Endocrine System: Mechanisms and Glands

• Endocrine glands are classified based on the chemical nature of their secretions.
• Aims include describing the histology of endocrine glands, identifying the pituitary gland's components, and understanding the structure/function of specific glands.

Endocrine System

• The endocrine system is made up of a variety of ductless glands.
• Endocrine glands release hormones into surrounding connective tissue, which then enter the bloodstream via capillaries to reach target organs.
• Target organs are specific due to molecules having an effect on specific organs.
• Only cells with matching hormone receptors respond, conversely organs lacking those receptors do not respond.
• Paracrine control involves cells acting on adjacent cells with specific receptors.
• Autocrine control is when cells have receptors for the hormones they produce.

Distribution of Endocrine Glands

• Endocrine glands are distributed in three ways.
• First, there are major endocrine glands, including the pituitary, thyroid, parathyroid, adrenal, and pineal glands.
• Second, there are organs containing groups of endocrine cells, such as the Islets of Pancreas, gonads (testes/ovaries), thymus, and hypothalamus.
• Last, there are isolated endocrine cells/APUD/neuroendocrine systems found in the lining epithelium of the GIT and respiratory tract.

Development of Glands

• Glands develop as cords of epithelial cells from a membrane surface, which then invaginates and forms two parts, proximal and distal, into underlying connective tissue.
• In endocrine glands, the proximal part disappears, whereas the distal part forms the secretory islands near blood capillaries, with the glands pouring secretions directly into bloodstream.

Types of Endocrine Glands

• Endocrine glands are categorized into cord and clump types or follicular types.
• In the cord and clump type cells are arranged in irregular cords or clumps permeated by capillaries, secreting directly outward into capillaries.
• In the follicular type cells are arranged in follicles surrounded by capillaries, secreting inward and storing secretions outside the cells within the follicles utilizing an "extra-cellular method". -Most endocrine glands such as the pituitary, adrenal, parathyroid, and pineal glands utilize the “intra-cellular method”.
• Example of the follicular type gland is the thyroid gland.

Major Endocrine Glands

• The key components of the endocrine system include the pituitary gland/hypophysis cerebri, hypothalamus, thyroid and parathyroids, adrenals, and pineal gland.

Pituitary Gland/Hypophysis Cerebri

• The pituitary gland, also known as the hypophysis cerebri, is the master gland of the endocrine system.
• The pituitary gland is under the control of the hypothalamus.
• The pituitary gland is suspended from the floor of the 3rd ventricle.
• It is small, about the size of a pea, and has a reddish-grey ovoid shape.
• Pituitary gland has a transverse measurement of 12 mm, an antero-posterior measurement of 8 mm, and a weight of 500 mg.
• It is located in the Sella Turcica.

Subdivisions of the Pituitary Gland

• The pituitary gland is subdivided into the anterior pituitary/adenohypophysis and the posterior pituitary/neurohypophysis.
• Both parts differ embryologically, morphologically, and functionally.
• The anterior pituitary/adenohypophysis has glandular tissue, while the posterior pituitary/neurohypophysis has neural tissue.
• The entire gland is covered by a connective tissue capsule.
• The hypophysis consists of glandular (adenohypophysis) and nervous portions (neurohypophysis).
• At low power, can identify adenohypophysis (anterior pituitary) and pars anterior (pars distalis;).
• Can also identify Pars intermedia, remnant of rathke pouch rudimentary in human beings, pars tuberalis and neurohypophysis.
• Pars nervosa (posterior pituitary) consist of median eminence and infundibulum.

Anterior Pituitary (Adenohypophysis)

• Further divided into 3 parts: Pars Tuberalis, Pars Distalis, and Pars Intermedia.
• Both parts differ embryologically, morphologically, and functionally.

Posterior Pituitary (Neurohypophysis)

• Consists of 3 parts: median eminence of tuber cinereum, pituitary stalk and infundibular stalk, and pars nervosa.

Connections of the Pituitary Gland

• The hypothalamo-hypophyseal tract consists of axons extending from supraoptic and paraventricular nuclei of the hypothalamus, terminating in the pars nervosa.
• This tract carries and releases hormones from the hypothalamus to the Pars Nervosa for capillary uptake.

Blood Supply of the Pituitary Gland

• The superior hypophyseal artery supplies the pituitary gland.
• There exist also the inferior hypophyseal artery.
• The superior hypophyseal artery forms the primary capillary plexus in the pars tuberalis, median eminence, and infundibular stem.
• The hypophyseal portal veins transport these hormones from the hypothalamus to adenohypophysis.
• Secondary capillary plexus are in the pars distalis.
• Hypophyseal veins are also found in the blood supply of the pituitary gland.
• Regulatory hormones synthesized by hypothalamic neurons control the secretory activity of the parenchymal cells in the pars distalis/adenohypophysis.
• The hypothalamo-hypophyseal portal system carries hormones synthesized by neurons, transporting them to the neurohypophysis.

