• The science of the development
of the individual during the
embryonic stage and,
• by extension, in several or even
all preceding and subsequent
stages of life cycle.
General definitions of embryology
•The branch of biology that deals with
the formation, early growth, and
development of living organisms
•Study of the formation and
development of an embryo and
Why medical students should study Embryology?
• Know how a single cell develops into a newborn,
containing numerous tissues and organs.
• Help us understand many complicated facts of adult
• Explain why some children are born with organs that
• Appreciate the factors responsible for maldevelopment to
assist us in preventing or treating such abnormalities.
Some preliminary consideration
• Embryo = developing individual during 1st two months.
• Fetus = developing individual from 3rd month until birth.
• Gametes : are produced by testes and ovaries.
• Testis : produces gametes ( spermatozoa ).
• Spermatogenesis : process of producing spermatozoa.
• Ovary : produces gametes ( ova ).
• Oogenesis : process of producing ova.
• Fertilization : takes place when one spermatozoon enters an
• Zygote : the fused ovum and sperm.
What is Embryology?
Embryology is the study of the development of an individual before birth.
• Every individual spends 266 days—
38 weeks ( to be exact )
• One celled structure pass stages to become organism having
billions of cells, numerous tissues and organs are formed and
come to function in perfect harmony.
The most spectacular of these changes occur in the 1st two months ( the unborn
baby acquires its main organs and just begins to be recognized as human ).
• 1st two months = Embryo.
• 3rd month until birth = fetus.
• 1st 28 days post birth = Newborn baby .
• A structure in the nucleus containing a linear thread of DNA,
which transmits genetic information and associated with
RNA and histones; during cell division.
• Chromatid =
one of the paired chromosome strands, joined at the
centromere, which make up a metaphase chromosome,
resulting from chromosome reduplication during the DNA
synthetic phase of interphase.
• The more readily stainable protein of the cell nucleus, forming a network
of nuclear fibrils.
It is a DNA attached to a protein primarily (histone) structure base and is the
carrier of the genes in inheritance.
It coils to form the metaphase chromosome.
A segment of DNA molecule that contains all the information required for
synthesis of a product ( polypeptide chain RNA molecule ).
It is biologic unit of hereditary, self reproducing and transmitted
from parent to progeny.
Each gene has a specific position (locus) on the chromosome.
THE NUCLEIC ACID IN WHICH THE SUGAR DEOXYRIBOSE, constituting the
primary genetic material of all cellular organism and the DNA viruses;
and occurring predominantly in the nucleus.
DNA is duplicated by replication and it serves as atemplate for synthesis of
ribonucleic acid (RNA ) (transcription).
Some facts about chromosomes
• Number of chromosomes in each cell is fixed for a given
species and in man it is (46).
• This is referred to as diploid (or double number ).
• In spermatozoa and ova the number of chromosomes in only
half the diploid number (23), this is called haploid or half
• There are (46) chromosomes in human cell (44) autosomal
(2) are sex chromosomes.
• (44) autosomal chromosomes = (22) pairs.
• The chromosomes forming a pair being exactly alike.
• Other facts
• Characters of parents are transmitted to offspring through
codes borne on strands of DNA.
• Genes are made up of such strands of DNA.
• Genes are located on chromosomes.
• A typical cell contains (46) chromosomes = diploid number.
• A gamete contains (23) chromosomes = haploid number.
• The diploid number of chromosomes is restored as a result
Significance of chromosomes
• The entire human body develops from the fertilized ovum.
• The fertilized ovum contains all the information necessary
for formation of the numerous tissues and organs of the
body, and for their orderly assembly and function.
• Each cell of the body inherits all directions that are
necessary for it to carry out its functions through life.
• This tremendous volume of information is stored within the
chromosomes of the cell.
Significance of chromosomes(continue)
•The nature and functions of a cell depend on the
proteins synthesized by it
•So the genes control the development and
functioning of cells by determining what types of
proteins will be synthesized within them.
