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Thread: Anterior Lobe Alpha Cells 4 picture - Endocrine Histology Atlas

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    Default Anterior Lobe Alpha Cells 4 picture - Endocrine Histology Atlas

    The endocrine system consists of the ductless glands. These glands produce chemical messengers called hormones which pass into the bloodstream for circulation throughout the body. Hormones will excite or inhibit the activity of target organs or tissues. Whether or not a target tissue will be affected by a hormone depends upon the presence of specific receptor molecules on the membranes or in the cytoplasm of the target cells.

    When a hormone is present in excess the number of receptor molecules on the target cells will decrease. This "down regulation" will reduce the responsiveness of the target cells to the hormone. However, when a hormone is at a very low level in the blood, the number of receptor molecules on the target cells may increase. This is known as "up regulation". As a result, the target tissue becomes more sensitive to the lower levels of that hormone.

    Basically there are three types of hormone molecules:

    1. Protein hormones are large, complex molecules made up of many amino acids joined together. These molecules range in size from those hormones composed of 8 to 10 amino acids, e.g., oxytocin and vasopressin (ADH) to the very large molecules of insulin and growth hormone.

    2. Amine hormones are derived from a single modified amino acid, e.g., the catecholamines (epinephrine and dopamine) are produced from tyrosine.

    3. Steroid hormones are manufactured from cholesterol, a fat soluble substance.

    The glands of internal secretion (endocrine) are concerned with the control and coordination of processes which are widespread in the body, such as, Metabolism, Growth, Homeostasis, Adaptation to Stress and Sexual Reproduction. Although each endocrine gland has specific functions, all are interdependent. The activity or lack of activity of one gland usually influences the rest of the system.

    Control of Hormone Secretions

    Hormone secretion is stimulated and inhibited by three types of signals:

    1. Neuronal signals are basically nerve impulses which control hormone secretion. For example, sympathetic nerve stimulation of the adrenal medulla causes the release of epinephrine into the blood.

    2. Hormonal signals - Hormones can stimulate other endocrine tissues to release their hormones. ACTH from the anterior pituitary stimulates the release of steroids from the adrenal cortex.

    3. Humoral signals - The presence or absence of a particular chemical substance in the blood can bring on the release of a given hormone from an endocrine gland. An example of this type of mechanism can be seen in the effect of high blood calcium producing the release of calcitonin from the thyroid.

    Negative and Positive Feedback

    The most common method of controlling endocrine secretions is through negative feedback. When a stress, such as low blood sugar, triggers the release of a hormone (glucagon in this case) a response is produced in the body, i.e., an increase in blood sugar. If the response reduces the stress, then the secretion of hormone will also be reduced - negative feedback. Less often, the effect of the hormone will enhance or intensify the original stress. In this positive feedback condition, the mechanism is usually shut off by an outside event. For example, the release of the hormone oxytocin increases during labor due to the increased pressure on the uterine wall. The birth of the baby shuts down the mechanism.

    PITUITARY GLAND - The Anterior Lobe

    The pituitary gland is attached to the median eminence of the hypothalamus by the stalk-like infundibulum. The anterior lobe of this gland accounts for about 75% of the weight of the pituitary and is derived from a pocket in the roof of the embryonic mouth (Rathke's pouch). In addition to a direct arterial supply, the anterior lobe of the pituitary receives blood through the hypothalamic-hypophyseal portal system from the hypothalamus. Secretion of the hormones produced by the anterior pituitary is controlled by Releasing and Inhibiting factors released from the hypothalamus into the hypothalamic portal vessels. The principal hormones produced by the anterior lobe of the pituitary are:

    1. Corticotropin (ACTH) - is released in response to all forms of stress, i.e., pain, trauma, cold, hypoglycemia, fear and anger. It targets the cells of the adrenal cortex. Under the influence of ACTH, the adrenal cortex (zona reticularis) releases cortisol along with some aldosterone. Over-production of ACTH leads to Cushing's disease.

