Pregnancy, Development and Lactation

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Pregnancy, Development and Lactation

• Chapter 18

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Species Development

• Species can only be perpetuated if pregnancy,
  development of offspring and lactation are
  appropriately carried out.

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Fertilization

• Copulation- the act of breeding allowed by the
  female during estrous (heat) period.
• Usually in a mounted position
• Intromission- the insertion of the penis into the
  vagina.
• Ejaculation- when the semen is deposited in the
  upper portion of the vagina
• Horse and pig deposit semen directly into uterus
  through open cervix.

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Transport of Spermatozoa

• Start actively swimming as soon as deposited
  trying to make their way to oviducts.
• Would take over an hour under own power
• Are helped out by uterine contractions
• Copulation causes posterior pituitary gland to
  release oxytocin which causes smooth muscle of
  reproductive tract to contract helping
  spermatozoa to the ovum.

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Capacitation

• Series of changes that spermatozoa undergo in the
  female reproductive tract to increase chances of
  fertilization.
• Changes of ion movement through cell membranes
• Increase in cells metabolic rates
• Increase in rate of use of simple sugars for
  energy production.
• Allows acrosome enzymes to be released.

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Fertilization of the Ovum

• Spermatozoa are programmed to seek out something
  large and round and attempt to penetrate it.
• Some try to fertilize non-ovum things.
• Once ovum is found, many spermatozoa may swarm
  around it and start tunneling through the layers.
• Process aided by enzymes of acrosome.
• Once one spermatozoa penetrates ovum, change in
  membrane prevents any other sperm from entering.

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The Zygote

• Once ovum is fertilized, it becomes a zygote.
• Male pronucleus- the nucleus of the male
  spermatozoan immediately after fertilization.
• Female pronucleus- the nucleus of the ovum
  immediately after fertilization.
• Each pronucleus carries haploid number
• Join together to get diploid number
• This joining establishes genetic information for
  offspring.

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Cleavage

• The rapid division of the zygote once single
  nucleus has been established.
• Cell divides rapidly but overall size remains the
  same because are dividing so quickly do not have
  time to grow.
• Once zygote is a solid mass of cells, is in
  morula stage.
• During this time, zygote is moving down ovum to
  uterus
• Propelled by cilia and muscular contractions

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Blastocyst

• Cells of morula stage continued to divide and
  hollow cavity is formed in center of zygotic
  cell.
• Once bump of cells on one side is developed, this
  is now the blastocyst.

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Implantation

• The means by which the blastocyst makes itself a
  home by attaching itself to the lining of the
  uterus (endometrium).
• Once blastocyst comes to rest beside uterine
  lining, enzymes produced by blastocyst dissolve
  away some lining and implants itself into this
  pit in the lining.
• Placenta begins to form as soon as implantation
  occurs.
• Way of transporting oxygen and nutrients to
  blastocyst.

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Terminology

• Embryo-what developing offspring is referred to
  during early part of pregnancy.
• Fetus- What developing offspring is referred to
  during later part of pregnancy.

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The Placenta

• Life-support system of the developing fetus.
• Fetus is a parasite on mother during pregnancy.
• Grows along with fetus to enable for appropriate
  transfer of waste products, etc.

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Structure of the Placenta

• Multi-layered, fluid-filled membranous sac.
• Develops around embryo and is connected to it by
  umbilical cord.
• Smaller connections between outermost layer of
  placenta and lining of uterus.
• This is where exchange of nutrients and wastes
  takes place.
• Fetal and maternal blood does not mix but runs in
  close proximity to one another.

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Structure of Placenta Continued

• 3 layers of placenta
• 1. Amnion
• Forms amniotic sac directly around fetus
• 2. Allantois
• Outside amniotic sac and forms allantoic sac.
• 3. Chorion
• Outside of allantoic sac and attaches to uterine
  lining.
• Linked to fetus by umbilical cord.

