Presentation on theme: "The Ovum Like any ordinary cell in its structure. It is large oval cell which varies from 117-142 µ in diameters. Coverings: a.has 2 membrane: an inner."— Presentation transcript:
The Ovum Like any ordinary cell in its structure. It is large oval cell which varies from µ in diameters. Coverings: a.has 2 membrane: an inner thin called vitelline membrane & an outer thick transparent membrane named zona pellucida. b. Corona radiata: 2 or 3 layers of cells which surround the zona pellucida when the ovum is shed from follicle.
Its cytoplasm is called ooplasm, it is nucleus, the germinal vesicle, its nucleolus is called the germinal spot. The cytoplasm contains a nutritive which nourishes the embryo in the early stages of development. Transport of Ovum: In the uterine tube the ovum passes along by: a. Activity of the cilia of tube. b. Muscular contraction of the tube which increases during ovulation.
Types of Ova A. According to the amount of yolk : 1. Oligolecithal eggs: small-sized eggs, contain a very small amount of yolk. found in amphioxus and eutherian mammals. 2. Mesolecithal eggs: eggs contain moderate amount of yolk, found in amphibian (frogs). 3. polylecithal eggs: eggs contain enormous amount of yolk and are found in insects, reptiles and birds.
B. According to the distribution of yolk: 1. Isolecithal eggs: the amount of yolk is found regularly distributed throughout the egg cytoplasm. As in oligolecithal eggs, that have a little amount of yolk. 2. Telolecithal egg: have a polarized distribution of yolk in the cytoplasm and are found in the mesolecithal and polylecithal eggs. the yolk due to its gravity is concentrated more in one hemisphere than in the other.
may be: a. Moderately telolecithal (eggs of amphibian). b. Heavy telolecithal (eggs of reptiles & birds). 3. Centrolecithal eggs: in insects, the yolk is concentrated in the center of the egg and the active cytoplasm forms a thin peripheral layer around the yolk.
Functions of Yolk Yolk (reserve material) that formed of lipid droplets and glycogen granules) is used for two purposes: a. Supply of energy. b. Synthesis of the products required for elaboration of the embryonic body. Also, the yolk influence on the size of egg, differentiation of ooplasm, patterns of cleavages and on the morphogenetic movements of blastomeres during gastrulation.
Egg Membranes membranes produced outside the plasma membrane of egg. vary in different animal groups. There are several ways to classify these membranes, but the simplest way is to group them according to the origin as follows:
1. Primary Membranes: Formed in the ovary between the egg plasma membrane and follicle cells. They are formed either by the egg or follicle cells as: - Vitelline membrane: has been given different names in different animals. Ex, in amphibian and birds it is in close contact with ooplasmic surface until the egg is fertilized at which time it separates from this surface, forming a tough fertilization membrane. - In fishes is called chorion. - In reptiles and mammals named zona pellucida.
2. Secondary egg membranes: Is secreted outside the primary egg membrane by a layer of follicle cell that surrounds the oocyte (by the ovarian tissue) before the egg leave the ovary It occurs in the form of a chitinous shell surrounding the egg in insects and called chorion, in this case it have a micropyle for sperm entry. It may secretes female sex hormones. As in mammals like membrana granulosa & corona radiata.
3. Tertiary egg membranes: Formed during passage of the egg in the oviduct or during the presence of ovum in the uterus. a. As the egg of the amphibian, three uniform layers of albumen (jelly) are deposited around it. These jelly envelopes hold the egg together in masses, protect the eggs from infection and make them unappetizing to predators. b. In reptiles, birds tertiary membranes make up the envelopes of the egg. External to vitelline membrane are added the white of egg (albumen), two shell membranes and a porous calcareous shell.
Significance of Egg Membranes: 1. Provide the protection to the contents of eggs of developing embryos from different ecological hazards (variable ph, temperature, radiations, pollutions). 2. prevent polyspermy i.e. fertilization by more than one sperm. 3. Help in sperm adhesion. 4. For maintaining normal cleavage of the egg.
Fertilization Fertilization, the process by which male and female gametes nuclei fuses together to produce diploid zygote. Types of Fertilization 1. External: Eggs are librated in water. Occurs outside the female genital system. Female laid a large number of eggs, them the male pour its sperms in the same region in water e.g. in fish and amphibian.
