Everett Anderson

Sperm differentiation in the sea urchins Arbacia punctulata and Strongylocentrotus purpuratus

The morphology of the testes and the development of sperm cells in the sea urchins Arbacia punctulata and Strongylocentrotus purpuratus have been investigated with techniques of light and electron microscopy. Developing spermatids are connected by intercellular bridges which persist until the sperm cells are nearly mature. Formation of the acrosomal vesicle is associated with the Golgi complex. During differentiation the chromatin of the spermatid nucleus condenses. Concomitantly, the spermatid nucleus becomes morphologically specialized, i.e., its apex becomes indented (subacrosomal region) and its base invaginated (centriolar fossa). The single large mitochondrion which constitutes the middle-piece of the spermatozoon presumably develops by a fusion of the randomly dispersed mitochondria. During early stages of development spoke-like satellites are associated with the proximal and the distal centrioles. The distal centriole, unlike the proximal one, is associated with a bundle of fibers. In the spermatozoon, the proximal centriole is situated in the centriolar fossa; the distal centriole is found between the base of the nucleus and the mitochondrion.

The origin and structure of the tertiary envelope in thick-shelled eggs of the brine shrimp, Artemia.

The shell glands of the brine shrimp Artemia are composed of clusters of shell gland units. Each unit usually consists of two rather oblong cells separated by a lumen. Each lumen communicates with the uterus by a duct, formed by a neck cell and a small number of duct cells. Often, two or more ducts join before entering the uterus. The plasma membrane facing the lumen between gland cells is thrown into short microvilli. Elsewhere, the plasmalemma lacks such specializations. The two gland cells are joined together by maculae adhaerentes and septate desmosomes. The nuclei of these cells are large and contain abundant chromatin, arranged in a reticular pattern. The cytoplasmic matrix contains glycogen, lipids, mitochondria, a few microtubules, Golgi complexes, and large amounts of rough endoplasmic reticulum. When Artemia produces thick-shell eggs, the material for the outer portion of the shell (the tertiary envelope) is synthesized in the shell glands. During this synthesis numerous membrane-bounded secretory granules are formed that contain lipoprotein. Prior to their liberation the granules show a decrease in density and their membranes fuse with the plasmalemma adjacent to the lumen between the gland cells. Other granules fuse with those already being liberated, thereby enlarging the lumen of the shell gland unit. The tertiary envelope that is deposited around an embryo is at first homogeneous. Later it becomes filled with alveolus-like structures, the surfaces of which are composed of hexagonal porous units, through which the alveoli interconnect. Still later, in the gastrula stage, these interconnections are lost, and a more homogeneous outer layer is added.

An ultrastructural analysis of fertilization in the surf clam, Spisula solidissima. I. Polar body formation and development of the female pronucleus

Breakdown of the germinal vesicle in inseminated eggs of the surf clam, Spisula solidissima , consists of a vesiculation of its nuclear envelope and the condensation of its chromatin. Adjacent to the disrupting germinal vesicle, two asters develop, move to either side of the chromosomes, and form the first meiotic apparatus. The meiotic apparatus moves to one side of the zygote so that one aster is located at the cortex (peripheral aster) while the other is situated medially (inner aster). At anaphase the dyads separate and those that are attached to the peripheral aster move into a cytoplasmic protrusion, which will become the first polar body. Subsequently, the chromosomes remaining in the zygote are realigned on the metaphase plate of the second meiotic apparatus. Other than its chromatin content, the second meiotic apparatus appears to be morphologically equivalent to the first. The second polar body is formed in a manner similar to that observed in the production of the first; however, the second remains attached to the zygote via a cytoplasmic bridge at least until the time of the first cleavage. Both polar bodies appear to contain the same number and kinds of organelles and inclusions, although only the second polar body has its chromatin limited by a nuclear envelope. The maternal chromatin remaining in the zygote after the formation of the second polar body is reorganized into chromosome containing vesicles which later fuse and form the female pronucleus.

