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Early embryology: somite stage and limb buds

Embryology of spine and spinal cord

Respiratory embryology

Cardiovascular embryology

Development of the body cavities and the diaphragm

Embryology of abdominal contents

Urogenital system embryology

Embryology of the pharynx: branchial arches and derivatives

Illustrations are referenced to The Developing Human, Clinically Oriented Embryology, 8th ed. by Moore and Persaud, 2008, Saunders.



The intraembryonic mesoderm on each side of the forming notochord and neural tube thickens to form a longitudinal column of paraxial mesoderm. By the end of the 3rd week, the paraxial mesoderm divides into paired bodies called somites, located bilaterally of the neural tube (p. 64, fig. 4-10).


Week 4

Limb bud (p. 366, fig. 16-2)

Each limb bud consists of a mass of mesenchyme derived from the somatic mesoderm, covered by a layer of ectoderm. At the tip of each limb bud, ectodermal cells form an apical ectodermal ridge, which promotes growth and development of the limbs in the proximo-distal axis .  Fibroblast growth factors and T-box genes (tbx-4 and tbx-5) from the apical ectodermal ridge activate the mesenchymal cells at the posterior margin of the limb bud (the zone of polarizing activity).  This causes expression of the Sonic Hedgehog gene, which controls the patterning of the limb along the anterior-posterior axis.  Expression of Wnt7 from the dorsal epidermis of the limb bud and engrailed-1 (EN-1) from the ventral aspect specifies the dorsal-ventral axis

Week 5

Week 6 (p. 354, fig. 14-14; p. 371, fig. 16-7)

Further differentiation of the limb buds during week 6 (p. 367, fig. 16-3):

Week 7

Limb rotation begins (p. 373, fig. 16-9):

Week 8 (Last week of embryonic life; p. 372 fig. 16.8)

At the beginning of week 8,

At the end of week 8, there are distinct regions in the limbs, with long fingers and distinct toes.

FETAL PERIOD (p. 5-6, fig. 1-1 and 1-2)

Weeks 9-12

Weeks 34-38

A dermatome is the area of skin innervated by a single spinal nerve and its dorsal root ganglion (p. 373, fig. 16-10).

Development of the innervation of the limbs

Development of the blood supply to the limbs

Limb buds are supplied by branches of the intersegmental arteries arising from the aorta (p. 374, fig. 16-11).
Initially, a primary axial artery and its branches supply the limb bud and a peripheral marginal sinus drains it.

In the upper limb,

In the lower limb,

updated 8/25/2008

Embryology of the spine and spinal cord

The AXIAL SKELETON is formed by the :

Development of the vertebral column

Precartilaginous (mesenchymal) stage

During week 4, mesenchymal cells from the sclerotome of the somites are found in 3 main areas (The Developing Human, 8th ed., p. 345):

1. Around the notochord

Each sclerotome consists of loosely packed cells cranially and densely packed cells caudally (The Developing Human, 8th ed., p. 345)

The notochord degenerates and disappears where it is surrounded by the vertebral body.

2. Surrounding the neural tube

These mesenchymal cells form the vertebral arch (The Developing Human, 8th ed., p. 345).

3. In the body wall

These mesenchymal cells form the costal processes which develop into ribs in the thoracic region.


The cartilaginous stage

During week 6, chondrification centers appear in each mesenchymal vertebra (The Developing Human, 8th ed., p. 346).

Chondrification spreads until a cartilaginous vertebral column is formed.

The bony stage

Ossification of the typical vertebrae begins during the embryonic period and ends by year 25 of life.

Prenatal period

2 (ventral and dorsal) primary ossification centers for the centrum fuse to form one.

3 primary ossification centers at the end of the embryonic period (The Developing Human, 8th ed., p. 346):

At birth, each vertebra consists of 3 bony parts connected by cartilage (The Developing Human, 8th ed., p. 346).

