A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Saccule: (in the inner ear) it is the cavity of the vestibule of the inner ear that connects with the cochlea. In the saccule, there are receptors sensing the gravity and linear acceleration of the head.
Salivary gland: it is a type of exocrine gland found in the posterior and ventral parts of the oral cavity. There are several salivary glands. Parotid, sublingual and submaxilar glands are the mayor salivary glands in mammals. They release serous and mucous components forming altogether the saliva.
Salt gland: (in plants) it is a type of gland found in halophyte plants. Salt glands gather and store the salt of the plant body. Once the salt gland cells break down, the salt are laid on the surface of the epidermis.
Scala media: (medial ramp or cochlear duct) it is a canal of the middle ear placed between the scala tympani and scala vestibuli. Scala media is filled with endolymph and contains the Corti's organ, a structure that senses the endolymph currents caused by the sound.
Scala tympani: (or tympanic ramp) it is a canal in the cochlea of the inner ear that is located below the scala media. The scala tympani is filled with perilymph.
Scala vestibular: (or vestibular ramp) it is a canal in the cochlea of the inner ear that is located above the scala media. The scala vestibular is filled with perilymph.
Schizogeny: (in plants) it is a way of aeriferous parenchyma cells differentiation that produces characteristic large intercellular spaces. These spaces are generated during tissue differentiation and does not involve death of cells. Lysogeny is another way to get large intercellular spaces, but involving cell death.
Schwann cell: it is a type of glial cell of the peripheral nervous system. Shwann cells form the myelin sheaths around peripheral nerves. They are also involved in supporting and repairing the peripheral nervous system.
Sclereids: (in plants) it is a type of cell that forms the sclerenchyma, which is a type of supporting tissue. Sclereids have thick and lignified secondary walls, and show a variety of forms, such as isodiametric, star-like morphology, branched, and many others. They are named according to their shape. Sclereids are found in stems, leaves, fruits and seeds, either isolated or in sheets.
Sclerenchyma: (in plants) it is a supporting tissue with cells having thick and lignified secondary walls, so they are dead cells. Sclerenchyma is composed of two cell types: sclereids and sclerenchyma fibers. The supporting role of sclerenchyma is important in plant structures that stop growing in length, where sclerenchyma also provides elasticity. Although sclerenchyma is found in plant organs with primary and secondary grow, its function is more important in primary growth structures. It is abundant in stems, leaves and roots.
Sclerenchyma cell: See sclerenchyma fiber.
Sclerenchyma fiber: (in plants) it is a type of cell that forms the sclerenchyma tissue. Sclerenchyma fibers are long cells, with sharp ends, and a more or less thick secondary wall with a variable level of lignification. Sclerechyma fibers are classified regarding their location in the cell. Thus, extra-xilary fibers are found in the phloem (phloem fibers), in the cortex (cortical fibers) or around the vascular bundles (perivascular fibers). Xilary fibers are those found in the xylem.
Sclerotic: it is the outermost tunic of the eye. It is also known as fibrous tunic.
Sebaceous gland: it is type of gland that releases substances known as cebum. Sebaceous glands are found associated to hair follicles and release their content into the canal of the hair follicle. In the skin regions without hair follicles, such as lips and some regions of the reproductive organs, sebaceous glands can release their content directly to the skin surface.
Secondary cell wall: (in plants) it is a layer of the cell wall found between the primary cell wall and the plasma membrane. So, it is the last layer to be synthesized. The secondary cell wall is usually thicker than the other cell wall layers and appears when the cell stop growing, and usually cells die after the secondary cell wall is completed. It is mostly composed of cellulose, hemicellulose and lignin, which provides great stiffness. The secondary cell wall is the main component of wood.
Secondary endosperm: (in plants) it is the endosperm of angiosperm seeds. The secondary endosperm is originated from the fusion of the two polar haploid nucleus of the female gametophyte with the generative nucleus of the male gametophyte. Therefore, the secondary endosperm is a triploid tissue that functions as an energy store, which is mobilized during seed germination.