    2. Somatotropin (Growth hormone) - is released during the active growth years and throughout life. A powerful stimulant for its release is the condition of hypoglycemia. Growth hormone has a wide variety of metabolic effects. In general, it operates through two pathways. First, it has a direct effect on the metabolism of proteins, fats and carbohydrates. These activities lead to an increase in blood sugar, cellular uptake of amino acids and lipolysis. Indirectly, growth hormone stimulates the liver to release a class of growth stimulating substances called Somatomedins. The somatomedins target the growing tissues of the body, especially the skeletal system.
    Anterior Lobe Alpha Cells picture attachment.php?s=7b83a548b47095cd5f0d15b2e1dfd506&attachmentid=1326&d=1439065475
    Hyposecretion of growth hormone during childhood leads to dwarfism. Oversecretion during childhood produces unchecked body growth - giantism. Oversecretion during adulthood produces the condition called acromegaly in which the victim exhibits overgrowth of the mandible ("lantern jaw") and brow ridges.

    3. Thyrotropin (Thyroid Stimulating Hormone) - acts on the thyroid gland to stimulate the synthesis and secretion of thyroxine. A major factor leading to the release of TSH would be a hypothermic condition in the body. An autoimmune condition which prevents the thyrotropic cells of the anterior pituitary from recognizing the level of TSH in the blood leads to oversecretion of TSH. This is Grave's disease, a condition marked by overproduction of thyroxine, exophthalmia and goiter.

    4. Prolactin or lactogenic hormone initiates and maintains milk secretion. It also seems to be responsible for the stability of the ovarian corpus luteum. Prolactin release is triggered by a releasing factor from the hypothalamus. The inhibiting factor for prolactin appears to be dopamine. During the late secretory phase of the menstrual cycle, a drop in the blood level of estrogens and progesterone leads to an increased release of prolactin. The resulting increase in lactogenic activity may be responsible for the breast tenderness many women experience at this time.

    5. Gonadotropic hormones - The anterior pituitary releases two hormones which target the gonads:
    a. Follicle Stimulating Hormone (FSH) - facilitates the development of ovarian follicles and sperm production. Its release is controlled partly by blood levels of gonadal steroids.
    b. Luteinizing hormone (LH) - controls ovulation, the development of the corpus luteum and estrogen secretion in the female. In the male, this hormone is called Interstitial Cell Stimulating hormone and stimulates the testis to produce testosterone from the cells located between the semeniferous tubules (interstitial cells).

    Pituitary Gland - The Posterior Lobe

    The posterior pituitary or neurohypophysis derives from the hypothalamus of the embryonic brain. The activity of the posterior lobe is controlled directly by nerve cells whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus. The posterior pituitary secretes the hormones vasopressin (ADH) and oxytocin. Vasopressin is released under the conditions of low blood volume (hypovolemia) and increased osmotic pressure of the blood. The latter condition may be due

    1. loss of body water from sweat, expired air, feces (especially during
    diarrhea), excessive urine production (diabetes).

    2. Lack of fluid intake.

    3. Increased intake of salt.

    Vasopressin is produced in cells of hypothalamic nuclei and travels within the axons of these cells through the infundibular stalk. It is stored and released from the posterior lobe and causes an increased reabsorption of water by the kidney tubule cells into the blood.

    Oxytocin is also produced by the hypothalamic nuclei and released from the posterior pituitary. The stimuli for this release include sensory nerve impulses from the uterus during the last stages of pregnancy. This will result in a reflexive contraction of uterine muscle leading to the birth of the child. Another source of stimuli for the release of this hormone originates from the nipples of a women who is nursing a child. Oxytocin will cause a contraction of the myoepithelial tissue in a lactating mammary gland forcing milk into the baby's mouth (milk letdown)


    Last edited by Medical Photos; 08-08-2015 at 08:24 PM.

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