With the exception of birds, most non-mammalian vertebrates depend on the production of a relatively large number of eggs with only a small proportion surviving. Mammals have evolved a system wherein relatively few eggs are produced, but the survival rate is relatively high. This is accomplished by a combination (in all except the prototheres) of prenatal care and, in most, post-natal care.
In the generalized early development in eutherian mammals, the early cleavages produce the blastocyst, a hollow ball of cells. A cluster of cells intrude into the cavity of the blastocyst: the inner cell mass. This eventually will produce the embryo and some extraembryonic membranes. The outer layer of the blastocyst is the trophoblast. The trophoblast eventually develops into the chorion of the placenta and associated structures.
The placenta forms the connection between mother and child, allowing for nutrition, gases, and excretionary materials to move by diffusion between embryonic and maternal circulatory systems. Bacteria and many large molecules are prevented from passing from the maternal bloodstream to that of the embryo. The placenta also produces hormones important to maintain the pregnancy. Different types of placentae characterize different mammalian groups. The choriovitelline placenta is considered the most primitive. It's found in all marsupials except the bandicoots (Peramelidae). In this, the yolk sac (in egg-laying vertebrates, the yolk sac grows to enclose the yolk) is greatly enlarged to form the placenta. The embryo doesn't implant deeply into the uterine lining but sinks into a shallow depression. To considerable extent, nourishment is by "uterine milk", a substance secreted by the wall of the uterus and then absorbed by the embryo. There also is some nourishment transferred between the maternal blood in the uterine mucosa and blood vessels in the yolk sac.
The chorioallantoic placenta occurs in the Peramelidae and in all eutherians, although transfer between maternal and embryo tissues in peramelids is less than in the eutherians. In eutherians, the blastocyst eventually sinks into the endometrium (the lining) of the uterus. Villi (cylindrical projections) rapidly develop, pushing farther into the endometrium with local breakdown of the endometrium; the blastocyst cells absorb the breakdown products for nourishment until the villi are fully developed and embryonic circulatory system is functional. Vascularization of the uterus in the region of implantation proceeds rapidly and material is exchanged across the embryonic and maternal circulatory systems. The villi produce a huge surface area for this (estimated at a length of ca. 48 km in humans).
Four extraembryonic membranes develop. The chorion mentioned above completely surrounds the embryo and the other membranes. The amnion grows to enclose the embryo, encasing it in "its own private swimming pool" (and thus eliminating the necessity to laying eggs in the water). The empty yolk sac is responsible for the early production of blood cells. The allantois begins as out outgrowth of the embryonic gut and is incorporated into the umbilical cord, producing the blood vessels connecting placenta and embryo for the transfer of gases, nutrients, and waste matter.
There are a number of placental types named according to the degree of separation of embryonic and maternal blood. For example, in the epitheliochorial placenta, the epithelium of the chorion rests in pockets in the endometrium, contacting endometrial epithelium. In this case, blood of the two individuals is separated by tissues of the blood vessels and intervening connective tissue. This type occurs in lemurs, suids, equids, and whales. On the other hand, in the hemochorial placenta, maternal blood directly bathes the villi; this type is found in some insectivores, bats, higher primates, and some rodents. The most extreme type is found in rabbits and some rodents, where only the endothelial lining of the villi blood vessels separates fetal and maternal blood (the hemoendothelial placenta). Villi distribution of the chorion also varies among mammals and has its own set of terminology.
Various cycles governed by hormones cued by environmental events (and to a degree, innate) typify mammalian reproduction. The ovarian cycle has a follicle growth phase with release of the ovum. The follicle is the egg surrounded by specialized ovarian cells. The second phase is the development of the corpus luteum. The cycle apparently is controlled largely by the pituitary and ovary secretions of hormones. FSH (follicle-stimulating hormone) is produced by the pituitary as is LH (luteinizing hormone). These stimulate growth and initiate ovarian secretion of estrogen which in turn stimulates more pituitary production of LH. This increased LH starts production of LTH (luteotropic hormone) and reduces pituitary production of FSH. These events result in release of the ova from the follicles; the corpus luteum is formed from the now emptied follicle. LTH from the pituitary maintains the corpus luteum which begins to produce progesterone. The latter prepares the uterus for implantation. If fertilization doesn't occur, the corpus luteum diminishes and estrogen and progesterone production eases, allowing the pituitary to resume production of FSH and thus initiating another ovarian cycle. A number of variations on the sequence occur. During the ovarian cycle, the uterus varies according to the stage of the cycle. Before ovulation, the endometrium becomes thicker: the proliferation phase. For many species a period of "heat" (estrus), an increase in female receptivity, occurs at the end of the proliferation uterine phase, but before ovulation. Following ovulation, the endometrium continues development and also become highly vascularized; this is the progestational phase of the uterine cycle. In most mammals, if fertilization doesn't occur, the endometrium shrinks and the vascularization decreases; the period of receptivity is short. The uterine cycle in such animals is referred to as the estrus cycle. Mammals that have a single estrous cycle per year are monestrous; those that have more are polyestrous.
The primate cycle is somewhat different than that described and is called the menstrual cycle. Differences include bleeding at the time of endometrial breakdown, regular ovulations throughout the year, and a tendency toward receptivity over an extended length of time. Except in humans, though, copulation tends to be cyclic, occurring only during the time of ovulation, when the female vulva is swollen.
If pregnancy occurs, various hormones work to preserve the pregnancy. Early on, chorionic gonadotropin is critical; this is produced by the embryo. Progesterone from the corpus luteum maintains the uterus thickness and vasculization and decreases possible contractions. In many mammals, including humans, much of the later hormonal support for pregnancy is taken over by the placenta. In most, perhaps all, placentals, relaxin causes relaxation of pelvic ligaments and the pubic synthesis. Toward the end of pregnancy, oxytocin produced by the hypothalamus increases and eventually results in uterine contractions for birth. Several other changes in hormone concentrations also appear to be involved.
Estrogen and progesterone from the placenta during pregnancy result in growth of mammary gland tissue, with eventually milk production stimulated and regulated by prolactin from the anterior lobe of the pituitary. Reduction of placental hormones at parturition removes an inhibitory effect, allowing milk production. Continued milk production depends on the suckling stimulus to the central nervous system.
There is a great dichotomy between embryonic development in most marsupials and in placentals. Marsupials are born in virtually an embryonic state (compared to placentals). After a very short gestation period, the newborns climb from the birth canal with relatively highly developed forelimbs, though most of the body basically is embryonic. They remain attached to nipples until organogensis, etc., is completed. In placentals, the fetus is relatively complete at birth, with most post-birth development being growth and some further development of the brain (in some).
A prominent hypothesis regarding the difference has to do with the immune system. Embryos carry some paternal antigens, and the immune system would attack these if these antigens were introduced into the mothers. In at least some marsupials, the eggshell membranes are retained through about 2/3 of the gestation period. It is thought that these membranes provide protection from the immune system of the mother. It's suggested that the birth shortly after the loss of this protection is an adaptation to prevent immunological attack.
In placentals, a layer of non-cellular material (the zona pellucida) surrounds the early embryonic stages, separating the embryo from the maternal tissues. Later, the trophoblast tissues are thought to be protective, with the chorion producing a gonadotropin blocking an immune response.
It's suggested that the greater morphological diversity of placentals versus marsupials may be because of differences in early development. The marsupial needs to have forelimbs adapted to crawling to the nipples; this presumably prevents such adaptations as flipper for swimming or wings for flying.
Last Update: 4 Feb 2008
Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso