Embryonic Event Research Articles

Spontaneous regression of patent omphalomesenteric duct: From a fistula to Meckel's diverticulum

A newborn with a patent omphalomesenteric duct is described in this report. This anomaly is a very rare one. Regression of the omphalomesenteric duct occurs as a normal embryonic event in the intrauterine life between the fifth and ninth week of gestation; however, in this case regression occured spontaneously in the neonatal period and resulted in a Meckel's diverticulum with no connection with the umbilicus. This is the first case to be reported.

Patterning of connective tissues in the head: discussion report

ABSTRACT The three papers presented by Noden, Thorogood and Lumsden in this session encompassed the connective tissues as broadly defined, i.e. soft (fibrous) connective tissue, cartilage, bone, muscle and the dental tissues, enamel and dentine, and utilized a variety of experimental techniques on both avian and mammalian embryos to explore specificity and patterning of the vertebrate head. Whether similar developmental processes pattern homologous structures in different Vertebrate classes (Amphibia, Aves, Mammalia) was discussed with reference to patterning of the cranial musculature, chondrocranium and dental tissues. A number of challenging ideas emerged during this session. Does the premigratory neural crest consist of a homogenous population of totipotent cells or of subpopulations of bi- or tripotential cells? Is fundamental patterning of the head an early embryonic event, perhaps specified during primary embryonic induction or the consequence of neuroepithelial folding, brain growth, inductive interactions and/or spatially and temporally distributed extracellular matrix products? Can the fact that mesoderm and angioblasts do not display distinctive patterning that relates to their particular embryologic origins be extrapolated to patterning in general? How does the documentation of an ondontogenic trunk neural crest in mammals affect our theories of how patterning mechanisms arose or were modified during vertebrate evolution?

Embryology of Bladder Exstrophy

A hypothesis in respect to the teratogenesis of bladderexstrophy and its variants is offered. The central feature of this hypothesis is the abnormal persistance of the caudal position of the insertion of the body stalk on the embryo. As a consequence of this, the normal advance and interposition of mesenchymal tissue to the midline becomes impossible. The cloaca cannot be translocated backwards into the body cavity and the cranial end of the cloacal membrane remains in contact with the inferior aspect of the low-set body stalk. This, in contrast to the previously proposed abnormal rostral extention of the cloacal membrane, causes a wedge-effect resulting in the lateralization of the abdominal wall structures and also in the prevention of the midline fusion of the genital hillocks (labioscrotal or genital folds). A cloacal membrane normally is an unstable structure lacking mesoderm, and it retains these characteristics in the superficial and infraumbilical position to be described. It has a strong tendency to disintegrate. It may rupture at variable times and to a variable extent. The consequence of such an embryonic event is either a typical bladder exstrophy or one of the variants of the exstrophy malformation. Three different variants are presented that the proposed embryologic hypothesis can readily explain.

Embryology of bladder exstrophy

A hypothesis in respect to the teratogenesis of bladder exstrophy and its variants is offered. The central feature of this hypothesis is the abnormal persistence of the caudal position of the insertion of the body stalk on the embryo. As a consequence of this, the normal advance and interposition of mesenchymal tissue to the midline becomes impossible. The cloaca cannot be translocated backwards into the body cavity, and the cranial end of the cloacal membrane remains in contact with the inferior aspect of the low-set body stalk. This, in contrast to the previously proposed abnormal rostral extension of the cloacal membrane, causes a wedge-effect resulting in the lateralization of the abdominal wall structures and also in the prevention of the midline fusion of the genital hillocks (labioscrotal or genital folds). A cloacal membrane normally is an unstable structure lacking mesoderm, and it retains these characteristics in the superficial and infraumbilical position to be described. It has a strong tendency to disintegrate. It may rupture at variable times and to a variable extent. The consequence of such an embryonic event is either a typical bladder exstrophy or one of the variants of the exstrophy malformation. Three different variants are presented that the proposed embryologic hypothesis can readily explain.

Evidence for two active X chromosomes in germ cells of female before meiotic entry.

THERE is considerable evidence that, as a result of some early embryonic event, the genes of one X chromosome in each somatic cell of the human female are not expressed1. Studies of somatic tissues and cultured cells (including fibroblast clones) from human embryos, heterozygous for the common electrophoretic variant of glucose-6-phosphate dehydrogenase (G6PD), indicate that a single active X is present in cells from various tissues, at least by 5 weeks from conception2. On the other hand, the presence of the heteropolymorphic form of the enzyme in oocytes of the heterozygous adult female3 and a 16-week-old foetus4 is compelling evidence that, in meiotic stage germ cells, there are two active X chromosomes; however, the basis for the two active chromosomes in oocytes is not clear. It is conceivable that germ cell progenitors escape inactivation because inactivation occurs when cells destined to become germ cells have already been imprinted. Alternatively, only a single X chromosome may be active in all cells of the early zygote, with the second X activated only in germ cells at some time after their differentiation. The third possibility, that germ cells are subject to inactivation, but that reactivation occurs when meiosis commences, has been suggested by Gartler et al. because they did not observe the heteropolymer in ovaries from a 12-week-old human foetus heterozygous for the G6PD variant5,6. The ovary at that developmental stage contains approximately 23% germ cells, with a preponderance of premeiotic and leptotene germ cells7. But the implication that two functional X chromosomes are required for the onset of mammalian meiosis is not supported by evidence that XO mice produce normal gametes8. Using human foetal material, we have now obtained evidence against the third possibility.

