Gametogénesis

ABSTRACT

Gametogenesis is the production of haploid sex cells (in
humans, ovum and spermatozoa) that each carry one-half the
genetic compliment of the parents from the germ cell line of each
parent.

The production of ovum is termed oogenesis and the
production of spermatozoa is called spermatogenesis. Both
oogenesis and spermatogenesis provide a mechanism through which
genetic information may be passed to offspring. The fusion of
spermatozoa and ova during fertilization results in a zygote with
a fully restored diploid genome.

The production of male and female gametes a highly
complex and coordinated sequence of a mitotic division, two
meiotic divisions, cytoplasmic apportionment (divisions) and
cellular differentiation.

In eukaryotic organisms the gametes are derived from
primordial germ cells, which enter the gonads during early
development. During embryogenesis, the primordial germ cells are
determined early in development by the presence of a cytoplasmic
component termed germ plasm. Once germ cells are determined they
follow a different maturation and, of course, genetic function,
than do the remaining somatic cells of the body. Primordial germ
cells are the stem cells that, via mitosis, supply both
spermatogonia and oogonia.

In humans, spermatogenesis starts with a diploid (2N)
spermatogonium that carries the full genetic compliment of 46
chromosomes (22 autosomal pairs, one X and one Y sex
chromosomes). The spermatogonium represents the germ cell line
from which all sperm cells are derived. Sequentially, the process
of spermatogenesis via mitosis produces a primary spermatocyte
that is also diploid (2N) and then via meiosis, two secondary
spermatocytes that are haploid (N). The haploid secondary
spermatocytes carry 22 autosomes and either an X or a Y sex
chromosome. The secondary spermatocytes each undergo a second
meiotic division to form a total of four haploid spermatids.
Subsequently, nurtured by surrounding somatic cells, through the
process of cellular differentiation the four spermatids produce 4
sperm cells capable of motility and fertilization

In human females the germ cell line is represented by
the diploid (2N) oogonium that carries the full
female genetic compliment of 22 autosomal pairs and two X
chromosomes. Mitotic division yields a diploid primary oocyte.
Meiotic divisions then produce one female gamete–the ovum. In
humans, the first meiotic division is suspended in the diplonema
stage during embryonic development. Meiosis resumes, one ovum at
a time following puberty and during the ovulatory period of the
menstrual cycle. Maturation proceeds with the production of
haploid (N) secondary oocytes with 22 autosomal
chromosomes and an X sex chromosome (the sex chromosome must be
an X chromosome because normal human females carry two X
chromosomes and no Y chromosomes). Also formed is a haploid polar
body that is nearly devoid of cytoplasmic contents. This is a
fundamental difference between male and female Gametogenesis. In
males, there is a nearly equal divison of cytoplasm to the
gametes, in females the cytoplasmic contents are preserved for
the eventual "egg" or ovum. Extraneous genetic material is
removed via polar bodies. Another meiotic division results in the
production of an ootid and yet another polar body (the eventual
number of polar vies associated with an ovum may equal as many as
three if the first sloughed off polar body undergoes a subsequent
division. Cellular differentiation of the ootid yields an ovum
ready for fertilization. In many cases, however, the last
maturational processes are accelerated because in human females,
meiosis II is usually completed after fertilization.

1.
INTRODUCCION

La reproducción es la capacidad que los seres
vivos tienen para perpetuarse y dar lugar a otros individuos
semejantes a ellos y constituye una función básica
de todo organismo.

Los detalles del proceso reproductivo varían
mucho según los organismos, pero existen dos mecanismos
por los cuales se propagan o multiplican: uno es la
reproducción asexual, donde los nuevos organismos
provienen de un progenitor, el otro es la reproducción
sexual, donde los nuevos organismos provienen de la
combinación genética de dos células llamadas
gametos aportadas por dos progenitores.

La reproducción sexual esta conformada por dos
etapas, una primera etapa también llamada
gametogénesis, en la cual se produce el gameto o unidad
reproductora mediante el proceso de meiosis y la segunda etapa o
fecundación durante la cual el gameto masculino y el
femenino se unen para formar el huevo o cigoto.

Las células haploides que están
especializadas para la fusión sexual reciben el nombre de
gametos. Típicamente se forman dos tipos de gametos: uno
es grande e inmóvil y se denomina oocito (o huevo) y el
otro es pequeño y móvil y se denomina
espermatozoide. Durante la fase diploide que sigue a la
fusión de gametos, las células proliferan y se
diversifican formando un organismo pluricelular
complejo.

2.
GAMETOGÉNESIS

Gametogénesis es el proceso inclusivo por el cual
células diploides experimentan meiosis para producir
gametos haploides altamente diferenciados y especializados.
Aunque la formación de gametos difiere en cada sexo el
espermatozoide y el ovulo son homólogos e involucran
transformaciones morfofisiológicas. Ciertamente el
espermatozoide debe reconocer y acoplarse a componentes
específicos de la zona pelucida del ovulo para que la
fertilización ocurra