Adrenal Gland (Hormones) - Structure, Functions, Disorders

• Introduction
• Structure of Adrenal Gland
• Hormones of Adrenal Gland
• Functions of Adrenal Gland
• Diseases and Disorders of Adrenal Gland

Introduction

The adrenal glands, also known as suprarenal glands, are a pair of structures situated atop the kidneys, responsible for the production of various steroid hormones.

• Comprising the adrenal cortex and adrenal medulla, these glands exhibit distinct structures and functions, making them a fascinating subject in the field of biology.
• Being an endocrine gland, the adrenal glands release hormones directly into the bloodstream, facilitating their transportation to target cells and organs.
• The adrenal cortex and adrenal medulla, characterized by differences in embryological origin, functionality, and structure, are so dissimilar that some propose considering them as separate glands.
• Functioning as a crucial endocrine organ, the adrenal gland secretes vital hormones such as mineralocorticoids, glucocorticoids, and steroid hormones like androgens.
• These hormones play pivotal roles in metabolic processes, reproduction, and overall body homeostasis.
• The embryonic derivation of the adrenal cortex traces back to the embryonic mesoderm, while the adrenal medulla originates from the nervous tissue of the embryo.
• While the adrenal cortex is indispensable for sustaining life, the adrenal medulla is not essential for survival.

Structure of Adrenal Gland

The adrenal glands, also known as suprarenal glands, are paired structures situated above each kidney, with a pyramid-like shape that may vary among individuals based on age and physiological conditions. These glands are enveloped by a protective fibrous capsule and a cushion of fat, shielding them from internal shocks.

Adrenal Cortex:

The adrenal cortex, a vital component of the adrenal gland, is predominantly composed of glandular epithelium and is divided into three distinct zones: zona glomerulosa, zona fasciculata, and zona reticularis.

• Zona Glomerulosa: Positioned as the outermost layer, it consists of a thin arrangement of columnar cells forming an arched pattern. The zona glomerulosa produces the hormone aldosterone.
• Zona Fasciculata: Located in the middle and representing the thickest zone, it comprises columns of secretory cells surrounded by numerous capillaries. This zone is responsible for the production of glucocorticoids.
• Zona Reticularis: Serving as the innermost layer, it is composed of polyhedral cells arranged in linear or round nests or clumps. While predominantly producing glucocorticoids, some species may also generate sex steroids known as androgens.

Adrenal Medulla:

The adrenal medulla is a smaller portion of the adrenal gland, completely surrounded by the adrenal cortex. Unlike the adrenal cortex, it originates from neural tissues during embryonic development and is considered a part of the sympathetic nervous system.

Hormones of Adrenal Gland:

The adrenal medulla contains medullar chromaffin cells situated within blood-filled capillaries and sinusoids.

These medullary chromaffin cells are modified postganglionic sympathetic neurons that synthesize catecholamines such as epinephrine (adrenaline) and norepinephrine.

Adrenal Cortex Hormones:

Mineralocorticoids:

• Produced by the zona glomerulosa cells of the adrenal cortex, mineralocorticoids, such as aldosterone, regulate concentrations of mineral salts (Na+ and K+) in extracellular fluids.
• Essential for maintaining blood volume and pressure, aldosterone stimulates the distal tubules of nephrons, promoting reabsorption of Na+ and water while increasing K+ elimination in urine.
• Regulatory effects last approximately 20 minutes, ensuring control of plasma electrolyte balance.

Glucocorticoids:

• Crucial for life, glucocorticoids, including cortisol, cortisone, and corticosterone, impact energy metabolism.
• Primary function involves influencing the activity of vasoconstrictors to maintain blood glucose and pressure.
• Cortisol, predominant in humans, modifies gene activity on target cells, with secretion following a specific pattern.
• Regulated by a negative feedback mechanism involving ACTH from the pituitary gland, stimulated by hypothalamic corticotrophin-releasing hormone.
• Prime metabolic effect is inducing gluconeogenesis to limit glucose use, mobilizing fatty acids, and proteins for energy.

Gonadocorticoids/Androgens/Sex Hormones:

• Androgens, weak sex hormones like androstenedione and dehydroepiandrosterone, produced by the adrenal cortex, are converted into potent sex hormones.
• Secreted in insignificant amounts compared to gonadal production after puberty.
• Associated with axillary and pubic hair development in both genders.
• In females, responsible for sex drive and contribute to most postmenopausal estrogen production.

Hormones from the Adrenal Medulla: 

Epinephrine and Norepinephrine:

• The adrenal medulla responds to stress by secreting two hormones: epinephrine and norepinephrine.
• These hormones are produced in a ratio of 4:1, with epinephrine being the dominant component. Epinephrine exhibits greater potency in stimulating metabolic activities, leading to bronchial dilation and increased blood flow to skeletal muscles.
• Conversely, norepinephrine primarily influences peripheral vasoconstriction and contributes to the regulation of blood pressure.
• The adrenal medulla's hormone release induces a brief, short-lived response to stressors.

Functions of the Adrenal Gland:

The roles of the adrenal gland are dictated by the specific hormones it produces. Here are some key functions:

• Mineralocorticoids: Regulate sodium and potassium ions in the blood and extracellular fluids.
• Cortisol: Produced by the adrenal cortex, cortisol regulates metabolism, the immune system, and the cardiovascular system.
• Stress Adaptation: Hormones from the adrenal gland aid the body in adapting to heightened stress conditions.
• Androgens: Androgens produced by the adrenal gland are converted into sex hormones crucial for reproductive system development.
• Adrenal Medulla Hormones (Catecholamines - Epinephrine and Norepinephrine): Contribute to the "fight or flight" response, playing a pivotal role in responding to stress.

Adrenal Gland Diseases and Disorders:

Cushing’s Syndrome:

• Caused by hypersecretion of cortisol, the primary glucocorticoid hormone of the adrenal cortex.
• Hypersecretion may result from adrenal tumors or excessive ACTH production by the pituitary.
• Characterized by adiposity in the face, neck, and abdomen, along with extensive tissue protein breakdown.
• Osteoporosis, increased gluconeogenesis, hyperglycemia, and glycosuria may occur.

Adrenal Hypoplasia:

• Arises from the underdevelopment of the adrenal cortex due to various clinical conditions.
• Primary hypoplasia leads to the underdevelopment of the adrenal gland and hyposecretion of adrenal hormones.
• Secondary hypoplasia, less common and severe, affects only the gland's secretions.

Addison’s Disease:

• Results from the destruction of the adrenal gland, causing hyposecretion of glucocorticoids and mineralocorticoids.
• Autoantibodies formed by the immune system against cortical cells contribute to the gland's destruction.
• Symptoms include muscle weakness, fatigue, hypoglycemia, and increased skin pigmentation.

Adrenocortical Adenomas:

• Cortisol-producing adenomas are caused by elevated levels of cAMP or protein kinase A.
• Typically benign, these adenomas may hypersecrete various hormones of the adrenal gland.
• Adenoma formation is triggered by ACTH binding to receptors, inducing protein kinase A release, a pathway for cortical cell formation.