The placenta (Greek, plakuos = flat cake) named on the basis of this organs appearance. The placenta a mateno-fetal organ which begins developing at implantation of the blastocyst and is delivered with the fetus at birth.
There are essentially 3 separate aortic/venous circulatory systems: umbilical, systemic and vitelline. The umbilical system is lost at birth, the vitelline contributes to the portal system and the systemic (embryonic) is extensively remodelled to fom the the cardiovascular system.
The placenta at birth has recently been seen as a new source for cells in bone marrow replacement therapy in many diseases. (More? Stem Cells - Cord Blood)
Page Links: Introduction | Recent Findings | Embryonic Placenta | Placental Classification | Reading | Computer Activities | Objectives | Learning activities | Placental Types | Placental Layers | Development Overview | Wharton's jelly | Blood flow through the Embryo | Parturition | Terms | Glossary |References | About Notes
Related Pages: Villi Development | Maternal Decidua | Placental Abnormalities | Stage 13/14 | Stage22 | Placental Histology | Placental Vascular Beds | Blood | Blood Vessels | Birth | Stem Cells - Cord Blood
The human placenta is a hematopoietic organ during the embryonic and fetal periods of development. Bárcena A, Kapidzic M, Muench MO, Gormley M, Scott MA, Weier JF, Ferlatte C, Fisher SJ. Dev Biol. 2009 Mar 1;327(1):24-33. Epub 2008 Dec 3. PMID: 19073167
"The placenta-derived precursors were fetal in origin, as demonstrated by FISH using repeat-sequence chromosome-specific probes for X and Y. The number of CD34(++)CD45(low) cells increased with gestational age, but their density (cells per gram of tissue) peaked at 5-8 wk, decreasing more than sevenfold at the onset of the fetal phase (9 wk of gestation). In addition to multipotent progenitors, the placenta contained myeloid- and erythroid-committed progenitors indicative of active in situ hematopoiesis."
Abramowicz JS, Sheiner E. Ultrasound of the Placenta: A Systematic Approach. Part I: Imaging. Placenta. 2008 Feb 8;
"A methodical sonographic evaluation of the placenta should include: location, visual estimation of the size (and, if appearing abnormal, measurement of thickness and/or volume), implantation, morphology, anatomy, as well as a search for anomalies, such as additional lobes and tumors. Additional assessment for multiple gestations consists of examining the intervening membranes (if present)."
Mori M, Ishikawa G, Luo SS, Mishima T, Goto T, Robinson JM, Matsubara S, Takeshita T, Kataoka H, Takizawa T. The cytotrophoblast layer of human chorionic villi becomes thinner but maintains its structural integrity during gestation. Biol Reprod. 2007 Jan;76(1):164-72. Epub 2006 Oct 11.
"According to the embryology and placentology literature, during the first trimester, the cytotrophoblast (CTB) layer that is subjacent to the syncytiotrophoblast (STB) and supported by a basal lamina is nearly complete, but later, it becomes discontinuous. ...In full-term placenta, the cell surface of the CTB layer was spread over the basal lamina but was not interrupted. Morphometric analysis showed that throughout the villous tree, 80% of the continuity of the CTB layer of full-term placenta and 90% of that of first-trimester placenta were preserved. Gestation was accompanied by unique structural change in the basal domain of the trophoblast layer. The initially cuboidal-shaped CTB cells were transformed to flat cells with many cellular processes that, together with those of the adjacent STB, eventually covered the trophoblast basal lamina in a complex network of interdigitations." (More? Villi Development)
Vogel P. The current molecular phylogeny of Eutherian mammals challenges previous interpretations of placental evolution. Placenta. 2005 Sep-Oct;26(8-9):591-6. "....epitheliochorial placenta evolved at least three times in a convergent manner."
The embryo can be seen enclosed in the amniotic membrane with the umbilical cord to the left. Within the cord the placental blood vessels are visible and branching into finer vessels before they enter the fetal side of the main placental structure.
The fetal side of the placenta is relatively smooth and is continuous with the choriononic membrane.
To the far left of the image, placental villi can be seen radiating out from placenta facing towards the maternal side.
Note also the small yolk sac (bottom centre) covered in a fine network of anastomosing vitelline blood vessels.
