Striated Muscle

Stratum corneum. Outer layer of epidermis of vertebrate skin. Cells undergo CO rn i f i c at i o n (keratinization) and die, becoming worn

STREPSIPTERA (stylopids). Small endopterygote insects; females 1 en-doparasitic; males free-living, short-lived, with large metathorax, anterior wings haltere-like, hrnd wings large and fan-shaped. Females degenerate, apodous and larviform, enclosed in persistent, larval cuticle. Several forms parasitize hymenopterans. 'Larvae emerge from host and probably find new hosts by waiting on flowers or through contact in nest. Probable affinities with the Coleoptera.

Streptococcus. Genus of non-spore-producing, Gram-positive bacterium, forming long chains. Many are -harmless colonizers of milk, Some commensal in the vertebrate gut; but the pyugenic group are human pathogens, some'" producing haemolysins, destroying erythrocytes. The viridans streptococcal group lives usually non-pathogenically in the upper respiratory tract, but can cause serious infections (somechronic), and may cause bacterial arthritis.

Streptomycin antibiotic inhibiting translation of mRNA on prokaryotic, -but not eukaryotic, ribosomes and can be used to distinguish these translation sites. Streptomycin resistance is conferred upon prokaryotes by plasmid-borne transposon. See

ANTIBIOTIC RESISTANCE ELEMENT,CHLORAMPHENICOL.

STRETCH RECEPTOR. See muscle spindle, proprioceptor.

Striated muscle (skeletal/voluntary/striped muscle). Contractile tissue, consisting in vertebrates of large elongated muscle fibres formed by fusion of my o b lasts to form syncytia. The cytoplasm of each fibre is highly organized, producing conspicuous striations at right angles to its long axis, and contains numerous longitudinal fibrils (imyofibrils), each with alternating bands (A, anisotropic, I isotropic), H zones and Z discs caused by distributions of ac tin and myosin myofilaments and of oc-actin in (see Fig. 61). The cross-striations of a whole fibre result from similar bands lying side by side. Each fibre is bounded by a sarcolemma (plasmalemma and basement membrane), the plasmalemma of which is deeply invaginated into the fibre forming transverse tubules (T-system), generally between the Z discs and H zones in insects but over the Z discs in vertebrates. These bring membrane depolarizations right into the fibre, ensuring uniform contraction. Mitochondria abound between myofibrils. The endoplasmic reticulum is modified to form a confluent system of sacs (sarcoplasmic reticulum) controlling calcium ion concentration.

On stimulation, a striated muscle fibre contracts by shortening and thickening. Fibres are bound together by connective tissue to form muscle tissue, and bring about locomotion by moving the skeleton, to which they are attached in vertebrates by tendons. See m usc le off.

1 micron
Complete Diagram Sarcomere

Fig. 61 (a) Diagram of the ultrastructure of a vertebrate striated muscle fibre showing a complete sarcomere and two adjacent parts of sarcomeres. Several such fibres together, with appropriate connective tissue, form a muscle fascicle several of which in turn comprise a striated muscle as shown in (b).

Fig. 61 (ft) Four striated muscle fascicles and surrounding connective tissue forming a small muscle.

Fig. 61 (a) Diagram of the ultrastructure of a vertebrate striated muscle fibre showing a complete sarcomere and two adjacent parts of sarcomeres. Several such fibres together, with appropriate connective tissue, form a muscle fascicle several of which in turn comprise a striated muscle as shown in (b).

Fig. 61 (ft) Four striated muscle fascicles and surrounding connective tissue forming a small muscle.

539 SUBSTRATE

539 SUBSTRATE

contraction, cardiac muscle, smooth muscle, neuro-muscular junction.

Strobilation. Process of transverse fission which produces proglottides from behind the scolex of a tapeworm and ephyra larvae from the jellyfish scyphistoma (Scyphozoa). Regarded as a method of asexual reproduction in the latter, but less commonly in the former. The whole ribbon-like chain of tapeworm proglottides may be referred to'as a strobila.

STROBILUS, Cone. Reproductive structure comprising several modified leaves (sporophylls), or ovule-bearing scales, grouped terminally on a stem.

Stroma. (Bot.) (1) Tissue-like mass of fungal hyphae, in or from which fruit bodies are produced. (2) Colourless matrix of the c h lor o ■ plast, in which grana are embedded. (Zool.) Intercellular material (matrix), or connective tissue component of an animal organ.

Stromatolites. Macroscopic structures produced by certain blue-green algae (c y a n0ba cter i a) where there is deposition of carbonates along with trapping and binding of sediments. Predominantly hemispherical in shape, they possess fine concentric laminations produced by growth responses to regular (often daily) environmental change. Fossil stromatolites occur from early Precambrian (more than 3000 Myr BP) to the Recent period.

Style. Slender column of tissue arising from top of ovary and through which pollen tube grows.

Subarachnoid space. Area between the arachnoid and the pia mater, filled with cerebrospinal fluid. See meninges.

