,

the majority of excretory tubules
have disappeared.

• In the male, a few of the caudal

tubules and the mesonephric
duct persist and participate in
formation of the conduit for
sperm from the testes to the
urethra. In the female, these
ducts regress.

Metanephros

• appears in the 5

week.

• develops from the lower lumber

and sacral portions of the
nephrogenic cord

• Its excretory units develop in the

same manner as in the
mesonephric system.

• The development of the duct

system differs from that of the
other kidney systems.

Collecting System

• Collecting ducts of the

permanent kidney develop

from the ureteric bud ( an

outgrowth of the mesonephric

duct close to its entrance to

the cloaca ).

• The bud penetrates the

metanephric tissue, which is

molded over its distal end as a

cap.

• Subsequently the bud dilates,

forming the primitive renal

pelvis, and splits into cranial

and caudal portions, the

future major calyces

• Each calyx forms two new buds which continue to subdivide until

12 or more generations of tubules have formed.

• The tubules of the second order enlarge and absorb those of the

third and fourth generations, forming the minor calyces of the renal

pelvis.

• During further development, collecting tubules of the fifth and

successive generations elongate considerably and converge on the

minor calyx, forming the renal pyramid

• The ureteric bud gives rise to the ureter, the renal pelvis, the major

and minor calyces, and approximately 1 to 3 million collecting

tubules

Excretory System

• Each newly formed

collecting tubule is
covered at its distal end by
a metanephric tissue cap .

• Under the inductive

influence of the tubule,
cells of the tissue cap form
small vesicles, the renal
vesicles, which in turn give
rise to small S-shaped
tubules.

Excretory System

• Capillaries grow into the

pocket at one end of the S
and differentiate into
glomeruli.

• The proximal end of each

nephron forms Bowman's
capsule, which is deeply
indented by a glomerulus.

Excretory System

• The distal end forms an open connection with one of the

collecting tubules.

• Continuous lengthening of the excretory tubule results in

formation of the proximal convoluted tubule, loop of
Henle, and distal convoluted tubule

• Hence, the kidney develops from two sources:

Metanephric mesoderm, which provides

excretory units

Ureteric bud, which gives rise to the collecting

system.

• There are approximately 1

million nephrons in each
kidney. They are formed until
birth.

• Urine production begins soon

after differentiation of the
glomerular capillaries, which
start to form by the 10th week.
The definitive kidney becomes
functional near the 12th week.

• During fetal life, the kidneys are

not responsible for excretion of
waste products, since the
placenta serves this function.

• At birth, the kidneys have a

lobulated appearance, but
the lobulation disappears
during infancy as a result of
further growth of the
nephrons, although there is
no increase in their number.

Renal agenesis

may arise if the interaction between the metanephric mesoderm and
the ureteric bud fails to occur

• Unilateral renal agenesis

often causes no symptoms

and is usually not discovered

during infancy because the

other kidney usually

undergoes compensatory

hypertrophy and performs

the function of the missing

kidney.

• Unilateral renal agenesis

should be suspected in

infants with a single

umbilical artery

Bilateral renal agenesis

•

is associated with

oligohydramnios (small

amount of amniotic

fluid) because little or no

urine is excreted into the

amniotic cavity.

• Most infants with

bilateral renal agenesis

die shortly after birth or

during the first months

of life

Congenital polycystic kidney disease

 numerous cysts form.
 Autosomal recessive polycystic

kidney disease is a progressive

disorder in which cysts form

from collecting ducts. The

kidneys become very large, and

renal failure occurs in infancy or

childhood.

 Autosomal dominant polycystic

kidney disease ,cysts form from

all segments of the nephron and

usually do not cause renal failure

until adulthood. The autosomal

dominant disease is more

common but less progressive

than the autosomal recessive

disease .

(A) Surface view of a fetal kidney with multiple cysts

(B) Section of the kidney in A, showing multiple cysts.

Duplication of the ureter

 results from:

• early splitting of the ureteric

bud . Splitting may be partial or

complete, and metanephric

tissue may be divided into two

parts, each with its own renal

pelvis and ureter.

• In rare cases, one ureter opens

into the bladder, and the other

is ectopic, entering the vagina,

urethra, or vestibule .This

abnormality results from

development of two ureteric

buds. One of the buds usually

has a normal position, whereas

the abnormal bud moves down

together with the mesonephric

duct. Thus it has a low, abnormal

entrance in the bladder, urethra,

vagina, or epididymal region.

Position of the Kidney

• The kidney,

initially in the pelvic region

, later shifts to a more

cranial position in the abdomen.

• This ascent of the kidney is caused by:
1. diminution of body curvature
2. growth of the body in the lumbar and sacral regions

• The position of the kidneys becomes fixed once they come

into contact with the suprarenal glands in the 9

week

where they attain their adult position

• In the pelvis, the metanephros receives its arterial supply from a pelvic branch

of the aorta.

• During its ascent to the abdominal level, it is vascularized by arteries that

originate from the aorta at continuously higher levels.

