ZOOHCC - 601: Developmental Biology (Theory)
Gametogenesis
Gametogenesis is the process by which specialized reproductive cells
called gametes are formed. Gametes are responsible for sexual reproduction
and are necessary for the fusion of genetic material from two individuals
during fertilization, resulting in the formation of a new organism.
In humans, gametogenesis occurs in the gonads, which are the testes in
males and the ovaries in females. The process of gametogenesis differs
between males and females:
Spermatogenesis (Male Gametogenesis): Spermatogenesis is the process of
producing sperm cells or spermatozoa. It begins at puberty and continues
throughout a male's life. The key steps involved in spermatogenesis
are:
a. Germ Cell Division: Specialized cells called spermatogonia undergo
mitotic divisions to produce a large number of cells called primary
spermatocytes.
b. Meiosis: Primary spermatocytes undergo two rounds of meiotic division,
resulting in the formation of four haploid cells called spermatids.
c. Spermiogenesis: Spermatids undergo a process called spermiogenesis,
during which they undergo structural changes to develop into mature
spermatozoa.
Oogenesis (Female Gametogenesis): Oogenesis is the process of producing
eggs or ova. It begins before birth and continues until menopause. The key
steps involved in oogenesis are:
a. Germ Cell Division: Specialized cells called oogonia undergo mitotic
divisions during fetal development to produce primary oocytes.
b. Meiosis I: Primary oocytes enter meiosis I but pause at prophase I
until puberty. Each month, one primary oocyte is stimulated to complete
meiosis I, resulting in the formation of a secondary oocyte and a polar
body.
c. Meiosis II: If the secondary oocyte is fertilized, it proceeds to
meiosis II, resulting in the formation of a mature ovum and another polar
body.
Spermatogenesis
It is the process of sperm cell development in males, leading to the
production of mature spermatozoa (sperm cells). It occurs in the
seminiferous tubules within the testes and involves a series of cellular
divisions and differentiations.
Here is an overview of the stages and key steps involved in
spermatogenesis:
Spermatogonia
Spermatogonia are the undifferentiated cells located in the outer lining
of the seminiferous tubules. They undergo a process called mitosis to
maintain the pool of stem cells. Some spermatogonia differentiate into
primary spermatocytes.
Primary Spermatocytes: Primary spermatocytes are diploid cells resulting
from the differentiation of spermatogonia. They undergo the first
meiotic division (meiosis I) to produce two haploid secondary
spermatocytes.
Secondary Spermatocytes: Secondary spermatocytes are haploid cells
resulting from the first meiotic division. They quickly enter the second
meiotic division (meiosis II) without going through an intervening
interphase. This division produces four haploid cells called spermatids.
Spermatids: Spermatids are haploid cells resulting from the second
meiotic division. They are initially round in shape and have a limited
capacity for movement. During spermiogenesis, spermatids undergo
extensive structural changes to become mature spermatozoa.
Spermatozoa: Spermiogenesis is the final stage of spermatogenesis, where
spermatids differentiate into mature spermatozoa. Key transformations
include:
a. Acrosome Formation: The Golgi apparatus within spermatids forms the
acrosome, a cap-like structure containing enzymes essential for
fertilization.
b. Head and Tail Formation: The nucleus of the spermatid condenses and
elongates to form the head of the spermatozoon. The centrioles in the
spermatid's cytoplasm give rise to the microtubules that make up the
tail, allowing for motility.
c. Shedding of Excess Cytoplasm: Excess cytoplasm is shed, leaving a
compact and streamlined structure.
The mature spermatozoa are released into the lumen of the seminiferous
tubules and subsequently move into the epididymis for further maturation
and storage until ejaculation.
spermiogenesis is the final phase of spermatogenesis, specifically
focused on the maturation and differentiation of spermatids into fully
formed and functional spermatozoa.
It is important to note that spermatogenesis is a continuous process in
males, producing millions of sperm cells each day. The entire process,
from spermatogonia to mature spermatozoa, takes approximately 64 to 72
days.
Oogenesis
Oogenesis is the process of egg cell or ovum development in females.
It involves the maturation of oocytes within the ovaries, leading to
the production of mature eggs capable of fertilization. Oogenesis
differs from spermatogenesis, as it has a more limited and finite
supply of oocytes that are present at birth and gradually decline over
a woman's reproductive lifespan.
Here is an overview of the stages and key steps involved in oogenesis:
Oogonia: Oogonia are the undifferentiated cells present in the ovaries
of a developing female fetus. They undergo mitotic divisions to
increase their numbers during fetal development. These cells
eventually differentiate into primary oocytes.
Primary Oocytes: Primary oocytes are diploid cells resulting from the
differentiation of oogonia. They enter a phase of cell growth and
remain arrested in prophase I of meiosis until puberty.
Ovarian Cycle: During the monthly ovarian cycle, a small number of
primary oocytes are stimulated to resume development. However,
typically, only one primary oocyte fully matures during each menstrual
cycle.
Meiosis I: The fully matured primary oocyte undergoes meiosis I. As a
result of meiosis I, the primary oocyte divides into two cells: a
secondary oocyte and the first polar body. The secondary oocyte
retains almost all of the cytoplasm, while the first polar body
contains a small portion of cytoplasm and eventually degenerates.
Meiosis II: If fertilization occurs, the secondary oocyte proceeds to
meiosis II. However, meiosis II is only completed if the secondary
oocyte is penetrated by a sperm cell. Completion of meiosis II results
in the formation of a mature ovum (egg) and a second polar body.
Fertilization: If the mature ovum is fertilized by a sperm cell, it
combines its genetic material with that of the sperm, forming a
zygote. The zygote then undergoes further development and cell
divisions to form an embryo.
