Glossary: R
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Radial initial cells: (in plants) it is a type of cell found in the vascular cambium meristem. Radial initial cells differentiate into the radial elements of the xylem and phloem.
Radicle: (in plants) it is the root primordium found in the embryo of plant seed that gives rise to the plant root. During germination, radicle is the first to grow and develops. Radicle is located in the opposite site to plumule, which develops into the shoot of the plant.
Ray parenchyma: (in plants) it is a set of parenchyma cells that are arranged as spokes in the secondary xylem when the stem is observed in transverse sections.
Receptacle: (in plants) it is the part of flowers where the sepals, petals and stamens are inserted. The relative position of the ovary and the receptacle give three types of flowers: superous, half-inferior and inferous ovary. In some species, the receptacle contribute to form some part of the fruit.
Red cell: see erythrocyte.
Regular dense connective tissue: it is a type of dense connective tissue with abundant collagen fibers oriented in a particular direction. The orderly arrangement of the collagen fibers is intended to resist unidirectional mechanical forces. Regular dense connective tissue can be found in tendons, joint ligaments (excepting those in the vertebral column), in the sheaths covering some muscles, and in the cornea.
Reticular connective tissue: it is a type of connective tissue with abundant reticular fibers in the extracellular matrix. Reticular fibers are contain collagen type III and carbohydrates, and are the main support for cells and the rest of the extracellular matrix of the reticular connective tissue. This tissue can be found in the bone marrow, thymus, spleen and lymphatic ganglia.
Reticular dermis: it is the deeper layer of the dermis. Reticular dermis is irregular connective tissue with thicker collagen fibers. It also contains a dense net of blood and lymphatic vessels.
Reticular fiber: (or reticulin) it is a type of fiber found in the extracellular matrix of the connective tissues. Reticular fibers are mostly made up of type III collagen and form a network that supports some soft organs, such us lymphoid organs, liver, and bone marrow. Although type III collagen is found in other types of organs, reticular fibers are more frequent in these organs.
.Retina: it is the nervous structure that senses and processes the light that gets into the eye of vertebrates. Retina is the inner tunica of the eye tunicas. It is formed after an outgrowth of the encephalon that folds to form a cup-like structure. The wall of the cup has two layers, one pigmented (outer) and one neuronal (inner). The neuronal layer is the retina itself. Retina is made up of several layers of neurons and a layer of photoreceptors that can be either cones or rods.
Retroperitoneum: it is the space of the abdominal cavity found just behind the peritoneum.
Rhizodermis: it is the epidermis of the root.
Ritidome: it is the outer layer of periderm that sheds out periodically, usually every year.
Rod: (in the retina) it is a type of photoreceptor, sensory cell of the retina of the eye. Rods are responsible for detecting light intensity.
Root: (in plants) it is the organ that fix the plant body to the ground, and is responsible for getting water and minerals from the soil. In some species, the root is a storage organ. There are two types of root system: axonomorph and fasciculated. Roots may show primary grow and secondary grow. The root apical meristem is found in the root tips for promoting root elongation, and close to the tip there are the root hairs, epidermic structures specialized in absorbing water.
Root cap: (in plants) it is a hood-like structure that covers the primary root apical meristem. Root cap is made up of cells that release mucilaginous substances for lubricating the growing tip of the root. Besides the protective role, root cap also contains cells that are able to detect gravity and therefore provide the positive gravitropism to roots.
.Root hair: (in plants) it is an epidermal specialized cell found in the root of plants (do not confuse with trichomes). Root hairs are localized in the maturation region, near the tip of the root. Root hairs are unicellular and filiform, and their main role is to get water and minerals from the soil into the plant.
Root meristem: (in plants) it is the meristem found at the tips of the roots.
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.Root system: it is the set of roots of a plant. There are two major types of root tap systems: tap (branched) and fibrous root (non branched).
Ruffini corpuscle: it is a type of encapsulated receptor found in deep regions of the body. Ruffini corpuscle shows slow adaptation to stimuli so that it is able to respond to long lasting signals. They are located in joint capsules where they sense the rotational position. They work as thermoreceptors, mechanoreceptors and pain receptors.