Histology of the Endocrine System

Overview of the Endocrine System

  • The endocrine system is crucial for communication between different parts of the body, using hormones to transmit information.

Information Transmission

  • Information from one part of the body can be transmitted to remote parts through:

    1. The Nervous System:

    • Components: Dendrites, axons, synapses.

    • Characteristics: Quick response but short-lived.

    1. The Endocrine System:

    • Components: Hormones, receptors in target organs, second messenger systems.

    • Characteristics: Slow to initiate but results in prolonged responses.

Structure of the Endocrine System

  • The endocrine system consists of clusters of cells (glands) that secrete hormones directly into the circulatory system to reach distant target organs.

  • Glands can signal each other in a sequential manner, forming what is known as an axis.

    • Examples of axes include:

      • Hypothalamic-pituitary-gonadal axis.

      • Hypothalamic-pituitary-adrenal axis.

Major Endocrine Glands

  1. Pituitary Gland (Hypophysis)

  2. Thyroid Gland

  3. Parathyroid Glands

  4. Suprarenal (Adrenal) Glands

  5. Pineal Gland

  6. Islets of Langerhans in the pancreas (scattered cell groups embedded within the exocrine pancreas).

Detailed Examination of Major Endocrine Glands

Pituitary Gland (Hypophysis)

  • Divided into two lobes:

    1. Anterior Lobe (Adenohypophysis)

    2. Posterior Lobe (Neurohypophysis)

  • Structure includes:

    • Anterior pituitary

    • Posterior pituitary

    • Pars tuberalis, pars intermedia, pars distalis

  • The anterior pituitary is the source of several important hormones which are regulated by the hypothalamus and includes:

    • Growth Hormone (GH)

    • Prolactin (PRL)

    • Tropic hormones like Thyroid-Stimulating Hormone (TSH), Adrenocorticotropic Hormone (ACTH), Follicle-Stimulating Hormone (FSH), and Luteinizing Hormone (LH).

  • The posterior pituitary consists of:

    • Unmyelinated nerve fibers from the supraoptic and paraventricular nuclei of the hypothalamus.

    • Pituicytes - glial cells assisting in hormone release.

Cell Types in the Anterior Pituitary (Adenohypophysis)

  1. Chromophils: Approximately 50% of cell population.

    • Acidophil cells: Display larger, clearer cell borders and highly acidophilic cytoplasm.

    • Basophil cells: Other half of Chromophils.

  2. Chromophobes: The remaining 50%, small rounded cells with scant cytoplasm.

Cell Types in the Posterior Pituitary (Neurohypophysis)

  • The posterior pituitary primarily releases:

    • Oxytocin (produced by paraventricular nucleus) - regulates childbirth and lactation.

    • Antidiuretic Hormone (ADH) (produced by supraoptic nucleus) - regulates water retention and blood pressure.

  • Herring bodies are accumulations of hormone-bound granules seen via light microscopy.

  • The volume of the posterior pituitary is composed of about 25% Pituicytes.

Thyroid Gland

  • Structure: Comprises two lateral lobes connected by an isthmus, which are further divided into lobules and follicles surrounded by reticular fibers and blood capillaries.

  • Each follicle features:

    • One layer of cuboidal epithelium around colloid containing thyroglobulin, a precursor for hormones Triiodothyronine (T3) and Thyroxine (T4).

    • Principal (follicular) cells, with granular basophilic cytoplasm, may contain thyroglobulin droplets.

    • Parafollicular (C) Cells: Located between follicular cells, responsible for secreting calcitonin, which suppresses bone reabsorption.

Parathyroid Gland

  • Anatomy: Small, oval bodies closely related to the thyroid, covered by a thin fibrous capsule.

  • Hormonal role: Secretes Parathyroid Hormone (PTH), playing a crucial role in regulating calcium and phosphate levels in the blood.

Cell Types in Parathyroid Gland

  • 1. Chief Cells (Principal Cells): The most common type, secretes PTH.

  • 2. Oxyphil Cells: Larger, acidophilic, present in small groups.

  • 3. Adipose Cells: Present as well.

Adrenal Glands

  • Structure: Covered by a dense connective tissue capsule and consists of two main regions:

    1. Cortex (outer)

    2. Medulla (inner with many blood vessels).

Zones of the Adrenal Cortex

  1. Zona Glomerulosa: Produces mineralocorticoids (e.g., Aldosterone) to regulate mineral balance and blood pressure.

  2. Zona Fasciculata: Dominates the middle layer, secreting glucocorticoids (e.g., Cortisol).

  3. Zona Reticularis: Surrounding the medulla, secreting androgens and estrogens.

Adrenal Medulla

  • Comprises chromaffin cells that secrete catecholamines such as epinephrine and norepinephrine (fight or flight hormones).

Pineal Gland (Epiphysis Cerebri)

  • Small, pinecone-shaped gland responsible for secreting melatonin, which regulates sleep-wake cycles.

  • Composed of:

    • Pinealocytes: Irregular shape with long branching processes, characterized by large nuclei.

    • Glial Cells: Form an interwoven network around pinealocytes.

Islets of Langerhans (in Pancreas)

  • Scattered, spheroidal masses of cells, more abundant towards the tail of the pancreas, containing five cell types:

  1. Alpha Cells (A Cells): 20% of population, secrete glucagon.

  2. Beta Cells (B Cells): 70%, secrete insulin.

  3. Delta Cells (D Cells): 5%, secrete somatostatin.

  4. Epsilon Cells: Secrete ghrelin, stimulating feeding behavior.

  5. PP Cells: Secrete pancreatic polypeptide (PP) to promote satiety and reduce food intake.

Summary on Hormonal Control and Feedback

  • Hormonal release is regulated by stimulating hormones from the hypothalamus, which can include:

    • Growth Hormone Releasing Hormone (GH-RH) stimulating GH.

    • Dopamine, which inhibits Prolactin.

    • Thyrotropin-Releasing Hormone (TRH) stimulating TSH.

    • Gonadotropin-Releasing Hormone (GnRH) for FSH and LH.

    • Corticotropin-Releasing Hormone (CRH) stimulating ACTH and LPH.

Conclusion

  • The endocrine system's intricate network and the interplay between glands and hormones play essential roles in physiological regulation, emphasizing the importance of homeostasis. The complexity of this system signifies its fundamental impact on health and disease management.