Postnatal period

The halves of the vertebral arch fuse during years 3-5.
The laminae of the arch first unite in the lumbar region and the progression moves cranially.
The vertebral arch articulates with the centrum at cartilaginous neurocentral joints (The Developing Human, 8th ed., p. 346).
These articulations permit the vertebral arches to grow as the spinal cord enlarges.
The neurocentral joints disappear when the vertebral arch fuses with the centrum during years 3-6.

After puberty

5 secondary ossification centers appear (The Developing Human, 8th ed., p. 346):

The vertebral body is a composite of the superior and inferior annular epiphyses and the mass of bone between them. It includes the centrum, parts of the vertebral arch and the facets for the heads of the ribs.

All secondary centers unite with the rest of the vertebra around year 25.


Ossification of atypical vertebrae

Exceptions to the typical ossification of vertebrae occur in C1, C2, C7, lumbar vertebrae, sacrum and coccyx.

Examine the entire vertebral column because an apparent extra or absent vertebra in one segment may be compensated by an absent or extra vertebra in an adjacent segment (ex: 11T and 6 L vertebrae).

Development of the spinal cord

The nervous system develops from an area of embryonic ectoderm called the neural plate which appears during week 3 (The Developing Human, 8th ed., p. 382).

The underlying notochord and adjacent mesoderm induce the formation of the neural plate. The neural tube and the neural crest differentiate from the neural plate.


Central nervous system

The spinal cord is formed from the neural tube caudal to somites 4.

Proliferation and differentiation of the neuroepithelial cells in the developing spinal cord produce thick walls and thin roof and floor plates. A shallow longitudinal sulcus limitans appears in the lateral walls of the spinal cord and separates the dorsal alar plate from the ventral basal plate (The Developing Human, 8th ed., p. 386).

Mesenchyme surrounding the neural tube condenses to form the primitive meninx.

Positional changes of the developing spinal cord

In the embryo, the spinal cord extends the entire length of the vertebral canal and the spinal nerves pass through the intervertebral foramina near their levels of origin.

This relationship does not persist because the spine and the dura mater grow more rapidly than the spinal cord. The caudal end of the spinal cord comes to lie at relatively higher levels.

Positional changes of the developing spinal cord (The Developing Human, 8th ed., p. 390)

Congenital malformations:

Spina bifida occulta (The Developing Human, 8th ed., p. 391, 392 fig. 17-14)

Spinal dermal sinus

Intramedullary dermoids are tumors arising from surface ectodermal cells incorporated into the neural tube during closure of the caudal neuropore.

Spina bifida cystica (The Developing Human, 8th ed., p. 393)

Spina bifida

updated 9/8/2008


The lower respiratory system (from the pharynx down)

The laryngotracheal groove deepens into a diverticulum ventrally which enlarges distally into a lung bud (The Developing Human, 8th ed., p. 200, fig. 10-2). The diverticulum becomes separated from the primitive pharynx by longitudinal trachoesophageal folds which fuse to form the trachoesophageal septum, dividing the foregut into the ventral laryngotracheal tube and the dorsal esophagus.

A fistula (The Developing Human, 8th ed., p. 202, fig. 10-5, 10-6) may exist connecting trachea and esophagus and resulting in abnormal communication between the 2.

The lung bud develops into 2 endodermal bronchial buds (The Developing Human, 8th ed., p. 202, fig. 10-7) which grow into the pericardioperitoneal cavities, the primordia of the pleural cavities.

PLEURAE (The Developing Human, 8th ed., p. 202, fig. 10-7)

LUNG DEVELOPMENT (The Developing Human, 8th ed., p. 204, fig. 10-9; p. 205, fig. 10-10)

1) Pseudoglandular period (5-17 weeks)

By week 17 all major elements of the lungs have formed except for those involved with gas exchange. The lungs look like an endocrine organ. No respiration is possible!

2) Canalicular period (16-25 weeks)

The lumen of the bronchi and terminal bronchioles become larger and the lungs become vascularized. By week 24, respiratory bronchioles have developed and respiration becomes possible, although the chances of survival are slim.

