Parathyroid gland histology

Architectonics and histological structure

The parathyroid gland has a distinct tissue organization, traditionally divided into the stromal component and functional parenchyma, as is typical for parenchymatous organs.

Externally, the organ is covered by a dense connective tissue capsule that performs protective and shape-forming functions. Numerous septa composed of loose fibrous connective tissue (LFCT) extend inward from this outer capsule into the gland. These partitions create the internal supporting framework of the organ. A dense network of blood vessels runs within the loose fibrous connective tissue, ensuring an adequate blood supply necessary for proper endocrine function.

A characteristic and extremely important diagnostic feature of the parathyroid gland stroma is the obligatory presence of adipocytes. Fat cells are located within the connective tissue septa and help to visually identify the organ on a histological slide.

The main working component of the gland is the parenchyma. It is formed by clusters of specialized epithelial cells. Unlike some other glands, the epithelium here does not form continuous sheets, but rather specific structural units. The cells are organized into elongated epithelial cords, gathered into compact groups, or form characteristic rosette-like structures.

Age-related morphology and pseudofollicles

The histological picture of the parathyroid gland does not remain static throughout a person's life. The most noticeable structural remodeling is observed in elderly patients.

During the aging process, specific formations begin to appear in the organ's parenchyma, which are morphologically described as follicle-like structures. A dense content accumulates inside these cavities, which visually resembles the thyroid colloid very closely upon microscopic examination.

However, when studying this topic, it is critically important to understand the physiological difference: unlike true colloid, which serves as a hormone depot, the fluid within the follicle-like structures of the parathyroid gland has absolutely no hormonal activity. This is merely an age-related morphological tissue change that carries no additional endocrine load.

Embryonic development and gland classification

In the general histology course, special attention is paid to the sources of development of the endocrine system. Although all endocrine glands are closely linked functionally and form a single regulatory network of the body, their embryonic origins vary drastically.

Parathyroid glands develop from the endoderm of the pharyngeal pouches. The thyroid gland and thymus form from this same embryonic source.

To understand the general picture of the endocrine system's development, one should remember the classification of glands based on their formation sources:

• Endodermal glands of intestinal origin: this group includes the endocrine part of the pancreas.
• Mesodermal glands: they develop from the coelomic epithelium. These include the adrenal cortex, as well as the endocrine part of the gonads.
• Ectodermal glands of sympathetic origin: the adrenal medulla and paraganglia are formed from this source.
• Neurogenic (ectodermal) glands: their development is inextricably linked with the formation of the diencephalon. This group includes the hypophysis (pituitary gland) and pineal gland.

Neoplasm nomenclature (ICD-10 and WHO)

In clinical practice and pathological anatomy, the histological structure of the organ is inextricably linked with the classification of its tumors. According to the International Classification of Diseases, malignant neoplasms of the parathyroid gland are coded under C75.0.

This category belongs to the broader class "Other endocrine glands and related structures" (C75), which also includes:

• C75.1 — Pituitary gland lesions.
• C75.2 — Craniopharyngeal duct tumors.
• C75.3 — Pineal gland neoplasms.
• C75.4 — Malignant tumors of the carotid body.

Adrenal neoplasms are classified separately (category C74). For example, the code C74.1 denotes malignant neoplasm of the adrenal medulla, and C74.9 denotes a tumor of an unspecified part of the organ.

According to the WHO histological classification of tumors (2017 revision), specific morphological types of neoplasms developing from ectodermal structures of sympathetic origin are distinguished: pheochromocytoma has the code 8700/3, and paraganglioma has 8693/3.

Class and exam questions

What else to study on this topic

• Cellular composition of epithelial cords and compact groups of the parenchyma
• Mechanisms of formation of age-related follicle-like structures
• Histological differences between pheochromocytoma (8700/3) and paraganglioma (8693/3)
• Development of endocrine glands from the coelomic epithelium (mesoderm)
• Morphology of malignant neoplasms of other endocrine structures (ICD-10: C75)

Other topics in the "Additional Topics" section

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