Crescent-shaped Structure Research Articles

Ultrastructural studies of sporulation in Bacillus sphaericus

Spore septum formation in Bacillus sphaericus 9602 occurs 2 h after the end of exponential growth at one end of the vegetative cell, which retains a uniform diameter. The apparently rigid spore septum contains an inner cell wall layer which disappears when the sporulation septum "bulges" into the mother cell cytoplasm. This process occurs simultaneously with terminal swelling at the end of the cell containing the spore septum. It is suggested that the inner cell wall layer is peptidoglycan and that its dissolution and the terminal swelling are consequences of a localized autolysis. Engulfment of the forespore by membrane proliferation results in the production of a forespore surrounded by two flexible, closely apposed membranes. These membranes appear to become more rigid as a peptidoglycan-like layer appears between them, concomitant with the condensation of the forespore nucleoid into a crescent-shaped structure. After nuclear condensation, visible development of distinct cortex, primordial cell wall, and spore coat layers begin, and the forespore cytoplasm assumes an appearance similar to that of a refractile spore. The spore coats consist of an amorphous inner layer, a lamellar midlayer, and a structured outer layer. As cortex synthesis and spore coat assembly continue, exosporium development commences close to that portion of the mother cell plasma membrane which surrounds the forespore. The exosporium is lamellar and in tangential section is seen to have a hexagonal arrangement of subunits. The timing of these morphological events has the expected correlation with the appearance of unique enzyme activites required for cortex synthesis.

STUDIES OF PHYSIOLOGICALLY ACTIVE ARTHROPOD SECRETIONS: VI. EVIDENCE FOR A SEX PHEROMONE IN FEMALEORGYIA LEUCOSTIGMA(LEPIDOPTERA: LYMANTRIDAE)

AbstractThe results of behavioural, chemical, and histological studies are reported showing the presence of a sex attractant in femaleOrgyia leucostigmaJ. E. Smith. A method of bioassaying the attractancy of the female moths by the use of 4-ft-long glass tubes is given. Extraction of female abdominal tips with dichloromethane yields material biologically active when tested against maleO.leucostigma.The pheromone-producing gland is a dorsally situated, crescent-shaped structure formed by modification of the epidermal cells in the intersegmental membrane between the eighth and ninth abdominal segments. The glandular cells are goblet-shaped and are arranged in an unusual manner.

Electron Microscope Studies on the Carpal Organ of the Pig

The eccrine sweat gland located in the subcutaneous tissue at the palmar side of carpal or metacarpal regions of forelegs of the pig (the carpal organ) was studied with the electron microscope. Besides the description of its fine structures, dicussions on the secretion mechanism of this gland are presented in this paper. The results obtained are summarized as follows.1. Two different cell types of the glandular epithelium which were previously pointed out by light microscopy (KITAMURA, 1957), namely the dark and clear cell, are clealy distinguished from each other under the electron microscope. The designation of two cell types may depend on the previous light microscopic observation, though appearances by light- and electron microscopy may often be reverse.2. Within the cytoplasm of the dark cell, there exist a great number of secretory granules or vacuoles. The electron density of these granules or vacuoles is so low that the electron optical appearance of this cell type is rather clear, in contrast to the image by light microscopy of stained preparations, in which the secretory granules are always darkly tinted. The secretory granule is surrounded by a dense smooth membrane (probably one type of the smooth surfaced variety of the endoplasmic reticulum). As it grows and ripens, its internal substance shows a marked decrease in density and the fusing of neighbouring granules occurs, then it converts into the secretory vacuole.3. A small number of mitochondria with high electron density are present in the dark cell cytoplasm. The rough surfaced variety of the endoplasmic reticulum is occasionally observed to be localized at the infranuclear zone. Furthermore, small dense particles (microsomes) and microvesicles are abundant within the cytoplasm, especially in the interstices among the secretory grauules. It was forcibly convinced from many electron micrographs that granules of varying sizes and densities might be transitional forms from one to another of each of the microsomes, microvesicles, secretory granules and secretory vacuoles.The GOLGI apparatus consists of GOLGI vesicles, several pairs of GOLGI membranes and GOLGI vacuoles, which are probably produced either by the expantion of GOLGI vesicles or by the terminal vacuolation of flattened sacs, viz. GOLGI double membranes. It is difficult, in some cases, to distinguish the GOLGI vacuole from the secretory granules.4. The secretory granules of the clear cell are generally opaque to the electron beam and are small in number. These often situate in the vicinity of the crescent-shaped endoplasmic reticulum (rough surfaced). Somtimes, extremely dense granules with irregular outline were observed; they might be identified as fat droplets.5. As compared with the dark cell, far more numerous mitochondria are contained in the clear cell. They have marked limiting membranes and internal ridges (cristae mitochondriales), both of which are electron dense and show a double membraned structure. There have been recognized various transitional steps from the mitochondria to the secretory granules of the clear cell.6. In the clear cell cytoplasm, were observed to be scattered many microsomes and thread-like or tubular structures. The latter may be considered to be produced by chaining of microsomes, and sometimes they aggregate in a compact mass of peculiar crescent shape, which is found to surround the nucleus at the supra- and paranuclear sides maintaining a roughly uniform distance apart the nucleus. This crescent-shaped structure was found under the ordinary light microscope to be strongly basophilic. The constituent elements of such a crescent-shaped mass possess the characteristic of typical endoplasmic reticulum (rough surfaced); but in the case that thread-like or tubular structures are dispersed throughout the cytoplasm, the characteristics of the endoplasmic reticulum are obscure in such distributed bodies.