3) Terminal sac period (24 weeks to birth)

Surfactant reaches adequate levels 2 weeks before birth.

Adequate pulmonary vasculature and sufficient surfactant are critical to the survival of premature infants.

4) Alveolar period (late fetal period to 8 years)

95% of the mature alveoli develop after birth. A newborn infant has only 1/6 to 1/8 of the adult number of alveoli and the lungs look denser in an x-ray.

Developing lungs at birth are half filled with amnotic fluid. The fluids in the lungs are cleared:

updated 9/14/2008


The cardiovascular system begins to develop during week 3.

Mesenchymal cells derived from the mesoderm form endothelial tubes which join to form the primitive vascular system (The Developing Human, 8th ed., p. 286, fig. 13-1).


Heart develops from splanchnic mesenchyme in the cardiogenic area.

Bilateral cardiogenic cords

Surrounding mesenchyme thicken to form the myoepicardial mantle (future myocardium and epicardium) separated from the endothelial heart tube (future endocardium) by the gelatinous cardiac jelly (The Developing Human, 8th ed., p. 294, fig. 13-8).

The future heart develops dilatations and constrictions resulting in 4 chambers (The Developing Human, 8th ed., p. 296-298):

The truncus arteriosus is continuous caudally with the bulbus cordis, and enlarges cranially to form the aortic sac from which the aortic arches arise (The Developing Human, 8th ed., p. 296, fig. 13-10).

The sinus venosus receives (The Developing Human, 8th ed., p. 296, fig. 13-10):

3 systems of paired veins drain into the primitive heart:

The bulbus cordis and the ventricle grow faster and the heart bends upon itself, forming a bulboventricular loop (The Developing Human, 8th ed., p. 294, fig. 13-8 E).

The atrium and sinus venosus come to lie dorsal to the bulbus cordis, truncus arteriosus and ventricle (The Developing Human, 8th ed., p. 297).

At the same time, the heart invaginates into the pericardial cavity (The Developing Human, 8th ed., p. 295).

The dorsal mesocardium which attaches it to the dorsal wall of the pericardial cavity degenerates and forms the tranverse pericardial sinus (The Developing Human, 8th ed., p. 294, fig. 13-8).

First heartbeat occurs at 21 to 22 days and originates in the muscle, forming peristalsis-like waves beginning in the sinus venosus.

By the end of week 4 coordinated contractions of the heart results in unidirectional flow:

The heart divides into 4-chambered heart between weeks 4 and 7.

1) Endocardial cushions (The Developing Human, 8th ed., p. 297-298) form on the dorsal and ventral walls of the atrioventricular canal. At week 5, they approach each other and fuse, dividing the atrioventricular canal into right and left canals.

2) Atria are partitioned successively by the septum primum and the septum secundum (The Developing Human, 8th ed., p. 299-301). The latter is an incomplete partition and leaves a foramen ovale. The foramen ovale has a valve formed from the degeneration of the cranial portion of the septum primum.

Before birth the foramen ovale allows blood to pass from the right atrium into the left atrium; reflux is prevented by the valve (The Developing Human, 8th ed., p. 301).

After birth the foramen ovale normally closes by fusion of the septum primum and the septum secundum.

3) The sinus venosus develops a left horn which becomes the coronary sinus (The Developing Human, 8th ed., p. 302) and a right horn which will be incorporated into the right atrium. The smooth part of the right atrium, the sinus venarum, is derived from the sinus venosus whereas the muscular part, the auricle, is derived from the primitive atrium. The 2 portions are separated internally by the crista terminalis and externally by the sulcus terminalis.

4) The primitive pulmonary vein and its 4 main branches become partially incorporated into the left atrium (The Developing Human, 8th ed., p. 303). This results in the 4 pulmonary veins. The portion derived from the original left atrium retains a trabeculated apperance.

5) The ventricles become partitioned by a crescentic fold which is open cranially until the end of week 7 (interventricular foramen; The Developing Human, 8th ed., p. 304). The interventricular septum is formed of a central membranous part and a surrounding muscular part. After closure, the right ventricle communicates with the pulmonary trunk and the left ventricle with the aorta.