It is important to note that oogenesis is a continuous process, with a
small number of oocytes being selected for maturation and release each
month during a woman's reproductive years. However, the vast majority
of oocytes do not fully develop and undergo atresia, a process of
degeneration.
Overall, oogenesis is the process by which female germ cells develop
into mature eggs, allowing for the possibility of fertilization and
subsequent development of an embryo.
Types of eggs
On the basis of the amount of yolk eggs are classified as follow:
1. Alecithal Eggs: If yolk is not present in the egg, it is known as
alecithal egg. e.g., The eggs of
eutherian mammals
2. Microlecithal Egg: These are small sized eggs containing less or
negligible amount of yolk
than the amount of cytoplasm. e.g., Eggs of Hydra, Sea urchin,
Amphioxus, Tunicates.
3. Mesolecithal Egg: If amount of yolk is moderate in eggs, these
eggs are named as
mesolecithal eggs. e.g., the eggs of Amphibians, Dipnoi and
Petromyzon.
4. Macrolecithal or Megalecithal or Polylecithal Egg
This type of egg contains large amount of yolk several time greater
than cytoplasm. e.g.,
Reptiles, Birds, Prototheria.
On the Basis of the distribution of yolk
A)Isolecithal or Homolecithal Eggs: In isolecithal eggs very small
amount of yolk is present.
The yolk in such eggs is scattered almost uniformly throughout the
ooplasm. e.g.,
Echinoderms, Amphioxus, mammals.
B)Telolecithal Eggs: In telolecithal eggs, moderate or large
quantity of yolk is present but the
distribution of yolk is not uniform. The yolk is concentrated more
towards the vegetal pole.
C) Centrolecithal Egg:In these types of eggs, yolk is concentrated
in the center of egg. The
surface of the yolk is covered by a thin peripheral layer of
cytoplasm. e.g., Eggs of Insects and
some hydrozoa.
Mosaic and Regulative Eggs
a. Mosaic Egg or Determinate egg:
In this type of eggs every portion of egg is predetermine before
fertilization for embryonic
development. If any part of egg is removed, the related organ does
not originate and a defective
embryo is formed. The remaining portion of the egg cannot make
compensatory development.
e.g., Molluscs and Ascidians.
b. Regulative Egg/Indeterminate egg:
The developmental potentialities of eggs are not predetermined. The
normal development is not
affected by removal of a small portion of the egg, or even one or
two early blastomeres.
On the Basis of Shell
These are of two types (a) Cleidoic eggs (b) Non-cleidoic eggs
2
(a) Cleidoic egg: These eggs are covered by thick and hard shell for
protection. The shell is
permeable for gases. e.g., Reptiles and Birds.
(b) Non Cleidoic egg: These eggs are without shell e.g., All
viviparous animals and oviparous
animals which lay eggs in water have non-cleidoic egg.
Egg Membranes
The oogenesis is followed by formation of protective egg membrane
around the ovum. Except
sponges and coelenterates almost all the animals have a protective
membrane surrounds the ovum.
These membranes are produced by the egg itself or by the follicle
cells of the ovary or by the
oviduct of the female. On the basis of their origin the egg
membranes are classified into three
types.
1. Primary membranes
2. Secondary membranes
3. Tertiary membranes
I. Primary membrane: These membranes develop in the ovary, between
the oocyte and follicle
cells around the plasma membrane. Primary egg membranes are secreted
either by follicle cells
or by oocyte and sometimes by the contribution of both. The primary
membranes have different
structure in the different animals. They are following types:
A) Vitelline Membrane: In some animals egg primary membrane is known
as vitelline
membrane. It is very thin and transparent and contains
mucopolysaccharides and fibrous
protein. The space between vitelline membrane and the plasma
membrane is called
perivitelline space, filled with a fluid called perivitelline fluid.
Commonly found in Eggs of
Insects, Molluscs, Echinoderms, Amphibians, Birds etc.
B) Chorion: It is found in the eggs of lower chordates like Teleost
fishes. It is a product of
surface ooplasm.
C) Zona Radiata: The primary membrane of egg of the shark, some bony
fishes, some
amphibians and some reptile has a radiated appearance produced by
the surface of ooplasm
called zona radiata.
D) Zona Pellucida: It is a glycoprotein layer surrounding the plasma
membrane of mammalian
oocyte secreted by follicle cells.
II. Secondary Membranes
The secondary membrane is formed as a basement membrane by a layer
of the follicle cells that
surround the ovum. These membranes are usually tough and
impermeable. The secondary
membranes are as follows:
a. Chorion: This is a common outer covering in the eggs of insects,
ascidians and
cyclostomes. It is found outside the vitelline membrane. As the
chorion is tough and
impermeable, it is provided with one or more openings called
micropyles through which
the sperms enter the egg.
b. Corona Radiata: It is found in mature mammalian eggs around the
ovum outside the
zona pellucida . This membrane is formed by columnar, radially
arranged layer of follicle
cells.
3 III. Tertiary Membranes
The tertiary membranes are secreted by the cells of oviduct when
ovum moves through the
oviduct.
a. White Albumen
It surrounds the vitelline membrane of hen egg and formed by the
secretions of anterior
part of the oviduct, consists of water and protein.
b. Shell Membrane: The two shell membranes of keratin cover the
albumen in the egg
of hen.
c. Shell: The shell is the outer porous covering formed by calcium
carbonate and
collagen found in eggs of reptiles and birds.
d. Jelly Coat: The amphibian eggs are covered by albumen (jelly) as
gelatinous covering called jelly coat. These jelly envelopes attach
the eggs in masses and protect them from infection and insulate
them.
