Animal Reproduction
Gametogenesis
The process by which gametes are
formed is known as Gametogenesis.
When
an
individual
becomes
sexually mature, production of
mature germ cells begins. The
gonads first undergo a period of
rapid growth, after which they are
known as Auxocytes (a gameteforming cell) or Meocytes. In the
male they are called spermatocytes
and in the female oocytes. (Oocyte is
a female germ cell in the process of
development.
The
oocyte
is
produced in the ovary by an
ancestral cell called an oogonium
and gives rise to the ovum (the egg)
which can be fertilized.(oogonium is
a cell in the ovary that develops into
an
oocyte)
So
we
have
spermatogenesis in the male and
oogenesis in the female. By this
process ultimately each auxocyte
gives rise to four daughter cells.
Oogenesis
The female auxocyte or primary
oocyte is very much like the primary
spermatocytes. But the period of
growth prior to the maturation
divisions is prolonged where large
amounts of yolk are formed. The
nature of the maturation division is
also different, for the first division,
instead of giving rise to two
secondary oocytes of equal size,
produces one very large cell and one
excessively minute cell called first
polar body. In the second division
the large secondary oocyte again
divides into one large cell and one
very small cell – the second polar
body. At the same time the small first
polar body may or may not divide
into two equal cells. In this way four
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cells are produced (or three if the
smaller polar body fails to develop).
These latter are incapable of
fertilization and degenerate. As they
are formed at the animal pole of the
egg, the polar bodies are often of
practical value in orienting the egg.
Unlike the male gametes whose
maturation is completed within the
testis the oocytes do not complete
their maturation until after leaving the
ovary and in vertebrates at least, the
second division is not completed
until the entrance of the sperm. The
significance of this situation is not
understood although it has been
suggested that the egg shall be
freshly matured at the time of the
union of the pronuclei.
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Animal Reproduction
A good looking egg/oocyte
Spermatogenesis
Like oogenesis, spermatogenesis
also passes through three equivalent
stages: (1) The first is the
proliferation of the primitive germ
cells. (2) The second is the growth
period leading to rapid enlargement
or growth, (3) The third or the final
stage is the maturation stage when
the important nuclear changes occur.
Spermatozoa
arise
from
the
epithelial cells that leave the tubules
of the testis. A study of the cross
section of the normal mature
mammalian testis usually reveals all
stages of spermatogenesis. The
cells within a tubule are of two
distinct types.
They are:
1. The Sustenacular cells, also
called nurse cells, or Sertoli
cells which supply nutrition to
the maturing spermatids and
possibly produce estrogen.
2. The second are the male
germ cells. They are found in
various stages of maturation
and are arranged in a layered
order.
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A very immature egg with tightly
packed corona and cumulus cells
around
The spermatogonia and Sertoli cells
are relatively inactive until the bull
reaches puberty, which occurs when
the average bull calf is 9 – 12
months
of
age.
Then
the
spermatogonia begin to multiply
which are of two types in the bull:
The A type, which divide only to
produce other spermatogonia and
the B type, which divide by a similar
mitotic division to form two primary
spermatocytes. After growth and
enlargement
of
the
primary
spermatocytes, two nuclear divisions
follow. The first gives rise to two
secondary spermatocytes from each
spermatocyte and the second to two
spermatids from each secondary
spermatocyte. Thus from each
primary
spermatocytes
four
spermatids are formed.
The primary spermatocytes while
forming secondary spermatocytes go
through reduction division. Thus
each
secondary
spermatocyte
contains only one half of the number
of chromosomes in the primary
spermatocytes and it is not any half
but one member from each pair of
homologous chromosomes.
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Animal Reproduction
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Prior to the above separation and
reduction in chromosome number,
each chromosome starts to split
lengthwise.
At this time there usually occurs
twisting of the tetrad which allows for
the exchange of chromatin material
between the members of the tetrad,
i.e. crossing over.
The pair members, each of which is
partially split, come together and
form what is known as tetrad.
Primary spermatocyte
Primary spermatocyte
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Animal Reproduction
College of Natural Resources
The secondary spermatocytes after
its formation go through a second
meiotic division. This process
involves homotypic or equational
division and the separation of the
earlier formed diads. This results in
the formation of spermatids, which
are
smaller than either the
spermatogonia
or
primary
spermatocytes and differ from them
in having half the original number of
chromosomes
The newly formed spermatid then
enters a series of morphological
changes by which it becomes
converted into an elongated element
with the chromatin material packed
in the head, followed by a slender
middle piece and behind the middle
piece is a whip like tail.
Primary spermatocyte
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Animal Reproduction
College of Natural Resources
Secondary spermatocyte
Secondary spermatocyte
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