Egg Characteristics: Totipotency, Polarity & Symmetry

General Characteristics

• An egg is generally spherical and enclosed in protective or nutrient-containing membranes.
• A zygote (fertilized egg) has all nuclear and cytoplasmic components of other cells, plus some unique characteristics.

Totipotency

• An egg is a totipotent cell, meaning it can give rise to any cell type of an organism, even an entire organism.
• The zygote is the best example of a totipotent cell.
• This totipotency is lost during embryonic development.

Polarity

• Vegetal Pole: Rich in yolk
• Animal pole: Site of emission of polar bodies, poor in yolk, contains the nucleus and active cytoplasm.

Symmetry

• Before fertilization, the egg exhibits axial symmetry.
• After fertilization, most coelomate metazoa develop bilateral or dorso-ventral symmetry during embryonic development.

Food Reserves

• Eggs import or produce food reserves, mainly yolk, for the early oogenesis consumed during embryonic development.
• Food reserves can be of exogenous or endogenous origin and consist of protein, lipid, or carbohydrate.

Heterogeneous

• The distribution of egg components, such as ribosomes, mitochondria, and yolk, occurs in a gradient system.
• RNA Gradient: RNA is more abundant in the animal pole, increasing from the vegetal pole (VP) to the animal pole (AP).
• Yolk Gradient: Yolk is more abundant in the vegetal pole, increasing from the animal pole (AP) to the vegetal pole (VP).

Vitellogenesis

• The egg initiates and directs development.
• An egg can import, develop and store reserves in its cytoplasm.
• Yolk in the egg's cytoplasm serves as a nutritive reserve for the embryo at the beginning of development.
• Accumulation occurs mostly during meiotic prophase I, subdivided into previtellogenesis and vitellogenesis.

Previtellogenesis

• This is a phase of slow oocyte growth.
• A large amount of rRNA, mRNA, and tRNA, ribosomal and functional proteins are synthesized and stored.
• Nuclear sap is produced; the nucleus increases in size and becomes a germinal vesicle.

Vitellogenesis

• Also known as yolk deposition
• This involves the accumulation of nutritional reserves in the oocyte's cytoplasm, starting during the diplotene stage of prophase I of meiosis.
• Vitellus, yolk, or deutoplasm are lipoproteins composed of proteins, phospholipids, and a small amount of glycogen.
• Exogenous origin
• Yolk accumulates in the egg cytoplasm during growth and maturation as yolk platelets.
• The reserve material is from the maternal organism, accumulating during the growth and maturation phase of oogenesis and being used during embryonic development.
• Vitellogenesis occurs when oocytes reach the diplotene stage of prophase I of meiosis. Amphibian oocytes can remain blocked for years in prophase 1 diplotene.
• The yolk is mainly composed of vitellogenin, a protein synthesized in the liver and transported to the ovaries via blood circulation.
• All the events are controlled by hormonal interactions between the hypothalamus and the ovaries cells.
• In the mature oocyte, vitellogenin becomes phosvitin and lipovitellin forming vitelline (yolk) platelets.

Types of Eggs based on Yolk

• The type of egg is determined by the amount of yolk and its distribution in the cytoplasm.
• Egg size and cleavage depend on yolk content.
• Lecithal: Having yolk

Alecithal/Microlecithal

• Small sized egg, very poor in yolk.
• Found in placental mammals.

Oligolecithal (oligo=few)

• Possess a small amount of yolk uniformly distributed throughout the cytoplasm.
• Examples include some echinoderms (starfish, sea urchin) and amphioxus.

Heterolecithal (hetero=different)

• Contain a moderate amount of yolk heterogeneously distributed in the cytoplasm.
• More yolk is found in the vegetal pole than the animal pole.
• Active cytoplasm is concentrated at the animal pole.
• Found in amphibians (frogs).

Telolecithal (telo=at the end)

• Large egg, rich in yolk at the vegetal pole.
• Active cytoplasm and nucleus occupy a small region in the animal pole (blastodisc).
• Found in reptiles, birds, and some fish (trout).

Centrolecithal (centro=in the middle)

• Moderate size.
• Yolk is concentrated in the center of the egg.
• Cytoplasm is present as a thin peripheric layer on the surface, near the membrane.
• The nucleus is surrounded by little cytoplasm (ooplasm) inserted between yolk blocks.
• Example: Insects.