Usmle Embryology

Front 1

Factors involved in development of the testes

Back 1

Sry gene on Y chromosome encodes testes-determining factor (TDF); testosterone by the Leydig cells; Mullerian-inhibiting factor by Sertoli cells.

Front 2

Describe spermatogenesis

Back 2

Dormant primordial germ cells differentiate into type A stem cell spermatogonia in puberty; some type A spermatogonia differentiate into type B which enter meiosis; primary and secondary spermatocytes and spermatids are formed; spermatids mature into sperm.

Front 3

Describe oogenesis

Back 3

Primordial germ cells differentiate into oogonia in the 4th week; by 5 months all oogonia enter meiosis I forming primary oocytes that remain arrested in prophase of meiosis I until puberty; at puberty, a primary oocyte completes meiosis I; secondary oocyte becomes arrested in metaphase of meiosis II and ovulation occurs; following fertilization in the fallopian tube, secondary oocyte completes meiosis II forming a mature oocyte and a polar body.

Front 4

Ectopic pregnancy

Back 4

Risk factors are endometriosis, PID, tubular pelvic surgery. Uterine bleeding, abdominal pain, last menses 60 days ago, hCG+, culdocentesis shows intraperitonial blood.

Front 5

Blastula

Back 5

Less than 32 cells beginning on day 2.

Front 6

Morula

Back 6

32 cells on day 4

Front 7

Where does fertilization occur?

Back 7

Ampulla of fallopian tube

Front 8

Blastocyst

Back 8

When fluid secreted within the morula forms the blastocyst cavity on day 5. Inner cells is embryoblast (embryo), outer cells are trophoblast/placenta

Front 9

Implantation

Back 9

Occurs on day 6 on the posterior wall of the uterus which is in progestational phase.

Front 10

Bilaminar embryonic disk

Back 10

Epiblast and hypoblast form during second week from differentiation of embryoblast.

Front 11

Changes during second week

Back 11

Bilaminar embryonic disk forms; amniotic cavity and yolk sac form. Syncytiotrophoblast grows into endometrium reaching blood vessels.

Front 12

hCG

Back 12

Produced by syncytiotrophoblast stimulates progesterone production by corpus luteum. Important until week 8 when placenta produces progesterone.

Front 13

Extraembryonic mesoderm

Back 13

Derived from epiblast during 2nd week. Extrambryonic somatic mesoderm lines cytotrophoblast and forms connecting stalk. Extraembryonic visceral mesoderm covers the yolk sac.

Front 14

Hidatiform mole

Back 14

Blastocyst without embryo. Trophoblast continues to insert itself in uterus. Preeclampsia during 1st trimester, high hCG (>100,000), enlarged uterus and bleeding.

Front 15

Choriocarcinoma

Back 15

Malignant trophoblast tumor. Very high levels of hCG. Metastasizes to liver or brain.

Front 16

Sacrococcygeal teratoma

Back 16

Tumor arises from remnants of primitive streak and contains various types of tissue

Front 17

Gastrulation

Back 17

Week 3. Primitive streak forms within epiblast; Epiblast becomes ectoderm and gives rise to neuroectoderm and neural crest; Hypoblast becomes mesoderm and gives rise to paraxial mesoderm (35 pairs of somites), intermediate and lateral mesoderm; Epiblastic cells migrate through primitive streak to form endoderm between ecto and mesoderm

Front 18

Acrosomal reaction

Back 18

Spermal enzymes are released to allow penetration of zona radiata and zona pellucida.

Front 19

Placenta previa

Back 19

Placenta attaches to lower uterus. Uterine vessels rupture with profuse bright red bleeding.

Front 20

Tracheoesophageal fistula

Back 20

Blind esophagus is MCC. Polyhydramnios, gagging, cyanosis, reflux aspiration and pneumonitis.

Front 21

Respiratory distress syndrome and hyaline membrane

Back 21

Lack of surfactant in premature babies and products of diabetic mothers. Dyspnea, tachypnea, nasal flaring and cyanosis. Lecithin:sphyngomyelin < 2. Rx.: surfactant and dexamethasone

Front 22

Lung development: glandular stage

Back 22

Weeks 5-17. Respiration not possible.

Front 23

Lung development: canalicular stage

Back 23

Weeks 13-25. Respiration not possible. Premature fetuses rarely survive.

Front 24

Lung development: terminal sac stage

Back 24

Weeks 24-birth. Type I and II pneumocytes present, respiration possible. Premature fetuses born between weeks 25-28 survive in intensive care,

Front 25

Lung development: alveolar stage

Back 25

Birth-8 years. Respiratory bronchioles, terminal sacs, alveolar ducts and alveoli increase in number.

