Development of Midgut loop:

1
Development of Midgut loop

• Derivatives of midgut loop are
  1-Small intestine, including
  most of duodenum.
  2-Cecum appendix.
  3-Ascending colon.
  4-Right 2/3 of
  transverse colon.
• All these derivatives are supplied by superior
  mesenteric artey (artery of midgut)

2
Development of midgut loop

• At the biginning of 6th week, the midgut
  elongates to form a venteral U-shaped midgut
  loop projecting into extra-embryonic coelom of
  proximal part of umbilical cord this called
  physiological umbilical herniation.
• At this stage the arrow indicates communication
  of intraembryonic coelom (peritoneal cavity) with
  extraembryonic coelom.
• At this stage ,the abdominal cavity is
  temporarily too small in comparison to
  relatively massive liver kidnes to contain the
  developing rapidly growing intestines.

3
Development of midgut loop

• The midgut loop joined with Yolk sac through yolk
  stalk or vitelline duct until 10th week.
• Midgut loop consists of 2 limbs, cranial limb
  caudal limb. (A)
• Cranial limb grows rapidly forming the small
  intestines.(B)
• Caudal limb gives rise to cecal diverticulum, the
  primordium of cecum appendix. ( C )

4
Rotation of midgut loop

• While midgut loop is in the umbilical cord, it
  rotates 90 degrees counterclockwise around axis
  of superior mesenteric artery. (A),(A1)
• This brings the cranial limb to the right and
  caudal limb to left. (B), (B1).

5
Return of midgut to abdomen
(reduction of physiological hernia)

• During 10th week the small intestine (formed of
  cranial limb) returns first to abdomen due to
  enlagement of abdominal cavity .this is called
  reduction of physio-logical midgut hernia.(CD)
• large intestine (formed from caudal limb)
  undergoes a further 180 degree counterclockwise
  rotation to occupy right side of abdomen.
  (D),(D1)
• Ascending colon becomes recognizable as posterior
  abdominal wall elongates. (E)

6
Fixation of various parts of intestines

• At first the dorsal mesentry is in the median
  plane. As the intestines lenghthen,and obtain
  their final position, some mesentries are fused
  with posterior abdominal wall.
• Mesentry of ascending descending colon
  disappears, so ascending descending colon
  becomes fixed to posterior abdominal wall and
  they become retro-peritoneal. (B)
• Transverse mesocolon persists and fuses with
  posterior wall of greater omentum and maintains
  its mobility. (C)

• A,Venteral view of the intestine Prior to
• Fixation.
  
• B,T.S showing areas of fusion.
  
• C, Sagittal S. showing greater omentum over-
  
• hanging the transverse colon, arrows indicate
  
• areas of subsequent fusion.

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Fixation of various parts of intestines

• With rotation of stomach, duodenum pancreas are
  pressed against posterior abdominal wall by the
  colon.
• Adjacent layers of peritoneum fuse and disappear,
  so most of duodenum pancreas become
  retroperitoneal.

Intestines prior to fixation

• E, T.S after disappearance of mesentry of
  ascending descending colon.
• F, sagittal S.showing fusion of layers of
  greater omentum and fusion of greater omentum
  with mesentery of transverse colon.

Intestines after fixation
8
Development of Cecum Appendix

• At 6th week, cecal diverticulum appears as a
  swelling on the anti-mesenteric border of caudal
  limb of midgut loop. (A)
• Cecal diverticulum gives rise to cecum
  appendix.(B)
• Appendix is intially a small diverticulum of the
  cecum, arising from distal end of cecum.(B)
• As ascending colon elongates, appendix elongates
  and may be retrocecal or retrocolic or pelvic
  appendix. In 64 of people, it is retrocecally.

• A,6 weeks . B, 8 weeks. C,12 weeks
  
• D,at birth appendix is long and is continuous
  with apex of cecum. E, After birth
  (adult appendix) It is short as a
  result that the appendix enters medial side of
  cecum.

9
Congenital Omphalocele

• It is a persistence of herniation of abdominal
  contents into proximal part of umbilical cord
  due to failure of reduction of physiological
  hernia to abdominal cavity at 10th week.
• Herniation of intestines occurs in 1 of 5000
  births herniation of liver intestines occurs
  in 1 of 10,000 births.
• It is accompanied by small abdominal cavity.
• The hernial sac is covered by the epithelium of
  the umbilical cord, the amnion.
• Immediate surgical repair is required.

