The mature ovum is covered by a plasma membrane that is surrounded by protective accessory egg membranes. These membranes include a primary membrane like the vitelline membrane in many animals, which adheres closely to the plasma membrane. Secondary membranes are produced by follicle cells surrounding the ovum, and tertiary membranes are secreted by the oviduct when the egg passes to the exterior. The various egg membranes provide protection and help prevent polyspermy in mammals.
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Structure of mature ovum
1. Structure of Mature Ovum
G. Ramasubbu
Assistant Professor
SBK College, Aruppukottai
2. Structure of Mature ovum
•The Mature egg is covered by a cell membrane or
plasmalemma.
•In most animals except sponges and coelenterates the
plasma membrane is surrounded by certain special protective
accessory egg membranes.
•Depending on their origin these may be subdivided into
following types.
4. • Primary membrane.
• These membrane develop in the ovary by the
ovum itself between the oocyte and follicle
cells around the plasmamembrane.
• In insect, mollusca, amphibians and birds the
primary membrane is knows as vitelline
membrane. In teleost fishes it is usually called
the chorion.
5. • The primary envelop of shark, some bony fishes, some
amphibians and some reptiles have striated
appearance and is called zona-radiata.
• In mammals, the unstriated and modified zonaradiata
is called zonapellucida.It is formed by the joined efforts
of ova and follicle cell.
• All these primary membrane usually adhere closely to
the surface of the plasma membrane of oocyte. But at
later stage a fluid filled space may appear between the
egg plasma membrane and the primary membrane is
called pre-viteline space.
6. Secondary membranes
These are produced as a basement membranes
by layer of follicle cells surrounding ovum.
A) In insect ascidians and cyclostomes
membrane is secreted by the follicle cells on
the top of vitelline membrane. This second
membrane is also called the chorion.
B) An egg while escaping from the mammalian
ovary carries with it on the surface of zona
pellucida a layer of follicle cells known as
corona radiata.
7. Tertiary membranes
• These membranes are secreted by oviduct of
the maternal genital organs when the egg
passes from the ovary to the exterior.
• The oviparous sharks and rays the egg is
surrounded by allbumen and a hard shell
secreted by shell glands of the oviducts.
• The shell is drawn out into egg and sometimes
serves to make the eggs to adhere to one
another and submerged objects such as
plants.
8. • Most complicated egg membranes are found
in the egg of birds where no less than five
membranes can be distinguished.
• In addition to innermost vitelline membrane
which is primary in nature , Albumen, Shell
membranes, and Shell.
9. Significance of egg membranes
• The extraneous envelope of egg of different
ooviparous vertebrates provide the protection to
the contents of egg from different ecological
hazards.
• Zona pellucida in viviparous mammals check
polyspermy, prevent egg fusion and maintains
necessary means of maintaining normal cleavage
of egg.
• The cortical region of cytoplasm are spherical
bodies, the cortical granules surrounded by
simple membrane and contain acid
mucopolysaccharide.
10. • Golgi body is related to the secretory activity
whereas the mitochondrial aggregated into
mitochondrial cloud. They are bags of oxidative
enzymes involved in cell respiration.
• Ovum has a minute centrosome close to the
nucleus. It is active during cell division.
• Typical endoplasmic reticulum in the form of
double lamellae and numerous vesicle with
ribosome attached to their surfaces present in a
mature ovum.
11. • The yolk is nutritional but it also exercises
influences on the size and type of eggs,
pattern of cleavages morphogenetic
movements of blastomeres during
gastrulation and the mode of development .
• Nucleus is essential to the life , growth and
reproduction of a cell.