Mammalian Structure and Function: Reproduction

  1. Characteristics of living mammals


    1. Mammary glands*

        Two basic types in eutherians

        e.g., cows e.g., primates
        mammary tissue (structurally similar to primate gland) mammary tissue (=lobules)
        cistern collecting ducts
        teat nipple

    2. Complex placenta* (except monotremes, most marsupials)

    II. Some comparisons of mammalian infraclasses

    1. Similarities in reproduction between mammalian infraclasses:

      1. Internal fertilization
      2. Nourish young with milk from mammary glands

    1. Differences: mode of reproduction

      Prototheria (Monotremes) Metatheria (Marsupials) Eutheria (Placentals)
      Lay eggs Give birth to live, partially developed young Give birth to well developed young
      Young left in nest or pouch-like abdominal groove, no nipples Young carried on nipples, often in pouch Young usually not carried attached to nipples
      No placenta Usually a simple placenta, but some species more complex than others Complex placenta
      Highly altricial young

      1. Lay (usually) 1 egg that hatches in 9 days
      2. After hatching it lives in its mother's pouch (really just a groove in the abdomen) for 12 weeks
      3. Spines at 8 weeks
      4. At 12 weeks eyes open and baby leaves pouch for burrow
      5. Weaned at 20 weeks

      1. Lays 1 or 2 eggs in a burrow nest
      2. Incubates 12 days then hatches
      3. Hair at 7 weeks
      4. Eyes open at 9 weeks
      5. Weaned at 16 weeks
      Highly altricial young
      • Highly altricial young
      • Short gestation (8-43 days)
      • Young born extremely small relative to adult size

        Dasyurus (quoll) infant 1/4,250 of adult size
        kangaroo infant 1/33,400 of adult size

      • Organs not completely developed

        1. Heart: incomplete separation of the ventricles
        2. Lungs: no alveoli
        3. Naked
        4. Blind

      • Well-developed front limbs for grasping fur
      • Move to pouch or (if pouchless) grab ahold of teats
      • Teat swells in its mouth to make a semi-permanent attachment
      • Stays in pouch 1 week - 1 year
      • Leaves pouch at weight roughly equivalent to birthweight of a similarly-sized placental
      Altricial or precocial young

    III. Basic anatomy and physiology

    1. Female

      1. Structures of eutherian mammals

        1. ovary (2) - produce ova (singular= ovum)
        2. oviducts or fallopian tubes (2) - through which ovum travels
        3. uterus (1 or 2) - in which ovum implants and develops
        4. cervix (1 or 2) - seals uterus from external environment during fetal development and through which neonate passes at birth
        5. vagina (1) - connects internal reproductive tract with the external world

      1. Contrast with monotremes and marsupials

        1. Monotremes
          possess a cloaca
        2. Marsupials
          Dual vaginal canals; ureters join reproductive tract to form urogenital sinus
          Medial vaginal canal
          (usually temporary)

      1. Types of eutherian reproductive systems





      Uterus 2 2 separate uterine horns 1 uterus with 2 horns + body 1
      Cervix 2 1, but has 2 separate openings (one to each uterine horn) 1 1
      Vagina 1 1 1 1
      Examples rodents, rabbits pigs, carnivores insectivores, most ungulates, bats, some primates edentates, primates

      1. Neural and hormonal control of the estrous cycle

        FSH=follicle stimulating hormone
        LH=luteinizing hormone
        1. Development of uterine tissue and ovum within a follicle (FSH, E)
        2. Ovulation (LH)

          1). Spontaneous
          2). Induced

        1. Ruptured follicle becomes corpus luteum, secretes P
        2. If conception occurs, P maintains pregnancy
          Special case - pseudopregnancy
        3. If conception doesn't occur

          1). Corpus luteum regresses
          2). Uterine lining regresses
          3). Cycle starts over

      1. Contrast with primate menstrual cycle

    1. Male

      1. Structures (Fig. 10.2)

        1. Testes (2) - produce sperm

          1) Scrotal
          2) Abdominal (held entirely within the body cavity)
          ex. monotremes, edentates, elephants, cetaceans
          3) May be seasonal

        1. Epididymus (2) - in which sperm are stored
        2. Vas deferens (2) - through which sperm pass
        3. Urethra (1) - connects to outside, also carries urine
        4. Accessory glands - add secretions to sperm to make semen

