HISTOLOGÍA - Glándula Tiroides (Células Foliculares, Parafoliculares y Función de Hormonas Tiroideas

Histology of the Thyroid Gland

Introduction to the Class

• The instructor introduces the class and mentions that resources such as summaries, slides, and brochures on anatomy, histology, neuroanatomy, and physiology are available on their Patreon to aid students in understanding medical subjects more easily.

Overview of the Thyroid Gland

• The thyroid gland is described as a bilobed endocrine gland located at the anterior part of the neck, covering parts of the trachea and larynx. Its name derives from its shield-like shape.

• Embryologically, the thyroid develops during the fourth week of gestation from endodermal tissue in the primitive pharynx. It has a unique structure with two lateral lobes connected by an isthmus and sometimes a pyramidal lobe.

Cellular Composition

• The thyroid contains two main cell types: follicular cells and parafollicular cells (C cells). Parafollicular cells originate from neural crest-derived epithelial cells migrating into the thyroid.

• Follicular cells differentiate around week nine of gestation. This dual embryological origin highlights both endodermic and ectodermic contributions to thyroid development.

Structural Anatomy

• The thyroid consists of parenchyma (functional tissue) and stroma (supportive tissue). The stroma includes a dense irregular connective tissue capsule with reticular fibers forming about 40 lobules within the gland.

• A deeper layer of connective tissue known as pretracheal fascia also envelops the thyroid but should not be confused with its stroma.

Functional Units: Thyroid Follicles

• Each functional unit of the thyroid is called a thyroid follicle, measuring approximately 50 to 500 micrometers in diameter. A healthy adult's thyroid can contain around 20 million follicles.

• Follicles are cystic structures lined by epithelial cells containing colloid—a gelatinous substance where T3 and T4 hormones are synthesized.

Hormonal Functionality

• The follicular epithelium is simple cuboidal in nature; it plays a crucial role in synthesizing hormones that regulate basal metabolic rate while parafollicular cells contribute to calcitonin production for calcium regulation.

• Unlike other endocrine glands that store secretions intracellularly, the thyroid stores its hormones extracellularly within follicle lumens.

Cell Types Within Thyroid Tissue

• Follicular cells make up about 99.9% of all thyroid cells; they are responsible for producing T3 and T4 hormones while being directly adjacent to follicle lumens.

• Parafollicular or C-cells represent only about 0.1% of total cell population but play essential roles in hormone secretion related to calcium homeostasis.

Understanding Thyroid Follicular Cells and Hormone Production

Structure and Function of Follicular Cells

• The cells surrounding the thyroid follicle are crucial for understanding their functional state; they appear cuboidal to columnar based on activity levels.

• These basophilic cells have spherical nuclei and a robust rough endoplasmic reticulum, essential for synthesizing thyroglobulin, a glycoprotein derived from tyrosine.

• Follicular cells possess TSH receptors on their basal plasma membrane, allowing them to respond to thyroid-stimulating hormone (TSH) levels secreted by the pituitary gland.

• TSH regulates hormone production in response to body needs, signaling when to increase or decrease thyroid hormone synthesis.

Parafollicular Cells and Calcitonin Production

• Parafollicular cells (C-cells or clear cells), larger than follicular cells, are responsible for producing calcitonin but do not contact the follicle's lumen.

• These cells feature a prominent Golgi apparatus, indicating their role in processing and packaging hormones.

Hormones Produced by Follicular Cells

• Follicular cells secrete three main hormones: T4 (thyroxine), T3 (triiodothyronine), and RT3 (reverse triiodothyronine).

• The terms "iodothyronines" refer to these hormones being composed of two tyrosine molecules attached to three or four iodide ions.

Synthesis and Release of Thyroid Hormones

• Approximately 93% of thyroid secretion is T4, with 7% as T3 and less than 1% as RT3; iodine from diet is critical for this synthesis.

• Iodine is typically ingested through iodized salt, absorbed in the intestine, while thyroglobulin is synthesized from tyrosine within follicular cells.

Transport and Action of Thyroid Hormones

• In circulation, T3 and T4 bind to plasma proteins like thyroxine-binding globulin (TBG), albumin, and transthyretin before reaching tissues.

• Enzymes called deiodinases convert most circulating T4 into the more potent T3 form within tissues; thus, while T4 is abundant in secretion, T3 has greater biological activity.

Physiological Role of Thyroid Hormones

• Thyroid hormones regulate basal metabolic rate by controlling chemical reactions' speed within the body; they significantly influence energy expenditure.

Hormonal Functions and Effects on Growth

Role of Thyroid Hormones in Development

• The thyroid hormone plays a crucial role in the osteoarticular system, particularly in children, contributing to normal bone growth and development during fetal life and early childhood.

• It promotes the growth and development of the central nervous system; congenital hypothyroidism can lead to conditions like cretinism, characterized by stunted growth and poor neurological development.

Cardiovascular and Respiratory Effects

• Increases heart rate and contraction strength, enhancing cardiac output.

• Stimulates respiratory function by increasing both the frequency and depth of breaths.

Metabolic Impact of Thyroid Hormones

• Enhances hepatic gluconeogenesis while simultaneously stimulating lipolysis (fat breakdown) and proteolysis (protein breakdown).

Calcitonin: Function and Mechanisms

• Calcitonin, secreted by parafollicular cells, is also known as tirocalcitonina; it should not be confused with procalcitonin, which serves as an inflammatory marker.

• The primary function of calcitonin is to lower blood calcium levels through two mechanisms:

• It decreases bone resorption by inhibiting osteoclast activity.