Ultrastructure of developing muscle in the upper limbs of the human embryo and fetus

Background: The ultrastructure of the myogenesis, which proceeds along with the appearance of muscle-specific proteins and isozymes, has not been fully described in the upper limb of staged human embryos. Methods: Eight human embryos (Carnegie stage 14–22) and two fetuses (11 and 12 weeks of gestation) were fixed with 5% glutaraldehyde, 4% paraformaldehyde, and 0.2% picric acid in 0.1 M phosphate buffer, pH 7.2. The upper limbs were dissected out and processed for transmission electron microscopy, and sections of the biceps brachii muscle were cut and examined. Results: At stage 14, the myoblasts were loosely scattered in the ventral proximal region of the upper limb bud and had a small amount of cytoplasm with a few intracellular organelles. At stage 16, the myoblasts were spindle shaped and oriented parallel to the axis of the upper limb bud. These cells had irregularly shaped nuclei with prominent nucleoli, rough endoplasmic reticulum (ER), and mitochondria, but no myofilaments were observed. At stages 17–19, rough ER, free ribosomes, and mitochondria increased in number and thick and thin filaments with faint Z-lines appeared in the peripheral cytoplasm of the myotube. The plasma membranes of some neighboring myotubes were continuous, suggesting that these cells were in the initial stages of the fusion process. At stage 22, the striated pattern of the myofilaments became evident and tubular structures appeared around them and near the plasma membrane. In the fetus at the 11th week, the basal lamina began to surround the myotubes, and T-tubules with sarcoplasmic reticulum were observed. Dyads and triads were observed in the myotube of the 12th week fetus. Conclusion: These findings suggest that rapid myogenesis occurs during the late embryonic period in human upper limbs and that the ultrastructural characteristics of mature myotubes are established during the early fetal period. © 1995 Wiley-Liss, Inc.     

Meckel's cartilage in the human embryo and fetus

This work studied the development of the ventral part of Meckel's cartilage in a series of human embryos (classified in stages) and fetuses. These stages appeared particularly important: stage 16, appearance of Meckel's cartilage; stage 20, beginning of membranous ossification of mandible; and stage 23, end of the embryonic period (8th week). The primitive bony nodule which develops from the embryonic mesenchyme appears as a double bony layer forming a groove containing the neurovascular bundle, into which the dental lamina is also invaginated. It was concluded that during the fetal period, the cartilage participates in the formation of the body of the mandible in an area close to the mental foramen via endochondral ossification. The cartilage disappears in parallel with the development of ossification by the sixth month. © 1994 Wiley-Liss, Inc.     

Control of the internal anal sphincter (manometric study with human subjects)

Summary Manometric studies of the functions of the internal anal sphincter were performed in 73 children. Twenty-one out of these patients were normal subjects, the others had neurological lesions either central (sacral meningocele, dorsal cord transections) or peripheral (Hirschsprung's disease). It was found that the functions of the internal sphincter do not depend upon mechanical factors and are subject to nervous control. The pathways for control of the basal sphincteric tone are discussed. It is likely that this tone depends not only of the sympathetic pathways, but also of fibers of the sacral outflow. It is shown that the recto-anal inhibitory reflex is specific and independent of peristalsis. Arguments are given to prove that this reflex is an activity proper to the intra-mural plexus which is, however, subject to and regulated by the sacral cord.     

The morphogenesis of human sphincter urethrae muscle

Summary The morphogenesis of the sphincter urethrae muscle was studied in human ontogeny. Muscles of 65 embryos and fetuses, 7 newborns, 3 children and 3 adults of both sexes were examined histologically and by means of microdissection. Three developmental stages can be recognized in terms of morphogenetic events, histogenesis and development of sexual dimorphism. In the sexually indifferent stage (up to approximately 50 mm crown-rump length), the sphincter urethrae primordium is formed by a shallow arch apposed only to the ventrolateral wall of the urethra. The primordium extends from the level of the urogenital diaphragm up to the vesicourethral transition. It consists of a condensation of mononuclear cells. Myotubes appear in fetuses of 30 mm crown-rump length. During the second stage (until birth) sexual dimorphism develops in conjunction to the formation of the prostate and vagina. In this stage, the sphincter urethrae muscle fibres gradually extend to the posterior wall of the urethra. At the same time cranially situated muscle fibres project to the lateral wall of the prostate, whereas in females caudally located muscle fibres attach to the lateral wall of the vagina. In this way the sphincter achieves the sexually distinct form. The definitive arrangement develops in the third morphogenic stage (after birth), in which a complete muscle ring is formed by encircling the urethra in the infraprostatic part in males and in the upper, larger part of the sphincter in females. The sphincter urethrae muscle is located inside the sling of the puborectalis muscle in both sexes, but no muscle fibres connect them to one another. A part of the puborectal muscle fibres orginates from the connective tissue plate lying on the ventral face of the sphincter in the level of the bladder neck. From this arrangement it can be suggested that fibres of the puborectalis muscle participate in the ensurence of urinary continence.     