10
Umbilical Hernia

• The intestines return to abdominal cavity at 10th
  week, but herniate through an imperfectly closed
  umbilicus
• It is a common type of hernia.
• The herniated contents are usually the greater
  omentum small intestine.
• The hernial sac is covered by subcutaneous tissue
  skin.
• It protrudes during crying,straining or coughing
  and can be easily reduced through fibrous ring at
  umbilicus.
• Surgery is performed at age of 3-5 years.

11
Ileal (Meckel) Diverticulum

• It is one of the most common anomalies of the
  digestive tract, present in about 2 of people.
• It is a small pouch from the ileum, and may
  contain small patches of gastric pancreatic
  tissues.The gastric mucosa often secretes acid
  producing ulceration bleeding.
• It is the remnant of proximal part
  nonobliterated part) of yolk stalk (or vitelline
  duct).
• It arises from antimesenteric border of ileum,1/2
  meter from ileocecal junction.
• It is more common in males.
• It is sometimes becomes inflamed and causes
  symptoms that mimic appendicitis.
• It may be connected to the umbilucus by a
  fibrous cord or Omphalo-enteric fistula
  (vitello-intestinal duct remains
  patent and faecal matter is carried through the
  duct into umbilicus).

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Hindgut

• Derivatives of hindgut are
  
• 1-left 1/3 of transverse colon.
  2-Descending colon
  sigmoid colon.
• Part of hindgut dilate to form Cloaca which gives
  rise to
  
  3-Rectum.
  
  4-Superior part of anal canal.
  
  5-Certain urogenital structures
  (epithelium of urinary bladder most of
  urethra)
• Inferior mesenteric artery is the artery of
  hindgut.

13

• Partitioning of cloaca into rectum and
  uro-genital sinus by urorectal septum.
• A,C,E cloaca at 4,6 and 7 weeks.
• B,D,F cloacal region.note degeneration and
  disappearance of the postanal or tailgut (B) as
  the rectum forms from dorsal part of cloaca.
• B1,D1,F1, T.S.of cloaca.

14
Development of Rectum Anal canal

• Cloaca, the expanded terminal part of hindgut,
  receives allantois ventrally.
• cloacal membrane, is composed of endoderm of
  cloaca ectoderm of proctodeum (or anal pit).
• Cloaca is divided into dorsal ventral parts
  by a mesenchyme-urorectal septum- between
  allantois and hindgut, producing infoldings of
  lateral walls of cloaca.

• These folds fuse forming a partition that divides
  cloaca into
• 1-Rectum cranial (superior) part of anal canal,
  dorsally.
  2-Urogenital sinus, ventrally.

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Development of Rectum Anal Canal

• At 7th week, urorectal septum fused with cloacal
  membrane, dividing it into
  1-dorsal anal membrane.
  2-ventral urogenital membrane.
• Area of fusion of urorectal septum with cloacal
  membrane gives rise to perineal body.
• Urorectal septum also divides cloacal sphincter
  into anterior posterior parts. The posterior
  part becomes external anal sphincter/ and
  anterior part develops into superficial
  transverse perineal,bulbospongiosus and
  ischiocavernosus muscles that supplied by one
  nerve,the pudendal nerve.

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Development of Rectum Anal Canal

• Mesenchymal proliferations produce elevations of
  the surface ectoderm around anal membrane. So the
  anal membrane is located at the bottom of an
  ectodermal depression-the proctodeum (or anal pit
  ).
• Anal membrane usually ruptures at the end of 8th
  week, bringing distal part of anal canal into
  communication of amniotic cavity.

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Development of Anal Canal

• Superior 2/3 of anal canal derived from hindgut,
  whereas inferior 1/3 of anal canal is derived
  from proctodeum.
• Pectinate line -located inferiorly to anal valves
  is the junction of epithelium derived from
  ectoderm of proctodeum endoderm of hindgut. It
  is the former site of anal membrane.
• White line / or anocutaneous line, 2 cm superior
  to anus,where the anal epithelium changes from
  columnar to stratified squamous cells.
• At anus, epithelium is keratinized and continuous
  with skin around anus. Wall of anal canal is
  derived from splanchnic mesenchyme.

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Development of Anal Canal

• Because of different embryological origin of
  anal canal, superior and inferior parts of anal
  canal are supplied by different arteries ,nerves
  and have different venous and lymphatic drainage.
  This is important clinically in spread of cancer
  cells.
• Superior rectal artery (continuation of inferior
  mesenteric,artery of hindgut / inferior rectal
  artery superior rectal vein/ inferior rectal
  vein- inferior mesenteric L.N/
  superficial inguinal L.N.
• Tumors in superior part of anal canal are
  painless (supplied by autonomic N.S) and arise
  from columnar epithelium, / whereas those of
  inferior part are painful (supplied by inferior
  rectal N.) and arise from stratified squamous
  epithelium.

