Physiology of Glandular tissues

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Types of Intercellular Communication

• The nervous system uses two types of intercellular communication: electrical and chemical signaling.
• Electrical signaling involves direct action of an electrical potential, while chemical signaling involves neurotransmitters such as serotonin and norepinephrine.

Endocrine System

• The endocrine system uses chemical signaling to send messages throughout the body.
• Hormones are secreted by endocrine glands into the extracellular fluid, where they bind to receptors on target cells.
• Hormone signaling requires more time than neural signaling to prompt a response in target cells.
• Hormones are produced by glandular tissue, which also produces other secretory products such as sweat, saliva, and digestive enzymes.

Types of Chemical Signaling

• Autocrine signaling: a chemical messenger stimulates the cell that originally secreted it, and sometimes nearby cells of the same type.
• Paracrine signaling: a chemical messenger acts locally on nearby cells of a different type.
• Hemocrine or endocrine signaling: a chemical messenger is secreted into the bloodstream and affects cells that are distant from its source.
• Neurocrine signaling: a chemical messenger is released locally in a synaptic gap, and activates an adjacent cell.
• Neuroendocrine signaling: a hormone is released both locally and distantly.

Classification of Hormones

• Hormones can be classified into three types based on their chemical structure: amino acid-derived, peptide, and lipid-derived.
• Amino acid-derived hormones are derived from the amino acids tryptophan and tyrosine, and have names that end in "-ine".
• Peptide hormones are composed of polypeptide chains, and include molecules that are short polypeptide chains, small proteins, and large glycoproteins.
• Lipid-derived hormones are derived from cholesterol, and include steroid hormones such as testosterone and estrogen.

Endocrine Glands and Their Hormones

• The pituitary gland produces several hormones, including growth hormone, prolactin, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone.
• The thyroid gland produces thyroxine and triiodothyronine, which stimulate basal metabolic rate.
• The parathyroid gland produces parathyroid hormone, which increases blood calcium levels.
• The adrenal gland produces several hormones, including aldosterone, cortisol, and epinephrine.

Transport of Hormones in the Blood

• Protein and peptide hormones are transported in the plasma in dissolved form.
• Steroid and thyroid hormones are transported in the blood through association with various types of proteins.
• Carrier proteins have high affinity for specific hormones, but low capacity due to low plasma concentration.
• Albumins have low affinity for steroid hormones, but high capacity due to high plasma concentration.

Hormone Receptors

• Hormone receptors are present on cells in much greater numbers than required for eliciting a biological response.
• Hormone receptors can be present on the cell membrane or embedded in the cell membrane.
• Lipid-soluble hormones bind to intracellular receptors to activate transcription, resulting in a slow response.
• Plasma-soluble hormones bind to cell surface receptors, which activate a second messenger, resulting in a rapid response.

Factors Affecting Target Cell Response

• Presence of a significant level of a hormone can cause target cells to decrease their number of receptors (downregulation).
• Prolonged absence of a hormone can cause target cells to increase their number of receptors (upregulation).
• Hormones can interact with each other to affect the response of cells, through permissive, synergistic, or antagonistic effects.

Metabolism of Hormones

• Peptide hormones are degraded by the liver and cleared by the kidney with half-lives of a few minutes.
• Steroid hormones and thyroid hormone circulate bound to protein carriers, which extends their half-life in the blood.
• Metabolism of steroids involves reduction, followed by conjugation with sulfates and glucuronides, allowing them to be excreted in urine.

Glandular Tissues

• Glandular tissues are excitable tissues that respond to stimuli with a specific reaction – the production of a specific secretion.
• Glands are divided into two main categories:
  • Exocrine glands: have ducts and secrete to the surface (e.g., sebaceous and sweat glands, salivary glands)
  • Endocrine glands: do not have ducts; their secretions (hormones) are secreted into the bloodstream

Exocrine Glands

• Retain ducts to body surfaces
• Examples:
  • Sweat glands (sweat)
  • Ceruminous glands (cerumen)
  • Salivary glands (saliva)
  • Mammary glands (milk)
  • Lacrimal glands (tears)
  • Sebaceous glands (sebum)
  • Liver and pancreas (exocrine part)

