Developmental mechanisms in heterospory

The megasporangium of Selaginella sulcata (Desv.) Spring contains approximately equal numbers of megasporocytes of two kinds that can be distinguished on size and ultrastructure. These are called viable and non-viable. During the prophase of meiosis the non-viable megasporocytes degenerate by a process corresponding to cellular autophagy. One viable megasporocyte completes the meiotic cycle and the others persist, presumably as diploid cells, in the post-meiotic megasporangium; and it is shown that in S. plana (Desv.) Hieron an exine is formed on these. The possible significance of the particular disposition of the mitochondria and of intranuclear vesicles in the viable megasporocytes is discussed.     

The adaptive value of heterospory: Evidence from Selaginella

Heterospory was a pivotal evolutionary innovation for land plants, but it has never been clear why it evolved. We used the geographic distributions of 114 species of the heterosporous lycophyte Selaginella to explore the functional ecology of microspore and megaspore size, traits that would be correlated with many aspects of a species’ regeneration niche. We characterized habitats at a global scale using leaf area index (LAI), a measure of foliage density and thus shading, and net primary productivity (NPP), a measure of growth potential. Microspore size tends to decrease as habitat LAI and NPP increase, a trend that could be related to desiccation resistance or to filtration of wind‐borne particles by leaf surfaces. Megaspore size tends to increase among species that inhabit regions of high LAI, but there is an important interaction with NPP. This geographical pattern suggests that larger megaspores provide an establishment advantage in shaded habitats, although in open habitats, where light is less limiting, higher productivity of the environment seems to give an advantage to species with smaller megaspores. These results support previous theoretical arguments that heterospory was originally an adaptation to the increasing height and density of Devonian vegetative canopies that accompanied the diversification of vascular plants with leaves.     

Developmental mechanisms in heterospory. III. The plastid cycle during megasporogenesis in Isoetes.

The megasporocyte of Isoetes englemanni at the leptotene-zygotene interval of meiosis contains 4 disk-shaped proplastids about 12 mum in diameter. The disposition of these organelles in the cell is such that each of the four megaspores delimited during cytokinesis contains a single proplastid. During prophase and following their incorporation into the spores, the proplastids are undergoing fission by budding. The buds are first discernible as low surface evaginations which contain a complement of granular somal material, some wefts of tubular membrane and osmiophilic globuli, in addition to a number of vesicles derived by invagination from the inner membrane of the proplastid envelope. As the evaginations emerge they enlarge and the link with the parent body is reduced to a narrow channel. At this stage one or more of the vesicles derived from the proplastid envelope comes into register with the lumen of the channel. One vesicle is transported into the lumen, elongating as it passes through. The passage of the vesicle into the channel destroys the connexion between the matrix of the evagination and the stroma of the proplastid. The occurrence in the cytoplasm around the proplastid of bodies not connected to the proplastid, but identical in structure to the evaginations and carrying a membranous tail suggests that the evaginations are released by abscission of the channel close to the surface of the parent body. After release the bodies undergo division by constriction. Regression of the tail follows division in those bodies which are regular in outline and in which the matrix is ultrastructurally similar to the stroma of the parent organelle. The process does not seem to occur in co-existing forms which have assumed an irregular outline and have a less-opque matrix. The more mature megaspore of Isoetes contains proplastids up to 4 mum in greatest dimension. The stroma in these is dense and granular and contains membrane-bound vesicles, osmiophilic globuli, starch granules and wefts of tubular membrane. There is no evidence that the large budding organelle persists to this later stage in development. The resemblance of the plastids in the more mature megaspore to the bodies produced by evagination earlier in development suggests a common identity. The observations and interpretations lead to the proposition that the plastids in Isoetes englemanni are autonomous. This situation contrasts with the one described for another heterosporous haploid dioecious pteridophyte, Marsilea vestita, where nucleocytoplasmic interaction has been interpreted as the de novo creation of plastids and mitochondria following the elimination by autophagy of the organelles inherited at meiosis. It is suggested that an explanation to account for the 2 different mechanisms might be sought in regard to the degree of developmental success enjoyed by the individual megaspores in the 2 plants. In Isoetes all 4 megaspores of every tetrad survive and develop, while in Marsilea the mature megasporangium contains a single functional megaspore.     

