Gametogenesis, fertilization and types of eggs

By – Dr. Mafatlal M. Kher
Developmental biology: Gametogenesis and Fertilization
Date & Time : Sunday, 29 August 2021
Semester : III
Program : B.Sc. Biotechnology
School : School of Science
Subject : BSBO303

Developmental Biology?
Developmental biology is the study of the process by which animals and
plants grow and develop.
2

Developmental Biology (Definition):-
 Developmental Biology enquires about the fundamental processes that underpin the
fertilisation of an egg cell and its step-by-step transformation into the fascinating
complexity of a whole organism.
 Developmental Biology is the study of the processes by which organs grow and develop.
Modern developmental biology studies the genetic control of cell growth, differentiation
and morphogenesis, which is the process that gives rise to tissues, organs and anatomy,
but also regeneration and ageing.
 Developmental biology is the study of the process by which animals and plants grow and
develop.
 Developmental Biology is the causal analysis of the cellular mechanisms that drive
processes of growth, pattern formation and morphogenesis. 5

Development (Definition):-
Development is the process by which a complex multicellular organism arises from a
single cell. It involves an:
 Increase in cell number,
 Differentiation,
 Pattern formation
 Morphogenesis,
 As well as net growth.
Development is a gradual process, so the complexity of the embryo increases
progressively.
6

Gamete
Zygote
Growth Cell
Division
Differentiation Pattern
formation
Morphogenesis
Increase
in Size
Increase
in Cell
number
Diversification
of Cell types
Organisation Generation of
Shapes and
Structures
Adult
Gamete
An overview of developmental process
7

Why we study developmental
Biology
8

Why we study developmental biology?
Population Health
Food security/Crop productivity
How?
How?
Developmental Biology can provide key strategies to improving crop and plant
cultivation
Most of diseases are associated with defects during early developmental stages.
9

 Identification of birth defects, has uncovered:-
 Many links with human disease.
 Many diseases that only manifest symptoms during adulthood are the result of defects that occurred
during:-
 Embryogenesis or
 result from the inappropriate reactivation of developmental pathways.
 Many basic science discoveries that were originally made by Investigating development have now become
relevant in the context of :-
 Clinical research.
 Cancer is perhaps the best example of a disease where mechanisms and genetic pathways that were
originally identified as important for normal development, were later shown to be defective in the disease
state. For example:-
 The establishment of cell polarity is critical during embryogenesis for the determination of cell fate and is accomplished by
the asymmetric distribution of macromolecules, including RNAs, proteins, and organelles.
 The failure to maintain cell polarity is a hallmark of advanced tumour's and recent evidence suggests that loss of
cell polarity plays an important role in the initiation of cancer.
10

Why we study developmental
Biology: Scope
11

 What processes lead to fertilisation and the initiation of development?: How can we
overcome infertility and childlessness?
 How do single fertilised egg cells, or later on groups of progenitor cells, generate
the enormous cellular diversity of an organism and its organs and tissues?: How do
stem/progenitor cells generate whole tissues or organs – for example in
regeneration or tissue engineering, and how does wound healing work?
 How do cells, which originate from common ancestors and contain the same
genetic information, adopt different fates?: How do cells change their identities and
behaviours – for example in cancer?
 How do tissues and their cells know when to stop growing?: How can cells evade
growth control – for example in tumour growth?
 How is the formation of different cells/tissues coordinated in space and time?: What
are the mechanisms underlying birth disorders?
Fundamental questions asked by developmental biologists – and how
they translate into biomedical application
12

Who came first hen or egg?
?
13

Hen or Egg?
 Archaeopteryx fossils, which are the oldest generally accepted as birds, are around
150 million years old, which means that birds in general came after eggs in general.
That answer is also true— the egg comes first —when you narrow it down to
chickens and the specific eggs from which they emerge.
 The first amniote egg—that is, a hard-shelled egg that could be laid on land, rather
than remaining in water like the eggs of fish or amphibians—appeared around 312
million years ago.
14

Gametogenesis
The process in which cells undergo meiosis to
form gametes.
15

Gametogenesis: Importance
 The process leads to formation of germ cells or gametes.
 The normal body cells known as somatic cells are diploid (2n) where as the
germ cells are haploid (n).
 During fertilization one halpoid sperm unites with one haploid ovum to form a
normal diploid somatic cell thus keeping the chromosome number constant
generation after generation.
 During first maturation division, the reshuffling of paternal and maternal genes
take place resulting in variation.
16

