The human digastric muscle: patterns and variations with clinical and surgical correlations.

The digastric muscle is located in the suprahyoid region on each side and frequently exhibits two muscular bellies (anterior and posterior) linked by an intermediate tendon. The paired digastric muscles act together either depressing the mandible or elevating the hyoid bone; therefore acting as a single muscle with important physiological roles. In the present study, the digastric muscle has been analyzed bilaterally in 74 adult human cadavers. A computerized morphometrical investigation of the digastric muscles has been performed (Image Pro Plus software package, Media Cybernetics, USA) and the resulting quantitative data have been statistically assessed (SPSS 11.0 for Windows, USA). We hereby propose an original morphological classification that encompasses five types (I-V) for the anterior belly (AB); three types (I-III) for the intermediate tendon (IT); and two types (I-II) for the posterior belly (PB) of the human digastric muscle. In each digastric muscle, the aforementioned anatomical types have been characterized according to the muscular bellies and intermediate tendon. Consequently, as a result of the combinations of those diverse types, individual digastric muscles have been considered as pertaining to distinctive morphological patterns (named from A to J). Cases with absence of either AB or PB have been included in patterns K and L and would be more appropriately defined as monogastric muscles. This innovative classification provides clear-cut anatomical parameters for interpreting morphological variants of the digastric muscle with relevant clinical and surgical correlations.

Experimental chronic entrapment of the sciatic nerve in adult hamsters: an ultrastructural and morphometric study

Entrapment neuropathy is a group of clinical disorders involving compression of a peripheral nerve and interference with nerve function mostly through traction injury. We have investigated the chronic compression of peripheral nerves as an experimental procedure for detecting changes in ultrastructural nerve morphology. Adult hamsters (Mesocricetus auratus, N = 30) were anesthetized with a 25 percent pentobarbital solution and received a cuff around the right sciatic nerve. Left sciatic nerves were not operated (control group). Animals survived for varying times (up to 15 weeks), after which they were sacrificed and both sciatic nerves were immediately fixed with a paraformaldehyde solution. Experimental nerves were divided into segments based upon their distance from the site of compression (proximal, entrapment and distal). Semithin and ultrathin sections were obtained and examined by light and electron microscopy. Ultrastructural changes were qualitatively described and data from semithin sections were morphometrically analyzed both in control and in compressed nerves. We observed endoneurial edema along with both perineurial and endoneurial thickening and also the existence of whorled cell-sparse structures (Renaut bodies) in the subperineurial space of compressed sciatic nerves. Morphometric analyses of myelinated axons at the compression sites displayed a remarkable increase in the number of small axons (up to 60 percent) in comparison with the control axonal number. The distal segment of compressed nerves presented a distinct decrease in axon number (up to 40 percent) comparatively to the control group. The present experimental model of nerve entrapment in adult hamsters was shown to promote consistent histopathologic alterations analogous to those found in chronic compressive neuropathies

Experimental chronic entrapment of the sciatic nerve in adult hamsters: an ultrastructural and morphometric study.

Entrapment neuropathy is a group of clinical disorders involving compression of a peripheral nerve and interference with nerve function mostly through traction injury. We have investigated the chronic compression of peripheral nerves as an experimental procedure for detecting changes in ultrastructural nerve morphology. Adult hamsters (Mesocricetus auratus, N = 30) were anesthetized with a 25% pentobarbital solution and received a cuff around the right sciatic nerve. Left sciatic nerves were not operated (control group). Animals survived for varying times (up to 15 weeks), after which they were sacrificed and both sciatic nerves were immediately fixed with a paraformaldehyde solution. Experimental nerves were divided into segments based upon their distance from the site of compression (proximal, entrapment and distal). Semithin and ultrathin sections were obtained and examined by light and electron microscopy. Ultrastructural changes were qualitatively described and data from semithin sections were morphometrically analyzed both in control and in compressed nerves. We observed endoneurial edema along with both perineurial and endoneurial thickening and also the existence of whorled cell-sparse structures (Renaut bodies) in the subperineurial space of compressed sciatic nerves. Morphometric analyses of myelinated axons at the compression sites displayed a remarkable increase in the number of small axons (up to 60%) in comparison with the control axonal number. The distal segment of compressed nerves presented a distinct decrease in axon number (up to 40%) comparatively to the control group. The present experimental model of nerve entrapment in adult hamsters was shown to promote consistent histopathologic alterations analogous to those found in chronic compressive neuropathies.

