Placental mammals

Monotremes are oviparous or egg-laying mammals. Marsupials are ovoviviparous, meaning that the egg is large and has a yolk adequate to nourish the embryo during its early development, but it remains unattached to the uterus wall. Gestation in marsupials is necessarily short, therefore, and the young are born in an immature, fetal stage. They make their way to the mother’s pouch and continue to grow, nourished by the mother’s milk. All other mammals, termed placental, are viviparous. The small egg, lacking food substance, becomes attached to the uterine wall, and the developing embryo is nourished by the mother’s blood passing through a placenta. This process allows longer gestation, and as a result, the young are born in a more advanced (precocial) state of development.

The placental mammals form a diverse and successful group that includes the insectivores (such as shrews, hedgehogs, and moles), bats, sloths, anteaters, armadillos, primates (to which humans belong), rodents, rabbits, whales, dolphins and porpoises, carnivores (such as cats, dogs, and bears), seals, aardvarks, elephants, hyraxes, manatees, uneven-toed mammals (such as tapirs, horses, and rhinoceroses), and even-toed (cloven-hoofed) mammals such as pigs, camels, deer, sheep, cattle, and goats.

Placenta Structure

The term “placenta” comes from the Latin meaning “flat cake” and is used to describe the flat structure (in most animals) which attaches the developing embryo and fetus to the wall of the uterus. The term was first used in 1559 CE, although knowledge of the placenta, at least in humans, goes far back into antiquity. Reference to it may be found in many ancient texts and drawings, including the Old Testament books of the Bible. Early Egyptians considered the placenta to be the seat of the external soul.

The placenta is the organ responsible for transmitting materials between mother and fetus before birth—the only bridge between them. In many species, it has important endocrine functions, producing hormones necessary for the development of the fetus or for the maintenance of the pregnant state. It is essentially a product of the developing ovum and the mother. The placenta is commonly referred to as the afterbirth, extruded from the mother’s uterus at the end of the birth process.

When the ovum is released by the ovary and fertilized by sperm, it eventually comes to rest in the hollow cavity of the uterine horn (one of two chambers) or the uterus (single chamber). While it moves toward that area, it begins to divide, forming a ball of cells known as a morula. As development continues, the ball becomes hollow, and it is then referred to as a blastocyst. Within this hollow blastocyst, a few cells protrude into the cavity, forming a knob. These are the only cells that will eventually develop into the embryo and fetus. The rest of the cells are responsible for forming the supportive structures, including the chorion, the placenta, and the umbilical cord (which attaches the embryo to the placenta).

Among the eutherian (placental) mammals, a variety of placental configurations occurs. In many species, the entire chorionic sac becomes connected to the uterine wall, and the transfer of materials between the maternal and fetal compartments occurs over the whole surface. In other species, a much more specialized system develops. Here, parts of the chorion (a membrane equivalent to the one that lines the shell of reptile and bird eggs) become highly specialized, establishing an intimate relationship with the uterine tissues. Thus, the transfer of materials occurs only in one select region of the chorion, referred to as the placenta. It is these tissues, with their flattened, cakelike appearance, from which the name derives.

The most important feature of the placenta is the close contact between the fetal blood vessels in the placenta and the maternal blood vessels in the uterine wall. While it is a common misconception that fetal and maternal blood mix or flow together, this is not a correct picture. What actually occurs is that the two blood systems come close to each other, at which point materials that can diffuse out of one vessel may diffuse into another. Thus, transfer of materials from mother to fetus, and vice versa, can occur. The closer the two blood pools come to each other, the better it is for transfer. Nutritional, respiratory, and excretory products are transferred.

Placenta Types

In the epitheliochorial placenta, as found in hoofed mammals, such as Artiodactyla and Perissodactyla, the wall of the uterus retains its surface epithelium. The minimum separation of bloods is four cells thick (two epithelial cell layers and the endothelium of the blood vessels). Thus, it is vital to have a large surface area to allow adequate movement of materials. While this is a large improvement over the marsupial system, it is 250 times less efficient at salt transfer than the placenta used by humans.

The separation between mother and fetus is reduced in carnivores and sloths. Here, the chorion invades the uterine epithelium and comes in direct contact with the epithelium of the maternal blood vessels, allowing a more uniform transfer of materials. The most advanced form, showing minimal separation, is the hemochorial placenta, found in humans, rodents, bats, and most insectivores. Here, the maternal blood vessel walls are chemically broken down, and the invading chorion is now in direct contact with the maternal bloodstream. Because this is so much more efficient for materials exchange, the size of the interacting surfaces can be much reduced.

In addition to the exchange of materials, the placenta plays an important role in immunology. Without the placenta, the mother’s body would reject the developing embryo like any other foreign body. It is this tolerance of the embryo that separates the placental mammals from the marsupials and allows for gestation periods to be extended. Fetuses in placental mammals receive antibodies from their mothers, thus enhancing their early immunity to disease.

