2. 1. Animal Reproduction
and Development
In the animal kingdom, each species has its
unique adaptations for reproduction.
Asexual reproduction produces genetically
identical offspring (clones), whereas in
sexual reproduction, the genetic material of
two individuals combines to produce
offspring that are genetically different from
their parents. 2
3. 1.1. Embryonic development in Animals
How did you become the large, organized
multicellular individual that you are today?
Development!
• Development describes the changes in an
organism from its earliest beginnings through
maturity.
• Your body consist of over 30 trillion cells, but you
began as a single cell: a fertilized egg, or zygote.
• As an animal embryo develops, its cells divide,
grow, and migrate in specific patterns to make a
more and more elaborate body (plant cells perform
differential expansion instead of migration).
3
4. • Embryonic Development, known as Embryogenesis is the process by
which the embryo is formed and develops into a fetus.
• It starts with the fertilization of the ovum (or egg) by sperm, the fertilized
ovum is referred to as a zygote.
1.1.1. Definition of embryology
Different scholars defined embryology as accordingly
The study of the embryo and its development from a single-celled
zygote (fertilized ovum) to the establishment of fetus.
It is the studies of formation and early development of living
organisms.
The study of developmental events that occur during the prenatal
stage.
The branch of biology concerned with the study of embryos and
their development.
4
5. Ontogeny: all the developmental events that occur during
the existence of a living organism
Phylogeny: it pertains to the evolutionary history or
development of a group of organisms, such as a tribe or a
racial group.
Phylogeny vs. ontogeny
Both phylogeny and ontogeny deals with the origin and the
development of organisms.
o They are both concerned with the developmental
histories.
However, ontogeny is different from phylogeny in a way that it
looks through the historical development of an organism within
its own timeline (e.g. from its simplest to the most complex
form) and not on its evolutionary history.
Thus, ontogeny is to the development of an individual
organism as phylogeny is to the evolution of a species. 5
6. 1.1.2. BRANCHES OF EMBRYOLOGY
1. DESCRIPTIVE EMBRYOLOGY: This field of embryology
is associated with the morphological description of different
embryonic stages in the ontogenetic development of
individuals of different species, which involves the initial
work of embryologists till 18th century.
2. COMPARATIVE EMBRYOLOGY: It embraces the
comparative study of embryology of different animal
groups.
3. EXPERIMENTAL EMBRYOLOGY: It involves all those
studies that attempt to understand the various fundamental
mechanism in the development of different animals, like
fertilization, Cleavage, Gastrulation, Embryonic induction,
determination and differentiation.
6
7. 4. CHEMICAL EMBRYOLOGY: This branch
of embryology includes all those studies which
employ various biochemical, biophysical and
physiological techniques for understanding
embryological events at molecular level.
5. TERATOLOGY: It is the branch of
embryology concerned with the study of
malformations or birth defects. The
substances that cause birth defects are called
tetratogens. Eg. Phocomalia (poorly developed
arms child), Ectomalia (arm less child)
7
8. 1.1.3. History of embryology
Embryonic development has been a source of
wonder…
Aristotle’s (384-322 B.C.) studies – a shift
from superstitions to observation.
Galen (130-200 A.D) – learned about advanced fetuses
but the minute dimensions resisted analysis
DeGraafin 1672– describedovarianfollicle
Hamm and Leeuwenhoek in 1677 – have
seen the spermcells
8
9. Theory of Preformation
Spermists - sperm contained new individual in miniature and
onlynourishedintheovum
Ovists- thought the same and that the seminal fluid only
stimulatesit.
Bonnet (1745)– discoveredeggs ofsome insects undergoing
parthenogenesis
Spallanzani (1729-1799) – demonstrated that both male and
female sex products are necessary for the initiation of
development
Wolff (1733–1794) – thesis on epigenesis
(embryological development occurs through progressive
growth and differentiation) 9
10. Von Baer (1828) – discovered mammalian egg, first
emphasized that the more general basic features of any
animal group appear earlier in the development than do
special features of different members of the group
Von Baer’s law (Demonstrated existence of germ
layers)
• The formulation of cell theory byMatthias
• Schleiden and Theodore Schwann laid down the
foundationof modern embryology as a science.