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The umbilical cord

• Link between the fetus and nutrient and waste
  exchange structures of the placenta.
• Cordlike in structure
• Contains umbilical arteries (2) and umbilical
  vein.
• Umbilical arteries carry unoxygenated, waste
  filled blood from fetus to placenta
• Umbilical vein carries nutrient and oxygen rich
  blood back from placenta to fetus.
• Contains drainage tube from fetus urinary
  bladder (urachus)
• Tube runs from the cranial tip of fetus bladder
  through umbilical cord to allantoic sac.
• Fetus does not produce urine but do produce
  fluid that must be eliminated.

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Attachment to the Uterus

• Where chorion attaches to lining of the uterus.
• Type of attachement varies among species and is
  one of four types
• 1. Diffuse Attachment
• 2. Cotyledonary Attachment
• 3. Zonary Attachment
• 4. Discoid Attachment.

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Diffuse Attachment

• Means that attachment sites are spread diffusely
  over the whole surface of the placenta and the
  whole lining of the uterus.
• No small, limited areas of attachment.
• Found in pigs and horses
• Detaches easily from uterine lining and is passed
  after the delivery of the newborn.

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Cotyledonary Attachment

• Most complicated type and is somewhat opposite of
  diffuse attachment.
• Areas of attachment are small, separate, and
  numerous.
• Placentome- attachment sites.
• Cotyledon- area on surface of the placenta
• Caruncle-area on surface of uterus
  (mushroom-like).
• Cotyledon and caruncle interdigitate with one
  another.
• Each placentome must separate completely for
  placenta to pass after birth.
• If not completely passed, may be retained and can
  cause other problems.

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Zonary Attachment

• Placenta attaches to the uterus in belt-shaped
  area that encircles the placenta.
• Found in dogs and cats
• Detaches easily after delivery.

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Discoid Attachment

• Area of attachment between placenta and uterus is
  a single-disk shaped area.
• Found in humans and other primates

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Pregnancy

• Also called gestation period
• Time from implantation to delivery of the
  newborn.
• Judge time by time since fertlization.
• Is divided into three segments called trimesters
• 1st- period of the embryo implanting and
  organizing and placental development
• 2nd- embryo now called fetus and is fetal
  development period. Parts are taking shape and
  differentiating.
• 3rd- fetal growth. All parts grow dramatically

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Gestation Periods

• Species Approx Gestation Period
• Cat 2 mo (56-69 days)
• Dog 2 mo (59-68 days)
• Cow 9 mo (271-291 days)
• Elephant 21 mo (615-650 days)
• Ferret 6 w (42 days)
• Goat, Sheep 5 mo (143-155 days)
• Hamster 3 w (19-20 days)
• Horse 11 mo (321-346 days)
• Human 9 mo (280 days)
• Pig 3 mo, 3 w, 3 d (110-116 days)
• Rabbit 1 mo (30-32 days)

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Parturition

• Birth process
• Lungs of newborn become functional
• Goes from parasite to independent being.
• Parturition is triggered by size and weight of
  uterus and changing hormone levels.
• Progesterone of dam declines
• Progesterone has kept myometrium from contracting
• Increased levels of glucocorticoid hormones
  stimulate rise in estrogen levels
• These increase sensitivity to oxytocin, released
  from posterior pituitary gland.
• Oxytocin stimulates contractions which starts
  labor process

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3 Stages of Labor

• 1. Uterine Contractions
• Presses fetus against uterus
• Causes cervix to gradually dilate
• Dam may appear restless in this stage.
• 2. Delivery of the Newborn
• Combination of uterine and abdominal muscle
  contractions
• Water or amniotic and allantoic sacs rupture.
• 3. Delivery of the placenta
• Placenta separates from wall of the uterus and is
  expelled by uterine contractions.
• Dam often eats the placenta.

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Labor Continued

• Multiparous animals the second and third stages
  of parturition intermix with one another.
• Newborn and placenta are delivered alternatively.
• Next newborn will not be delivered typically
  until previous placenta has been expelled.

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Parturition normal presentation
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Dystocia

• difficult birth
• Most common cause is that fetus is too large to
  pass or is in wrong orientation for delivery.
• May have to Repell the fetus or deliver through
  Cesarean section.
• If fetus is dead, may have to be removed in
  segments- called Embryotomy.