Gametogenesis
Spermatogenesis Oogenesis
17

Spermatogenesis
Spermatogenesis is the process by which haploid spermatozoa develop
from germ cells in the seminiferous tubules of the testis. This process
starts with the mitotic division of the stem cells located close to the
basement membrane of the tubules
18

Spermatogenesis: Process
• The entire process of spermatogenesis can be divided into
two phases:
1. Formation of Spermatids:
A. Multiplication phase
B. Growth phase
C. Maturation phase
2. Spermiogenesis (Spermatoleosis):
20

2n
(Mitosis)
n n
(Second Meiosis)
n n n n
2n
(First Meiosis)
2n
Spermatogonia/
Spermatogonium
46
Primary spermatocyte
46
Secondary spermatocyte
23
Spermatids
23
Sperms/Spermatozoa
23
1.
Formation
of
Spermatids
2. Spermiogenesis
B. Growth phase
C. Maturation phase
Spermatogenesis
(n) (n) (n) (n)
21

Spermatogenesis:
1. FORMATION OF SPERMATIDS:
 The male gonad known as testis is the site of spermatogenesis.
 In each vertebrate a pair of testes remains attached to dorsal body
wall by a connective tissue called mesorchium.
 Each testis is formed of thousands of minute elongated and coiled
tubules called seminiferous tubules.
 The inner lining of seminiferous tubules is called as germinal
epithelium and is made of primordial germ cells (Primary germ cells)
as well as some supporting nutritive cells.
 The primordial germ cells give rise to spermatids through the following
steps:-
B. Growth phase
C. Maturation phase
22

Spermatogenesis:
1. FORMATION OF SPERMATID:
A. Multiplication phase:
 The primary germ cells multiply by repeated mitotic division.
 The cells produced after the final mitotic divisions are known as spermatogonia or
sperm mother cells.
B. Growth phase
 The spermatogonia do not divide for sometime but increase in size by
accumulating nutritive materials from the supporting cells.
 In mammals such supporting cells are called cells of Sertoli.
 The enlarged spermatogonia are now called primary spermatocytes.
C. Maturation phase
 During the phase of maturation, the primary spermatocytes divide by meiosis
consisting of two successive divisions.
 The first division is reductional or disjunctional reducing the chromosome
number from ‘2n’ to ‘n’. These cells are called secondary spermatocytes.
 Second division is equational resulting in formation of four daughter cells called
spermatids. 23

Spermatogenesis:
2. SPERMIOGENESIS (SPERMATOLEOSIS): This is the second phase of spermatogenesis during which the
spermatids produced at the end of first phase are metamorphosed into sperm cells. The spermatid is a typical cell
containing a nucleus and cytoplasmic organelles such as mitochondria, golgi bodies, centriole etc, but the nucleus
only contains haploid number of chromosomes.
Spermiogenesis changes: (Spermatid to
Sperm)
 The large spherical nucleus becomes smaller
by losing water and usually changes its shape
into elongated structure.
 The Golgi bodies condense into a cap called
acrosome in front of the nucleus.
 Nucleus and the acrosome combined to form
the head of the developing sperm.
 Cytoplasm with mitochondria and centrioles
move downwards and form the cylindrical
middle piece behind the head.
 The two centrioles of middle piece develop
axial filaments which are bunched into a
single thread and extend behind in the form of
a long vibratile tail.
 Thus, spermatid is transformed into a motile
sperm divisible into head, middle piece and tail. 24

2. SPERMIOGENESIS (SPERMATOLEOSIS): Spermatids undergo morphological changes to form spermatozoa
25

26

27

28

Role of Sertoli cells in Spermatogenesis
At all stages of differentiation, the
spermatogenic cells are in close contact
with Sertoli cells which are thought to
provide structural and metabolic support
to the developing sperm cells. they
support the developing gametes in the
following ways:
• Maintain the environment necessary
for development and maturation.
• Secrete substances initiating meiosis
• Secrete supporting testicular fluid
• Protect spermatids from the immune
system of the male
29

