1
The greater omentum is derived from which of the following embryonic structures?
A. Dorsal mesoduodenum
B. Dorsal mesogastrium
C. Pericardioperitoneal canal
D. Pleuropericardial membranes
E. Ventral mesentery
Explanation:
The correct answer is B. Both the omental bursa and the greater omentum are derived from the dorsal
mesogastrium, which is the mesentery of the stomach region.
The dorsal mesoduodenum (choice A) is the mesentery of the developing duodenum, which later
disappears so that the duodenum and pancreas come to lie retroperitoneally.
The pericardioperitoneal canal (choice C) embryologically connects the thoracic and peritoneal canals.
The pleuropericardial membranes (choice D) become the pericardium and contribute to the diaphragm.
The ventral mesentery (choice E) forms the falciform ligament, ligamentum teres, and lesser omentum.
A PSA level is drawn from a 54-year-old man as part of a routine health evaluation. Which of the
following embryonic structures gives rise to the organ being screened for carcinoma?
A. Genital tubercle
B. Processus vaginalis
C. Testis cords
D. Tunica albuginea
E. Urogenital sinus
Explanation:
The correct answer is E. It is recommended that PSA levels (prostate-specific antigen) be
measured annually in men over age 50 to screen for prostatic carcinoma (and to record a
baseline level). The prostate is immediately derived from the prostatic urethra, which is
derived from the urogenital sinus.
The genital tubercle (choice A) gives rise to the glans penis.
The processus vaginalis (choice B) is a coelomic extension into the scrotal swelling that
carries with it extensions of the body wall to form the inguinal canal during the descent of the testes.
The testis cords (choice C) are composed of primitive germ cells, which give rise to
spermatogonia, and sex cord cells, which differentiate into Sertoli cells.
The tunica albuginea (choice D) is derived from mesenchyme and condenses to form the fibrous
connective tissue capsule of the testis.
A newborn baby has projectile vomiting shortly after each feeding. It is determined that there is
obstruction of the digestive tract as a result of an annular pancreas. Annular pancreas is a
result of an abnormality in which of the following processes?
A. Rotation of the dorsal pancreatic bud around the first part of the duodenum
B. Rotation of the dorsal pancreatic bud around the second part of the duodenum
C. Rotation of the dorsal pancreatic bud around the third part of the duodenum
D. Rotation of the ventral pancreatic bud around the first part of the duodenum
E. Rotation of the ventral pancreatic bud around the second part of the duodenum
Explanation:
The correct answer is E. The ventral pancreatic bud normally rotates around the duodenum to
fuse with the dorsal pancreatic bud. Both pancreatic buds form from evaginations from the
second part of the duodenum, hence the rotation is around the second part of the duodenum. The
normal rotation is around the right side of the embryonic duodenum. Annular pancreas results
from the ventral pancreatic bud dividing and rotating around both the right and left sides of
the second part of the duodenum, thus encircling it.
The dorsal pancreatic bud (choices A, B, and C) does not rotate around the duodenum and
therefore is not the cause of annular pancreas.
The ventral pancreatic bud does not form from the first part of the duodenum (choice D) and
therefore does not rotate around this part of the duodenum.
A newborn infant has some of its abdominal viscera protruding through a defect in the abdominal wall.
Which of the following is the likely cause of this defect?
A. Failure of the intestinal loop to retract from the umbilical cord
B. Failure of the yolk stalk to degenerate
C. Failure of peritoneal fusion
D. Incomplete fusion of the lateral body folds
E. Umbilical herniation
Explanation:
The correct answer is D. During the fourth week of development, the lateral body folds move
ventrally and fuse in the midline to form the anterior body wall. Incomplete fusion results in
a defect that allows abdominal viscera to protrude from the abdominal cavity, a condition known
as gastroschisis.
During development, the midgut normally herniates into the umbilical cord and then subsequently
retracts into the abdominal cavity. Failure of the intestinal loop to retract from the
umbilical cord (choice A) results in omphalocele.
Failure of the yolk stalk to degenerate (choice B) results in an ileal (Meckel's) diverticulum
or a vitelline fistula or cyst. In the early embryo, the gut tube is connected to the yolk sac
by a narrow connection known as the yolk stalk. Normally, this connection degenerates.