Secondary growth: (in plants) it is a type of growth of stems and roots, which is characteristic of some groups of plants, mainly woody plants. Secondary growth is a growth in diameter as a result of the activity of the secondary meristems.
Secondary meristem: (in plants; or lateral meristem) it is a type of meristem that allows the grow in thickness of plants. Secondary meristems are not present in some grasses, in organs without primary grow, or in some organs like leaves. There are two types of secondary meristems: vascular cambium and cork cambium.
Secondary phloem: (in plants) it is a type of phloem that develops from the vascular cambium during the secondary grow of plant organs. Secondary phloem is composed of several types of cells, such as sieve cells or sieve tubes, companion cells, axial and radial parenchyma cells, and fibers.
Secondary root: (in plants) it is the root with secondary grow, that is, when secondary xylem and phloem are produced by the vascular cambium. The vascular cambium activity leads to an increase of the root girth. Primary and secondary grow may be observed in the same root: the primary grow (primary root) is close to the root tip, and the secondary grow (secondary root) can be observed more distant from root apex.
Secondary xylem: (in plants) it is a vascular and support tissue in plants showing secondary growth. The secondary xylem is derived from the vascular cambium meristem. The conducting cells in the secondary xylem are tracheids and tracheae. The growing rings of tree stems are layers of xylem cells, that make most of the wood.
Secretion: it is the release of substances from the cell to the extracellular space and to the body surfaces. These substances have a physiological role. Sometimes, it is difficult to distinguish between secretion and excretion (excretion is the release of waste substances) because some waste substances may have a function..
Seed: (in plants) it is an organ of plants that contains the embryo surrounded by the endosperm and seed coats. The seed is developed from the flower ovary.
Semicircular canals: they are ducts with semicircular morphology that form part of the inner ear, and are found inside the temporal bone of the head. There are three: superior, posterior and lateral. A membranous structure called membranous labyrinth lines the inner surface of semicircular canals. In each canal, there is a thickening known as ampulla, which is a sensory receptor for detecting the movement of the endolimph, and therefore the movement of the head. Semicircular canals work as sensory strucuture that sense the position of the body to maintain the posture body balance.
Seminal coat: see episperm
Septal organ of Masera: it is a sensory structure found in the floor of the nasal cavity of some animal species.
Serous gland: it is a type of gland with secretory cells releasing an aqueous fluid that mainly contains enzymes, is isotonic with the blood plasma, and is colorless. Parotid and submandibular salivary glands are examples of serous glands.
Serous secretion: it is a type of secretion mostly consisting in enzymes, salts, and water.
Smear: it is a layer of cells spread on a slide. Cells are usually coming from a suspended solution of cells. It is the best technique to observed blood cells. It is then called blood smear.
Sepal: (in plants) it is the modified leaf that forms the calyx of flowers.
Serous acinus : it is a group of cells that form the secretory units of many exocrine glands. These cells show a purple cytoplasm when stained with haematoxylin-eosin. The nucleus is rounded and not close to the basal membrane. Water and enzymes are the main components released by serous acini. Serous acini are abundant in those glands involved in digestion, such as salivary glands and pancreas.
Sesile: (in plants) it is a type of leaf that lacks petiole and is attached to the stem directly through the blade.
Sieve areas: (in plants)they are cell wall specializations of the sieve cells and sieve tubes of the phloem of plants. Sieve areas are groups of pores that laterally connect adjoining cells. The size of the pore is usually larger than the common plasmodesmata and the inner surface of the canal is partially covered by calose.
Sieve cell: it is a type of cell found in the phloem of pteridophytes and gymnosperms. Sieve cells transport nutrients and organic molecules through the plant body.
Sieve element: (in plants) it is the conducting cell of the phloem. There are two types of sieve elements: sieve cells gymnosperms and sieve tubes in angiosperms. Sieve elements are living cells that are connected to each other by the sieve plates of their cell walls and form long rows of conducting cells.
Sieve plate: (in plants) it is a specialization of the terminal walls of the phloem sieve tubes. Sieve plates contain pores of about 15 µm that connect consecutive sieve cells.
