FIGURE 2 | Key stages in the development of the mammalian head.

From the following article:

Genetics of craniofacial development and malformation

Andrew O. M. Wilkie & Gillian M. Morriss-Kay

Nature Reviews Genetics 2, 458-468 (June 2001)


Genetics of craniofacial development and malformation

a–d | Scanning electron micrographs. a | Neural-fold-stage rat embryo, embryonic day (E)8.5 (equivalent to mouse E8.0, and human 20 days post-ovulation); anterior (rostral) is to the left. The major part of the neural tissue present at this stage is the future brain region; the preotic sulcus (arrowhead), divides the future brain into two parts: forebrain plus midbrain plus rostral hindbrain (future rhombomeres 1 and 2) and the more caudal hindbrain (rhombomeres 3–8). The arrow indicates the approximate position of the brain–spinal cord boundary. The most posterior (caudal) region is the primitive streak (ps). b | Eight-somite stage rat embryo, E9.0 (equivalent to mouse E8.5, human 22 days). Neural-tube closure is initiated at the seven-somite stage in rodent and human embryos, in the region of the brain–spinal cord junction; it progresses in both rostral and caudal directions (large arrows). The second groove, caudal to the preotic sulcus (arrowhead), is the otic sulcus (small arrow), which marks the position of rhombomere 5. In the rostral forebrain (TELENCEPHALON), the concavity of the optic sulcus (os) can be seen. (S1, first somite.) c | Eight-somite-stage rat embryo with surface ectoderm removed to reveal migrating neural crest (blue): frontonasal/first-arch crest and hyoid crest cells just starting to emigrate. The mandibular part of the first branchial arch (m) enlargens as the crest cells enter it; the heart (ht) is intimately associated with the developing head at this stage. (Image courtesy of Seong Seng Tan.) d | Formation and onset of fusion of the facial processes: human 41 days post-ovulation (equivalent to mouse E12.0). The facial processes form by the accumulation of neural-crest-derived mesenchyme around the mouth (maxillary and mandibular components of the first branchial arch, mx and md, respectively) and nasal pits (medial and lateral nasal swellings, mn and ln, respectively). Fusion of the maxillary and medial nasal swellings forms the upper lip; the two medial nasal swellings together form the premaxillary region of the upper jaw, which in humans forms the philtrum of the lip, the primary palate and the upper incisor teeth. (e, eye.) e | Formation of the secondary palate: human seventh and tenth week post-ovulation (equivalent to mouse E12.5 and E14.5–15.5, respectively). The palatal shelves form from the internal aspect of the maxillary swellings; they are at first vertically orientated, either side of the tongue; as the tongue descends they reorientate to a horizontal position and then grow together, to fuse with each other and with the nasal septum. f | Ossification of the skull vault and formation of the cranial sutures: E16 mouse head (equivalent to approx4 months post-ovulation in humans). RNA in situ hybridization showing expression of the bone differentiation marker, Spp1 (secreted phosphoprotein 1). (Bones: f, frontal; ip, interparietal; n, nasal; p, parietal. Sutures: c, coronal; l, lambdoid; m, metopic; s, sagittal. e, eye.) (Image courtesy of Sachiko Iseki.) For conversion of embryonic days to Theiler and Carnegie stages, see link to the Atlas and Database of Human Developmental Anatomy.

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