Genes play an important role in the development of tissues and organs of
•Characters ( traits) of an individual are determined
by genes carried on his (or her) chromosomes.
•Half of these characters are inherited from the
father and half from the mother.
Significance of chromosomes CONTINUE
• Each chromosome bears on itself a very large number of
• Genes are made up of a nucleic acid called DNA and all
information is stored in the molecules of this substance.
• Genes are involved in synthesis of proteins.
• Proteins are the most important constituents of our body,
they make up the greater part of each cell and intercellular
substance. ‘Enzymes, hormones and antibodies are also
• In resting cell, the chromosomes aren’t visible under light microscope.
• During cell division the chromatin network becomes condensed into a
number of chromosomes. (visible under light microscope)
• Chromosome is made up of two
( rod shaped structure) called chromatids.
Each chromatid has two arms.
• Chromatids unite at an area called
centromere ( kinetochore ).
Chromosome structure (continue)
Individual chromosome differs from one another
in total length, relative length of the two arms
and in various other characteristics and these
differences enable us to identify each
chromosome individually .
Classification of chromosomes in this way is
Karyotyping makes it possible for us to detect abnormalities in
chromosome number or in the individual chromosome.
• This term is used interchangeably with cell
• but strictly speaking it refers to nuclear division,
• whereas cytokinesis refers to division of
• multiplication is an essential feature for embryonic development
and is necessary after birth for growth and for replacement of
• Chromosomes with the nuclei of cells carry genetic information that
controls the DEVELOPMENT and FUNCTIONING of various cells and
• When cell divides the daughter cells must have
chromosomes identical in number ( and in
genetic content ) to those in the mother cell.
• This type of division is called mitosis
Cell Division (continue)
• A different kind of cell division is called MEIOSIS occurs during
the formation of gametes.
• MEIOSIS consist of two successive divisions.
• The cells resulting from the first and second MEIOTIC divisions
(gametes) differ from other cells of the body in that:
1. The number of chromosomes is reduced to half the normal
2. The genetic information in various gametes produced isn’t
•Multiplication of cells takes place by cell division.
• The usual method of cell division, seen in most tissues.
•Daughter cells resulting from mitotic division are similar to
the parent cells and have the same number of chromosomes
•Many cells of the body have limited span of functional
activity, At the end of functional activity, cells undergo
division into two daughter cells.
•The daughter cells in turn have their own span of activity;
followed by another division.
•The period between two successive division is called
Stages of MITOSIS
• MITOSIS IS DIVIDED INTO FOUR STAGES
The best phase for understanding is
• During a specific period of the interphase, the DNA
content of the chromosome is duplicated.
• Another chromatid is refers to all
stages of the
formed.(identical to the original cell cycle
one) other than
• Chromosome now is made up of two
• In Early interphase = chromosomes are in the
form of extended threads.
• In Late interphase DNA of each chromosome
has undergone duplication.
• The chromatin of the chromosome
becomes gradually more and more
• That means the chromosome becomes
recognizable as a thread-like then as a
rod like appearance.
• Toward the and of prophase, the two
chromatids become distinct and the
chromosome now has its typical
• The two centerioles separate and
move to opposite poles of the cell.
• The two centerioles produce a number
of microtubules that pass from one
centeriole to the other and form a
• The nuclear membrane breaks down.
• The nucleoli disappear.
• With the formation of spindle,
chromosomes move to a
position midway between the
two centerioles. ( at the
equator of the cell)
• Each chromosome
becomes attached to
microtubules of the
spindle by its
• The centromere splits
longitudinally into two,
• The chromatids now
• The cell contains (46)
pairs of chromosomes.
• One chromosome moves along
the spindle to either pole of the
• Two daughter nuclei are
formed by appearance of
• Chromosomes gradually elongate
and become indistinct.
• Nucleoli reappear.
• Centeriole is duplicated at
this stage or in the early
• The division of the nucleus is
accompanied by the division of the
cytoplasm = (CYTOKINESIS). In this
• THE ORGANELLES are presumably
• Each daughter cell comes to have a
full complement of them.
special kind of cell division takes place in the
testis and ovary for formation of gametes.
