Keith R. Porter

Cytoplasmic fibrils in living cultured cells. A light and electron microscope study.

SummaryBy a combined light and electron microscopic study, the structure and behavior of the stress fibers of cultured rat embryo cells are described. From an analysis of movie records of living cells it is seen that the stress fibers are in a state of flux, continually altering their dimensions and dispositions within the cell. However, compared to most other cellular movements, these rates of change are slow. By electron microscopy it is shown that the stress fibers consist of bundles of close packed elongate 75 Å filaments, arranged just beneath the plasma membrane adjacent to the cells plane of attachment and that similar filaments, forming a loose framework, permeate the cytoplasmic matrix. On the basis of careful light and electron microscopic comparisons, it is concluded that the filamentous structure shown within the cytoplasm of glutaraldehyde/osmium-fixed cells is a generally accurate representation of the structure of the living cell cytoplasm. It seems likely that the stress fibers are concerned in stabilizing areas of cellular attachment as well as with resisting forces that stretch the cell. The suggestion is made that, by controlling cytoplasmic viscosity and responding to cytoplasmic microtubules, the diffuse framework of filaments helps to determine the form of the cell and that, by a coordinate dynamic activity of its filaments, it provides the motive power for the various forms of cellular and intracellular movements.

Morphology of Microtubules of Plant Cell

The microtubules present in the cortices of plant cells are found to have a wall made up of slender filamentous subunits, probably 13 in number, which have a center-to-center spacing of about 45 angstrom units. Thus the micromorphology of these tubules is similar to that reported for the 9 + 2 fibrils of rat sperm flagella. This structure can be used to account for a primary function of motion attributable to the microtubules.

Microtubules and pigment migration in the melanophores ofFundulus heteroclitus L.

SummaryThe dermal melanophores ofFundulus heteroclitus L. have been investigated by light and electron microscopy with the purpose of revealing the mechanisms controlling pigment migration. As predicted by earlier studies, the nerve endings of a double innervation were found adjacent to and in synaptic relation to the melanophore surface. Not expected were the large number of small pits or invaginations present in the cell surface. These appear identical to the so-called micropinocytotic vesicles found generally in cells of the vascular endothalium and smooth muscle. In chromatophores they are more reasonably interpreted as receptor sites for neurohormones than as uptake and transport mechanisms.Observations made on the kinetics of pigment migration within the processes of these melanophores indicate that the granules move along relatively fixed channels arranged parallel to the long axes of the processes. Examined at fine structure levels, the zones of cytoplasm around these channels are found to be populated by microtubules about 225 Å in diameter aligned parallel to the direction of pigment movement. These long slender elements are present in the processes regardless of whether the melanin is concentrated in the cell center or dispersed. It is reasoned from these and other observations that the microtubules function as cytoskeletal elements which help maintain the extended form of the melanophore arms and at the same time define the channels in which the pigment moves. The possible role of the tubule in generating the motive force for pigment migration is discussed.

Centriole replication during ciliogenesis in the chick tracheal epithelium.

SummaryMorphological aspects of centriole replication as displayed in tracheal epithelial cells of 15 to 19-day-old chick embryos during ciliogenesis were examined by electron microscopy. It was found that centriole replication and development take place in a finely fibrous region around the two mature centrioles of the diplosome. Procentrioles in early stages of development are observed in clusters. The core of each cluster is occupied by one or more cylindrical structures which gradually disappear as the procentrioles mature. Some of the procentriolar clusters are attached directly to the walls of the diplosomal centrioles by the cylindrical cores. This observation suggests that all of the clusters may form initially in close association with the diplosomal centrioles.The earliest recognizable procentrioles are short, cylindrical structures that have no microtubules. Later microtubules appear, first as singlets, then doublets and finally as triplets. The singlet becomes the innermost microtubule of the triplet. Following the assembly of the nine triplets, the procentrioles separate from the clusters, elongate to their mature length and acquire rootlets, ciliary vesicles and cilia. The observation that all the procentrioles in each cell are in the same stage of development indicates that all the centrioles or presumptive basal bodies required by this cell are produced at the same time.

Specializations of the Unit Membrane

The three-layered unit membrane is a structure showing several modifications not only in its thickness but also in the nature and amount of materials associated with its outer and inner surfaces. Indeed, the presence of associated layers is so constant that one is led to wonder whether the unit membrane as commonly defined should not be regarded as merely the central component in a complex and highly variable structure which marks cell boundaries. The variation in thickness first pointed out by Ledbetter (1962) for the plant cell has since been described by Sjostrand (1963) among others, in the animal cell. In plant cells, for example the plasma membrane and that of the tonoplast measure about 100–120 A, whereas the limiting membranes of the mitochondria and chloroplasts fall in the 50–70 A range. This kind of variation is now so well known that it requires neither illustration nor further comment.

