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UNIT 3: DEVELOPMENTAL

BIOLOGY

BY; DR. LUNA PHUKAN

TH

SEM GENERAL

GAMETOGENESIS: SPERMATOGENESIS

AND OOGENESIS

Gametogenesis occurs when a haploid cell (n) is

formed from a diploid cell (2n) through meiosis.

We call gametogenesis in the male

spermatogenesis and it produces spermatozoa.

In the female, we call it oogenesis. It results in

the formation of ova.

An organism undergoes a series of changes

throughout its life cycle. Gametogenesis

(spermatogenesis and oogenesis), plays a crucial

role in humans to support the continuance of

generations.

Gametogenesis is the process of division of diploid

cells to produce new haploid cells. In humans, two

different types of gametes are present. Male

gametes are called sperm and female gametes are

called the ovum.

Spermatogenesis: Sperm formation

Oogenesis: Ovum formation

The process of gametogenesis occurs in the

gonads and involves the following steps:

Multiple mitotic divisions and cell growth of

precursor germ cells

• Two meiotic divisions (meiosis I and II) to

produce haploid daughter cells

• Differentiation of the haploid daughter cells to

produce functional gametes

Oogenesis

• Oogenesis describes the production of female gametes (ova) within the ovaries (and, to a lesser extent, the oviduct)
• The process begins during foetal development, when a large number of primordial cells are formed by mitosis (~40,000)
• These cells (oogonia) undergo cell growth until they are large enough to undergo meiosis (becoming primary oocytes)
• The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles

• The secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube)
• The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte
• If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body)
• Once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote

Formation of Polar Bodie

Timing of the process In spermatogenesis, the production of gametes is a continuous process that begins at puberty and continues until death In oogenesis, the production of gametes is a staggered and finite process: It begins before birth (prenatally) with the formation of a fixed number of primary oocytes (~40,000) It continues with the onset of puberty according to a monthly menstrual cycle It ends when hormonal changes prevent the further continuance of the menstrual cycle (menopause)

Summary of the Differences between Spermatogenesis and Oogenesis

FERTILIZATION : SPERM EGG

INTERACTION : ACTIVATION OF EGG:

GAMETE FUSION IN SEA URCHIN

Fertilization is a multi-step process that involves

the interaction of a mature, capacitated

spermatozoon and ovulated egg. ... Sperm and egg

plasma membranes bind and fuse, resulting in

incorporation of sperm contents into the egg

cytoplasm. Fusion triggers egg activation and

establishment of a block to polyspermy.

SUMMARY: Fertilization is a cell-cell

recognition process that occurs between two

distinct cells: a small

asymmetric and motile sperm cell and a large

and nonmotile egg. The stages of fertilization

can be divided intofour processes:

1 ) sperm preparation, 2) sperm-egg

recognition and binding, 3) sperm-egg fusion

and 4) fusion

of sperm and egg pronuclei and activation of

the zygote.

Discuss the sequential nature of fertilization in which

ordered changes in the gametes “drive”

the process of fertilization toward completion.

2. Explain the role of specialized sperm and egg surface

structures in fertilization.

3. Describe how egg and sperm receptors were

identified.

4. Explain the current state of knowledge about sperm-

egg membrane fusion and how sperm

components are incorporated into the egg.

5. Describe how polyspermy is prevented and the

fertilized egg is activated for development.

GLOSSARY:

Capacitation: The process by which the sperm

becomes capable of fertilizing an egg.

Acrosome Reaction: A regulated exocytotic event in

which an apical vesicle in the sperm head fuses with

the sperm plasma membrane. The acrosome

reaction is triggered in response to egg factors.

Acrosin: A serine protease released during the

acrosome reaction.

Cortical Reaction: A regulated exocytosis in which

apically localized vesicles (cortical granules) in the

egg fuse with plasma membrane after fertilization.