4. Development of the skull. Growth of the bones. Malformations

Learning objectives

  • From which embryonic tissues do the various parts of the skeleton originate from?
  • What is mesenchyme?
  • Describe endochondral ossification
  • Which bones are formed by endochondral ossification?
  • Describe intramembranous ossification
  • Which bones are formed by intramembranous ossification?
  • What are the parts of the skull?
  • From which embryonic tissues does the membranous neurocranium originate from?
  • What are the sutures and fontanelles and what are their functions?
  • When do the fontanelles close?
  • From which embryonic tissues does the cartilaginous neurocranium originate from?
  • From which embryonic tissues does the viscerocranium originate from?
  • What is cranioschisis?
  • What is cranial meningocele and meningoencephalocele?
  • What is craniosynostosis?
  • What is achondroplasia?

Growth of bones

The axial skeleton, consisting of the skull, vertebral column, ribs, and sternum is generally formed from paraxial mesoderm. The appendicular skeleton, consisting of the skeleton of the limbs, the shoulder girdle, and the pelvic girdle is generally formed from lateral plate mesoderm.

The cells of the sclerotome of the somites form a new type of tissue called mesenchyme. Mesenchyme, also called embryonic connective tissue, is a polymorphous and loosely organized tissue. Mesenchymal cells can differentiate into fibroblasts (produce extracellular matrix), chondroblasts (produce cartilage), and osteoblasts (produce bone).

Neural crest cells in the head region also form mesenchyme and contribute to the formation of the skull.

Bone is formed by one of two methods of ossification: endochondral ossification or intramembranous ossification.

Endochondral ossification

In endochondral ossification mesenchymal cells first create a hyaline cartilage “model” of the bone, which is later converted to bone. This type of ossification is characteristic for long bones of the limbs, and the base of the skull.

Mesenchymal cells first differentiate into chondroblasts, which form the cartilage model of the bone to be formed. The cartilage model is surrounded by a perichondrium, in which mesenchymal cells will differentiate into osteoblasts. These osteoblasts will form a bone collar of bone around the diaphysis of the cartilage model.

The cartilage of the model will then function as a framework for osteoblasts to form new bone. This process begins at the diaphysis and proceeds in the direction of both epiphyses, forming two ossification zones.

Intramembranous ossification

In intramembranous ossification mesenchymal cells directly form bone, without a model. This type of ossification is characteristic for flat bones, like the calvarium, mandible, and clavicle.

In this type of ossification, no chondroblasts are involved. Instead, mesenchymal cells directly differentiate into osteoblasts.

Development of the skull

The skull is divided into two parts: the neurocranium and the viscerocranium. The viscerocranium involves the skeleton of the face, while the neurocranium involves the rest.

The neurocranium has two parts: the membranous part, consisting of flat bones which surround the brain as a vault, and the cartilaginous part, which forms the bones of the base of the skull.

The membranous neurocranium

The membranous neurocranium is derived from neural crest cells and paraxial mesoderm. Mesenchyme derived from these sources form the membranous neurocranium by intramembranous ossification.

At birth the flat bones of the skull are separated by narrow seams of connective tissue; these structures are called sutures. At points where more than two bones meet fontanelles are formed, of which there are 6. The largest is the anterior fontanelle. There is also the posterior fontanelle, and there are two sphenoidal fontanelles and two mastoid fontanelles.

The presence of these fontanelles and sutures allow the skull to change shape slightly, without which it would be impossible to pass through the birth canal. After birth the sutures and fontanelles close. The posterior fontanelle closes after a couple of months while the anterior fontanelle closes within 18 months.

The cartilaginous neurocranium

Also called the chondrocranium, this part of the neurocranium is formed be endochondral ossification. The bones which lie anteriorly to the sella turcica arise from neural crest cells while those that lie posteriorly to this point arise from the occipital sclerotomes of the paraxial mesoderm.

The viscerocranium

The viscerocranium arises from neural crest cells from the two pharyngeal arches. The first pharyngeal arch gives rise to the maxilla, zygomatic bone, palatine bone, and mandible. It also forms the Meckel cartilage, from which the malleus and incus are formed.

The second pharyngeal arch gives rise to the stapes and the styloid process of the temporal bone.

Relevant malformations

Cranioschisis

Cranioschisis refers to when the cranial vault fails to form, causing the brain to be exposed to amniotic fluid. This results in anencephaly as the brain tissue degenerates. This is incompatible with life.

Cranial meningocele and meningoencephalocele

Cranial meningocele refers to when the meninges herniate through small defects in the skull. If brain tissue also herniates it’s called a meningoencephalocele. These can be treated surgically.

Craniosynostosis

Craniosynostosis refers to the premature fusion of one or more cranial sutures. It only causes asymptomatic abnormal shape of the head, but surgery is recommended to prevent intracranial complications.

Achondroplasia

Achondroplasia is a condition caused by defective receptor for FGF, which impairs endochondral ossification, but not intramembranous ossification. The result is very short limbs but a normal sized head.

Summary

  • From which embryonic tissues do the various parts of the skeleton originate from?
    • The axial skeleton is generally formed from paraxial mesoderm
    • The appendicular skeleton is generally formed from lateral plate mesoderm
  • What is mesenchyme?
    • Mesoderm (embryonic connective tissue) is a tissue from which fibroblasts, chondroblasts, and osteoblasts arise
  • Describe endochondral ossification
    • Mesenchymal cells first create a hyaline cartilage “model” of the bone, which is later converted to bone
    • Two ossification zones are formed in the diaphysis, which proceed in the direction of the epiphyses
  • Which bones are formed by endochondral ossification?
    • Long bones of the limbs, base of the skull
  • Describe intramembranous ossification
    • Mesenchymal cells directly form bone without a cartilage model
  • Which bones are formed by intramembranous ossification?
    • Calvarium, mandible, clavicle, etc.
  • What are the parts of the skull?
    • Neurocranium
      • Membranous neurocranium
      • Cartilaginous neurocranium
    • Viscerocranium
  • From which embryonic tissues does the membranous neurocranium originate from?
    • From neural crest cells and paraxial mesoderm
  • What are the sutures and fontanelles and what are their functions?
    • The sutures are narrow seams of connective tissue between flat bones of the skull
    • The fontanelles are points where more than two flat bones of the skull meet
    • The sutures and fontanelles allow the skull to change shape during birth
  • When do the fontanelles close?
    • The posterior fontanelle closes after a couple of months while the anterior fontanelle closes within 18 months.
  • From which embryonic tissues does the cartilaginous neurocranium originate from?
    • The bones which lie anteriorly to the sella turcica arise from neural crest cells
    • The bones which lie posteriorly to this point arise from the occipital sclerotomes of the paraxial mesoderm
  • From which embryonic tissues does the viscerocranium originate from?
    • First pharyngeal arch
      • Maxilla
      • Mandible
      • Zygomatic bone
      • Palatine bone
      • Meckel cartilage
        • Malleus
        • Incus
    • Second pharyngeal arch
      • Stapes
      • Styloid process
  • What is cranioschisis?
    • A condition where the cranial vault fails to form, causing anencephaly
  • What is cranial meningocele and meningoencephalocele?
    • Conditions where meninges or meninges and brain herniate through a small defect in the skull, respectively
  • What is craniosynostosis?
    • A condition where one or more sutures fuse prematurely
  • What is achondroplasia?
    • A condition characterised by dysfunctional endochondral ossification but normal intramembranous ossification, causing short limbs and stature but normal head

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5. Development of the blood vessels. Malformations

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