Structure of wall layers in Selaginella kraussiana microspores (Lycophyta)

A similarity was found in both construction and ultrastructure between the two exospore layers in microspores of Selaginella kraussiana. The exospore is made up of two different kinds of rods. One of the kinds of rods are large, 100–150 nm in width, while the other are tubular rods 10–15 nm in diameter. The large rods are wider at the base of the spines than in the upper part, possibly due to flattening or compression. Both the outer and the inner exospores have a stranded surface that is very pronounced in the microspores of this species. Fibrous strands persisting the scanning electron microscope and transmission electron microscope (TEM) fixations were observed on the spore surface proximally and through perforations (exospore channel openings). This net of fibres penetrates and fills the space of the cavities within large channels through the outer and inner exospore and within the gap. According to our interpretation, these strands would be produced by the tapetum and are probably related to the nourishment of the developing microspores. Contrast varies in TEM sections after cytochemical stains, but this appears to be due to transitory substances, e.g. carbohydrates, rather than to be a substantial difference in basic composition between inner and outer exospore layers.     

Evidence for fluid-phase pinocytosis of extrahepatic bile duct cells isolated from normal rats in culture.

In order to elucidate the physiological function of extrahepatic bile duct cells, we isolated epithelial cells from the rat extrahepatic bile duct by digesting resected segments of the extrahepatic bile duct with 0.15% trypsin in ice-cold Ca(2+)-free Hanks' balanced salt solution supplemented with 0.25 mM EDTA overnight. As a result, the epithelial cells were collected as aggregates and attached to culture dishes coated with type I collagen. Approximately 95% of the cells cultured for 24 hrs were found to be positive for gamma-glutamyl transpeptidase and cytokeratin-19, but negative for vimentin. These characteristics were identical to the features of rat extrahepatic biliary epithelial cells in situ. Ultrastructurally, the cells were long and columnar in configuration on the 2nd day in culture, and possessed numerous microvilli at the apical surface and well-developed junctional complexes at the lateral surface. These findings also indicate that the cells maintain an epithelial nature and are morphologically polarized. When the cells were exposed to a low dose of horseradish peroxidase (HRP) on the 2nd day in culture, which was followed by fixation and treatment with 3-3'-diaminobenzidine, HRP was found preferentially in the cytoplasmic vesicles near the apical surface. HRP was then observed in the intercellular spaces; however, the electron-dense tracer, ruthenium red, did not permeate into the intercellular spaces, and HRP was found in neither cytoplasms nor intercellular spaces when the cells were incubated in HRP-containing medium at 4 degrees C for 30 min. These results suggest that the extrahepatic bile duct epithelial cells are involved in the reabsorption of bile constituents.     

Developmental Mechanisms in Heterospory: Features of Post-meiotic Regression in Selaginella

Post-meiotic regression in the sporangia of Selaginella sulcataresults in tetrads comprising small and large spores. From germinationtests it has been found that the small spores are abortive andthe large ones fertile, a circumstance which suggests an innateconstitutional difference between them. Densitomctric measurementsshow that the cytoplasm in the prophase megasporocyte is polarized.At cytokinesis this cytoplasm is simultaneously partitionedinto four cells, equal in size, but presumably not in composition.Initially, all four megaspores in the tetrad grow at the samerate, but later, development becomes differential with one,or more usually two spores increasing rapidly in size and theothers much more slowly. The onset of this phase is signalledby a difference in protoplasmic staining and it is the megasporeswith a densely-staining protoplast which develop to fertility. A comparable process of regression occurs also in the microsporangium;the spores develop differentially after a period of equal growth.Examination of young microspore tetrads shows that daughterspores differ in the density of free cytoplasmic ribosomes.It is suggested that this is a consequence of cither disproportionateinheritance of ribosomes from the parental cell resulting frompolarization or, more probably, since the deficiency is notre-dressed in later development, an inherited difference inthe ability to effect ribosome synthesis after prophase elimination.Endoplasmic reticulum-ribosome complexes, in other organismsbelieved to be implicated in cell degeneration, o0ccur in themicrospore cytoplasm.     

