GAMETOGENESIS
Professor
Alfred Cuschieri
Department of
Anatomy
University of
Malta
Objectives
o
Explain the significance and
importance of meiosis in sexual reproduction
o
Identify the different stages of gametogenesis
in males and females in micrographs of testis and ovary
o
Name the stages at which the first
and second meiotic divisions take place
o
Outline the stages of
spermiogenesis
o
Name the functions of Sertoli cells
and granulosa cells
o
Define the stages at which meiotic
arrest in oocytes normally occurs
o
Identify the components of
developing ovarian follicles and of the oocyte at the time of ovulation
o
Discuss the mechanism of
non-disjunction in numerical chromosome aberrations.
Sexual Reproduction
Sexual reproduction involves the
formation of male and female gametes
and the the mechanisms necessary for the gametes to come together and
fuse to form one cell that represents the beginning of a new individual with a
distinct genetic identity.
Preparation for pregnancy involves two
main programs of events:
1. Gametogenesis
o
the process of formation of the male
and female gametes
o occurs in the gonads (ovary or testis)
2. Cyclic changes in the female genital tract
o the ovarian cycle
o the uterine cycle
Prepration for pregnancy has three important practical applications
1. Controlling undesired pregnancies - Contraception
2. Treating Infertility - Assisted conception
3. Transmission
of genetic disease
- transmission of inherited traits
- disorders of meiosis
All these applications have profound medical, social and ethical implications
One Essential Question: Why is sexual reproduction necessary?
The main purpose of sexual reproduction is the formation of offspring who are genetically different from one another and from their parents.
Meiosis is the fundamental process underlying sexual
reproduction. It involves two essential
outcomes:
1. Reduction
Division the
process in which each gamete receives a haploid set (n) of chromosomes and
genes.
The diploid
number (2n) is restored on fusion of two gametes.
2. Rearrangement of genes on the maternal and paternal
chromosomes.
This
ensures that the offspring are genetically different from one another.
Meosis involves four main events,
and two cell divisions. (In the following
diagram only one pair of homologous chromosomes is shown, to represent 23 pairs
in humans).
DNA replication precedes meiosis, and occurs in the S phase, as in
all cell divisions. Recall that the
chromosomes in the parent cell contains a diploid set of chromosomes (2n
chromosome number and 2c amount of DNA).
Following replication, there are still 2n chromosomes, but each
chromosome consists of two chromatids, and has 2c DNA.
2. Pairing of
homologous chromosomes and crossing over of chromosome segments occur during prophase
of meiosis. They are crucial event in meiosis.
The complex of a homologous chromosome pair consisting of four
chromatids is a tetrad. The result of
crossing over is that there are now four recombinant chromatids.
3. Separation of chromosomes occurs as a result of the first meiotic
division. The two resulting daughter cells each have one of a pair of replicated
chromosomes, or a haploid set (n) with a total of 2c DNA.
4. Separation of chromatids
occurs as a result of the second meiotic division, and give rise to four
daughter cells, each containing a haploid set of chromosomes (1n; 1c) amount of
Meiosis occurs in the germ cells. As a result of meiosis, four daughter
cells or gametes, are produced, each
containing one of a pair chromosomes and
all containing different chromosomes.
Although the above diagram illustrates only crossing over between the
adjacent chromatids, in fact crossing over also occurs between the two “outer”
chromatids, and at different sites from the other pair, so that all four
daughter cells are different from one another and from their parents. Pairing
of the two “outer” chromatids is possible because, in 3 dimensions these would
also be adjacent to one another.
Genetic
Imprinting
In all diploid cells of an individual the chromosomes occur in homologous pairs. One chromosome of each pair is derived from the mother and the other from the father. The maternal and paternal chromosomes are morphologically indistinguishable but have important functional implications because the expression of some genes is dependent on whether they are on the maternal or the paternal chromosome. This is termed genetic imprinting
Gametogenesis
Gametogenesis is the process of formation of gametes from the germ cells in the testes and ovaries. Many principles of gametogenesis are the same in both males and females, and will be considered first. Gametogenesis is divided into four phases:
1.
Extra-gonadal origin of primordial germ cells
2.
Proliferation of germ cells by mitosis
3.
Meiosis
4.
Structural and functional maturation of the ova and spermatozoa
Primordial Germ Cells
o Are the earliest precursors of all
germ cells
o
Are
formed in the early stages of embryonic development
o
Are
first recognizable close to the hindgut as large cells with high alkaline
phosphatase
o
Proliferate
and migrate into the gonad (testis or ovary)
o
Differentiate
into male or female germ cells (determined by sex chromosomes)
The nomenclature of the developmental stages of gametogenesis is similar in male and female germ cells. It is summarised in the following diagram.