Glossary: O
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
Olfactory bulb: it is a part of the pallium of the encephalon of vertebrates. The olfactory bulb receives primary olfactory affences from the olfactory epithelium. It therefore processes odorous information.
Olfactory nerve: (nerve I) it is the nerve that contain the axons of the olfactory receptors located in the main olfactory epithelium. Axons of the olfactory nerve cross the skull throught the cribiform plate and end in the olfactory bulb forming the olfactory glomeruli.
.Olfactory receptor neuron: it is a type of sensory neuron involved in olfaction. Olfactory receptor neurons are found in the olfactory epithelium, in the nasal cavity. They recognize the airborne olfactory molecules. This recognition is transformed in electric information, which is transmitted to the olfactory bulb via the olfactory nerve.
Open collateral: (in plants) it is a type of vascular bundle organization. Open collaterals show the sequence: inner xylem, procambium in the middle, and outer phloem.
.Optic nerve: (nerve II) it is the nerve that conveys the information of the light gathered by the retina. The optic nerve is made up of axons from retinal ganglion cells. They make synaptic contacts in the thalamic lateral geniculate nucleus and in the superior colliculus.
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.Organ: it is a set of tissues organized between each other to form a differentiated structure that performs one or several functions in the organism. For instance, kidneys, heart, lungs and gut are animal organs, and flowers, root and leaves are plant organs.
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.Organ of Corti: it is a sensory structure found in the cochlea, in the inner ear. The organ of Corti is a mechanoreceptor for detecting the vibration of the endolymph, which are transformed in electrical information. It is made up of an epithelial layer containing ciliated cells.
.Osmophore: (in plants) it is a secretory structure that produces the fragrance of plants by releasing volatile substances.
Osteoblast: it is a cell type found during of bone formation and remodeling. Ostoblasts are the main responsible for the synthesis and mineralization of bone extracellular matrix. Osteoblasts cannot divide and when they get surrounded by extracellular matrix, they differentiate into osteocytes, the mature cell of bone tissue. It can be said that osteoblasts are an intermediate stage into the whole differentiation process of bone cells that starts as bone osteoprogenitor mesenchymal cells and ends up as osteocytes.
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,Osteoclast: it is a type of cell that resorpts the bone during bone tissue remodeling. Osteoclast release enzymes and low the pH to degrade bone extracellular matrix. They are multinucleate and polarized cells that belong to the macrophage cell system in animals. Osteoclast differentiate from cells located in the bone marrow.
Osteocyte: it is a type of cell found in the mature bone. Osteocytes are differentiated from osteoblasts and they cannot divide. Oscteocytes maintain the mature bone extracellular matrix and live in cavities known as bone lacunae. Neighbor bone lacunae are connected by small ducts named bone canaliculi. Osteocytes send cellular extensions through bone canaliculi to contact and communicate with neighbor osteocytes.
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.Osteon: it is the structural unit in some parts of the compact bone of some mammal species. Each osteon is made up of extracellular matrix lamellae (from 4 to 20) arranged concentrically around a central canal called Haversian canal. Blood vessels and nerves run through the Havers canals. Between the lamellae, the oocytes are located inside the cavities known as bone lacunae. There are bone canaliculi (small ducts) connecting neighbor bone canaliculi and osteocyte thin cellular extensions inside bone canaliculi that allow the communication between adjoining osteocytes. Osteon is also known as Haversian system.
,Ostiole: (in plants) it is the pore between the two guard cells of stomata. The ostiole allows the exchange of gas and water between the environment and the inner tissues of plants (mostly in leaves). The pore diameter can be regulated by increasing or decreasing the size of guard cells.
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.Outer ear: it is the most external part of the ear consisting of the pinna (auricle) and the external auditory canal, which communicates the outer environment with the tympanic membrane.
.Ovary: (in plants) it is the part of the pistil where the female gametophyte is developed and where fertilization takes place. The ovary contains the ovules, which become the seed, and the fruit in many species.
.Ovule: (in plants) it is the structure of the ovary of the flower that contains the macrospore and the female gametophyte. The ovule becomes the seed.
Ovule integument: (in plants) it is the coat surrounding the embryo in angiosperms. The ovule integument, also known as episperm, is divided in two layers that develop into the testa and tegmen of seeds.