Endocrine Glands

• Divided into three categories based on localization:
  • Peripheral endocrine glands
  • Diffuse endocrine glands
  • Central endocrine glands

Peripheral Endocrine Glands

• Anatomical formations whose basic function is the synthesis and secretion of hormones into the blood
• Examples:
  • Thyroid gland (T3, T4)
  • Parathyroid glands (parathormone - PTH)
  • Thymus (thymosin and other hormones that stimulate the development of T lymphocytes)
  • Gonads (testes and ovaries)
  • Adrenal glands (cortex and medulla)
  • Pancreas (endocrine part - Islets of Langerhans)

Diffuse Neuroendocrine Glands

• Include neuroendocrine cells scattered throughout the body that synthesize and secrete hormones
• Examples:
  • Cells of adrenal gland medulla
  • Enterochromaffin cells
  • Thyroid gland C cells
  • Hypophysis endocrine cells
  • Pancreas endocrine cells

Central Endocrine Glands (Neurohemal Organs)

• Store their secretory products (neurohormones) in a special chamber until stimulated to release it by a signal from the nervous system
• Examples:
  • Hypothalamus
  • Hypophysis (pituitary gland)
  • Epiphysis

Hypothalamus

• Link between the nervous and endocrine systems
• Receives information about the internal and external environment of the body
• Regulates and coordinates activity of various tissues and organs
• Produces:
  • Vasopressin (antidiuretic hormone, ADH)
  • Oxytocin
  • Liberins (stimulating hormones)
  • Statins (inhibiting hormones)

Hypophysis (Pituitary Gland)

• Composed of three parts:
  • Adenohypophysis (anterior pituitary): secretes tropic hormones that regulate the action of several peripheral endocrine glands
  • Intermediate pituitary (pars intermedia): produces melanotropic hormone (melanocyte stimulating hormone)
  • Neurohypophysis (posterior pituitary): stores and secretes hormones produced by the hypothalamus (oxytocin and vasopressin)

Hypothalamic-Hypophyseal System

• Connection between the hypothalamus and the hypophysis
• Humoral connection: blood flows from the hypothalamus to the adenohypophysis, transporting liberins and statins
• Neural connection: axonal transport of hormones from the hypothalamus to the neurohypophysis

Hypothyroidism

• Occurs due to problems with thyroid hormone production (iodine deficiency, tumor, or atrophy of thyroid follicular cells)
• Can also occur due to failure of the hypothalamus to secrete TRH or failure of the adenohypophysis to secrete TSH
• Characteristics:
  • Reduced basic metabolism processes
  • Lowered body temperature
  • Reduced heart rate
  • Weight gain
  • Hair loss and changes in skin and hair color
  • Prone to skin infections
  • Reduced libido and infertility
  • Very low mental activity and slower reflexes
  • Can lead to myxedema or mucus edema

Iodine Sources

• Found in food:
  • Sea cabbages
  • Cod liver
  • Shrimps
  • Milk
  • Eggs
  • Prunes (dried plums)
  • Cranberries
  • Strawberries
  • White bread
  • Rhubarb

Adrenal Glands

• Consists of two independent secretion glands:
  • Adrenal medulla (modified ganglion of the sympathetic nervous system)
  • Adrenal cortex (three parts from histology)
• Each part has a separate blood supply and innervation
• Adrenal cortex part is necessary for life-support, especially mineralocorticoid aldosterone

Corticosteroids

• Produced in the adrenal cortex
• Types:
  • Mineralocorticoids (zona glomerulosa)
  • Glucocorticoids (zona fasciculata)
  • Sex steroids (zona reticularis)
• Regulation of synthesis:
  • Under the influence of the adenohypophysis (ACTH)

Zona Glomerulosa

• Produces mineralocorticoids (aldosterone and deoxycorticosterone)
• Absolutely necessary for life-support
• Regulates the reabsorption of electrolytes (mainly Na+ and K+) from the nephron tubules
• Aldosterone secretion is regulated by:
  • Renin-angiotensin system
  • Blood potassium concentration
  • Blood sodium level
  • ACTH stimulation

