(14) Puberty

Question 1

(Puberty)

Puberty is the period during which reproductive competence is attained. It culminates in the production of functionally competent gametes in sufficient numbers for fertility, and in normal reproductive behavior.

For convenience, puberty is often referred to as occurring at a specific time, such as production of 50 x 106 sperm in bulls, menarche in girls, and first ovulation in females of other species. However, the physiological changes culminating in these milestones start well before they are achieved, and sexual maturation continues long after “puberty” (by these definitions) in both sexes. Sperm number, % motile and % with normal morphology increase in males. In females, fertility increases for some time after the first ovulation, at least partly due to functional differentiation of the uterus under the influence of ovarian steroid hormones. So in reality the process is gradual, in humans taking 4-5 years.

Question 2

1. Puberty represents what?

If we split the early life of an animal into neonatal, juvenile, pubertal and mature periods then:…

1. GnRH neurones in the late fetal and neonatal periods are capable of and do secrete GnRH.
2. In the juvenile period GnRH secretion decreases and is maintained at very low levels, but at this stage:

if you specifically stimulate GnRH neurons they can do what?

If supplied with GnRH, the pituitray of juvenile animals responds by doing what?

Will the ovaries develop in response to LH and FSH

Answer 2

1. re-activation of the Hypothalamo-pituitary-gonadal axis
2. secrete GnRH

LH and FSH (pituitary is functional)

yes (gonads are capaable)

So during the juvenile state, the hypothalamo-pituitary-gonadal axis is held in check by suppression of GnRH secretion from the hypothalamus and puberty isn’t so much an initial activation of the system as a reactivation.

Question 3

How the GnRH pulses are held in check during the juvenile period and then re-activated again during puberty is still an area of active research and the controls seem to have different degrees of importance between sexes and potentially species.

1. At the core level there appears to be some kind of developmental clock that does what?
2. The clock then receives input from various permissive signas that may do what?

but can’t do what?

3. example of some cues?

Answer 3

1. times the genetic program for puberty and sets a lower age limit on it’s occurence
2. delay puberty

cannot make earlier

3. energy balance, photoperiod, social cues

(The identity of the developmental clock and the integrating mechanism between it and the permissive signals is still unknown.)

Question 4

(Mechanism for Control of GnRH Neuron Activity)

1. critical change for puberty is what?
2. how suppressed during juvenile?
3. When these alterations do not involve gonadal steroid feedback… they are represented by what hypothesis?
4. When they do, explanation is termed what?

Answer 4

1. increase in GnRH pulses in the hypothalamus
2. decreased stimulatory inputs on GnRH and increased inhbitory inputs
3. the “direct drive hypothesis”
4. the “gonadostat hypothesis”

Question 5

(The Direct Drive Hypothesis)

1. In this scenario puberty is simply the result of what?
2. Is it steroid dependent?
3. How is inhibition released?

some of indirect influence by what?

4. What are the inhibitory influences?
5. Stimulatory Influences?
6. Is inital event a decline in inhibition or increase in stimulation?

Answer 5

1. an increased drive for GnRH secretion, a function of brain centers
2. no (same sequence occurs in intact and gonadectomized animals)
3. Cell types in hypothalamus are either stimulatory or inhibitory… changes are made to activate (direct or indirect)

inhibition of kisspeptin neurons in the juvenile state

4. GABA (main one, including female primates)

NPY (main one in male primates)

5. glutamate (main one)

NE?

NPY? (in female primates)

Astroglial cells

6. don’t know

Question 6

(The Gonadostat Hypothesis)

1. In this model the secretion of GnRH during juvenile is kept in check by what?
2. In these species, ovariectomy of juvenile causes what?
3. The final phases of puberty are controlled by what?
4. hypothesis fits fairly well for what species?

Answer 6

1. negative feedback inhibition from gonadal steroids
2. increase in GnRH to pubertal levels
3. changing sensitivty of the hypothalamus to gonadal steroid feedback.

(During the juvenile period, sensitivity to inhibitory steroid feedback (estrogen in females) is very high. During puberty this sensitivity decreases (resetting of the gonadostat), GnRH secretion increases and acts on the pituitary to give increases in FSH and LH which then allow full follicular development of the ovaries.)

4. sheep and cattle (esp female)

In many species it is postulated that both systems (direct drive and gonadostat) have a role and there may be a difference between the sexes. Thus in male primates direct drive appears operational while in females a combination of direct drive and gonadostat theories are required to explain the observations.

Question 7

(Permissive Signals Altering the time of puberty)

Timing of puberty represents an interaction of genetic and environmental factors. The earliest possible puberty for an individual is genetically limited, how close the animal goes to achieving this lower limit is probably related to when it achieves a set level of somatic development. Thus the initiation of puberty is generally more closely tied to body growth than to chronologic age. Essentially, if you haven’t reached these markers you are not ready to reproduce (certainly not ready for a successful pregnancy) so making the developmental clock wait until the permissive signals are achieved is a protective mechanism.

Restricting dietary intake retards growth and delays the time of puberty.