Structure of the sperm
1. The Head: It has two important features.
 The acrosome (derived from Golgi apparatus)
contains hydrolytic enzymes which are released
when the sperm reaches an ovum. These enzymes
digest the outer membrane of the egg (proteins
and complex sugars) , allowing penetration of the
sperm.
 The nucleus (haploid) contains a single set of
chromosomes derived from the male. This will
include either an 'X' or 'Y' chromosome, because
of the way the XY separate during meiosis.
2. Neck (The Middle piece): Behind the head,
contains numerous mitochondria. These respire
sugars in the semen to generate ATP in order to
provide the energy for movement of the tail.
3. Tail: (Flagellum) contains microfilaments running
the length of the tail. Rhythmic contraction of the
filaments causes the tail to wave and move against
the fluid environment, providing forward motion.
30

Oogenesis
Oogenesis starts with the process of developing primary oocytes, which
occurs via the transformation of oogonia into primary oocytes, a process
called oocytogenesis. Oocytogenesis is complete either before or shortly
after birth.
31

Oogenesis
The process of formation of a mature female
gamete (ovum) is called oogenesis. It occurs
in the ovaries (female gonads). It consists of
three phases: Multiplication, Growth and
Maturation.
32

Oogenesis: Process
1. Multiplication:
 During foetal development, it should be noticed that certain cells present in the
germinal epithelium of the female ovary are bigger than others. These are
primordial germ cells.
 Hence, these primordial germ cells divide mitotically and creates a couple of
million oogonia or mother egg cells in each ovary present in the foetus. The
oogonia multiply by the mitotic divisions and form the primary oocytes which
pass through the growth phase.
 There are no more oogonia which are formed after birth.
35

Oogenesis: Process
2. Growth: The growth phase of the oogenesis is comparatively longer than the
growth phase of the spermatogenesis.
 In the growth phase, the size of the primary oocyte increases enormously.
 In the primary oocyte, large amount of fats and proteins becomes accumulated in
the form of yolk and due to its heavy weight (or gravity), it is usually concentrated
towards the lower portion of the egg forming the vegetative pole.
 The portion of the cytoplasm containing the egg pro-nucleus remains often
separated from the yolk and occurs towards the upper side of the egg forming the
animal pole.
 The cytoplasm of the oocyte becomes rich in RNA, DNA, ATP and enzymes.
Moreover, the mitochondria, Golgi apparatus, ribosomes, etc., become
concentrated in the cytoplasm of the oocyte.
 The nucleus becomes large due to the increased amount of the nucleoplasm and
is called germinal vesicle.
 When the growth of the cytoplasm and nucleus of the primary oocyte is
completed, it becomes ready for maturation phase.
36

Oogenesis: Process
3. Maturation: The maturation phase is accompanied by the maturation or meiotic
division.
 Here after the meiotic division of the nucleus, the cytoplasm of the oocyte
divides unequally to form a single large-sized haploid egg and three small haploid
polar bodies or polocytes at the end.
 This type of unequal division has the great significance for the egg.
 If the equal divisions of the primary oocyte might have been resulted, the stored
food amount would have been distributed equally to the four daughter cells and
which might prove insufficient for the developing embryo.
 Therefore, these unequal divisions allow one cell out of the four daughter cells to
contain most of the cytoplasm and reserve food material which is sufficient for the
developing embryo.
 It is divided into two phases first and second maturation.
37

Oogenesis: Maturation
3.1 First maturation division:
 During the first maturation division or first meiosis, the homologous chromosomes of
the primary oocyte nucleus pass through the pairing or synapsis, duplication, chiasma
formation and crossing over. Soon after, the nuclear membrane breaks and the bivalent
chromosomes move towards the opposite poles due to contraction of chromonemal fibres.
 A new nuclear envelope is developed around the daughter chromosomes by the
endoplasmic reticulum. After the karyokinesis, the unequal cytokinesis occurs and a small
haploid polar body or polocyte and a large haploid secondary oocyte or ootid are formed.
3.2 Second maturation division:
 The haploid secondary oocyte and first polocyte pass through the second meiotic division.
Due to the second meiotic division, the secondary oocyte forms amature egg and a second
polocyte.
 By the second meiotic division, the first polocyte also divides into two secondary polocytes.
These polocytes ooze out from the egg and degenerate while the haploid egg cell becomes
ready for the fertilization.
38