The gametes resulting from meiosis have the
haploid number of chromosomes (23).
The various gametes formed don’t have the
same genetic content.
• Consists of two successive divisions.
• They are called the 1st and 2nd meiotic divisions.
• During the interphase preceding the 1st division;
DUPLICATION OF DNA as in mitosis.
Another chromatid identical to the original one is formed.
• Each chromosome is made up of two chromatids.
First Meiotic Division
The Prophase is prolonged and is
usually divided into a number of stages as
• Chromosomes become visible ( as in mitosis).
• Chromatids aren’t distinguished.
• Pairing =two chromosomes lie
parallel to each other.
• Synapsis = conjugation = pairing
• Chromatids become distinct.
• The bivalent has (4)
chromatids is called a
• There are (2) central and (2)
peripheral chromatids one
from each chromosome.
• The (2) central chromatids
become coiled over each
other at a number of points
= CROSSING OVER.
• Chromatids become
adherent at crossing points,
these points are called
• Exchange of genetic
material between chromatids.
• The two chromosomes of
• bivalent move apart
• This result in crossing over “BREAK” at the points of crossing over.
• Loose pieces become attached to the opposite chromatid.
• As in mitosis
the (46) chromosomes
become attached to the
spindle at the equator.
• The two chromosomes
of pair being close to
• ( differs from that in mitosis).
• There is no splitting
of the centromeres.
• One entire chromosome
of each pair moves to
each pole of the spindle.
• Daughter cells have (23)
made up of two
• Two daughter
nuclei are formed.
• The division of the
nucleus is followed
by division of the
cytoplasm Telophase- the final
stage of meiosis
Second Meiotic Division
• The first meiotic division is
followed by short interphase.
• There is no duplication of DNA.
(Such duplication is unnecessary as
chromosomes of cells possess two
• The 2nd meiotic division is similar to mitosis.
• The daughter cells aren’t identical in
genetic content because of the crossing
over has occurred during the 1st division .
Significance of Meiosis
WHY ARE NO TWO PERSONS ALIKE?
EXCEPT IDENTICAL TWINS NO TWO PERSONS ARE ALIKE!
1. There is a reduction of the number of chromosomes from
diploid to haploid. At time of fertilization the diploid number
is restored. This provides consistency of chromosome
number from generation to generation.
2. Chromosomes from mother and father are distributed
between the daughter entirely.
3. Crossing over results in thorough SHUFFLING of genetic
material (ova and spermatozoa all have a distinctive genetic
4. A THIRD STEP OF SHUFFLING TAKES PLACE AT FERTILIZATION.
• The development of defects in an embryo
• Abnormalities in development can result due to exposing the
embryo to certain agents (chemical or physical).
• The list of teratogens keeps increasing.
• Some particular organs are most sensitive to teratogens when
they are passing through critical phase in their development.
• This period of great susceptibility to teratogens differs from organ
• In early stages of development , the age reckoned in days.
• Later when the events are less dramatic, age can be expressed in
weeks or months. However , the exact age of embryo isn’t always
• An estimate can be made by observing the size of the embryo
(expressed as C.R. length).
Some other feature like the number of somites.
• There are numerous references to the timing of embryonic
events (most commonly in terms of C.R. length).
• The disadvantage of doing so is that it adds yet one more
complication to understanding of an already intricate
• Drown-rump length =
the length of any embryo, fetus, or infant
from the crown of the head to the breech;
used in estimating the age of the embryos
from the fourth to the eighth week .
• It is the equivalent of sitting vertex height in
• One of the paired, block like masses of
mesoderm, arranged segmentally alongside
the neural tube of the embryo, forming the
vertebral column and segmental musculature;
called also mesodermic, mesoblastic,
primitative, primordial, or protovertebral
This shows prophase, prometaphase,
metaphase, anaphase and telophase
Aster short fibers produced by cells during
mitosis and meiosis.
next lecture = spermatogenesis and oogenesis