Repro physiology 10 – Puberty in animals

The attainment of puberty in females is normally defined as the

time of first estrus

The attainment of puberty in females is normally defined as the time of first estrus what are the factors

• Ability to reproduce

• Age at first estrus (heat)

• Age at first ovulation

• Age at which the female can support pregnancy

The onset of puberty involves the

hypothalamic-pituitary-ovarian

axis.

At birth the ovaries in a calf may contain up to

150,000 primary or

primordial follicles, each follicle consists of an oocyte surrounded by a

single layer of granulosa cells but no thecal cells

Soon after birth, the ovaries start to produce

growing follicles, which

consist of an oocyte with two or more layers of granulosa cells and a

basement membrane

In the growing female, the hypothalamus secretes

increasing levels of

GnRH, which acts on the pituitary gland resulting in increased secretion

of FSH and LH

The low level of estrogen produced by

these follicles

stimulates the development of secondary sexual

characteristics

Pre-pubertal animals do not produce

the preovulatory surge in LH

that is typically seen in mature females

The pre-ovulatory LH surge is

dependent upon the ability of

the hypothalamic-pituitary system to respond to estrogen

Some follicles become luteinized and secrete

low levels of

progesterone, which is a requirement for the development of the

capacity to produce a pre-ovulatory LH surge

This cycle continues, with more FSH resulting in more

estrogen release and thus an increase in the LH surge

Thus, in the physiological sense, puberty is the attainment of

that degree of maturity of the reproductive system which

permits a pre-ovulatory surge in LH to be attained in response

to estrogen secreted by large follicles in the ovary

in the female, attainment of puberty is more

of a sudden

event, whereas in males it is a gradual process

The first ovulation will be accompanied by a silent heat –

why?

• No prior exposure to progesterone

• Female requires progesterone-priming before estrogen can induce

full estrus behavior

Changes in organs near puberty: Uterine horns length (mm)

58

Changes in organs near puberty: Uterus weight (g)

72

Changes in organs near puberty: Ovaries weight (g)

32

Factors affecting the onset of puberty

• Hormonal

• social

• genetic

• season of birth

• body weight

• environment

• nutriotional

Factors affecting the onset of puberty: GnRH pulses determine puberty: Prepubertal period

• minimal GnRH release

• FSH and LH low

• minimal to no folliculogenesis

Factors affecting the onset of puberty: GnRH pulses determine puberty: Puberty

• increase in the pulse frequency and amplitude of GnRH release

• increase in FSH and LH pulses

• folliculogenesis occurs

Do larger breeds within a species reach puberty at the same weight as smaller breeds?

• Larger breeds have a heavier weight requirement

• Beef heifers reach puberty when they are 55-65% of mature weight, while dairy heifers mature when they are 30-40% of mature weight

Body weight is the most important factor (why?)

Typically, puberty is reached between 40 and 60% of mature weight

• Live weight is a good indicator that:

• they can handle pregnancy and lactation

• they are large enough to give birth safely

Factors that inhibit body growth will also tend to inhibit the onset

of puberty:

• birth/rearing status (e.g., single vs. twin)

• dam age (since females reared as multiples or by young dams will tend to grow more slowly than those reared as singles or by older dams)

• environmental conditions (e.g., poor feed supply or quality)

• nutrition/trace mineral deficiencies

• parasitism

Does timing of birth affect onset of puberty?

Yes

• if lamb/kid born in late spring/summer it will not reach puberty till

it’s second autumn

• this is due to the species being seasonal breeders

In non-seasonal animals, date of birth can affect

when puberty occurs due to the availability of feed

• i.e., cattle born in spring reach puberty faster than those born in

autumn - due to feeding levels

• These effects are due mainly to live weight

Factors affecting the onset of puberty: genetic

• Genetic factors are also very important in determining the time

at which puberty occurs

• Prolific breeds (i.e., those which tend to produce many young ones)

generally reach puberty at an earlier age and at a lower proportion of

mature weight than do less prolific breeds

Prolific breeds tend to reach puberty earlier why?

• more sensitive to positive triggers (i.e., produce more hormones)?

• for a given level of hormones more responsive?

Can we breed for an earlier onset of puberty?

• crossbreeding results in earlier puberty than parent average

• selection for early puberty?