• Ernst Haeckel (1834 -1919) –drafted the Biogenetic Law
of MullerandHaeckel– Haeckel’s Lawof Recapitulation
10
11. 1.1.4. Stages of embryonic development in
animals
• Fertilization: the process of a single sperm cell
combining with single egg cell to form a zygote.
• Cleavage: rapid, multiple rounds of mitotic cell
division where the overall size of the embryo does
not increase. The developing embryo is called
a blastula following completion of cleavage.
• Gastrulation: the dramatic rearrangement
(movement) of cells in the blastula to create the
embryonic tissue layers. These tissue layers will go
on to produce the tissues and organs of the adult
animal.
• Organogenesis: the process of organ and tissue
formation via cell division and differentiation.
11
12. 1. Fertilization
•The initial event in development in
sexual reproduction.
–Union of male and female gametes.
–Recombination of paternal and maternal
genes.
–Restoration of the diploid number (two sets
of chromosomes).
• Zygote is the diploid cell resulting from
fertilization. 12
13. 2. Cleavage and morula formation
• Since the goal of development is the production of a
multicellular organism, many cells must be produced
from the single-celled zygote.
• This task in animal reproduction is accomplished by
cleavage, which is a series of consecutive cell
divisions.
• Cleavage is a series of extremely rapid mitotic
divisions wherein the enormous volume of zygote
cytoplasm is divided into numerous smaller cells
immediately after fertilization.
• These cells are called blastomeres, and by the end of
cleavage, they generally form a sphere known as a
blastula.
13
14. • During cleavage, almost no growth occurs
between consecutive divisions, and the total
volume of living matter does not change
substantially; as a consequence, the size of the cells
is reduced by almost half at each division.
• At the beginning of cleavage, cell divisions tend to
occur at the same time in all blastomeres, and the
number of cells is doubled at each division.
• As cleavage progresses, the cells no longer divide
at the same time.
14
17. 3. Blastula formation
• Blastula is a hollow sphere of blastomeres surrounding a
cavity.
• The process of formation of blastula is known as
blastulation.
• The cavity of blastula is called blastocoel.
• It contains 32-64 cell stage.
• The blastula of various groups of animals differs in form
and structure depending upon a variety of factors such
as the size of the amount and distribution of yolk etc.
• The following categories of blastulae have been
recognized in different groups of animals
17
18. I. Coeloblastula
It is a hollow blastula containing a large spacious blastocoel.
The blastocoel is filled with a fluid containing
mucopolysaccharides (see the figure below).
18
The blastula of echinoderms and amphioxus
resulting from holoblastic equal cleavage, equal
coeloblastula .
The blastula of frog Holoblastic resulting from
unequal cleavage, unequal coeloblastula.
19. II. Stereoblastula
Embryo produced by spiral cleavage, characterized by the
absence of a blastocoel; formed by Embryos of annelid
worms, turbellarian flatworms, nemertean worms, and
all mollusks except cephalopods, certain fishes (see
image below).
This type of blastula is composed of an aggregate of
larger sized and relatively lesser number of cells
without or with extremely small blastocoelic space in the
center.
19
20. III. Discoblastula
Disco-blastula consists of a disc shaped mass of
blastomeres overlying a large yolk mass.
This blastula is the result of meroblastic discoidal
cleavage as in most fishes, reptiles and birds.
There is no blastocoel, instead a slit like cavity
called sub germinal cavity appears in between the
blastoderm and the yolk mass.
20
21. IV. Blastocyst
It is the blastula stage of mammals, and consists of
a hollow spherical vesicular blastula, containing
an inner cell mass at the animal pole.
The embryo develops from the inner cell mass,
and the outer single layer of cells which encloses
the blastocoel is called the trophoblast.
The trophoblast establishes relations with uterine
wall and helps in nutrition of the developing
embryo.