Classification of placenta is on the basis of histological (microscopic) structural organization and layers between fetal and maternal circulation, giving 3 main groups:
The presence of these three differing types of placenta have also been used to describe the pattern mammalian evolution. See also Placental Layers
(More? UCSD Medicine Comparative Placentation)
Links below are to internal and external online resources. Note that the dynamic nature of the web means that some links change over time.
Embryo Images Unit: Early cell Populations
NCBI Bookshelf: Developmental Biology (images) Circulatory system of a 4-week human embryo | Transfer of oxygen from the mother to the fetus in human embryos | Redirection of human blood flow at birth | The Structure of Placentas | cellular layers potentially between the fetal and maternal blood cells | four main types of placenta in different species |
Online Resources: UCSD Medicine Comparative Placentation
Discoid in humans, mice, insectivores, rabbits, rats, and monkeys.
Zonary in dogs, cats, bears and seals.
Cotyledenary in cows, deer, goat, and giraffe.
Diffuse in horses, pigs, camels, lemurs, opossums, kangaroos, and whales.
Haemochorial placentas (maternal blood cells directly contacts fetal chorion) in humans, rats, and mice.
Endotheliochorial placentas (4 layer) in dogs, cats, seals, and ferrets.
Epitheliochorial placentas (6-layer) in pigs, cows, horses, and sheep.
(List modified from Dev Biology after Renfree, 1982 | cellular layers potentially between the fetal and maternal blood cells )
Trophoblast cells are the major source of placental hormones.
Placental growth hormone (PGH) is mainly expressed in the syncytiotrophoblast cells (PGH differs from pituitary derived growth hormone by 13 amino acids). extravillous cytotrophoblast - arise from anchoring villi invade the uterine spiral arteries, generating fibrinoid material and endovascular trophoblastic cells. syncytiotrophoblast
Fetal Blood Vessels
At least 2 phases of development during pregnancy driven by vascular endothelial growth factor (VEGF):
Fibrinoid - said to exist as 2 forms of extracellular matrix:
Endoreplication - rounds of nuclear DNA replication without intervening cell or nuclear division (mitosis).
Cytokines - of maternal origin also act on placental development.
Natural Killer (NK) cells - are present in the maternal decidua in large numbers close to the extravillous trophoblast cells.
Maternal Decidual Cells are differentiated by progesterone from endometrial stromal cells (fibroblast-like). These cells change morphology (become enlarged and polygonal) and secrete various factors (prolactin, insulin-like growth factor binding protein-1, tissue factor, interleukin-15, and vascular endothelial growth factor).
After implantation cells within the developing placenta (syncitiotrophoblasts) synthesize and secrete Human chorionic gonadotrophin (hCG) into the maternal bloodstream. The main function of serum hCG is to maintain the corpus luteum in the maternal ovary and therefore maintain the early pregnancy, that is block the menstrual cycle.
Weeks after Last Menstrual period (LMP)
Days after Fertilization
hCG level for Singleton (mIU/ml or IU/L)
0 to 5
14 (next period due)
5 to 426
18 to 7340
1,080 to 56,500
Weeks 7 to 8
35 to 42
7,650 to 229,000
Weeks 9 to 12
49 to 70
25,700 to 288,000
Weeks 13 to 16
77 to 100
13,300 to 254,000
Weeks 17 to 24
4,060 to 165,400
Weeks 25 to birth
3,640 to 117,000
Weeks 4 to 6 neonatal
Less than 5
The historic diagram shows the general organisation of blood flow in the fetus including the connection with the placenta.
Maternal Blood | -> umbilical vein -> liver -> anastomosis -> sinus venosus -> atria ventricles-> truncus arteriosus -> aortic sac -> aortic arches-> dorsal aorta-> pair of umbilical arteries | Maternal Blood
Embryonic flow is shown on the stage 13/14 embryo G6 section.
Fetal blood flow through Fetus and Placenta
Wharton's jelly is a placental cord (umbilical cord) gelatinous connective tissue composed of myofibroblast-like stromal cells, collagen fibers, and proteoglycans. It is seen at parturition when it increases in volume (myxomatous, connective tissue embedded in mucus) to assist closure of placental blood vessels.
Matrix cells from Wharton's jelly have recently been identified as a potential source of stem cells.
This placental cord substance is named after Thomas Wharton (1614-1673) an English physician and anatomist who first described it.
Search Pubmed Now: placenta[TITL]+development[WORD]+review[WORD] (2005: 39 Reviews)
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