HISTOLOGY OF THYROID

Histology of the Thyroid Gland

Introduction

The thyroid gland is a vital endocrine organ that plays significant roles in regulating the metabolism of the body.

Gross Features

  • Location: The thyroid gland is situated antero-inferior to the larynx.

  • Structure: It consists of two lobes united by an isthmus and is highly vascularized.

  • Development: The thyroid gland develops from the foregut endoderm during early embryonic life.

  • Function: Its primary functions include the synthesis, storage, and secretion of thyroid hormones thyroxine (T4) and tri-iodothyronine (T3), which are crucial for regulating the basal metabolic rate in various body cells.

  • Parafollicular Cells: Another important cell type in the thyroid is the parafollicular cell, which produces the polypeptide hormone calcitonin, which has a mild regulatory effect on calcium metabolism in the body.

Histology

  • Thyroid Parenchyma: The thyroid gland's parenchyma is characterized by millions of epithelial thyroid follicles of variable diameters, each surrounded by a simple epithelium and containing a central lumen filled with acidophilic colloid. Large quantities of secretory products are stored within the colloid; notably, this stored hormone can maintain the body's metabolic rate for approximately three months without additional synthesis.

  • Composition of Colloid: Thyroid colloid contains glycoprotein thyroglobulin, which serves as a precursor for the active thyroid hormones.

  • Capsule and Lobules: A fibrous capsule covers the thyroid gland, extending into the parenchyma to delineate lobules while conveying blood vessels, nerves, and lymphatics. The follicles themselves are separated by well-vascularized sparse reticular connective tissue.

  • Follicular Cells: The follicular cells, also known as thyrocytes, display variability in shape ranging from squamous to low columnar, with their size and characteristics being influenced by activity levels controlled by thyroid stimulating hormone (TSH). Glands in active states exhibit more low columnar epithelium, while hypoactive glands tend to have more squamous epithelium. Thyrocytes demonstrate apical junctional complexes, rest upon a basal lamina, and possess organelles indicating active protein synthesis, secretion, phagocytosis, and digestion.

Cell Composition and Functionality

  • Nucleus and Organelles of Thyrocytes: The nucleus of thyrocytes is typically round and centralized. Basally, these cells are rich in rough endoplasmic reticulum (RER), while their apical portion contains a Golgi complex, secretory granules, numerous phagosomes, lysosomes, and microvilli.

  • Parafollicular Cells (C Cells): Found either inside the basal lamina of the follicular epithelium or as isolated clusters between follicles, parafollicular cells differentiate from the neural crest and are generally larger than follicular cells. They possess a reduced amount of RER and Golgi complexes but contain numerous small granules filled with calcitonin, which stains less intensely. The secretion of calcitonin is stimulated by elevated calcium ion (Ca²⁺) levels in the blood and works to inhibit osteoclast activity.

Production of Thyroid Hormones and Its Control

  1. Production of Thyroglobulin: This precursor for thyroid hormone is synthesized in the RER and modified in the Golgi apparatus of thyrocytes, containing tyrosyl residues critical for hormone synthesis. It is released from the apical vesicles of thyrocytes into the follicular lumen.

  2. Uptake of Iodine: The Na/I symporter (NIS) at the basolateral cell membrane of thyrocytes allows for a concentration of dietary iodine up to 30-fold compared to plasma, vital for hormone production. An apical iodide/chloride transporter, known as pendrin, facilitates the transport of iodide from thyrocytes into the colloid.

  3. Iodination of Tyrosyl Residues: Within the colloid, iodination of tyrosyl residues in thyroglobulin occurs through a membrane-bound thyroid peroxidase located on the microvilli surface of thyrocytes, oxidizing iodide to iodine.

  4. Formation of T3 and T4: The coupling reaction of two iodinated tyrosines within thyroglobulin leads to the formation of T3 and T4.

  5. Endocytosis of Iodinated Thyroglobulin: The thyrocytes endocytose iodinated thyroglobulin, yielding endocytic vesicles that merge with lysosomes, where lysosomal proteases degrade it and liberate active thyroid hormones T3 and T4.

  6. Secretion of Thyroid Hormones: T3 and T4 are then secreted at the basolateral domains of the thyrocytes into the bloodstream.

Transport and Regulation of Thyroid Hormones

  • Carriage in Blood: Thyroid hormones are transported in circulation bound to proteins such as thyroxine-binding globulin or albumin.

  • Proportions and Activity: T4 constitutes approximately 90% of the circulating thyroid hormones and has a half-life of about one week, while T3 has a half-life of approximately 1.5 days. Despite T4 being more abundant, T3 is more biologically active.

