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Week 3

GASTRULATION is the formation of the trilaminar embryonic disc from a bilaminar disc. EPIBLAST becomes ECTODERM (dorsal), HYPOBLAST becomes ENDODERM (volar)

The PRIMITIVE STREAK at the dorsal aspect of the embryonic disc is formed. The cranial end thickens to form the PRIMITIVE KNOT, defining the cranial end.  (Primitive streak remnants can form Teratomas)

The primitive streak produces MESENCHYMAL (MESOBLASTIC) cells destined to form the MESODERM by blending endoderm and ectoderm. This will produce connective tissue, bone, blood vessels, blood cells, muscles and the G.U. system.

ECTODERM forms the NOTOCHORD – at the cranial end of the PRIMITIVE STREAK. The Notochord is the structure around which the spinal cord will develop (remnants can form Chordomas).

NEURULATION – The Notochord induces the overlying ectoderm to form the NEURAL PLATE (the primordium of the CNS). The thickened plate then invaginates centrally to form the NEURAL GROOVE, with thickened NEURAL FOLDS (day 18). The folds fuse dorsally forming the NEURAL TUBE, which will become the spinal cord.

As the neural folds fuse some of the ectodermal cells lying along the crest of each neural fold attach to neighbouring cells & become the NEURAL CREST cells, which differentiate to begin the formation of the peripheral nervous system, the automatic nervous system and Schwann cells.

SOMATISATION – SOMITES are formed from MESODERM and they begin to line both sides of the NOTOCHORD. Eventually they will form 42-44 pairs. The SOMITES continue their developmental process and soon become a lateral dermatome, a medial myotome and a ventral scleratome. This becomes in due time, the basis of the skin, muscle and skeletal elements, respectively.





Weeks 4 to 6

The embryo FOLDS laterally & transversally forming a C-shaped cylindrical embryo.

The APICAL ECTODERMAL RIDGE exerts an inductive influence on the limb mesenchyme that promotes growth & development of the limbs.

UPPER LIMBS – At this time the limb buds also develop. The upper extremity, with pronated forearms, appears first – and begins to rotate externally. For more detail see Upper limb Congenital Disorders

LOWER LIMBS – The lower extremity appears slightly later than the upper extremity and begins to rotate internally.

Developmental disturbances during this period gives rise to major congenital malformations. Early suppresion of limb development causes AMELIA (complete abscence of a limb); Late suppresion causes MEROMELIA (partial abscence).





Weeks 7 to Birth

By week 7 the ten finger rays appear and continue to differentiate till week 12 – 13 when the hands appear.

During this initial 12 week period the formation of the body's solid framework also begins. The beginning process involves MESENCHYMAL AGGREGATION into a cartilage prototype. Gradually but systematically each cartilage model becomes solid bone. This process applies to all bones except those formed through intra-membranous ossification such as the skull.

By week 12 the PRIMARY CENTRES of OSSIFICATION in the DIAPHYSES of most bones has appeared. Most SECONDARY CENTRES for OSSIFICATION are not present however, until after birth.




Vertebral formation at 3-5 weeks with segmentation occurring at 6-8 weeks

Each vertebrae forms from 2 adjacent sclerotomes and so becomes an inter segment structure
Notochord degenerates but between vertebrae it persists to form the nucleus pulposus
During the 6th week chondrification occurs
Two centres in each centrum fuse at the end of the embryonic period resulting in one centre evident in the centrum after this period (defects here -> hemi vertebrae)
Centres in the neural arches fuse with each other and the centrum
The two 1/2's of the neural arch usually fuse in the first year and these to the centrum in ~ the 3rd to 6th year
At about puberty five secondary centres appear (upper and lower body, and one in the tip of the TPs and spinous process)

Ossification begins in the embryonic period and ends at about 25 years.
Longitudinal growth is via superior and inferior apophysis.

Horizontal growth is by periosteal apposition
Spinal canal enlarges by growth of the pedicles and posterior elements, enlarging rapidly from birth to five years and more slowly from 5-10 years.
The spinal canal reaches its final dimentsions relatively early compared with the continual growth of the rest of the vertebral structures
The final height of vertebral column is reached
girls by 11 – 13 years
boys by 14 – 16 years
Boys grow ~ 3.5cm and girls 1.5cm after Risser IV



There are 80 bones in the axial skeleton & 126 bones in the appendicular skeleton, for a total of 206 bones.


