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(BQ) Part 2 book BRS Cell biology and histology presents the following contents: Endocrine system, skin, respiratory system, digestive system - oral cavity and alimentary tract, digestive system - glands, the urinary system, female reproductive system, male reproductive system, special senses.
Endo crine System I OVERVIEW-THE ENDOCRINE SYSTEM A The endocrine system is composed of several ductless glands, cl usters of cells within certain organs, and isolated single endocrine cel ls, known as the diffuse neuroendocrine system (DNES) cells, located in the epithelial lining of the respiratory and gastrointestinal systems (discussed in chapters 15 and 6, respectively) B Glands of the endocrine system include the pituitary gland (and a region of the brain known as the hypothalamus), as well as the thyro id, parathyroid, adrenal, and pineal glands Additional components of the endocrine system, such as the Islets of Langerhans, adipose tissue, female gonads, and male gonads, are discussed in the pertinent chapters C Function The endocrine system secretes hormones into nearby capillaries and interacts with the nervous system to modulate and control the body's metabolic activities Hormones are chemical messengers that are carried via the bloodstream to distant target cells They include low-molecular-weight water-so luble proteins, polypeptides, and amino acids (e.g., insulin, glucagon, follicle-stimulating hormone [FSH]) and l i pid-soluble substances, principally the steroid hormones (e.g., progesterone, estradiol, and testosterone) A Water-soluble hormones interact with specific cell surface receptors on target cells, which communicate a message that generates a biological response by the cell G protein-li nked receptors are used by some hormones (e.g., epinephrine, thyroid-stimulating hormone [TSH], serotonin) Binding of the hormone to the G protein-linked receptor leads to the production of a second messenger that evokes a target-cell response Catalytic receptors are used by insulin and growth hormone Binding of the hormone to the catalytic receptor activates protein kinases that phosphorylate target proteins B Lipid-soluble hormones diffuse across the plasma membrane of target cells and bind to specific receptors in the cytosol or in the nucleus, forming hormone-receptor complexes that regulate transcription of deoxyribonucleic acid (DNA) 221 222 BRS Cell B iology and H istology The pituitary gland lies below the hypothalamus in a bony housing known as the hypophyseal fossa, a depression in the se lla turcica of the sphenoid bone located in the base of the middle cranial fossa of the skull The hypothalamus is a region of the diencephalon of the brain; it possesses nuclei that are struc turally and functionally linked to the pituitary gland The structural link is via a series of axons whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus These axons form the hypothalamo-hypophysea l tract and terminate in the pars nervosa of the pituitary gland, where they store and, when needed, release their hormones The functional connection is via releas ing hormones that are synthesized in the arcuate, pa raventri cular (and medial paraventricular), peri ventricular, ventromedial, and dorsal nuclei of the hypothalamus These hormones are released by the neurons located in these nuclei, enter a capillary bed, and make their way, via the hypophyseal porta l system t o a second capillary b e d i n the anterior lobe of the pituitary gland, leave the capillary bed, and bind to their respective target cells in the anterior pituitary The pituitary gland is a relatively small gland, weighing only about 0.6 g in men and g in women who are pregnant or who have given birth to two or more children The pituitary is divided into two major subdivisions, the adenohypophysis and the neurohypophysis (Figure ) Each sub division is derived from a distinct embryonic analog, which is reflected in its unique cellular constitu ents and functions A The adenohypophysis is also called the anterior pituitary gland (Figures and 13.2) It originates from an ectodermal diverticulum of the stomodeum (Rathke pouch) The adenohypophysis is subdivided into the pars d ista l is, pars i ntermedia, and pars tuberalis The pars distalis is supported by a connective tissue capsule and framework It consists of irregular cords composed of two types of parenchymal cells, chromoph ils and chromophobes, lying adjacent to fenestrated capillaries a Chromophils (Figures and 13.3) (1 ) Overview Chromophils are parenchymal cells that stain intensely because of their hormone-containing secretory granules They synthesize, store, and release several hormones They are regulated by specific stimulatory and inhibitory hormones produced by neurons, referred to as neurosecretory cel ls, in the hypothalamus These hormones are conveyed to the pars distalis via a system of portal blood vessels originating in the median eminence (2) Types Chromophils are classified into two types, acidophils and basophils, depending on the dyes they bind using special histological stains With hematoxylin-eosin stain, the distinction between the two cell types is much less obvious (a) Acidophils (Tables and 13.2) bind acidic dyes and often stain orange or red They are small cells of two subtypes: somatotrophs and mammotrophs Somatotrophs constitute about 50% of the chromophils and produce somatotropin (g rowth hormone) They are stimulated by somatotropi n-releasing hormone and are inhibited by somatostatin Mammotrophs (lactotrophs) constitute about 10% of the chromophil population, except in multiparous women, where they may be as high as 30% Mammotrophs produce prolactin, which is stored in small secretory granules They are stimulated by prolacti n-releasing hormone and thyrotropi n-releasing hormone (TRH ) and are inhibited by dopamine (until re-identified that it was designated as prolacti n-inhibiting hormone) (b) Basophils (Tables 13.1 and 13.2) bind basic dyes and typically stain blue They include three subtypes: corticotrophs, thyrotrophs, and gonadotrophs Corticotrophs constitute about 10% of the chromophil population They produce pro-opiomela nocortin (POMC) whose by-products are adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormone (MSH), and lipotropic l!iitJ'!1tilU Endocrine System Neu rosecretory c e l l s Parave ntricular nuclei located i n hypothalamus secrete releas i n g and i n h i bitory hnnmnr''"' � Hyperglyce m i a 223 Hypothalamus M uscle Elevation , / JC Adipose tissue of free Mammary fatty acids gland · � Ovu l at i o n : p rogestero n e secretion Milk secretion F I G U R E 13.1 An i l l u stratio n of the p itu ita ry g l a n d , s h owing its c o n n ecti o n s with the hypoth a l a m u s, the h o r m o n e s it r e l e a ses, a n d th e effe cts of th e s e h o r m o n e s o n o r g a n s a n d tis s u e s of the b o dy A D H , a ntid i u reti c h o rm o n e ; ACTH, a d re n o c o rtic otro p i c h o r m o n e ; TS H , thyro i d -sti m u lati n g h o r m o n e ; FS H , fo l l i c l e -sti m u lating h o r m o n e ; LH, l ute i n iz i n g h o r m o n e I From Gartner LP H iatt JL Color Atlas o f Histology 5th ed Baltimore M D : Lippincott William & Wilkins; 2009:206.) hormone (LTH ), a precursor of�-endorphin They are stimulated by corticotropin releasing hormone Thyrotrophs constitute about 5% of the chromophil population, produce TSH, and are stimulated by TRH 224 BRS Cell B iology and H istology Source of hypophysiotropic hormones (releasing factors) , Source of ADH and oxytocin -�·�� Wit.li? � '