Zona Fasciculata

• Produces glucocorticoids (cortisol and corticosterone)
• Stimulate the release of glucose from the liver into the blood and gluconeogenesis
• Suppress allergic reactions in the body
• Suppress inflammatory reactions
• Reduce the number of eosinophils
• Increase the body's resistance to harmful effects
• Increase gastric hydrochloric acid secretion

Zona Reticularis

• Produces androgens (male), estrogens (female), and progesterone (female)
• Important for animals (and humans) until reaching sexual maturity
• Hyperfunction contributes to the onset of sexual maturity, including premature puberty

Gonads

• Produce gametes and sex hormones
• Sex hormones:
  • Estrogens (female)
  • Androgens (male)
• Regulation of gonadal hormones:
  • By gonadotropic hormones of the adenohypophysis (FSH, LH, LTH)

Female Sex Hormones

• Produced in ovaries and placenta
• Types:
  • Estrogens (estradiol and estrone)
  • Progesterone
• Functions:
  • Stimulate the growth and development of female gonads
  • Stimulate the growth and development of genital organs
  • Promote the proliferation of the uterine mucosa
  • Inhibit the development of atherosclerosis in blood vessels

Male Sex Hormones

• Produced in testicular interstitial Leydig cells
• Types:
  • Testosterone
  • Androsterone
• Functions:
  • Stimulate protein synthesis and body weight gain
  • Stimulate the growth, development, and functions of genital organs
  • Determine the expression of sexual instinct

Endocrine Glands Independent of the Hypothalamo-Hypophyseal System

• Parathyroid glands (PTH)
• Thyroid gland C cells (calcitonin)
• Pancreas (insulin, glucagon)
• Thymus (thymosin)
• Adrenal medulla (epinephrine, norepinephrine)

Pancreatic Hormones

• The pancreas has four types of cells: α cells (~25%), β cells (~60%), δ cells (~10%), and PP cells.
• These cells produce hormones that regulate glucose levels in the blood.

Insulin and Glucagon

• α cells produce glucagon, which increases blood glucose levels.
• β cells produce insulin, which decreases blood glucose levels.
• Insulin is a polypeptide formed from 51 amino acids and is synthesized in the rough endoplasmic reticulum.
• Insulin is essential for life and affects carbohydrate, protein, and fat metabolism.

Insulin Synthesis and Secretion

• Insulin is synthesized as preproinsulin in the ribosomes, then cleaved to proinsulin in the Golgi apparatus.
• Proinsulin is cleaved into equimolar amounts of insulin and C-peptide in the secretory granules.
• Insulin is released into the blood when blood glucose is elevated, and β-cells uptake glucose through facilitated diffusion.
• Insulin release is regulated by the concentration of glucose in the blood.

Regulation of Insulin Secretion

• When blood glucose is elevated, β-cells uptake glucose, and the concentration of ATP increases, closing K ion channels and causing depolarization.
• This leads to the opening of Ca ion channels, breakdown of insulin and C-peptide granules, and release of insulin and C-peptide into the blood.
• In normoglycemia, insulin is secreted minimally, but along with insulin and C-peptide, β-cells also secrete amylin, which inhibits gastric emptying and promotes satiety.

Glucose Transport

• Glucose is transported from the small intestine and kidneys into the blood through active transport and facilitated diffusion.
• Glucose transport in insulin-dependent tissues (myocytes, adipocytes, and cardiomyocytes) requires insulin to bind to insulin receptors and change the permeability of the cell membrane.
• Insulin stimulates the entry of glucose into cells and promotes its intracellular metabolism, activates lipogenesis, and stimulates glycogen production in muscles.

Effect of Eating on Insulin Secretion

• Between meals, the concentration of glucose in the blood and β-cells is low, and insulin secretion is minimal.
• After a meal, glucose concentration increases, and insulin synthesis and release increase, ensuring glucose transport and use in insulin-dependent tissues.
• In non-insulin dependent tissues, such as nervous tissue, glucose is transported by GLUT-3 transport-proteins without insulin receptor stimulation.

