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Puberty
It is a Sexual Maturity or Reproductive Competence, Ability for an animal to
release gametes and to manifest complete sexual behavior sequence. the
Gradual process Not a single event and Occurs over a period of time
Reproductive endocrine system maturity
1. Hypothalamic-Pituitary-Gonad Axis
Initiated by hypothalamic neuron development Release of GnRH and
Feedback mechanisms evolve
2. Influences
Development of threshold body size Exposure to a variety of environmental
and social cues Genetics of the Animals
Onset of Puberty
A. Age at Puberty (Farm Animals)
Puberty Age
Species
Male
Cattle
11 mo. (7-18)
Sheep
7 mo. (6-9)
Swine
7 mo. (3-8)
Horses
14 mo. (10-14)
Female
11 mo. (9-24)
7 mo. (4-14)
6 mo. (3-7)
18 mo. (12-19)
B. Age at Puberty (Other Mammals)
Rabbits
3 to 4 months
Cat
8 to 9 months
Dog
10-12 months
Humans
10-16 years
Endocrine Mechanism of Puberty
A. Maturation of Hypothalamus
1. Pituitary and Gonad can function before puberty Exogenous hormones
2. Hypothalamus need to mature for puberty to occur Gradual process
Proportional to growth
B. Maturation of GnRH release centers (Hypothalamus)
1. Two release centers
a. Tonic release
hormones
center Sensitive to negative feedback of steroid
b. Surge release
surge Ovulation
center Sensitive to positive feedback of estradiol LH
C. Tonic Center
1Circulating gonadotropins increase pulsatility
2. Frequency controlled by Pulse Generator
a. Senses internal and external stimuli
i. Internal--Nutrients, Metabolites, Steroid Hormones, etc.
ii. External Day length, Pheromones, etc.
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3. Steroid hormones undergo gradual rise to adult levels
D. Surge Center
1. Characteristics of prepubertal female: Lack of gonadal estradiol to
stimulate surge center and Lack of sensitivity to estradiol at surge center
2. Increase in tonic pulsatility: increases gonadal estradiol(GnRH,LH/FH,
Increase Follicular Development Increase Steroidogenesis (Estradiol)
3. Increase in sensitivity to estradiol
a. Sensitivity increases with age/body weight
i. Increase in estrogen receptor numbers
b. Estradiol much reach threshold concentration to cause surge
Positive Feedback
Parturition
Parturition is the act or process of giving birth to offspring.
The terms used to describe parturition vary with the species of
animal it is being used to describe. The following are
examples of parturition terminology:
A
A
A
A
A
dog whelps and gives birth to puppies
cow calves and give birth to a calf
sow farrows and gives birth to piglets
ewe lambs and give birth to lambs
horse foals and give birth to a foal
Stages of Parturition
Just as there are 3 phases of pregnancy, there are also 3
stages of parturition. It is important to recognise each stage
and monitor the dam to make sure she is progressing from one
stage to the next in a timely fashion.
The First Stage of Parturition is the Preparatory Stage. During
this stage, a number of important things happen, including:
*Positioning of the fetus for birth - The normal position of a
fetus is with the front feet pointing out the cervex, right-side
up with the chin resting on forelegs. With cattle, sheep and
horses, any other position is considered an abnormal position
and may result in dystocia.
*Dilation of cervix
*Exposure of fetal membranes through the vulva with possible
rupture
In the cow, this stage can last for up to 6 hours. During this
time, it is inadvisable to attempt to assist the dam with
delivery. If, however, the dam does not progress to the second
stage of parturition within 6 hours, it is likely that there are
problems with positioning of the fetus that will require
assistance. In a full breech position, where the rump is
presented to the cervix and the hind feet are tucked under the
calf's belly, the cervix will not dilate properly due to lack of
stimulation by the nose and front feet of the fetus.
The Second Stage of Parturition is the Expulsion Stage.
During this stage, the following takes place:
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Uterine contractions intensify, leading to
Abdominal pressing by the dam, followed by
Expulsion of the fetus
In the cow, this stage of parturition can take up to 4 hours; in
the mare, the foal must be delivered within 15 minutes of the
rupture of the amniotic sac, or the foal is likely to sufficate.
The ewe usually delivers each lamb at about 15 minute
intervals. The sow normally delivers piglets at approximately
5-6 minute intervals. One should assist the dam only after the
cervix is fully dilated or you may damage the cervix and
uterus.
The Third Stage of Parturition is the "Cleaning" Stage. During
this stage, the placental membranes (afterbirth) are expelled.
This happens after each birth in cattle, sheep and horses,
however the sow may deliver all the piglets from one uterine
horn, expell the placental membranes from that horn and then
deliver the pigs from other horn, or she may deliver all of the
piglets before she "cleans". It is very important to make sure
that the animal finishes the third stage of parturition, because
any remnants of placental membranes left in the uterus will
serve as a nidus for infection and could lead to the death of
the dam. In the horse, the placental membranes should be
expelled within 15 minutes of expulsion of the fetus. Swine
can take up to 1 hour to expell the placental membranes, and
cattle may take as much as 12 hours to deliver the placental
membranes. After the appropriate length of time, if the
membranes are not expelled, the animal has a retained
Placental Hormones
In addition to its role in transporting molecules between
mother and fetus, the placenta is a major endocrine organ. It
turns out that the placenta synthesizes a huge and diverse
number of hormones and cytokines that have major influences
on ovarian, uterine, mammary and fetal physiology, not to
mention other endocrine systems of the mother. This section
focuses only on the major steroid and protein hormones
produced by the placenta. Additional details on placental
endocrinology can be found in the Placental Hormones section
of the Endocrine System text.