        1) Prostate
        2) Bulbo-urethral (Cowper's)
        3) Vesicular (seminal vesicles)
        4) Function of secretions

        1. Penis - may contain a bone ( baculum)

    1. Placenta = fusion of maternal and fetal tissues

      1. Functions

        1. Transfer of nutrients from mother to fetus
        2. Gas exchange
        3. Production of hormones

      1. Membranes

        1. Chorion
        2. Allantois
        3. Amnion
        4. Comparison to egg membranes

      2. Types (Fig. 10.10)

        1. Choriovitelline
        2. Chorioallantoic

      3. Patterns of attachment in placental mammals based on shape of contact zone

        1. Diffuse
        2. Cotyledonary
        3. Zonary
        4. Discoid/bidiscoid

      4. Six potential layers of cells between maternal blood and fetal blood in chorioallantoic placentas

        • maternal blood
        • (1) maternal endothelial cells of blood vessels
        • (2) maternal connective tissue of uterus
        • (3) maternal epithelium of uterus
        • (4) chorion of fetal placenta
        • (5) fetal connective tissue
        • (6) fetal endothelial cells of blood vessels
        • fetal blood

      5. Invasiveness of placenta (has implications for efficiency of nutrients and gasses between fetus and mother)

        1. Epitheliochorial
        2. Endotheliochorial
        3. Hemochorial
        4. Name of invasiveness type tells you which tissues are in contact

          • epithelio = maternal epithelium
          • endothelio = maternal blood vessel cells
          • hemo = maternal blood
          • chorial = fetal chorion membrane

      6. Evolution of patterns of attachment

        1. Hemochorial placentas evolved very early in the mammalian lineage and are the most widespread
        2. Discoid attachment is ancestral
        3. Humans show the ancestral pattern of both attachment type and invasiveness

  1. Lactation
    1. Evolution
    2. Endocrine control
    3. Milk composition
      1. fat
      2. protein
      3. carbohydrate (lactose)
      4. colostrum
    4. Evolution of lactose tolerance

IV. Interesting mammalian reproductive strategies

  1. Usual sequence
    copulation -> ovulation-> fertilization -> implantation + pregnancy -> birth

  2. Variants

    1. Delayed fertilization
      copulation -> females store sperm -> fertilization

    2. Delayed implantation
      copulation -> ovulation -> fertilization-> early development -> dormancy -> implants and finishes development

    3. Delayed development
      copulation -> fertilization -> implantation -> dormancy -> resumption of development

    4. Embryonic diapause
    5. Spontaneous abortion

Marsupial-Placental Dichotomy

I. Why do marsupials and placentals differ in their mode of reproduction?

Marsupials Placentals
very short gestation usually longer gestation
neonate incompletely developed neonate anatomically complete

  1. Why do marsupials get expelled from the uterus so early? May relate to immunology

    1. Immune system recognizes self/not-self
    2. Fetus is not-self because it receives 1/2 its genome from the father
    3. How do animals get around this problem?

      1. Egg laying vertebrates: separation of fetus/mother; fetus nourished with yolk sac
      2. Marsupials: shell membrane
      3. Eutherians: trophoblast

    4. Hormone production by placenta enables longer gestation

      1. Chorionic gonadotropin maintains the trophoblast
      2. Other compounds suppresses immune response from maternal cells
  1. Ramifications of long pregnancy

    1. Precocial young
    2. Increased structural diversity (eg limb structure)

II. Are marsupials inferior?

  1. Lines of reasoning that they are:

    1. Less diversity of habitat types
    2. Less diversity of locomotion
    3. Less diversity of foraging
    4. No really big marsupials
    5. Social organization is less complex
    6. Not as speciose
    7. Most diverse and numerous in Australia, where there is negligible competition with eutherians

  1. Is the competitive disadvantage the result of their mode of reproduction?

    1. Maybe. Their mode of reproduction limits the environments in which they can live.
    2. Maybe not. Other factors include:

      1. Cerebral cortex of marsupials is smaller and develops more slowly
      2. Learning and behavioral flexibility is less developed in marsupials.
      3. Behavior is less diverse.
      4. They have a small number of chromosomes, which may make them less evolutionarily flexible.
      5. Their reproductive rate is lower.

  1. An alternate view is that marsupials are not necessarily at a competitive disadvantage in all circumstances. Possible advantages of a marsupial reproductive stratgey include:

    1. Low energy requirements, spread out of a long period of time
    2. Can quickly replace lost young