Adenohypophysis

• Composed of the Pars Distalis & Tuberalis
• Consist of cells which are arranged in clumps or irregular cords.
• Separated by fenestrated capillaries and sinusoids.
• Forms 75% of the adenohypophysis

Anterior Pituitary (Distalis/Anterior Pars)

• Cells are divided into two classes based on staining: chromophils ("color-loving") and chromophobes ("color-fearing").
• Chromophils are further classified into acidophils and basophils.
• Each cell type produces only one hormone.
• Parenchyma consists of secretory epithelial cells arranged in anastomosing cords and clusters.
• Characterized by abundant sinusoidal capillaries with fenestration.
• Based on affinities for Hematoxylin and eosin (H&E), cells are classified as chromophobes/chromophil cells.
• Chromo = Stain/color/pigment, Philia = Attraction & Phobe/Phobia = Fear.
• Chromophil cells are easily stainable and chromophobe cells are less stained.

Acidophil Cells

• Stained with acidic dyes, like eosin and orange G.
• Round cells with cytoplasmic granules.
• Based on hormone products, divided into somatotrophs, which produce growth hormone, and mammotrophs, which produce prolactin.

Basophil Cells

• stained with basic dyes, like Hematoxylin and PAS.
• Poligonal with cytoplasmic granules
• Based on hormone divided in trotrophs producing TSH, gonadotrophs producing FSH & LH, corticotrophs producing ACTH.

Adenohypophysis - Pars Intermedia

• Poorly developed in humans has basophilic cells arranged in irregular clumps, produces melanocyte-stimulating hormones.
• Characterized by colloid-filled vesicles lined by cuboidal cells and remnants of Rathke's pouch.
• Other secretory cells and chromophobes are also present.

Neurohypophysis

• A down growth from the hypothalamus, exhibits characteristics of nervous tissue.
• Axon terminals in this area originate from hypothalamic cells, producing ADH (vasopressin) in the supraoptic nucleus, and oxytocin in the paraventricular nuclei.
• Neuronal cell bodies are located in the hypothalamus.
• The pars nervosa contains no neuronal cell bodies.
• It consists of glial cells or pituicytes, which are variable in cell size and shape.
• Herring bodies: Large, unmyelinated axon terminals with many neurosecretory granules, and characterized by a rich capillary network present in pars nervosa.
• Contains hormones synthesized in the hypothalamus. it also consists of Unmyelinated axons of neurosecretory cells with cell bodies located in supraoptic and paraventricular nuclei of hypothalamus are present. also has supporting cells called Pituicytes.

Disruptions of the Pituitary Gland

• Gigantism: Overproduction of growth hormone in adults before epiphyseal closure.
• Acromegaly: Overproduction of growth hormone in adults after cessation of bone growth.
• Diabetes Insipidus: Deficient secretion of ADH, resulting in the excretion of a large volume of dilute urine with a low specific gravity (below 1.010), polyuria and polydipsia

Thyroid Gland

• The thyroid gland looks like a shield.
• Special features include manufacturing, storing, and releasing hormones when required, dependence on external environment and is rich in blood flow.

Microscopic Features of the Thyroid

• Gland is covered by a capsule, septa extends from the capsule divide the gland in to lobules and convey blood vessels, nerves and lymphatics into gland.
• Has a stroma covered by a capsule, from those capsule septa extend to convey blood vessels, nerves and lymphatics into gland.
• Parenchyma contains hollow, spheroidal structures called Thyroid follicles and Para-follicular cells.

Thyroid Follicles

• The basic structural & functional unit of the thyroid gland and is spherical in shape.
• Size vary between 50 – 500μm and More than 20 million exist in one gland.
• It has single layer of cells that rest on basement membrane.
• The lumen contains gelatinous homogeneous material called colloid that stain Pink.
• Contains large glycoprotein Precursor, a constituent of thyroid hormones called Thyroglobulin.

Characteristics of Follicles

• Resting Gland: Lumen has follicle full of colloid, lined by simple squamous epithelium.
• Highly Active Gland: Lumen has has less colloid, lined by simple columnar epithelium.
• Moderately Active Gland: Most common, follicle hasmoderate amount of colloid, lined by simple cuboidal epithelium.