Three centres of ossification
Ilium appears first at ~ 2 months foetal life
Ischium at three months and the pubis at 4 months
Ischial and pubic rami fuse at about 7 years and the 'Y' shaped cartilaginous physis of the three bones fuses soon after puberty (15 years)



Centre for the shaft appears in the 8th week of intra uterine life and the centre of the lower end appears at the end of the 9th month (the time of birth) and unites with the shaft at about 20 years
Centre appears in the head at 1 year of age, the greater trochanter at 3 years and the lesser trochanter at 12 years and fuse with the shaft at about 18 years



Centre appears at 3 years and ossification is complete soon after puberty



The primary centre of the shaft appears at about 8 weeks intra uterine life and the upper epiphysis appears immediately after birth and joins with the shaft at about 20 years
The secondary centre for the tuberosity may appear at about puberty
The distal epiphysis ossifies at the second year and joins the shaft at about 18 years



Primary centre appears at 8 weeks foetal life
The centre of the proximal end ossifies at about the 4th year and fuses with the shaft at about 20 years, the lower end at about the 2nd year and fuses with the
shaft at about 18 years



Bones of the tarsus are ossified at birth, the calcaneus at 6/12, the talus at 7/12 and the cuboid at 9/12 inter uterine life (ossification is medico legal evidence of maturity)
Secondary centres in the calcaneum appear at about 10 years and fuse at about 18 years
The navicular ossifies in the 4th year
Cuneiforms ossify, lateral in the 1st year, medial in the 3rd and intermediate in the 4th years of life
Metatarsals (epiphysis of the 1st is at the base and at the head of the other toes) ossify 2 – 3 years later than those of the hand at about the 5th year, and fuse at about 18 years
Secondary centres may also appear in the lateral tubercle of the talus, the tuberosity of the navicular and the styloid process of the 5th metatarsal



Ossifies in membrane and appears before any cartilage is evident in the body
The first bone of the skeleton with two centres ossifying in the fifth week which rapidly fuse
Elongation occurs at the sternal end and a cartilaginous epiphysis appears late in the teens and fuses a couple of years later



Forms by chondrification of mesenchyme in the 6th week and bony centres appear in the 8th week in the glenoid angle
The centre at the base of the coracoid appears at about 10 years and fuses with the glenoid at puberty
At birth the blade and spine are ossified
Secondary centres appear in the acromion, coracoid, medial border and inferior angle at about puberty and are fused by the age of 25 years



Cartilaginous at 6/52 with the primary centre of ossification appearing in the 8th week
Secondary centres appear at both ends

Upper end:
Head in the 1st year
Greater tuberosity in the 3rd year
Lesser tuberosity in the 5th year
These fuse together by the 7th year and to the shaft at about 20 years
Lower end:
Capitellum in the 2nd year
(Radial head in the 4th year)
Medial epicondyle in the 5th year
Trochlea in the 12th year
Lateral epicondyle in the 13th year
Union with the shaft occurs at about 18 years



Appears in cartilage at 6 weeks and the primary centre of ossification appears in the 8th week
The distal end is the growing end and secondary centres appear early in the second year with the centre for the head appearing in the 4th year which fuse with the shaft at about 18 years



Cartilage at 6 weeks and the primary centre appears in the shaft at about 8 weeks
The head ossifies at about 6 years and fuses with the shaft at about 20 years. The small olecranon epiphysis appears at about 8 years and fuses at about 18 years (does not involve the articular surface)



Each carpal bone ossifies from one centre the largest (capitate in the first year and then one each year until the 7th with the pisiform becoming ossified in the 10th year

  • Capitate: 1st year
    Hamate: 2nd year
    Triquetral: 3rd year
    Lunate: 4th year
    Trapezium: 5th year
    Scaphoid: 6th year
    Trapezoid: 7th year
    Pisiform: 10th year

Shafts of metacarpals and phalanges ossify in utero
The epiphysis is at the base of the thumb metacarpal and the phalanges and through the neck of the 2nd, 3rd, 4th and 5th metacarpals
The ungual tuberosities of each distal phalanx ossifies in membrane



Vertebrae develop from sclerotomes which surround the notochord and neural tube in a sheath of mesoderm

A series of cartilaginous rings appear in the mesodermal sheaths and each ring ossifies in 3 centres, the centrum and two halves of the neural arch

The centre of the centrum is double but the two halves rapidly fuse, failure of formation of one half® hemivertebrae
Each one ossifies by the 8th week and by the third year all these parts are jointed
the epiphysis for the body appears as bony rings soon after puberty and fuse with the bodies in the early twenties

Secondary centres also appear in the tips of the spinous and transverse processes and the mamillary body of T12 which also fuse in the early 20's

Costal elements of the cervical and lumbar vertebrae do not have separate centres of ossification but an occasional centre in C7 or L1® formation of cervical or lumbar ribs

Costal elements form the anterior and lateral margins of the vertebral foramen of cervical vertebrae and in the sacrum form the lateral mass which does have separate ossification centres which appear in the 6th month of foetal life and fuse with the neural arch elements at about 5 years and with each other and the sacral bodies at about 20 years

The lower part of the dens ossified at birth and fuses with the centrum of C2 at 4 years of age. The cartilaginous apex has begun to ossify at this time and fuses with the rest of the dens at 12 years


VACTERLS (previously VATER)

  • = nonrandom unexplained assocaiation of system defects
  • Vertebral anomalies – failures of segmentation, congenital scoliosis
  • Anal anomalies – imperforate, anal atresia
  • Cardiac abnormalities
  • TracheoEosophageal fistula
  • Renal anomalies
  • Limb anomalies – usually radially based – from hypoplastic thumb to club hand
  • Single umbilical artery




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