Subcutaneous. Immediately below dermis of vertebrate skin (i.e. the hypodermis). Such tissue is usually loose connective tissue, blood vessels and nerves and generally contains fat cells (see ad i pose TISSUE). In many tetrapods, also includes a sheet of striated muscle (panniculus carnosus) to move skin or scales.

Suberin. Complex mixture of fatty acid oxidation and condensation products present in walls of cork and most endodermis cells, rendering them impervious to water.

SUBERIZATION. Deposition of suberin.

Subspecies. Formal taxonomic category used to denote the various forms (types), usually geographically restricted, of a polytypic species. See 1nfraspecific variation.

Substrate. (1) Substance upon which an enzyme acts. (2) Ground or other solid object on which animals walk or to which they are attached. (3) -Material on which a microorganism is growing, or solid surface to which cells in tissue culture attach.

subtidal 540

subtidal 540

subtldal. Zone in sea or ocean extending from low-tide mark to edge of-continental shelf.

succession. Progressive change in composition%-of a community of organisms, e.g. from initial, colonization of a bare area (primary succession), or of an already established community (secondary succession), towards a largely stable climax. See sere.

Succulent. Type of xerophytic plant whidh stores water within its tissues and has a fleshy appearance (e.g. cacti).

succus ENTERicus (intestinal juice). Digestive juice (pH about 7.6 in humans) containing enterokinase, peptidases, nucleases, suc-rase^ etç,, allsecreted.by. the-glandular crypts of Liebeikuhn between , „,

. intestinal villi. Completes hydrolysis of food molecules begun higher in the gut. About 2r3 litres per day secreted in humans. See digestion.

Sucker. Sprout produced by roots of some plants, giving rise to a new plant.

sucrase (invertase). See sucrose.

Sucrose- (cane, suqar). Non-reducing_ disaccharide, comprising one glucose and one fructose moiety, linked between Cx of glucose and C2 of fructose. Abundant transport sugar in plants. Digested by the enzyme sucrose (myertase) and dilute mineral acids to glucose and fructose. See invert sugar. !

suctoria. Predatory ciliates, ciliated only in larval stage. See ciliata.

sulcus. (1) Longitudinal furrow, as in groove containing trailing flagellum in dinoflagellates; (2) thin furrowed area of pollen wall, notably in cycads and Ginkgo pollen.

Summation. Additive effect at synapses when arrival of one or a few presynaptic impulses is insufficient to evoke a propagated response but a train of impulses can do so. Termed temporal summation when impulses arrive at the same synapse* spatial summation when at different synapses of the same cell. In, conjunction with nervous inhibition and facilitation it enables fine control over an animal's effector responses. See nervoÛS intégrât ion, synergism, transmitter.

Supergene. Group of gene loci with mutually reinforcing effects upon phenotype that have come (through selection) to lie on the same chromosome, increasingly tightly linked so as to be inherited as a block (e.g. supergenes for the heterostyle or homostyle system in primroses (Primula) and shell colour and banding pattern in the snail Cepaea). See POLYMORPHISM.

541 SUTURE

Superior ovary. See receptacle .

Supernumerary chromosome (accessory chromosome):' Chromosome additional, to normal karyotype of the species (B chromosome in botany). Either not homologous, or only partially homologous, with members of normal karyotype. In some populations of a species most of the individuals carry such chromosomes; in others, frequency is low. Most are heterochromatic. Their presence does not seem to affect markedly the individual's appearance. Geographical distribution often non-random.

Superspecies. Informal taxonomic category usually applied to al-lopafric arrays of species where evidence suggests common ancestry

' and where the species are sufficiently similar. Such species complexes tend to be discoverable with difficulty due to cryptic distinguishing characteristics between the species (usually sibling species), such as Anopheles gambiae and Simulium damnosum complexes. See dna probe.

Supination. See pronation.

Suppressor mutation. (1) (Intragenic.) A mutation fnucleotide addition or deletion) at a site in a. chromosome sufficiently close to a prior nucleotide deletion or addition to restore the reading frame and thus suppress the effect of the original mutation. (2) (Intergenic.) A mutation at one locus which prevents the expression of a mutation at another locus. (3) (Uncommonly) a mutation preventing local or complete crossing-over in meiatic cells.

Some suppressor genes seem to be responsible for preventing oncogenic phenotype in normal cells; their loss may result in ONcogene expression. See ABERRANT chromosome BEHAVIOUR (1), mutation, hypostasis.

Suprarenal gland. See adrenal gland.

Survival value. Characters and genes are said to have positive survival value if they increase an individual's f itness, and (less commonly) negative survival value if they decrease it. Neutral survival value is attributable to characters and genes with no effect on fitness. See NATURAL SELECTION.

Suture. Area of fusion of two adjacent structures. (1) (In flowering plants) the line of fusion of edges of carpel is known as a ventral suture. Mid-rib of carpel is known as the dorsal suture, not implying any fusion of parts to form it but to distinguish it from the ventral (true) suture. (2) Junction between the irregular interlocking edges of adjacent skull bones, or between plates of hardened cuticle of exo-skeleton. Suture-lines occur on shells of ammonites, marking edge of a septum with side-wall of shell. (3) (Surgical) to sew a wound together.

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