Two Neck Phenotype

Normal Mammalian Neck Development:

In mammals, the neck develops in coordination with the rest of the trunk. In short, the primitive streak moves from the posterior to the anterior of the embryo creating a grove (“streak”) in the tissue. The streak signals the surrounding to migrate inward, toward the center of the embryo. This migration and convergence of the epiblast toward the streak and eventually through it creates more specialized tissue layers, such as endoderm and mesenchymal mesoderm.

When the streak reaches its anterior most limit it begins to move back from where it came from (the posterior). At this point the leading edge of the migrating cell mass is referred to as Hensen’s Node (HN). HN’s main task is to lay down the notochord as it moves toward the posterior of the embryo which, in turn, creates a gradient of maturity with the more mature area of the embryo lying toward the anterior pole.

Once HN has passed and the notochord has been laid, the ectoderm on both sides of the midline begins to thicken and then elevate. Following elevation these two cell populations hinge toward the midline, meet, combine, and eventually separate from the overlying ectoderm to create a hollow neural tube separate from the overlying ectoderm it was derived from. Additionally, neural crest cells appear at the ectoderm-neural tube boarder, near the midline. These cells are also derived from ectoderm. Following their formation the overlying epidermis induces them to express Slug and RhoB proteins by secreting BMP-4 and BMP-7. This induction is highly important because this causes the neural crest cells to lose their tight junctions and and N-Cadherin expression, thus allowing them to migrate. If Slug or RhoB protein expression is suppressed or halted, the neural crest cells will fail to migrate.

Next, the segmental plate which lies laterally of the neural tube on both sides and is composed of mesoderm begins to segment (or pinch off) into individual somites at the rate of 1 pinched off somite for every 90 minutes. This temporal clock is thought to be regulated by the Lfng, Hes1, Hes7, Hey2 genes (Erol, et al., 2003).

Picture showing the "pinching off" of somites.

Used and modified with permission © K Tosney

Now that the neural tube, somites, and various tissue layers are present, the embryo is ready to continue forming its central nervous system and begin forming its peripheral nervous system, vertebra, and (more toward the posterior) viscera, ribs and other anatomic features of the mammal (see organogenesis).  

The actual formation of the neck in mammal, as us Peppeteers think of it, has mostly to do with their neural tube, somites, and neural crest cells. The somites, of which part will become vertebra, not only are segmented themselves, but also lead to segmentation of the migrating neural crest cells and thus, the autonomic nervous system (ANS).

Neural Crest Migration:

After the somites have formed they further differentiate into three main tissues: dermatome, myotome, and sclerotome. If the somite is yet to differentiate and form these three tissues, the neural crest will not migrate from the neural tube. It isn’t until the somites form the three cell populations mentioned above that the neural crest cells begin to migrate from their starting positions. Once the neural crest cells sense sclerotome they begin to migrate into it. This migration, however, does not happen at all locations along the somites. Only the anterior most pole of each somite allows for the migration of neural crest cells. This is due to the posterior inhibitory somite tissue that expresses ephrin proteins which the neutral crest cells can recognize with their eph receptors. Thus, the somites create a segmental pattern along the anterior-posterior (AP) axis. Furthermore, since the PNS is derived from neural crest cells it exhibits a segmental pattern also.

Picture showing the migration of neural crest cells.

Used and modified with permission © K Tosney

Somites:

To enable flexibility within the neck (and back), the neck is composed of vertebra. These so called vertebra are develop from medial sclerotome in an interesting fashion. To form the vertebra the medial sclerotome divides horizontally (in a transecting fashion). Following this division the anterior half of one joins with the posterior half of the adjacent segment. Since somites are present on both sides of the neural tube, the same process occurs on both sides and, synchronously, fuse at the dorsal and ventral midline surrounding the neural tube (Peck, et al., 2003).

In order for the sclerotome to form the vertebral column, Sonic hedgehog (SHH) expression appears to be obligatory (Bonaventure, 2003). The notochord contributes the SHH. Additionally, different vertebral segments are coded for by Hox gene expression (Bonaventure, 2003). Following cartilaginous vertebra formation, the cartilage ossifies due to the implantation of osteoblasts into the cartilage at the ossification center. The osteoblasts secrete a calcium salt matrix which mineralizes the bone.

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Puppeteer Development

As described in the eyes and brain sections, the neural tubes which the Puppeteer’s necks develop from the diencephalon where the formation of the optic vesicle causes the underlying neural tissue to undergo rapid mitosis. So, neck development in us is different than mammalian neck development. Our neck’s neural tissue is not derived from the tissue running parallel to the primitive streak and Hensen’s Node. Instead, they are formed as described above and migrate toward the anterior pole. During the migration of the neural tubes, overlying ectoderm must “stretch” toward the anterior to accommodate the developing neural tubes. Additionally, the migration of the neural tubes stimulates the mesenchymal mesoderm to undergo mitosis. Thus, when the neural tubes reach their desired height, the surrounding ectoderm and mesoderm have compensated and are sitting in the same position as we see in mammals.

Following the migration of the neural tube and subsequent compensation of the ectoderm and mesoderm, the somites begin to form bilaterally, as they do in mammal. At about the same time the neural crest cells are expelled from the neural tube due to induction of these cells by the overlying ectoderm (the gene/cue is yet to be determined). At this point the migration of the neural crest cells begin in the segmented pattern seen in mammals, only after they have been induced by the myotome to express RhoB and Slug proteins. In Puppeteers this segmental pattern is due to the expression of Ephrin proteins on the posterior sclerotome. In Puppeteers, the migration of neural crest cells in predominately responsible for the formation of sensory neurons within the necks. Once the neural crest cells have migrated the sclerotome begins to form the vertebra and, like in mammal, the sclerotome splits and the bottom of one segment fuses with the top of another. Following this fusion, the vertebra encompass the neural tube and fuse medially. Your necks are now formed. Since the necks contain vertebra they are highly flexible, yet sturdy, and pack enough strength to protect the afferent optical signals entering the brain. Afterall, a Puppeteer who can’t see is no Puppeteer at all!

Sources:

Bonaventure J. “Skeletal development in human: a model for the study of developmental genes.” Atlas of Genetics and Cytogenetics in Oncology and Haematology. University Hospital, 2003.

Erol B. Kusumi K. Lou J. Dormans JP. “Etiology of Congenital Scoliosis.” UPOJ (15) Pages 37-42. 2002.

Peck WW. Hesselink JR. Barkovich JA. “Pediatric Spinal Anomalies.” University of California – San Diego, Dept. of Neuroradiology. 2003.

Online at: http://spinwarp.ucsd.edu/NeuroWeb/Text/sp-140.htm

Tosney K.W.  “Neural Crest.”  Biology 208: Embryology, Ann Arbor. 2005. 

Tosney K.W.  “Somites”  Biology 208: Embryology, Ann Arbor. 2005.