Histology Sheet 23 PDF

23 Noor Hamadneh Jeyad Alkhzouz Ghada Abu el Ghanam  Endocrine system The endocrine system consists of ductless glands that secrete chemical messengers called hormones directly into the bloodstream or the extracellular fluid. These hormones regulate physiologica...

23 Noor Hamadneh Jeyad Alkhzouz Ghada Abu el Ghanam  Endocrine system The endocrine system consists of ductless glands that secrete chemical messengers called hormones directly into the bloodstream or the extracellular fluid. These hormones regulate physiological processes and maintain homeostasis by influencing the activity of target cells throughout the body. Hormones: A hormone is a chemical substance produced and secreted by one cell or group of cells. It travels via the circulatory system or extracellular fluid to influence the activity of: o Cells in another part of the body (endocrine signaling). o Nearby cells (paracrine signaling). o The same cell that secreted it (autocrine signaling). Hormonal Effects: Hormones typically act more slowly compared to the nervous system but have widespread and longer-lasting effects. While the nervous system is highly specific and precise in its control (e.g., a direct nerve impulse affects a specific target), hormones are released into the bloodstream and affect any cell that has the appropriate receptor. Hormone-Receptor Interaction: For a hormone to exert its effect, it must bind to a specific receptor located on or within a target cell. Receptors: o Are specialized proteins that specifically recognize and bind to their corresponding hormone. o Trigger a series of intracellular events upon hormone binding (e.g., activating enzymes, changing gene expression). Cells lacking the specific receptor will not respond to the hormone, ensuring specificity of the hormonal action. Examples of Hormonal Functions: Growth Hormone (GH): o Plays a critical role in growth and development, especially in children. o Targets muscles, bones, and other tissues, stimulating cell growth and division. o Many cells in the body have receptors for growth hormone, leading to its widespread effects. Thyroid Hormones (T3 and T4): o Produced by the thyroid gland, these hormones regulate metabolism, energy production, and growth. o Even though thyroid hormones circulate throughout the bloodstream, they primarily affect tissues sensitive to metabolic regulation. Endocrine glands: Endocrine glands are composed of specialized unicellular or multicellular cells that secrete hormones directly into the bloodstream or extracellular fluid, without the use of ducts. These glands are typically associated with fenestrated capillaries, which facilitate the efficient exchange of hormones into the blood. Some of the major endocrine glands include: 1. Pituitary gland (regulated by hormones produced by the hypothalamus). 2. Pineal gland 3. Thyroid gland 4. Parathyroid 5. Adrenal Gland Hypothalamus: The hypothalamus acts as the central regulator of the endocrine system by maintaining constant communication and feedback with the pituitary gland, which in turn regulates other endocrine glands in the body. Participates in the endocrine system by two mechanisms: 1. Direct Hormone Secretion: The hypothalamus produces two hormones: antidiuretic hormone (ADH) and oxytocin. These hormones are transported to and stored in the posterior pituitary gland, where they are later released into the bloodstream. 2. Regulation of Anterior Pituitary Hormones: The hypothalamus controls the secretion of anterior pituitary hormones through the release of: a. Releasing hormones: Stimulate the anterior pituitary to secrete specific hormones. b. Inhibitory hormones: Suppress the release of specific anterior pituitary hormones. Pituitary gland: - Weighs 0.5 g in adults. Lies below the brain in a small cavity on the sphenoid bone, called Sella turcica (in the hypophyseal fossa). - Pituitary is formed in the embryo partly from the developing brain, and partly from the oral cavity. - It starts developing from the early weeks of the embryo.  