Ovulation
 Ovulation occurs in secondary oocyte stage.
 During lactation period of six moths from birth follicular development is inhibited by prolactin
hormone.
 So there is no menstrual cycle during lactation period.
 Ovulation occurs alternately from ovaries.
 So one ovary releases six eggs in a year.
39

Think?
 A women of 35 year age who was married at 25 is having one children, calculate?
 The number of Egg she released till now.
 Total polar bodies released .
 Total number of 1st polar body released.
 Total number of 2nd polar bodies released.
(HINT: lactation period is six month)
40

Fertilization
Male Gamete + female Gamete = Zygote
41

External fertilization
 External fertilization is a type of fertilization in which the fusion male and female gametes occur
externally of the female body. Hence, external fertilization needs water to facilitate their
fertilization. Therefore, external fertilization mainly takes place in wet environments.
 Unlike the internal fertilization, both, male and female gametes release into the environment,
especially to the water, so that male gametes can swim towards the female gametes for fusion. The
release of eggs and sperm into the water is known as spawning.
 Hence, the organisms which show external fertilization should live in water or they should go to
watery environments for reproduction. Special feature of the male gametes is that they are motile.
42

External fertilization: advantage
 External fertilization is, however, a simple reproductive strategy which does not require the
involvement of any hormones or mating rituals.
 Large number of offspring is produced.
 It is easier to find mates as the gametes just need to be released and can drift away with wind or
water.
 External fertilization results in greater genetic variation.
43

External fertilization: Limitations
 However, there are some limitations associated with external fertilization such as the requirement
of releasing a large number of gametes, having a low survival rate of the embryo, lack of parental
care, etc.
 Environmental factors and timing are key challenges to the success of external fertilization.
 While in the water, the male and female must both release gametes at similar times in order to
fertilize the egg.
 Gametes spawned into the water may also be washed away, eaten, or damaged by external factors.
44

Internal fertilization
 Internal fertilization is the union of an egg cell with a sperm during sexual reproduction inside the
female body..
 During the internal fertilization, male organism deposits its gametes inside the female organisms.
Therefore, the union of the male and female gametes occurs inside the female body. Once the
fertilization completes, the zygote develops within the female organisms until the birth of the
offspring.
 This type of fertilization is common in birds, reptiles and mammals.
45

Internal fertilization: Methods
 Copulation: This is common in mammals, reptiles, some birds, some fish and certain groups of
animals. In this method, a penis or an intromittent organ is introduced into the vagina or cloaca.
 Cloacal kiss: This is common in birds, whereby two animals press their cloacas together while
transferring sperm.
 Spermatophore: salamanders, spiders, some insects and some molluscs undertake internal
fertilization by transferring a spermatophore, a bundle of sperm from male to the female.
46

Internal fertilization: Offspring's
 Oviparity: In this mode of internal fertilization, fertilized eggs are laid outside the body of the
mother. The egg receives nourishment from the York. For example fish, amphibians, reptiles and
all birds.
 Viviparity: In this mode of internal fertilization, the young ones develop inside the mother and
receive nourishment through the placenta.
 Ovoviviparity: In this mode of internal fertilization, eggs are retained in female and the embryo
receives nourishment from the York. When they hatch, the young ones are fully developed. This is
common in some bony fish, sharks, lizards, snakes etc.
47

Internal fertilization: benefits
 The chances of a successful fertilization are high.
 The mates are selective.
 Chances of survival of the offspring are high.
 Hence, this mode of fertilization mainly protects the female gamete.
 Furthermore, the embryo gets more protection from predation and harsh environmental conditions,
so it has a higher survival rate.
 Also, there is no need of producing a large number of female gametes (eggs) since they locate
inside the female body.
48

Internal fertilization: Limitation
 Internal fertilization takes a long time for the fetus to develop, the mother must carry the foetus
until birth.
 Very less number of offspring's are produced.
 Parents can have a chance of getting sexually transmitted diseases.
 It is sometimes difficult for the male and female to come into intimate contact.
 Males must produce a large number of sperms.
 It requires close coordination between males and females in terms of behaviour and physiology
which requires extensive hormonal controls.
49