Fertilization in Ginkgo biloba

1. At the time of shedding, the pollen grain of Ginkgo usually contains four cells: the first prothallial, second prothallial, generative, and tube cells. After germination of the pollen grain the first prothallial cell aborts, while the generative cell divides, forming the body and stalk cells. During a late stage of development of the body cell, two blepharoplasts are formed, and the nucleus changes from a spherical to a lens-shaped structure. While the nucleus is changing shape, two vacuole-like structures appear close beneath the blepharoplasts, and a conspicuous granule becomes visible beside the nucleus. 2. Chromatin in the nucleus of the body cell is diffuse but condenses into chromosomes during division. Each spermatozoid cell includes one of the blepharoplasts, which changes from a rounded to a crescent-shaped structure and then forms the cilial band. The vacuole-like structure separates from the blepharoplast and moves to the side opposite the cilial tail. Later it becomes impregnated with chromatin and apparently forms the head of the spermatozoid. One half of the granule near the nucleus of the body cell is included in each spermatozoid. The newly formed spermatozoid is flexible and flows out from the pollen tube. 3. Free-nuclear divisions occur simultaneously in early stages, and more than a thousand nuclei are formed before the initiation of wall formation in the megagametophyte. Wall formation proceeds centripetally. In the free-nuclear stage the cells of the spongy tissue of the nucellus surrounding the megagametophyte may be multinucleate, with extremely large polyploid nuclei. This tissue is absorbed by the developing megagametophyte. 4. Archegonial initials, usually two in number, appear superficially at the micropylar end of the megagametophyte, each containing a large central nucleus. Each initial divides periclinally to form a cover cell and an inner central cell. The cover cell divides anticlinally to form two neck cells. The central cell gradually enlarges, while adjacent cells form a well-organized jacket layer. Just before division of the central cell the two neck cells divide, forming four neck cells, which swell to form a hemispherical protuberance. The central cell divides unequally into the small ventral-canal cell and large egg. 5. When the spermatozoid is released from the pollen tube, the egg cell forms a small projection which extends between the neck cells. As soon as the spermatozoid becomes attached to the egg projection, the latter retreats, leaving a path through the neck. Only the head of the spermatozoid, which includes the chromatin, penetrates the egg cytoplasm, where it forms a globular mass. Simultaneously, the chromatin in the egg nucleus forms another group of granules. The union of the two sets of granules is considered to be the actual fertilization. 6. The first division of the zygote is perpendicular to the axis of the ovule. In telophase an obscure cell plate is visible but becomes less evident later. Free-nuclear division continues until 256 nuclei are formed. Wall formation in the proembryo is simultaneous. As the embryo continues to develop, cells differentiate into the cotyledons and the apical meristems of root and stem.