Extracellular matrix molecules play diverse roles in the growth and guidance of central nervous system axons.

Axon growth and guidance represent complex biological processes in which probably intervene diverse sets of molecular cues that allow for the appropriate wiring of the central nervous system (CNS). The extracellular matrix (ECM) represents a major contributor of molecular signals either diffusible or membrane-bound that may regulate different stages of neural development. Some of the brain ECM molecules form tridimensional structures (tunnels and boundaries) that appear during time- and space-regulated events, possibly playing relevant roles in the control of axon elongation and pathfinding. This short review focuses mainly on the recognized roles played by proteoglycans, laminin, fibronectin and tenascin in axonal development during ontogenesis.

Extracellular matrix molecules play diverse roles in the growth and guidance of central nervous system axons

Axon growth and guidance represent complex biological processes in which probably intervene diverse sets of molecular cues that allow for the appropriate wiring of the central nervous system (CNS). The extracellular matrix (ECM) represents a major contributor of molecular signals either diffusible or membrane-bound that may regulate different stages of neural development. Some of the brain ECM molecules form tridimensional structures (tunnels and boundaries) that appear during time- and space-regulated events, possibly playing relevant roles in the control of axon elongation and pathfinding. This short review focuses mainly on the recognized roles played by proteoglycans, laminin, fibronectin and tenascin in axonal development during ontogenesis

Molecular tunnels and boundaries for growing axons in the anterior commissure of hamster embryos.

We have analyzed the immunohistochemical expression of chondroitin sulfate proteoglycan (CSPG), fibronectin (FN), laminin (LN), tenascin (TN), and glial fibrillary acidic protein (GFAP) along the anterior commissure (AC) of hamster embryos (n=175; from embryonic day (E)12 to E16). Frozen sections were cut at different planes from embryonic brains between E12 and E16, treated for immunohistochemistry, and observed under epifluorescence microscopy. During the pre-crossing stage (E12-E13), CSPG was expressed as a sagittal stratum between the interhemispheric fissure and the prospective AC region. TN appeared rostral to the third ventricle and along the medial subventricular zone of the lateral ventricles. LN and FN both presented a faint expression, and GFAP was not detected. Although AC axons started crossing the midline region (E13.5-E14), CSPG, FN, LN, and, much less intensely, GFAP circumscribed the AC bundle, forming a tunnel through which AC fibers elongate. TN was no longer seen at the midplane but remained visible laterally. During the post-crossing stage (E14.5-E16), CSPG and TN were no longer seen at the midline, although both could be observed between the AC limbs, seeming to form boundaries for AC lateral growth. LN and FN were then absent near the AC bundle. During this late stage, GFAP expression became most intense, forming a distinct tunnel around the AC. We have shown that the expression of extracellular matrix molecules and GFAP follow a time- and space-regulated course related to AC development, plausibly representing influential factors for growth and guidance of commissural fibers.

Topographic organization in the anterior commissure of developing hamsters.

The present study has examined the existence of a topographic organization in the anterior commissure (AC) of developing hamsters. Fluorescent carbocyanine crystals (DiI and/or DiA) were implanted into different rostrocaudal and dorsoventral sectors of the paleocortex of hamsters ranging in age from E15 to P10 (E16 = P1 = date of birth). The cerebral hemispheres of each brain were cut horizontally and sagittally, respectively, and the sections were observed under a fluorescence microscope coupled to a computerized reconstruction system. A distinct topographic organization of AC fibers was observed along the rostrocaudal axis starting at E15, and continued unchanged thereafter. These results support the hypothesis that the orderly pattern of AC fibers is achieved by active positioning during the first days after crossing the midplane rather than by a regressive sculpting from an initially disorganized pattern.