Sieve tissue: (in plants)see sieve elements.
Sieve tube: (in plants) it is the conducting cell of the phloem in angiosperms. Sieve tubes transport organic molecules through the plant body. They form long rows of cells connected by the sieve plates.
Simple epithelium: it is a type of epithelium consisting in just one layer of cells. The cells may be squamous, cuboidal or columnar.
Simple gland: it is a type of gland with a non-branched excretory duct. Simple glands show only one secretory unit that releases substances into one excretory duct, which leads to the body surface.
Skeletal muscle: it is a type of muscle made up of skeletal striated muscle cells, which are long and multinucleated cells, with the nuclei located near the plasma membrane. These cells form muscles attached to bones, excepting the tongue, face and some others. Skeletal muscles are responsible for the voluntary movements and locomotion. Skeletal striated muscle cells are also known as myocytes or muscle fibers.
Smooth muscle: it is a type of muscle made up of muscle cells that do not show transverse striations when observed at light microscopy. Smooth muscle cells are elongated, much shorter than the skeletal muscle cells, and showing a central nucleus. They are involuntary contracted by the autonomous nervous system. For instance, smooth muscle is found in the wall of the digestive tract and blood vessels.
Solitary tract nucleus: it is a sensory nucleus found in the rhombencephalon. The axons that carry the information of the taste buds of the oral cavity enter the encephalon in two nerves: VII and IX. These axons innervate the solitary tract nucleus, which sends axons to the thalamus nuclei. From the thalamus, the gustatory information is sent to the gustatory cortex.
Soma: (in neurons) it is the part of the neuron where the nucleus and most organelles are found. The soma is the thicker part of the neuron. Dendrites and the majority of axons arise from the soma.
Somatic peripheral nervous system: it is the set of nervous elements that carry sensory information to the central nervous system and those coming from the central nervous system that innervate muscle cells and move the body.
Spermatophyte: it is the group of plants characterized by producing seeds (they were also known as phanerogam). Spermatophytes are divided into gymnosperms (plants without true flowers) and angiosperms (plants with true flowers).
Spinal cord: it is the most caudal region of the central nervous system. The spinal cord is found in the medullary canal, surrounded by vertebrae. The spinal cord is a long structure divided in segments, each of one bearing a dorsal root with sensory nerves and a ventral root with motor nerves. Sensory information travels along the spinal cord from the body to the encephalon and nerves from the encephalon to the body carry motor information. In transverse view, the spinal cord is divided in the external white matter and the internal gray matter.
Spinal nerve: it is a nerve that enters or quits the spinal cord.
Spiral ganlion: see Corti's ganglion
Spleen: it is a lymphoid organ found in the upper part of the abdominal cavity, near the diaphragm. The spleen shows an ovoid and elongated shape, triangular in transverse sections. It is the largest lymphoid organ accounting for about 0,2 % of the total body weight. Together with the liver, the spleen shows an intense red color. This color is related to its functions: storage and remove of blood cells. Thus, the spleen removes erythrocytes (hematolysis) and platelets, but also storages platelets, monocytes and other blood cells. At same time, during the fetal period, it is involved in the production of lymphocytes, and other blood cells (hematopoiesis).
Spongy parenchyma: (in plants) it is part of the mesophyll, found in the lower part of the leaf. Spongy parenchyma contains photosynthetic parenchyma cells that leave large and numerous intercellular spaces, which facilitate the gas diffusion. Spongy parenchyma is a type of aerenchyma. It is also known as lacunar parenchyma.
Stamen: (on plants) it is the male organ of flowers. Stamen consists of an anther, containing the pollen grains, which are the male gametophyte with the microspores, and a filament that connects the anther with the flower receptacle. The set of stamens of a flower is known as androecium.
Stapes: it is a bone found in the tympanic cavity of the middle ear. Stapes, incus and malleus are the middle ear bones. The tympanic membrane moves the malleus, that moves the incus, that moves the stapes. The stapes is in contact with the oval window which transmits the vibrations to the vestibule and the cochlea. In the cochlea the vibrations are transformed in sound information.