6) During week 5, the bulbus cordis and the truncus arteriosus become divided by an aorticopulmonary septum into the definitive pulmonary trunk and aorta (The Developing Human, 8th ed., p. 307, 317). Valves develop from proliferation of the subendocardial tissue.

The primitive atrium acts as a temporary pacemaker. But the sinus venosus soon takes over.

The critical period of development is from day 20 to day 50 after fertilization.

Improper partitioning of the heart may result in defects of the cardiac septa, of which the ventricular septal defects are most common (25% of congenital heart disease).

Membranous ventricular septal defect (most common):

Muscular septal defect:

Absence of interventricular septum is rare and results in a 3-chambered heart called cor triloculare biatriatum.

The tetralogy of Fallot consists of (The Developing Human, 8th ed., p. 316):

Cyanosis is an obvious sign but may not be present at birth.

Aortic arches

The lymphatic system begins to develop around week 5 (The Developing Human, 8th ed., p. 334).

FETAL CIRCULATION (The Developing Human, 8th ed., p. 328-329)

After birth, the foramen ovale, ductus arteriosus, ductus venosus and umbilical vessels are no longer needed and they close (The Developing Human, 7th ed., p. 373, fig. 14-47).

The right ventricular wall is thicker in the newborn but by the end of month 1, the left ventricular wall is thicker.

The fetal circulation is designed to carry oxygenated blood from the placenta to the fetal circulation, bypassing the lungs.

updated 9/14/2008

Development of the body cavities and the diaphragm

The Developing Human - Clinically Oriented Embryology - Moore and Persaud, 8th edition - Chapter 8

The intraembryonic coelom is the primordium of the embryonic body cavities and begins to develop near the end of week 3 (fig. 8-1). By the beginning of week 4, it is a horseshoe-shaped cavity in the cardiogenic and lateral mesoderm.

The curve of the horseshoe represents the future pericardial cavity (fig. 8-2B) and its lateral limbs represent the future pleural and peritoneal cavities (fig. 8-2C).

During folding of the embryonic disc in week 4, the lateral parts of the intraembryonic coelom are brought together on the ventral aspect of the embryo (fig. 8-2F).

Until week 7, the embryonic pericardial cavity communicates with the peritoneal cavity through paired pericadioperitoneal canals (fig. 8-4C-D).

During weeks 5 and 6, partitions form near the cranial and caudal ends of these canals:

The diaphragm forms from (figs. 8-7, 8-8 & 8-9):

1) the septum transversum,

2) the pleuroperitoneal membranes,

3) the dorsal mesentery of the esophagus,

4) the body wall.

A posterolateral defect of the diaphragm results in congenital diaphragmatic hernia (figs.8-10, 8-11, 8-12) and is due to failure of fusion between the pleuroperitoneal membranes and other diaphragmatic components.

updated 09/25/2008

Embryology of the abdominal contents
The Developing Human - Clinically Oriented Embryology - Moore and Persaud, 8th edition - Chapter 11

The primitive gut forms during week 4 when the embryo folds and incorporates the dorsal part of the yolk sac (fig. 11-1).

The FOREGUT gives rise to:

Because, trachea and esophagus have a common origin, imcomplete partitioning of the trachoesophageal septum results in stenoses or atresias, with or without fistulas between them.

Development of the liver

The liver bud or hepatic diverticulum is formed from an outgrowth of the endodermal epithelial lining of the foregut (fig. 11-5). The epithelial liver cords and primordia of the biliary system which develop from the hepatic diverticulum, grow into the mesenchymal septum transversum (fig. 8-9). Between the layers of the ventral mesentery, derived from the septum transversum, these primordial cells differentiate into the parenchyma of the liver and the lining of the ducts of the biliary system.

Development of the duodenum

Congenital duodenal atresia is due to the failure of vacuolization and recanalization (week 8; fig. 11-6). This process occurs following the normal solid stage of the duodenum (week 5). Obstruction of the duodenum can also be caused by an annular pancreas (fig. 11-11), resulting from parts of the pancreas developing around the duodenum.