Topographic organization in the anterior commissure of developing hamsters

The present study has examined the existence of a topographic organization in the anterior commissure (AC) of developing hamsters. Fluorescent carbocyanine crystals (DiI and/or DiA) were implanted into different rostrocaudal and dorsoventral sectors of the paleocortex of hamsters ranging in age from E15 to P10 (E16 = P1 = date of birth). The cerebral hemispheres of each brain were cut horizontally and sagittally, respectively, and the sections were observed under a fluorescence microscope coupled to a computerized reconstruction system. A distinct topographic organization of AC fibers was observed along the rostrocaudal axis starting at E15, and continued unchanged thereafter. These results support the hypothesis that the orderly pattern of AC fibers is achieved by active positioning during the first days after crossing the midplane rather than by a regressive sculpting from an initially disorganized pattern.

The prenatal development of the anterior commissure in hamsters: pioneer fibers lead the way.

The prenatal development of the anterior commissure (AC) was studied in 130 hamster embryos with ages varying from E12 to E16 (E1 = day of conception and E16 = P1 = day of birth) by use of carbocyanine crystals (DiI, DiA and/or DiO) implanted into different rostrocaudal segments of the paleocortex. On E12 and E13, many AC axons were seen with tortuous trajectories pointing towards the midline (precrossing stage). On E13.5 and E14, most AC fibers abutted the midsagittal plane, led by a few pioneer axons that grew as far as 500 microns ahead into the opposite hemisphere (crossing stage). Pioneers were present in most brains at these ages irrespective of the rostrocaudal position of the carbocyanine crystal. Somata of pioneer axons could be identified by retrograde labelling. They were characteristically immature neurons, located either in the olfactory peduncle or in the superficial layers of the olfactory cortex. On E14.5 and E15, pioneers and followers were seen close to the targets and on E15.5 and E16 interstitial budding occurred, and arborization started within the olfactory peduncle and the paleocortex (postcrossing stage). If the existence of pioneer fibers represents something more than a stochastic phenomenon, their appearance in the developing AC may reflect the operation of signals at the midline and/or in the contralateral hemisphere that either accelerate the growth of pioneers, or decelerate the growth of followers.

Pioneer axons in the anterior commissure of hamster embryos.

The existence and the morphological characteristics of pioneer axons in the anterior commissure (AC) were investigated in hamster embryos using fluorescent carbocyanine crystals implanted into the paleocortex. On embryonic days 12-13 (E12-E13), a group of tortuous fibers was seen close to the midline. After this group of fibers reached the midline on E13.5-E14, pioneer axons were seen in the AC, and from E14.5 to E15.5 both pioneers and followers approached the targets, the former ahead of the latter by about 500 microns. Arborization took place as soon as the followers reached the targets (E15.5-E16). The pioneer cells were labeled retrogradely and were seen in the most superficial layers of the paleocortex, along the rostrocaudal extent of the hemisphere. The fact that pioneers appear only during or after crossing suggests the existence of factors either in the midline region and/or in the opposite hemisphere, which selectively accelerate the growth of the pioneers or decelerate the growth of the followers.

Pioneer axons in the anterior commissure of hamster embryos

The existence and the morphological characteristics of pioneer axons in the anterior commissure (AC) were investigated in hamster embryos using fluorescent carbocyanine crystals implanted into the paleocortex. On embryonic days 12-13(E12-E13), a group of tortuous fibers was seen close to the midline. After this group of fibers reached the midline on E13.5-E14, pioneer axons were seen in the AC, and from E14.5 to E15.5 both pioneers and followers approached the targets, the former ahead of the latter by about 500 µm. Arborization took place as soon followers approached the targets, the former ahead of the latter by about 500 µm. Aarborization took place as soon as the followers reached the targets (E15.5-E16). The pioneer cells were labeled retrogradely and were seen in the most superficial layears of the paleocortex, along the rostrocaudal extent of the hemisphere. The fact that pionners appear only during or after crossing suggests the existence of factors either in the midline region and/or in the opposite hemisphere, which selectively accelerate the growth of the pioneers or decelerate the growth of the followers