During development, certain peritoneal organs fuse with the posterior abdominal wall to become
secondarily retroperitoneal. Failure of this peritoneal fusion (choice C) will result in
certain organs that are normally immobile being mobile (e.g., mobile cecum).
Umbilical herniation (choice E) results from abdominal viscera protruding through a weakness in
the abdominal wall after development. Such protrusions are covered by subcutaneous fascia and
skin, distinguishing them from gastroschisis.
Autopsy of a baby who died minutes after birth demonstrates intestines within the thoracic cavity. Which
of the following most probably accounts for these findings?
A. Heterotopia
B. Hypertrophic pyloric stenosis
C. Large sliding hernia
D. Maldevelopment of the lungs
E. Partial absence of the diaphragm
Explanation:
The correct answer is E. In some infants, there is a weakness or partial-to-total absence of
the diaphragm (often on the left) which, if sufficiently large, can permit herniation of
abdominal contents into the thorax. Small defects are usually surgically correctable, but large
ones may cause rapidly lethal respiratory embarrassment in the newborn.
Heterotopia (choice A) is the presence of small amounts of normal tissue in abnormal sites.
Congenital hypertrophic pyloric stenosis (choice B) can cause persistent vomiting in infancy,
but does not cause herniation of intestines into the thorax.
Death in infancy due to intestinal herniation into the thorax is not related to sliding hernia
(choice C).
Maldevelopment of the lungs (choice D) is a result rather than the cause of this baby's problems.
During development, the formation of the kidney is induced by which of the following structures?
A. Allantois
B. Mesonephric duct
C. Mesonephros
D. Metanephric duct
E. Urogenital ridge
Explanation:
The correct answer is D. The metanephric duct (also known as the ureteric bud) is a
diverticulum of the mesonephric duct. It grows to the metanephric mass of the urogenital ridge.
It induces the development of the metanephros, which will give rise to the excretory units of
the definitive kidney. The metanephros, in turn, induces the metanephric duct to divide into
the calyces and the collecting tubules.
The allantois (choice A) is an endodermal diverticulum of the yolk sac. It becomes incorporated
into the urogenital sinus and contributes to the formation of the urinary bladder. It is not
involved with the formation of the kidney.
The mesonephric duct (choice B) is a mesodermal duct into which the mesonephric tubules drain. The
mesonephric duct is the same as the pronephric duct, but at a later stage of development. The
mesonephric duct develops into the ductus deferens in the male. The metanephric duct is a
diverticulum of the mesonephric duct.
The mesonephros (choice C) is the embryonic kidney, which functions in the embryo before the
metanephros (or definitive kidney) develops. The mesonephros develops from the urogenital ridge.
The urogenital ridge (choice E) is a longitudinal elevation of the intermediate mesoderm. The
embryonic and adult kidneys, as well as the gonads, develop from this mesoderm.
Which of the following developmental abnormalities might account for unexplained small intestinal
bleeding?
A. Central nervous system heterotopia
B. Gastric heterotopia
C. Pancreatic heterotopia
D. Parathyroid heterotopia
E. Thyroid heterotopia
Explanation:
The correct answer is B. Heterotopic rests are small areas of normal tissue in abnormal sites.
These are usually clinically insignificant unless they form a noticeable mass or are
misdiagnosed in a biopsy (raising suspicion of metastatic cancer). One exception is a gastric
heterotopia, which typically occurs in the small intestine, and can produce enough acid to
cause a peptic ulcer in adjacent mucosa. The ulcer may be a source of gastrointestinal
bleeding.
Heterotopias of the CNS (choice A), parathyroid (choice D), and thyroid(choice E) can occur,
but would not be expected in the small intestine and would not be likely to cause bleeding if present.
Pancreatic heterotopia (choice C) can occur in the small intestine, but does not cause bleeding.
When examining a histological section of a normal ovary, a technician notices an oocyte surrounded by
several layers of follicular cells. A small antrum is present. Which of the following is the
correct term for the entire structure, composed of the oocyte, follicular cells, and antrum?
