Group 6 Endocrine System: Growth Hormone
E-Facilitator: Cathy
Learning Outcomes
  • Identify and locate the endocrine gland which secretes the Growth Hormone (GH)
  • Discuss normal GH physiology over the lifespan
  • Describe homeostatic imbalances that are caused by hyper or hyposecretion of GH
  • Highlight the effects seen in your elderly dental patients with the cessation of GH production and secretion

Identify and locate the endocrine gland which secretes the Growth Hormone (GH)

Growth hormone (GH) is protein hormone produced by cells called somatotrophs of the anterior lobe of the hypothalamus (Chiras 1995, Marieb 2007). The Hypothalamus lies just above the pituitary gland which is in the brain (Figure 1.1). It contains the receptors that that monitor blood level of hormones, nutrients and ions. The pituitary gland is attached to the underside of the brain by a thin stalk . About the size of a pea, the pituitary lies in a depression in the base of the skull, the sella turcica.The gland is divided into two major parts: the anterior pituitary and the posterior pituitary (Chiras 1995).

Growth hormone is secreted by the anterior pituitary or adenohypophysis which is controlled by the region of the brain , the hypothalamus (Chiras 1995). This gland also secretes at least five other different hormones directly into the blood. (Campbell & Reece 2005). .
pituit1.gif
Figure 1.1: Location of the pituitary gland relative to the rest of the brain
Reference: http://www.becomehealthynow.com/article/bodyendocrine/733/

Discuss normal GH physiology over the lifespan

GH stimulates cell growth. Primary targets are muscle and bone, where GH stimulates amino acid uptake and protein synthesis. It also stimulates fat breakdown in the body (Chiras 1995). The release of GH is controlled by two hormones: a releasing hormone and an inhibiting hormone. The releasing hormone circulates to the anterior pituitary and stimulates the release of GH. The inhibitng hormone prevents the release of GH (Tortora 1994).

GH acts on a wide range of target tissues and has both tropic and nontropic effects.
  • Its major tropic action is to signal the liver to release insulin-like growth factors which circulates in the blood and directly stimulate bone and cartilage growth. In the absence of GH, the skeleton of an immature animal tops growing. Injecting the hormone into the experimentally deprived of its own supply partially restores the animal's growth.
  • GH also exerts diverse metabolic effects that tend to raise blood glucose, thus opposing the effects of insulin. (Campbell & Reece)

GH secretion undergoes a daily cycle. As the highest blood levels are present during sleep and strenuous exercise. It is no wonder that sleep is so important to a growing child (Figure 1.2). Growth hormone secretion declines gradually with the increase in age.

sleeping.jpg
Figure 1.2: Sleep is so important to a growing child


Reference: http://www.co.cowlitz.wa.us/health/environmentalhealth/PHEPR/StopGerms.html
GH is controlled by a negative feedback loop. Hypoglycemia, that is, low blood sugar level can stimulate the secretion of GH. When blood sugar level is low, the hypothalamus is stimulated to secrete the releasing hormone which causes the release of GH. This hormone will in turn raise blood sugar level by converting glycogen into glucose and releasing it into the blood. As soon as the blood sugar level returns to normal, releasing hormone secretion shuts off. This is a negative feedback system (Tortora 1994). Its release can also be stimulated by through the nervous system. For example, stress and exercise (Chiras 1995).

Secretion of GH peaks at the end of the adolescent growth spurt. Genetic difference control when this will occur and thus account for differences in height.
The rate of growth is not continuous nor are the factors responsible for promoting growth same throughout the growth period. GH plays no role in fetal development as it is largely reliant on the secretion of hormones from the placenta which is determined by genetic and environmental factors. After birth, GH and other nonplacental hormonal growth factors begin to play an important role at regulating growth.

Children display two periods of rapid growth
  • Postnatal growth spurt, during their first 2 years of life
  • Pubertal growth spurt, during adolescence.
From the age of 2 until puberty, the rate of growth is linear. Some evidence indicates that GH secretion is elevated during puberty and thus may contribute to growth acceleration during this time (Sherwood 2004)

GH secretion is continual even in adults whom growth has already ceased. This implies that this hormone has important influences other than on growth. In addition to promoting growth, GH has important metabolic effects and enhances the immune system.

Metabolic actions unrelated to growth

GH increases fatty acid levels in the blood by enhancing the breakdown of triglyceride fat stored in adipose tissues and it increases blood glucose levels by decreasing glucose uptake by muscles. The metabolic effect of GH is to mobilise fat stores as a major source of energy while conserving glucose for glucose-dependent tissues like the brain. This metabolic pattern is suitable for maintaining the body during prolonged fastings.

Growth-promoting actions of GH on soft tissues

GH promotes growth in soft tissues by
  1. increasing the number of cells (Hyperplasia).
GH increases the number of cells by stimulating cell division and preventing apoptosis, which is programmed cell death.
2. increasing the size of the cells (Hypertrophy).
GH increases the size of cells by favouring synthesis of protein, the main structural component of cells (Sherwood 2004)

Growth-promoting actions of GH on bones

GH promotes growth of bone in both thickness and length. It stimulates osteoblast activity and proliferation of epiphyseal cartilage, thereby making space for more bone formation. GH can promote lengthening of long bones as long as the epiphyseal plate remains cartilaginous. At the end of adolescence, under the influence of the sex hormones these plates ossify so that the bones can no longer lengthen despite further presence of GH. Thus, after the plates are closed the individual does not grow any taller (Sherwood 2004).


