In the interhemal area, the maternal vessels and fetal vessels are situated just below the basement membranes of the endometrium and trophectoderm without the destruction of endometrial tissue 22 , 23 Fig. However, the endometrium and trophectoderm are thin and deeply indented by the blood vessels as pregnancy proceeds, resulting in shorter diffusion distances across the epitheliochorial placenta 2.
The depths between the chorionic folds, the so-called areolus, are lined by tall, columnar trophoblasts areolar trophectoderm that are actively phagocytic Fig. Uteroferrin, an iron-containing glycoprotein, is released from the endometrial glands to the lumen, taken up by the areolar trophectoderm, and then transferred to the fetus, as an iron source Many endometrial glands are observed under the endometrium Fig.
Pig minipig placenta. Epitheliochorial type placenta. Dogs have an endotheliochorial and zonary type of placenta 26 Fig. Histologically, the placenta of dogs is composed of the labyrinth zone, the junctional zone and the glandular zone Fig. The labyrinth zone is composed of trophoblastic lamellae, in which cytotrophoblasts and syncytiotrophoblasts cover the maternal vessels Fig.
The maternal vessels are surrounded by a noncellular layer, which is positive for periodic acid-Schiff PAS stain and Alcian blue stain. The fetal vessels deeply indent the trophoblasts.
The junctional zone is an area of transition between the labyrinth zone and gland zone Fig. The trophoblasts, which show tall columnar cells in monolayers with microvilli on the free surface, invade into the endometrial gland cavity. Particularly, the deep part of the junctional zone is called the sponge zone Fig.
The glandular zone is composed of the remnants of endometrial glands. These glands become distended by retained secreted function as the result of obstruction of their mouths by penetrating trophoblasts Fig. Marginal hemophagous zones filled with maternal blood develop at both edges of the placenta or in the middle of the placenta 22 Fig.
They are lined by high columnar trophectoderm showing active phagocytosis and digestion of erythrocytes, and are considered to have a relationship with placental iron transport Dog placenta. Endotheliochorial type placenta. Rats and mice have a hemotrichorial and discoid type of placenta 9 , 28 Fig. Histologically, the placenta of rats and mice is composed of the labyrinth zone, the basal zone, the decidua and the metrial glands 29 , 30 , 31 , 32 Fig.
In the labyrinth zone, there are three layers of trophoblasts, separating the maternal blood spaces from the fetal blood vessels 2 , 18 Fig. The outer trophectoderm, which comes into direct contact with the maternal blood, is referred to as cytotrophoblasts with a microvillous surface. Under this trophectoderm, there are two layers of syncytiotrophoblasts. The basal zone is comprised of three types of differentiated cells: 1 spongiotrophoblasts, 2 trophoblastic giant cells and 3 glycogen cells Fig.
The spongiotrophoblasts are present immediately above the trophoblastic giant cell layer located at the materno-fetal placental interface. The glycogen cells form multiple small cell masses and develop into glycogen cell islands in midgestation, and then most of them disappear before parturition. The decidua is comprised of the mesometrial decidual cells ultimately, and plays essential roles in the development of the vascularized decidual-placental interface.
The metrial gland is located in the mesometrial triangle of the pregnant uterus from early gestation and is fully developed in midgestation, leading to regression before parturition 30 , It is composed of decidualized endometrial stromal cells, uterine natural killer cells, spinal-shaped arteries, trophoblasts originating from glycogen cells, and fibroblasts 34 , 35 Fig.
The yolk sac is composed of epithelial cells and mesodermal cells Fig. Because the parietal yolk sac ruptures in midgestation, the inside of the visceral yolk sac becomes exposed to the intrauterine cavity and is called a reversed yolk sac placenta, which functions throughout pregnancy.
Rat placenta. Hemotrichorial type placenta. Rabbits have a hemodichorial and bidiscoid type of placenta 22 , 28 , 36 Fig. In the labyrinth zone, there are two layers of trophoblasts, an outer and inner layer separating the maternal blood spaces from the fetal blood vessels 2 , 22 , 39 Fig. The outer trophectoderm, which comes into direct contact with the maternal blood, is comprised of the syncytiotrophoblasts, which are joined to the underlying cytotrophoblast layer by adhesion junctions.
The inner trophectoderm is one layer of cytotrophoblasts overlying fetal blood vessels. The junctional zone is composed of glycogen cells containing PAS-positive substances Fig. These cells are transiently detected in midgestation, and disappear before parturition. The decidua originates from stromal cells of the mesometrial endometrium and is divided into the zone of necrosis and the zone of separation in midgestation. The zone of necrosis develops with dilated blood vessels as pregnancy advances.
This zone is detected under the junctional zone and is composed of necrotic tissue. The zone of separation becomes thinner without necrosis as pregnancy advances Fig. The structure and functions of the yolk sac placenta are the same as those of rats and mice Fig. Rabbit placenta. Hemodichorial type placenta. Cynomolgus monkeys have a hemomonochorial and bidiscoid type of placenta 28 Fig.