Development (read only): o Neural component is the neurohypophysis bud growing downward from the floor of the future diencephalon as a stalk (for infundibulum) that remains attached to the brain. o Oral component arises as an out pocketing of ectoderm from the roof of the primitive mouth and grows cranially forming a structure called the hypophyseal (Rathke) pouch. ▪ Base of this pouch eventually constricts and separates from the pharynx, its anterior wall then thickens greatly, reducing the pouch’s lumen to a small fissure. - Because of its dual origin, pituitary consists of two glands: 1. The posterior Neurohypophysis: retains many histological features of the brain tissue and consists of: a. Pars nervosa (large part) b. Smaller infundibulum (Stalk attached to the hypothalamus at the median eminence) 2. Anterior adenohypophysis: derived from oral ectoderm, has three parts: a. Large pars distalis (anterior lobe) b. Pars tuberalis (wraps around the infundibulum c. Thin pars intermedia adjacent to the posterior pars nervosa. Both united anatomically but with different functions. ____________________________________________________________________ The hypothalamic-hypophyseal portal system: Two important systems: - The hypothalamic-hypophyseal tract is a neural pathway that connects the hypothalamus to the posterior pituitary gland (neurohypophysis). It facilitates the transport of neurohormones produced in the hypothalamus to the posterior pituitary, where they are stored and released into the bloodstream. o Blood stream is mainly supplied by the Inferior hypophyseal arteries - Pituitary gland still retains neural connection to the hypothalamus at the base of the brain. - In addition to the vascular portal system carrying small regulatory peptides from the hypothalamus to the adenohypophysis, a bundle of axons called the hypothalamic- hypophyseal tract courses into the neurohypophysis from two important hypothalamic nuclei: o Supraoptic nuclei: synthesizes ADH (antidiuretic hormone) o Paraventricular nuclei: synthesizes oxytocin. - Both hormones undergo axonal transport and accumulate temporarily in the axons of the hypothalamic-hypophyseal tract before their release and uptake by capillaries branching from the inferior arteries. - Blood supply derives from two groups of vessels coming off the internal carotid artery and drained by the hypophyseal vein. o Superior hypophyseal arteries: supply the median eminence and the infundibular stalk. ▪ These divide into a primary plexus of fenestrated capillaries that irrigate the stalk and median eminence. ▪ These capillaries then rejoin to form venules that branch again as a larger secondary capillary plexus in the adenohypophysis. ▪ These vessels form the hypothalamic-hypophyseal portal system that carries neuropeptides from the median eminence the short distance to the adenohypophyses where they either stimulate or inhibit hormone release by endocrine cells there. o Inferior hypophyseal arteries: provide blood mainly for neurohypophysis. - The hypothalamic-hypophyseal portal system is a specialized vascular network that connects the hypothalamus to the anterior pituitary gland (also called the adenohypophysis). This system enables the hypothalamus to regulate the function of the anterior pituitary through the transport of hormones via the bloodstream. o Blood stream is mainly supplied by the superior hypophyseal arteries. Secretions of hypothalamus: To understand the secretions of the hypothalamus and why they’re important, just a quick recap: 1. Connection to the Neurohypophysis (Posterior Pituitary) - Hypothalamic-Hypophyseal Tract: Composed of axons extending from the supraoptic and paraventricular nuclei of the hypothalamus. ▪ These axons transport hormones produced in the hypothalamus to the posterior pituitary for storage and release. - Hormones Released: o ADH (Antidiuretic Hormone, also called Vasopressin): Regulates water retention in the kidneys and maintains blood pressure. o Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding. 2. Connection to the Adenohypophysis (Anterior Pituitary) - The hypothalamus regulates the anterior pituitary using releasing and inhibitory hormones secreted into the portal circulation (a system of capillaries connecting the hypothalamus and anterior pituitary). - Releasing Hormones: o Thyrotropin-Releasing Hormone (TRH): Stimulates the release of TSH (Thyroid- Stimulating Hormone). o Gonadotropin-Releasing Hormone (GnRH): Stimulates FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone) secretion. o Growth Hormone-Releasing Hormone (GHRH): Stimulates growth hormone release. o Corticotropin-Releasing Hormone (CRH): Stimulates ACTH (Adrenocorticotropic Hormone) release. - Inhibitory Hormones: o Prolactin-Inhibitory Hormone (Dopamine, PIH): Inhibits prolactin secretion. o Growth Hormone-Inhibiting Hormone (Somatostatin, GHIH): Inhibits growth hormone secretion. - Hormonal Targets and Actions: o Prolactin (PRL): Stimulates milk production. o ACTH: Stimulates the adrenal cortex to produce glucocorticoids (like