Reproduction

General differences between Metatherians and Eutherians

• Female reproductive tract

  • Two uteruses = Can have offspring in right uterus developing at a different rate than offspring in left uterus

• Metatherians have a very short gestation period

• Dynamic alternative

  • Extremely short gestation period

  • Extremely long lactation period

Placenta Functions

Anchor fetus to uterus

Nutrient and waste exchange

Oxygen in, carbon dioxide out

Produce hormones. to regulate the fetus and mother

2 major types of placentas

1. Chorio-vitelline

2. Chorio-allantoic

Chorio-vitelline Placenta

• Placental attachment seen in marsupials/metatherians

• Blastocyst does not implant deeply in uterine wall

• Embryo is nourished by uterine milk

• Reasons why gestation is short

  • Mechanical weakness

  • Inefficient nutrient exchange

• Immunosuppression issue

Chorio-allantoic Placenta

Blastocyst adheres to uterus, sinks deeply into uterine wall, villi project into uterine wall, uterus becomes highly vascularized in that area of implantation = substantial amount of surface area

Types of chorioallantoic attachment come down to the extent that the fetus shares a circulatory system with the mother

Types of Chorio-allantoic Attachments

1. Epitheliochorial

2. Syndesmochorial

3. Endotheliochorial

4. Hemochorial

5. Endothelioendothelial

6. Hemoendothelial

Epitheliochorial

All 6 layers of tissue are present

• Mom and baby have capillary wall, connective tissue, uterine epithelium/trophoblast

Least intimate connection

Found in lemurs, pigs, horses, whales, etc

Syndesmochorial

5 layers

• Baby: capillary wall, connective tissue, trophoblast

• Mom: capillary wall & connective tissue (NO UTERINE EPITHELIUM)

Seen in ruminants

Endotheliochorial

4 layers

• Only blood vessel is left in baby

• Maternal endothelium

• Fetal endothelium

• Connective tissue

• Trophoblast

Seen in carnivores

Hemochorial

3 layers

• Trophoblast

• Connective tissue

• Fetal endothelium

Endothelioendothelial

2 layers (?)

• Endothelium

Hemoendothelial

1 layer

• Chorionic endothelium

Seen in rabbits and some rodents

2 categories of afterbirth

Deciduate

Nondeciduate

Nondeciduate Afterbirth

Villi pull out of uterine wall, leaving no uterine scars (like velcro), seen in least intimate connections (ex. epitheliochorial)

Deciduate Afterbirth

Part of uterus where placenta is attached is pulled away, causes some bleeding and placental scars, seen in most intimate connections (ex. hemoendothelial)

Placenta Shapes

• Diffuse

  • Most intimate

  • Villi closely attached

• Cotyledonary

  • In evenly spaced groups/patches of villi

• Zonary

  • Continous band of villi

• Discoidal

  • One or two disc-shaped areas of villi

Uteri of Prototherians, Metatherians, & Eutherians

• Monotremes (Prototherians)

  • No uterus

• Marsupials (Metatherians)

  • Lock and key type uterus structure

• Eutherians

  • “Normal” uterus

  • Simpler system without side chambers

Types of Eutherian Uteri

Distinguished by the degree of fusion of the two “horns” (oviducts)

• Duplex

• Bipartite

• Bicornuate

• Simplex

Duplex

Fallopian tubes share different cervices instead of sharing a single cervix

Seen in rabbits, rodents, aardvarks, hyraxes

Bipartite

No common uterus space, just two horns (oviducts)

Babies are split between oviducts

Seen in cetaceans

Bicornuate

Smaller uterus body, horns of uterus are larger

Seen in carnivores, insectivores, chiroptera

Simplex

Fallopian tubes, big uterus, cervix, and birth canal

Seen in humans and other greater primates

Estrous Cycle

Period where reproduction is possible

In humans, it is the menstrual cycle

In every other animal, it is the estrous cycle = “heat”

Estrous Cycle Characteristics

• Significant period when hormones allow for copulation

• Female goes into heat/ovulation on day 1 (as opposed to the buildup in humans)

Sequences in Successful Breeding

• Spontaneous Ovulation

  • Egg is released from ovary without external stimulation, such as copulation

  • Occurs naturally

  • Seen in humans

• Induced Ovulation

  • Release of eggs (ovulation) is triggered by external stimuli, such as copulation

• Delayed Fertilization

  • Development of embryo is temporarily paused, and the blastocyst (early embryo) remains in the uterus in a dormant state, rather than immediately implanting

• Delayed Implantation

  • Fertilization occurs, but zygote waits to implant in uterine wall

• Embryonic Diapause

  • Delay is AFTER implantation

Oviparous

Fetus gets nutrients from yolk in egg

Viviparous

Fetus gets nutrients from placental attachment

Ovoviviparous

Eggs are retained in the mother’s body, fetus gets nutrients from yolk in egg, hatching in female’s body, emerges as if live birth

Benefits of being Viviparous

Offspring survival

Prolonged post-partum period