A. Corpus luteum
B. Graafian follicle
C. Primary follicle
D. Primordial follicle
E. Secondary follicle
Explanation:
The correct answer is E. Follicles in different stages of maturation have different
appearances. The most primitive follicles, primordial follicles (choice D), are inactive
reserve follicles that contain primary oocytes (arrested in prophase of first meiotic division)
surrounded by a single layer of flattened follicular cells. Primary follicles (choice C), the
next stage, are slightly larger and contain a central oocyte surrounded by one or several
cuboidal follicular cells. When several small spaces in the follicular mass fuse to form the
antrum (follicular cavity), the follicle is termed a secondary follicle (choice E). The
secondary follicles continue to enlarge, and develop a more complex structure that includes
cumulus oophorus, corona radiata, theca interna, theca externa, and zona pellucida. The
Graafian follicle (choice B) is the mature form of the follicle, which extends through the
entire cortex and bulges out at the ovarian surface. After it ruptures and releases the ovum,
the corpus luteum (choice A)develops as the cells of the follicle and the theca interna cells
enlarge, become epithelioid, and secrete estrogen. The granulosa lutein cells contain yellow
pigment and secrete progesterone. If pregnancy does not occur, the corpus luteum eventually
degenerates; if pregnancy occurs, it is maintained throughout the pregnancy.
Which of the following cell types is derived from neuroepithelial cells?
A. Astrocytes
B. Enterochromaffin cells
C. Melanocytes
D. Odontoblasts
E. Schwann cells
Explanation:
The correct answer is A. Astrocytes and oligodendrocytes are both derived from glioblasts,
which, in turn, are derived from neuroepithelial cells. Other neuroepithelial cell derivatives
include neuroblasts and ependymal cells.
All the other choices are derived from neural crest cells. Other neural crest derivatives
include the neurons of the parasympathetic and sympathetic ganglia (including the adrenal
medulla), the dorsal root ganglia of the peripheral nervous system, the sensory ganglia of
cranial nerves V, VII, IX, and X, and the leptomeninges (pia and arachnoid).
Which of the following embryonic structures gives rise to the adrenal cortex?
A. Ectoderm
B. Endoderm
C. Mesoderm
D. Mesonephros
E. Neural crest cells
Explanation:
The correct answer is C. The mesoderm gives rise to the adrenal cortex. In addition, it also
gives rise to connective tissue, cartilage, bone, muscle, blood and lymph vessels, kidneys, gonads,
serous membranes lining body cavities, and the spleen.
The ectoderm (choice A) gives rise to the central nervous system, peripheral nervous system,
epidermis and its appendages, mammary glands, pituitary gland, tooth enamel, and the neural crest.
The endoderm (choice B) gives rise to the parenchyma of the tonsils, thyroid and parathyroid
glands, thymus, liver, pancreas, the epithelial lining of the GI and respiratory tracts,
urinary bladder, urethra, and auditory tube.
The mesonephros (choice D) functions as an interim kidney in the embryo.
The neural crest cells (choice E) give rise to cells of the spinal and cranial nerves,
autonomic ganglia, melanocytes, leptomeninges, connective tissue and bone of branchial arch
origin, and the adrenal medulla.
A neuroscientist is investigating the development of the nervous system. In his experiments, he
microinjects a dye into the embryo of an animal subject in vivo. After birth, he performs
histological studies to determine the destination of the dye. In one animal subject, he locates
the dye in the dorsal horn of the spinal cord. Which of the following locations in the embryo
was the most likely site of the injection?
A. Alar plate
B. Basal plate
C. Neural crest
D. Rostral end of neural tube
E. Sulcus limitans
Explanation:
The correct answer is A. The spinal cord arises from the caudal end of the neural tube. During
development, an alar and a basal plate is formed, separated by a longitudinal groove called the
sulcus limitans (choice E). The alar plate forms the dorsal (posterior) part of the spinal cord
and becomes the sensory or afferent portion of the cord. The basal plate (choice B) is the
ventral (anterior) part of the cord, and becomes the motor, or efferent, portion of the spinal
cord, and therefore would contain anterior horn cells.
