•  It is a unique organ that develops in mammalians for the development of the fetus.
  • It is an apposition of fetal membranes to the endometrium to permit physiological exchange
  • between the fetus and the mother.


The placenta is composed of two parts:
1. The fetal placenta or allantois chorion
2. The maternal placenta or endometrium


  • The yolk sac or amniotic chorion acts as primitive placenta for a few weeks in the early
    embryonic period.
  • Allantois develop as a diverticulum of hind gut and fuses with the chorion (trophoblastic
    capsule of the blastocyst) to form the chorioallantoic placenta.
  • The blastocyst gets attached to the endometrium and the fetal membranes including the
    allantois chorion develop during the first month or more of gestation.
  • At this time the villiform projections of the chorion and the maternal crypts in the
    endometrium are rudimentary, small and friable and the nutrition is from the uterine
  • The easy separation of the two structures i.e., (maternal and fetal placentae) is prevented,
    not until the end of the first third of gestation because they do not become sufficiently
    intimate and complex.



  •  In epitheliochorial placentas, (cow, sheep, mare and sow) where the interdigitation of
    microvilli of the chorion or trophoblast and endometrial epithelium are closely apposed, no
    extensive degeneration or deposition of fibrinoid is present.
  • Therefore, in the former an acellular mechanical barrier and in the latter, the absence of
    trophoblastic antigenicity offer reasonable explanations for the retention of the placental
  • The inability of the immunologically active maternal cells to penetrate in to fetal circulation
    may also be important.
  • The sire contributes half of the genetic make up of the fetus and placenta and hence there
    should be sufficient tissue incompatibility to induce an immune reaction in the dam and
    subsequent rejection of the conceptus.
  • Wynn postulated that the greater the trophoblastic invasiveness (man and rodents), the
    greater the necrosis of both the chorionic and endometrial tissue. As a result there is
    development and deposition of a mechanical acellular barrier of acid mucopolysaccharide
    in man and rats having haemochorial placenta.

Classification of Placenta:


  • A.Deciduate or conjoined, Seen in man and rodents and in a slightly modified form in the dog and
  • In this type, the decidua composed of portions of the maternal epithelium or endothelium, submucosa, decidual cells and the fetal placenta are shed at parturition leaving the portion of the endometrium
  • B.Indeciduate or Non-deciduate
    • Seen in swine, horses and
    • In this type, the fetal membranes and placenta are expelled at the time of parturition, leaving the endometrium intact except in ruminants in which only the surfaces of the carcuncles are devoid of epithelium after the caruncles sloughs about 6–10 days following



It is divided in to 4 general types based on their shape as:

  • Diffuse
  • Cotyledonary
  • Zonary
  • Discoidal
  • It is found in wide range of species, including pigs, horses, camels, lemurs, whales, dolphins, kangaroos and
  • The villi of the chorion are distributed more or less evenly over the entire surface of the chorionic
  • The villi interdigitate with corresponding depressions or villi in the uterine epithelium, and physiological exchange take across these
  • The most striking feature of the ontogeny of the fetal membranes in the pigs is that the membranes undergo a rapid and dramatic elongation between days 6 and 12 of gestation, during which time the 2 mm spherical vesicle grows in to a filament of up to 1 m in
  • This elongation is due to a proliferation of trophoblastic tissue
  • Although it is well recognized that the fetal placenta can produce gonadotrophic hormones during pregnancy in many species, the horse always appeared to be an exception to the general


  • The cotyledonary placenta is characteristic of the ruminants; instead of being uniformly distributed over the entire surface of the chorion, the chorionic villi are clumped together in to well developed circular regions known as cotyledons.
  • These cotyledons develop only in those regions of the chorion that overlie predetermined aglandular areas of the endometrium known as the
  • The fetal cotyledon and maternal caruncle unite to form a placentome and these placentomes are the only sites of maternal-fetal exchange, the inter-cotyledonary chorion being devoid of villi and unattached to the


  • It is characteristic of the carnivores, and is the result of an aggregation of chorionic villi to form a band that encircles the equatorial region of the chorionic sac.
  • It may be complete, as in dog and cat, or incomplete, as in
  • The yolk sac persists as a vestigial structure floating in the allantoic fluid, whilst the chorioallantois remains as an oblong, fluid filled sac, with its girdle of placental
  • Incomplete zonary placenta may resemble the single or double discoid type, but the zonary placenta always has a central or marginal effusion of the maternal blood (the haemophagous organ).


  • It is found in a mixed group of mammals, including man and mouse, bats and rats, rabbits, hares.
  • The chorionic disc may be single (man) or double (monkey).
  • We should remember, however, that not all primates have interstitial
Classification       based       on       the       tissues       or       structures       that       intervene between the maternal and fetal blood


  1. Epitheliochorial type: (Horse, Pig, Cow and Sheep)

In this six structures: the endothelium, connective tissue, epithelium of the endometrium and the trophoblast or the chorion, mesenchyme and and endothelium of the fetal tissue separate the maternal and fetal blood.