Front 26

Embryonic heart dilation: truncus arteriosus

Back 26

Develops into aorta and pulmonary trunk

Front 27

Embryonic heart dilation: bulbus cordis

Back 27

Develops into the smooth part of right and left ventricles

Front 28

Embryonic heart dilation: primitive ventricle

Back 28

Develops into trabeculated parts of right and left ventricles

Front 29

Embryonic heart dilation: primitive atrium

Back 29

Develops into trabeculated parts of right and left atria

Front 30

Embryonic heart dilation: sinus venosus

Back 30

Develops into smooth part of right atrium, left coronary sinus and oblique vein of left atrium

Front 31

Development of aorticopulmonary septum

Back 31

Neural crest cell origin. Elongate with spiral rotation and divides truncus arteriosus into aorta and pulmonary trunk

Front 32

Transposition of the great vessels

Back 32

Neural crest cells fail to spiral rotate. Aorta will be in right ventricle, pulmonary artery in left ventricle. Right to left shunt with early cyanosis. ASD, VSD or PDA are protective.

Front 33

Tetrallogy of Fallot

Back 33

Neural crest cells migrate improperly to truncus arteriosus causing pulmonary stenosis, right ventricular hypertrophy, overiding aorta, VSD. Right to left shunt with early cyanosis. Severity of pulmonary stenosis is prognostic.

Front 34

Persistent truncus arteriosus

Back 34

Neural crest cells migrate incompletely with only partial formation of AV septum. Right to left shunt with early cyanosis.

Front 35

Development of the atrial septum

Back 35

The septum primum grows towards AV septum temporarily forming foramen primum. SP fuses with AV septum obliterating FP. The foramen secundum opens up within SP. Septum secundum forms to the right of the SP forming foramen ovale through which oxygenated blood from inferior vena cava is shunted during fetal life. After birth SP and SS fuse to form atrial septum. Excesive resorption of SP or SS leads to ASD.

Front 36

Development of interventricular septum

Back 36

Muscular IV septum grows from the floor towards AV cushions forming foramen ovale. The membranous IV septum closes formamen ovale by the fusion of right bulbar ridge, left bulbar ridge and AV cushinons. Failure of right bulbar ridge, left bulbar ridge and AV cusions tu fuse lead to a membranous ventricular septal defect.

Front 37

Development of the primitive gut tube

Back 37

Its formed by incorporation of the yolk sac into the embryo. Epithelial lining and glands of mucosa are from the endoderm. Lamina propria, muscularis mucosae, submucosa, muscularis externa and adventitia are from the mesoderm.

Front 38

Pyloric stenosis

Back 38

Hypertrophy of the muscularis externa closes the pylorus. Projectile nonbilious vomiting and small knot at the right costal margin.

Front 39

Extrahepatic biliary atresia

Back 39

Lumen of biliary ducts fail to recanalize and are occluded. Jaundice, white-colored stools, dark urine.

Front 40

Annular pancreas

Back 40

The ventral and dorsal pancreatic buds form a ring around duodenum causing obstruction.

Front 41

Duodenal atresia

Back 41

Lumen of duodenum fails to recanalize and is occluded. Polyhydramnios, bile vomit, distended stomach.

Front 42

Omphalocele

Back 42

Midgut loop fails to return to the abdominal cavity forming light gray shiny sac at the ombilical cord filled with loops of small intestine

Front 43

Ileal Meckel diverticulum

Back 43

Remnant of vitelline duct that forms a blind pouch on the antimesenteric border of ileum. Could produce inflammation and ulceration or bleeding with iron deficiency anemia.

Front 44

Vitelline fistula

Back 44

Vitelline duct persists forming a fistula between the intestinal lumen and the outside of the body at the umbilicus. Drainage of meconium from umbilicus.

Front 45

Malrotation of the midgut

Back 45

Partial rotation results in abnormal position of viscera. Associated with volvulus.

Front 46

Colonic aganglionosis (Hirchsprung)

Back 46

Failure of the neural crest to form myenteric plexus in sigmoid and rectum. Loss of peristalsis, constipation, obstipation, abdominal distention, dilation of proximal colon, delayed pasage of meconium.

Front 47

Development of the liver

Back 47

Hepatic diverticulum evaginates from endoderm of foregut at second portion of duodenum, enters mesentery. Distal part becomes liver and gallbladder, proximal part becomes biliary ducts. Mesentery between liver and gut becomes lesser omentum and between liver and ventral body wall it becomes falciform ligament.

