Anatomy: Human development- embryology
NB: Really good website for all this stuff is:
http://www.embryology.ch/genericpages/moduleembryoen.html - I would just look at this to be honest!
Understand the anatomical features of
o the male and female gametes and
o The male and female reproductive tracts
enzymes
Motile, uses lots of
ATP. Tail provides
propulsion
Stationary
Lots of nutrients, large
cytoplasm
ATP from
mitochon
dria
HAPLOID Genetic
material, including either
X or Y sex chromosome
HAPLOID Genetic
material, with only X sex
chromosome
Secretions
Semen =
sperm +
secretions
Activates sperm
Sperm produced here
Stores sperm – 13 m long
Fertilisation most often occurs in the fallopian
tube. Then the fertilized egg travels down and
embeds in the uterine wall. If it doesn’t
migrate it’s called an ectopic pregnancy and
must be aborted to avoid permananent
damage or death of the mother
Endometrial epithelium = single layered
prismatic epithelium of uterus
Endometrium = inner lining of uterus, where
zygote embeds
Myometrium = muscular layer of uterus
Overview of fertilization
http://www.embryology.ch/anglais/bvueEmbr/evorimplantation/03.html
Time
after
fert.
24
hours
Day 4
Developmental
stage
Characteristics
Location
Zygote
After fertilization and before cleavage
Morula
After cleavage. ‘Ball’ of blastomeres
Day 6
Blastocyst
Day 8
Day12
Complete
implantation
Embryonic disc
undergoes
gastrulation
Day 21
Axial canal formed
Day 23
Notochord
separates
Cells differentiate into embryoblast and
trophoblast
Cytotrophoblast, syncytiotrohphoblast,
epiblast and hypoblast
Bilaminar embryonic disc. The epiblast
(upper layer) → 3 layers endoderm,
ectoderm and mesoderm.
GASTRULATION
Invagination of epiblast cells migrate from
primitive node
Notochord is separated now
Fallopian tube at
ampulla
Travelling down
fallopian tube
Uterine wall
Endometrium
Attached to
endometrium by a
connecting stalk
“”
“”
NB: The embryonic period comprises the first 8 weeks of pregnancy. It is divided into a
preembryonic phase (from the 1rst to the 3rd week), in which the three germinal layers arise,
and into the embryonic phase proper (from the 4th to 8th week), in which the embryonic organ
anlagen arise.
Understand the process of fertilization
1
2
3
4
5
6
Stage of fertilization
Sperm goes through
corona radiata
Penetration of zona
pellucida
Fusion of plasma
membranes of oocyte
and sperm
Oocyte undergoes 2nd
meiotic division. AND
formation of female
pronucleus
Formation of male
pronucleus
Zygote is formed!
Description
Acrosome releases enzymes, which digest the corona radiata
until the sperm reaches the zona pellucida
Acrosomal enzymes again digest – this time the enzyme is
acrosin. A reaction occurs that prevents any other sperm
entering the egg (oocyte) – the zona pellucida is made
impermeable
Plasma membranes fuse, but nuclear (genetic) fusion hasn’t
yet occurred.
Ova are suspended after Meiosis 1, and so when the oocyte is
fertilized by the sperm it undergoes Meiosis 2 - Just a thing
you need to remember. The sperm has already completed
Meiosis 2.
Haploid nucleus of sperm contains either X or Y sex
chromosome, therefore father determines sex of offspring
(oocyte always contains X).
Fusion of the pronuclei creates a new DIPLOID zygote, with
46 chromosomes, half contributed from the oocyte (mother)
and half contributed from the sperm (father)
Know the terms:
1. Zygote noun: A diploid (2n i.e. 46 chromosomes) cell formed from the fusion of two
haploid (n i.e. 23 chromosomes) gametes: the sperm fertilising the egg. This very early
stage lasts until the first cleavage.
NB: Cleavage is repeated mitotic cell divisions. The dividing cells are called blastomeres.
2. Morula noun from the Latin for ‘mulberry’: After the first cleavage, it’s called a morula.
It’s a ball of 16-32 blastomeres which are also considered stem cells, as they are
undifferentiated cells and could potentially become any cell in the body. This stage lasts
until blastocyst formation.
3. Blastocyst noun: A hollow, thin-walled structure containing a cluster of cells called the
embryoblast (or inner cell mass) which will become the embryo. The outer layer
(trophoblast) of the blastocyst will become the placenta and other supporting tissues. A
blastocyst contains some cells that have differentiated (changed).
Explain the process of implantation of the blastocyst
.
The blastocyst attaches itself to the uterine wall, on the endometrial epithelium. Below the
endometrial epithelium in the endometrium itself are capillaries and endometrial glands. These
two structures supply blood (O2) and secrete required nutrients respectively.
The trophoblast (‘troph’ means relating to nutrition and feeding) digests the uterine wall, and
transports the nutrients it obtains back into the blastocyst.
