Comparative Vertebrate
Reproduction
Part VII. Fertilization and
Cleavage
Introduction and Overview
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Fertilization
External Fertilization
 Internal Fertilization
 Mechanism of Fertilization
 Gamete Fusion
 Sperm Incorporation
 Activation
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Cleavage
Holoblastic Cleavage
 Meroblastic Cleavage
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Fertilization
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External Fertilization
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Spawning
Pelagic or Broadcast Spawning – The least
specialized of the spawners; large number of sperm
and ova are released synchronously in the water
column.
 Collective Spawners – Males and females come
together to release sperm and ova at the same place
and time in a restricted volume of water. (e.g.,
herring below)
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Benthic or Terrestrial Spawners – Ova are fertilized
in a very specific location such as nests and holes
where males are able to easily locate and fertilize
them. (e.g., nest-building sticklebacks, wrasses,
cichlids, etc.)
(Sea lamprey male creates
spawning hole for female.)
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An advantage of releasing ova in a confined
space is that females can restrict male access
to the space thereby increasing the efficacy of
female choice (e.g., interesting variations in
mouth-breeding cichlids)
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Among cyclostomes, many teleosts, and anuran
amphibians, males and females ensure a high rate of
fertilization by bring cloacae together at the time
gametes are released.
In frogs, males perform cloacal apposition during
spawning by grasping females with their forelimbs
(e.g., axillary and inguinal amplexus – See below).
Internal Fertilization
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Distribution of Internal Fertilization
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Occurs in all elasmobranchs, some osteichthyans
(including the coelacanth), all gymnophione
amphibians, one species of anuran, most urodeles,
and all amniotes.
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Mechanisms of Internal Fertilization
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Some fishes use highly specialized modifications of the
urogenital papilla and anal fins.
Elasmobranchs use claspers in conjunction with sac-like
siphons to introduce sperm suspensions (sperm plugs) into
female reproductive tract
The tailed frog, Ascaphus truei, uses a posterior extension
of the cloacal wall.
Lizards and snakes use hemipenes; turtles and crocodilians
use a single median penis
Most birds use cloacal apposition placing sperm over
inverted female cloacal structures. In flightless birds,
ducks, geese, swans, and others use a modified region of
the cloacal wall which is inserted within the female’s
cloaca. Male ducks have a highly derived cloacal wall that
is used as a true intromittent organ.
Monotremes possess a penis that is similar to that of turtles.
Copulation in marsupials and eutherial mammals involves a
single penis. (See Part IV for details)
Sperm Storage in Females
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The period of storage varies according to taxa.
Among internal fertilizing fishes
(e.g., poeciliids) store sperm over
extended periods, and sperm from
one mating can be used to fertilize
successive clutches of ova.
 In most urodeles, sperm are stored
within specialized structures of the
spermathecae derived from the cloacal
walls.
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Season Sperm Storage
in Spermathecal Tubules
of a Salamander
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Among reptiles, sperm storage occurs within
modified glandular structures of the vagina (e.g.,
anoline lizards) or of the proximal oviduct (e.g.,
garter snakes).
Two sites of sperm storage called sperm nests are
found in birds.
Sperm storage allows for a decoupling of two major
reproductive events: mating and ovulation. In sperm
storing species, these events need not occur within a
discreet period of time and may be separated by quite
a long interlude.
Another important aspect of sperm storage is that a
female can defer fertilization and the onset of
embryogenesis until environmental conditions are
favorable.
Sperm Capacitation
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Among the fishes, amphibians, birds, and prototherial
mammals, sperm are capable of fertilizing an ovum at the time
of their release from the male genital tract.
In marsupials and eutherial mammals, however, sperm are
inactivated, or decapacitated, by peptide secretions of the
epididymal duct prior to their ejaculation.
In order to fertilize an ovum, decapacitated sperm must be
reactivated.