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Congenital Megacolon (hirschsprung disease)

• It is a dilated segment of the colon, due to
  failure of development of parasympathetic
  ganglion cells distal to the dilated part but the
  dilated part has a normal ganglion cells.
• The dilatation results from failure of neural
  crest cells to migrate into wall of colon during
  5-7th week, leading to loss of
  peristalsis in the aganglionic segment of colon,
  so no movement of intestinal contents.
• Mostly,only the rectum sigmoid colon are
  involved.
• It is the most common cause of neonatal
  obstruction of colon

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Imperforate Anus (membranous anal atresia)

• A thin membrane separates the anal canal from the
  exterior. this membrane is thin enough to bulge
  on straining and appears blue due to presence of
  meconium above it.
• It results from failure of anal membrane to
  perforate at the end of 8th week.
• The anus is at normal position.
• It is more common in males.

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22
Development of Urogenital System

• It develops from intermediate mesoderm. A,B
• During folding, this mesoderm comes venterally.
  C,D
• A Urogenital ridge (longitudinal elevation of
  mesoderm) forms the urinary genital systems.
  D,F
• Nephrogenic cord giving rise to urinary system,
  and the gonadal ridge gives rise to genital
  system.
• In adult males , these 2 systems are closely
  associated anatomically e.g.urethra conveys both
  urine semen.
• In adult females, urethra vagina open into same
  space- the vestibule between labia minora.

• A,dorsal view of 3rd week embryo. B,T.S, C,
  lateral view of 4th week embryo. D,T.S during
  folding, showing nephrogenic cord of mesoderm.
  E,lateral view during 4th week. F,T.S showing
  meeting of lateral folds ventrally, notice
  position of urogenital ridges nephrogenic cords.

23
Development of Kidneys

• The 1st set of kidneys- the pronephroi- are
  rudimentary and non-functional.
• The 2nd set of kidneys the mesonephroi are
  well developed and function for a short time
  during the early fetal life.
• The 3rd set the metanephroi- become the
  permanent kidneys.
• Pronephroi
  -transitory , nonfunctional strucutres
  appear early in 4thweek.
  found in cervical region.
  -it consists of pronephric tubules
  ducts which open into cloaca.
  -The rudimentary
  pronephroi degenerates, but most of pronephric
  ducts persist to utilize by next set of kidneys.

• 3-sets of excretory systems in an embryo of 5th
  week.
• A,lateral view. B, ventral view.

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Development of Kidneys and Ureters

• Mesonephroi
  -They appear late in 4th week they are
  caudal to rudimentary pronephroi.
  -Found in thoracic upper
  lumbar regions. -They
  consist of mesonephric ducts mesonephric
  tubules,that acquire a tuft of capillaries to
  form a glomerulus.
  -They are well developed and function
  during the embryonic early fetal life.until
  permanent kidneys develop.

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• A, lateral view of 5th week embryo showing early
  mesonephros appearance of metanephric
  diverticulum (ureteric bud), the primordium of
  metanephros (permanent kidney).
• B, T.S showing nephrogenic cords, which give rise
  mesonephric tubules.
• C to F, T.S showing stages of development of
  mesonephric tubules between 5th and 11th week.
• Note that mesenchymal cell cluster in
  nephrogenic cord develops a lumen to form
  mesonephric vesicle. The vesicle becomes S-shaped
  mesonephric tubule and joins the mesonephric
  duct.
• Medial end of tubule is invaginated by blood
  vessels to form glomerular capsule (Bowman
  capsule) and capillaries projecting into this
  capsule is the glomerulus.

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• Mesonephroi -The
  mesonephric kidneys consist of glomeruli
  mesonephric tubules.
  -The tubules open into mesonephric ducts, which
  were originally the pronephric ducts and open
  into cloaca.
  -Mesonephroi degenerate by end of 1st trimester.
  -The
  mesonephric tubules In female gives
  rise epoophoron paroophoron, while, in male
  become efferent ductules of testes.
  -The mesonephric
  ducts In females degenerates
  leaving duct of epoophoron Garteners, while in
  males become duct of epididymis ductus deferens.

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Development of permanent Kidneys and Ureters

• Metanephroi -are the
  primordia of permanent kidneys, which develop
  early in 5th week from mesoderm.
  -They develop from
  1-metanephric diverticulum (ureteric bud) ,it is
  an outgrowth of mesonephric duct near cloaca. (A)
  
  2-metanephric mass of intermediate mesoderm
  (metanephric blastema), is derived from caudal
  end of nephrogenic cord.