The theory of pinocytosis. Factors determining the dynamics of pinocytosis in capillary endothelium

Peculiarities of the mechanism and conditions of pinocytosis vesicules formation in capillary endothelium are considered in relation to: 1) the size of a protein cluster which is formed from plasma proteins on the endotheliocyte plasma membrane surface, and initiates a caveole formation; 2) the value of intracapillary hydrostatic pressure providing energetics of the caveole transition in the vesicula and its comming off. The bound parameters of vesicules formation are calculated in comparison with the well-known experimental data. It is suggested that in other types of endocytosis the initial phase of the process (caveoles formation) is also connected with absorption of the protein molecules on the plasma membrane and their clusterisation. A possible explanation is given to the fact of vesicles quantitative increase in endothelium in hypertension.     

Developmental staging of maize microspores reveals a transition in developing microspore proteins

A method for the preparation of developmentally staged microspores and young pollen from maize (Zea mays) has been devised. The preparations are of sufficient purity and quantity for biochemical analysis, including the analysis of steady-state protein and RNA populations associated with each stage. A major transition in protein populations occurs during the developmental period that encompasses microspore mitosis, the asymmetric nuclear division producing the vegetative and generative nuclei. Several differences between early and late stage proteins can be detected by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two-dimensional gel electrophoresis of proteins reveals that over half of the steady-state proteins differ between the younger and older stages, either quantitative or qualitative. One protein that increases in relative abundance about fourfold is actin. In vitro translation of RNA isolated from staged microspores demonstrates changes in microspore gene expression during the same developmental period.     

Morphogenesis in Selaginella: II. Auxin Transport in the Root (Rhizophore)

The rhizophore of Selaginella willdenovii Baker develops from the ventral angle meristem. The morphological nature of this organ has been in dispute. The purpose of this investigation was to obtain physiological evidence to support the contention that the rhizophore is a root and not a shoot. This was accomplished by studying the movement of ³H-indoleacetic acid and ¹⁴C-indoleacetic acid in Selaginella rhizophores. In 6-millimeter tissue segments, twice as much radioactivity accumulated in acropetal receivers as in basipetal. During 1 hour of transport in intact roots auxin traveled twice as far in the acropetal direction as basipetal. A significant amount of radioactivity transported in the tissue was found to co-chromatograph with cold indoleacetic acid. Decarboxylation accounted for 10% loss of activity from donors. The data provide sufficient physiological evidence that this organ is morphogenetically a root.     

Evidence that pinocytosis in lymphoid cells has a low capacity

In contrast to adherent cells, human B and T lymphoblasts, marmoset monkey T lymphoblasts, and mouse T lymphoblasts do not form monolayers and have a poor ability to pinocytose. After a 10-min incubation of lymphoblasts at 37 degrees C, the level of internalized medium reached a plateau. During this time, lymphoblasts pinocytosed 3-4 femtoliters (1 fl = 10(-15) l) of medium per cell as calculated by the quantity of the entrapped pinocytic marker 5(6)-carboxyfluorescein. The levels of pinocytosed liquid did not increase during a subsequent 90-min incubation of cells at 37 degrees C. Adherent HeLa cells took up 27 fl of medium per cell per hour. Other types of adherent cells were reported by others to pinocytose 20 to 90 fl of medium per cell per hour. The process of pinocytosis in lymphoblasts appeared to be reversible since cells which were pre-loaded with carboxyfluorescein and then incubated at 37 degrees C in fresh medium lost the marker almost completely within 40 min. Similar results were obtained with horseradish peroxidase as the pinocytic marker. Further evidence that lymphoblasts have a low capacity for pinocytic internalization relative to adherent cells was obtained from the observation that Namalwa lymphoblasts were approximately 100 times more resistant to the cytotoxic action of the protein toxin gelonin than the adherent HeLa cells. Gelonin is a ribosome-inactivating toxin which is not capable of binding to cells, and its only mode for internalization appears to be pinocytosis. Ribosomes in cell lysates of the two lines were equally sensitive to gelonin. It is speculated that the poor pinocytic ability of lymphoid cells may reflect a fundamental difference between adherent and non-adherent cells and that this may impede the targeting of drugs into lymphoid cells.     

