The primary reproductive process in female mammals is the production of eggs (ova) from follicles in the ovary. In a non-pregnant female mammal, production of eggs is typically a cyclical process, although there are varying degrees of seasonal restriction such that some female mammals do not show repeated cycles. Seasonality of reproduction in mammals is mainly governed by annual variation in rainfall and vegetation, and hence becomes increasingly common at high latitudes. In many mammals, seasonality of reproduction is indirectly triggered by annual variation in day length, but in some mammals, it is a direct response to rainfall or food avail-
ability. The typical ovarian cycle of mammals begins when one or more follicles ripen to the point where the egg can be released (ovulation). Following ovulation, the residue of the follicle is converted into a corpus luteum (yellow body), which produces progesterone that maintains at least the early part of pregnancy. The basic stages of the ovarian cycle are common to all mammals, with a follicular phase preceding ovulation and a luteal phase afterwards. However, there is a fundamental difference between different mammal groups with respect to the occurrence of ovulation and changes in the ovary. In many mammals, ovulation occurs only if mating takes place (induced ovulation), and therefore formation of a corpus luteum also requires mating. Some species show a slightly different condition in which ovulation takes place without mating, but mating is necessary for formation of a corpus luteum (induced luteinization). In both cases, mating is required for a corpus luteum to form, such that without mating ovarian cycles are confined to follicular phases and are correspondingly short. By contrast, in other mammals both ovulation and formation of a corpus luteum occur regardless of whether mating takes place (spontaneous ovulation). In these species, cycles always include combined follicular and luteal phases and are correspondingly long. Induced ovulation and induced luteinization are found in mammals that breed relatively rapidly, as is the case with most insectivores, tree shrews, many rodents, and numerous carnivores. On the other hand, spontaneous ovulation is typical of mammals characterized by slow breeding, such as hoofed mammals, cetaceans (whales and dolphins), hystricomorph rodents, and primates.
Following ovulation in marsupials and placentals, the egg travels down the oviduct, where fertilization will take place if the female has been inseminated. The fertilized egg (zygote) begins to divide as it completes its journey down the oviduct. By the time the zygote reaches the uterus, it has transformed into a hollow ball of cells (blastocyst). In placentals, the blastocyst is ready to implant in the wall of the uterus as the first stage in the development of placentation that will nourish the developing embryo/fetus. (By definition, a developing embryo becomes a fetus when recognizable organs are formed.) In both marsupials and placentals, development of the embryo/fetus within the uterus involves four embryonic membranes that play different roles. The chorion is the outermost membrane and remains intact throughout development right up to birth. Hence, any nutrients supplied by the mother to the developing offspring must first of all pass through the chorion. In all placental mammals, the chorion is in intimate contact with the wall of the uterus in the placenta. A second embryonic membrane, the amnion, surrounds the developing embryo/fetus throughout pregnancy and its fluid content (am-niotic fluid) provides a protective hydrostatic cushion. The
remaining two embryonic membranes, the yolk sac (vitelline sac) and the allantois, play a crucial role in transfer of nutrients from the mother and in transfer of waste products in the opposite direction across the placenta, to be disposed of by the mother. In the enclosed egg of reptiles, which has been retained by monotremes, the vitelline sac contains a nutrient-rich yolk that is absorbed by blood vessels running over the surface of the sac, while the allantois stores waste products deposited by similar superficial waste products. When a reptile or montreme emerges from the egg, the waste-filled al-lantois is shed. In the development of the embryo and fetus from the yolk-poor egg in marsupials and placentals, nutrients must be provided directly by the mother and waste products must be removed in some way. In a fascinating reversal of function, the superficial blood vessels of the yolk sac in marsupials and placentals absorb maternal nutrients arriving from outside rather than absorbing yolk from inside. In many cases, the superficial blood vessels of the allantois also absorb nutrients coming from the mother as a substitute for the original function of depositing waste products inside the sac. In accordance with the original functional adaptations, in marsupials and placentals the blood vessels of the yolk sac typically develop their exchange role first (chorio-vitelline placenta), while blood vessels of the allantois do so secondarily (chorio-allantoic placenta).