cortisol). o TSH: Stimulates the thyroid gland to release thyroid hormones (T3 and T4). o FSH and LH: Regulate reproductive functions. o GH: Promotes growth, metabolism of lipids and carbohydrates. o ADH and Oxytocin: Stored in the posterior pituitary, secreted into the bloodstream for functions like water retention and uterine contraction. Summary of the Hypothalamic-Pituitary Axis: - Hypothalamic Hormones (Releasing/Inhibitory): o Regulate anterior pituitary hormone secretion. - Hypothalamic-Neurohypophyseal Hormones: o ADH and Oxytocin are synthesized in the hypothalamus and stored in the posterior pituitary. o This system integrates the nervous system and endocrine system, controlling vital functions such as growth, metabolism, reproduction, and stress response. Neurohypophysis - Consists of: o Pars nervosa o Infundibular stalk - Doesn’t contain cells that synthesize their own hormones. - Composed of neural tissues containing 100000 of unmyelinated axons of large secretory neurons with cell bodies in supraoptic and paraventricular nuclei of hypothalamus. - Also present are high numbers of branched glial cells called pituicytes that resemble astrocytes and are most abundant cells in posterior pituitary. - Stores: o ADH (vasopressin) produced by supraoptic cell body. o And oxytocin made by paraventricular cell body. - These are transported axonally to pars nervosa where they accumulate in axonal dilations called neurosecretory bodies (Herring bodies). o Faint eosinophilic structures. - Neurosecretory bodies contain membrane enclosed granules with either oxytocin or ADH bound to carrier proteins called neurophysin 1 and 2 o Neurophysin 1: carries Oxytocin o Neurophysin 2: carries ADH - Nerve impulses along the axons trigger the release of the peptides from the neurosecretory bodies for uptake by the fenestrated capillaries of the pars nervosa, and the hormones are then distributed to general circulation. - ADH released in response to increase blood tonicity, sensed by osmoreceptor cells in hypothalamus. o Hypothalamus then stimulates ADH synthesis in supraoptic neurons. - Oxytocin stimulates contraction of uterine smooth muscles during childbirth and the myoepithelial cells in mammary gland alveoli. Dark: anterior lobe (secretory cells) Light: posterior lobe (axons) Adenohypophysis: The three parts of the adenohypophysis are derived from the hypophyseal pouch. Pars Distalis - Accounts for 75% of the adenohypophysis and has a thin fibrous capsule. - Main components are cords of well-stained endocrine cells interspersed with fenestrated capillaries and supporting reticular CT. - Common stains suggest two broad groups of cells with different staining affinities: o Chromophils: secretory cells in which hormone is stored in cytoplasmic granules. ▪ Also called basophils and acidophils based on their affinities for basic and acidic dyes. Acidophils either secrete growth hormone (somatotropin) or prolactin, and are called: o Somatotrophs (somatotropic cells): constitute about half the cells of pars distalis (most abundant). o Lactotrophs (lactotrophic cells) Basophilic cells are: o Corticotrophs (target cells in adrenal cortex): main hormone synthesized is pro-opiomelanocortin (POMC), which is cleaved into: ▪ Adrenocortical tropic hormone (ACTH) ▪ B-lipotropin (B-LPH) o Gonadotrophs (target cells in gonads): secrete two different glycoproteins: ▪ FSH (follicle stimulating hormones) ▪ Luteinizing hormones (LH( o Thyrotrophs (target cells in thyroid gland): least abundant Chromophobes: stain weakly. - Hormones produced by pars distalis have important functional activities including: o Regulating almost all other endocrine glands o Ovarian functions o Sperm production o Milk production o Metabolism of muscle, bone and adipose tissue Pars Tuberalis - Smaller funnel shaped region surrounding the infundibulum of neurohypophysis - Most of the cells there are gonadotrophs Pars Intermedia - A narrow zone lying between pars distalis and pars nervosa. - Contains: o Basophils (corticotrophs): express POMC, but cleave it differently from cells in pars distalis, producing mainly smaller peptide hormones. o Chromophobes o Small colloid-filled cysts derived from lumen of embryonic hypophyseal pouch - Best developed and active during fetal life Major Cell types of the anterior pituitary (know the hormones and what produces them not their functions):  ‫‪Read only slides:‬‬ ‫‪Pituitary Organogenesis:‬‬ ‫تمت كتابة هذا الشيت صدقة جارية عن روح والدة زميلنا عمرو رائد من دفعة تيجان‬ ‫دعواتكم لها بالرحمة والمغفرة‬ ‫‪Thank you‬‬