A,lateral view of 5-week embryo,showing the
primordium of metanephros. B to E, showing
development of metanephric diverticulum (5th to
8th week) and its derivatives.
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Development of Permanent Kidneys and Ureters

• Metanephric diverticulum
  -is the primordium of ureter, renal
  pelvis, calices, collecting tubules. C to E
• It penetrates metanephric mass, forming a cap
  over its upper end.
• Upper end of ureteric bud enlarges to form pelvis
  of ureter, from which major calyces develop.
• Each major calyx undergoes repeated branching.
• 3rd 4th generations of ducts give rise to minor
  calyces, while other generations form collecting
  tubules.

A,lateral view of 5-week embryo,showing the
primordium of metanephros. B to E, showing
development of metanephric diverticulum (5th to
8th week) and its derivatives.
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Stages of Nephrogenesis and
differentiation of nephrons

• A, Nephrogenesis commences around the beginning
  of 8th week.
• BC, the metanephric tubules, the primordia of
  nephrons, become continuous with the arched
  collecting tubules to form uriniferous tubules
  (excretory units).
• D, formation of more nephrons. Note that a
  uriniferous tubule consists of 2 parts
  1-nephrons (metanephric tubules) are
  derived from metanephric mass of mesoderm, 2-
  that of collecting tubules are derived from
  metanephric diverticulum (ureteric bud).

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Stages of Nephrogenesis (development of
nephrons)

• The end of each arched collecting tubule induces
  clusters of mesenchymal cells in the metanephric
  mass of mesoderm to form metanephric vesicles.
  A
• These vesicles elongate to become metanephric
  tubules. B C
• Proximal ends of these tubules are invaginated by
  glomeruli. D
• Renal corpuscles (glomerulusglomerular capsule)
  proximal convoluted tubule loop of Henle distal
  convoluted tubule constitute a Nephron. D

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Fetal Kidneys

• Fetal kidneys are subdivided into lobes and still
  indicated in the kidneys of a newborn infant.
• External lobulation of kidneys usually disappears
  by the end of 1st postnatal year.
• The nephron formation is complete at birth,
  except in premature infants.
• Functional maturation of kidneys occurs after
  birth. Glomerular filtration begins during 9th
  fetal week and the rate of filteration increases
  after birth.

• Photograph of kidneys suprarenal glands of
  28-week fetus. Note the large size of suprarenal
  glands.
• 2 weeks after birth, suprarenal glands reduce to
  half this size.

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Positional changes of kidneys

• A to D, 6 to 9th week, diaphrammatic ventral
  views of abdominopelvic region of embryo fetus
  showing medial rotation and ascent of kidneys
  from pelvis to abdomen.
• A B, showing the kidneys in the pelvis.
• C D, showing ascending of kidneys and changing
  of their blood supply and rotation of the hilum
  is directed anteromedially.
• D, At 9th week, kidneys come in contact with
  suprarenal glands in abdomen and stop their
  ascend, so they receive permanent renal arteries
  from aorta.

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Positional changes of kidneys

• Initially the metanephric kidneys (primordial
  permanent kidneys) lie close to each other in the
  pelvis. A
• The kidneys gradually come to lie in abdomen and
  move farther apart. B,C
• The kidneys attain their adult position by 9th
  week. D
• Initially hilum of kidney faces ventrally, by
  9th week, it is directed anteromedially. D
• Initially, renal arteries are branches of common
  iliac arteries in the pelvis. AB
• When the kidneys ascend in abdomen, they receive
  renal arteries from distal end of aorta then at
  higher level they receive new branches from
  aorta. CD

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Cystic kidney disease

• Polycystic kidney disease
  -is an autosomal recessive disorder (ARPKD)
  that is diagnosed at birth or in utero by
  ultrasonography.
  -It is bilaterally ,both
  kidneys contain many small cysts, which result in
  renal insufficiency.
  -Death of infant
  usually occurs shortly after birth.
• Multicystic dysplastic kidney disease (MDK)
  -results
  from dysmorphology during development of renal
  system, due to failure of metanephric
  diverticulum (forming collecting tubules) to join
  metanephric mesoderm (forming metanephric
  tubules)
  -It is now
  believed that cystic structures are wide
  dilatation of parts of the nephron loops (of
  Henle) -it
  is unilateral in 75 of cases, it contains fewer
  cysts of different size in the same kidney, and
  the outcome for children is generally good.