Because monotremes still lay eggs, they are commonly labeled Prototheria to distinguish them from the Theria (marsupials and placentals), which all have live births. Although the fertilized egg is retained within the mother's body for the initial phase of development in marsupials, the impression is often given that there is no placentation in marsupials. It is, indeed, true that in all marsupials a shell membrane is present over the chorion at least for the major part of pregnancy. Widespread use of the name "placental mammal" has unfortunately tended to reinforce the false impression that placen-tation is lacking in all marsupials. In fact, some form of placentation is developed in certain marsupials and a few of them, such as the bandicoot (Perameles), even develop a relatively advanced chorio-allantoic form of placentation. For this reason, many mammalogists prefer the terms metatherian for marsupials and eutherian for placentals, derived from the formal names Metatheria and Eutheria. The fact remains, how ever, that proper formation of a placenta is characteristic of all eutherians, whereas it has secondarily been developed only in some marsupials, so continued use of the easily understandable term "placental mammal" is surely acceptable.
In placental mammals, there is considerable variation in the form of the definitive chorio-allantoic placenta, although as a general rule each order, or at least suborder, of mammals tends to have a particular kind of placentation. Following Grosser (1909), a basic classification of types of placentation into three major categories, reflecting different degrees of invasiveness, can be made with respect to the relationship between the chorion and the inner wall of the uterus. In the least invasive type of placentation, the placenta is diffuse and the chorion is simply apposed to the inner epithelial lining of the uterus. It is labeled epitheliochorial placentation. In the other two kinds of placentation, invasion of the uterine wall occurs to some degree and the placenta is accordingly relatively localized (discoid). When moderate invasion occurs, the uterine wall is broken down in the region of the placenta and the chorion comes into contact with the walls of maternal blood vessels (en-dothelium). This type of placentation is called endothelio-chorial. In the most invasive form of placentation, the walls of
the maternal blood vessels are themselves broken down in the region of the placenta, such that the chorion is directly bathed by maternal blood (haemochorial placentation). Epitheliocho-rial placentation is found in a few insectivores and it is uniformly characteristic of hoofed mammals, whales and dolphins, hyraxes, and strepsirrhine primates (lemurs and lorises). En-dotheliochorial placentation is found in some insectivores, tree shrews, carnivores, sloths, anteaters, armadillos, elephants, and sea cows. Haemochorial placentation is found in many insectivores, rodents, bats, and haplorhine primates (tarsiers, monkeys, apes, and humans).
The evolutionary history of the three basic types of placentation is still subject to debate. It is often stated that the least invasive, epitheliochorial kind of placentation is the most primitive. This seems to be only logical, as the initial development of placentation must surely involve simple superficial contact between the chorion and the inner lining of the uterus. Because it is regarded as primitive, the epitheliochorial placenta is also often believed to be inefficient, notably with respect to development of the brain. By contrast, the highly invasive, haemochorial type of placentation is commonly thought to be very advanced and efficient. Human beings have the largest brain size (relative to body size) found among mammals and they also have highly invasive haemochorial placentation, so this is often seen as proof of the advanced nature of that very invasive type of placentation. However, many mammals with endotheliochorial or haemochorial placenta-tion have relatively small brains, while dolphins, which have noninvasive epitheliochorial placentation, come a close second to humans with respect to relative brain size. In fact, there is much to be said for the alternative interpretation that ancestral placental mammals already had a moderately invasive type of placenta, following a long previous history of development. According to this view, noninvasive epitheliochorial and highly invasive haemochorial types of placentation rep resent divergent specializations away from a moderately invasive ancestral condition. It is noteworthy that during pregnancy, mammals with epitheliochorial placentation show great proliferation of uterine glands in the wall of the uterus. These uterine glands produce a nutrient secretion (so-called uterine milk) that is absorbed by special structures (chrionic vesicles) on the surface of the chorion. The selective advantages of the different basic types of placentation have yet to be identified. However, it is clear that the degree of invasiveness of the pla-centation has little to do with development of large-bodied offspring or of offspring with relatively large brains. Instead, it seems likely that the degree of invasiveness of the placenta reflects a trade-off between the advantages of an intimate pla-cental connection between the mother and her developing offspring and the disadvantages of potential immunological conflict between the mother and her embryo/fetus.
The process of spermatogenesis typically takes place throughout the life span of male mammals, although it may be subject to periodic interruption in those species with a seasonal pattern of breeding. Spermatogenesis occurs as a wavelike process along the seminiferous tubules and completion of
sperm development in any one region takes between several days and a number of weeks. After transfer to the epididymis, the sperm are then stored until ejaculation takes place.
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