Developmental mechanisms for the generation of telencephalic interneurons

Interneurons, which release the neurotransmitter γ-aminobutyric acid (GABA), are the major inhibitory cells of the central nervous system (CNS). Despite comprising only 20-30% of the cerebral cortical neuronal population, these cells play an essential and powerful role in modulating the electrical activity of the excitatory pyramidal cells onto which they synapse. Although interneurons are present in all regions of the mature telencephalon, during embryogenesis these cells are generated in specific compartments of the ventral (subpallial) telencephalon known as ganglionic eminences. To reach their final destinations in the mature brain, immature interneurons migrate from the ganglionic eminences to developing telencephalic structures that are both near and far from their site of origin. The specification and migration of these cells is a complex but precisely orchestrated process that is regulated by a combination of intrinsic and extrinsic signals. The final outcome of which is the wiring together of excitatory and inhibitory neurons that were born in separate regions of the developing telencephalon. Disruption of any aspect of this sequence of events during development, either from an environmental insult or due to genetic mutations, can have devastating consequences on normal brain function.     

Stimulation of pinocytosis in the macrophage by lymphocyte mediators: II. Evidence that the stimulating factor is MAF,and studies on the cellular requirements for response to the mediator

Partial characterization of a lymphocyte-derived mediator which stimulates macrophage pinocytosis of horseradish peroxidase is described. The mediator elutes with Sephadex G-100 fractions ranging from 25,000–68,000 molecular weight. It is neuraminidase sensitive and has a buoyant density characteristic of glycoproteins. These properties are identical with those of migration inhibitory factor and macrophage-activating factor, suggesting that they may be the same material. Using a variety of inhibitors, certain cellular and metabolic functions of the mediator-induced stimulatory process were investigated. Anaerobic glycolysis is critical to any stimulation, whereas submaximal stimulation can occur in the absence of an intact aerobic respiratory pathway. Protein synthesis is required for stimulation of pinocytosis. However, neither microtubule nor microfilament function appears necessary for the mediator to induce a stimulatory response as assessed by enhancement of pinocytosis.     

Evidence for pyroelectric and piezoelectric sensory mechanisms in the insect integument.

Quantitative pyroelectric (PE) and piezoelectric (PZE) measurements were carried out on the insect integument of live Blaberus giganteus (cockroach) and on dry integument preparations of the same species. Voltage responses to optical pulses of 10--500 ms, absorbed in the live integument, were PE: interference filter measurements showed the responses to be proportional to the absorbed thermal radiation flux and independent of the wavelength. The voltage/time-course of the responses was in agreement with theoretically calculated PE signals. Voltage responses to mechanical pulses were PZE. The responses of the inner and outer integument surfaces always had opposite electric signs. The polar character of the integument was confirmed by means of a separate dielectric heating method. To explain these results, we hypothesize that the PE properties are for the most part localized in the two outermost layers (outer and inner epicuticle) of the integument, which consists mainly of polar lipids and proteins. Parallel alignment of these polar molecules perpendicular to the integument surface is very likely. PE and PZE responses, therefore, will not only occur in live insects but will also be measurable in dead, dry integument preparations as long as the polar tissue texture remains intact. Due to its polar texture, the insect integument will react to rapid changes in temperature, illumination, or uniaxial pressure in the same way as nonbiological PE materials, where the voltage responses depend on dX/dt (X, pressure or temperature). It seems clear, therefore, that the well-known physiological reactions of various arthropods to such physical outside influences may be related to the PE property of their integument.     

Developmental mechanisms of gyrification

Folding of the cerebral cortex is a fundamental milestone of mammalian brain evolution associated with dramatic increases in size and complexity. Cortex folding takes place during embryonic and perinatal development and is important to optimize the functional organization and wiring of the brain, while allowing fitting a large cortex in a limited cranial volume. Cortex growth and folding are the result of complex cellular and mechanical processes that involve neural stem progenitor cells and their lineages, the migration and differentiation of neurons, and the genetic programs that regulate and fine-tune these processes. Here, we provide an updated overview of the most significant and recent advances in our understanding of developmental mechanisms regulating cortical gyrification.