Comparative Vertebrate Comparative Vertebrate ReproductionReproduction

Part VII. Fertilization and Part VII. Fertilization and CleavageCleavage

Introduction and OverviewIntroduction and Overview Fertilization Fertilization

External FertilizationExternal Fertilization Internal FertilizationInternal Fertilization Mechanism of FertilizationMechanism of Fertilization Gamete FusionGamete Fusion Sperm IncorporationSperm Incorporation ActivationActivation

CleavageCleavage Holoblastic CleavageHoloblastic Cleavage Meroblastic CleavageMeroblastic Cleavage

Fertilization Fertilization External FertilizationExternal Fertilization

SpawningSpawning Pelagic or Broadcast SpawningPelagic or Broadcast Spawning – The least – The least

specialized of the spawners; large number of sperm specialized of the spawners; large number of sperm and ova are released synchronously in the water and ova are released synchronously in the water column.column.

Collective SpawnersCollective Spawners – Males and females come – Males and females come together to release sperm and ova at the same place together to release sperm and ova at the same place and time in a restricted volume of water. (e.g., and time in a restricted volume of water. (e.g., herring below) herring below)

Benthic or Terrestrial SpawnersBenthic or Terrestrial Spawners – Ova are fertilized – Ova are fertilized in a very specific location such as nests and holes in a very specific location such as nests and holes where males are able to easily locate and fertilize where males are able to easily locate and fertilize them. (e.g., nest-building sticklebacks, wrasses, them. (e.g., nest-building sticklebacks, wrasses, cichlids, etc.)cichlids, etc.)

(Sea lamprey male creates (Sea lamprey male creates

spawning hole for female.)spawning hole for female.)

An advantage of releasing ova in a confined An advantage of releasing ova in a confined space is that females can restrict male access space is that females can restrict male access to the space thereby increasing the efficacy of to the space thereby increasing the efficacy of female choice (e.g., interesting variations in female choice (e.g., interesting variations in mouth-breeding cichlids)mouth-breeding cichlids)

Among cyclostomes, many teleosts, and anuran Among cyclostomes, many teleosts, and anuran amphibians, males and females ensure a high rate of amphibians, males and females ensure a high rate of fertilization by bring cloacae together at the time fertilization by bring cloacae together at the time gametes are released.gametes are released.

In frogs, males perform cloacal apposition during In frogs, males perform cloacal apposition during spawning by grasping females with their forelimbs spawning by grasping females with their forelimbs (e.g., (e.g., axillaryaxillary and and inguinal inguinal amplexusamplexus – See below – See below). ).

Internal FertilizationInternal Fertilization Distribution of Internal FertilizationDistribution of Internal Fertilization

Occurs in all elasmobranchs, some osteichthyans Occurs in all elasmobranchs, some osteichthyans (including the coelacanth), all gymnophione (including the coelacanth), all gymnophione amphibians, one species of anuran, most urodeles, amphibians, one species of anuran, most urodeles, and all amniotes.and all amniotes.

Mechanisms of Internal FertilizationMechanisms of Internal Fertilization Some fishes use highly specialized modifications of the Some fishes use highly specialized modifications of the

urogenital papilla and anal fins.urogenital papilla and anal fins. Elasmobranchs use claspers in conjunction with sac-like Elasmobranchs use claspers in conjunction with sac-like

siphons to introduce sperm suspensions (sperm plugs) into siphons to introduce sperm suspensions (sperm plugs) into female reproductive tractfemale reproductive tract

The tailed frog, The tailed frog, Ascaphus trueiAscaphus truei, uses a posterior extension , uses a posterior extension of the cloacal wall.of the cloacal wall.

Lizards and snakes use hemipenes; turtles and crocodilians Lizards and snakes use hemipenes; turtles and crocodilians use a single median penisuse a single median penis

Most birds use cloacal apposition placing sperm over Most birds use cloacal apposition placing sperm over inverted female cloacal structures. In flightless birds, inverted female cloacal structures. In flightless birds, ducks, geese, swans, and others use a modified region of ducks, geese, swans, and others use a modified region of the cloacal wall which is inserted within the female’s the cloacal wall which is inserted within the female’s cloaca. Male ducks have a highly derived cloacal wall that cloaca. Male ducks have a highly derived cloacal wall that is used as a true intromittent organ.is used as a true intromittent organ.

Monotremes possess a penis that is similar to that of Monotremes possess a penis that is similar to that of turtles.turtles.

Copulation in marsupials and eutherial mammals involves Copulation in marsupials and eutherial mammals involves a single penis. a single penis. (See Part IV for details)(See Part IV for details)

Sperm Storage in FemalesSperm Storage in Females The period of storage varies according to taxa.The period of storage varies according to taxa.