Front 48

Development of the pancreas

Back 48

Two pancreatic buds evaginate from foregut endoderm at second portion of duodenum. The dorsal bud grows into dorsal mesentery and forms duct of Santorini, neck, body and tail. The ventral bud rotates around gut tube and enters dorsal mesentery forming duct of Wirsung and inferior head of pancreas.

Front 49

Development of the spleen

Back 49

Originates from cells of mesentery behind the stomach. Mesentery around spleen becomes visceral peritoneum of spleen and mesentery between spleen and gut forms gastrosplenic ligament. Mesentery between the spleen and dorsal body wall becomes splenorenal ligament and parietal peritoneum.

Front 50

Rotations of the gut

Back 50

Foregut rotates 90 degrees such that dorsal side and spleen rotate to the left and lesser omentum and liver to the right. Midgut herniates out into umbilical cord and rotates 270 degrees around anteroposterior axis.

Front 51

Development of the pronephros

Back 51

Appear in week 4 at the cervical intermediate mesoderm. Dissappears at the wend of the 4th week.

Front 52

Development of the mesonephros

Back 52

Appear in week 5 at the throracic and lumbar intermediate mesoderm. The medial end forms Bowman's capsule and tubules. The lateral end open into mesonephric Wolffian duct. Mesonephros regresses in the third month. Wolffian ducts persist and form male structures.

Front 53

Metanephros

Back 53

The ureteric bud evaginates from the mesonephric duct and forms the collecting ducts, minor calyx, major calyx, renal pelvis and ureter as it penetrates and induces the metanehpric mass. The metanephric mass is an evagination from lumbar intermediate mesoderm and will differentiate into Bowman's capsule and tubules.

Front 54

Mesonephric Wolffian ducts

Back 54

Develop into ductus epididymidis, ductus deferens and ejaculatory duct

Front 55

Urorectal septum

Back 55

Divides the cloaca into the anorectal canal and the urogenital sinus by week 7

Front 56

Development of the bladder

Back 56

The bladder derives from the upper part of the urogenital sinus and is initially continuous with the allantois (which becomes obliterated). The mesonephric duct enters the dorsal bladder wall. The inner epithelium of the bladder is from the endoderm. The smooth muscle is from splachnic mesoderm.

Front 57

Allantois

Back 57

Duct initially continuous with urogenital sinus and bladder, becomes an obliterated fibrous urachus (or median umbilical ligament) which connects the apex of the bladder to the umbilicus.

Front 58

Development of male urethra

Back 58

Prostatic, membranous and proximal penile urethra develop from lower end of urogenital sinus. Distal penile urethra is derived from the ectodermal cells of the glans penis.

Front 59

Development of the female urethra

Back 59

The upper 2/3 from mesonephric ducts and lower 1/3 from the urogenital sinus.

Front 60

Renal agenesis

Back 60

Failure of one or both kidneys to develop due to early degeneration of ureteric bud.

Front 61

Renal cysts or polycystic kidney disease

Back 61

Abdominal mass, flank pain and hematuria later in life. Associated with cysts of the liver, pancreas and lungs.

Front 62

Horseshoe kidney

Back 62

Fusion of both kidneys at inferior poles. Gets trapped behind inferior mesenteric artery as it ascends.

Front 63

Double ureter

Back 63

Caused by splitting of ureteric bud. Urine dribbles out and result in incontinence.

Front 64

Patent urachus

Back 64

No obliteration of allantois. May cause drainage of urine through umbilicus in patients with obstruction of prostatic urethra or prostatic hyperplasia.

Front 65

Female pseudohermaphrodite

Back 65

46XX, ovarian tissues, masculinization of external genitalia. Due to congenital adrenal hyperplasia.

Front 66

Male pseudohermaphrodite

Back 66

46XY, testicular tissue, stunted genitalia development. Due to low testosterone (5 alfa-reductase deficiency), low Mullerian inhibiting factor or insensitivity to androgens.

Front 67

5 alfa reductase deficiency

Back 67

No conversion of testosterone to dihydrotestosterone. Underdevelopment of penis and scrotum.

Front 68

Complete androgen insensitivity

Back 68

46XY with female external genitalia. Mutation in androgen receptor gene.

Front 69

Hypospadias

Back 69

Uretheral folds fail to fuse and urethral orifice opens into ventral surface of penis.

Front 70

Epispadias

Back 70

Urethral orifice opens into dorsal surface of the penis.

Front 71

Cryptorchidia

Back 71

Testes fail to descend within 3 months of birth and may result in infertility