PLACENTAL MAMMALS
LIVE BIRTH AND THE PLACENTA
Most vertebrates lay eggs. In reptiles and birds, the embryos are surrounded
by a layer of albumen, a shell membrane, and a shell. These layers are present in monotreme mammals
which lay eggs and some may be retained in live-bearing mammals as well. Marsupial embryos are surrounded by a zona pellucida,
albumen, and a shell membrane. Placental
embryos have a zona pellucida and a thin layer of albumen but have no
trace of a shell or shell membrane (Mossman, ). In marsupials the keratinous shell membrane
is usually lost after the embryo reaches the uterus but may last until
birth in some species (Mossman, ).
Most bony fish and amphibians lay eggs before
they are fertilized, and thus development occurs outside the body of the
female. In most cartilaginous fish,
reptiles, and all birds fertilization occurs inside the body of the female.
While most of these groups lay eggs, reproduction through live
birth (viviparity) is known. Viviparity seems to have evolved separately
about 100 times in amniotes: once in ancestral mammals, once in ichthyosaurs,
and about 100 times in lizards in snakes, in 16 of the 27 families. (Thompson, 2002). Skinks are the largest family of squamates
(with about a third of lizard species). Many skinks are viviparous and
some have evolved placenta. The only 5 groups of squamates with complex
placentae are skinks, including members in 2 of the 4 subfamilies. Viviparity and oviparity can not only be found
in the same family, the can exist in the same subfamily, such as the skink
subfamily Lygosominae. Within subfamily Lygosominae, there are 3 species
groups. The Eugongylus species
group contains both oviparous and viviparous species and the viviparous
species have at least 2 types of placenta (Thompson, 2002).
In
addition to these amniotes, there are viviparous cartilaginous fish, actinopterygian
bony fish, sarcopterygian bony fish (coelocanths), and amphibians (Mossman,
p.3). Thus, it is evident that
a number of anamniote lineages have evolved vivparity separately as well.
These mechanisms of live birth in these various cases are physiologically
different and are isolated in separate lineages (such as in the mechanisms
of live birth in the two bony fish Heterandria
and Cymatogaster) (Mossman, p. 13).
It
is interesting that all amniote embryos share the same extraembryonic
membranes whether they develop in eggs or inside the body of the female. Amniotes share four extra-embryonic membranes:
the yolk sac, allantois, amnion, and chorion. In all amniotes, these extra-embryonic membranes
develop much faster than the embryo itself and an early embryo invests
more cells into these membranes than into what will later become the embryo’s
body. In the illustrations used
in this chapter, the amnion will be depicted with a light blue, the chorion
with red (and the chorionic cavity with a light red), the yolk sac with
yellow, and the allantois with green.
In some illustrations, the maternal tissue will be represented
with purple.
In the following illustration of a oviparous
lizard (egg-laying; the egg shell is not included in the drawing), the
primitive amniote condition is seen. Development
occurs in the fluid of the amnion. The yolk sac provides the nourishment for the
developing embryo and fetus and the allantois collects wastes which develop
during development. The chorion
performs gas exchange with the outside world through the egg shell. Monotremes (the platypus and echidnas) reproduce
by laying eggs and their extra-embryonic membranes perform the same essential
function as observed in reptiles.
AMNION
The
amnion doesn’t vary much in amniotes.
It surrounds the embryo and is filled with an amniotic fluid. As a result, amniote embryos can develop in
a watery environment as do the embryos of fish and most amphibians. This amnion was critical in the adaptation of
amniotes to terrestrial environments.
Although humans are terrestrial animals, we spend our first 9 months
surrounded by fluid. In some mammals
it fuses to the chorion (such as anthropoid primates including humans)
while in others it fuses with the allantois (as in carnivores and artiodactyls)
(Mossman, p. 59).
Cat
Embryo
CHORION
In birds and reptiles, the chorion is the
extraembryonic membrane which lies just deep to the eggshell and performs
gas exchange between the developing embryo and the outside world. In viviparous animals, the chorion performs
gas exchange between the embryo and the environment of the uterus, inside
the body of the female. In placental
mammals, the chorion composes the fetal portion of the placenta.
What is a placenta? Placental mammals possess a chorioallantoic
placenta, in which the placenta is composed of maternal uterine tissue
and fetal chorionic tissue with blood vessels derived from the allantois. This is not the only type of placenta, however.
In some placental mammals, the yolk sac may contribute blood vessels
to the placenta at an early stage creating a transient choriovitelline
placenta. If the definition of
placenta is widened to refer to maternal and fetal tissues which interact
and allow for the exchange of substances, then placentas are not confined
to placental mammals. In fact,
at least a few members of all vertebrate groups except for birds, jawless
fish (represented by two modern kinds), and monotremes possess placentas
(Mossman, p. 3)
In some fish, the pericardium may be expanded
to the point where it forms a pseudoamnion and a pseudochorion around
the neck. In Heterandria
In one frog, extensions to the gills function
for materials exchange while the larvae develop in pouches on the back
of the parent. In the viviparous
salamander Proteus, larval gill
filaments contact the uterine mucosa.
Of course, there are other ways to provide nutrients for the young
in utero. In the genus Salamandra, 40-60 eggs are produced of which only one or two are born
alive. The young feed on the remaining
eggs and even some maternal blood once their yolk is exhausted (Mossman,
p. 22-4).
In amphibians, the gills and tailfin may serve
as placental structures (Mossman, ). Most
vivparous snakes and lizards possess some sort of placenta, including
a chorioallantoic placenta in some lizards (Mossman, ).
Shark embryos vary in their nutrition as is evident in the adjacent drawing.
The following list illustrates the variety
structures that can exist to provide nutrients and/or gas exchange for
a developing fetus:
1)
the yolk sac provides nutrients
2)
a primitive placenta is formed
between folds of the yolk sac and the maternal endometrium
3)
a primitive placenta is formed
from folds of the yolk sac, filaments from the yolk stalk, and the maternal
endometrium
4)
uterine villi contact expanded
gill filaments
5)
uterine villi grow into the
fetal spiracles and secrete materials into the pharynx
6)
the continual ovulation of
under-developed eggs provides fetuses with a food source they can eat
7)
the yolk sac contents are
supplemented with additional yolk around the fetus which it can ingest
(Mossman,
p. 29-31)
Thus, placental mammals are not the only
group of vertebrates which have evolved a placenta. Mammals (named for the mammary glands which
nourish the young after birth) are not the only vertebrates in which parents
can produce nutrients for the young. In
pigeons, for example, the hormone prolactin (the same hormone which causes
milk production in mammals) causes the formation of “crop milk” which
is produced in the crop to nourish the young.
The earliest placental mammals are known
from the Cretaceous.