Comparison External fertilization Internal fertilization
Definition A type of fertilization in which male and
female gamete fuse with each other at
the outside of the female body.
A type of fertilization in which male
and female gametes fuses when
they are inside the female body.
Occurrence Externally from the female body Inside the female body
Wastage of gametes Higher Lower
Number of gamete
produced
Large number Small number
Survival rate of the
embryo
Low High
Parental care No Yes
Water Required Not required
External v/s internal fertilization
50

Comparison External fertilization Internal fertilization
Forms External fertilization occurs in the
external environment.
Three modes of internal fertilization are
oviparity, viviparity and ovoviviparity.
Syngamy Further process of development
(syngamy) occurs outside the body.
Further process of development
(syngamy) occurs inside the body only.
Examples Corals, Hydra, Fish and
Amphibians
Reptiles, Birds and Mammals
External v/s internal fertilization
51

Fertilization: Mechanism
Fusion of haploid nuclei of two gametes resulting in the restoration of diploid
chromosomal number is called fertilization
52

Fertilization events:-
1. Site of fertilization: In human being, fertilization takes place mostly in the ampullary
region of the oviduct (Fallopian tube).
2. Arrival of sperms: Male discharge semen into female vagina close to the cervix during
copulation. This is called insemination. A single ejaculation of semen may contain 300
million sperm.
3. Movement of sperms: From the vagina the sperms travel upto the uterus but only a few
thousand find their way into the opening of the fallopian tubes. Primarily, contraction of
the uterus and fallopian tubes assists in sperm movement but later on they move by their
own motility.
53

Fertilization events:-
4. Arrival of secondary oocyte: In humans, the secondary oocyte is released from the
mature Graafian follicle of an ovary (ovulation). The oocyte is received by nearby
fallopian funnel and sent into the fallopian tubes by movements of fimbriae and their
cilia. The secondary oocyte can be fertilized only within 24 Hrs after its release from the
ovary. The secondary oocyte is surrounded by many sperms but only one sperm succeeds
in fertilizing the oocyte. Sperm enters into the oocyte at the time of second meiotic
division. The second meiotic division is completed by the entry of sperm into the
secondary oocyte. After this, secondary oocyte is called as ovum.
5. Capacitation of sperms: The sperm in the female’s genital tract are made capable of
fertilizing the egg by secretion of female genital tract. These secretion of the female
genital tract remove coating substances deposited on the surface of the sperms
particularly on the surface of the sperm particularly those on acrosome. Thus, the receptor
sites on the acrosomes are exposed and sperm become active to penetrate the egg. This
phenomenon of sperm activation in mammals is known as capacitation. It takes about 5-
6 Hrs for capacitation.
54

Fertilization events:- Physical and chemical reactions
1. Acrosomal reaction: After ovulation, the secondary oocyte reaches the fallopian tube
(oviduct). The capaciated sperm undergo acorsomal reaction and releases various
chemicals contained in the acrosome. These chemicals are collectively called as sperm
lysisins. Due to acrosomal reaction, plasma membrane of sperm fuses with the plasma
membrane of the secondary oocyte so that sperm content enters the oocyte. Binding of the
sperm to the secondary oocyte induces depolarization of the oocyte plasma membrane.
Depolarisation prevents polyspermy (entry of more than one sperm into the oocyte). It
ensures the monospermy (entry of one sperm into the oocyte).
55

Cortical
reaction
57

2. Cortical reaction: Just after the fusion of sperm and plasma membrane of oocyte, the
secondary oocyte shows a cortical reaction. The cortical granules are present beneath the
plasma membrane of the secondary oocyte. These granules fuses with the plasma
membrane of the oocyte and release their contents including cortical enzymes between
the plasma membrane and zona pellucida which also prevents entry of sperms
(polyspermy).
58