  1. Syndesmochorial: (Ruminants)

All tissues of the previous type are present with the exception of the maternal epithelium. The loss of uterine epithelium was previously considered to occur in the placentomes in this type by phagocytosis and cytolysis by the cells of the trophoblast.

  1. Endotheliochorial: (Dog and Cat)

This has four structures seperating the maternal and fetal blood. i.e., the endothelium of the uterine vessels and the chorion, mesenchyme and endothelium of the fetal tissues.

  1. Haemochorial: (Man and Rodents)




Classification of Chorioallantoic Placentas

Chorio Villous Pattern Maternal-Fetal


Loss of Maternal

Tissue at Birth

Pig Diffuse Epitheliochorial None (non-deciduate)
Mare Diffuse and Micro-


Epitheliochorial None (non-deciduate)
Sheep, goat,

cow, buffalo

Cotyledonary Epitheliochorial None (non-deciduate
Dog, cat Zonary Endotheliochorial Moderate (deciduate)
Man, monkey Discoid Hemochorial Extensive (deciduate

Placental Attachment

Attachment or fusion of the placenta to the endometrium of the uterus.

Species Day of Gestation

Cow 30 – 35

Ewe 18 – 20

Mare 50 – 60

Sow 12 – 20

Implantation is the invasion of the embryo into the endometrium where the embryo and placenta continue to develop. This type of placentation is observed in humans, primates and rodents. Dogs and cats have a semi-invasive placentation.

Placental Anatomy and Function

The primary function of the placenta is to accommodate the fetus throughout gestation and to allow for nutrient transfer from the maternal circulation to the fetal circulation so the fetus can grow and develop. It is important to remember that the maternal and fetal circulatory systems never mix. Additionally, the individual components of the placenta have specific functions.


Yolk Sac Nutrient supply for the early developing embryo. Becomes vestigial as gestation


Amnion Protects fetus from injury, provides lubrication for parturition, and serves as a

reservoir for urine and waste.

Allantois Fuses with chorion (chorio-allantoic placenta), carries blood vessels of umbilical

cord, which attaches fetus to allantois, and is a reservoir for nutrients and waste.

Chorion Attaches to uterus, absorbs nutrients from the uterus, and allows maternal/fetal

gas exchange. Produces hormones.


  • The placenta functions as a multi-organ performing many functions and substituting for the fetal:
    • Gastro intestinal tract
    • Lung
    • Kidney
    • Liver, and
    • Endocrine
  • In addition, the placenta separates the maternal and fetal organism, thus ensuring the separate development of the


  • The blood of fetus and dam never come in to direct contact. Yet, the two circulations are close enough at the junction of chorion and endometrium so that oxygen and nutrients can pass from the maternal blood to the fetal blood, and waste products in the opposite direction.
  • The placental membrane controls the transfer of a wide range of substances by several processes.


  • Simple diffusion: The movement of molecules from an area of high concentration to an area of low
  • Most molecules of physiologic importance are transferred by some active transport, thus they can be “Pumped” against a concentration gradient allowing the embryo to accumulate higher concentrations of nutrients that exist in the maternal


Group Physiologic role Substances Exchange mechanism
I Maintenance of biochemical homeostasis or protection against

sudden fetal death

Electrolytes, water and respiratory gases Rapid diffusion
II Fetal nutrition Amino acid, sugars and most

water soluble vitamins

Predominantly by active

transport system

III Modification of fetal growth or the

maintenance of pregnancy

Hormones Slow diffusion
IV Immunologic or toxic importance Drugs, anaesthetics, plasma proteins, antibodies and whole cells Rapid diffusion Pinocytosis or leakage through pores in placental




There many similarities between gases exchange across the placenta and the lungs.

  • The major difference, however, is that in the placenta it is a fluid to fluid system whereas, in the lungs it is a gas to fluid system.
  • The process of transfer of O2 from maternal to fetal blood involves its dissociation from the maternal blood, its diffusion through the placental membrane and finally its combination with fetal
  • The umbilical arteries carry deoxygenated blood from the fetus to the placenta, while the umbilical veins carry oxygenated blood from the placenta to the


  • The gas exchange in the placenta takes place through four basic systems.
    • Concurrent
    • Counter current
    • Multivillous
    • Pool
  • The efficiency of oxygen exchange varies with the particular system. It is greatest in the counter current system and least in the concurrent system. The efficiency of the multivillous system is intermediate between the above mentioned systems. In the pool system, gas exchange is less than in the multivillous system but is comparable to a concurrent
  • It is difficult to ascertain which of these systems is primarily involved in a particular species and probably some species may contain more than one


  • The oxygenated blood from the dam is carried to the fetus through the tributaries of umbilical veins whereas the tributaries of umbilical artery carry the deoxygenated blood from the
  • Water moves very freely between the dam and
  • Of the total substances absorbed by the fetus 77% is
  • However, the water moves from the dam to fetus against the osmotic gradient and low concentration of plasma


