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The placenta is an ephemeral (temporary) organ present in female placental vertebrates during gestation (pregnancy), but a placenta has evolved independently also in other animals as well, for instance scorpions and velvet worms.


Filtration and transfer

The placenta receives nutrients, oxygen, antibodies and hormones from the mother's blood and passes out waste. It forms a barrier, the placental barrier, which filters out some substances which could harm the fetus. Many substances are not filtered out, however, including alcohol and some chemicals associated with smoking cigarettes. Several types of viruses, such as Human Cytomegalovirus, may also cross this barrier; this often leads to various degrees of birth defects in the infant.

Metabolic and endocrine activity

In addition to the transfer of gases and nutrients, the placenta also has metabolic and endocrine activity. It produces, amongst other hormones, progesterone, which is important in maintaining the pregnancy; somatomammotropin (also known as placental lactogen), which acts to increase the amount of glucose and lipids in the maternal blood; oestrogen; relaxin, and human chorionic gonadotrophin HCG. This results in increased transfer of these nutrients to the fetus and is also the main cause of the increased blood sugar levels seen in pregnancy.

After delivery

When the fetus is delivered, the placenta is delivered afterwards (and for this reason is often called the afterbirth). After delivery of the placenta the umbilical cord is usually clamped and severed or may be left attached to fall off naturally which is referred to as a Lotus Birth. In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of prostaglandin on the uterus after birth. This is known as Placentophagy. The site of the former umbilical cord attachment in the center of the front of the abdomen is known as the umbilicus, navel, or belly-button.

Human placenta baby side
Human placenta uterine side

Top: Human placenta shown a few minutes after birth. The side shown faces the baby with the umbilical cord top right. The white fringe surrounding the bottom is the remnants of the amniotic sac. Bottom: A different placenta displays side that connects to the uterine wall..

Species variation

All mammals other than monotremes and (most) marsupials utilize placentas in reproduction, and are known as placental mammals. Also, some species of snakes utilize placentas.

The shape and exchanging surfaces of placental mammals varies according to species.

  • Ruminants have cotyledonary placenta that is really many small placentas where the fetus' cotyledons interface with the dams' caruncle forming a placentome.
  • Carnivores have a zonary placenta.
  • Perissodactyles have a micro-cotyledonary that grossly ressembles diffuse placentas.
  • Primates have discoid placentas.


Additional images

Wikibooks Cookbook has an article on


External links

Mammalian development of embryo and development and fetus (some dates are approximate - see Carnegie stages) - edit

Week 1: Zygote | Morula | Blastula/Blastomere/Blastosphere | Archenteron/Primitive streak | Blastopore | Allantois | Trophoblast (Cytotrophoblast | Syncytiotrophoblast | Gestational sac)

Week 2: Yolk sac | Vitelline duct | Bilaminar disc

Week 3: Hensen's node | Gastrula/Gastrulation | Trilaminar embryo Branchial arch (1st) | Branchial pouch | Meckel's cartilage | Somite/Somitomere | Germ layer (Ectoderm, Endoderm, Mesoderm, Chordamesoderm, Paraxial mesoderm, Intermediate mesoderm, Lateral plate mesoderm)

Histogenesis and Organogenesis

Circulatory system: Primitive atrium | Primitive ventricle | Bulbus cordis | Truncus arteriosus | Ostium primum | Foramen ovale | Ductus venosus | Ductus arteriosus | Aortic arches | Septum primum | Septum secundum | Cardinal veins

Nervous system: Neural development/Neurulation | Neurula | Neural folds | Neural groove | Neural tube | Neural crest | Neuromere (Rhombomere) | Notochord | Optic vesicles | Optic stalk | Optic cup

Digestive system: Foregut | Midgut | Hindgut | Proctodeum | Rathke's pouch | Septum transversum

Urinary/Reproductive system: Urogenital folds | Urethral groove | Urogenital sinus | Kidney development (Pronephros | Mesonephros | Ureteric bud | Metanephric blastema) | Fetal genital development (Wolffian duct | Müllerian duct | Gubernaculum | Labioscrotal folds)

Glands: Thyroglossal duct

Uterine support: Placenta | Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) | Amniotic sac (Amnion, Chorion)

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