Expression of cathepsin K in the human embryo and fetus.

Cathepsin K is a protease with high collagenolytic and elastinolytic activity. Its cellular expression was previously thought to be restricted to osteoclasts and osteoclast-mediated bone resorption. In this study, the expression of cathepsin K in the human embryo and fetus was demonstrated by immunohistochemistry, in situ hybridization, and by Northern blotting of fetal tissue extracts. Besides osteoclasts and chondroclasts and their precursors, epithelial cells of various organ systems expressed significant amounts of this enzyme. Respiratory and gastrointestinal mucosa, including bile duct epithelia and urothelia, showed high levels of cathepsin K expression. With the exception of the urothelium, showing a more homogenous expression pattern, the protease was usually accentuated in the surface cell layers of pithelia. In summary, these findings in the human embryo and early fetus demonstrated a significant expression of cathepsin K in different epithelial cell types besides osteoclasts. The functional aspects of cathepsin K expression in nonosteoclastic cells and potential conclusions on physiological and pathological conditions in the embryo-fetal or adult organism remain to be investigated. Dev Dyn 1999;216:89-95.     

Origin of the styloglossus muscle in the human fetus

The origin of the styloglossus muscle was histologically studied bilaterally in nine human fetuses (18 sides). In all cases, the muscle originated in Reichert's cartilage, which gives rise to the temporal styloid process. We identified three types of variation: type A, an accessory muscle fascicle originating from the mandibular angle, found in 7 cases (12 sides); type B, where the styloglossus muscle was attached to the mandibular angle by fibrous tracts, found in three cases (4 sides); and type C, where an accessory muscle fascicle arose from the fibrous tract connecting Reichert's cartilage to the mandibular angle; found in one case. In all cases (2 sides), the styloglossus muscle was innervated by the hypoglossal nerve. Relationships between the styloglossus muscle and vasculonervous elements of the prestyloid and retrostyloid spaces were analysed.     

The urethral sphincter muscle in the male

The male urethral sphincter is a striated muscle in contact with the urethra from the base of the bladder to the perineal membrane. The individual muscle fibers are 25 to 30% smaller than fibers of associated muscles and are embedded in connective tissue which obscures the visibility of the whole muscle. The muscle primordium is laid down around the urethra prior to the development of the prostate. Subsequently, the prostate develops as a diverticulum of the urethra and grows into the developing sphincter, thinning the overlying musculature. With the onset of puberty, accelerated growth of the prostate displaces the sphincter, with atrophy of the overlying muscle, resulting in what may appear to be isolated segments of the sphincter muscle distributed around the prostate. The prostate overgrows the anterior portion of the urethra and the associated sphincter muscle. There is no distinct superior fascia of the so-called urogenital diaphragm separating the sphincter muscle from the prostate. The fascia of the sphincter muscle is inseparable from the prostatic sheath, is oriented vertically, and passes through the urogenital hiatus to unite with the fascia of the pudendal canals at the isochiopubic rami. Thus, the sphincter muscle is a component of a bladder-urethra-prostate-sphincter unit which lies within the pelvis, in the urogenital hiatus, and rests upon the perineal membrane. The concept of a urogenital diaphragm is not borne out by this study.     

