Evolution of Viviparity

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Evolution of Viviparity E. C. Amoroso -
Reproduction is one of the cardinal attributes of
all living things
and although the end result in all instances
is the same, the ways by
which it is accomplished may
be quite different. Irrespective of the
ordinal rank, living
creatures face much of the same problems in
their efforts to survive as
individuals and as species. Viviparity
encompasses a broad span of adaptations for the
retention of young in the
dam during early embryonic development.
Species must adjust according to their
environment and seasonal changes. These changes
have been made through the use of the endocrine
system. However, it is the tissues and not really
the hormones that have developed to react.
Seen in fish to man
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How did the ovary evolve, and how was its
evolution connected to that of eutherian
viviparity? How did oocytes become enclosed in
follicles?
Irving
Rothchild
Four unique characteristics of the mammalian
follicle 1) Mature follicle is vesicular 2)
Granulosa cells more differentiated at follicle
maturity 3) Contains more total cells compared
to other vertebrates 4) In eutherians, oocyte is
smallest of all vertebrates and contains no yolk
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Was the loss of vitellogenin gene expression a
predecessor to Viviparity?
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Follicular Stages of Maturation. Depiction of
representative primordial (pig) (a), primary
(pig) (b), preantral (hamster) (c), incipient
antral (mouse) (d), small antral (hamster) (e)
and Graafian (pig) (f) follicles.
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How did the mammalian ovary change from the large
number of oocytes released in spawning
vertebrates?
Follicular atresia is a common fact of ovarian
development in almost all mammalian species.
The oocyte and the granulosa cells are both
involved with the survival of the oocyte in
development. (Obligatory Symbiosis)
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Developing Tertiary Follicle
Tertiary Follicle
Granulosa Cells
Antrum
Nucleus
Primary Oocyte
Theca Interna
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The Corpus Luteum of Eutherians is Unique

1. Granulosa Cells become luteinized and theca
   interna.
2. A large number of cells form the CL.
3. Cumulus oophorus not part of the CL formation.
   (except Marsupials)
4. Endothelial Cells invade and angiogenesis allows
   formation of dense capillary network.

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Before true viviparity could be attained, the
problem of endocrine adaptation for retention of
fertilized eggs had to be solved.
A prerequisite to internal gestation - Internal
fertilization (also evolution of intromittent
organs for copulation). Suppression of Ovulation
during pregnancy. Intrauterine care of young
until they attain greater degrees of development.
Also, the need for placental development to
absorb nutrients. Adoption of the corpus luteum
as an endocrine gland. Development of
receptor! Signaling by the conceptus to dam to
maintain pregnancy Maternal Recognition of
Pregnancy. Participation of biochemical
secretions of the conceptus in extending
gestation beyond the length of the normal estrous
cycle. Abrogation of the maternal immune response
against the conceptus (Fetal Allograft) Involveme
nt of the fetus in the initiation of
parturition. Endocrine control of lactation in
preparation for nutrient requirements of
newborn.
1. 2. 3. 4. 5. 6. 7. 8. 9.
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Key Alterations to Progress to Eutherian
Viviparity

