Gametogenesis
gametogenesis meaning | gametogenesis definition | gametogenesis class 12
| gametogenesis notes | gametogenesis and types of eggs | gametogenesis and spermatogenesis | gametogenesis and oogenesis
The process of formation of gametes in sexually reproducing
animals is called gametogenesis.
The sperm is the male gamete and the ovum is the female
gamete.
Gametes are formed from the primitive germ cells of gonads.
Spermatogenesis
In the testicles, the male gonads contain seminiferous
tubules lined with germinal epithelium.
The cells of the germinal epithelium pass through
spermatogenesis and produce sperm.
Germ cells contain nerve cells or Certoli cells that nourish
the developing sperm.
Stages of spermatogenesis
There are three stages of spermatogenesis, e.g. Multiplication stage, growth stage and maturity
stage.
Germinal cells are called primitive germ cells
A. Multiplication stage:
Repetitive mitotic division produces a large number of sperm
which are double (2n).
B. Growth Stage:
Food accumulates in the sperm and increases in size. This is
now called primary spermatocyte.
C. Maturity stage:
Primary sperm cells first pass through the meiotic division.
At the end of this division, two haploid secondary sperms are formed. The
secondary spermatocyte passes through the second meiotic division and produces
sperm.
Spermatids are immobile and inefficient.
In the spermatogenic process they pass through metamorphosis
and mature into dynamic sperm.
Each haploid spermatogonium thus produces four haploid sperm
which are then converted into sperm.
Changes in spermatogenesis
Increase the length of the sperm
Distinction of centriole in proximal and distal.
Formation of axial filament from distal centriole
Formation of spiral coiled mitochondria.
Formation of acrosomes from Golgi complex.
Structure of the sperm (spermatozoa)
The tiny, elongated haploid motile male gamete is about
0.055 mm [60p] long.
Produced by the process of spermatogenesis.
Their viability lasts for seventy-two hours, but ovulation
can occur within the first 12 to 14 hours.
Parts of sperm
The sperm consists of head, neck, middle piece and tail.
Head
The head is the main body which is flat and oval and has a
large nucleus and an acrosome.
The acrosome is formed from the Golgi complex. It secretes
the hyaluronidase enzyme that helps the egg penetrate during fertilization.
The acrosome of the nucleus and the front half are covered
with fibrillar sheath.
Neck
The neck is a small area with two centrioles.
The proximal centriole plays a role in the first cleavage of
the zygote.
The axial filament of sperm is formed by distal centrioles.
Middle piece
The middle piece acts as a power house for sperm.
It consists of several spiral mitochondria or Nebenkern
wrapped around an axial filament.
The mitochondria provide energy for the sperm to swim in the
female genitalia at a speed of about 1.5 to 3 mm per minute.
The posterior half of the nucleus, the neck, and the middle
part of the sperm are covered with sheath.
Tail
The tail is composed of cytoplasm and has long, slender and
pointed structure.
Axial filament is a fine thread-like structure formed from
the distal center and extends to the center and tail.
There are nine auxiliary fibers around two intermediate
longitudinal axial fibers.
The tail helps the sperm to swim.
gametogenesis meaning | gametogenesis definition | gametogenesis class 12
| gametogenesis notes | gametogenesis and types of eggs | gametogenesis and spermatogenesis | gametogenesis and oogenesis
Oogenesis
It is process of formation of the haploid female gamete i.e. egg or ovum from the diploid germinal epithelium. It involves the process of meiosis (and mitosis).
This process can be divided into three stages
Phases of oogenesis
- Multiplication phase
- Growth phase
- Maturation phase
- Multiplication phase
- Multiplication phase
In this stage, the primary germ cells of the ovary are PGCs (2n) undergo mitosis repeatedly to make
Millions of gametes mother cells or oogonial cells (2n).
It is completed in the process embryonic stage of human females
2. Growth phase
Some of the oogonia stop division and begin to increase in size and form the primary oocytes (2n).
Cellular organelles like ER, golgi appratus and mitochondria increase in number
3.Maturation phase
Structure of secondary oocyte
It is rounded,
nonmotile and haploid female gamete.
It is
non-cleidoic (without shell) and microlecithal (yolk is present in very small
quantity).
It is
approximately 0.1mm (100 microns) in size.
The nucleus of
the egg appears large and is called germinal vesicle.
Typical nucleus
or pronucleus is formed at the time of fertilization.
The cytoplasm of
egg is also called ooplasm.
It is devoid of
centrioles.
The egg is
surrounded by various coverings.
- Vitelline membrane
- Perivitelline space.
- Zona pellucida
- Corona radiata.
- Layer of granulosa cells.
The egg membrane is called vitelline membrane.
It secretes a non-cellular Glycoproteinous membrane, zona pellucida on its outside.
Adhering to the outer surface of zona pellucida are several radially elongated cells forming the corona radiata.
These cells are derived from the innermost layer of granulosa cells.
They are firmly held to the zona pellucida and to each other by hyaluronic acid (mucopolysaccharide).
Between the vitelline membrane and the zona pellucida is a fluid filled perivitelline space.
The egg shows polarity.
The side having germinal vesicle and first polar body is called animal pole while the side opposite to it is called vegetal pole.
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