Search for: All records

INTRODUCTION:The gastrulation brain homeobox (Gbx) genes are essential for patterning and maintenance of neurons along the anteroposterior axis of the developing neural tube. Knockout (ko) of Gbx2 results in neonatal lethality associated with neurological and other defects. To understand pathologies, ko studies are not realistic as gene loss usually results in death before birth. Gbx2 neo/neo mutant mice express 6-10% of the wildtype Gbx2 expression levels. They have milder malformations than ko mice but lack the cerebellar vermis and mandibular division of the trigeminal nerve (3rd division of 5th cranial nerve = V3), among other defects. These Gbx2 neo/neo mutant mice die perinatally as they are unable to suckle due to lack of motor innervation via V3 to the muscles of mastication. Muscle cells develop largely without interaction with their motor nerve. However, the final differentiation of myocytes requires interactions with nerve fibers. Unexpectedly, the muscles of mastication appear normal in Gbx2 neo/neo mutant mice, despite the lack of V3. METHODS:We performed microdissections of neonate wildtype and Gbx2neo/neo mutant mice. Additionally, we embedded mutant and wildtype mouse embryos in paraffin, serial sectioned them (7 μm), stained the sections with Azan staining, and analyzed specimen microscopically. RESULTS:Current analysis of the data to identify where nerve fibers in the muscles of mastication originate is in process. We favor the origin of these fibers from the facial nerve (7th cranial nerve), which has several overlapping territories with the trigeminal nerve. CONCLUSIONS:Muscles require innervation for their final differentiation steps. The loss of a nerve can result in the invasion of another nerve into the territory of the lost one, which rescues muscle differentiation but not muscle function. IACUC Ziermann Med-20-02, Funding NSF #2000005 to Ziermann. 

Birth defects affect one in every 33 babies born in the United States each year. A developmental biologist at Howard University in the US, Dr Janine M. Ziermann is studying the head, neck, and heart to find out how head and heart birth defects form. 

A key player in brain and neural crest development is the gastrulation-brain-homeobox (Gbx) transcription factor family member, Gbx2. During the early stages of gastrulation, Gbx2 RNA is broadly expressed in the prospective hindbrain and posterior region of the embryo. Later it becomes restricted to a sharp transverse band at the interface between the prospective midbrain and hindbrain, and is maintained in the anterior hindbrain in the developing neuroaxis (Bouillet et al. 1995; Li & Joyner 2001; Martinez-Barbera et al. 2001). Gbx2 regulates diverse developmental processes, including anteroposterior patterning within the mid-/hindbrain boundary and anterior hindbrain (Burroughs-Garcia et al. 2011). Expression of Gbx2 is required for the correct formation of rhombomeres r1-r3 (Wassarman et al. 1997). Loss of Gbx2 function in mouse embryos (Gbx2-/-), results in aberrant neural crest cell patterning leading to defects in neural crest derivatives and to abnormalities in the central nervous system, craniofacial, and cardiovascular components (Byrd & Meyers 2005). Li et al. (2009) demonstrated that Gbx2 is a direct target of the neural crest inducer Wnt, and is essential for neural crest induction. Together, these studies show that Gbx2 resides upstream in the genetic cascade controlling neural crest development and directly regulates the expression of key molecules involved in the migration and survival of neural crest cells that differentiate into neural and other components (e.g., connective tissue) of the head and heart. It was shown that Gbx2neo/neo mouse embryos, in which wild-type levels of Gbx2 expression is reduced to 6-10% of normal, are useful to further elucidate the complexity concerning the role of Gbx2 in anterior hindbrain development (Waters & Lewandoski 2006). Among other malformations, in Gbx2neo/neo embryos the mandibular branch of the trigeminal nerve (CNV3) is absent. CNV3 innervates the muscles of mastication (e.g., pterygoids, masseter, temporalis). However these muscles are needed to suckle and neonate (P0) Gbx2neo/neo mice are not able to suckle and die perinatally (Langenbach & van Eijden 2001). Here we describe the anatomy of the trigeminal ganglion and the trigeminal nerves in neonate Gbx2neo/neo mice and evaluate if there are differences in the muscles of mastication in these mice as compared to wildtype specimens. We expected that we find clear abnormalities in the thickness of the masseter, temporalis, and other muscles innervated by CNV3. However, this is not the case, indicating that the innervation of a muscle is not, as previously thought, needed for the differentiation of the muscles. Histological analyses will give insights into the muscle cell structure and if this is altered in the Gbx2neo/neo mice, which could be related to the loss of motor innervation. The research was funded by NSF EiR HBUC 18-522 awarded to JMZ (#2000005) and STW (#1956450). Bouillet et al. (1995). Dev Dyn, 204: 372-82. Burroughs-Garcia et al. (2011). Dev Dyn, 240: 828-38. Byrd & Meyers (2005). Dev Biol, 284: 233-45. Langenbach & van Eijden(2001). Am Zool, 41: 1338-51. Li et al. (2009). Development, 136: 3267-78. Li & Joyner (2001). Development, 128: 4979-91. Martinez-Barbera et al. (2001). Development, 128: 4789-800. Wassarman et al. (1997). Development, 124: 2923-34. Waters & Lewandoski (2006) Development, 133: 1991-2000. Funding or Support Information: The research was funded by NSF EiR HBUC 18-522 awarded to JMZ (#2000005) and STW(#1956450).