Glossary: D
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
Defensin: (in plants) it is a type of proteins for defending plants against parasites or herbivores. Defensins are found in the seed coats and are toxic or repellent.
Dehiscent: (in plants) it is a type of fruit. Dehiscent fruits are split open at maturity and release the seed. For instance beans and poppy.
.Dendrite: (in neurons) it is the part of the neuron specialized in receive and process information coming from other neurons. This information is transferred through many synapses. The result of the information processing is transmitted to the neuronal soma. Neurons may have a variable number of dendrites and the set of dendrites o a neuron is known as dendritic tree. Dendrites are tubular-like expansions emerging from the soma and may branch many times with different morphology depending on the neuronal type. Many dendrites show little expansions called spines, that are places for synaptic contact.
.Dense connective tissue: it is a type of connective tissue characterized by the high proportion of collagen fibers compared to the amorphous component of the extracellular matrix. Collagen fibers are highly compacted which gives it a dense appearance, hence the name dense connective tissue. Furthermore, there are more collagen fibers than cells, which are mostly fibroblasts. Dense connective tissue is divided into two types, regular and irregular, depending on whether the fibers of the extracellular matrix are ordered or disordered, respectively.
Dermal papilla: (in the skin) it is a tissular projection from the dermis toward the epidermis. Dermal papillae are observed in the thick skin that are those skin regions under heavy mechanical stress. For instance, dermal papillae can be found in the palms and soles.
.Dermis: it is one of the layers of the skin. Dermis is found just below the epidermis and above the hypodermis. It is connective tissue and can be divided into two sublayers: papillar dermis (loose connective tissue), close to the epidermis, and reticular dermis (irregular dense connective tissue). The secretory units of glands that release their content to the skin surface are found in the dermis, such as sweet glands. Dermis provides mechanical support and carries the blood vessels that nurse the epidermis. Most sensory receptors of the skin are found in the dermis.
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Descemet's membrane: It is a membrane foun in the cornea (in the eye). Descement membrane is located between the stroma and the endothelium of the cornea.
.Desmosome: (macula adherens) it is type of cell-cell junction. Desmosomes are established between adjoining cells. They are patch like adhesions complexes that are intracellularly connected with the intermediate filaments of the cytoskeleton. The extracellular domain of cadherines, the adhesion proteins of desmosomes, are responsible for cell-cell recognition and adhesion.
,Diarch root: (in plants) it is a type of root with the vascular bundle showing two rows, or poles, of protoxylem.
Dicotyledon: (or dicot) it is a group of plants that have two embryonary leaves, or cotyledons, in the seed and during the germination. Dicot plants show roots with lateral branches. The stem develops secondary growth, with vascular tissue arranged as a cylinder. Dicto flowers are composed of calyx, corolla, gynoecium and androecium, and the number of petals is generally multiple of 4 or 5.
Diffuse endocrine system: it comprises the endocrine neurons that are scattered through the body, mainly in the gut and respiratory epithelia. They do not form typical endocrine glands.
Dilator muscle: (in the eye) it is the muscle that changes the pupil diameter. Dilator muscle consists of myoepithelial cells radially arranged. They are innervated by sympathetic neurons of the superior cervical ganglion.
Dioecy: plant species that produce microspores and macrospores in different individuals.
Discontinuous endothelium: (or sinusoidal endothelium or sinusoid) it is a type of endothelium characterized by having large spaces between the endothelial cells, so that molecules and cells can easily cross it. Discontinuous endothelium is found in the liver, bone marrow and spleen.
.Dormancy: (in plants) it is a period of time where seeds show a very low metabolic rate, i.e., very few chemical reactions. During dormancy, seeds don't need much oxygen, energy and water. Seeds can stay in dormancy stage for a long time. Dormancy period finishes when the environment is suitable for germination.
Dorsal root: (in vertebrates) it is the fascicle of nerves that enter the spinal cord dorsally. Dorsal roots carry sensory information. There is one dorsal root in each spinal cord segment.
Drupe: it is a type of fruit showing a very hard endocarp, with stone-like consistency. The endocarp becomes the stone of the fruit that encloses the seed. For instance, the peach.
.Dry fruit: (in plants) it is a type of fruit. It is membranous or hard, non-well developed and with little hydration. There are three types of dry fruits: 1. Dehiscent fruits that develop from only one carpel, like leguminous fruits. 2. Dehiscent fruits coming from several carpels, for example the capsule fruits such as those in some species of the genus Hypericum. 3. Indehiscent fruits form compact fruit, for example caryopsis fruits such as most grasses.
Dura mater: it is the outer layer on meninges. Meninges are layer of tissue that overly the central nervous system: encephalon and spinal cord. Arachnoids and pía mater are the other two inner layers of meninges.