The trophoblast is now composed of two layers:
 The syncytiotroblast – invades the endometrial connective tissue and interrupts the
menstrual cycle by releasing beta hCG. This is picked up by early pregnancy tests and
indicates the embryo has implanted.
 The cytotrophoblast – covers the embryo
The embryo uses these two structures to completely embed itself in the uterine wall.
The embryoblast differentiates into a two-layered embryonic disc
 Hypoblast – support structure
 Epiblast – actually becomes the baby
Understand the development of the ectoderm, endoderm + mesoderm.
Gastrulation (“formation of the gut”)
1. Blastula → 2. Gastrula with different germ layers
Blastocyst becomes a gastrula. A gastrula is a trilaminar (three-layered) stage of embryo
development, where the three germ layers are endoderm, ectoderm and mesoderm. The
embryonic disc (remember it’s bilaminar i.e. epiblast and hypoblast) undergoes these
changes:
1.
Loss of symmetry
2
Formation of
primitive streak
3
Epithelial cells
become
endoderm,
ectoderm and
mesoderm
Formation of proximal-distal (caudal-cranial) and anterior-posterior
axes. Direction signaled by nodal signalling
Forms from ‘caudal’ or proximal end and grows towards ‘cranial’ or
distal end. This establishes the bilateral symmetry of the embryo.
The caudal or proximal end will be the ‘tail’ end of the embryo
(caudal – tail-like)
Along the primitive streak epithelial cells migrate down inside the
embryo to become a new layer, called the endoderm, pushing the
hypoblast out of the way (this goes on to form the amnion.) The
epiblast keeps moving and forms a second layer, the mesoderm.
The top layer is now called the ectoderm.
Summary of what’s just happened to the embryonic disc:
Embryonic disc is bilaminar, it has two layers:
 Epiblast → primitive streak and ectoderm → endoderm → mesoderm
 Hypoblast → endoderm of umbilical vesicle → extraembryonic mesoderm (i.e. support
structures)
What do these three newly formed layers of the embryo become?
Ectoderm
 Epidermis
 Central and peripheral nervous systems
 Eye
 Inner ear
Mesoderm
 Skeletal muscles
 Blood cells and lining of blood vessels
 Smooth muscle
 Cardiovascular system
 Bones, tendons and ligaments
 Dermis
Endoderm
 epithelial lining of gastrointestinal tract
 Epithelial lining of respiratory tract

Some glandular cells
This is what the whole structure looks like at this point:
Formed from
epiblast
Formed from hypoblast
Understand the development of the notochord and explain its significance
Overview: http://embryology.med.unsw.edu.au/Medicine/BGDlabfertilization10.htm
The notochordal process begins as a fold of ectoderm extending cranially. The sequence of
differentiation:
Notochordal process → notochordal plate → notochord.
It occurs during GASTRULATION:
“Migrating cells produce a thick knob at one end of the primitive streak. Their continued forward
movement from this so-called primitive knot [node] produces a dense band that becomes the rodlike
notochord” – encyclopedia Brittanica
http://www.britannica.com/EBchecked/topic/474953/prenatal-development/63787/Formation-of-the-three-primary-germlayers?anchor=ref526542
In detail:
The notochordal process elongates by invagination from the primitive pit. It’s a hollow tube
called the notochordal canal ventral to (under) the ectoderm.
It then forms a notochordal plate which is continuous with the endoderm.
Finally the notochordal plate in-folds and forms the notochord. The notochord regulates cellular
differentiation. It plays an organisational role in central nervous system development. It will later
become part of the vertebral column.
This is different to the process that creates the neural tube. The notochord induces the
differentiation of the neuroblast. This process is called neurulation. The median part of the
epiblast thickens and forms a groove and afterwards a tube (neural tube), out of which the
central nervous system will arise.
From http://www.embryology.ch/anglais/bvueEmbr/vueembryo.html
Understand the relationship of fertility to the patency of the fallopian tube
Patent fallopian tubes = unblocked or open
The fallopian tube is much more complex than a simple pipe, and the lining of the tube is folded
and lined with microscopic hair like projections called cilia which push the egg and embryo
along the tube.
Any factor that affects the motility or patency of the fallopian tube can cause infertility. Tubal
disease has been implicated in 15-20% of couples presenting with infertility.
Tubal damage usually occurs through pelvic infection, and this is called pelvic inflammatory
disease (PID). This causes the fallopian tubes to lose patency. Some of the causes of pelvic
infection that can be pinpointed are:





Sexually transmitted diseases (e.g. Gonorrhea, Chlamydia)
Infection after childbirth, miscarriage, termination of pregnancy ( MTP) or IUD
(intrauterine device) insertion
Post-operative pelvic infection (e.g. perforated appendix, ovarian cysts)
Severe endometriosis
Tuberculosis
Besides causing blocked tubes, any pelvic inflammatory disease can also produce bands of
scar tissue called adhesions, which can alter the functioning of the fallopian tubes.
From http://www.drmalpani.com/book/chapter12a.html
And http://www.trichurheart.com/images/Infertility.pdf