• The lower vessels usually degenerate, but some may remain.
• Initially the hilum of the kidney faces ventrally; however, as the kidney

relocates (ascends), it rotates medially almost 90 degrees. By the 9

week, the

hilum is directed anteromedially.

Abnormal Location of the Kidneys

• During their ascent, the kidneys

pass through the arterial fork

formed by the umbilical arteries,

but occasionally one of them fails

to do so. Remaining in the pelvis

close to the common iliac artery,

it is known as a pelvic kidney

• Sometimes the kidneys are

pushed so close together during

their passage through the arterial

fork that the lower poles fuse,

forming a horseshoe kidney . The

horseshoe kidney is usually at the

level of the lower lumbar

vertebrae, since its ascent is

prevented by the root of the

inferior mesenteric artery

Accessory renal arteries

• are common
• they derive from the persistence of embryonic

vessels that formed during ascent of the
kidneys.

• These arteries usually arise from the aorta and

enter the superior or inferior poles of the
kidneys

Common variations of renal vessels

• A and B, Multiple renal arteries.
• Note the accessory vessels entering the poles of the kidney.
• The polar renal artery, illustrated in B, has obstructed the ureter

and produced an enlarged renal pelvis.

Bladder and Urethra

• During the 4

-7

week of development, the Cloaca

divides into the urogenital sinus anteriorly and the anal
canal posteriorly .

• The Urorectal septum is a layer of mesoderm between

the primitive anal canal and the urogenital sinus. The tip
of the septum will form the perineal body .

Three portions of the urogenital sinus can be distinguished

 the upper part ( the urinary bladder)
 Is the largest part.
 Initially, the bladder is continuous with the allantois
 when the lumen of the allantois is obliterated, a thick fibrous

cord, the Urachus, remains and connects the apex of the

bladder with the umbilicus . In the adult, it is known as the

median umbilical ligament.

The next part is

the pelvic part of the urogenital sinus
 is a rather narrow canal
 in the male , it gives rise to the prostatic and

membranous parts of the urethra

The last part is

the phallic part of the

urogenital sinus

 It is flattened from side to

side, and as the genital
tubercle grows, this part of
the sinus will be pulled
ventrally .

 Development of the phallic

part of the urogenital sinus
differs greatly between the
two sexes.)

 During differentiation of the cloaca, the caudal portions of the

mesonephric ducts are absorbed into the wall of the urinary
bladder

 As a result of ascent of the kidneys,
 the orifices of the ureters move farther cranially;
 those of the mesonephric ducts move close together to enter

the prostatic urethra and in the male become the ejaculatory
ducts

Trigone of the bladder

• Since both the mesonephric ducts and ureters originate in

the mesoderm, the mucosa of the bladder formed by
incorporation of the ducts (the trigone of the bladder) is also
mesodermal.

• With time, the mesodermal lining of the trigone is replaced

by endodermal epithelium, so that finally the inside of the
bladder is completely lined with endodermal epithelium

• In infants and children, the urinary bladder,

even when empty, is in the abdomen.

• It begins to enter the greater pelvis at

approximately 6 years of age, but it does not
enter the lesser pelvis and become a pelvic
organ until after puberty.

Urethra

• The epithelium of the urethra in both sexes originates in the

endoderm;

• the surrounding connective and smooth muscle tissue is derived

from splanchnic mesoderm.

• At the end of the third month, epithelium of the prostatic urethra

begins to proliferate and forms a number of outgrowths that

penetrate the surrounding mesenchyme.

• In the male, these buds form the prostate gland.
• In the female, the cranial part of the urethra gives rise to the

urethral and para-urethral glands.

Bladder Defects

 A: Urachal fistula occurs when the lumen of the

intraembryonic portion of the allantois persists . It may cause

urine to drain from the umbilicus

 B:Urachal cyst If only a local area of the allantois persists,

secretory activity of its lining results in a cystic dilation

 C: Urachal sinus occurs when the lumen in the upper part

persists

Exstrophy of the bladder

• is a ventral body wall defect in which the bladder mucosa is

exposed.

• Epispadias is a constant feature
• Exstrophy of the bladder may be caused by a lack of

mesodermal migration into the region between the umbilicus
and genital tubercle, followed by rupture of the thin layer of
ectoderm.

Summary

 Embryologically and anatomically, both urinary and genital systems

are intimately interwoven. Both systems are derived from

intermediate mesoderm and their excretory ducts enter the

cloaca.

 Three kinds of kidney systems appear during intrauterine life
 pronephros: rudimentary and nonfunctional
 mesonephros : may function for a short time during the early fetal

period

 metanephros: forms the permanent kidney.
 the kidney develops from two sources:
 Metanephric mesoderm, which provides excretory units
 Ureteric bud, which gives rise to the collecting system.
 The upper part of urogenital sinus distinguished into urinary

bladder

 The pelvic part of urogenital sinus gives rise to the prostatic and

membranous parts of the urethra.

 The phallic part of the urogenital sinus differs greatly between the

two sexes

Thank you
Next Lecture: Genital system