Corpus: Gland: Unterschied zwischen den Versionen

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Definition

Glands are organs, or in some cases individual cells (e.g., goblet cells), capable of synthesizing and releasing specific substances.

Depending on the site of release, glands are classified into:

• Exocrine glands, which secrete their substances onto internal or external surfaces
• Endocrine glands, which release their secretions into the bloodstream

Embryology

The development of glands begins with epithelial cells. In the case of exocrine glands, these cells grow into the underlying tissue in a cone-like fashion and eventually form tubular structures. As the gland develops, these tubes branch repeatedly, resulting in large glands with complex, tree-like structures.

Histology

Except for unicellular glands, all glands consist of specialized epithelial cells known as glandular epithelium, located in the functional tissue (parenchyma). Glands are sometimes surrounded by connective tissue capsules.

• Endocrine glands typically consist of clusters of cells surrounded by dense networks of blood vessels
• Exocrine glands, in contrast, follow a shared structural pattern. The actual production of secretions occurs in the glandular bodies (glandular corpus), which are found at the end of the gland and enclose the secretory ducts in a hemispherical configuration.

The excretory duct system begins at the glandular body and ends on the surface. In some glands, additional duct sections can be observed:

• Intercalated ducts with cuboidal (isoprismatic) epithelial cells
• Striated ducts with columnar epithelium, so named because of the high concentration of mitochondria in the duct cells

These ducts can modify the secretion by absorbing substances from or adding substances to the primary fluid.

A distinct histological feature is found in seromucous glands. The glandular bodies in these glands are composed of low-columnar epithelium, which surrounds the excretory ducts and produces a viscous secretion. On top of this structure, columnar cells form so-called Ebner crescents, which are responsible for the production of the thin, serous component of the secretion.

Classification

Glands can be categorized based on several morphological and functional criteria. Key classifications include:

1. By number of cells

• Multicellular glands (e.g., parotid gland)
• Unicellular glands (e.g., goblet cells)

2. By secretion target

• Endocrine glands: Secrete into the vascular system (e.g., hormones)
• Exocrine glands: Secrete onto internal or external surfaces
• Paracrine glands: Secrete into the surrounding tissue

3. By secretion mode

• Eccrine glands: Secrete without a noticeable loss of cytoplasm
• Merocrine glands: Secrete with minimal cytoplasmic loss
• Apocrine glands: Secrete by shedding parts of the cell, including the apical membrane
• Holocrine glands: Secrete by disintegrating the entire cell

4. By structure

• Acinar glands: Round, berry-shaped structures with columnar epithelial cells and narrow ducts
• Alveolar glands: Spherical structures with low epithelial cells and wide-lumened ducts. These can be further subdivided into:
  • Simple alveolar glands
  • Branched alveolar glands
• Tubular glands: Tubular structures with sometimes highly convoluted ducts. These can be further subdivided into:
  • Simple tubular glands
  • Branched tubular glands

Mixed forms, such as tubuloalveolar and tubuloacinar glands, also exist.

5. By secretion type

• Mucous glands: Secrete thick, viscous mucus
• Serous glands: Secrete watery, thin fluid
• Seromucous glands: Secrete a mixture of watery and viscous substances
• Mucosal glands: Predominantly secrete mucus but may have mixed secretions

Important glands in the human body

Glands are present throughout the body. Key examples include:

Exocrine glands

• Skin
  • Sweat glands
  • Sebaceous glands
  • Ceruminous glands
• Digestive tract
  • Salivary glands
  • Weber glands
  • Gastric glands
  • Brunner glands
  • Exocrine pancreas
• Respiratory tract
  • Tracheal glands
  • Bronchial glands
  • Bowman glands
• Eye
  • Lacrimal glands
  • Krause glands
  • Wolfring glands
  • Meibomian glands
  • Glands of Moll
• Genital organs
  • Prostate
  • Seminal vesicles
  • Bartholin glands
  • Bulbourethral glands
  • Paraurethral glands
  • Uterine glands
• Mamma

Endocrine glands

• Thyroid gland
• Parathyroid glands
• Pituitary gland
• Pineal gland
• Leydig cells in the testes
• Pancreatic islet cells
• Enterochromaffin cells