Statoacustic nerve: see vestibulochoclear nerve.
Statocyst: (in plants) it is a type of cell found in the root cup of plants. Statocysts contain dense bodies, mostly starch grains in amyloplasts (statolisths), that are responsible for the positive gravitropism during root growth.
Stem: (in plants) it is the organ of plants that supports the leaves, flowers, seeds, fruits, and branches. In addition, the stem may perform photosynthesis or function as a storage place. It is divided longitudinally into nodes and internodes. In the nodes the buds or axillary meristems produce the leaves and branches. The stem apical meristem is found at the tip of the stem, from which the axillary buds and the different tissues of the main stem are generated. All stems initially have primary growth, but some develop secondary growth.
Stem cell: it is a type of undifferentiated cell that can differentiate into functional cells of the organism. There are several types of stem cells, but they can be divided into two broad categories: embryo and adult stem cells. Embryo stem cells are found in the inner cell mass of the embryo at the blastocyst stage. They can differentiate into each cell type of the organism. So, they are pluripotent cells. Adult stem cells are found in almost every tissue at post-embryo stages, including adults. They give rise to some types of cells depending on the tissue they are located in. So, they are multipotent cells. All stem cells have two properties: differentiate into functional cells and auto-renewing, that is, they can divide by mitosis and produce new stem cells.
Stereocilium: it is a type of filiform cellular expansion of the free surface of some cells. Stereoclilia may function as sensory structures. In some organs like the inner ear, stereocilia are mechanosensory receptors. Stereocilia have nothing to do with cilia because they are modified microvilli and their inner structure is made up of actin filaments, and they cannot move.
Stigma: (in plants) it is the part of the pistil of the flower where the pollen grains are attached to during pollinization. The pollinic tube outgrows from the pollen grains into the stigma. Style is the part of the pistil connecting the stigma and the ovary.
Stipule: (in plants) it is a small leave or scale that develops at the insertion point of leaves with the stem.
Stoma: (in plants) it is a structure found in the epidermis of leaves, mostly in the lower epidermis. Stomata can also be found in the stem of herbaceous plants. Stomata is made up of two adjoining cells known as guard cells, which form a pore that communicates the plant tissues and the external environment. Just below the two guard cells, there is a subtesmotal chamber in the parenchyma. The number and shape of stomata is widely variable depending on the plant species and their habitats. The main function of stomata is the regulation of plant transpiration and the exchange of CO2 y O2 between the plant and the environment.
Stomatal crypt: (in plants) it is a cavity or depression of the epidermis surface of leaves, usually found in the lower or abaxial surface. Stomata are found in the stomatal crypts. These depressions are common in xerophyte plants.
Storage parenchyma: (in plants) it is a type of parenchyma specialized in storing substances. Storage parenchyma may accumulate proteins, lipids or carbohydrates, either solved in the cytosol or as aggregates like starch.
Stratified epithelium: it is a type of epithelium made up of more than one layer of cells. According to the shape of the cells of the outermost layer, there are squamous, cuboidal and columnar stratified epithelia. Epidermis is a keratinized stratified squamous epithelium because the outermost layer, the stratum corneum, consists of death keratinocytes.
Stratum basale: it is the innermost layer of the stratified epithelia. Epithelial stem cells are located in the stratum basale. These cells make possible the continuous renewing of the eptihelial cells. Although there are stem cells in all types of epithelia, the stratum basale is clearly distinguished in the stratified squamous epithelium.
Stratum corneum: it is the outermost layer of the keratinized stratified squamous epithelium, just above the stratum granulosum. Stratum corneum is made up of flattened dead cells filled with keratins. Its main function is to protect against dehydration, pathogens and mechanical insults. The superficial layers of the stratum corneum are continuously detached, at the same time that new ones are added in its inner surface. The thickness of this stratum depends on the body region and it is thicker in those areas under heavy rubbing or mechanical stress. The stratum corneum is found in the epidermis of the skin.