Describe homeostatic imbalances that are caused by hyper or hyposecretion of GH

Hypersecretion and hyopsecretion of GH can result in structure abnormalities. (Marieb, 2007) The differences between normal GH secretion, hypersecretion (too much) and hyposecretion (too little) is expressed easily in the Figure 1.3 below.

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Figure 1.3: Differences of GH secretion
Reference: http://cache.eb.com/eb/image?id=93313&rendTypeId=34
  • Hypersecretion of GH during childhood can lead to gigantism, in which a person grows unusually tall - as tall as 8 feet (2.4m) - though body proportions remain relatively normal. (Figure 1.4)
  • Excessive production of GH in adulthood, a condition known as acromegaly, stimulates bony growth in a few tissues that are still responsive to the hormone , such as in the face, hands and feet. This is caused by a adenohypophyseal tumor that continues to produce and secrete excessive amounts of the GH. The tumor can be surgically removed but the additional effects it had on the body can not be reversed.

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Figure 1.4: Homeostatic Imbalance- Hypersecretion of GH

Reference:http://english.people.com.cn/200703/29/images/09030428143236189988.jpg

Hyposecretion of GH in childhood retards long-bone growth and can lead to pituitary dwarfism. Individuals with this disorder generally reach a height of only about 4 feet (1.2 m), though body proportions remain relatively normal. If diagnosed before puberty, pituitary dwarfism can be treated successfully with human GH (Figure 1.5). An autopsy of a 78-year-old dwarf found he had an autosomal recessive gene trait from hypothalamic GH deficiency which could have been the cause the individuals decreased size and growth (Aminoff et. al, 2004).

wee-man.jpg
Figure 1.5: Homeostatic Imbalance: hyposecretion of GH

Reference: http://wee-man.com/gallery/c.jpg

Highlight the effects seen in your elderly dental patients with the cessation of GH production and secretion//

The effects that can be seen in the cessation of growth hormone either being in a patient that is of an elderly age can be observed as:
  • a decrease of lean body mass
  • reduced development of bone mass
  • usually a decrease in fat mass
  • frailty
  • muscle deterioration
  • increased frequency of fractures
  • fracturing of jaw during tooth extraction
  • disordered sleep

As the speed of GH secretion from the anterior pituitary is highest around puberty it declines progressively thereafter. The cause of the normal age-related decrease in GH secretion is not well understood, but is thought to result from the GH-inhibiting hormone. Research has now been widely established on the effects of GH on the elderly for its beneficial growth of skeletal muscle, but even though there is an increase in muscle mass their strength is not directly affected. A study by Rudman et al., 1990 was conducted where men were over the age of 60 years underwent a GH treatment to see the effects on the human body which had the following results: 8.8% increase in lean body mass, 14.4% decrease in adipose-tissue mass, and a 1.6% increase in average vertebral bone density and vast increase of skin thickness of 7.1%. (Bowen 2003 & Lee, Liao, Mukherjee 2004).

Some other effects that can be seen in elderly patients is the condition known as acromegaly where during adulthood the patient had an excessive production of GH which stimulated bony growth in tissues that are still responsive to the hormone such as in the face, hands and feet (Figure 1.5). This could have an effect on the treatment for the elderly patient becasue if they have excessive growth on their face, cheeks or jaw they may need specialist dental treatment.

uchr_01_img0062.jpg
Figure 1.5: The condition acromegaly - excessive growth of the face, hands or feet

Reference: http://www.faqs.org/health/images/uchr_01_img0062.jpg

Reference List
Aminoff, M, Boller, F, Swaab, D (2004) Handbook of Clinical Neurology: The human Hypothalamus, Volume 80, 3rd Series, Volume 2, Elsevier, Netherlands
Bowen R, (2003) Growth Hormone and Aging, The Hypothalamus and Pituitary Gland: Introduction and Index, viewed online 13/9/2007
http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/hypopit/ghaging.html
Campbell, N.A., & Reece, J.B. (2005). Biology (7ed.International ed.). San Francisco: Pearson Education Inc.
Chiras, D.D, (1995) Huamn Biology Health, Homeostasis, and the Environment (2 ed) West Publishing Company, United States of America.
Lee, K, Liao, L, Mukherjee, J (2004) Does growth hormone prevent aging in the healthy elderly with low serum insulin-like growth factor-I? Mechanisms of Ageing and Development Volume 125, Issue 4, Pages 291-295, Viewed online 13/9/2007
<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T31-4BRJYSK-2&_user=162644&_coverDate=04%2F30%2F2004&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000013138&_version=1&_urlVersion=0&_userid=162644&md5=68b6621220512f3b0437d2abde3defe1>
Marieb, E, Hoehn K, (2007) Human Anatomy & Physiology (Seventh Edition) San Francisco, Pearson Educataion, Inc.
Sherwood, L. (2004) Human Physiology From Cells ro Systems (5 ed) Brooks/Cole Thomson Learning Inc. United States of America.
Tortora, G.J. (1994) Introduction to the Human Body. The essentails of human anatomy and physiology (3 ed) Haper Collins College Publishers, New York.