Lecture Notes in Human Anatomy - Hryhorieva O.A., Svitlytskyi A.O. 2020

The Doctrine of Viscera (Splanchnologia)
Anatomy of Internal Organs
Endocrine System

Lecture Plan:

1. Pituitary Gland.

2. Pineal Gland.

3. Thyroid Gland.

4. Parathyroid glands.

5. Thymus.

6. Adrenal Glands.

7. Endocrine part of the Pancreas (pancreatic islets).

8. Endocrine PARTS OF THE Gonads.

Endocrine glands are ductless glands that secrete their products (Hormones) directly into the Blood or Lymph. Every organ in The Human Body is under dual control—by The Nervous system and the endocrine glands. Thus, the Endocrine System, together with the nervous system, regulates all vital Functions in the human body. All endocrine glands are functionally interconnected and constitute a single system, in which the pituitary gland plays a leading role.

The endocrine glands include the pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, adrenal glands, the Endocrine portion of the pancreas (pancreatic islets), and the endocrine parts of the gonads.

The pituitary gland is located in the hypophyseal fossa of the sella turcica of the Sphenoid bone. It measures 8-10 mm in length, 12-15 mm in width, and weighs 0.5-0.65 g. By the onset of Puberty, the weight of the pituitary gland nearly doubles. The Development of the gland continues up to 30-40 years of age, after which its weight decreases and cellular changes occur. The pituitary gland produces 22 hormones. It is divided into two lobes: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The anterior lobe is larger and consists of three parts: the distal, intermediate, and tuberal parts. The pituitary gland is enclosed in a capsule. The parenchyma of the anterior lobe consists of glandular Cells interspersed with sinusoidal capillaries. The posterior lobe is composed of neuroglial cells, nerve fibers extending from the neurosecretory nuclei of the Hypothalamus, and neurosecretory bodies. The pituitary gland is the master endocrine gland because it produces hormones that regulate the development and functions of other endocrine glands.

The pineal gland (epiphysis cerebri) is located in the region of the superior colliculi of the Midbrain (belonging to the epithalamus) and is connected to the thalamus. It is an unpaired, egg-shaped, spherical, or conical organ weighing about 0.2 g. It secretes hormones that suppress pituitary activity until puberty and participates in The regulation of nearly all METABOLISM/4.html">Types of Metabolism. Externally, the pineal gland is covered by a capsule from which trabeculae extend inward, dividing the parenchyma into lobules. The parenchyma consists of glandular cells, known as pinealocytes, and glial cells.

The Thyroid Gland resembles a trident in shape and lies in the neck region at the level of the 5th-7th cervical vertebrae. Anteriorly, it is bordered by the Infrahyoid Muscles, and posteriorly by the Larynx and Trachea. It is the largest unpaired endocrine gland, with a mass of 30-40 g. The gland consists of two lateral lobes and an isthmus. Externally, it is covered by a fibrous capsule that fuses with the larynx and trachea. Trabeculae extend from the capsule, dividing the parenchyma into lobules. The glandular tissue (parenchyma) is composed of thyroid follicles surrounded by a dense network of BLOOD AND LYMPH capillaries, through which 5-6 liters of blood flow every hour. The follicles contain colloid and a protein containing iodine known as thyroglobulin.

The parathyroid glands are located on the posterior surfaces of the thyroid lobes, occasionally embedded within the thyroid tissue itself. They are round or oval in shape, typically four in number—two superior and two inferior. The total mass of the parathyroid glands ranges from 0.13 to 0.36 g. Each gland is enclosed in its own fibrous capsule.

The thymus is located in the thoracic cavity and partially in the neck, with its anterior surface adjacent to the posterior surface of the manubrium and body of the Sternum (up to the level of the 4th costal Cartilage). It belongs to the superior Mediastinum; posteriorly, it is bounded by the aorta, pulmonary trunk, SUPERIOR VENA CAVA, and left brachiocephalic vein. The thymus is a fork-shaped, unpaired organ. Its mass reaches 35-40 g during its period of maximum development (ages 10-15), decreasing to 13-15 g in older age. The thymus consists of two lobes, right and left. Externally, it is covered by a Connective Tissue capsule from which interlobular septa extend, dividing the parenchyma into lobules. The anatomical unit of the thymus is the lobule, featuring a peripheral cortex and a central medulla. The cortex is densely packed with thymic lymphocytes (thymocytes), while the medulla contains thymic corpuscles (Hassall's corpuscles), which are densely packed epithelial cells. The thymus is the site where T-lymphocytes differentiate from stem cells originating in the Bone Marrow.

The adrenal glands are paired Organs located in the retroperitoneal adipose tissue above the superior poles of the corresponding Kidneys, at the level of the 11th-12th thoracic vertebrae. The right Adrenal gland is triangular, while the left is crescent-shaped. Three surfaces are distinguished: anterior, posterior, and inferior, with the hilum located on the anterior surface. Externally, each gland is enclosed in a fibrous capsule. Beneath the capsule lies the cortex, which consists of three zones: the zona glomerulosa, zona fasciculata, and zona reticularis. The center of the gland is occupied by the medulla, formed by large cells of two types. The cortex produces hormones grouped into three categories, while the medulla secretes adrenaline and noradrenaline.

The endocrine portion of the pancreas is represented by pancreatic islets (islets of Langerhans), which are clusters of epithelial cells accounting for no more than 1/100 of the total pancreatic mass. Although distributed throughout the pancreas, they are most numerous in the tail region and are responsible for producing the hormones Insulin and Glucagon.

The gonads include the Testes in males and the Ovaries in females. The Endocrine Component of the Testis consists of interstitial endocrinocytes (Leydig cells) located between the convoluted seminiferous tubules, which produce the hormone testosterone. In the Ovary, the endocrine tissue is found in the granulosa layer of developing follicles and among the interstitial cells, producing estrogens. Additionally, the corpus luteum produces the hormone progesterone.