The epitheliochorial placenta, being only in contact with the uterine wall and not being invasive, is readily shed by the uterus when the fetus is born. There is no damage to the maternal tissue. The more invasive types of placenta, including the hemochorial type, can only be lost by separation through the uterine tissues. Thus, birth in species with hemochorial placentas is of necessity associated with some degree of maternal bleeding. In fact, in many hemochorial placentas, the blastocyst actually digests (chemically) the endometrial lining of the uterus and comes to lie completely within it. The endometrium then heals over the blastocyst, which then grows, fully surrounded by the endometrium. The true placenta forms on the deep pocket of the endometrium where the blastocyst lies. When the fetus is expelled from the uterus (birth), it ruptures the now very thin layer of stretched endometrium that covers the chorion. The placenta separates from the uterus due to the rupture of the uterine blood vessels and tissues when the uterus contracts down after the expulsion of the fetus. Bleeding between the uterus and placenta produces a clot that eventually seals the broken blood vessels and forms the basis for endometrial repair.

Most mammalian placentas, regardless of type, have some type of endocrine (hormonal) function. While the specific hormones may vary from species to species, two, in particular, are found in most placental mammals. Chorionic gonadotropin is a hormone secreted by the placenta which acts upon the ovary to increase progesterone synthesis. Progesterone, in return, is responsible for maintaining pregnancy during the early phases. Placental lactogen, another hormone secreted by most placentas, acts on the mother to stimulate mammary gland development. This occurs throughout gestation so that the mammary glands are ready for suckling by the time the offspring is born.

Gestation Periods

The length of gestation varies tremendously among placental mammals. In elephants, the gestation period is between eighteen and twenty-two months. However, size alone does not determine gestation length. The giant among all mammals, the blue whale, has a gestation period of ten to twelve months, not appreciably longer than the human (nine to ten months).

Many bats, bears, mice, otters, and other mammals can delay implantation, also called embryonic diapause, in which the fertilized ovum remains dormant in the blastula stage (about eighty cells), considerably extending the gestation period and delaying birth until the optimal season of warm weather or abundant food is present. Often, a placenta is not found during this period of delay. Thus, the gestation period of the fisher, a small North American carnivore with delayed implantation, is forty-eight to fifty-one weeks, or about the same as that of the blue whale. Gestation varies from twenty-two to forty-five days in squirrels, twenty to forty days in rats and mice, two to seven months in porcupines, six months in bears, and fourteen to fifteen months in giraffes.

Gestation length is constrained by the size of the skull, which will fit through the maternal pelvis. Where agility, speed, or long distances of travel put a premium on the mother’s athleticism, gestation length is often shortened, and the birth weight of the offspring will be low. Animals having long gestation periods, or whose young mature slowly and are suckled for a long time, generally do not breed as often as others. Many species of mice breed repeatedly throughout the spring, summer, and fall. They have a gestation period of about twenty days and are mature and ready to breed twenty-one days after birth.

Many others, such as bears, deer, coyotes, and weasels, breed only once a year. Animals that have a breeding season usually have a gestation period that accommodates the timing of the birth, ensuring there will be ample food and no extreme temperatures that could harm the newborn. Environmental conditions and the adaptability of various species to these conditions play a large role in breeding cycles. It is advantageous for young to be born during the season of least severe weather and to be weaned when food is most abundant. Many tropical mammals breed and give birth throughout the year, whereas in temperate or cold climates, young are usually born in the spring or summer.

Similar factors also influence the number of young born in each litter among different species. Their growth rate until weaned, mortality rates, nutrition, inbreeding, adult activity cycles, genetics, and environmental factors help determine the litter size. A general rule for mammals is that the number of nipples the mother has indicates her maximum litter size and half of that number is the average litter size for the species. This is not without exceptions, but it is a guideline. Mice have an average of six to eight pups in a litter but can have as many as fourteen. A litter size under three is uncommon in most rodents. Seals, whales, and most large mammals bear only one young at one time. Some species, like the tamarins and marmosets, usually have twins rather than a single young, though three is uncommon in primates. Among mammals, Madagascar's tailless tenrec has the largest litter, with a litter of thirty-two observed in the wild, though the average is between fifteen and twenty. There have also been reports of the naked mole rat having litters of over thirty.

Principal Terms

Chorion: the outer cellular layer of the embryo sac of reptiles, birds, and mammals; the term was coined by Aristotle

Embryo: a young animal that is developing from a fertilized or activated ovum and that is contained within egg membranes or within the maternal body

Endometrium: an inner, thin layer of cells overlying the muscle layer of the uterus

Fetus: a mammalian embryo from the stage of its development where its main adult features can be recognized, until birth

Maternal: referring to the female parent

Ovum: an unfertilized egg cell

Uterus: in female mammals, the organ in which the embryo develops

Viviparous: producing young that are active upon birth (often referred to as live birth); the embryo is nurtured within the uterus

Bibliography

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