The perivitelline fluid is an

extracellular fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a lower ...

found in the eggs of most

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. Ther ...

and constitutes the main source of nutrition and defense for their

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s. It replaces the egg

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

of other animals, which in snail eggs is reduced to non-nutritive

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

aceous granules with putative

enzymatic Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

function. During

embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

the perivitelline fluid is ingested macropinocytotically by the embryos and the resulting

phagosome In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs). A phagosome is formed by the fusion of the cell mem ...

s fuse with β-granules containing

hydrolytic Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

s, which digest the perivitelline fluid components.

Origin

The perivitelline fluid is synthesized by the albumen gland of female snails (also known as albumen gland-capsule gland complex or uterine gland), an accessory gland from the reproductive tract. Fertilized

oocyte An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...

s enter the albumen gland and, on their way out, are coated with the perivitelline fluid. The amount of perivitelline fluid per egg vary considerably among species. However, the amount of perivitelline fluid per egg is constant within a given species. In this regard, it has been shown in ''

Pomacea ''Pomacea'' is a genus of freshwater snails with gills and an operculum, aquatic gastropod mollusks in the family Ampullariidae, the apple snails. The genus is native to the Americas; most species in this genus are restricted to South America. ...

apple snail Ampullariidae, commonly known as the apple snails, is a family of large freshwater snails, aquatic gastropod mollusks with a gill and an operculum. These snails simultaneously have a gill and a lung as functional respiratory structures, which ...

s that during the reproductive season, when the nutrient precursors decrease in the albumen gland due to successive ovipositions, females tend to reduce the number of eggs per clutch but not the amount allocated to each egg.

Composition

The perivitelline fluid contains predominantly

galactogen Galactogen is a polysaccharide of galactose that functions as energy storage in pulmonate snails and some Caenogastropoda. This polysaccharide is exclusive of the reproduction and is only found in the albumen gland from the female snail reproducti ...

s, and

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

Carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...

is invariably the most abundant component of the perivitelline fluid. Specifically, the eggs of most gastropod accumulate the polysaccharide

, which would provide the main energy source for the developing embryo. A small amount of soluble

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

was also detected in some species. Proteins, called

perivitellins Perivitellins are egg proteins found in the perivitelline fluid of many gastropods. They are multifunctional complexes providing the developing embryo with nutrition, protection from the environment, and defense against predators. Despite the cen ...

, are the second most abundant component of the perivitelline fluid. Perivitellins are also a source of nutrients for snail embryos and play a role in protection against

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

s and predators, and include non-digestible perivitellins, toxins and

protease inhibitors Protease inhibitors (PIs) are medications that act by interfering with enzymes that cleave proteins. Some of the most well known are antiviral drugs widely used to treat HIV/AIDS and hepatitis C. These protease inhibitors prevent viral replicat ...

. These proteins were thoroughly studied in apple snails from the genus ''

'', where they were originally grouped in two most abundant protein fractions perivitellin-1 or PV1,

perivitellin-2 Perivitellin-2 (PV2) is a pore-forming toxin present in the egg perivitelline fluid of the apple snails '' Pomacea maculata'' (PmPV2) and ''Pomacea canaliculata'' (PcPV2). This protein, called perivitellin, is massively acumulated in the eggs (~2 ...

or PV2 (comprising approximately 70% of total protein), and a heterogeneous fraction dubbed perivitellin-3 or PV3 fraction. Recent proteomic analyses, however, showed that the perivitelline fluid from ''

'' snails has between 34-38 different proteins with a wide variety of functions. Lipids are a minor component, mostly represented by membrane lipids, indicating that snails do not use lipids as a major energy reserve during reproduction. Apart from structural lipids, some eggs also contain carotenoid pigments, notably

astaxanthin Astaxanthin is a keto- carotenoid within a group of chemical compounds known as terpenes. Astaxanthin is a metabolite of zeaxanthin and canthaxanthin, containing both hydroxyl and ketone functional groups. It is a lipid-soluble pigment with r ...

. These lipidic pigments have been associated with antioxidant and photoprotective functions, and also provide ''

'' eggs with the typical bright color that would function as a warning signal (i.e.

aposematism Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...

) to deter predators. Among the inorganic components,

ion is the most abundant in the perivitelline fluid. As these snails have direct development, calcium needs to be stored to allow the snail to develop the shell during

organogenesis Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal orga ...

. Besides, calcium is the main component of the

eggshell An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats. Diversity Worm eggs Nematode eggs present a two layered structure: an external vitellin layer made of chitin that confers mechanical ...

of those snails with aerial oviposition.

References

{{reflist Gastropod anatomy Embryology