Development of the pancreas

The pancreas is formed by dorsal and ventral pancreatic buds (fig. 11-10) originating from the endodermal lining of the foregut. When the duodenum rotates to the right, the ventral pancreatic bud moves dorsally and fuses with the dorsal pancreatic bud. The ventral pancreatic bud forms most of the head of the pancreas and the dorsal pancreatic bud forms the rest. If the duct systems from each pancreas fail to fuse, an accessory pancreatic duct forms.


The MIDGUT gives rise to:

The midgut forms a U-shaped intestinal loop herniating into the umbilical cord during week 6 because of the lack of room in the abdomen : This is the physiological umbilical herniation (fig. 11-13, 11-14).


Omphalocele (fig. 11-17), malrotations and abnormalities of fixation result from failure of return or abnormal rotation of the intestines in the abdomen. Because the gut is normally occluded during weeks 5 and 6 due to rapid mitotic activity of its epithelium, stenosis, atresias and duplications (fig. 11-24) may result if the recanalization fails to occur or occur abnormally.

Various remnants of the yolk stalk may persist such as Meckel's (ileal) diverticulum (fig. 11-21; fig. 11-22) which can become inflamed and produce pain.

The Hindgut gives rise to:

The inferior part of the anal canal develops from the proctodeum (fig. 11-26).

The caudal part of the hindgut (the cloaca; fig. 11-25) is divided by the urorectal septum into the urogenital sinus and rectum. The urogenital sinus gives rise to the urinary bladder and urethra. The rectum and superior anal canal are separated from the outside by the anal membrane which breaks down by the end of week 8.


updated 09/25/2008

Urogenital system embryology

The Developing Human - Clinically Oriented Embryology - Moore and Persaud, 8th edition - Chapter 12

The urogenital system develops from:

The intermediate mesoderm used to lie lateral to the somites, then moved away from the somites during the lateral fold. It forms the urogenital ridge (fig. 12-1F) which is comprised of:

3 successive sets of kidneys develop:

The metanephros develops mesodermally from the metanephric diverticulum or ureteric bud which is a dorsal outgrowth from the mesonephric duct near the cloaca (fig. 12-6).

The metanephric diverticulum or ureteric bud penetrates the metanephric mesoderm in the caudal part of the nephrogenic cord and stimulates the formation of the metanephric mass or cap (fig. 12-9).

The metanephric mesoderm gives rise to the nephrons (glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle and distal convoluted tubule; fig. 12-7). The cortex of the kidney in the newborn contains mostly undifferentiated mesenchyme; the nephrons continue to develop several months after birth.

Ascension of the kidneys (fig. 12-10): The kidneys are first located in the pelvis ventral to the sacrum but gradually ascend to the abdomen. They reach the adult position by week 9 having touched the suprarenal glands (fig. 12-10). This is due to the disproportionate growth between the lumbar and sacral regions: the sacral region grows faster than the lumbar region.

The kidneys rotate 90 degrees from anterior to medial.

During their ascension, the blood supply changes continuously so that an adult may have 2 to 4 renal arteries (fig. 12-11).

The suprarenal glands ( fig. 12-27):

The urinary bladder develops from the urogenital sinus and the surrounding splanchnic mesenchyme (fig. 12-20). The urogenital sinus is comprised of 3 regions:

The female urethra and almost all of the male urethra have the same origin.

The glans penis in the male develops from the ectodermal glandular plate (figs. 12-24, 12-25)

Developmental abnormalities of the kidney and excretory passages are common:

updated 10/06/2008

THE GENITAL OR REPRODUCTIVE SYSTEM develops in close association with the urinary or excretory system.

Genetic sex is established at fertilization, but the gonads do not begin to attain sexual characteristics until week 7. Early genital development is referred to as the indifferent stage of sexual development: the external genitalia do not acquire distinct masculine or feminine characteristics until week 12.