2. Internal: Occurs in animals that have a well developed reproductive system, animals may be: a) Oviparous: zygote develops in a shell e.g. birds. b) Viviparous: zygote develops inside uterus e.g. mammals. The intrauterine life is about 21 days in the rat, 70 days in the in the Guinea pig while its about 280 days in human.
It have 4 major steps: Contact and recognition between sperm and egg. (same species) Regulation of sperm entry into the egg. (only one and inhibiting the others) Fusion of the genetic material of sperm and egg. Activation of egg metabolism to start development.
Sperm Capacitation (enhanced sperm function) Results from secretion in the females reproductive tract. Freshly ejaculated sperm are unable or poorly able to fertilize. Rather, they must first undergo a series of changes (4 sets of molecular changes) known collectively as capacitation. 1. The sperm cell membrane may be altered by changing its lipid composition. (lowering its cholesterol) 2. Particular proteins or carbohydrates on the sperm surface are lost.
3. Certain proteins are phosphorylated. (in other meaning activated) 4. The membrane potential of the sperm is dramatically lowered ( from -30 mV to about - 50mV) Finally, and in brief Capacitation appears to destabilize the sperm's membrane to prepare it for the acrosome reaction, only capacitated sperms can pass through the corona cells and undergo the acrosome reaction.
Recognition of Egg and Sperm: There are 2 problems: 1. How can sperms and eggs meet in a dilute concentration.(water)?? 2. What mechanisms prevents sperms of one species from trying to fertilize eggs of other species.??? Two mechanisms evolved to solve these difficulities: 1. Species-specific attraction of sperm. 2. Species-specific sperm activation.
Sperm Activation (The Acrosomal Reaction) The acrosomal reaction is the discharge of hydrolytic enzymes from a vesicle in the acrosome of a sperm cell. On 2 steps: 1. The fusion between the acrosomal vesicle and sperm plasma membranes (an exocytosis that results in the release of the contents of the acrosomal vesicle). 2. The extension of the acrosomal process.( by polymerization of globular actin molecules into actin filaments)
Initiated by soluble egg jelly or by contact with the egg itself. Can activated artificially by increasing the calcium concentration.
First: Contact and Recognition The plasma membrane of the sea urchin egg is surrounded by the vitelline layer and the thicker, outer jelly coat (zona pellucida in mammals) The acrosome reaction: The acrosome of the sperm releases proteolytic enzymes. Acrosome digests a path through the external coverings. The acrosomal process of the sperm contact the vitelline envelope of the egg. If sea urchin gametes are of the same species, a protein called bindin on the acrosome adheres to a specific receptor on the vitelline membrane.
Step 2: Sperm and Egg fuse (Sea Urchins) The microvilli of the egg membrane form a fertilization cone Sperm is drawn into the cone. Gamete plasma membranes fuse and the sperm is drawn into the egg cell
Preventing entry of more than one sperm (Polyspermy) Fast block to polyspermy involves egg plasma membrane action potential, prevents additional sperm from entering. (-70µ to +10µ) also release of Ca ions. The slow block to polyspermy: In mammals, the zona pellucida sperm receptors are modified to prevent further entry of sperm Some organisms do not block polyspermy. In salamanders, many sperm enter, only one survives, the rest are degraded
Step 3: Activation of the Egg Prior to fertilization, the egg is in a quiescent state, arrested in metaphase of the second meiotic division. Upon binding of a sperm, the egg rapidly undergoes a number of metabolic and physical changes that collectively are called egg activation. 1. Aerobic respiration increases in the egg. 2. Enzyme systems become activated. 3. A burst of protein synthesis begins. 4. In most animals, the nucleus undergoes the second division of meiosis.
In many eggs it is possible to physically stimulate an egg to undergo activation, and even division
Step 4: Pro-nuclear Fusion Once the second polar body is ejected, the female pronucleus can fuse with the male pronucleus This is the genetic beginning of a new organism The haploid genetic complements of the two pronuclei form a 2N nucleus, which prepares the nucleus, and cell, for cleavage.
The Cortical Reaction The fusion of egg & sperm membranes stimulates a series of changes in the eggs cortex known as a cortical reaction. Chemical reactions change the eggs cortical granules. Granules fuse with the plasma membrane releasing enzymes separating the vitelline layer from the plasma membrane. Swelling lifts the vitelline membrane forming the fertilization membrane. Prevents penetration by other sperm
The Zona Reaction Animation
Acrosomal, Cortical Reactions
Thank you for your time. Questions? Dr. Abdelalim Gadallah