Stratum granulosum: it is the layer of cells located just above the stratum spinosum of the stratified squamous epithelia. In the epidermis, the stratum granulosum consists in 3 to 5 sheets of flattened cells with a cytoplasm full of basophil granules containing keratohyalin. This content is important to form the upper stratum corneum in keratinized stratified squamous epithelium.
Stratum spinosum: it is a cell layer of the squamous stratified epithelium. Stratum spinosum is located between the stratum basale and the stratum granulosum. It is the thicker stratum in this type of epithelium. It is clearly visible in the epidermis, where keratinocytes show many interdigitations that look like spines, that is why the name spinosum.
Striated muscle: it is a type of muscle composed of striated muscle cells. These muscle cells show transverse striations when visualized at light microscopy, which are the result of the orderly disposition of the actin and myosin filaments in the cytoplasm. There are two types of striated muscle cells: skeletal muscle cells, which form the skeletal muscles, and cardiac muscle cells, which form the heart walls.
Style: (in plants) it is the part of the pistil of the flower that connects the stigma with the ovary.
Subcommissural organ: it is a group of cells found in the dorsal wall of the third encephalic ventricle, under the posterior commissure. Subcommissural organ synthesizes the Reissner fiber of the central canal, a long row of glycoproteins that reaches the caudal levels of the spinal cord. The subcommissural organ is one of the subventricular organs, which are cell populations located at different rostrocaudal levels of the ependyma of the encephalon.
Suberin: (in plants) it is a polymer found in the cell wall of plant cells. Suberin consists in long chain fatty acids that are laid in the inner part of the primary cell wall. Suberin provides hydrophobicity to the cell wall.
Subsidiary cell: (in plants) it is a type of cell that surrounds the stomata guard cells. Subsidiary cells may help with the opening of the stoma pore. For example, during low CO2 in the mesophyll, they favor the stomata opening by decreasing their own volume.
Substomatic chamber: (in plants) it is a cavity in the parenchyma tissue, just below the guard cells of the stomata. Substomatic chamber favors the exchange of gases and water between the parenchyma tissue and the environment through the stoma pore.
Superior cervical ganglia: it is a ganglion of the sympathetic system that innervates the muscles for eye pupil dilation. Superior cervical ganglion makes pupil increase in diameter.
Superior ovary: (in plant flowers) it is the ovary located above the receptacle (structure where stamens, sepals and petals are inserted).
Supporting cell : (in the olfactory epithelium) it is a type of cell found among the olfactory receptor neurons of the olfactory epithelium. Supporting cells provide support and some kind of electrical isolation to receptors cells.
Supporting cells: (in the taste buds) it is type of cell that surrounds the sensory cells of the taste buds. They are also known as subtentacular cells.
Suspensor: (in plants) it is the structure that connect the embryo to other tissues of the female gametophyte. The suspensor is the way for food to reach the seed.
Sweat gland: it is a type of gland found in the integument of many mammal species. Sweat glands release an aqueous substance, known as sweat, to the surface of the skin. There are eccrine sweat glands that release their content directly to skin surface, and apocrine sweat glands that release their content into the hair follicle canal.
Sympathetic ganglion: it is a type of peripheral ganglion (autonomic nervous system). Paravertebral ganglia are sympathetic gangli.
Symplast: (in plants) it is the interior of plant cells. Symplastic communication is the exchange of molecules between adjoining cells through plasmodesmata, so that molecules do not quit the cytoplasm. That is, it is direct cytoplasm-cytoplasm communication.
Synapsis: it is a cellular structure for communication between neurons. A neuron may make synaptic contacts with thousands of neurons. A synapse consists of a presynaptic element that releases neurotransmitters. These neurotransmitters diffuse through a synaptic gap and activate receptors locate in the postsynaptic element, that leads to electric changes in the postsynaptic neuron.
Syncitium: it is a cytoplasm that contains several nuclei. The fusion of several cells, as it happens to the skeletal muscle cells, or an atypical division with nuclear division without cytoplasm split (no cytokinesis) lead to a syncitium.