Embryology of the Axial Skeleton

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Questions and Answers

What is the fate of the notochord in the region of the bodies of vertebrae?

It forms the vertebral arch

It forms the central part of the intervertebral discs

It degenerates (correct)

It forms the annulus fibrosus part of the intervertebral discs

What is formed by the mesoderm surrounding the notochord?

Annulus fibrosus (correct)

Vertebral arch

Nucleus pulposus

Costal processes

At what stage do the chondrification centers appear?

At puberty

At 25 years

At the end of 8th week

At 6th week (correct)

What is the result of the failure of two embryonic neural arches to fuse with each other?

Spina Bifida Signup and view all the answers

When do the 5 secondary ossification centers appear?

At puberty Signup and view all the answers

When do all centers unite?

At 25 years Signup and view all the answers

What is the purpose of giving folic acid to mothers prior to conception?

To reduce the risk of spina bifida Signup and view all the answers

What is the most severe and complex form of spina bifida?

Spina bifida cystica Signup and view all the answers

What is the characteristic of spina bifida occulta?

Only one vertebra is affected Signup and view all the answers

What is the term for the protrusion of a sac containing meninges and cerebrospinal fluid?

Meningocoele Signup and view all the answers

What is the term for the formation of ribs from the ventral extensions of the sclerotomic mesenchyme?

Development of the ribs Signup and view all the answers

What is the most common rib anomaly?

Accessory lumbar rib Signup and view all the answers

What is the main source of mesenchyme that forms bones and cartilages?

Paraxial mesoderm Signup and view all the answers

What is the process by which mesenchymal cells differentiate into chondroblasts?

Chondrogenesis Signup and view all the answers

What is the term for the imprisonment of chondroblasts within the intercellular substance of developing cartilage?

Chondrocytes Signup and view all the answers

What is the classification of cartilage based on the presence of collagen or elastic fibers?

Hyaline cartilage, fibrocartilage, and elastic cartilage Signup and view all the answers

What is the process by which the vertebral column develops from somites?

Somitogenesis Signup and view all the answers

What is the term for the two-layered blastula becoming a three-layered gastrula?

Gastrulation Signup and view all the answers

What percentage of cases is an accessory cervical rib found in?

0.5-1% Signup and view all the answers

What is the condition caused by pressure on the lower trunk of the brachial plexus and subclavian artery?

Superior thoracic outlet syndrome Signup and view all the answers

How do the two sternal bars develop during the development of the sternum?

They are converted into cartilage to form cartilaginous sternal bars Signup and view all the answers

At what age is the fusion of the sternebrae complete?

25 years Signup and view all the answers

What is the process by which the cartilaginous model of the sternum is formed?

Chondrification Signup and view all the answers

At what age does the xiphoid process typically fuse completely with the sternum?

40 years Signup and view all the answers

What is the primary source of the mesenchyme that develops into the membranous neurocranium?

Neural crest cells and paraxial mesoderm Signup and view all the answers

What is the percentage of adult brain size reached by the age of 4 years?

75% Signup and view all the answers

What type of ossification forms the membranous bones that constitute the vault of the skull?

Membranous ossification Signup and view all the answers

What is the term for the narrow seams of fibrous tissue in the newborn skull?

Sutures Signup and view all the answers

What is the reason for the relatively small size of the viscerocranium in the newborn skull?

Small size of facial bones, particularly the jaws and virtual absence of paranasal air sinuses Signup and view all the answers

What is the source of the cartilaginous skeleton that forms the neurocranium?

Neural crest cells migrating into the pharyngeal arches Signup and view all the answers

Study Notes

Development of the Axial Skeleton

The axial skeleton consists of the skull (cranium), vertebral column, sternum, and ribs.
Development of the vertebral column begins with the development of the notochord during gastrulation.

Fate of the Notochord

In the region of the bodies of vertebrae, the notochord degenerates.
Between the bodies of vertebrae, the notochord forms the central part, nucleus pulposus, of the intervertebral discs.
The annulus fibrosus part of the intervertebral discs is formed by the mesoderm surrounding the notochord.

Development of the Vertebral Column

The vertebral column develops from somites (42 pairs) derived from paraxial mesoderm.
The notochord induces the formation of the sclerotomes from the paraxial mesoderm.
The sclerotomes grow dorsally around the neural tube and form the vertebral (neural) arch.
Ventrolaterally, costal processes develop that give rise to ribs in the thoracic region.

Ossification of the Vertebral Column

Ossification of the vertebral column occurs in two stages: chondrification and ossification.
Chondrification centers appear at 6 weeks (cartilage).
By the end of 8 weeks, the 3 primary ossification centers appear (bone).
The 5 secondary ossification centers appear at puberty.
Fusion of bony halves of vertebral arch occurs at 3-5 years.
Fusion of centrum with vertebral arch occurs at 4-6 years.
All centers unite around 25 years.

Spina Bifida

Spina bifida is a neural tube defect that occurs when two embryonic neural arches fail to fuse with each other.
It is one of the most serious vertebral defects.
It occurs in 1 per 1000 births and may be prevented in many cases by giving folic acid to mothers prior to conception.

Types of Spina Bifida

Spina bifida occulta: the closed type, only one vertebra is affected, and skin overlying it is intact.
Spina bifida cystica: the open type, which is the most severe and complex form of spina bifida.

Development of the Ribs

Ribs are formed from the ventral extensions of the sclerotomic mesenchyme that forms the vertebral arches.
Accessory ribs may develop, such as accessory lumbar rib and accessory cervical rib.

Development of the Sternum

The sternum develops from two mesenchymal vertical plates on either side of the midline.
These plates are called mesenchymal sternal bars.
The two sternal bars fuse with each other in a craniocaudal sequence to form the cartilaginous model of the sternum.
The cartilaginous model of the sternum gets ossified to form the sternum.

Ossification of the Sternum

Ossification of the sternum occurs in a craniocaudal sequence during 5th, 6th, 7th, 8th, and 9th months.
Manubrium and body ossify by five double centers from above downward.
The fusion is complete by 25 years of age.
Xiphoid process appears during the 3rd month and fuses completely with the sternum at about 40 years.

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Take this quiz to test your knowledge on the development of the axial skeleton from mesenchyme, including chondrogenesis and the formation of cartilages and bones. Learn about the three sources of mesenchyme and how they contribute to the skeletal system.