Testes and ovaries are derived from the mesodermal epithelium (mesothelium) lining the posterior abdominal wall, the underlying mesenchyme and the primordial germ cells.

The primordial germ cells form in the wall of the yolk sac during week 4 (fig. 12-30). They later migrate into the developing gonads at week 6 and differentiate into the definitive germ cells (oogonia/spermatogonia).

The reproductive organs in both sexes develop from primordia that are identical at first.

At first both the male and the female have 2 pairs of genital or sex ducts: the mesonephric (wolffian - medial) and paramesonephric (müllerian - lateral) ducts (figs. 12-33 and 12-34).

Gonadal sex is determined by the Y chromosome, which exerts a positive testis-determining action (TDF) on the indifferent gonad.

Persons with true hermaphroditism (ovo-testes - very rare) have both ovarian and testicular tissue and variable internal and external genitalia.

Errors in sexual differentiation cause pseudohermaphroditism.


updated 10/06/2008


The Developing Human - Clinically Oriented Embryology - Moore and Persaud, 8th edition - Chapter 9

The branchial apparatus consists of (figs. 9-3, 9-4):

Most congenital malformations of the head and neck originate during transformation of the branchial apparatus into its adult derivatives.

The primitive mouth or stomodeum is separated from the primitive pharynx by the buccopharyngeal (oropharyngeal) membrane (fig. 9-1E). This membrane ruptures at about day 24 (fig. 9-1F), bringing the primitive gut into contact with the amniotic fluid cavity.

Branchial arches develop early in week 4 as neural crest cells migrate to the future head and neck region.

By the end of week 4, 4 pairs of branchial arches are visible, the 5th and 6th being small. Branchial arches are separated by the branchial grooves and are numbered in a craniocaudal sequence (fig. 9-3).

Initially, each pharyngeal arch consists of mesenchyme derived from the intraembryonic mesoderm and is covered with ectoderm externally and endoderm internally.

Neural crest cells migrate into the arches, creating the swellings of the arches and contributing to the arches, even though they are of ectodermal origin. Neural crest cells give rise to specific skeletal structures.

The mesenchyme in the arches give rise to muscles.

A typical branchial arch contains (fig. 9-3C):

Derivatives of the branchial arch cartilages (fig. 9-5B)

1st branchial (mandibular) arch cartilage develops :

2nd branchial (hyoid) arch cartilage develops:

3rd branchial arch cartilage develops into the greater cornu and inferior part of the body of the hyoid bone.

4th and 6th branchial arch cartilages fuse to form the laryngeal cartilages, except for the epiglottis which forms from the mesenchyme in the hypobranchial eminence (from the 3rd and 4th branchial arches).

Derivative of the branchial arch nerves (fig. 9-7):

Derivatives of the branchial arch muscles (fig. 9-6):

1st branchial arch:

2nd branchial arch

3rd branchial arch

4th and 6th branchial arches:

PHARYNGEAL POUCHES (fig. 9-8) develop between the branchial arches (1st pouch is found between the first and second branchial arches). There are 4 pairs, the 5th is absent or very small.

The endoderm of the pharyngeal pouches and the ectoderm of the branchial grooves contact each other to form the branchial membranes separating the pharyngeal pouches and the branchial grooves.

Derivatives of the pharyngeal pouches (fig. 9-9)

1st pharyngeal pouch expands into a tubotympanic recess (fig. 9-8B).

2nd pharyngeal pouch contributes to the formation of the palatine tonsil (fig. 9-8) and the epithelial lining of the fauces.

3rd pharyngeal pouch contributes to the formation of the inferior parathyroid glands (week 5- bulbar portion; fig. 9-8) and the thymus (elongate portion). which migrate inferiorly (past the superior parathyroid glands of the 4th pouch).

4th pharyngeal pouch contributes to the formation of the superior parathyroid gland (bulbar portion) and the parafollicular cells or calcitonin cells of the thyroid gland (elongate portion - ultimobranchial body).

updated 11-03-2008


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