The neural crest (choice C) develops into multipolar ganglion cells of autonomic ganglia,
pseudounipolar cells of spinal and cranial nerve ganglia, leptomeningeal cells, Schwann cells,
melanocytes, chromaffin cells of the adrenal medulla, and odontoblasts.
The brain forms from the rostral end of the neural tube (choice D).
A newborn boy does not pass meconium until 48 hours after his birth. Two weeks later his mother reports
that he has not been passing stool regularly. Anorectal manometry reveals increased internal
anal sphincter pressure on rectal distention with a balloon. Radiographic studies reveal
massive dilation of the colon proximal to the rectum. This indicates a developmental
abnormality in which of the following embryonic tissues?
A. Ectoderm
B. Endoderm
C. Neural crest
D. Neural ectoderm
E. Splanchnic mesoderm
Explanation:
The correct answer is C. The baby has Hirschsprung's disease, which is due to an absence of
ganglion cells in the wall of the colon. Neural crest cells contribute to the formation of many
adult structures. Among these are all of the postganglionic neurons of the autonomic nervous
system and the sensory neurons of the peripheral nervous system.
Ectoderm (choice A) forms the epidermis of the skin and the parenchymal cells of glands
associated with the skin such as the sweat glands, sebaceous glands, and mammary glands.
Endoderm (choice B) forms the epithelial lining of the gut tube and the parenchymal cells of
glands associated with the gut tube, such as the liver and pancreas.
Neural ectoderm (choice D) forms the central nervous system, the somatic motor neurons of the
peripheral nervous system, and the preganglionic neurons of the autonomic nervous system.
Splanchnic mesoderm (choice E) forms the visceral peritoneum, visceral pleura, visceral
pericardium, and the stroma and muscle of the wall of the gut, among other structures.
Physical examination of a young boy reveals discharge of urine from the umbilicus. The physician
concludes that the urachus has failed to fuse. Which of the following structures is the normal
adult remnant of the fused urachus?
A. Lateral umbilical fold
B. Medial umbilical fold
C. Medial umbilical ligament
D. Median umbilical fold
E. Median umbilical ligament
Explanation:
The correct answer is E. The urachus is the derivative of the allantoic duct, which passes
from the urogenital sinus to the umbilical cord. Normally, this duct fuses and is no longer
patent. The adult derivative is the median umbilical ligament, which lies in the midline along
the interior surface of the anterior abdominal wall. It passes from the upper end of the
bladder to the umbilicus.
The lateral umbilical fold (choice A) is the fold of parietal peritoneum that covers the
inferior epigastric artery and vein on the interior surface of the anterior abdominal wall.
The medial umbilical fold (choice B) is the fold of parietal peritoneum that covers the medial
umbilical ligament, the adult derivative of the umbilical artery (see below).
The medial umbilical ligament (choice C) is the adult derivative of the distal portion of the
umbilical artery. The umbilical artery arises from the internal iliac artery. It passes along
the bladder and then the anterior abdominal wall to reach the umbilicus. Prenatally, this
artery carries fetal blood to the placenta, where it gains oxygen and nutrients. Postnatally,
the proximal part of the umbilical artery remains patent and supplies blood to the superior
surface of the bladder. Distal to the bladder, the artery becomes fibrotic and is known as the
medial umbilical ligament.
The median umbilical fold (choice D) is the fold of parietal peritoneum that covers the median
umbilical ligament, the adult derivative of the urachus.
A male infant presents with flattened facial features, low set ears, and deformities of the feet. The
lungs are underdeveloped. The pregnancy was complicated by severe oligohydramnios. The most likely cause
of this condition is a malformation of the
A. paramesonephric ducts
B. pronephros
C. ureteric buds
D. urogenital sinus
E. urorectal septum
Explanation:
The correct answer is C. This infant suffers from Potter syndrome, caused by an absence of both
kidneys. This is often caused by a failure of the ureteric buds to develop. The ureteric buds
form the ureters, renal pelvis, calyces, and collecting tubules. The collecting tubules induce
the formation of metanephric vesicles, which differentiate into the tubular components of the
nephron.
The paramesonephric ducts (choice A) form the uterine tubes, uterus, and the upper third of the
vagina.
The pronephros (choice B) is a transient collection of cells that disappears during the fourth week of