Reproduction

Regulation of Reproduction:
Nerves, Hormones, and Target Tissues

It is the interplay between these two systems, the nervous system and the endocrine system (or nerves and hormones), that controls and regualtes reproduction. Hormones originate from endocrine glands (ductless glands) and enter the blood and cause the cells in the target tissues (with specific receptors for only that hormone) to produce new hormones. If the tissue has lots of receptors the hormones have a big effect but if the tissue has only a few receptors, the hormone has a much smaller effect.

1) Simple Neural Reflex (neural control) (fast and immediate response)

• nerves acting on muscles
• nerves stimulated by heat, touch, smell, visual
• sends message through the spinal cord to the brain
• target tissue is what's getting affected
• muscles for sexual behavior and ejactulation
• muscles for scrotal tone
• scrotal sweat glands

2) Neuroendocrine Reflex (combination of nerve stimulation and hormone) (medium speed)

• Glossary definition: A reflex initiated by stimulation of sensory neurons that causes the release of a neurohormone from neurosecretory cells.
• stimulation of the teat ends (nerves) by the calf when trying to suck or sucking (pressure, moisture) causes oxytocin to be let out of the posterior lobe of the brain to contract smooth muscle and let the cow's milk down. Same sort of process with a milker's hands or a milking machine.
• Males when the see and smell a female in heat, the oxytocin in let out of their brain causing the smooth muscle in the tail of their epididymus to contract and and push the sperm out to the accessory sex glands.

3) Endocrine Control (hormones only, nerves are not involved) (slower to happen but lasts longer)

• folliculargenesis (maturation of follicles)
• estrus cycle
• EX: leydig cells stimulated to make testosterone

Classifying Hormones
        Hormones can be classified by their source, mode of action, and biochemical classification.

• NEUROPEPTIDE- hormones made by nerve cells in the brain and a short chain of amino acids
• a hormone that releases other hormones
• EXAMPLE:
• Gonadotropin Releasing hormone (GnRH)
• Oxytocin
• stored in the posterior pituitary
• Water soluble- to get them absorbed they must be injected in the bloodstream. If fed to the animal, their body will break it down in the digestive tract (because it's made of amino acids, like meat) before it can get to the target tissue. Won't absorb well through the skin either.

• GLYCOPROTEINS- (proteins with sugar)
• EXAMPLE:
• Follicle Stimulating hormone (FSH)
• stimulates sertoli cells in the male
• Luteinizing hormone (LH)
• in the female, it brings on ovulation and stimulates the CL to produce P4
• in the male, it stimulates the leydig cells to make testosterone
• Inhibin and Activin
• Water soluble- to get them absorbed they must be injected in the bloodstream. If fed to the animal, their body will break it down in the digestive tract (because it's made of amino acids, like meat) before it can get to the target tissue. Won't absorb well through the skin either.

• PROTEINS-
• EXAMPLES:
• Prolactin
• a hormone secreted  in the anterior lobe-pituitary (lactotroph cells) that stimulates lactogenesis and maternal behavior
• Placental lactogen
• a hormone produced by the placenta that stimulates lactogenesis in the dam and fetal growth

• STEROID HORMONE (comes from cholesterol)
• EXAMPLES:
• testosterone (T) produced by leydig cells in the testes of the male and in the female it is produced in the theca interna cells
• progesterone (P4) produced in the CL and placenta- relaxes smooth muscle, sustains pregnancy, keeps GnRH down
• estradiol (E2) estrogen produced by the dominant follicles during the estrus cycle
• Fat soluble- can be given orally, absorbed through skin, or injected as well

• PROSTAGLANDIN
• EXAMPLES:
• prostaglandin F2 (PGF) destroys and gets rid of the CL
• produced in the uterine endometrium and the vesicular glands
• prostaglandin E (PGE) promotes CL
• produced in the ovaries, uterus, and embryonic membrane
• Water and fat soluble- dual purpose

Feedback Mechanisms:

• Negative feedback- one thing going up holds another thing down
• when P4 is high it holds GnRH down
• Positive feedback- one thing goign up causes another thing to go up
• when E2 is high it causes GnRH to go up

Half-life:

• the period of time for one half of a substance to be destroyed or removed from the body
• some hormones have very short lives and others longer lives

Pheromones

A substance that acts like hormones but is technically not. It is considered a pheromone if it gets a response from the opposite sex.