Reactivation of decapacitated sperm is known as capacitation
and involves changes in the lipid structure of the acrosomal
membrane that are initiated by high concentrations of calcium
within the female genital tract. In some sperm storing species,
capacitation does not occur immediately upon introduction
into the female reproductive tract. Instead, sperm remain
fertilization incompetent during their periods of storage and
are capacitated as needed.
This mechanism in sperm storing species remain unclear..
Ovum Transport in Females
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In most vertebrates, ova are transported through the female
genital tract prior to fertilization. After ovulation, ova are
picked up by ciliated coelomic funnels or funnel-like ostea.
Ovulation releases ova directly into the coelomic cavity. The
mechanisms by which macrolecithal ova are picked up by
ostea are unclear.
The mechanisms for microlecithal ova are better understood.
In humans, the ovum is released from the follicle along with
its adherent layer of follicular cells and follicular fluids. Upon
its release, the follicular fluid rapidly gels and adheres to the
outer ovarian tunic and nearby fimbriated surfaces of the
infundubulum. The gel helps hold the ovum against the ovary
and prevents it from being lost (except in cases of ectopic cysts
outside the female genital tract). Ciliary action gradually
draws the gel containing the ovum into the osteum.
Mechanism of Fertilization
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Sperm Chemotaxis – In
cyclostomes and most
teleosts, a chemical attractant
is released by the secondary
accessory envelope. This
substance is released in the
vicinity of where sperm enter
of envelope at the micropyle.
Sperm orient their
movements toward the source
of this chemotaxic agent in a
process known as sperm
chemotaxis. Most of these
compounds also increase
sperm motility.
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Sperm swim into
the micropyle
Trout Micropyle
Agglutination and Acrosomal Reaction
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Fertilization takes place in a series of discrete steps (next slide). The
sperm actually interacts with the egg on three separate levels: first
with the cumulus cells and the hyaluronic acid extracellular matrix
(ECM) in which they are embedded, secondly with the egg's own
ECM, called the zona pellucida (ZP), and finally with egg plasma
membrane. Sperm-ZP interactions are important, because the binding
of the sperm to specific ZP glycoproteins induces the sperm to
undergo the "acrosome reaction," the exocytosis of the acrosome
vesicle on the head of the sperm. The acrosome reaction has two
important results. First, enzymes released from the acrosome allow the
sperm to penetrate the ZP to gain access to the perivitelline space.
Secondly, new portions of the sperm membrane are exposed or
modified upon the acrosome reaction, including the inner acrosomal
membrane and the equatorial segment, regions of the sperm head that
can participate in initial gamete membrane binding or subsequent
sperm-egg membrane fusion (1-3). The acrosome reaction is
absolutely required for sperm-egg plasma membrane interactions to
occur, as only acrosome-reacted sperm can bind and fuse with the egg
plasma membrane (1).
Gamete Fusion
Gamete fusion requires binding between membranes of the
spermatozoon and the oocyte.
Sperm Incorporation – After gametes come in contact and
fuse, elements of the spermatozoon are actively drawn into
the oocyte cytoplasm by the female gamete.
Activation – Fusion of gametes initiates a series of
morphological and physiological changes in the newly
formed zygote that mark the beginning of embryonic
development. This is known as activation.
In most vertebrates, when sperm fuse with the oocyte, they
introduce into the oocyte cytoplasm a single haploid set of
chromosomes. This process is called monospermy.
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Polyspermy – Occurs when multiple sperm fuse to
the oocyte. Polyspermy is common in vertebrates
that produce large oocytes. Deleterious consequences
of this process (e.g., polyploidy) are avoided when
supranumerary sperm nuclei are blocked from fusing
with that of the oocyte.
Cleavage
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Cleavage is a relatively rapid series of successive mitotic
divisions that occur following fertilization.
Several types of cleavage among vertebrates reflect the pattern
of yolk distribution.
 Holoblastic – Total cleavage of the blastomeres.
 Meroblastic – Incomplete cleavage.
Early Cleavage Patterns
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Blastocyst in eutherian mammals.