Question 8

(Permissive Signals Altering the time of puberty)

1. To be able to initiate puberty an individual needs to achieve species-specific targets for growth or metabolism. The body detects achievement of the growth/somatic targets (or reduction in BMR) by peripheral signals. When these signals indicate the appropriate metabolic state has been achieved
   (“metabolic gate”), then puberty can proceed. The postulated peripheral signals are the same ones we’ve seen previously that mediate the nutritional effects on reproductive function…like what 4?

Answer 8

1. glucose, insulin, IGF-1 and leptin

(With the discovery of leptin, the kisspeptin system and their positive interactions, it was considered they were the dominant link between attaining the permissive goal for somatic development and onset of puberty. However it is now evident that there are many parallel and intertwined mechanisms involving additional signals and while the leptin→kisspeptin system is still involved, it probably doesn’t have primacy)

Remember that these signals of somatic development are only “permissive” of puberty, they do not directly cause it. Puberty itself will be initiated by a genetic program (developmental clock) sometime after they are reached.

In certain continuous breeders (humans, cows, pigs) the appropriate degree of somatic development is the only permissive signal you need to achieve in order to be able to initiate puberty. In other species there are other signals or cues involved, and actual size and age at puberty will depend on the interplay of these factors. They can largely be divided into seasonal and social (see below).

Question 9

(Seasonal Influences on Age at Puberty)

(photoperiod)

1. those species that are seasonal breeders can only attain puberty when?
2. Lambs are born in spring and the breeding season for sheep is fall/winter. Ewe lambs can achieve puberty at ~ 30 weeks (7.5 months) of age. Whether puberty is achieved in the first available breeding season after birth will depend on the interaction of what two factors?

Answer 9

1. within the breeding season
2. growth rate

their time of birth in the previous spring

(Their time of birth will decide how much time they have to achieve the necessary target growth before the end of the subsequent fall breeding season. Their growth rate will determine how long it takes them to achieve the target. Fast growing ewes will achieve puberty in their first season, slow growing ones won’t. Intermediate growth rate ewes may if born very early in the previous spring, but won’t if born later. They may well reach the metabolic target in the following spring or summer but will not go through puberty due to inhibitory photoperiod at this time. They will have to wait until the following fall. (Of course the real interaction here is they need to be big enough and have been exposed for long enough to non-stimulatory long days in order to become refractory to them → then they can get induced by short days – remember the annual cycle of ewes depends on becoming refractory to long days)

Question 10

(Seasonal Influences on Age at Puberty)

(Food Supply)

1. Seasonal variations in nutrition influence growth rate and thus the achievement of the metabolic gate. This may not matter so much if you are long-lived like a human, that now achieves puberty at 12 instead of 16 like 180 years ago - either way you still have lots of reproductive opportunities. However, it can be vital for short-lived species. Deer mice live ~ 18 weeks, with earliest puberty at ~ 5 weeks. If they are born late in the season when food is scarce, their only chance for reproduction is to try and overwinter to achieve success in Spring. Many members of short-lived species with strict photoperiodic controls may never achieve puberty before they die, because they grow too slowly if food is limiting.

Question 11

(Social Factors in the Attainment of Puberty)

1. these cues are what?
2. A pheromone in the urine of male mice accelarates puberty onset in young females. The pheromone is produced in response to androgens and is strongest in urine from dominant males (they have more testosterone).
3. Boar steroidal pheromones androstenone and androstenol will hasten puberty in gilts. Similar effects have been seen in goats, sheep and cattle.

The hastening of puberty on exposure to males is called what?

Answer 11

1. mostly pheromonal, and perhaps tactile
2. the Vandenberg effect

Question 12

(Sex Differences in Time of Puberty)

1. While ewe lambs are capable of puberty at about 30 weeks, ram lambs start the process at 10 weeks. This is due to sexually imprinted (brain sex) differences in the controls over GnRH secretion. Giving female fetuses testosterone in late gestation allows them to go through puberty at 10 weeks also. The testosterone removes the strict requirement for photoperiod alterations and thus puberty can occur earlier.
2. Are males or females of seasonally reproducing species less seasonal? why?

Answer 12

2. males (often enter breeding season earlier); time lag for sperm production

In other species (e.g. humans) females may enter puberty before males.

Question 13

(Genetic Influences on Age at Puberty)

1. Some of these differences will be mediated by what?

others will be due to what?

(cattle)

1. Bos Taurus vs. Bos indicus?
2. smaller beef (Angus) vs large European (Charolais)?
3. crossbreeding increases or decreases?
4. selecting for milk yield?
5. selection of bulls for large scrotal circumference results in what?

(Goats)

European breeds 6-8 months, Pygmy goats as early as 3 months.

(Sheep)

Prolific breeds (Finnsheep) earlier.

(Dogs)

1. generally, smaller or larger cycle earlier?

(Cats)

1. what percent mature weight?

Purebreds later than garden variety, Persians late

Answer 13

1. alterations in the developmental clock

time taken to achieve certain permissive cue goals, particularly somatic development/body composition

1. Bos Taurus is earlier
2. smaller beef
3. decreases age
4. decreases puberty
5. early puberty in daughters
6. smaller

Small breeds 6-10 months.
Large/giant breeds 18 months-2 years.

lots of individual variation within breed (laboratory beagles - 1st proestrus 6-13 months)

1. 80%, 6-9 months (5-12)