The cells in the blastula then rearrange themselves
spatially to form three layers of cells, through the
process called gastrulation. 21
22. 4. Gastrulation and formation of body layers
It is the most characteristic event occurring during the
third week of gestation, and the process that
establishes all three germ layers (ectoderm,
mesoderm, and endoderm) in the embryo.
Gastrulation begins at the posterior end of the
embryo and this is where the cells of the node arise.
During gastrulation, the blastula folds in on itself and
cells migrate to form the three layers of cells. Each
of the layers of cells is called a germ layer and will
differentiate into different organ systems.
Gastrula, with a hollow space that will become the
digestive tract.
22
23. Figure above, shows: Gastrulation is the process wherein
the cells in the blastula rearrange themselves to form the
germ layers. (credit: work by Abigail Pyne)
23
24. The ectoderm gives rise to the nervous
system.
The mesoderm gives rise to the muscle cells
and connective tissue in the body.
The endoderm gives rise to the gut and
many internal organs.
Diploblasts (animals with only two germ
layers) do not have mesodermal cells. These
animals, which include jellyfish and comb
jellies, have radial rather than bilateral
symmetry and have far fewer tissue types than
trophoblasts due the lack of a mesoderm 24
25. Figure, that shows the three germ layers give rise to different cell
types in the animal body. (credit: work by NIH, NCBI) 25
27. 5. Organogenesis
• Organogenesis is the development of the organs that begins
during the third to eighth week, and continues until birth.
Sometimes full development, as in the lungs, continues after
birth.
• Different organs take part in the development of the many
organ systems of the body, from the germ layers through the
process of differentiation.
• During differentiation, the embryonic stem cells express
specific sets of genes that will determine their ultimate cell
type, (E.g., some cells in the ectoderm will express the genes
specific to skin cells).
• As a result, these cells will take on the shape and characteristics
of epidermal cells. 27
28. The process of differentiation is regulated by
location-specific chemical signals from the
cell’s embryonic environment that sets play a
cascade of events that regulates gene
expression.
The cells in complex multicellular organisms
are organized into tissues, groups of similar
cells that work together on a specific task.
Organs are structures made up of two or more
tissues organized to carry out a particular
function, and groups of organs with related
functions make up the different organ systems.
28
29. 2. Postembryonic development
in animals
(humans as example)
Post-embryonic development refers to
the period of development after
embryogenesis, and normally includes
extensive growth, patterning,
morphogenesis and maintenance of the
adult form.
29
31. Four Main Types of Growth and
Development
i. Physical Development:
– body growth
ii. Mental Development:
– mind development
iii. Emotional Development:
– feelings
iv. Social Development:
– Interactions and relationships with others
• All four types above occur in each stage 31
32. Principles of Growth and Development
• Development is a continuous
• Development follows a pattern
• Developments proceeds from general to specific
responses
• The rate of development is not uniform throughout life
• Most of the traits are correlated in the process of
development
• Developmental is predictable
– There is wide individual difference in development
• Development is the product of the interaction
between the organism and his environment
• Development is cumulative
• Development is application oriented
32
33. Difference between Growth and development
Growth
• It is a narrow term referring
only to the physical growth
• Refers to increase in size,
height, weight, length etc
• It is quantitative in nature and
can be measured in terms of
meter, gram etc which are
standard units.
• Growth stops when maturity is
reached
• It is structural in nature
• Describes changes in
particular aspects of the body
Development
• It is much broader and
comprehensive term referring to all
aspects of human personality-
physical, social, mental, emotional
etc
• Refers to overall changes in shape,
form or structure.
• It is qualitative in nature and
difficult to measure
• It is a continuous and lifelong
process
• Development is functional in
nature resulting in efficiency and
maturity
• Describes changes in the organism
as a whole
33
34. Life Stages
i. Infancy
ii. Early childhood
iii. Late childhood
iv. Adolescence
v. Early adulthood
vi. Middle adulthood
vii. Late adulthood
34
35. 1. Infancy
• birth to 2 year
Infancy is defined as a beginning or the early period of
existence as an individual rather than as a parasite on
mother’s body.
