Anatomical and Histological Aspects of Endocrine system

The endocrine system The endocrine system includes all the gland which produced their hormonal secretion directly into the blood stream, therefore they usually known as ductless glands formed of groups of cells separated by numerous blood vessels.

They are of two types: (A) Pure endocrine glands: (1)Pituitary gland (2)Pineal gland (3) Hypothalamus are associated with the brain. (4) Thyroid gland (5)Parathyroid glands (6) Suprarenal glands (Adrenals)

(B) Organs containing endocrine cells: (1) Pancreas (2) Thymus (3) Gonads (T estis & Ovaries) They are also considered as mixed glands

Control of Secretion Rate CONTROL OF HORMONE SECRETION  Most hormones are controlled by negative feedback  Most hormones are not secreted at constant rate, but their secretion is regulated by three different methods 1.Humoral control: The action of a substance other than a hormone on an endocrine gland. As an increased blood glucose concentration stimulates insulin secretion from the pancreas

2.Nervous control : Stimuli such as stress activate sympathetic division of autonomic nervous system which stimulate the release of epinephrine and norepinephrine from the adrenal medulla. TSH [thyroid stimulating hormone] from anterior pituitary stimulates release of thyroid hormones from thyroid gland) 3.Hormonal control: Control of one endocrine gland by hormone secreted by another endocrine gland as TSH [thyroid stimulating hormone] from anterior pituitary stimulates release of thyroid hormones from thyroid gland)

I - Pituitary gland It is located in sella turcica of sphenoid bone, inferior to hypothalamus

It is composed of two lobes: 1)Posterior pituitary (neurohypophysis = Pars Nervosa): Secretes neurohormones 2) Anterior pituitary (adenohypophysis) –Consists of three areas with indistinct boundaries: pars distalis, pars intermedia, pars tuberalis

Neurohypophysis (Posterior Pituitary) Pars Nervosa  It is connected to hypothalamus by hypothalamo- hypophyseal tract  It consists of: Nerve fibers (axons of neurosecretory cells present in hypothalamus) Nerve fibers (axons of neurosecretory cells present in hypothalamus) Pituicytes (glial cells that support nerve fibers)Pituicytes (glial cells that support nerve fibers) Herring bodies: accumulation of secretionHerring bodies: accumulation of secretion Blood capillariesBlood capillaries

It acts primarily as a storage and releasing area for hormones actually which are secreted in hypothalamic nuclei.It acts primarily as a storage and releasing area for hormones actually which are secreted in hypothalamic nuclei. These hormones are: Oxytocin and Antidiuretic hormone (ADH or “vasopressin”)These hormones are: Oxytocin and Antidiuretic hormone (ADH or “vasopressin”) (1) OXYTOCIN HORMONE in pregnant or nursing (lactating) women: 1- It stimulates contraction of smooth muscle of uterine wall during labor 2- It stimulates ejection of milk in lactating mothers

(2) Antidiuretic Hormone (ADH) Functions of ADH: 1- It directly affects blood pressure - acts as powerful vasoconstrictor  increases blood pressure (hence name “vasopressin”) 2- It affects water balance: acts on tubules of kidney to increase reabsorption of water  less water lost in urine

Disorders of ADH secretion :Disorders of ADH secretion : –Hyposecretion due to damage of hypothalamic nucleus or neurohypophysis--> diabetes insipidus - excessive urine production (polyuria) and thirst –Hypersecretion --> SIADH (syndrome of inappropriate ADH secretion) - water retention, headache, cerebral edema, weight gain, hypoosmolarity

Adenohypophysis (Anterior Pituitary)  It is linked to hypothalamus via hypothalamo- hypophyseal portal circulation.  It carries regulatory hormones from hypothalamus to pituitary. Which are either releasing hormones stimulate secretion of pituitary hormonesreleasing hormones stimulate secretion of pituitary hormones inhibitory hormones inhibit secretioninhibitory hormones inhibit secretion  It consists of epithelial cells separated by blood capillaries

–It Consists of three areas with indistinct boundaries: pars distalis, pars intermedia, pars tuberalis (1) Pars Distalis Cells of pars distalis can be differentiated on the basis of the staining properties of the granules in the cells. These granules contain the hormones.