• Reptilian form of viviparity to Marsupial
  viviparity

Marsupial implantation to parturition last no
longer than 4 to 10 days.
Progesterone is not involved with maintenance of
pregnancy.
The marsupial thus lost the ability to hold the
embryo in the uterus for a long period of time.
Developed the use of the pouch and milk
secretion to complete development.
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II. Marsupial to Eutherian Viviparity
Uterus regained the ability to hold the embryo
in the uterus Uterus developed the ability to
respond to progesterone. The corpus luteum
developed the ability to respond to extrinsic
stimuli.
Thus Rothchild theory is that eutherian
viviparity actually returned to a reptilian form
with notable exceptions. Placentation more
prolonged and advanced. Eutherian trophoblast
and placentation, may be improvisations from
marsupials that made prolonged intrauterine
gestation possible.
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Evolution of viviparity must have involved
increasingly longer retention of the conceptus
with the body of the mother so that they might
develop to a more mature stage.
Pointed out that when mammals evolved phenomenon
of utero-gestation, the chosen place for shelter,
the uterus was developed form part of the oviduct
which existed as a channel that had for its
purpose, the efficient transportation and
discharge of eggs, not retention and maintenance
Corner (1947) -
Acquirement of a viviparous relationship between
parent and offspring may involve
Primitive - Simple storage of young for advanced
embryo development - embryos development through
yolk reserves (species - molluses, annelids and
cockroaches). Although development occurs in
mother, no nutrients provided by her. Advanced -
Through formation of the placenta are nourished
by the secretions provided by the maternal
system. Simple type - Teste fly - single egg in
uterus with special structures serve to nourish
larva. Milk gland has a nipple similar to
marsupial. It also provides antibodies like
colostrum in ungulates. No ovulation until young
removed from uterus.
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Examples of adaptations in invertebrates
Peripatus (Worm-like anthropod) Each egg becomes
implanted in a newly formed segment of the
oviduct (similar to yolk sac placenta). As it
develops, remains attached and is displaced
distally while new embryos are added proximally.
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Without hard shell Remain in maternal system for
a period of time
Adaptations in Fishes Although most
elasmobranches are oviparous, a few are
ovoviviparous and some show advanced conditions
of viviparity. Chief adaptations 1.
2. 3. 4.
5. 6.
Egg-layers
In some live-bearing fish, the ovary provides
housing and nourishment of young. Caused by lack
of ovulation stimulation? At later stages become
sensitive to ovulating- gonadotropins or
estrogen. In some fish corticosteroids involved
(Stress?)
Internal Fertilization Development of Mullerian
Duct as an organ for development of
young Development of fetal Yolk Sac as a Placenta
- Degree of uterine secretions and liver is
negatively correlated. Loss of Shell - Need for
respiration and water uptake - need to increase
vascular supply. Development of corpora lutea on
ovaries. Involvement of progesterone?? Amphibian
- CL produce progesterone Involvement of
pituitary gonadotropins? Pituitary-Gonadal
interactions. Control of parturition - Sharks -
Relaxin
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Adaptations in Reptiles Various reptiles have
developed rudimentary placentas that may be the
fore-runner to that inherited by mammals.
Reptiles have a chorion, allantois and a yolk
sac. Chief adaptation 1.
2. The involvement of progesterone is
reptilian pregnancy remains equivocal. It should
be clearly indicated that although observations
seem to deny ovarian participation in maintenance
of gestation in reptiles, evidence demonstrated
that the ovaries are capable of producing steroid
hormones.
Is progesterone involved with inhibiting
parturition and slowing embryo growth which
occurs at a more accelerated rate without it?
Estrogen stimulates vascularity.
Corpora lutea form and function to produce
progesterone (both in oviparous and viviparous
species). CL function throughout gestation unlike
birds in which they are short-lived. Development
of Chorioallantoic membrane. Pituitary control of
stimulating progesterone. Stress - increase
ACTH--increase P4. Parturition - arginine
vasotocin, PGF2a, oxytocin
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Hisaw (1959) suggested that the next stage in
evolution of viviparity leading to conditions
found in mammals was the adaptation of the
corpus luteum into the family of endocrine glands.
He suggested that synthesis of progesterone, on
a small scale, was the first step in
specialization. This is probably the most
important contributing factor to viviparity in
mammals. It added a luteal phase with follicular
phase that terminated at ovulation, thus forming
the stages of the estrous cycle.
Adaptations in Mammals The unique feature in
mammals was the establishment of a trophic
and respiratory placental connection between the
dam and conceptus. Through evolution, the
placenta of mammals also developed into
an endocrine organ
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Examples
Monotremes (Only species that lay eggs) - Egg is
small and yolk insufficient to support
development. Evidence that endometrial gland
secretions are absorbed by the egg until shell
is deposited. Egg is layed with involution of
the corpora lutea. Egg transport controlled by
CL as well as suppressing ovulation and
stimulating endometrial development. Involved in
development of mammary gland (much like pig).
Luteinizing Hormone involved with
ovulation. Marsupials Progesterone stimulates
uterine endometrial development. Duration of
pregnancy does not exceed the length of the
estrous cycle. Placenta does secrete
progesterone. Kangaroos and Wallabies have
developed the ability to hold the embryo dormant
for a period of time known as Embryonic
Diapause. Lactation inhibition of
cyclicity. Oxytocin can inhibit progesterone from
CL - May function to inhibit embryo growth during
embryonic lactational diapause. Ovx
delays parturition!
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Eutherian The previous species the development
of viviparity has evolved the interaction
between pituitary gland, gonad, and very
little from the conceptus. However, in majority
of mammals above marsupials, gestation is longer
than the estrous cycle. Therefore, there was a
need to extend the length of embryo
storage. Lengthening of gestation was
accomplished by either prolonging the functional
life-span of the CL or by the placenta acquiring
endocrine capability. Example
Human, monkeys, mare, sheep and guinea
pigs Hypophysectomy does not interrupt pregnancy
in mice and rats This coincides with the time
that chorioallantoic placental circulation is
established to secrete mouse placental lactogen
(mPL). In rat CL (cycle 4 days) maintained by
prolactin after breeding then mPL
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Apart from structural modification in the
arrangement of the female genital ducts, mammals
have uniformly adopted endocrine mechanisms to
solve many problems posed by viviparity. Five
components are involved 1. 2.
3. 4. 5.
Corpus luteum - Progesterone Hypothalamus -
Hypophysis - LH and Prolactin Placenta - PL,
Steroids, hCG, eCG Uterine endometrium -
Secretions, PGF2a Immunoglogical Protection
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Mammals needed to develop signal for Maternal
Recognition of Pregnancy
Embryonic Signals vary from species to species -
Regression of CL in majority of species is
achieved through release of a uterine
luteolysin (PGF2a) Cow, Ewe -
Interferon t Gilt -
Estrogen Mare - Protein?
Women - hCG Mouse, Rat -
Placental Lactogen
Attachment, or fusion, between the placenta and
uterine surface constitutes a naturally
occurring instance of tissue transplantation,
which with prolonged gestation period, represents
an allograft that breaks one of the fundamental
laws of transplantation immunology.
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"Facts don't make a theory it's the theory that
makes facts visible" (source unknown). In other
words, how we look at a fact can be more
important than the fact itself.
Irving Rothchild