Cytological events leading to the formation of the two-cell stage in the rabbit: Association of the maternally and paternally derived genomes

The cytological transformations leading to the establishment of the two-cell stage in the rabbit have been studied with techniques of light and electron microscopy. Subsequent to their migration the male and female pronuclei become centrally located and juxtaposed. Each pronucleas appears to be nearly identical in size and in structure and contains a fibrous nucleoplasm in which fine-textured and granular aggregations are embedded. Later the pronuclei become closely apposed and their proximal surfaces flatten and become highly convoluted. Along the proximal margin of the pronuclear envelopes small vesicular structures are produced. They are filled with a flocculent material and appear to be budded-off into the cytoplasm. During further morphogenesis, the pronuclei produce an extensive array of projections which interdigitate with one another to form complex configurations. Concomitantly, the blebbing activity of the pronuclear envelopes ceases and the chromatin condenses into large reticular aggregations, primarily in the region where the pronuclei are intimately associated. Association of the maternal and paternal genomes is effected when the two pronuclear envelopes break down at widely spaced intervals leaving regions of intact double-membrane structures (pronuclear envelope). Microtubules (spindle) are associated with the condensed chromatin and course through the disrupted regions of the pronuclear envelope. The two groups of chromosomes move together and assume positions on the first cleavage spindle. Thus, in the rabbit, there is an absence of a zygote nucleus.

The origin and structure of the secondary coat of the egg of Drosophila melanogaster.

SummaryThe techniques of light and electron microscopy have been employed in a study of the protective coverings of the egg of Drosophila melanogaster. Data obtained during this investigation suggest the involvement of the follicle cells, in the production of one of these coverings and justify its classification as a secondary coat. The secondary coat of D. melanogaster is highly organized and has been divided into three Zones (I, II, IIII). The follicle cells enveloping the oocyte exhibit two phases of secretory activity each involving hypertrophy of the Golgi complex and rough endoplasmic reticulum, and the production of protein and polysaccharide components. The first phase concerns the elaboration of the material which gives rise to the homogeneous lamina referred to as Zone I. The second results in the release of an electron dense component which becomes organized into two laminae separated by struts or pillars; this construction is referred to as Zone II. At the completion of this secretory phase, the follicle cells assume a squamous morphology and a third Zone, composed of a homogeneous substance, appears between the follicle cells and Zone II.

Spermiogenesis in the surf clam Spisula solidissima with special reference to the formation of the acrosomal vesicle

Fine structural studies of sperm differentiation in the surf clam Spisula solidissima have demonstrated the development of the acrosomal vesicle and the organization of the nucleus and the mitochondria in the spermatozoon. In early spermatids, proacrosomal vesicles arising from several Golgi complexes coalesce to form one acrosomal vesicle. This moves to the anterior region of the nucleus and becomes elongated, cone-shaped, and invaginated at its adnuclear surface. The axial rod, a subacrosomal structure, is initially recognized in mid-spermatids as an unorganized mass of fibers. These fibers become arranged in parallel and extend anteriorly into the acrosomal vesicle. In early spermatids, the nucleus contains peripheral aggregates and irregular filaments of electron opaque chromatin. Eventual coalescence of these chromatin masses yields a compact and barrel-shaped sperm nucleus. Ellipsoid mitochondria, which are randomly distributed throughout the cytoplasm of early spermatids, become circumnuclear and intimately apposed to the nuclear envelope. The mitochondrial population is reduced from many to four in number, presumably through a fusion process.

Structural and cytochemical features of the sperm of the cephalopod Octopus bimaculatus

The aboral region of the spermatophore of the cephalopod Octopus bimaculatus consists of three layers which surround a highly coiled sperm mass. Groups of filamentous spermatozoa, oriented head to tail, are embedded within the flocculent matrix of the sperm mass. The screw-shaped acrosome of the spermatozoon is composed of a granular substance, aggregated at periodic intervals to form striations that are situated perpendicular to the long axis of the sperm. Surrounding the acrosome is a granulofibrillar substance (periacrosomal material). The nucleus is a long cylindrical structure surrounding a PAS-positive rod (extranuclear rod) which ends blindly in its anterior region. The extranuclear rod lies in a lumen bounded by the nuclear envelope. At the base of the nucleus is a centriolar fossa containing a centriole embedded in a fibrous matrix. Extending from the centriole is the axial filament complex, surrounded by nine outer fibers that course through the major portion of the sperm tail. The middle piece is composed of a number of columns of mitochondria which are circumferentially positioned around the anterior region of the sperm tail. The posterior limit of the middle piece is marked by an annulus of electron opaque material. Glycogen particles fill the intracellular space between the plasma membrane and the outer fibers of the sperm tail.