Or in humans, it is the period of life between birth and the
acquisition of language approximately one to two years later.
It is the period of extra-uterine life, roughly the first year
after birth.
The transition from intra-uterine to extra-uterine existence
requires many critical changes, especially in the
cardiovascular and respiratory systems.
35
36. The body grows rapidly during infancy; total length
increases by approximately one half, and weight is
usually tripled.
By one year of age, most infants have six to eight
teeth.
• Emotional development – show anger, distrust,
happiness, excitement, etc.
• Social development – self-centeredness concept of the
newborn to recognition of others in their environment
– Infants are dependent on others for all needs
An infant age one month or younger is called a
neonate (newborn). 36
37. 2.CHILDHOOD
• This is the period between infancy and puberty.
• The primary (deciduous) teeth continue to appear
and are later replaced by the secondary
(permanent) teeth.
• During early childhood, there is active ossification
(formation of bone), but as the child becomes older,
the rate of body growth slows down. Just before
puberty, however, growth accelerates - the
prepubertal growth spurt.
37
38. 2.1. Early Childhood
• 2-6 years
• Emotional development – develop self-awareness
and recognize the effect they have on other people
and things.
– Children feel impatience and frustration as they
try to do things beyond their abilities.
– This lead to temper and tantrums.
• Social development – at beginning of stage very self-
centered one year old to sociable six year old.
– Strong attachment to parents. Needs are food,
shelter, protection, love and security. 38
39. 2.2. Late Childhood
• Age: 6-12 years old
• Conflict – Industry vs. inferiority
• Physical development– slow but steady.
– Muscle coordination is well developed and children
can engage in physical activity that require complex
motor-sensory coordination
• Mental development – developing quickly and
much of the child’s life centers around school.
– Reading and writing skills are learned, understand
abstract concepts like honesty, loyalty, values and
morals
39
40. 2.3.Late Childhood
• Late childhood: 6-12 years
• Emotional development- the child achieves greater
independence and a more distinct personality.
• Fears are replaced by the ability to cope.
• Social development – changes from activities by
themselves to more group oriented.
– They are more ready to accept the opinions of others and
learn to conform to rules, and standards of behavior.
• Needs are the same as infancy and early childhood
along with the need for reassurance, parental
approval, and peer acceptance.
40
41. 3. PUBERTY(Adolescence)
• Adolescence: 12-20 years
Puberty is the period when humans become
functionally capable of procreation
(reproduction).
• In females, the first signs of puberty may be
after age 8; in males, puberty commonly
begins at age 9.
• Conflict – Identity vs. Role Confusion
41
42. Physical development
– growth spurts, muscle coordination slows.
– Development of sexual organs and secondary sexual
characteristics (puberty).
– Secretion of sex hormones leads to the onset of
menstruation in girls and the production of sperm and
semen in boys.
– Body shape and form changes.
Mental development
– Development primarily involves an increase in
knowledge and sharpening of skills.
– Learn to make decisions and accept responsibility for
actions. 42
43. Emotional development
– Emotional development is often stormy and in conflict.
Adolescents try to establish their identities and independence.
– They respond more and more to peer group influences.
Social development
– Spending less time with family and more time with peer groups.
– They attempt to develop self-identity and independence and seek
security from their peers.
– Toward the end of this stage they develop a more mature attitude
and develop patterns of behavior that they associate with adult
behavior.
» Need for reassurance, support and understanding.
– Problems that develop in this stage can be traced to conflict and
feelings of inadequacy and insecurity.
43
44. 4. ADULTHOOD
• It is the period in the human lifespan in which full
physical and intellectual maturity and growth
have been attained.
• It can be:
i. Early adulthood: 20-40 years
ii. Middle adulthood: 40-65 years
iii. Late adulthood: 65 years and up
• Ossification and growth are virtually completed
during early adulthood
• Brain development continues into early adulthood,
including changes in gray-matter volume
44
