传统的发育学理论认为牙根的发育与上皮跟鞘的形成有关，而后者会断裂形成Malassez上皮剩余（ERM).上皮跟鞘由颈环发育而成，内外釉上皮在颈环处相互融合形成上皮跟鞘，而其内侧的被认为含有上皮干细胞的星网状侧细胞消失，因此牙根不能一直生长。啮齿类动物的切牙可以终生生长，它被分为了牙冠发育模式的唇侧和牙根发育模式的舌侧，唇侧的上皮包括一个颈环结构，其内含有星网状层细胞，可以不断的分化为成釉细胞并形成釉质。而切牙的舌侧被认为是牙根发育的模式，最终形成ERM. 我们发现切牙的舌侧的颈环结构内也包括少量的星网状层细胞，推测其可能发挥干细胞龛的作用。于此类似我们通过观察不断生长的Sloth磨牙和K-14-Eda转基因小鼠的切牙发现，他们在发育过程中始终保持星网状层细胞结构，不形成上皮根鞘，但可以形成ERM.因此我们推测上皮根鞘并不是启动牙根发育的必须结构。除此之外，我们认为，牙冠发育较之牙根发育：例如釉质的形成较之牙骨质的形成以及成釉细胞的分化较之ERM的形成是由是否保存上皮干细胞来独立调控的。这种发育的可调控性可能说明在牙齿发育和进化过程中的多样性。

Evol Dev. 2008 Mar-Apr;10(2):187-95.

Observations on continuously growing roots of the sloth and the K14-Eda transgenic mice indicate that epithelial stem cells can give rise to both the ameloblast and root epithelium cell lineage creating distinct tooth patterns.Tummers M, Thesleff I.
Institute of Biotechnology, Viikki Biocenter, University of Helsinki, Finland. mark.tummers@helsinki.fi

Root development is traditionally associated with the formation of Hertwig's epithelial root sheath (HERS), whose fragments give rise to the epithelial cell rests of Malassez (ERM). The HERS is formed by depletion of the core of stellate reticulum cells, the putative stem cells, in the cervical loop, leaving only a double layer of the basal epithelium with limited growth capacity. The continuously growing incisor of the rodent is subdivided into a crown analog half on the labial side, with a cervical loop containing a large core of stellate reticulum, and its progeny gives rise to enamel producing. The lingual side is known as the root analog and gives rise to ERM. We show that the lingual cervical loop contains a small core of stellate reticulum cells and suggest that it acts as a functional stem cell niche. Similarly we show that continuously growing roots represented by the sloth molar and K14-Eda transgenic incisor maintain a cervical loop with a small core of stellate reticulum cells around the entire circumference of the tooth and do not form a HERS, and still give rise to ERM. We propose that HERS is not a necessary structure to initiate root formation. Moreover, we conclude that crown vs. root formation, i.e. the production of enamel vs. cementum, and the differentiation of the epithelial cells into ameloblasts vs. ERM, can be regulated independently from the regulation of stem cell maintenance. This developmental flexibility may underlie the developmental and evolutionary diversity in tooth patterning.