Some Aspects of Morphogenesis in Cuscuta gronovii

More than 100 primary root segments from dormant seeds, 100 radicular segments from immature embryos, 450 whole immature embryos, and a number of whole seedlings of C. gronovii were studied under in vitro cultivation in the presence of such growth-regulating factors as: coconut milk, casein hydrolysate, auxins, kinetin, adenine, gibberellic acid, and additional B vitamins as supplements to White's medium. In normal embryogeny early swelling of the basal pole of the embryo and profuse production of epidermal hairs upon germination are superficial characteristics of Cuscuta which resemble seedling root growth in other plants. At no time is there evidence of a primary-root meristem, nor at any time do cotyledons develop. It was not possible to induce root formation experimentally from any of the starting embryonic materials, nor from stems which were cultured to maturity culminating in the production of flowers after 6 months of in vitro cultivation, nor from callus derived from the radicular pole of the embryo. The undifferentiated radicular pole frequently gave rise to rapidly growing callus from which new shoots were formed spontaneously, although some immature embryos completed normal embryogeny. Embryo size and subsequent type of development were not strictly correlated but there was a tendency toward the following patterns. Embryos less than 0.4 mm in length lost their original form, callused evenly throughout and became masked by development of buds over the entire surface. Embryos 0.4–1.2 mm were most apt to retain their original form; callus developed at the radicular pole but not at the shoot pole. Embryos 1.5 mm and larger frequently gave rise to radicular callus but also grew normal shoots. Embryo inoculants which had begun to coil were most apt to complete normal embryogenesis. No correlation could be drawn between the addition of specific growth regulators or combinations of regulators and specific patterns of development. It is suggested that in the evolution of Cuscuta from autotrophic ancestors, the loss of both cotyledons and roots was a single embryonic event and so complete that even under the influence of a highly modified biochemical environment it would be impossible to derive roots from dodder tissue. The usual interpretation of the dodder haustorium as a highly modified adventitious root in all probability is not valid. Is is further suggested, however, that this loss of root and cotyledon is not a necessary consequence of the evolution of the parasitic mode of nutrition, nor of the gross adult growth form.

SOME ASPECTS OF MORPHOGENESIS IN CUSCUTA GRONOVII

More than 100 primary root segments from dormant seeds, 100 radicular segments from immature embryos, 450 whole immature embryos, and a number of whole seedlings of C. gronovii were studied under in vitro cultivation in the presence of such growth‐regulating factors as: coconut milk, casein hydrolysate, auxins, kinetin, adenine, gibberellic acid, and additional B vitamins as supplements to White's medium. In normal embryogeny early swelling of the basal pole of the embryo and profuse production of epidermal hairs upon germination are superficial characteristics of Cuscuta which resemble seedling root growth in other plants. At no time is there evidence of a primary‐root meristem, nor at any time do cotyledons develop. It was not possible to induce root formation experimentally from any of the starting embryonic materials, nor from stems which were cultured to maturity culminating in the production of flowers after 6 months of in vitro cultivation, nor from callus derived from the radicular pole of the embryo. The undifferentiated radicular pole frequently gave rise to rapidly growing callus from which new shoots were formed spontaneously, although some immature embryos completed normal embryogeny. Embryo size and subsequent type of development were not strictly correlated but there was a tendency toward the following patterns. Embryos less than 0.4 mm in length lost their original form, callused evenly throughout and became masked by development of buds over the entire surface. Embryos 0.4–1.2 mm were most apt to retain their original form; callus developed at the radicular pole but not at the shoot pole. Embryos 1.5 mm and larger frequently gave rise to radicular callus but also grew normal shoots. Embryo inoculants which had begun to coil were most apt to complete normal embryogenesis. No correlation could be drawn between the addition of specific growth regulators or combinations of regulators and specific patterns of development. It is suggested that in the evolution of Cuscuta from autotrophic ancestors, the loss of both cotyledons and roots was a single embryonic event and so complete that even under the influence of a highly modified biochemical environment it would be impossible to derive roots from dodder tissue. The usual interpretation of the dodder haustorium as a highly modified adventitious root in all probability is not valid. Is is further suggested, however, that this loss of root and cotyledon is not a necessary consequence of the evolution of the parasitic mode of nutrition, nor of the gross adult growth form.