Histologically, the placenta of cynomolgus monkeys is composed of the placental villi, the chorionic plate, the basal plate and the decidua 31 , 40 Fig. The placental villi protrude into the intervillous space and are bathed directly in maternal blood. The anchoring villi are peripheral ones that are connected to the basal zone. The placental villous surface is composed of an outer continuous layer of syncytiotrophoblasts in contact with maternal blood and an inner discontinuous layer of cytotrophoblasts 2 , 41 Fig.
The stroma of the placental villi is composed of fetal vessels and mesenchyme. The chorionic plate is populated with mesenchymal cells within a fibrous connective tissue, and represents the cover of the intervillous space. Tree-like arranged placental villi arise from the chorionic plate Fig.
The basal plate is the bottom of the intervillous space and the junction of the endometrium with fetal tissues Fig. The basal plate is composed of extravillous cytotrophoblasts, endometrial stromal cells, decidual cells, etc. The placenta of cynomolgus monkeys is very similar to the human placenta 2.
Cynomolgus monkey placenta. Hemomonochorial type placenta. The fully formed placenta plays a major role in maintenance of nutrition for the fetus and in the secretory and essential regulatory functions for maintenance of pregnancy during the fetal period.
As described in this brief review of the anatomical placentas in some experimental animals, the composition of intervening cells in the interhemal areas is different between animal species. Molecules cross the placenta either by diffusion or some form of active or facilitated transport.
In the case of diffusion, the ability for molecules to cross the placenta in either direction is strongly influenced by the interhemal distance or the thickness of the cellular barrier between maternal and fetal blood. A small interhemal distance generally will increase the rate at which molecules can transfer between maternal and fetal blood, either by diffusion or active transport.
Thus, the number of cell layers separating the maternal from the fetal blood is considered to be important in modifying the transfer of nutrients and forming the materno-fetal barrier 42 , 43 , Actually, fatty acids and keto acids are readily transferred from dams to fetuses in the hemochorial placenta of rodents, rabbits and primates, whereas their uptake by ruminants, pigs and horses is very low In addition, the pig is not suitable as an informative model for the study of antibody therapeutics in embryo-fetal toxicity studies, since the pig placenta is impermeable to the passage of macromolecules such as immunoglobulins Also, it is known that there are at least three different mechanisms for iron transport, according to the structure of the maternal-fetal interface 46 hemochorial, penetration; endotheliochorial, phagocytosis; epitheliochorial, secretion.
On the other hand, it is known that there are regions of the pig placenta where the six cell layers of the maternofetal barrier become sufficiently thinned to equal the minimal interhemal distance of the three cell layers in a human placenta, although the mean interhemal distance in the pig placenta is greater than the mean in the human placenta There does not appear to be any difficulty in allowing for the passage of substances based simply on the number of layers separating the different blood supplies, even though there may be differences in transit times 8.
In addition, the disadvantage of the greater difficulty in passage of materials between organisms is partially overcome by a variety of mechanisms Therefore, it has been reported that the interspecies differences in the type of placenta do not play a dominant role in the placental transfer of most drugs, which is determined largely by placental blood flow At any rate, it should be considered that the histological structure separating the maternal blood from the fetal blood modifies the transfer of nutrients, and that the placental structure is one of the important factors for its permeability between different animal species.
In conclusion, the chorioallantoic placenta shows morphological diversity in experimental animals. In reproductive and developmental toxicity studies, careful attention should be paid to the histological structure of the interhemal area when extrapolating information concerning placental transfer characteristics to different animal species. The authors would like to thank Ms. Mayumi Kano Nihon Bioresearch Inc.
Tsuyoshi Yoshikawa and Dr. Kiyoshi Kobayashi, Ms. Kaori Maejima, Ms. Hiromi Asako, Mr. Atsushi Funakoshi, Mr. You know that female kangaroos have a pouch for the final development of their babies.
So, no, kangaroos are not placental mammals. What is a placental mammal? Like other female vertebrates, all female mammals have ovaries. These are the organs that produce eggs see Figure below. Therian mammals also have two additional female reproductive structures that are not found in other vertebrates. They are the uterus and vagina. Therian mammals are viviparous , giving birth to an embryo or infant rather than laying eggs.
The female reproductive system of all therian mammals is similar to that of humans. Therian mammals are divided into two groups: placental mammals and marsupial mammals. Each group has a somewhat different reproductive strategy. Placental mammals are therian mammals in which a placenta develops during pregnancy. Placental mammals give birth to relatively large and mature infants. Most mammals are placental mammals. The placenta is a spongy structure.
It consists of membranes and blood vessels from both mother and embryo see Figure below. The placenta passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placental mammals include such diverse forms as whales, elephants, shrews, and armadillos. They are also some of the most familiar organisms to us, including pets such as dogs and cats, as well as many farm and work animals, such as sheep, cattle, and horses.
And humans, of course, are also placental mammals. Placental mammals all bear live young, which are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, the placenta.
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