Development of axial skeleton system with diagram

Reading File

Finding Sources

Reading File

Loading Image

Reading File

Loading Image

Reading File

Loading Image

Reading File

Loading Image

Here is a comprehensive, textbook-based account of the development of the axial skeleton, drawn from The Developing Human: Clinically Oriented Embryology.

Development of the Axial Skeleton

The axial skeleton consists of the cranium (skull), vertebral column, ribs, and sternum. Its development begins in the third week of embryogenesis and continues into early adult life.

1. Overview: Somites and the Sclerotome

During the third week, intraembryonic mesoderm lateral to the notochord and neural tube thickens to form two longitudinal columns of paraxial mesoderm. These columns become segmented into blocks called somites (bead-like elevations along the dorsolateral embryo surface).

Each somite differentiates into two parts:

Part	Region	Derivative
Sclerotome (ventromedial)	Surrounds neural tube and notochord	Vertebrae, ribs
Dermomyotome (dorsolateral)	- Myotome region - Dermatome region	Muscle (myoblasts) - Dermis (fibroblasts)

Somite differentiation - paraxial mesoderm forming sclerotome and dermomyotome

Fig. 14.1 - Somite formation and differentiation. A: 18-day embryo (dorsal view). B: Paraxial mesoderm. C: Early somites at 22 days. D: Somite folding at 24 days. E: Sclerotome, myotome, and dermatome at 26 days.

2. Development of the Vertebral Column

Stages of Vertebral Development

Stage 1 - Mesenchymal (Precartilaginous) Stage

During the fourth week, sclerotomal cells migrate to three locations:

Around the notochord (future vertebral bodies)
Surrounding the neural tube (future vertebral arches)
Into the body wall (future ribs)

Each sclerotome has a cranial zone of loosely packed cells and a caudal zone of densely packed cells. The densely packed cells from one sclerotome fuse with the loosely arranged cells of the immediately caudal sclerotome to form the mesenchymal centrum (primordium of the vertebral body). This means each vertebral body is an intersegmental structure, derived from two adjacent sclerotomes - this is why spinal nerves lie between vertebrae and intersegmental arteries cross the vertebral bodies.

The notochord degenerates within the vertebral bodies but persists between them, expanding to form the nucleus pulposus of the intervertebral disc. The annulus fibrosus forms from surrounding mesenchyme. The neural arch (primordium of the vertebral arch) forms from mesenchyme surrounding the neural tube.

Vertebral column formation from sclerotomal cells

Fig. 14.6 - A: 4-week embryo showing sclerotome migration. B: Frontal section showing loosely and densely packed sclerotomal cells. C: Condensation forming the mesenchymal vertebra. D: Vertebral body formation, nucleus pulposus, and intervertebral disc.

Key molecular regulators:

TBX6, Hox, and PAX genes - regulate patterning along the anterior-posterior axis
Fat4 and Dchs1 (protocadherins) - mediate planar cell polarity and early chondrogenesis
Notch signaling - patterning of the vertebral column

Stage 2 - Cartilaginous Stage

During the sixth week, chondrification centers appear in each mesenchymal vertebra. The two centers in each centrum fuse to form a cartilaginous centrum by the end of the embryonic period. Centers in the neural arches fuse with each other and with the centrum. Chondrification spreads until a complete cartilaginous vertebral column is formed.

Stage 3 - Bony (Ossification) Stage

Ossification begins in the seventh week and is not complete until age 25.

Primary ossification centers (3 per vertebra):

1 in the centrum (from fusion of ventral + dorsal centers)
1 in each half of the neural arch (present by 8th week)

Secondary ossification centers (5 per vertebra, appear after puberty):

Tip of spinous process (1)
Tip of each transverse process (2)
Anular epiphyses - superior and inferior rims of vertebral body (2)

All secondary centers unite with the vertebra by approximately age 25.

Stages of vertebral development from mesenchymal to ossified

Fig. 14.7 - A: Mesenchymal vertebra at 5 weeks. B: Chondrification centers at 6 weeks. C: Primary ossification centers at 7 weeks. D: Thoracic vertebra at birth (3 bony parts). E-F: Secondary ossification centers after puberty.

Normal vertebral numbers: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (minor variations exist).

