Download - Gametogenesis Mona A. Elsafadi
Gametogenesis Mona A. Elsafadi Gametogenesis a process of
formation and development of specialized generative cells
(gametes)
Involves chromosomes and cytoplasm of the gametes. Prepares sex
cells for fertilization. Chromosomes number is reduced by half. The
shape of the cells is altered. Mona A. Elsafadi Meiosis is a
process of reduction division in which the number of chromosomes
per cell is cut in half and results in the formation of gametes.
Mona A. Elsafadi Meiosis Involves two meiotic divisions Takes place
in germ cells only.
Produces haploid gametes ( sperms and oocytes). Occurs during
gametogenesis Mona A. Elsafadi Meiosis Homologous chromosomes:
pairs of chromosomes, one inherited from each parent and are
similar in shape and size and have identical gene loci. X and Y
chromosomes are not homologues but they have homologous segments at
the tips of their short arms. They pair at these regions only. Mona
A. Elsafadi Meiosis Mona A. Elsafadi Meiosis I has three unique
features:
Synapsis: a process of forming complexes of homologous chromosomes
following chromosome replication. Homologous recombination
(crossing over): the genetic exchange process that occurs between
paired homologous chromosomes, so the sister chromatids in meiosis
II are not always identical. Reduction division: at the end of
meiosis each cell contains only half the original complement of
chromosomes. Mona A. Elsafadi Second Meiotic Division
Has no DNA replication. The haploid number of chromosomes is
retained. Each daughter cells has the reduced haploid number of
chromosomes with single chromatid chromosomes. Similar to an
ordinary mitosis except that the chromosome number of the cell
entering the second meiotic division is haploid. Mona A. Elsafadi
Importance of meiosis:
Provides constancy of chromosome number by reducing it from diploid
to haploid. Allows random assortment of maternal and paternal
chromosomes between the gametes. Relocates segments of maternal and
paternal chromosomes by crossing over of chromosome segments. Mona
A. Elsafadi Spermatogenesis The sequence of events by which
spermatogonia are transformed into spermatozoa (mature sperms) Mona
A. Elsafadi Spermatogenesis Purpose: to produce mature sperms which
are able to fertilize the oocytes to produce the zygotes. Location:
Seminiferous tubules of the testes Epididymis Duration: about 64
days in human Timing: begins at puberty and continues until death.
Mona A. Elsafadi Mona A. Elsafadi Stages of spermatogenesis
Spermatocytogenesis: at puberty spermatogonia proliferate by
mitosis to yield primary spermatocytes. Meiosis: Meiosis I:
formation of secondary spermatocytes (1N) Meiosis II: formation of
spermatids (IN) Spermiogenesis: maturation process by which
spermatids are transformed into mature sperms and supported by
Sertoli cells. Mona A. Elsafadi Spermiogenesis Mona A. Elsafadi
Sertoli Cells Lining the seminiferous tubules.
Support and nurture the germ cells. Involved in the regulation of
spermatogenesis. Mona A. Elsafadi Epididymis Elongated coiled duct
along the posterior border of the testis Continuous with the vas
deferens which transports the sperms to the urethra. Where sperms
are stored and become functionally mature after transporting
passively form seminiferous tubules. Mona A. Elsafadi Mature Sperm
Is a free-swimming, actively motile cell consisting of a head, a
neck and a tail Mona A. Elsafadi Sperms Head Forms most of the bulk
of the sperm.
Contains haploid nucleus. Contains Acrosome: Covers the anterior
two-third of the nucleus Caplike saccular organelle containing
several enzymes, which facilitate sperm penetration of the corona
radiata and zona pellucida during fertilization. Mona A. Elsafadi
Sperms Tail Provides the motility of the sperm.
Consists of three segments: Middle piece Principal piece End piece
The middle piece contains mitochondria, which provides ATP for
activity. Mona A. Elsafadi Genes and molecular Factors
Bcl-2 family proteins involved in: germ cells maturation germ cells
survival at different stages. Y chromosome: Essential for normal
spermatogenesis Microdeletions result in defective spermatogenesis
and infertility Mona A. Elsafadi Oogenesis Is the sequence of
events by which oogonia are transformed into mature oocytes. Mona
A. Elsafadi Oogenesis Timing: No. of oocytes: begins before
birth
Is completed after puberty Continues to menopause No. of oocytes: 2
millions primary oocytes at birth 40,000 remain at adolescence 400
become secondary oocytes and ovulated Mona A. Elsafadi Oogenesis
Mona A. Elsafadi Prenatal Maturation During early fetal life,
oogonia proliferate by mitosis. Before birth, oogonia enlarge to
form primary oocytes. Primordial follicle: primary oocyte
surrounded by a single layer of flattened epithelial cells Primary
follicle: enlarged primary oocyte surrounded by zona pellucida and
a single layer of columnar follicular epithelial cells Secondary
follicle: primary follicle with more than one layer of follicular
cells Mona A. Elsafadi Oocyte Maturation Mona A. Elsafadi Prenatal
Maturation First meiotic division
Begins before birth Completed after adolescence (11-19 yrs) Oocyte
maturation inhibitor: Secreted by primary oocytes maintains the
meiotic arrest Mona A. Elsafadi Postnatal Maturation The primary
oocytes remain dormant until puberty.
Shortly before ovulation, primary oocyte increases in size and
complete first meiotic division to form secondary oocyte with first
polar body. At ovulation, secondary oocyte begins second meiotic
division but arrested at metaphase. If fertilization occurs, second
meiotic division is resumed and produces second polar body. Mona A.
Elsafadi Postnatal Maturation Begins during puberty
One follicle matures and ovulates each month Long duration of first
meiotic arrest may cause: High frequency of meiotic errors
(non-disjunction) Primary oocytes to be vulnerable to environmental
agents (radiation) Mona A. Elsafadi Comparison of Gametes No
primary oocytes form after birth
Division of cytoplasm is unequal Massive immotile cells Surrounded
by zona pellucida and a layer of follicular cells. Only one kind of
secondary oocyte: 23,X Continuous production of primary
spermatocytes Division of cytoplasm is equal Microscopic motile
cells Sparse cytoplasm and have heads covered with acrosomes and
tails. Two kinds of normal sperm: 23,X and 23,Y Mona A. Elsafadi
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