Atlas of plant and animal histology

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Glossary: H


Hair follicle: it is an invagination of the epidermis of mammals that produces a hair. At the base of the hair follicle, there are cells that continuously synthesize the hair components, which associate to form and extend a hair. Sebaceous glands are associated to the hair follicle. They release a fatty solution to the hair follicle canal, through which the skin surface is reached.

♣ Animal tissues: Muscle.

Halophyta: it is a plant species that can live in environments with high salt concentration.

♣ Plant tissues: Glandular.

Hammer: see malleus.

Harvesian canal: it is the central canal of osteons. Osteons are structures made up of bone matrix and osteocytes found in the compact bone of some animal species. Nerves and blood vessels run through the Harvesian canals to innervate and irrigate, respectively, the bone tissue. Harvesian canals are parallel to the long axis to the bone epiphysis, and are connected between one another by the Volkmann's canals.

♣ Animal tissues: Bone tissue, Compact bone .

Harvesian system: see osteon.

Heart: it is the animal organ that pumps the blood through the vascular system, helped by the body movement. The heart is made up of cardiac muscle cells, which are only found in this organ. In mammals, the heart consists of 4 cavities: two ventricles that propel the blood, and two auricles that gather the blood from lungs and the rest of the body, respectively. Auricles are separated by inter-auricles septa and ventricles by inter-ventricular septa. Blood reflux of blood is prevented by valves situated in the exit openings to the heart cavities.

♣ Animal organs: Cardiovascular .

Hematid: see erythrocyte.

Hematopoiesis: it is the differentiation process that gives rise to all blood cell types. Hematopoiesis starts in stem cells found in the bone marrow of large bones.

♣ Animal tissues: Bone, Blood .

Hemicellulose: it is a family of polysaccharides of about 200 to 500 monosaccharides found in the cell wall of plant cells. The type of hemicellulose in the different cell walls and cell types is variable. Xyloglucan is the most abundant hemicellulose type. The hemicellulose molecule is similar to cellulose so that hydrogen bonds can be established between each other, which helps to make stronger cellulose fibers.

♣ Plant tissues: Introduction, Support.

Hemoglobin: it is protein responsible for transporting and exchange oxygen and carbon dioxide in the animal tissues. Hemoglobin is found in the erythrocytes.

♣ Animal tissues: Blood.
♣ Cell types: Erythrocytes.

Hesperidium: it is a type of fruit with exocarp made up of collenchyma containing glands, and an endocarp with juicy sacs. For example, oranges and lemons.

♣ Plant organs: Fruit .

Hilum: (in seed plants) it is a scar-like structure found in the seed coat. The hilum corresponds to the point where the ovary is connected with the funiculus. The funiculus it is a row of cells attaching the ovary to the ovule.

♣ Plant organs: Seed.

Histology: it is the part of science dealing with the tissues of organisms, mainly animals and plants.

Holocrine secretion: it is a type of secretion where the glandular cell breaks and all of its content is released.

♣ Animal tissues: Glandular epithelium .

Hormone: it is a type of molecule, released by some cells, that influences the physiology of cells located in other parts of the animal body or plant body. In animals, hormones are released by endocrine glands and cells, and are carried by the blood system or travel through tissues. In plants, hormones are named phytohormones and are moved by the vascular tissue or diffuse through parenchyma tissue.

♣ Animal organs: Hypothalamus.

Hyaline cartilage: it is the most frequent type of cartilage in vertebrates. Hyaline cartilage forms the fetal skeleton. In adults, it is found in tracheal rings, bronchii, nose, larynx, articular surfaces and joining areas between ribs and sternon. It shows an extracellular matrix enriched in type II collagen, but other collagen types are also present, as well as proteoglycans.

Hyaluronic acid: (or hyaluronan) it is a non-sulfated glycosominoglycan with a pair of monosacharydes, glucuronic acid and N-acethyl glucosamine, repeated hundreds or thousands of times. Therefore, it is an extremely long polymer. Hyaluronic acid is one of the major components of the extracellular matrix of most animal tissues. It is highly hydratable because there are many electric charges in the molecule, and the structure and chemical composition prevent the molecule to form small folds. Therefore, it occupies large volumes. Hyaluronic acid provides hydration, mechanical withstanding, and loose space that altogether favor the movement of molecules and cells through tissues. It is an atypical glycosominoglycan because it does not form chemical bonds with other molecules, but it associated with other tissular components through electrical forces.

♣ Animal organs: Senses.

Hyalocyte: it is a cell type found in the vitreous body of the eye. tipo celular que se encuentran en el cuerpo vítreo del ojo.

♣ Animal organs: Senses.

Hydathode: (in plants) it is a glandular structure that releases water into the surface of the aerial parts ofplants, mostly in leaves. The water appears as droplets. This mechanism is known as guttation. Hydathodes are found in plants with very little transpiration because they are found in adverse environments.

♣ Plant tissues: Glandular tissues.

Hydrophyte: (in plants) it is a type of plant adapted to aquatic environments. They are also known as aquatic plants. These plants are characterized by having less support tissues and develop aeriferous parenchyma for gas diffusion.

♣ Plant tissues: Aeriferous parenchyma .

Hypanthium: (in plants) it is a type of juicy parenchyma surrounding the pericarp of pome fleshy fruits. The hypanthium differentiates from the flower receptacle, not from the ovule. For example, it is present in the apple.

♣ Plant organs: Fruit .

Hypocotyl: (in plants) it is the part of the plant embryo below the insertion point of cotyledons. Hypocotyl encompasses the root and the portion of the stem below the cotyledon insertion points.

♣ Plant organs: Seed.

Hypodermis: (in plants) it is a type of tissue that can be found just below the epidermis. Hypodermis can be observed in different organs such as the root of xerophytes and pine leaves.

♣ Plant organs: Primary root, Pine leaf .

Hypodermis: (in animals) it is one of the tissue layers that form the integument of animals. Hypodermis is found just below the dermis and is mainly made up of adipose tissue, as well as connective tissue. The thickness of the hypodermis is different depending on the body region, but also may change with age and male/female. Hypodermis is also named adipose panicle or subcutaneous tissue.

♣ Animal organs: Integument, Thin skin , Thick skin .

Hypophysis: (or pituitary gland) it is an encorine gland found at the base of the encephalon of vertebrates. The hypophysis releases several types of hormones, many of them influence the activity of other endocrine glands. The hypophysis is a main place for communication between the nervous system and the endocrine system.

Hyponychium: it is the epithelium found below the free end of nails.

♣ Animal organs: Nail .
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