Follicular Epithelium

• It varies from is high cuboidal to low cuboidal and correlates to the secretory level and activity.
• Follicle cells have large, centrally- or basally-located nuclei, also its main function pertains to protein secretion.
• Involves storage and release of thyroid hormones via the protein thyroglobulin.
• Parrafollicular cells produces Single scattered or small, is responsible of calcitonin production.

Thyroid Stroma

• Is derived from the neural creat, and also lie as isolated clusters bewteen the follicles.
• The spaces between the follicles are filled by a specialized network of connective tissue called Stroma.
• The stroma is made up of delicate CT containing capillaries and lymphatics & sympathetic nerves, and capillaries lie closely in contact with the walls of the follicles.
• In short the stroma is for transfer of released hormone to the blood.
• Calcitonin secreted inhibits osteoclast.

Parathyroid Glands

• The parathyroid glands are small, yellowish-brown lentiform bodies.
• Situated along the posterior border of the thyroid gland, numbering between two to six, mostly four.
• Typically arranged as two superior and two inferior pairs.

Structure of Parathyroid Glands

• They are covered by a thin C.T. capsule that surrounds each gland and gives rise to internal septa.
• The gland consists of a mass of crowded single cells not arranged as follicles, are actually embedded in the capsule of the thyroid gland
• Present as a mass of crowded single cells, not arranged as follicles. It consist of 2 type of cells:Chief cells (principal cells) and Oxyphil cells.

Chief Cells

• They are Numerous, small cells with prominent nuclei and have pale, scant cytoplasmic staining: Secrete parathyroid hormone.

Oxyphil Cells

• The are single or clumps of larger cells with acidophilic (oxyphilic), cytoplasm with dark staining nucleus.

Adrenal/Suprarenal Gland

• Located at the upper pole of each kidney.
• Embedded in pararenal fat and fascia.
• The Volume of the cortex is about ten times more than medulla And is flattened and half-moon shape. Has two components Superficial Cortex and deep Medulla.
• Size is about between 4-6cm long, 1-2cm wide, 4-6mm thick in adults.

Adrenal Cortex

• Has two major structure cortex and medula and its Covered by capsule extend its septa into the substance of the gland.
• The cortex which constitutes major portion of gland and it secretes steroideal hormones.
• Cells are arranged in cords with intervening sinusoids. Zona Glomerulosa, Zona Fasciculata, Zona Reticularis.

Adrenal Cortex Zones

• Zona Glomerulosa is is Immediately inside the capsule constitute 10-15th and conist closely packed, rounded and inverted in v shaped cords of columna cells that consists of mineralocorticoid and aldosterone.
• The Zona Fasciculata constitutes 65-80 percents, consists of large polygonal cells, arranged in one and two cell thick and secrete glucocorticoids such as cortisol. The cells in here has numereous lipid doplets is vacuolated.
• Zona Reticularis Constitutes 10 1/5th which connect and separated by connective tissue. cells here is smaller but is in connected cords of cells to produced steroid sex hormone.

Adrenal Medulla

• Distinct from cortex, both functionally and embryologically.
• Develops from neural crest cells.
• Consists of two types of cells: chromaffin cells (pheochromocytes) and ganglion cells.

Chromaffin Cells

• The most numerous cells in the medulla are the ovoid-shaped secretory cells, arranged in clumps or cords surrounding the capillaries.
• Cytoplasm has containing secretory granules - stainable containing catecholamines (A & NA).

Ganglion Cells

• In addition to chromaffin cells, the medulla also contains few sympathetic ganglion cells, typically present in smaller amounts also present singly or in small groups.

Pineal Gland

• A gland that is a neuroendocrine gland regulating the body's rhythm.
• Responds to light intensity and duration.
• A flattened pine, cone-shaped structure which releases melatonin.
• The pineal gland consists of cells called pinealocytes and interstitial (glial) cells.

Cells of the Pineal Gland

• Pinealocytes (chief cells) are arranged in clumps within lobules by connective tissue septa from the pia mater and are neuro secretory cell and its cell body has short and long.
• Types of the cell includ light and dark pinealocytes.
• light is many large, dark is few and small.

Interstitial (glial) Cells

• The constituents which makes up five percent in the gland.
• Astrocytes present in between this structure and nuclei nature.

Corpora Arenacae or Brain Sand

• Consist of calcefied strcuctures in ther pineal and have areas in the Brain.
• Consist of calcium Phosphate and carbonates in organic matrix deposited in a concentric layer is made in an layer.

Explore the endocrine system, covering hormone transport, target cell specificity, and control mechanisms. Learn about endocrine glands and cell signaling methods. Test your understanding of hormone-receptor interactions.