Basically, there are two groups of cells in Pars distalis: Chromophobes and Chromophils. Chromophobes and Chromophils. Chromophobes: have no secretory granules. They are probably either undifferentiated cells or exhusted cells = Chromophobes 2= Basophils 3= Acidophils

Chromophils contain granules which stain either: Acidophilic (red) and are called acidophils (or alpha cells). Or Basophilic (blue) and are called basophils (or beta cells). Functions of Acidophils: it secretes 1- Growth hormone (GH) = somatotropin which act to: 1.Stimulate protein synthesis & growth in most tissues 2.Promote bone and cartilage growth 3.Stimulate glucose synthesis by liver Its function is stimulated by GHRH (growth hormone releasing hormone) and inhibited by Somatostatin

Increased its secretion leads to Giagantism before puberty:, and Acromegaly after puberty. Decreased secretion leads to Dwarfism 2- Prolactin (PRL): which acts to Stimulate milk formation by mammary glands. Increased its secretion leads to In ♀ : amenorrhea & infertility In ♂ : gynaecomastia. Decreased secretion leads to Decreased milk secretion after labour

Functions of Basophils: it secretes 1- Thyroid stimulating hormone (TSH) which act to: Stimulate thyroid gland to synthesize T3 & T4 Its function is stimulated by TRH (thyrotropin releasing hormone) Increased its secretion leads to Hyperthyroidism: Insomnia, ↑↑ heart rate, loss of body weight, irretability. Decreased secretion leads to Hypothyroidism: Laziness, ↑↑ body weight

2- Adrenocorticotropin (ACTH) = corticotropin which act to: Stimulates secretion of Corticosteroids from adrenal gland Its function is stimulated by CRH (corticotropin releasing hormone) Increased its secretion leads to cushing,’s syndrom. Decreased secretion leads to adrenal gland insufficiency

3- Gonadotrophins: they are 2 types. Both are stimulated by Gonadotropin releasing hormones (GRH) A- Follicle stimulating hormone (FSH) which act to: In ♀ : Stimulates growth of ovarian follicles In ♂ : Stimulates Spermatogenesis Decreased its secretion leads to In ♀ : amenorrhea & infertility In ♂ : infertility.

B- Luteinizing hormone (LH) : which act to: In ♀ : Stimulates ovulation & formation of corpus luteum In ♂ : Stimulates secretion of testosterone Decreased its secretion leads to In ♀ : amenorrhea & infertility In ♂ : ↓↓ ♂ sex characters

(2) Pars Intermedia: It secretes MSH (melanocyte stimulating hormone); It secretes MSH (melanocyte stimulating hormone); which stimulate melanin secretion. hyperpigmentation. Increased its secretion leads to hyperpigmentation. (3) Pars tuberalis. Most of these cells secrete gonadotropins

Thyroid Gland  It presents on thyroid cartilage of the larynx in front & in both sides of trachea.  It consists of 2 main lobes connected by isthmus  Its function is: 1.Secretion of Thyroxine: Tetraiodothyronine (T4) and Triiodothyronine (T3) {both control metabolism} 2.Secretion of Calcitonin Involved in calcium homeostasis Secretion

Each follicle has a center filled with acidophilic colloid which contains thyroid hormones and surrounded by 2 types of cells: 1- Follicular cells: 97 % of cells. They are cubical cells with central rounded nuclei and basophilic cytoplasm.`  Histologically It is formed of stroma & parenchyma. 1- Stroma: capsule (2 capsules), trabeculae (dividing it into lobules) & reticular network. 2- Parenchyma: composed of follicles separated by many capillaries.

2- Parafollicular cells: large pyramidal cells resting on basement membrane & not reach the lumen. They have basal oval nuclei & pale cytoplasm.

Functions of thyroid gland (A)Follicular cells secrete 2 hormones:  Triiodothyronine or T3 -less produced  Tetraiodothyronine or T4 more produced (B) Parafollicular cells. Secrete calcitonin which reduces [Ca2+] in body fluids when Ca levels are elevated

Functions of T3 & T4 1.Maintain normal rate of metabolism. 2.Increase the activity of Na + -K + pump which increases body temperature. 3. Growth and maturation of bone, hair, teeth, CT & nervous tissue require thyroid hormone. 4. Both T 3 and T 4 play a permissive role for GH and GH does not have its normal effect on tissues if T 3 and T 4 are lacking.

Regulation of T 3 and T 4 Secretion

Functions of Calcitonin: It decreases Ca 2+ concentration through its effect on: Bone: 1.Decreases osteoclast activity 2.Stimulates osteoblasts Intestine: inhibit Vitamin D so decrease Ca intestinal reabsorption Kidney: ↑↑ Ca ion excretion Regulation of Calcitonin Secretion Secretion triggered by high Ca 2+ concentration in blood.

