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Abaxial: surface of the plant leaves that do not face the Sun. In many plant species, this surface contains most of the stomata.
Abscission: aging process and shedding of leaves, fruits and flowers. In general, it is mediated by hormones such as cytokines and ethylene.
Abcisic acid: it is a plant hormone (phytohormone) mainly synthesized in roots and mature leaves after a period of hydric stress. It is also abundant in seeds, either because it is coming from other plant organs or because it is synthesized by seeds themselves. Abscisic acid is transported from roots and leaves by vascular bundles.
Acinar gland: it is a type of exocrine gland with secretory units organized in acini, which are bag-like structures with thick walls and very little inner empty space.
Adaxial: (in leaves) it is the surface of a leaf that faces the Sun. Sepals and petals of flowers also show an adaxial surface, usually toward the interior of the flower.
Adipocyte: it is the main cell type of adipose tissue. It functions as an energy storage by accumulating large amount of fatty acids. Adipocytes are round and big, depending on how much fat they storage. There are two main types of adipocytes: unilocular and multilocular. Unilocular adipocytes form the white fat, while multilocular adipocytes form the brown fat. Unilocular adipocytes contain a single large lipid droplet mainly composed of triacylglycerols that occupies most of the cytoplasm. The nucleus and remaining organelles are confined in a thin layer of cytoplasm near the plasma membrane. Multilocular adipocytes contain many small lipid droplets scattered in the cytoplasm and many mitochondria. In brown fat, mitochondria produce heat instead of ATP, and contribute to the brownish color of the brown fat.
Adrenal gland: it is a type of endocrine gland found onto the upper surface of the mammalian kidneys. Adrenal gland is made up of several regions organized from the periphery to the inner part, each to them releasing different types of hormones: mineralocorticoids by the zona glomerulosa, glucocorticoids by the zona fasciulata, androgens by the zona reticulata and catecholamines by the medulla.
Adventitia: it is the outer layer of connective tissue that lines many animal organs.
Aerenchyma: see parenchyma aeriferous.
Albuminous cell: it is a type of parenchymatic cell found in gymnosperm plants. Albuminous cells are companion cells morphologically and physiologically associated to sieve cells of the phloem by lateral pore fields.
Aleurone: see aleurone grains.
Aleurone grain: it is a type of protein depot found in the endosperm of seed in some plant species.
Alpha cell: it is a type of cell found in the Langerhans islets of the pancreas. Alpha cells release glucagon, which is a hormone that increases the levels of glucose in the blood.
Alveolus: it is a cavity or hollow surrounded by cells. Each of the hemispherical pit at the end of some ducts, like in some glands.
Ampullary crest: it is a projection or ridge of the epithelium of the semicircular canals in the inner ear. Ampullary crests form the sensory structures called ampullae. There are cells among the epithelial cells of the ampullary crests for detecting the movements of the endolymph. These movements are produced by the head movements. The information that sensory cells send to the brain helps keep the body balance.
Angiosperm: it is the group of vascular plants with seeds and flowers.
Angular collenchyma: it is a type of collenchyma with cells showing thickenings in their primary cell walls at the angles between adjoining cells. Angular collenchyma cells are densely packaged and there are not intercellular spaces. Angular collenchyma is usually found beneath the epidermis, forming a layer of cells sometimes referred as hypodermis.
Annular collenchyma: it is type of collenchyma tissue. Annular collenchyma cells have primary cell walls showing even thickness.
Anther: it is the part of the stamen of the flowers where microspores or male gametes are produced. Anther may be divided in two compartments called thecae. Each techae contains two microsporangia or polinic sacs.
Anticlinal: (in plants) it is a type of cell division where the new cell wall that separates the two new cells is perpendicular to the surface of the organ, usually a stem or a root. It can be radial when the new wall is parallel to the long axis of the organ or transversal when it is perpendicular to the long axis of the organ.
Apical dominance: it is the inhibition of the growth of the lateral branches of a plant caused by the apical part of the main shoot. This inhibition is produced by the auxin hormone, released by the apical part of the shoot. In this way, lateral branches grow when they get far from the main apical shoot, as the main shoot grows in lengths.
Aorta: one of the main arteries that emerges from heart and branches many times to form the system circuit, which drives blood to most parts of the body.
Apical: (in animas) it is the free domain of a cell, that facing a cavity or the exterior of the body. For example, epithelial cells have an apical domain in contact with the exterior of the body, the interior of the digestive duct, the interior of the blood vessels, the lumen of glandular secretory units and excretory ducts. Cells having an apical and a basolateral domain are called polarized cells because each domain performs different functions.
Apical: (in plants) those structures found at the ends of stems and roots.
Apoplast: (or apoplast communication) it is the space ouside de cells of plant tissues where molecules can diffuse. The apoplastic pathway is traveled by molecules that move through the intercellular spaces in plant tissues.
Artery: it is a type of vessel that drive blood from the heart to other organs. The artery wall is generally thick to counteract the blood pressure caused by the heart bits. Arteries are usually classified according to their size: large or elastic arteries, medium size or muscular arteries, and small or arterioles. They are made up of three tunics: the innermost tunica intima, tunica media and the external tunica adventitia.
Arteriole: it is a small diameter artery. Arterioles have one or two layers of smooth muscle cells and a diameter of about 30 µm. They control the blood flux toward the capillary network by the activity of the smooth muscles of the tunica media. Their wall is slightly contracted, so they can increase and decrease the diameter, and therefore the volume of the blood going through. Actually, arterioles are the main regulators of blood pressure. The histological organization of the arteriole wall is similar to the larger arteries. Thus, it has a tunica intima, a tunica media and a tunica adventitia.
Axon: it is a thin process that outgrows from a neuronal soma or from a primary dendrite. It transmits the electrical information generated in the soma to other cells: neurons or muscle cells. Axons are usually highly branched. Each branch is known as axon collateral, and the whole collaterals of an axon are called axonal tree. Along the thin axonal collaterals and in their tips, there are small swellings that work as presynaptic elements. Neurotransmitters are released from presynaptic elements to excite or inhibit the postsynaptic cell.