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Development of Urinary bladder

• Diagrams showing division of cloaca into
  urogenital sinus rectum, absorption of
  mesonephric ducts, development of urinary
  bladder, urethra, and urachus and changes in
  location of ureters. A,C,E,G,and H, lateral
  views. B,D,and F,dorsal views. A,B, 5th week.
  G,H, 12th week.

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Development of Urinary bladder

• A, the urorectal septum divides the cloaca into
  dorsal rectum and ventral
  urogenital sinus.
• C, the urogenital sinus is divided into 3 parts
  
  1-a cranial vesical part forms most of U.B.
  and is continuous with allantois..
  2-a middle pelvic part forms the urethra in
  bladder neck and prostatic urethra in males and
  entire urethra in females.
  3-a
  caudal phallic part that grows toward the
  genital tubercle.

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Development of Urinary bladder

• The bladder develops mainly from vesical part of
  urogenital sinus, but its trigon C.T is derived
  from caudal (distal) parts of mesonephric ducts.
• Epithelium of entire bladder is derived from
  endoderm of vesical part of urogenital sinus.
  The other layers of bladder wall develop from
  adjacent splanchnic mesenchyme
• Intially the bladder is continuous with
  allantois, then allantois constricts and becomes
  a thick fibrous cord, the urachus, which extends
  from apex of bladder to umbilicus and it is
  represented in adult by median umbilical
  ligament, which lies between the medial umbilical
  ligaments that are the fibrous remnants of
  umbilical arteries.

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Development of Urinary bladder

• As the mesonephric ducts are absorbed, the
  ureters open separately into the bladder (C to
  H).
• The orifices of mesonephric ducts move close
  together and enter the prostatic part of urethra,
  while caudal (distal) ends of mesonephric ducts
  degenerate in female and form the ejaculatory
  ducts seminal glands in males.
• In infant and children, the bladder even when
  empty, is in the abdomen. It begins to enter
  greater pelvis at 6th years of age and it becomes
  pelvic organ as enters lesser pelvis after
  puberty.

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Urachal anomalies

• A, urachal cyst results from the remnant of
  epithelial lining of urachus which persist in the
  superior end of urachus just inferior to
  umbilicus-the most common site. It may be small
  in 1/3 of cadavers or become infected and
  enlarged in living persons.
• B, urachal sinus, 2 types, one opens into
  bladder, the other opens at umbilicus and
  discharge serous fluid.
• C, very rarely the entire urachus remains patent
  and forms a urachal fistula that allows urine to
  escape from the umbilicus.

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Development of Urethra

• Epithelium of most of male urethra and entire
  female urethra is derived from endoderm of
  urogenital sinus.
• The epithelium of distal part of urethra in glans
  penis in male is derived from the surface
  ectoderm.
• The C.T and smooth muscle of urethra in both
  sexes are derived from splanchnic mesenchyme.

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Development of suprarenal gland

• A, 6 weeks, showing the mesodermal primordium of
  fetal cortex.
• B, 7 weeks, showing the addition of neural crest
  cells to form the medulla.
• C, 8 weeks, showing fetal cortex early
  permanent cortex beginning to encapsulate the
  medulla.
• D E, later stages of encapsulation of medulla
  by cortex.
• F, at birth, newborn infant, showing fetal cortex
  the 2 zones of permanent cortex zona
  glomerulosa zona fasciculata.
• G, 1 year, disappearance of fetal cortex and
  smaller size of the gland than at birth.
• H, 4 years, showing adult pattern of cortical
  zones -3 zones. Note that fetal cortex has
  disappeared and the gland is smaller than at
  birth.

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Development of suprarenal gland

• The suprarenal cortex develops from the mesoderm
  during 6th week by aggregation of mesenchymal
  cells on each side, between root of dorsal
  mesentry developing gonad.
• the cells of Fetal cortex are derived from the
  mesothelium lining the posterior abdominal wall.
  A
• The cells of medulla are derived from an
  adjacent sympathetic ganglion, which is derived
  from neural crest cells. Initially, these
  neural crest cells lie on the medial side of
  fetal cortex, then they are surrounded by fetal
  cortex and differentiated into secretory cells of
  medulla. B,C,D

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Development of suprarenal gland

• Later, mesenchymal cells from the mesothelium
  enclose the fetal cortex and give rise to
  permanent cortex. E,F,G
• Differentiation of cortical zones begins during
  the late fetal period. Z.glomerulosa Z.
  fasciculata are present at birth F ,
  but
  Z.reticularis is not recognizable untile end of
  3rd year. H
• Suprarenal glands of fetus, are larger than adult
  glands due to extensive size of fetal cortex,
  D,E, F. As the fetal cortex regresses during
  1st year, the suprarenal
  glands rapidly become smaller. G,H

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