8.14 Puberty and Growth Flashcards
What are the 4 distinct phases of growth
fetal, infantile, childhood and pubertal
Describe the fetal growth phase
The fetal growth phase is dependent on the uterine environment, factors such as placental sufficiency and maternal health which dictate growth factors
Describe the infantile growth phase
The infantile growth phase is dependent on nutrition, health and thyroid hormones. It is the fastest period of growth and by 2, half of the adult heigh is obtained. The rate of infantile growth is related to the rate of uterine growth but it is independent of growth hormone levels
Describe the childhood growth phase
Growth hormones are the dominant controlling factor in the childhood growth phase, it is also dependent on thyroid hormones, genetics and health. It is a longer, slower period of growth.
Describe the pubertal phase of growth.
It is dependent on the secretion of sex steroids (testosterone and oestrogen) and growth hormones. The pubertal growth phase is later in males than females
Describe the general patterns of growth
Growth is linear but episodic and the specific episodes have different influences. Growth is not unlimited
Describe the growth hormone-IGF axis
Growth hormone releasing hormone (GHRH) and somatostatin (SRIF) are the two hormones released from the hypothalamus that control the release of growth hormone. GHRH promotes the release of growth hormone from the pituitary gland and SRIF inhibits the release. Growth hormone promotes the release of IGF-1 (insulin like growth factor 1) from the liver. The IGF-1 then targets growth plates in long bones.
Describe the negative feedback loops that regulate growth hormone secretion
One central negative feedback loop is when SRIF neurons detect the growth hormones, they secrete SRIF back to the pituitary which in turn reduces the secretion of growth hormone.
One peripheral feedback loop is when IGF-1 reaches the anterior pituitary gland which then down regulates growth hormone production.
There are other more complex negative feedbacks with neurotransmitters in the CNS and with ghrelin and leptin hormones.
Describe the nature of release of the different growth related hormones throughout the day
There is pulsatile release of GHRH, with peaks during sleep. There is cyclic release of somatostatin. These contorl the release of growth hormones which is released at pulsatile points throughout the day, peaking during sleep with similar patterns to GHRH.
What are some factors that control the peaks and troughs of growth hormone release
time of day, puberty, exercise, sleep and interaction of drugs
Describe the downstream effects of IGF-1 on bone growth
IGF-1, along with other IGF binding proteins, have endocrine and paracrine effects that target the growth plates at long bones causing growth. In the bone growth, both IGF-1 and growth hormone allow for chondrogenesis. Growth hormone is needed to initiate and sustain long bone growth and IGF is needed to sustain hypertrophy and proliferation. After puberty the growth plates fuse and there is cessation of growth.
Describe the different ways of looking at growth defects in the growth hormone-IGF axis
The classical, hormone centric view that includes growth hormone deficiencies at the pituitary gland, growth hormone insensitivities at the peripheral tissue and IGF-1 defects at the growth plate.
The more modern view which focusses on IGF deficiencies. Either a primary IGF deficiency, which is associated with normal/increased growth hormone levels such as problems at growth hormone receptor, post receptor defects or IGF-1 defects. And a secondary IGF deficiency which is associated with low growth hormone levels such as hypothalamic and pituitary defects.
How are growth deficiencies viewed in a clinical context
There is a spectrum of disease causing short stature, that range from growth hormone deficiency to severe primary IGF deficiency
How is puberty started
Puberty is started by an increase in the pulsatile release of gonadotrophin releasing hormone (GnRH) which results in pulsatile changes in the secretion of LH and FSH
Briefly describe regulation of gonadotrophin release during puberty
Neuroendocrine regulation of gonadotrophin secretion is complex, but there is a balance of stimulatory and inhibitory signals. Both peripheral and CNS factors which influence the GnRH neuron, overall leading to the increased pulsatile secretion of gonadotrophins during puberty.
What are some physical changes in females during puberty
Physical changes in puberty are variable in time spans and intensity but the order remains the same. Some physical changes in females include maturation in uterus from a cylindrical shape to a pear shape and maturation in the ovaries leading to an increase in size.
What are some factors that impact the time of onset for puberty and its progression.
genetics, growth, body fat (obesity linked to earlier entry into puberty) , diet, stress, gonadal steroids, endocrine disruptors and energy expenditure
What are some timing markers for pubertal onset
for girls breast development - precocious is less than 8 years, early is 8-10 and delayed is 13+ years
for boys testicular enlargement - precocious is less than 9 years, early is 9-11 years and delayed is 14 + years
What is an interaction of sex hormones and growth that can explain puberty differences between boys and girls
Exposure to oestrogen has been linked with reduced growth hormone secretion thus causing growth plate fusion. This is a contributing factor to female male height difference. The patterns in serum IGF-1 levels also vary in boys vs girls
Describe the difference between growth and stature
Growth is a dynamic process with lots of variation and is different to stature as it includes types of growth other than height. The assessment of growth requires follow up in order to identify trends.
How is height distributed
Height is distributed on a normal curve and be compared to age to give percentiles
What are some factors behind atypical growth
-an underlying pathology (e.g neoplastic conditions, endocrine dysfunction or chronic diseases - these may have other symptoms other than growth)
-due to genetics or inheritance
-due to abnormal stature itself (e.g emotional and physchological upsets of lifestyle issues)