Sympatho-Adrenal System (SAS)

• The sympathetic nervous system and the adrenal medulla are connected anatomically and functionally, forming a single SAS.
• The SAS regulates the production of epinephrine and norepinephrine, which are stimulated by the sympathetic nervous system.

Glands

• Glands are classified into two types: endocrine and exocrine.
• Endocrine glands produce hormones that are secreted directly into the bloodstream.
• Exocrine glands produce secretions that are released through a duct or tube.

Exocrine Glands

• Types of exocrine glands:
  • Unicellular (e.g., goblet cells, mucous cells)
  • Multicellular (e.g., submandibular gland)
• Exocrine glands have a glandular portion and a duct portion, which can be classified as simple or compound.
• Modes of exocrine secretion:
  • Merocrine secretion (e.g., pancreatic acinar cells)
  • Apocrine secretion (e.g., mammary glands)
  • Holocrine secretion (e.g., sebaceous glands)

Sweat Glands

• Types of sweat glands:
  • Atrichial (formerly Eccrine) - found in the skin, functions in thermoregulation and excretion
  • Epitrichial (formerly Apocrine) - found in the armpits and around the anus, functions in releasing pheromones
• Sweat gland structure:
  • Secretory part (tubular, twisted into a ball, wrapped around by blood capillaries)
  • Excretory part (tubular, straight part)

Ceruminous Glands

• Specialized sweat glands located in the external auditory canal
• Produce cerumen (earwax) by mixing their secretion with sebum and dead epidermal cells
• Functions of cerumen:
  • Keeps the eardrum pliable
  • Lubricates and cleans the external auditory canal
  • Waterproofs the canal
  • Kills bacteria and traps foreign particles

Lacrimal Glands

• Produce tears, which have multiple functions:
  • Forms the watery part of the tear film
  • Cleanses the eyes
  • Moisturizes and lubricates the eyes
  • Rinses off allergens and irritants
  • Protects the surface of the eye against infections
• Tear film composition:
  • Oil layer (Meibomian glands)
  • Watery layer (tear gland)
  • Mucous layer (conjunctival goblet cells)

Mammary Glands

• Produce milk for the survival of newborn mammals
• Milk composition:
  • Casein (protein)
  • Lactose (carbohydrate)
  • Fat
  • Vitamins
  • Calcium
• Modes of milk secretion:
  • Apocrine at the beginning of secretions
  • Merocrine during intense lactation
  • Holocrine at the end of the lactation period
• Regulation of milk production:
  • Neural regulation (afferent part) induced by massage or other stimuli
  • Humoral regulation influenced by metabolic and stress hormones, directly by oxytocin and prolactin

Salivary Glands

• Types of salivary glands:
  • Large glands (sublingual, mandibular, parotid, zygomatic)
  • Small glands scattered throughout the oral cavity
• Saliva composition:
  • 99% water and 1% solids
  • Viscosity determined by mucin, glycoproteins, and other proline-containing proteins
  • Inorganic compounds (e.g., potassium, sodium, calcium, magnesium)
  • Metabolic end products (e.g., CO2, carbonic acid salts, urea)
  • Enzymes (e.g., ptyalin, salivary amylase)
• Functions of saliva:
  • Evaluates food
  • Aids in swallowing
  • Cleans the oral cavity
  • Protects against microorganisms
  • Protects against irritating substances
  • Participates in urea circulation (for ruminants)
  • Participates in thermoregulation

Regulation of Saliva Secretion

• Saliva secretion is regulated by the autonomic nervous system
• Both cholinergic and adrenergic neurotransmitters can stimulate the secretion of thick, protein-rich saliva
  • Cholinergic receptors (M-cholinergic) stimulate saliva secretion through Ca ion channels
  • Adrenergic receptors (α-adrenergic) stimulate saliva secretion through Ca ion channels
  • Adrenergic receptors (β-adrenergic) stimulate saliva secretion through cAMP

Sebaceous Glands

• Compound alveolar glands located in the dermis and open at the root of the hair
• Holocrine glands that secrete an oily substance called sebum
• Sebum secretion is stimulated by testosterone
• Increases during puberty
• Sebum consists mainly of triglycerides, cholesterol, esters, and other substances
• Forms a slightly oily film on the skin, keeping it elastic and preventing water loss

