Mammalian Reproduction
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Mammalian Reproduction Part II – Process. The Placenta. Physically anchors fetus to uterus Transport nutrients O 2 /waste to/from fetus Synthesizes hormones for maintaining pregnancy Maternal/fetal blood separate Fetus is a foreign object to mother Fetus carries ½ father proteins

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Mammalian ReproductionPart II – Process


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The Placenta

  • Physically anchors fetus to uterus

  • Transport nutrients O2/waste to/from fetus

  • Synthesizes hormones for maintaining pregnancy

  • Maternal/fetal blood separate

    • Fetus is a foreign object to mother

    • Fetus carries ½ father proteins

    • Different self-recognition proteins


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The Placenta

  • Protects fetus from bacteria and larger molecules

  • Eutherian placenta composed of fetal/maternal tissue

  • Eutherian placenta produces some nutrients

  • All mammals have a placenta of some type. Best developed in eutherians.


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Placental Types - Choriovitelline

  • Yolk-sac placenta

  • Most primitive

  • Monotremes/Marsupials (except peramelidae)

  • Vascularization between chorion and large yolk sac

  • Yolk sac enlarged and passes nutrients



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Placental Types - Choriovitelline

  • Monotremes:

    • Fetus receives nutrients from “uterine milk”

    • Once egg forms, it is laid (platypus) or transferred to pouch (Echidna)

    • Young hatch at undeveloped stage (altricial)


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Placental Types - Choriovitelline

  • Platypus

    • Eggs incubated for 11-12 days

    • Hatchlings are ~11mm long

    • Platypus do not have nipples, but have milk producing areas with hair tufts

    • Weaning at 21 days


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Placental Types - Choriovitelline

  • Echidnas:

    • Egg retained in pouch

    • Young ~1.5mm long

    • Nurse ~100 days

    • Ejected from pouch 50-60 days because spines form

    • Weaning at 140-150 days


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Placental Types - Choriovitelline

  • Marsupials:

    • Blastocyst sinks into depression in uterine mucosa

    • Contact strengthened by folding of blastocyst wall in contact with uterus

    • Embryo nourished by “uterine milk” secreted by uterine mucosa

    • Limited diffusion across placenta


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Placental Types - Choriovitelline

Marsupial Peramelid Placental


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Placental Types - Chorioallantoic

  • Peramelids (bandicoots) and all eutherians

  • Fusion involves chorion and allantois

  • Greater degree of intimacy between fetus and mother

  • Six tissue layers potentially involved:

    • M. endothelium F. epithelium

    • M. connective tissue F. connective tissue

    • Uterine epithelium F. endothelium


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Placental Types - Chorioallantoic

  • Peramelid Design:

    • Less effective than eutherian

    • Blastocyst rests on uterus on side with allantois

    • Large, vascularized allantois

    • Lacks villi between fetal/maternal tissue

    • Uterus becomes vascularized at contact point



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Placental Types - Chorioallantoic

  • Eutherian Design:

    • Blastocyst adheres, then sinks into uterus

    • Villi form and uterus becomes vascularized

    • Tissue may be eroded during implantation leading to different degrees of intimacy

    • Fetal contribution to placenta must be expelled from mother

      • Deciduous: afterbirth of fetal/maternal placenta

      • Non-deciduous: maternal placenta retained


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Placental Types – ChorioallantoicPlacental Types

Epitheliochorial Endotheliochorial Hemoendothelial

Syndesmochorial Hemochorial


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Placental Types – ChorioallantoicPlacental Types

  • Epitheliochorial:

    • All 6 layers

    • Least intimate

    • Loose association between maternal/fetal tissues

    • Non-deciduous

    • Pigs, horse, lemurs, whales


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Placental Types – ChorioallantoicPlacental Types

  • Syndesmochorial:

    • Uterine epithelium eroded

    • Deciduous

    • Ruminants (goats, sheep, cows)


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Placental Types – ChorioallantoicPlacental Types

  • Endotheliochorial:

    • Erosion of maternal connective tissue and uterine epithelium

    • Deciduous

    • Carnivores


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Placental Types – ChorioallantoicPlacental Types