Intracellular Impulse Conduction in Muscle Cells

A hypothesis, suggested previously by morphological studies, for impulse conduction from the sarcolemma to the contractile material via the sarcoplasmic reticulum is discussed. The relation of reticulum morphology and cell size to speed of contraction in smooth and striated muscle agrees with the hypothesis and thus supports it. Additional support comes from evidence concerning an unusual morphological relationship between the sarcolemma and contractile fibrils in striated muscle of amphioxus.

THE Z DISC OF SKELETAL MUSCLE FIBRILS.

SummaryThe structure of the Z disc has been studied in thin sections of striated muscle fibers from a wide variety of vertebrates. A common organization is found in all muscles examined. The disc shows a regular pattern made up of dense lines which seem to connect the actin filaments from adjacent sarcomeres. The lines are sometimes disposed to form a regular zigzag configuration; in other orientations with respect to the plane of the section the morphology is confused and, in still other images, the dense lines continuous with the actin filaments seem to go straight through the Z disc. In cross section this structure corresponds to a square pattern of considerable regularity. The intersections in the square pattern mark the location in the plane of the section of the actin filaments from adjacent sarcomeres. Dense lines form the edges of the squares and appear to represent condensations of Z-disc material, i.e., the lines in the zigzag. The possible origin of the structure as a product of the stretching of a membrane is discussed, together with functional interpretations of the Z disc.

A reinterpretation of the structure and development of the basement lamella: An ordered array of collagen in fish skin☆

Abstract An adepithelial basement lamella of 8–10 layers of apparently orthogonally arrayed collagen fibrils develops in the skin of Fundulus heteroclitus between the fifth and twelfth days after fertilization. Subsequent thickening of the lamella with growth until posthatching stages is the result of an increase in the number of rows of fibrils per layer, not in the number of layers. In contrast to the admuscular lamella, which forms in posthatching stages, the adepithelial lamella is acellular except in the sublamellar region. A morphological study of the skin of developing stages of Fundulus places the present “plywood” theory of basement lamella structure and development in doubt and supports a new “scindulene (shingle)” theory. Collagen fibril layers are observed to descend at a slight angle from the basement membrane, rather than to lie parallel to it as proposed by the plywood theory. Deposition of collagen in the basement lamella appears not to result from sequential switching and layering of collagen fibrils parallel to the basement membrane, as suggested by the plywood theory. Rather it appears that the collagen fibrils are inserted at end points into the basement membrane and are polymerized in one orientation only in any given area of the basement membrane and that adjacent areas are oriented at 105–110° to each other. Morphological changes with growth are accounted for by expansion of these “areas of insertion” in the basement membrane. Problems of variation in lamella thickness, crossover fibrils between different collagen layers, and the possible role of the basement membrane in the deposition of collagen into the lamellar structure are discussed.

The Plant Cell: Aspects of Its Form and Function

Publisher Summary This chapter discusses the plant cell and focuses on the aspects of its form and function. The cellular components characteristically observed in plant cells include mitochondria, proplastids, dictyosomes, nuclei, ribosomes, endoplasmic reticulum, and cell membranes. The mitochondria are ovoid to round organelles, although they also can be very irregular in shape as often seen in animal cells. The nuclei of plant cells are quite variable in size and shape, but most are typically spherical. The ribosomes of plants are small particles scattered in groups or clusters throughout the cytoplasm. Despite the universal acceptance of the physiological importance of the plasmodesmata, there are relatively few well-established evidences of their role. Their chief functions are related to the conduction of metabolites from one region of the plant to another and the movement of hormonal stimuli important in growth regulation. In nonvascular tissues such as those found in many mosses, liverworts, and fern gametophytes and in vascularized plants within regions comparatively isolated from phloem sieve cells, it seems very probable that the plasmodesmata are of particular importance.

Muscle Relaxation: Evidence for an Intrafibrillar Restoring Force in Vertebrate Striated Muscle

Observations of contracting muscle fibrils in cultured cells indicate that the force which restores the resting length of the sarcomere comes from the contractile elements themselves and not from external elasticity, as is now generally accepted. In light of biochemical studies on the contraction-relaxation cycle, it is postulated that the elongating force is one of internal elasticity in the sarcomere, which arises during contraction from the distortion of bonds between filaments and/or structural proteins. This mechanism of restoration may serve to establish optimal sarcomere length for production of maximum contractile force, and in cardiac muscle this mechanism may be a factor in ventricular filling.