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Parturition abnormal presentations
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Delivery
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Placental Delivery
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Whelping/ Parturition
Normal Presentation Presentation of two sacs
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Involution of the Uterus

• After parturition is complete, uterus gradually
  returns to nonpregnant size.
• Process is called involution.
• At placental attachment sites, endometrium
  sloughs into uterus and areas heal over.
• Myometrium contractions continue slowly, pushing
  contents through birth canal.
• Will pass from bright red blood, to dead tissue
  over course of weeks to about a month.

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Mammary Glands and Lactation

• Play an important role during neonatal period.
• Mammary glands are specialized skin glands.
• Produce colostrum and milk which are crucial to
  early life.
• Present in both males and females
• Females secrete appropriate hormones for them to
  become functional

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Species Differences

• Number, size and location varies from animal to
  animal.
• Cattle, sheep and goats only have one opening per
  teat
• Dogs have up to 20 openings per nipple.

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Number of Mammary Glands for Common animals
SPECIES NUMBER OF GLANDS
Cats Horses 10 2
Dogs Humans 10 2
Cattle Pigs 4 14
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Udder of the Cow

• Udder is term used for mammary glands.
• Exaggerated in size, but composition is similar
  to other animals mammary glands
• Four mammary glands (quarters)
• Quarters are completely separate units from each
  other
• Each quarter has its own milk-secreting systems
  and ducts leading down to separate teats
• Suspended by strong suspensory ligaments that
  allow it to stretch.
• Acts as a shock absorber.

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Udder Continued

• Mastitis- infection of the mammary gland
• Since are separate, is unlikely to spread from
  one quarter to another.
• Can spread through bloodstream.

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Alveoli and Duct System

• Alveoli- milk-secreting units of the mammary
  gland
• Alveolar duct- alveoli secrete milk into tube
• Similar in make up to Alveoli found in lungs.
• Ducts empty into large space called gland sinus
  which is continuous with the teat sinus which is
  where milk is extracted by suckling young.
• Tip of teat has streak canal- passageway from
  teat sinus to outside.
• Surrounded by elastic fibers and ringlike
  sphincter muscle that keeps it closed to prevent
  leakage.

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Mammary Gland Development

• Mammary glands develop in response to hormones
  produced at puberty
• Prolactin and Growth Hormone directly encourage
  mammary gland development
• Estrogen and progesterone encourage mammary
  alveoli and duct systems to develop
• Influenced by FSH and LH on ovaries
• Certain drugs may inhibit normal mammary gland
  development

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Lactation

• Process of milk production
• Begins at end of pregnancy and is obvious at time
  of parturition.
• Prolactin and Growth Hormone from anterior
  pituitary gland and hormones from adrenal cortex
  are involved with the starting of lactation.

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Colostrum

• First milk or premilk.
• Contains large amounts of proteins, lipids, and
  amino acids than milk and high levels of
  essential vitamins.
• Antibodies for defense
• Supplies important nutrients and defenses that
  newborn can not receive elsewhere.
• If does not receive within first few hours, body
  can no longer process appropriately.
• Has laxative effect to clear meconium ( first
  feces) from newborns intestinal tract.
• Involved with passive immunity from dam to
  newborn
• Those without appropriate colostrum tend to be
  weaker and do not grow as rapidly.
• Why we wait for vaccines.

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Maintenance and Lactation

• Lactation continues as long as mammary gland is
  emptied regularly.
• Physical stimulation of nipple.
• This sends nerve impulses to brain which
  continues stimulation of appropriate hormones for
  milk production.
• When nursing stops, signal stops.
• Will lead to involution of the mammary gland or
  drying up.

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Milk Letdown

• Immediate effect of nursing or milking
• Milk accumulates high up in mammary gland in
  alveoli and small ducts and does not move down
  until Milk letdown occurs.
• Oxytocin causes myoepithelial cells around the
  alveoli and small ducts to contract
• Squeezes milk down the large ducts and sinuses.
• Can take from few seconds to minute for milk to
  flow freely.

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