Physiology of Lactation

• Lactation is the combined process of milk secretion and milk removal
• Mammary gland structure:
  • Alveoli (acini): basic functional units
  • Lobules: clusters of alveoli
  • Lobes: collections of lobules
• Milk synthesis and secretion:
  • Alveolar cells produce milk
  • Myoepithelial cells contract to release milk
  • Milk letdown: oxytocin stimulates milk ejection
• Udder anatomy and blood flow:
  • Blood supply essential for mammary function
  • 500 liters of blood pass through the udder to produce 1 liter of milk
• Mammary gland development:
  • Rapid growth during fetal life, puberty, and pregnancy
  • Estrogen, progesterone, and prolactin stimulate growth and differentiation of secretory tissue

Lactation Performance

• Lactation curve: milk production increases to a peak and then decreases
• Maintenance of lactation:
  • Hormonal control of lactation: prolactin, oxytocin, and other hormones
  • Frequent milking or suckling stimulates milk production
• Milk composition:
  • Colostrum: high in fat, proteins, minerals, and vitamins; low in lactose
  • Regular milk: changes in composition throughout lactation
• Factors affecting milk composition:
  • Genetic factors
  • Interval between milkings
  • Stage of lactation
  • Age of the cow
  • Feeding regime
  • Disease
  • Completeness of milking### Milk Composition and Lactation
• Milk fat in marine mammals is used to build up blubber, a heat insulating layer of fat.
• In cows, lactose and dissolved ions make up most of the osmolarity, with 50% represented by lactose.
• Lactose and ion concentration are negatively correlated, meaning that if there is more lactose, there will be less ions.
• Lactose is the main determinant of the amount of milk secreted.
• Milk also contains vitamins, trace elements, and hormones needed by offspring.
• Milk composition is a factor that regulates growth rate, with faster-growing species having higher protein and mineral content in their milk.

Milk Ejection

• Milk ejection is the contraction of myoepithelial cells, which increases hydrostatic pressure in the alveoli.
• Teats have a high density of sensory nerve fibers, and sympathetic nerve fibers help regulate blood flow and contraction of teat sphincters.
• The mammary epithelial cells are not affected by the autonomic nervous system, but milk ejection is.
• When myoepithelial cells are relaxed, most of the secreted milk remains in the alveoli.
• During milking and suckling, myoepithelial cells contract and force milk from the alveoli into the excretory ducts.

Milk Ejection Reflex

• The milk ejection reflex is a neuroendocrine reflex that induces the emptying of mammary glands.
• The reflex is elicited by sensations on the udder and teats, which send nerve impulses to the CNS.
• Oxytocin is released from the posterior pituitary, binds to receptors on myoepithelial cells, and induces contraction.
• The contraction increases hydrostatic pressure in the alveoli, decreases resistance of the excretory ducts, and relaxes the sphincter muscle in the teat canal.
• It takes 60-90 seconds from initial stimulation to milk ejection, and oxytocin secretion lasts for a few minutes.

Lactation Curves

• Milk production increases in the early post-partum period, reaches a peak, and declines steadily.
• This is called a lactation curve.
• In dairy cows, peak lactation is usually 4-6 weeks after calving and declines until 40 weeks after calving.
• Some goats can start lactating in the absence of pregnancy.
• The lactation curve is influenced by factors such as genetic potential, feeding, milking regimes, and feeding regimes.

Metabolism during Lactation

• During lactation, uptake of glucose and amino acids in the udder and uterus is prioritized over other organs.
• Additional energy stores are taken from adipose and muscle tissue.
• Glucose utilization in other tissues is reduced, and amino acids are used for milk protein synthesis, with muscle proteins broken down and utilized.
• Homeorhesis, a coordinated redirection of metabolic resources, is regulated by hormones such as leptin, placental lactogen, PRL, and GH.

Pigeon Milk

• Pigeons and doves produce a milk-like substance, secreted by epithelial cells in the crop.
• The substance has a high protein and fat content, and contains IgA and some bacteria.
• It has a yellow cottage-cheese-like consistency and is regulated by PRL, similar to mammals.
• This is essentially bird lactation!