3. Sperm entry: At the point of contact with sperms, the secondary oocyte forms a
projection termed the cone of reception or fertilization cone which receives the sperms.
The proximal centriole of the sperm divides and forms two centrioles to generate the
mitotic spindle formation for the cell division. The mammalian secondary oocyte (egg)
doesn't have centrioles of its own.
4. Karyogamy (amphimixis): Sperm entry stimulates the secondary oocyte to complete the
suspended second meiotic division. This process a haploid mature ovum and a second
polar body. The head of the sperm which contains the nucleus separates from the middle
piece and the tail and becomes the male pronucleus. The second polar body and sperm
tail degenerate. The nucleus of the ovum is now called, the female pronucleus. The male
and female pronuclei move towards each other. Their nuclear membrane disintegrate.
Mixing up of chromosomes of sperm and ovum is called as Karyogamy or Amphimixis.
The fertilized ovum (egg) is now called Zygote. The zygote is diploid unicellular cell that
has 46 chromosomes in humans. The mother is now said to be pregnant.
5. Activation of egg: Sperm entry stimulates metabolism in the zygote. As a result, the rates
of cellular respiration and protein synthesis increases. Besides activating the egg another
role of sperm is to carry DNA to egg. 59

Prevention of polyspermy
Fertilization
60

Fertilization: Identical twins and Fraternal twins
64

Significance of fertilization
 It restores the diploid number of chromosomes, characteristic of species for e.g. Humans
(46 Chromosomes).
 Fertilization initiates cleavage.
 It introduces the centrioles which are lacking in the mature egg.
 Fertilization results in determination of sex in the embryo.
 It combines the characters of two parents, this introduces variations.
 Fertilization membrane developed after the entry of sperm prevents the entry of other
sperm into ovum.
66

Classification of Eggs: amount of yolk
1. Alecithal : When the egg contains no yolk, it is
called alecithal egg. Eg. The eggs of eutherian
mammals
2. Oligolecithal : When the egg contains small or
negligible amount of yolk it is said to be
microlecithal. Romer and Balinsky named these
eggs as oligolecithal eggs Eg'. Amphioxus,
Tunicates
3. Mesolecithal: In amphibian, Dipnoi and
Petromyzon the amount of yolk present is moderate
and is not high Hence these eggs are also named as
mesolecithal eggs.
4. Macrolecithal or Megalecithal or Polylecithal
Egg : When the egg contains large amount of yolk it
is said to be macrolecithal or megalecithal egg. It is
also called Polylecithal egg. Eg. Reptiles, Birds,
Prototheria (Monotremata) Egg laying mammals.
Nucleus Nucleus
Yolk
Nucleus
Yolk
Alecithal Oligolecithal
Macrolecithal Mesolecithal
68

Classification of Eggs: Distribution of yolk
Slightly Moderately Extremely
69

1. Isolecithal or Homolecithal Egg: In
isolecithal eggs, the very little amount of yolk
present is uniformly distributed throughout the
ooplasm (eg. echinoderms, Amphioxus,
mammals). This condition is usually observed in
eggs with very little amount of yolk.
70

2. Telolecithal Egg: In eggs containing moderate or large
quantity of yolk, the distribution of yolk is not uniform. lt
is concentrated more towards the vegetal pole. Such a type
of egg, in which the yolk is concentrated towards one pole,
is called telolecithal egg. Telolecithal eggs may further
classified into three types:
2.1 Slightly Telolecithal- This type of egg contains only a small
quantity of yolk which is distributed unevenly. The vegetal pole has the
highest concentration and the animal pole the lower (e.g. eggs of
fishes).
2.2. Moderately Telolecithal -This type of egg contains a moderate
quanilty of yolk which is distributed unevenly. Due to high
concenteration of yolk in the vegetal hemisphere, the nucleus is shifted
more towards the animal hemisphere (eg. amphibian egg).
2.3 Extremely Telolecithal -In this type of egg, due to the heavy
deposition of yolk, the entire vegetal hemisphere and a major portion of
the animal hemisphere are occupied by yolk. Due to this extremely
uneven distribution of yolk, the ooplasm and nucleus are displaced
towards the animal pole (eg. reptilian and avian eggs).
Vegetal pole
Animal pole
Animal pole
Vegetal pole
71

3. Centrolecithal Egg: Egg of many arthropods and
some coelenterates are described as centrolecithal.
They are relatively large and elongate and have a
very great amount of yolk. The nucleus lies at the
geometric centre of the yolk mass, surrounded by a
small amount of cytoplasm. A thin cytoplasmic layer
covers the surface of the yolk. Fine strands of
cytoplasm extend from the peripheral layer to the
zone occupied by the nucleus.
72

Gametogenesis, fertilization and types of eggs

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