3. Development of the Ribs

Ribs develop from the mesenchymal costal processes of thoracic vertebrae (visible in Fig. 14.7A as costal processes)
They become cartilaginous during the embryonic period and ossify during the fetal period
The original union site of costal processes with vertebrae is replaced by costovertebral synovial joints

Classification by attachment:

Type	Pairs	Attachment
True ribs	1-7	Directly to sternum via own cartilage
False ribs	8-10	Via cartilage of another rib
Floating ribs	11-12	No sternal attachment

4. Development of the Sternum

A pair of vertical mesenchymal bands called sternal bars develops ventrolaterally in the body wall
Chondrification occurs as they move medially
By 10 weeks, they fuse craniocaudally in the median plane to form cartilaginous models of:
- Manubrium
- Sternebrae (segments of sternal body)
- Xiphoid process
The manubrium also receives contributions from neural crest cells
Centers of ossification appear craniocaudally before birth (except the xiphoid process, which appears during childhood and may never fully ossify)

5. Development of the Cranium (Skull)

The cranium develops from mesenchyme around the developing brain and is divided into two parts:

Neurocranium (braincase)

a) Cartilaginous Neurocranium (Chondrocranium)

Forms the base of the skull via endochondral ossification. Multiple cartilages fuse sequentially:

Cartilage	Fuses to form
Parachordal cartilage (basal plate)	Base of occipital bone + foramen magnum
Hypophyseal cartilage	Body of sphenoid
Trabeculae cranii	Body of ethmoid
Ala orbitalis	Lesser wing of sphenoid
Otic capsules	Petrous and mastoid parts of temporal bone
Nasal capsules	Contributes to ethmoid

Ossification sequence: occipital bone → body of sphenoid → ethmoid bone

b) Membranous Neurocranium (Calvaria)

Forms the sides and top of the skull via intramembranous ossification in head mesenchyme. The flat bones (frontal, parietal, squamous temporal, squamous occipital) are separated in fetal life by:

Sutures - fibrous joints between bones
Fontanelles - 6 large fibrous areas at suture intersections

Cranial base development showing cartilaginous and membranous components

Fig. 14.8 - Superior views of cranial base development. A: 6 weeks (separate cartilages). B: 7 weeks (paired cartilages fusing). C: 12 weeks (fused cartilaginous base). D: 20 weeks (bone derivation shown by color).

Fontanelles and their closure times:

Fontanelle	Closure
Posterior	2-3 months after birth
Anterolateral (sphenoid)	2-3 months after birth
Posterolateral (mastoid)	End of first year
Anterior (largest)	End of second year
Frontal suture	Obliterated by ~8 years

The fontanelles allow molding of the calvaria during birth and rapid postnatal brain growth.

Fetal skull showing fontanelles and sutures

Fig. 14.9 - Fetal skull. A: Lateral view showing sphenoid and mastoid fontanelles, frontal and parietal eminences. B: Superior view showing anterior and posterior fontanelles, sagittal, coronal, and lambdoid sutures. C: 3D ultrasound at 22 weeks.

Viscerocranium (facial skeleton)

Derived largely from neural crest cells that migrate into the pharyngeal arches. Hox genes regulate neural crest migration and differentiation.

First pharyngeal arch (Meckel's cartilage): Malleus and incus (middle ear ossicles)
Second pharyngeal arch (Reichert's cartilage): Part of stapes, styloid process, lesser horn of hyoid
Third pharyngeal arch: Greater horn and lower body of hyoid

Membranous Viscerocranium

Intramembranous ossification of neural crest mesenchyme forms: maxilla, zygomatic bone, squamous temporal bone, most of mandible, and the nasal and lacrimal bones.

Summary Diagram: Components and Origins of the Axial Skeleton

AXIAL SKELETON
├── VERTEBRAL COLUMN
│   └── Source: Sclerotome (paraxial mesoderm → somites)
│   └── Stages: Mesenchymal (Wk 4) → Cartilaginous (Wk 6) → Ossification (Wk 7 → Age 25)
│   └── Key: Each vertebral body is INTERSEGMENTAL (from 2 adjacent sclerotomes)
│   └── Notochord → Nucleus pulposus (persists between vertebrae)
│
├── RIBS
│   └── Source: Costal processes of thoracic vertebrae (sclerotome)
│   └── Ossify during fetal period
│   └── Types: True (1-7), False (8-10), Floating (11-12)
│
├── STERNUM
│   └── Source: Sternal bars (body wall mesenchyme + neural crest for manubrium)
│   └── Fuse craniocaudally by 10 weeks
│   └── Ossifies craniocaudally (xiphoid process last)
│
└── CRANIUM
    ├── NEUROCRANIUM
    │   ├── Cartilaginous (chondrocranium) → Endochondral ossification → Skull BASE
    │   └── Membranous (calvaria) → Intramembranous ossification → Skull VAULT
    └── VISCEROCRANIUM
        ├── Cartilaginous → Neural crest + pharyngeal arches → Middle ear ossicles, hyoid
        └── Membranous → Neural crest mesenchyme → Maxilla, mandible, zygomatic, etc.

Source: The Developing Human: Clinically Oriented Embryology, Chapter 14 (Skeletal System Development), pp. 901-926. All diagrams are from the same source.

Clinical note: Defects in Notch-pathway genes can cause severe vertebral anomalies, including VACTERL syndrome (vertebral, anal, cardiac, tracheal, esophageal, renal, and limb defects). Residual notochord tissue can give rise to a chordoma, a rare slow-growing malignant tumor occurring at the cranial base or lumbosacral region.

This is a shared conversation. Sign in to Orris to start your own chat.