Fetal Development - 12 Weeks

Introduction

Images on this current page show head ossification occurring during the early fetal period (12 weeks approx 92 mm CRL in size). The head undergoes two different forms of ossification (endochondral and intramembranous) in separate regions of the skull. Furthermore the two images (lateral and medial) identify regions of cartilage development. Compare this 12 week fetus with the earlier 10 week Fetal and Carnegie stage embryos: size, head proportions, brain, head, skeletal development.

12 week Head

These are 2 views of the same 12 week 92 mm CRL human fetus head, double stained to show both cartilage (blue) and newly-formed bone (red).

Lateral View

In this lateral (external) view, note the distribution of new bone in the plates of the cranial vault, temporal bone, orbit, upper jaw (maxilla) and lower jaw (mandible) regions. (More? Notes Skull | Face) Bony regions in the lower jaw (mandible) region also show spaces where tooth formation is occuring. (More? Tooth Development)

Medial View

In this medial (internal) view, note the distribution of cartilage from the nasal region through the base of the skull to the atlas/axis (with new bone forming). See also the original Meckel's cartilage within the newly forming bony mandible.

Mandible

Selected extract of above images showing 2 views (lateral/external left, medial/internal right) of the mandible region.

Note: Cartilage in the neck where the hyoid bone will eventually form. Meckel's cartilage visible in the medial view within the newly forming bone.

Nose and Mouth

(Image: Virginia Diewert)

Selected medial head view showing key features of head musculoskeletal and neurological development. Note extensive nasal cartilage, nasal conchae, pituitary, secondary palate, oral cavity, tongue, mandible, hyoid, choana, oropharynx.

Terms

choana - the posterior nasal aperture which is the passageway from the back of one side of the nose to the throat. There are two choanae, one on either side of the nose.

mandible

nasal conchae - the thin scroll-shaped bony elements forming three groove-like air passages/chambers in the nasal cavities.

oropharynx - the oral (within the mouth) part of the pharynx which extends from the uvula to the level of the hyoid bone.

Also note the developing tongue musculature and its mandibular attachment site.