• secreted by animals to the outside of their body
• species can excrete them in their urine
• most of them are sensed by the olfactory system
• female dogs when they are in heat put off a pheromone that can attract male dogs miles away
• male goats put off a terrible smelling pheromone
• used in orchards to catch bugs because they put out an artificial pheromone to attract one specific sex
• cat urine repels mice and rats but if the rats and mice have toxoplasmosis they are attracted to the cat urine and then probably get killed

Puberty

When an animal becomes capable of reproducing successfully. Remember that puberty is a process.
The hypothalamus must be mature enough to drive everything.
The uterus is actually capable of carrying a pregnancy before puberty.
If a heifer is allowed to have a few heat cycles after reaching puberty, she is more likely to get pregnant when bred.

Some signs to show the onset of puberty
Males:

• semen test- if the droplets on the sperm cells tails are gone, the sperm is mature
• scrotal circumference
• behavior traits
• ejaculation usually precedes the ability to produce viable and mature sperm cells

Females:

• hormone test
• heat signs, cycling, behavior- they might show signs but not actually be releasing an egg
• size/age- not always very accurate, some reach it at 5 months and others not until 15 months

Differences in the Male and Female brain:

• Females
• In the female fetus: E2 produced in the ovaries is kept out the fetus's brain by the blood-brain barrier
• Females have a surge center as well as a tonic center
• Females really do have more brains :)
• Females let out a surge of E2 which causes GnRH to let out a surge of LH which causes her to ovulate. This happens about every 21 days 
• Males
• In the male fetus: T gets into the brain and turns into E2 and defeminizes the males hypothalamus and eliminates the GnRH surge center in the male. So the surge center which is present in the female does not develop in the male
• Males only have a tonic center
• Males have a steady pulse of LH which stimulates the leydig cells to produce testosterone

Average Age (Range) of Puberty:

• Bovine
• Males- 11 mo. (7-18)
• Females- 11 mo. (9-24)
• Equine
• Male- 14 mo. (10-24)
• Female- 18 mo. (12-19)
• Canine
• Male- 9 mo. (5-12)
• Female- 12 mo. (6-24)

Things that influence the onset of puberty

• Genetics and the breed of animal
• Environmental, external, or social factors
• Study of pigs that a group of females with a male nearby reached puberty faster, a bunch of females together reach puberty faster, but in isolation it will take longer
• Seasons of birth- hours of daylight in a day has an impact on horses and sheep but not really on cattle or pigs
• Size threshold, weight, or degree of "fatness"
• Feed and nutrition has a big impact probably more especially on the female
• A study was done of dairy heifers that were fed a high plane of nutrition and gained about 2 lbs of weight each day, reached puberty between 6-8 months of age and reached their target weight the next year. Compared to the other heifers that were fed lower and low planes of nutrition and reached puberty later (9-11 months and 12 months) Some of the heifers that didn't reach puberty until 12 months did not reach their target weight for successful parturition.
• Both body maturation and degree of fatness are important in regulating the of age of puberty onset
• It is important for a female to have reached her threshold body size before puberty can be reached. She might be capable of getting pregnant but not for carrying or delivering a fully developed fetus. She would not be considered to have gone through puberty yet then.
• Heifers that are born in the fall seem to reach puberty just a hair sooner than those born in the spring because in the heifers 2nd 6 months of maturing the weather and feed are more conducive to to puberty and weight gain. (better feed, warmer weather)
• Jersey's and Holsteins tend to reach puberty faster than beef cattle
• Brahman heifers are really slow at reaching puberty. They generally don't calve until they are 3 years old. Probably because of their environment: tropical areas with not very good nutrition, lots of grass but not very nutritious or digestible.
•  Beef Heifers- when they reach 60% of their mature body weight, research has shown that a large percentage of these have reached puberty.
• .

Some things you could do to bring puberty on earlier in heifers:

• better feed and nutrition for rapid gain
• lighting for longer days (artificial light)
• rumensin (feed additive that's cost effective)
• cross-breeds
• fall born heifers
• presence of a bull
• choose an early maturing breed
• genetics- select a sire with:
• low bw's
• large scrotal circumference
• good growth

A teacher at the grad school my teacher, Kerry Powell, attended had an interesting theory that goes as follows:

• 0 months----heifer is born
• 9 months----heifer gets breed
• 18 months----heifer calves
• 20 months----wean 2 month old calf
• 22-24 months----ship the heifer to the feedlot
• She sells as grade choice beef

This teachers thought was that dairymen have been weaning their two month old calves off of milk for decades. Why don't beef ranchers do the same? He claims that if these two month old calves if fed a high energy choice diet they would gain just as well if not better than if they were left on the cow.
This would be an expensive operation to run but supposedly the profit would out weigh the costs by a much bigger margin than before. When most heifers are just calving this heifer has already calved and is getting shipped out. Another idea was to do sexed semen so that you would have replacement heifers and to do twins. Split the embryo and sex the semen. It would be more expense and work but double the profits. Any thoughts?