  • Role of TSH: Thyroid stimulating hormone (TSH) serves as the main regulator influencing the anatomical and functional state of the thyroid follicles. TSH receptors are found abundantly on the basal membrane of thyrocytes.

Conditions Associated with Thyroid Gland

  • Common conditions include:

    • Goitre: An abnormal enlargement of the thyroid gland.

    • Hypothyroidism: A condition characterized by insufficient hormone production.

    • Hyperthyroidism: A state of excessive hormone production or activity.

Histology of Adrenal Gland

Introduction

This section discusses the adrenal glands, which also play critical roles in hormonal regulation and metabolic control.

Gross Features

  • Location: The adrenal (or suprarenal) glands are paired structures located near the superior poles of the kidneys, embedded in para-renal adipose tissue and fascia.

  • Size and Variability: The size and weight of the adrenal glands may change with age and physiological conditions. Typically, each gland measures about 4-6 cm in length, 1-2 cm in width, and 1-2 mm in thickness, with a combined weight of about 8 grams in adults.

  • Coating: Each gland is surrounded by a dense connective tissue capsule, which sends trabeculae into the gland's parenchyma. The stroma mainly consists of reticular fibers that support secretory cells and the microvasculature.

  • Regions: Each adrenal gland comprises two concentric regions: a yellowish adrenal cortex and a reddish-brown adrenal medulla. The cortex is approximately ten times larger than the medulla.

Structure of the Adrenal Gland

  • Histological Sections: A coronal section view reveals the distinct regions of the cortex and medulla delineated as follows:

    • Capsule

    • Cortex

    • Medulla

    • Blood Supply: The adrenal gland lacks a distinct hilum. The blood supply penetrates the capsule, creating a subcapsular arterial plexus. This plexus gives rise to arterioles that extend into the adrenal cortex and medulla, forming a network of fenestrated capillaries and sinusoids. The cortical capillaries supply the cells of the cortex before draining into the microvasculature of the medulla, where the medulla then receives arterial blood from medullary arterioles and venous blood from the cortical capillaries. Venous drainage is accomplished through the suprarenal veins.

Adrenal Cortex Overview

  • Cell Characteristics: Cells within the adrenal cortex exhibit features typical of steroid-secreting cells, with acidophilic cytoplasm enriched in lipid droplets and centrally located nuclei.

  • Ultrastructure: The cytoplasm of these cells demonstrates a wealth of smooth endoplasmic reticulum (SER), and the mitochondria often appear as spheres with tubular cristae. The cortex is comprised of three concentric zones:

    • Zona Glomerulosa: Comprises about 15% of the cortex, consists of rounded or arched cords of columnar or pyramidal cells with abundant capillaries, and secretes mineralocorticoids that regulate Na⁺, K⁺, and water uptake by renal tubule cells. The principal product is aldosterone, which is stimulated primarily by angiotensin II and increased plasma K⁺ levels.

Adrenal Cortex Layers

  • Zona Fasciculata: Occupies 65%-80% of the cortex, consists of long cords of large polyhedral cells, separated by sinusoidal capillaries, and primarily secretes glucocorticoids such as cortisol, primarily regulated by adrenocorticotropic hormone (ACTH). This zone may also produce small amounts of weak androgens.

  • Zona Reticularis: Forms about 10% of the cortex, composed of smaller cells arranged in networks. Cells in this zone are usually more heavily stained than those in other zones due to fewer lipid droplets and increased lipofuscin pigment. This zone also produces cortisol but mainly secretes weak androgens, including dehydroepiandrosterone (DHEA), which can be converted to testosterone. The secretion from this zone is under ACTH control.

Adrenal Medulla

  • Composition: The adrenal medulla consists of large, pale-staining polyhedral cells arranged in cords or clusters, supported by a reticular fiber network.

  • Blood Supply: A rich supply of sinusoidal capillaries fills the spaces between adjacent cellular cords, essential for functionality.

  • Chromaffin Cells: The medullary parenchymal cells, referred to as chromaffin cells, contain many electron-dense granules that store and secrete catecholamines, including epinephrine and norepinephrine. Approximately 80% of catecholamines released by the adrenal medulla is epinephrine.

  • Innervation: Chromaffin cells are innervated by preganglionic sympathetic neurons that modulate their activity.

Conclusion

Understanding the histology of the thyroid and adrenal glands provides a foundation for comprehending their physiological functions and the implications of various endocrine disorders.

Acknowledgment

Thank you for engaging with this comprehensive study of glandular histology, which serves as a crucial aspect of understanding endocrine system functionality and pathology.