Glandular epithelium- definition, structure, functions, examples

Glandular epithelium definition

腺上皮是一组组织，除了覆盖和衬里上皮，该组织专门用于生产和分泌各种大分子。由于其主要功能，腺上皮也称为分泌上皮。该上皮位于覆盖物和衬里上皮下方，要么由具有特定功能的细胞组成，要么由称为腺体的专门器官组成。腺体由上皮组成，该上皮在没有管道的情况下将物质分泌到管道（管），表面或最终成血液中。一些由专门细胞组成的单细胞腺可能被散布在简单的立方体，简单的柱状和假性上皮之间。

Structure of the glandular epithelium

Glands develop from covering epithelia in the fetus by cell proliferation and growth into the underlying connective tissue, followed by further differentiation.
The shape of the cells of the glandular epithelium can range from cuboidal to columnar as the squamous cells quite rare in the secretory epithelium.
The glandular epithelium of the endocrine glands is rich in vascularization as well as innervation.
The secretory cells in the glandular epithelium may synthesize, store, and release proteins (e.g., in the pancreas), lipids (e.g., adrenal, sebaceous glands), or complexes ofcarbohydrates和proteins.
The cells of some glands (e.g., sweat glands) have little synthetic activity and secrete mostly water and electrolytes (ions) transferred from the blood.

On the basis of the structure of the glands and the presence or absence of ducts, glands are classified into two types:

外分泌腺

外分泌腺secrete their products into ducts that release the secretions onto the surface of organs such as the skin surface or the lumen of a hollow organ.
外分泌腺remain connected to the covering epithelium via tubular ducts which are lined with lining epithelium and carry the secreted products into the site of action.
The effects of exocrine gland secretions are limited, and some of them would be harmful if they entered the bloodstream.
The exocrine glands are either unicellular or multicellular. Unicellular exocrine glands are made up of single-celled glands and mostly secrete mucus directly to the apical surface of the lining epithelium.
The multicellular exocrine glands are composed of several cells and form a specific macroscopic organ with branched or unbranched ducts.
每个外分泌腺由一个分泌单元和一个或多个管道组成。
The secretory unit is supported by the stroma of connective tissues with each unit connected to the duct system of the gland.
从结构上讲，多细胞外分泌腺进一步分为分泌单元的形状以及管道是分支还是无分支。
If the duct of the gland doesn’t have branches, it is called asimple gland.
If the duct is branched, the gland is acompound gland.
The glands with round secretory parts are calledacinar或者alveolar glands.
带有管状分泌部分的腺体是tubular glands.
Some glands have both round and tubular secretory parts and are calledtubuloacinar glands.
The exocrine glands are further divided into three types on the basis of the mechanism of release of secretion.
The glands that release their secretion from the cells in secretory vesicles via exocytosis are termed绿色腺体.
在分泌细胞的顶端表面积聚其分泌产物的腺体，后来将顶部部分从细胞的其余部分释放出来，以释放分泌物apocrine glands.
Finally, the glands in which the cells accumulate the secretory product in their cytosol, and as the cell matures, it ruptures to release the product, are termed theholocrine glands.
外分泌腺的分泌单元的细胞配有发达的内质网，高尔基体在不同成熟阶段的分泌颗粒填充的高尔基体。
In addition to secretory cells, epithelia of many exocrine glands (e.g., sweat, lachrymal, salivary, and mammary glands) contain contractile myoepithelial cells at the basal ends of the secretory cells.
The myoepithelial cells are abundant in actin filaments and myosins, which results in the formation of hemidesmosomes connecting the basement membrane as well as other epithelial cells.
在一组特定的外分泌腺，称为全分泌腺，随着分泌细胞的成熟，它破裂并成为分泌产物。
然后将隔离的电池替换为新单元。

内分泌腺

内分泌腺are the group of glands that do not have a duct system and thus, release their secretions directly into the bloodstream.
The secretions of endocrine glands, called hormones, enter the interstitial fluid and then diffuse into the bloodstream without flowing through a duct.
Endocrine secretions have far-reaching effects because they are distributed throughout the body by the bloodstream.
内分泌腺lose the connection to their original epithelium and therefore lack ducts.
Thin-walled blood capillaries adjacent to endocrine cells absorb their secreted hormone products for transport in the blood to target cells throughout the body.
The glandular epithelium of the endocrine glands lack myoepithelial cells and are specialized for either protein or steroid hormone synthesis.
The cells in the glandular epithelium are rich in the endoplasmic reticulum with a rather thin cell membrane.
The thin wall of the cell membrane allows the release of the hormones secreted by the secretory cells by diffusion.
The diffused hormones are then taken by the binding proteins present outside of the secretory cells.
负责信号传递激素从腺体到目标部位的受体的存在要么是靠近分泌激素的细胞或分泌细胞本身。
从组织学上讲，内分泌腺有三种类型：
- Trabecular type: In this type, the cells are arranged in cords like in the liver and the adrenal glands.
- Follicular type: In this type, the cells form spherical structures like in the thyroid glands.
- Disseminated type：在这种类型中，细胞分组或分别放置在睾丸中的leydig细胞等其他器官中。

腺上皮的功能

The most essential function of the glandular epithelium is secretion.

Secretion

外分泌腺secrete watery mucus that helps to reduce body temperature and maintain homeostasis.
Similarly, the glands like the sebaceous glands and the ceruminous glands secrete oily substances that help lubricate the surface of the covering epithelium providing protection against water loss and microbial invasion.
The salivary gland in the mouth and digestive glands (pancreas) in the small intestine produce digestive enzymes that help in the digestion of food and aids the absorption of nutrients.
The hormones secreted by the endocrine glands regulate many metabolic and physiological activities to maintain homeostasis and the proper functioning of the animals.
The glands like the pituitary and the thyroid glands are responsible for the overall growth and development of the body.
同样，肾上腺调节新陈代谢，免疫系统，血压以及对压力的反应以及其他基本功能。
生殖系统的卵巢和睾丸产生有性繁殖所需的配子。
The thymus gland in the thoracic cavity is responsible for the production of T-cells required for the removal of foreign antigens from the body.

腺上皮的例子

Examples of glandular epithelium include the epithelium of the endocrine glands like the pituitary gland at the base of the brain, the pineal gland in the brain, thyroid and parathyroid glands near the larynx (voice box), adrenal glands superior to kidneys, pancreas near the stomach, ovaries in the pelvic cavity, testes in the scrotum, thymus in the thoracic cavity.
外分泌腺包括皮肤的汗水，皮脂腺和陶瓷腺以及消化腺，例如唾液腺和胰腺。

References and Sources

Mescher AL (2016). Basic Histology. Fourteenth Edition. McGraw-Hill Education.
Tortora GJ and Derrickson B (2017). Principles of Physiology and Anatomy. Fifteenth Edition. John Wiley & Sons, Inc.
Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
Stefan TOTH. Glandular epithelium. Department of Histology and Embryology, P.J. Šafárik University, Medical Faculty, Košice.
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