Abnormalities of thyroid function (A) Hyperthyroidism: Increased levels of thyroid hormones in the blood. Symptoms may include weight loss, nervousness, tremors, excessive sweating, increased heart rate and blood pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid gland (goiter). The disease may be controlled with medications or by removal or destruction of the thyroid gland through surgery or radiation treatments.

(B) Hypothyroidism: Levels of thyroid hormones in the blood are abnormally low. (A) in a child the condition known as Cretinism, which, if untreated, results in a person of very small stature with learning disabilities. (B) In an adult, the condition known as myxoedema. Symptoms laziness fatigue, a slow heart rate, dry skin, weight gain, constipation, and, in children, slowing of growth and delayed puberty.

Parathyroid Glands

Functions of Parathormone (PTH) Regulates calcium homeostasis: Raises serum calcium, lowers serum phosphate - PTH increases calcium levels through: 1) stimulates osteoclasts to release calcium from bones 2) decreases secretion of calcium by the kidney 3) activates vit D, which stimulates uptake of Ca by the intestine

 Histologically: It is formed of stroma & parenchyma. 1- Stroma: capsule, trabeculae & reticular network. 2- Parenchyma: composed of cell cords separated by many capillaries). Each cord composed of 2 types of cells: Chief cells: 98%small produce PTH. They have deeply basophilic cytoplasm. Oxyphil cells 2%, large & acidophilic. Most likely they are immature or inactive chief cells

The Adrenal (Suprarenal) Glands It is formed of inner medulla; outer cortex Adrenal Cortex It is formed of groups of cells separated by blood sinusoids. They are arranged in 3 zones: –Zona glomerulosa –Zona fasciculata –Zona reticularis

1- Zona glomerulosa: -It is formed of columnar cells arranged in the form of arches. –Main hormone: Aldosterone. –General function: Maintain blood electrolyte balance (Na & K ions). It Increases rate of sodium reabsorption by kidneys increasing sodium blood levels –Main control: Na & K blood levels.

2- Zona Fasciculata –Cells are arranged in straight columns. Cells are steroid secreting acidophilic cells contain many SER, Mitochondria, cholestrol (dissolves during preparation so cells appear vacuolated and called Spongiocytes) –Main hormone: Cortisol (a glucocorticoid).

–General function of cortisol: 1.Gluconeagenesis: formation of glucose from metabolism of carbohydrates. 2.Lipolysis: breakdown of the lipids for production of energy 3.Stimulating breakdown of protein releasing amino acids for formation of enzymes. 4. Delay wound healing 5.Suppress immune responses 6.Decreases inflammatory response. –Main control: ACTH from anterior Pituitary.

3- Zona Reticularis –Cells are arranged in branching and anastomosing cords. –Hormones: Some cortisol and androgens (sex hormones). –Control of action: ACTH from anterior pituitary.

Regulation of Cortisol Secretion

1.Rapid weight gain: particularly of the trunk and face with sparing of the limbs (central obesity), on the back of the neck ("buffalo hump”), and on the face ("moon face").central obesitymoon face 2.Moodiness, irritability, or depression 3.Memory and attention dysfunction 4.Muscle and bone weakness 5.Osteoprosis 6.Diabetes mellitus 7.Hypertension Abnormalities of Adrenal cortex function Cushing’s syndrome: due to increased cortisone level, Its signs and symptoms are:

8.Immune suppression 9.Sleep disturbances 10.Menstrual disorders such as amenorrhea in womenamenorrhea 11.Decreased fertility in men 12.Hairsutism 13.Hypercholesterolemia Abnormalities of Adrenal cortex function

Adrenal Medulla Cells are arranged in branching and anastomosing cords separated by blood capillaries. It secrets neurohormones: epinephrine and norepinephrine. Their function is Acute response to stress

Main control: Preganglionic sympathetic innervation Effects of Epinephrine and norepinephrine: Increase heart rate and force of contraction. Cause blood vessels to constrict in skin, kidneys, gastrointestinal tract, and other viscera so increase blood volume. Causes dilation of blood vessels in skeletal muscles and cardiac muscles. Increases blood levels of glucose Increases fat breakdown in adipose tissue enhances the force of muscle contractions

The Pineal Gland Is small gland located near the center of the brain and attached to the posterior end of the roof of the third ventricle in the brain. It is stimulated by nerves from the eyes. It secretes Melatonin which Regulates human sleep-wake cycles

It is Stimulated by darkness, inhibited by light Functions of Melatonin 1.Inhibits early puberty 2.Day/night cycles: Timing of sleep, body temperature, appetite. 3.Antioxidant