Hormonal Regulation of Growth

Hormonal Regulation of Growth Objectives • Define hormone action • Explain the 5 types of hormone action • Know the function and effects of hormones in detail Hormone Action • Definition o Chemical messengers secreted by various tissues, not necessarily secreted by ductless glands. o Hormones act in an endocrine manner when secreted by cells and then transmitted via the bloodstream to act on distant target cells Types of action • Endocrine-secretions come from cells and are then transmitted via the blood stream Types of action • Neurocrine o Hormone is synthesized in a cell body of a neuron and stored in axons such as neurotransmmitters, but secreted into the bloodstream to act on distant target cells o A key regulator of animal growth and development by the hypothalamic-pituitary-peripheral gland axes • Neuroendocrine system o relies on close coordination between the nervous and endocrine system Types of action • Local conveyance- acting near the site of secretion o Paracrine- when a hormone from one cell is conveyed to an adjacent cell of different type over a short distance via interstitial fluid o Autocrine – where a hormone from one cell acts on itself or a neighboring cell of the same type o Intracrine – acts intracellularly and does not require secretion to alter the process Types of action • Tissue specificity o Allows hormones to act on target tissues without affecting other tissues or organs o Receptors – has an affinity for specific hormones that may be located at the cell o Hormones will bind and act through various enzyme systems, ion transport, or gene regulation o Negative feedback loops may also regulate hormonal function Chemical Nature of Hormones • Classification o Peptides/amino acid derivatives • Water soluble • Ex. Thyroxine, LH, FSH o Steroid/cholesterol derivatives • Ex. Estrogen, testosterone, progesterone • Fat soluble Hypothalamic-PituitaryPeripheral Gland Axis • Hypothalamus – the central organ of the neuroendrocrine system o Secretions regulate the secretions from the pituitary o Located at the base of the brain o Two sections: adenohypophysis and neurohypophysis Androgens • Two types: testicular and adrenal • Testicular hormones are testosterone and androstenone • Testosterone is produced in the Leydig cells of the testes. • Androstenone is a pheromone o Known to contribute to the boar taint odor in pork Androgens • Androstenone is stored in the salivary gland and accumulates in fat depots • Growth effects are seen by the influence of testosterone of bone and muscle. o This is seen by the increasing deposition of bone salts. Thus, increased bone mass is seen more in males Androgens • Muscle development is seen through androgen secretions in three ways o In utero, declines after birth, and increases at puberty o Prenatal androgens affects myogenesis o Castrated males have lower circulating GH than intact males o Androgens increase both protein synthesis and degradation, yet synthesis is stimulated more Androgens • Androgens synthesis induces the development of mature male characteristics such as: larger muscles in the forequarter, neck and crest region. • Castration diverts energy from growth of muscle development to fat deposition • Castration helps improve quality by less muscle and more fat development at an earlier age Estrogen • General classification for three hormones: Estrone, Estriol, Beta-estradiol • Responsible for: growth, maturation of repro tract, female behavior, mammary development • Impact: bone, fat, and muscle tissue growth • Females have shorter skeletons due to: earlier epiphyseal closure that is a result of chondrocyte proliferation and a function of bone formation Estrogen • Facilitates fat deposition • Anabolic for ruminants • Effective in castrate males for growth, yet is less effective in non-ruminants • Have little effect on intact males • In steers, estrogens increases muscle protein