Gametogenesis
Gametogenesis
It is a biological process by which diploid precursor cells undergo cell division and differentiation to form mature haploid gametes.
It includes two processes:
a)
Spermatogenesis
b)
Oogenesis
Spermatogenesis
It is a process by which the male gamete, known as sperm is formed.
In this process, each sperm is haploid, containing a single copy of each chromosome.
In order to create the haploid gamete, a cell undergoes the process of meiosis in which the genome is replicated and divided twice to produce four haploid gametes.
This process generally occurs in the seminiferous tubules of the testes and maturation in the epididymis where they are secreted in the form of semen along with glandular secretions.
The complete process of spermatogenesis in males are carried
out by the action of Leydig cells, hypothalamus and pituitary
gland.
The entire process of spermatogenesis can be divided into following two stages:
a)
Formation of Spermatids
b)
Spermiogenesis
1.
Formation of Spermatids
The cells of germinal epithelium which produce the spermatozoa are called Primordial germ cells. For the formation of spermatids, each primary germinal cell undergoes following stages:
a)
Multiplication phase
b)
Growth phase
c)
Maturation phase
Cryptorchidism
a)
Multiplication phase:
An undifferentiated primordial germ cell contains a large-sized, chromatin rich nucleus.
It divides by repeated mitotic cell divisions and produces the cells which ultimately become the spermatogonia.
Spermatogonia are diploid cells, found next to the basal membrane of the seminiferous tubules.
At the start of the process in the rat, the spermatogonia can be identified as A1 spermatogonia. This type of spermatogonia divides twice to form four A2 spermatogonia.
One of these four cells becomes a cell that remain dormant and does not divide again until another cycle of spermatogenesis begins.
Thus, this cell acts as a replacement for the cell that originated the
cycle and the process of spermatogenesis continues throughout the
adult life.
The other three A2 spermatogonial cells divide mitotically into six intermediate cells, which in turn divide mitotically into twelve B1 cells and then into twenty-four B2 cells.
In man, there are found seven distinct classes of spermatogonia and these can be identified by their cytological appearance and their diminishing size after each division.
b) Growth Phase:
After the last spermatogonial division, the nature of cell changes, instead of preparing for another mitosis, it grows by accumulating nourishing material obtained from germinal cells to become double in volume. Such cells are called Primary spermatocytes.
These diploid cells have to develop into haploid spermatozoa and
undergo meiosis, which constitutes the next phase.
a)
Maturation phase:
Primary spermatocytes enter into the prophase of first meiotic division which is followed by cytokinesis and leading to the formation of two haploid secondary spermatocytes.
Both secondary spermatocytes undergo meiosis II division and produce four haploid spermatids.
These spermatids are still not capable of functioning as male gametes, so to become functional spermatozoa, they have to undergo a process of differentiation or specialization.
2. Spermiogenesis
A sperm or spermatozoon is a very active and mobile cell, so, to provide great amount of mobility to the sperm, the superfluous materials of the developing spermatozoon are discarded and a high degree of specialization takes place in the sperm cell.
During spermiogenesis, two major parts of sperm, the head and the tail are
formed.
i)
Formation of head of spermatozoon: the nucleus and acrosome undergoes following changes to form a sperm head:
a) Changes in nucleus: from spermatid to spermatozoon, the nucleus loses its entire fluid content, all its RNA, nucleolus and most of its proteins. Only its haploid amount of DNA remains untouched. In some cases the proteins associated with DNA are replaced by an unusual class of small, basic proteins called Protamines. The general shape of nucleus also changes from spherical to narrow and elongated.
b) Acrosome formation: Acrosome of a spermatozoon is derived from
the golgi apparatus of a spermatid. Golgi apparatus consists of a series
of cisternae arranged concentrically around an aggregation of small
vacuoles. During acrosome formation, one or more vacuoles start
enlarging and inside the vacuole appears a small dense body called
pro-acrosomal granule. The granule increases further and becomes
the acrosomal granule. The vacuole now loses its liquid contents
and its wall spreads out over the acrosomal granule and front half of
the nucleus, covering them with a double membranous sheath.
D E F
ii)
Formation of the tail of spermatozoon: Centrosome of a spermatid after the II meiotic division consists of two cylindrical bodies called centrioles, lying at right angle to each other, just behind the sperm nucleus. A depression is formed in the posterior surface of the nucleus and one of the two centrioles is placed in it called as proximal centriole while the other called distal centriole serves as basal granule, which give rise to the axial filament of the flagellum.
• Most of the mitochondria of spermatids concentrate around the
distal centriole and proximal part of the axial filament, form the
neck and mid piece of the tail of spermatozoon.
Tail of sperm comprises an axial filament which remains differentiated into a principal piece and a tail piece.
Formation of spermatozoon from spermatid
2n Primordial germ cell Mitosis
1. Multiplication Phase
3. Maturation Phase 2. Growth Phase
4. Differentiation Phase
Oogenesis
Oogenesis is the process of female gamete formation i.e. formation of an oocyte from an oogonium.
The important events of oogenesis are:
a) Premeiotic or Proliferative phase
b) Meiotic phase
c) Maturation of Egg
a) PREMEIOTIC PHASE /PROLIFERATIVE PHASE:
Like spermatozoa, the oocytes arise from the primordial germ cells and multiply by mitosis to form population of oogonia. This phase occurs within the ovary, usually during embryonic, larval or foetal life.