Among internal fertilizing fishes Among internal fertilizing fishes

(e.g., poeciliids) store sperm over (e.g., poeciliids) store sperm over

extended periods, and sperm from extended periods, and sperm from

one mating can be used to fertilize one mating can be used to fertilize

successive clutches of ova.successive clutches of ova. In most urodeles, sperm are stored In most urodeles, sperm are stored

within specialized structures of thewithin specialized structures of the

spermathecae spermathecae derived from the cloacal derived from the cloacal

walls. walls.

Season Sperm StorageSeason Sperm Storagein Spermathecal Tubules in Spermathecal Tubules

of a Salamanderof a Salamander

Among reptiles, sperm storage occurs within Among reptiles, sperm storage occurs within modified glandular structures of the vagina (e.g., modified glandular structures of the vagina (e.g., anoline lizards) or of the proximal oviduct (e.g., anoline lizards) or of the proximal oviduct (e.g., garter snakes).garter snakes).

Two sites of sperm storage called Two sites of sperm storage called sperm nests sperm nests are are found in birds.found in birds.

Sperm storage allows for a decoupling of two major Sperm storage allows for a decoupling of two major reproductive events: mating and ovulation. In sperm reproductive events: mating and ovulation. In sperm storing species, these events need not occur within a storing species, these events need not occur within a discreet period of time and may be separated by quite discreet period of time and may be separated by quite a long interlude.a long interlude.

Another important aspect of sperm storage is that a Another important aspect of sperm storage is that a female can defer fertilization and the onset of female can defer fertilization and the onset of embryogenesis until environmental conditions are embryogenesis until environmental conditions are favorable.favorable.

Sperm CapacitationSperm Capacitation Among the fishes, amphibians, birds, and prototherial Among the fishes, amphibians, birds, and prototherial

mammals, sperm are capable of fertilizing an ovum at the time mammals, sperm are capable of fertilizing an ovum at the time of their release from the male genital tract. of their release from the male genital tract.

In marsupials and eutherial mammals, however, sperm are In marsupials and eutherial mammals, however, sperm are inactivated, or decapacitated, by peptide secretions of the inactivated, or decapacitated, by peptide secretions of the epididymal duct prior to their ejaculation.epididymal duct prior to their ejaculation.

In order to fertilize an ovum, decapacitated sperm must be In order to fertilize an ovum, decapacitated sperm must be reactivated.reactivated.

Reactivation of decapacitated sperm is known as Reactivation of decapacitated sperm is known as capacitation capacitation and involves changes in the lipid structure of the acrosomal and involves changes in the lipid structure of the acrosomal membrane that are initiated by high concentrations of calcium membrane that are initiated by high concentrations of calcium within the female genital tract. In some sperm storing species, within the female genital tract. In some sperm storing species, capacitation does not occur immediately upon introduction capacitation does not occur immediately upon introduction into the female reproductive tract. Instead, sperm remain into the female reproductive tract. Instead, sperm remain fertilization incompetent during their periods of storage and fertilization incompetent during their periods of storage and are capacitated as needed. are capacitated as needed.

This mechanism in sperm storing species remain unclear.. This mechanism in sperm storing species remain unclear..

Ovum Transport in FemalesOvum Transport in Females In most vertebrates, ova are transported through the female In most vertebrates, ova are transported through the female

genital tract prior to fertilization. After ovulation, ova are genital tract prior to fertilization. After ovulation, ova are picked up by ciliated coelomic funnels or funnel-like ostea.picked up by ciliated coelomic funnels or funnel-like ostea.

Ovulation releases ova directly into the coelomic cavity. The Ovulation releases ova directly into the coelomic cavity. The mechanisms by which macrolecithal ova are picked up by mechanisms by which macrolecithal ova are picked up by ostea are unclear.ostea are unclear.

The mechanisms for microlecithal ova are better understood. The mechanisms for microlecithal ova are better understood. In humans, the ovum is released from the follicle along with In humans, the ovum is released from the follicle along with its adherent layer of follicular cells and follicular fluids. Upon its adherent layer of follicular cells and follicular fluids. Upon its release, the follicular fluid rapidly gels and adheres to the its release, the follicular fluid rapidly gels and adheres to the outer ovarian tunic and nearby fimbriated surfaces of the outer ovarian tunic and nearby fimbriated surfaces of the infundubulum. The gel helps hold the ovum against the ovary infundubulum. The gel helps hold the ovum against the ovary and prevents it from being lost (except in cases of ectopic and prevents it from being lost (except in cases of ectopic cysts outside the female genital tract). Ciliary action gradually cysts outside the female genital tract). Ciliary action gradually draws the gel containing the ovum into the osteum.draws the gel containing the ovum into the osteum.

Mechanism of FertilizationMechanism of Fertilization Sperm Chemotaxis – Sperm Chemotaxis – In In

cyclostomes and most cyclostomes and most teleosts, a chemical attractant teleosts, a chemical attractant is released by the secondary is released by the secondary accessory envelope. This accessory envelope. This substance is released in the substance is released in the vicinity of where sperm enter vicinity of where sperm enter of envelope at the micropyle. of envelope at the micropyle. Sperm orient their Sperm orient their movements toward the source movements toward the source of this chemotaxic agent in a of this chemotaxic agent in a process known as process known as sperm sperm chemotaxischemotaxis. Most of these . Most of these compounds also increase compounds also increase

sperm motility.sperm motility.

