Gametogenesis | structure of sperm | NCERT | NEET | class 12

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This lecture is about
GAMETOGENESIS
The primary sex organs - the testis in the males and the ovaries in the
females -produce gametes, i.e, sperms and ovum, respectively, by the
process called gametogenesis. In testis, the immature male germ cells
(spermatogonia) produce sperms by spermatogenesis that begins at
puberty. The spermatogonia (sing. spermatogonium) present on the
inside wall of seminiferous tubules multiply by mitotic division and
increase in numbers. Each spermatogonium is diploid and contains 46
chromosomes. Some of the spermatogonia called primary
spermatocytes periodically undergo meiosis. A primary spermatocyte
completes the first meiotic division (reduction division) leading to
formation of two equal, haploid cells called
secondary spermatocytes, which have
only 23 chromosomes each. The secondary
spermatocytes undergo the second meiotic
division to produce four equal, haploid
spermatids (Figure 2.5). What would be the
number of chromosome in the spermatids?
The spermatids are transformed into
spermatozoa (sperms) by the process
called spermiogenesis. After
spermiogenesis, sperm heads become
embedded in the Sertoli cells, and are finally
released from the seminiferous tubules by
the process called spermiation.
Spermatogenesis starts at the age of
puberty due to significant increase in the
secretion of gonadotropin releasing hormone
(GnRH). This, if you recall, is a hypothalamic hormone. The increased
levels of GnRH then acts at the anterior pituitary gland and stimulates
secretion of two gonadotropins - luteinising hormone (LH) and follicle
stimulating hormone (FSH). LH acts at the Leydig cells and stimulates
synthesis and secretion of androgens. Androgens, in turn, stimulate the
process of spermatogenesis. FSH acts on the Sertoli cells and stimulates
secretion of some factors which help in the
process of spermiogenesis.
Let us examine the structure of a sperm. It
is a microscopic structure composed of a head,
neck, a middle piece and a tail (Figure 2.6).
A plasma membrane envelops the whole body
of sperm. The sperm head contains an elongated
haploid nucleus, the anterior portion of which
is covered by a cap-like structure, acrosome.
The acrosome is filled with enzymes that help
fertilisation of the ovum. The middle piece
possesses numerous mitochondria, which
produce energy for the movement of tail that
facilitate sperm motility essential for fertilisation.
The human male ejaculates about 200 to 300
million sperms during a coitus of which, for
normal fertility, at least 60 per cent sperms
must have normal shape and size and at least
40 per cent of them must show vigorous
motility.
Sperms released from the seminiferous
tubules, are transported by the accessory
ducts. Secretions of epididymis, vas deferens, seminal vesicle and
prostate are essential for maturation and motility of sperms. The seminal
plasma along with the sperms constitute the semen. The functions of
male sex accessory ducts and glands are maintained by the testicular
hormones (androgens).
The process of formation of a mature female gamete is called oogenesis
which is markedly different from spermatogenesis. Oogenesis is initiated
during the embryonic development stage when a couple of million gamete
mother cells (oogonia) are formed within each fetal ovary; no more oogonia
are formed and added after birth. These cells start division and enter into
prophase-I of the meiotic division and get temporarily arrested at that stage,
called primary oocytes. Each primary oocyte then gets surrounded by a
layer of granulosa cells and is called the primary follicle (Figure 2.7). A
large number of these follicles degenerate during the phase from birth to
puberty. Therefore, at puberty only 60,000-80,000 primary follicles are
left in each ovary. The primary follicles get surrounded by more layers of
granulosa cells and a new theca and are called secondary follicles.