In mammals and birds, primordial germ cells proliferate into a limited number of oogonia and this phase is completed once in the life of an organism long before sexual maturity. No proliferative oogonia are seen in an adult ovary.
All oogonia are not matured into functional eggs, only surviving oogonia usually begin meiosis and become primary oocytes.
This marks the transition between premeiotic and meiotic phase.
b) MEIOTIC PHASE:
Before oogonia enter meiosis, they increases in size and replicate their chromosome, leading to the formation of primary oocyte. Shortly after, prophase begins and replicated chromosomes condense, but prophase is markedly prolonged, lasting for months and even years.
Thus arrested prophase (diplotene stage) is the most peculiar feature of oocyte meiosis.
During meiotic prophase, the oocyte grows and acquires developmental information. The dual function of an egg, its role as a sexual recombinant and its morphogenetic potential are acquired during this prophase. This phase can be subdivided into following phases:
1. Growth phase of primary oocytes
2. Establishment of egg organization
1. Growth phase of primary oocytes: this phase involves the following events-
a) Growth and differentiation of oocyte: the processes of growth and differentiation of primary oocyte involve not only the accumulation of nutrients and other reserves necessary for embryonic development, but also genetic programming for development.
b) Magnitude of growth: the oocyte grows to enormous proportions normally to become the largest cell of the animal body. This growth is due not so much to an increase in the active cytoplasm, as to an increase in the reserves such as yolk.
c) Rate of oocyte growth: the time required to achieve the maximum size is not related to the increase that take place. The rate of growth of primary oocyte may be slow or higher.
d) Site of oocyte growth: the site of growth of primary oocyte may be different in different animals. In most instances, oocytes develop in a localized organ, an ovary. They may mature independently of surrounding ovarian cells or may be directly associated with follicle or nurse cell.
e) Mode of growth of oocyte: during growth phase of primary oocyte qualitative and quantitative changes take place both in the nucleus and the cytoplasm. These are:
i) Growth of nuclear substances: oocytes developing independently of nurse cell, swells to a volume and its newly extended lampbrush chromosomes are metabolically active. By contrast, oocyte surrounded by the nurse cells lack lampbrush chromosome. Instead, nurse cell nuclei enlarge, become metabolically active and synthesize gene products for storage in oocyte cytoplasm. The mRNA molecules which are synthesized during oogenesis are of 3 types; immediately translating mRNA, Masked mRNA (informosomes), heat- shocked mRNA (in amphibian oocytes).
Gene amplification, nucleolus and synthesis and storage of ribosomes: gene amplication is the process by which one set of genes is replicated selectively, while the rest of the genome remains constant. The nucleolus is the principal organelle for ribosome biosynthesis and assembly.
ii) Stockpiling of membranous organelles: membranous organelles of the oocyte such as mitochondria, ER and golgi apparatus make up an essential portion of egg’s developmental organization. Though some of these organelles are necessary for oocyte metabolism, the bulk is stockpiled in oocyte cytoplasm to sustain the developing embryo to hatching.
iii) Vitellogenesis (synthesis and deposition of yolk): yolk, the major nutritional reserve of the oocyte, accumulates during the vegetative growth phase and accounts for the rapid increase in size of the maturing egg. It is composed of protein, phospholipids and neutral fats. It may be Protein Yolk or Fatty yolk. It is of 2 types:
a) Granular yolk: invertebrates and lower chordates (Amphioxus and tunicates)
b) Yolk platelets: fishes, amphibians, reptiles and birds.
Yolk platelets of amphibian oocytes contain two main proteinaceous substances: Phosvitin (two molecules) and Lipovitellin (one molecule).
Yolk protein are synthesized in the amphibian liver (or vertebrate liver) and transported to the oocyte via circulation.
LIVER
Serum vitellogenin
Taken up by micropinocytosis
Formation of yolk proteins and yolk platelets Yolk platelets increases in size and migrate into the interior
In Rana pipiens oocytes, crystalline yolk platelets are seen within the mitochondrial matrix, and in molluscan eggs too mitochondria are transformed into yolk bodies.
In insects; yolk precursors are made by the cells of fat bodies or midgut and then are transformed to ovarian follicles for oocyte uptake.
Enzymes associated with yolk are phosphoprotein phosphatase, cathepsin stabilizing the yolk protein.
2. Establishment of egg organization: it includes following features-
a) Polarity
b) Symmetry
c) Polarplasm and ooplasmic segregation
d) Ooplasmic pigments
e) Cortical differentiation C) MATURATION PHASE:
Female oogenesis
Meiosis initiated once in a finite population of cells
One gamete produced per meiosis
Completion of meiosis delayed for months or years
Meiosis arrested at first meiotic prophase and reinitiated in a smaller population of cells
Differentiation of gamete occurs while diploid, in first meiotic prophase
All chromosomes exhibit equivalent transcription and recombination during meiotic prophase
Male spermatogenesis
Meiosis initiated continuously in a mitotically dividing stem cell population.
Four gametes produced per meiosis Meiosis completed in days or weeks
Meiosis and differentiation proceed continuously without cell cycle arrest
Differentiation of gamete occurs while haploid, after meiosis ends
Sex chromosomes excluded from
recombination and transcription during first meiotic prophase