  • Sodium is much restricted in the passage through placenta. Iron content is much higher in fetus, which is stored in liver, spleen and bone
  • The transfer of trace elements like copper is readily affected. This element also accumulates in the
  • Calcium and phosphorus enter the fetal blood against the concentration gradient. Depletion of calcium and phosphorus reserves from the dam occurs in cases of low plane of
  • Fetal blood comprises of 70–80% fructose whereas glucose predominates in maternal blood. Probably glucose is formed in the placenta and stored in the fetal liver which serves as a reserve energy
  • Placenta is not permeable to fat but the fatty acids and glycerol pass freely
  • Vitamin A, D and E are obstructed by placenta and hence their concentration in fetus is much lower. Since water soluble vitamins are synthesized in the rumen their concentration in fetus has not been


Placental Hormones

In addition to its role in transporting molecules between mother and fetus, the placenta is a major endocrine organ. It turns out that the placenta synthesizes a huge and diverse number of hormones and cytokines that have major influences on ovarian, uterine, mammary and fetal physiology, not to mention other endocrine systems of the mother.

This section focuses only on the major steroid and protein hormones produced by the placenta.


Steroid Hormones

Sex steroids are the best known examples of placental hormones. Two major groups are produced by all mammals:


Progestins: Progestins are molecules that bind to the progesterone receptor. Progesterone itself is often called the hormone of pregnancy because of the critical role it plays in supporting the endometrium and hence on survival of the conceptus.

The placenta of all mammals examined produce progestins, although the quantity varies significantly. In some species (women, horses, sheep, cats), sufficient progestin is secreted by the placenta that the ovaries or corpora lutea can be removed after establishment of the placenta and the


pregnancy will continue. In other animals (cattle, pigs, goats, dogs), luteal progesterone is necessary throughout gestation because the placenta does not produce sufficient amounts.

Progestins, including progesterone, have two major roles during pregnancy:

  • Support of the endometrium to provide an environment conducive to fetal survival. If the endometrium is deprived of progestins, the pregnancy will inevitably be terminated.
  • Suppression of contractility in uterine smooth muscle, which, if unchecked, would clearly be a disaster. This is often called the “progesterone block” on the myometrium. Toward the end of gestation, this myometrial-quieting effect is antagonized by rising levels of estrogens, thereby facilitating

Progesterone and other progestins also potently inhibit secretion of the pituitary gonadotropins luteinizing hormone and follicle stimulating hormone. This effect almost always prevents ovulation from occurring during pregnancy.


Estrogens: The placenta produces several distinct estrogens. In women, the major estrogen produced by the placenta is estradiol, and the equine placenta synthesizes a unique group of estrogens not seen in other animals. Depending on the species, placental estrogens are derived from either fetal androgens, placental progestins, or other steroid precursors.

With few exceptions, the concentration of estrogens in maternal blood rises to maximal toward the end of gestation. Two of the principle effects of placental estrogens are:

  • Stimulate growth of the myometrium and antagonize the myometrial-suppressing activity of progesterone. In many species, the high level of estrogen in late gestation induces myometrial oxytocin receptors, thereby preparing the uterus for
  • Stimulate mammary gland development. Estrogens are one in a battery of hormones necessary for both ductal and alveolar growth in the mammary

Like progestins, estrogens suppress gonadotropin secretion from the pituitary gland. In species like humans and horses, where placental estrogens are synthesized from androgens produced by the fetus, maternal estrogen levels are often a useful indicator of fetal well being.


Protein Hormones: Several protein and peptide hormones are synthesized in placenta of various species. They have effects on the mother’s endocrine system, fetal metabolism and preparation of the mother for postpartum support of her offspring.


Chorionic gonadotropins: As the name implies, these hormones have the effect of stimulating the gonads, similar to the pituitary gonadotropins. The only species known to produce a placental gonadotropin are primates and equines. The human hormone is called human chorionic gonadotropin or simply hCG. This hormone is produced by fetal trophoblast cells. It binds to the luteinizing hormone receptor on cells of the corpus luteum, which prevents luteal regression. Thus, hCG serves as the signal for maternal recognition of pregnancy. The first hormone you produced was hCG. Equine chorionic gonadotropin is also produced by fetal trophoblast cells. It is actually the same molecule as equine luteinizing hormone.


Placental lactogen: These hormones are molecular relatives of prolactin and growth hormone. These hormones have been identified in primates, ruminants and rodents, but not in other species.



Relaxin: Relaxin is a hormone thought to act synergistically with progesterone to maintain pregnancy. It also causes relaxation of pelvic ligaments at the end of gestation and may therefore aid in parturition. In some of the species in which relaxin is known to be produced, it is produced by the placenta, while in others, the major source is the corpus luteum. In some species, relaxin is produced by both the corpus luteum and placenta.


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