This document summarizes key stages of human embryology from fertilization through the first 3 weeks of development. It describes fertilization, where a sperm and egg fuse to form a zygote. It then discusses cleavage and blastocyst formation, implantation, and the establishment of the bilaminar and trilaminar embryos through gastrulation. The formation of the amniotic sac, yolk sac, and primitive streak are also summarized. Key events include fertilization, cleavage, blastocyst formation, implantation, gastrulation and the formation of the three germ layers (ectoderm, mesoderm, endoderm).

1. Embryology
GENERAL EMBRYOLOGY

2. EMBRYOLOGY:
• 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

4. Fertilization:
• 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
ampulla).

5. Fertilization:
• 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

8. Fertilization and the Events of the First 6 Days of
Development
Figure 3.3

9. Blastocyst:
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).

11. Implantation:
• 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
endometrium.

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
trophboblast
• Placenta: is made up of finger like process known
as villi and inetvillious space. An organ that joins
mother and off spring.

15. Placenta:
• 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
uterine lining.

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

18. Implantation of the Blastocyst
Figure 3.4

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
embryonic shape

20. The Primitive Streak
Figure 3.5e-h

21. Gastrutlation:
• 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