Trout MicropyleTrout Micropyle

Sperm swim intoSperm swim into

the micropylethe micropyle

Agglutination and Acrosomal ReactionAgglutination and Acrosomal Reaction Fertilization takes place in a series of discrete steps (next slide). The Fertilization takes place in a series of discrete steps (next slide). The

sperm actually interacts with the egg on three separate levels: first sperm actually interacts with the egg on three separate levels: first with the cumulus cells and the hyaluronic acid extracellular matrix with the cumulus cells and the hyaluronic acid extracellular matrix (ECM) in which they are embedded, secondly with the egg's own (ECM) in which they are embedded, secondly with the egg's own ECM, called the ECM, called the zona pellucidazona pellucida (ZP), and finally with egg plasma (ZP), and finally with egg plasma membrane. Sperm-ZP interactions are important, because the binding membrane. Sperm-ZP interactions are important, because the binding of the sperm to specific ZP glycoproteins induces the sperm to of the sperm to specific ZP glycoproteins induces the sperm to undergo the "undergo the "acrosome reactionacrosome reaction," the exocytosis of the acrosome ," the exocytosis of the acrosome vesicle on the head of the sperm. The acrosome reaction has two vesicle on the head of the sperm. The acrosome reaction has two important results. First, enzymes released from the acrosome allow important results. First, enzymes released from the acrosome allow the sperm to penetrate the ZP to gain access to the perivitelline space. the sperm to penetrate the ZP to gain access to the perivitelline space. Secondly, new portions of the sperm membrane are exposed or Secondly, new portions of the sperm membrane are exposed or modified upon the acrosome reaction, including the inner acrosomal modified upon the acrosome reaction, including the inner acrosomal membrane and the equatorial segment, regions of the sperm head that membrane and the equatorial segment, regions of the sperm head that can participate in initial gamete membrane binding or subsequent can participate in initial gamete membrane binding or subsequent sperm-egg membrane fusion (1-3). The acrosome reaction is sperm-egg membrane fusion (1-3). The acrosome reaction is absolutely required for sperm-egg plasma membrane interactions to absolutely required for sperm-egg plasma membrane interactions to occur, as only acrosome-reacted sperm can bind and fuse with the egg occur, as only acrosome-reacted sperm can bind and fuse with the egg plasma membrane (1). plasma membrane (1).

Gamete FusionGamete Fusion Gamete fusionGamete fusion requires binding between membranes of the requires binding between membranes of the

spermatozoon and the oocyte.spermatozoon and the oocyte.

Sperm IncorporationSperm Incorporation – After gametes come in contact and – After gametes come in contact and fuse, elements of the spermatozoon are actively drawn into fuse, elements of the spermatozoon are actively drawn into the oocyte cytoplasm by the female gamete.the oocyte cytoplasm by the female gamete.

ActivationActivation – Fusion of gametes initiates a series of – Fusion of gametes initiates a series of morphological and physiological changes in the newly morphological and physiological changes in the newly formed zygote that mark the beginning of embryonic formed zygote that mark the beginning of embryonic development. This is known as development. This is known as activation.activation.

In most vertebrates, when sperm fuse with the oocyte, they In most vertebrates, when sperm fuse with the oocyte, they introduce into the oocyte cytoplasm a single haploid set of introduce into the oocyte cytoplasm a single haploid set of chromosomes. This process is called chromosomes. This process is called monospermy.monospermy.

Polyspermy Polyspermy – Occurs when multiple sperm fuse to – Occurs when multiple sperm fuse to the oocyte. Polyspermy is common in vertebrates the oocyte. Polyspermy is common in vertebrates that produce large oocytes. Deleterious consequences that produce large oocytes. Deleterious consequences of this process (e.g., polyploidy) are avoided when of this process (e.g., polyploidy) are avoided when supranumerary sperm nuclei are blocked from fusing supranumerary sperm nuclei are blocked from fusing with that of the oocyte.with that of the oocyte.

CleavageCleavage CleavageCleavage is a relatively rapid series of successive mitotic is a relatively rapid series of successive mitotic

divisions that occur following fertilization.divisions that occur following fertilization. Several types of cleavage among vertebrates reflect the pattern Several types of cleavage among vertebrates reflect the pattern

of yolk distribution.of yolk distribution. Holoblastic Holoblastic – Total cleavage of the blastomeres.– Total cleavage of the blastomeres. MeroblasticMeroblastic – Incomplete cleavage. – Incomplete cleavage.

Early Cleavage PatternsEarly Cleavage Patterns

Blastocyst in eutherian mammals.Blastocyst in eutherian mammals.