Spermatogenesis and Oogenesis
Spermatogenesis and Oogenesis
Spermatogenesis and Oogenesis
2
nd
spermatocyte by 2
nd
meiotic division gives haploid
spermatids (4 in no.).
Spermiogenesis
- It is the morphological changes through which the spermatid is
transformed into a sperm.
The sperm
Fig. 8A: Drawing showing the main parts
of the human sperm (X1250). The head,
composed mostly of the nucleus, is partly
covered by the cap like acrosome (an
organelle containing enzymes). The tail of
the sperm consists of three regions: The
middle piece, principal piece, and an end
piece.
Scanning electron
micrograph of human
sperms
OOGENESIS
Oogenesis means differentiation of the
primordial germ cells (oogonia) that are
present in the cortex of the ovary into
mature ova. Oogenesis passes into two
stages of maturation: prenatal and
postnatal maturation:
1. Prenatatal maturation
- The primordial germ cells (oogonia) undergo
mitotic divisions then they are arranged in clusters
surrounded by a layer of flat epithelial cells.The flat
cells are called follicular cells and are derived from
the epithelium covering the ovary.
- Some oogonia differentiate into large primary
oocytes where DNA replication occurs and they
enter the prophase of the first meiotic division.
- The surviving primary oocytes become
surrounded by flat epithelial cells. Together they
are called primordial follicles.
2. Postnatal maturation
- Near birth, all the primary
oocytes have started
prophase of the first meiotic
division then they are arrested
till puberty.
- The number of primary
oocytes at birth ranges from
700,000 to 2 million. At
puberty only 400,000 primary
oocytes remain and about 400
only will be ovulated during
the female life time.
- With the onset of puberty, 5-15 primordial
follicles begin to maturate with each ovarian cycle.
- The primary oocyte begins to increase in size
and the surrounding flat cells change from flat to
cuboid and proliferate to produce a stratified
epithelium of granulosa cells.
- The follicle is now called the primary follicle. The
granulosa cells rest on a basement membrane
separating them from the stromal cells of the ovary
that form the theca folliculi.
- The granulosa cells and the oocyte secrete a
layer of glycoprotein on the surface of the oocyte
called the zona pellucida.
- Fluid-filled spaces appear between the granulosa
cells then they coalesce together forming the
follicular antrum and the follicle is now called the
secondary follicle. .
- The follicle enlarges and is called the Graafian
follicle
- The primary oocyte continues the first
meiotic division leading to two unequal
daughter cells with 23 chromosomes each
(22 autosomes + X). The large cell is called
the secondary oocyte and the small one
(having little cytoplasm) is the first polar
body. The secondary oocyte and the first
polar body enter the second meiotic division
without DNA replication.
- The second meiotic division is completed
only if fertilization occurs to give fertilized
oocyte (mature ovum) and a second polar
body that soon degenerates.
- The primary oocyte continues the first
meiotic division leading to two unequal
daughter cells with 23 chromosomes each
(22 autosomes + X). The large cell is called
the secondary oocyte and the small one
(having little cytoplasm) is the first polar
body. The secondary oocyte and the first
polar body enter the second meiotic division
without DNA replication (Fig. 8).
- The second meiotic division is completed
only if fertilization occurs to give fertilized
oocyte (mature ovum) and a second polar
body that soon degenerates.
The mature ovum
- It is a large oval cell that
varies from 117 142 in
diameter. It has two
membranes; the inner thin
one is the vitelline membrane
and the outer one is the zona
pellucida.
- The corona radiata is two
or three layers of cells
surrounding the zona
pellucida externally when the
ovum is shed from the
follicle. .
Fig. 14: Drawings (A-C) illustrating the movement of the uterine tube
that occurs during ovulation. Note that the fimbriated infundibulim of
the tube becomes closely applied to the ovary. Its finger like fimbriae
move back and forth over the ovary and sweep the secondary oocyte
into the infundibulum as soon as it is expelled from the ovarian follicle
and ovary during ovulation.
Comparison between the
male and female gametes
Sperm Oocyte (ovum)
- Size small (50 u) large (117-142 )
- Mobility highly motile immotile
- Membranes
------------------
covered by zona
pellucida and corona radiata
- Cytoplasm
Little
- abundant (contains
yolk granules for nutrition of
the embryo during the first
week of development)
- Chromosomes
2 types:
23,X or 23,Y
Only 23X