Steroid Hormones
Sex steroids are the best known examples of placental
hormones. Two major groups are produced by all mammals:
Progestins: Progestins are molecules that bind to the
progesterone receptor. Progesterone itself is often called the
hormone of pregnancy because of the critical role it plays in
supporting the endometrium and hence on survival of the
conceptus.
The placentae of all mammals examined produce progestins,
although the quantity varies significantly. In some species
(women, horses, sheep, cats), sufficient progestin is secreted
by the placenta that the ovaries or corpora lutea can be
removed after establishment of the placenta and the
pregnancy will continue. In other animals (cattle, pigs, goats,
dogs), luteal progesterone is necessary throughout gestation
because the placenta does not produce sufficient amounts.
Progestins, including progesterone, have two major roles
during pregnancy:
Support of the endometrium to provide an environment
conducive to fetal survival. And Suppression of
contractility in uterine smooth muscle.
Progesterone and other progestins also potently inhibit
secretion of the pituitary gonadotropins luteinizing hormone
and follicle stimulating hormone. This effect almost always
prevents ovulation from occurring during pregnancy.
Estrogens: The placenta produces several distinct estrogens.
In women, the major estrogen produced by the placenta is
estriol, and the equine placenta synthesizes a unique group of
estrogens not seen in other animals. Depending on the
species, placental estrogens are derived from either fetal
androgens, placental progestins, or other steroid precursors.
. Two of the principle effects of placental estrogens are:
Stimulate growth of the myometrium and antagonize the
myometrial-suppressing activity of progesterone and Stimulate
mammary gland development. Estrogens are one in a battery
of hormones necessary for both ductal and alveolar growth in
the mammary gland.
Protein Hormones
Several protein and peptide hormones are synthesized in
placentae of various species. They have effects on the
mother's endocrine system, fetal metabolism and preparation
of the mother for postpartum support of her offspring.
Chorionic gonadotropins: As the name implies, these
hormones have the effect of stimulating the gonads, similar to
the pituitary gonadotropins. The only species known to
produce a placental gonadotropin are primates and equids.
The human hormone is called human chorionic gonadotropin or
simply hCG. This hormone is produced by fetal trophoblast
cells. It binds to the luteinizing hormone receptor on cells of
the corpus luteum, which prevents luteal regression. Thus,
hCG serves as the signal for maternal recognition of
pregnancy. The first hormone you produced was hCG!
Equine chorionic gonadotropin is also produced by fetal
trophoblast cells. It is actually the same molecule as equine
luteinizing hormone.
Placental lactogens: These hormones are molecular relatives
of prolactin and growth hormone. These hormones have been
identified in primates, ruminants and rodents, but not in other
species.
The functions of placental lactogens are not well understood.
They are thought to modulate fetal and maternal metabolism,
perhaps mobilizing energy substrates for fetal use. In some
species they have been shown to stimulate function of the
corpus luteum, and to participate in development of the
mammary gland prior to parturition.
Relaxin: Relaxin is a hormone thought to act synergistically
with progesterone to maintain pregnancy. It also causes
relaxation of pelvic ligaments at the end of gestation and may
therefore aid in parturation. In some of the species in which
relaxin is known to be produced, it is produced by the
placenta, while in others, the major source is the corpus
luteum. In some species, relaxin is produced by both the
corpus luteum and placenta.
Animal Pregnancy Stage
The development of animals is extremely interesting, because
we get to see how it is similar and how it is different from the
development of human beings in the womb. In the
development of animals, the new life grows in its own right
and in preparation for its own children.
Early Cellular Changes
Immediately after conception occurs, cleavage of the cell into
many smaller cells occurs. They come together in a form
known as the blastula. The cells begin to take on different
positions in relation to one another. When they are doing this,
the cells-which are now an embyro- are referred to as being in the
gastrula stage. Due to this process, the cell has three germ
layers, known as the ectoderm, endoderm and mesoderm
Formation of Organs
One of the most fascinating processes of the pregnancy cycle
is the creation of organs in the womb. Organogenesis is the
process of organ and tissue formation. This process can occur
because the three germ layers are now interacting with one
another. Instead of the typical breakdown from organ to
tissues, we are looking at the build up process of these
materials. Once the organs have been formed, the cells begin
to move to where they are going to remain for the organism's
life. Virtually all of the distinct features of the body are formed
at this point. In a human's pregnancy, the sexual organs could
be used to determine the gender of the baby.
Preparation for the Future
Even at this early stage of life, the body begins to prepare
itself for future generations. Most of the cells in the body are
referred to as somatic cells. However, there are also germ
cells which are mainly composed of gametes. These cells are
for the developing organism's own sexual reproduction in the
future. The germ cells will go into the sexual organs of the
developing fetus. However, this process is a continual one.
Gametes, which allow the animal to produce a child during
sexual intercourse, will not appear until the creature is
sexually mature. Sexually maturity in animals is the equivalent
of puberty in human beings.