An ultrastructural analysis of fertilization in the surf clam, Spisula solidissima. II. Development of the male pronucleus and the association of the maternally and paternally derived chromosomes.

The events of fertilization involving the incorporation of the spermatozoa, the development of the male pronucleus, and the association of the maternally and paternally derived pronuclei have been investigated in the egg of Spisula solidissima. At the site of gamete plasma membrane fusion, a small fertilization cone is produced through which the contents of the spermatozoon passes. Entrance of the spermatozoon is accompanied by the vesiculation of sperm nuclear envelope. Subsequently the sperm chromatin is dispersed. A male pronuclear envelope and sperm aster are not formed until the maternal chromatin completes meiosis. Following these events, migration occurs and the male and female pronuclei come into proximity with one another. Later the pronuclear envelopes vesiculate and, concomitantly, the chromosomes condense into large reticular aggregates. The two groups of chromosomes become associated on what will become the metaphase plate of the first cleavage spindle. Thus, as in the rabbit and the mussel, no zygote nucleus is produced.

The anatomy of the peripheral nerve and its ensheathing artery in the horseshoe crab, Xiphosura (Limulus) polyphemus

The remarkable relationship between elements of the circulatory and nervous systems in Xiphosura (Limulus) polyphemus has been investigated by light and electron microscopy. The arterial wall consists of sheets of fibrocytes which alternate with fibrous lamellae. Each fibrous lamella, which is presumably derived from fibrocytes, may be divided into three zones: (a) two amorphous or finely granular regions which lie adjacent to fibrocytes and in which are embedded small filaments, and (b) a central fibrous zone whose constituent filaments are randomly oriented and posses an axial periodicity of 500 A. Unlike the blood vessels of the leech and earthworm, those of Limulus are not lined by endothelial cells. At irregular intervals, near the lumenal side of the artery, are groups of two or three striated muscle cells. The peripheral nerve trunk may be composed almost entirely of large axons or may contain both large and small ones. In the latter case, the small axons are centrally located within the trunk while the large ones are peripheral. In all cases, the large axons are always individually enclosed by one or more Schwann cells. The small axons, on the other hand, are encompassed by processes of glial cells. Thus the relation of the axons to glial elements in Limulus differs from that found in other invertebrates.

A comparative study of the concrement vacuole of certain endocommensal ciliates—a so-called mechanoreceptor

The concrement vacuole is a distinguishing cytoplasmic characteristic of ciliates of the families Buetschliidae and Paraisotrichidae. The vacuoles of the various species have some structural features in common; however, the distinctive morphologic qualities possessed by them permits their classification into two types: CV-1 (Buetschliidae) and CV-II (Paraisotrichidae). Two of the common features are: (a) oval concretions which give a positive reaction for calcium when subjected to the Alizarin red S test and which, in thin sections, display a laminated pattern, and (b) ciliary rootlets. The basic unit of each rootlet appears tubular and measures about 220–240 A in diameter. These rootlets are derived from the kinetosomes of a group of presumably specialized cilia and together define a part (Paraisotrichidae) or all (Buetschliidae) of the vacuoles boundary. Speculation is offered concerning possible modes by which the concrement vacuole could function as a statoreceptor. The kinetosomes which constitute the somatic infraciliature are attached to a subtending fibrous band whose constituent filaments are 60–70 A in diameter. This fibrous band, in addition to dividing the cytoplasm of both families into ectoplasm and endoplasm, partially encircles the CV-II type concrement vacuole. The cytopharyngeal armature is also composed of ciliary rootlets whose individual units are 220–240 A in diameter. Together these units form a hexagonal pattern. There is no anatomical association between the components of the cytopharyngeal armature and the vacuole. The cytoplasm of organisms from both families contains gastrioles, a contractile vacuole, a large complement of rough variety of endoplasmic reticulum, and a host of vesicles and agranular cisternae. The plasma membrane is endowed with a fuzzy coat; it is particularly prominent on organisms of the family Buetschliidae. This coat is PAS-positive, a reaction which is not abolished by prior treatment with α -amylase.