Detection of the immunoregulator p43-placental isoferritin in the human embryo and fetus

Human placental isoferritin (PLF) is known to exert an immunosuppressive activity in vitro and is involved in the down-regulation of the maternal immune system during pregnancy. We have investigated the presence of PLF in the human embryo and early fetus and its secretion into amniotic fluid (AF) and fetal blood. Immunohistochemistry was performed on 25 normal embryos and fetuses, at 7-22 weeks gestation, using the CM-H9 monoclonal antibody (mAb), generated specifically against the human p43-PLF protein. The amount of p43 was measured in AF of 81 fetuses at 11-22 weeks and in the blood of 19 fetuses at 15-22 weeks by means of an enzyme-linked immunosorbent assay with the same mAb. Positive p43-PLF immunostaining was found from 7 weeks gestation in proximal tubules of the primitive nephron and macrophages of the liver sinusoids, blood vessels and mesenchymal tissue. In the AF samples, p43-PLF was first detected at week 15 gestation and thereafter steadily increased with advancing gestation whereas in fetal blood, p43-PLF was below or just above the lower limit of the assay. The gap between the first appearance of 43-PLF in embryonic tissue and its secretion into the amniotic fluid is probably linked to maturation of the renal function. The detection of the p43-PLF immunomodulator protein in macrophages at a very early stage of embryonic development and its very low concentration in fetal blood suggests that its immunoregulatory role is limited to the feto-maternal interface.     

A study in organogenesis: The arterial supply of the anorectal region in the human embryo and fetus

The development of the anorectal region is based on differentiation of the terminal portion of the hindgut. The origin of anorectal malformations is still unknown but seems to occur very early in the embryologic period. To gain a better understanding of their development, a study of the arterial supply of the anorectal region was made in 26 embryos and 50 fetuses.     

The striated urogenital sphincter muscle in the female

Striated muscle associated with the female urethra and vagina constitute a continuous mass which appropriately may be called the urogenital sphincter. Though continuous, the muscle may be separated into two parts—one that surrounds the urethra, and the other surrounding the urethra and vagina. The individual muscle fibers are small and are embedded in connective tissue and infiltrated with smooth muscle which obscures the visibility of the muscle to gross dissection. Developmentally the muscle primordium is laid down around the urogenital sinus and urethra early, and foreshadows the anatomical arrangement that is maintained in the adult with little change. The urogenital sphincter muscle extends from the base of the bladder where it lies within the pelvic cavity and continues through the urogenital hiatus of the pelvic diaphragm to expand around the vagina in the perineum. Additional fibers attach to the ischiopubic rami and constitute a compressor of the urethra. As a result there is no superior fascia of the so-called “urogenital diaphragm” which closes off a deep perineal compartment or forms a floor of the urogenital hiatus.     

Membrane properties and the neuro-effector transmission of smooth muscle cells in the canine internal anal sphincter.

The most distal part of the circular muscle layer functions as the internal anal sphincter, which constitutes a high pressure zone at rest, but maintains a relaxed state during defecation. To elucidate such sphincter mechanisms of the smooth muscle cells, the circular muscle layer in the canine anal canal was examined within 2 cm from the anal verge. Both the mechanical and intracellular electrical activities were recorded simultaneously. The examined region could be divided into three different regions according to the pattern of spontaneous activity and innervation and consisted of an upper region (20-15 mm from the anal verge), a transitional region (15-5 mm from the anal verge) and a lower region (within 5 mm from the anal verge), respectively. The spontaneous membrane activity was characterized by ongoing slow potential changes and each potential change was associated with a phasic contraction in the three regions. The mean frequencies of spontaneous electrical activity were 6.8, 15.9, and 24.1 c/min in the upper, transitional and lower regions, respectively. In the transitional and lower region, muscle tone generation was observed. Transmural field stimulation (0.4 msec in pulse duration) evoked membrane depolarization and contractions in the lower region. The application of an alfa adrenergic blocking agent completely suppressed the generation of excitatory responses, leaving a long lasting hyperpolarization associated with relaxation. In the transitional and upper region, stimulation consistently evoked membrane hyperpolarization with relaxation. The characteristics of this hyperpolarization response varied among the three regions. The total duration of hyperpolarization increased distally, while the time to peak hyperpolarization became decreases in a reverse direction. These regional differences in the characteristics of spontaneous membrane activity and innervation indicate that the transitional and lower region might therefore function as the internal anal sphincter.