Genetic factors are also very important in determining the

time at which puberty occurs

• Puberty also tends to be delayed in inbred females

• Effects of inbreeding and crossbreeding on puberty are associated

with corresponding effects on body weight

Factors affecting the onset of puberty: social

• Interaction with the ‘others’ can affect onset of puberty e.g.,

• in pigs, gilts reared with young boars reach puberty earlier (and

at a lighter weight) than those reared in the absence of the male

• in addition, gilts reared in isolation (with no other females) reach

puberty later

• introduction of male (teasing) can be used to induce puberty in

females that are closer to puberty

Factors causing stress can delay

puberty

• i.e., shearing just prior to puberty

Temperature and nutrition: are they inter-related and can they affect puberty?

• yes

• high ambient temperature can delay onset of puberty

• some breeds not affected by this

Likely that the hypothalamus integrates a variety of

of internal and

external signals and, because of these signals, stimulates the

onset of puberty by secreting increased levels of GnRH

The role of the hypothalamus in

regulating puberty is thus similar

to its role in regulating breeding seasons

Does puberty mean sexual maturity in the female?

• No, sexual maturity occurs when the animal reaches its full reproductive

potential

Young females.......

• Often have irregular estrous cycles, lower ovulation rate, increased

fertilization failure and embryo loss

• Do not show the intense behavior (estrus) patterns of an adult, they do not seek or stand as readily for the male

• Have shorter estrus (heat)

The post-pubertal female

• They are only 40-60% of mature weight (i.e., they are still

growing themselves)

• Pregnancy and lactation could place a large strain on these

animals unless they are managed properly

Mammary gland growth

• Fetal, prepubertal, postpubertal, pregnancy and lactation

Milk production has a moderate to low

heritability in the dairy

cow reflecting the important and variable influence of the

environment

Repeatability is quite high, with first lactational performance

considered a

good indicator of lifetime performance

• This suggests that something happens between conception and the first

lactation that effects lifetime performance

Mammary gland development during

fetal life

Most of the major developmental changes that will occur

before birth have occurred by mid-gestation

At birth, the following are observed:

• Teats are well developed

• Secondary sprouts are canalized, but still have the solid core of cells

at the end. These are the cells that continue to grow and branch

• Growth of the sprouts is limited to the area around the gland cistern.

Only a few tertiary sprouts are present

• Non-secretory tissue is well formed (connective tissue, blood vessels,

lymph vessels)

• The male gland is similar to the female, but not as fully developed

• Secretory or glandular parts are not developed at this time

Sensitive period just prior to puberty:

• mammary gland is under a positive allometric growth spurt

• grows 3-5 more times rapidly than the rest of the body

• although the amount of tissue development is small, compared to

growth during pregnancy

• mammary growth before puberty is largely seen as an increase in

connective tissue, ductal growth, and growth of the fat pad

• the formation of a duct network will determine the

The high planes of nutrition that inhibit mammogenesis at

puberty are accompanied by

an accumulation of adipose

tissue in the mammary gland of cattle and sheep

Is the fat tissue crowding out the parenchymal tissue?

• Mammary tissue in heifers fed ad libitum was over 80% fat

• Heifers fed a restricted diet have around 65% fat, and 13% more

parenchymal tissue (tissue that will eventually become milk producing

tissue)

Mammary gland development post-puberty

• Rapid mammary growth continues through the first several

oestrous cycles after puberty has been reached

• After this oestrogens present during subsequent oestrous

cycles continues to stimulate mammary growth, although most

of the growth is lost through regression during the luteal phase

of each oestrous cycle

• Consequently, the number of oestrous cycles after puberty and

before pregnancy can influence total mammary growth

Majority of mammary growth occurs during pregnancy

• primarily due to simultaneously elevated blood concentrations of

oestrogen and progesterone

• a continuous, exponential process from conception to parturition

• the greatest increase in mass of parenchymal tissue occurring in late pregnancy

• elongation of mammary ducts, the formation of alveoli

• mammary epithelial cells complete differentiation and milk component synthesis begins

Mammary growth continues in

early lactation, but this growth

may account for less than 10% of total mammary development

in ruminants

Peak milk yield is dependent on

the number of milk secreting cells

Puberty in the male

It is a gradual process but physiologically similar to the process that

occurs in the female

Puberty in the male is defined as having occurred when:

• “testes are producing spermatozoa in large numbers, the separation

of the penis from the prepuce has been completed and the animal

has developed normal sexual behavior”

• Puberty occurs at about 30-60% of mature weight

Puberty in the male Other definitions

• Age when behavioral traits are expressed

• Age at first ejaculation

• Age when sperm first appear in the ejaculate