Human Anatomy - Kotsan I. Ya. 2009

Skeleton of the head
Embryogenesis of the human skull

The initial signs of Skull base development appear in the embryo at the beginning of the second month of intrauterine life as an accumulation of HEAD sclerotome Cells around the cranial end of the notochord, corresponding to the level of the Hindbrain vesicle. Subsequently, these Cell clusters transform into the cartilaginous plate of the skull base, which is divided into two parts: a posterior segmented part located along the sides of the cranial end of the notochord (parachordals), and an anterior unsegmented part situated rostral to the cranial end of the notochord, known as the trabeculae cranii.

Anterior and lateral to the cartilaginous skull base plate, the nasal and otic cartilaginous capsules develop initially unattached to it. Later on, these cartilaginous components fuse together, except for the space between the parachordals and trabeculae, which becomes overgrown only after the invagination into the cranial cavity of a pinched-off ectodermal portion of the stomodeum, serving as the source of adenohypophysis development. This gives rise to the chondrocranium (primordial skull), representing its phylogenetically ancient part and serving as the precursor for most of the BONES OF THE skull base.

The nasal capsule gives rise to the following bones: the ethmoid, lacrimal, nasal, Vomer, and inferior nasal conchae. The cartilaginous skull base plate develops into the sphenoid (except for the medial plate of its pterygoid process) and the Occipital bone (except for the upper part of the squamous portion). The otic capsule forms the petrous part and the mastoid process of the Temporal bone.

The bones of the cranial vault, which appear later in phylogeny, bypass the cartilaginous stage during embryonic development and form directly from a membranous matrix. Ossification of all these bones (the frontal, parietals, squamous and tympanic PARTS OF THE temporals, and the upper part of the occipital squama) begins at weeks 9–10 of Embryogenesis, significantly earlier than the bones of the skull base.

The facial Skeleton develops during embryogenesis on The basis of the pharyngeal (branchial) arches. The first branchial arch gives rise to the Maxilla, palatine, zygomatic, Mandible, as well as the malleus and incus of the Middle ear; the second branchial arch forms the Hyoid bone, the styloid process of the temporal bone, and the stapes of the middle ear.