• Embryology – study of the origin and
development of single individual
• Prenatal period
• Embryonic period – first 8 weeks
• Fetal period – remaining 30 weeks
3. • Embryogenesis starts with fertilization
• Fertilization: process of fusion of male
gamete(germ cell) sperm with female
gamete(germ cell) to form zygote
• Two sex cells fuse to form a new cell containing
genetic material derived from both parents.
• Restores the diploid number of chromosomes.
• Determines the sex of the organism
• Initiates cleavage.
• Occurs in the widest part of the uterine tube (the
• Millions of sperm cells are deposited in the female
reproductive tract during intercourse.
• Only a few hundred have a chance at fertilization.
• Only the first sperm to enter the secondary oocyte
is able to fertilize it.
• The remaining sperm are prevented from
penetrating the oocyte.
6. The Stages of Embryogenesis
• Cleavage. The zygote divides by mitosis to form a
multicellular structure called blastocyst.
• Gastrulation. The blastocyst cells form three
primary germ layers, which are the basic cellular
structures from which all body tissues develop.
• Organogenesis. The three primary germlayers
arrange themselves in ways that give rise to all the
organs within the body.
7. The Embryonic Period
• Week 1 – from zygote to blastocyst
• Conception – in lateral third of uterine tube
• Zygote (fertilized oocyte) moves toward the uterus
• Blastomeres – daughter cells formed from zygote
• Morula – at about 12–16 cell (blastomeres)stage
• Blastocyst –further division, fluid-filled structure –
about 60 cells
Formation of blastocyst: by 5th day, blastomere
reach to 107, small space appear between them
• The blastocyst is a sphere of about 150 cells, with
an outer layer (the trophoblast), a fluid-filled
cavity (the blastocoel), and a cluster of cells on the
interior (the inner cell mass).
• By the end of the first week after fertilization,
the blastocyst enters the lumen of the uterus.
• Implantation is the process by which the
blastocyst burrows into and embeds within the
14. Week 2 – The Two-Layered Embryo
• Bilaminar embryonic disc – inner cell mass
divided into two sheets
• Epiblast and the hypoblast
• At the site of implantation,the outer cell mass of
blastocyst start proliferating and give rise to
• Placenta: is made up of finger like process known
as villi and inetvillious space. An organ that joins
mother and off spring.
• Functions in exchange of nutrients, waste
products, and respiratory gases between the
maternal and fetal bloodstreams.
• Transmission of maternal antibodies to the
developing embryo or fetus.
• Production of hormones to maintain and build the
16. Week 2 – The Two-Layered Embryo
• Amniotic sac – formed by an extension of epiblast
• Outer membrane – forms the amnion
• Inner membrane – forms the amniotic sac cavity
• Filled with amniotic fluid
17. Week 2 – The Two-Layered Embryo
• Yolk sac – formed by an extension of hypoblast
• Digestive tube forms from yolk sac
• Tissues around yolk sac
• Gives rise to earliest blood cells and blood vessels
19. Week 3 – The Three-Layered Embryo
• Primitive streak –the embryonic disc become
elongated.The primitve streak is a midline
proliferative region of epiblast cells which may
break free from the epithelium and migrate
beneath the epiblast.
• Ingression: it’s a process by which cell become
part of the streak and then migrate away from it
• Gastrulation –formation of primitive streak is the
beginning of gastrulation, a process of where
epiblast give rise to a trilaminar structure with a
define cranio-caudal axis and formation of
• Occurs during the third week of development
immediately after implantation.
• One of the most critical periods in the
development of the embryo.
• Cells of the epiblast migrate and form the three
primary germ layers which are the cells from
which all body tissues develop.
• The three primary germ layers are called
ectoderm, mesoderm, and endoderm