  • Hemochorial:

    • Erosion of all maternal tissues

    • Fetal tissue in direct contact with maternal blood

    • Deciduous

    • Some insectivores, bats, higher primates, most rodents


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Placental Types – ChorioallantoicPlacental Types

  • Hemoendothelial:

    • All tissue eroded except fetal endothelium

    • Surrounded by maternal blood

    • Deciduous

    • Lagomorpha, some rodents


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Placental Types – ChorioallantoicPlacental Shape

  • Shape of the placenta determined by distribution of villi

    • Diffuse:

      • Villi over entire chorion

    • Cotylenodary:

      • Evenly spaced groups over entire chorion

    • Zonary:

      • Band of villi around equator of chorion

    • Discoidal:

      • Villi in one or two discs



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Birthing Process

  • Passage of embryo is difficult

  • Relaxin: hormone that relaxes pelvic ligaments and symphysis

  • Oxytocin: initiates contractions

  • Pelvic Passage:

    • Modifications to enlarge pelvis include resobtion of connective tissue joining pubic bones (gophers) or realignment of digestive tracts to be ventral to pelvis (moles)


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Birthing Process

  • When placenta tears away it leaves scars on uterus

  • Counting scars gives indication of number of young produced

  • Nulliparous = 0 young

  • Multiparous = multiple litters


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Patterns of ReproductionSpontaneous Ovulation

  • Spontaneous Ovulation (most mammals):

    • Ovulation

    • Copulation

    • Fertilization

    • Implantation

    • Gestation

    • Parturition

    • Lactation


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Patterns of ReproductionInduced Ovulation

  • hares/rabbits:

    • Copulation

    • Ovulation

    • Fertilization

    • Implantation

    • Gestation

    • Parturition

    • Lactation


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Patterns of ReproductionDelayed Fertilization

  • Temperate bats:

    • Copulation

    • DELAY

    • Ovulation

    • Fertilization

    • Implantation

    • Gestation

    • Parturition

    • Lactation

  • Good for species with long dormancy

  • Viable sperm stored through winter/hibernation

    • Up tp 70-80 days

    • Sperm attach to uterine wall, receive nutrients


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Patterns of ReproductionDelayed Implantation

  • Insectivores, rodents, bears, mustelids, seals, armadillos, some bats:

    • Ovulation

    • Copulation

    • Fertilization

    • DELAY

    • Implantation

    • Gestation

    • Parturition

    • Lactation

  • Obligate (bears) or facultative

  • Embryo develops to blastocyst, remains in uterus until spring then continues to develop


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Patterns of ReproductionDelayed Development

  • Blastocyst implants, but delays development

  • Jamaican Fruit Bat example

    • March April: Birth 1st young, estrous

    • Apr/May: pregnant and nursing

    • Jul/Aug: birth to 2nd young, estrous, new blastocyst implants

    • Sep-Mar: implanted young delays then grows


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Embryonic Diapause

  • Macropodid Marsupials

  • Mates and produces young (Joey)

  • If conditions good, reproduce again

  • Presence of joey causes 2nd embryo to arrest

  • 2nd young born when joey leaves pouch

  • Possible mating again to have 3 young

    • Joey outside pouch

    • Joey inside Pouch

    • Embryo arrested


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Special Problems of Reproduction

  • Cursiorial Species:

    • Predator exposure problems

    • Camouflaged young

    • Precocial young

  • Arboreal Species:

    • Less predator problems

    • Altricial young

    • More development may be needed for mental acuity


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Special Problems of Reproduction

  • Aquatic Species:

    • Cetaceans:

      • Birthing as a breach to prevent drowning

      • Rich milk for fast growth of blubber

    • Pinnipeds:

      • Come on land to birth

      • Need to return to sea for forage

        • Short lactation, high fat, maternal body reserves

        • Longer lactation, foraging trips by female


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Special Problems of Reproduction

  • Aerial Species:

    • Disproportionately large young

      • Up to 50% adult weight

    • Adult females larger than males, esp. wings

      • Some Females carry young

      • Others use nest


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