Embryology

Question 1

Define embryology?

Answer 1

The developmental events that occur during the prenatal period.

Question 2

Define the three periods of the prenatal period?

Answer 2

Pre-Embryonic period
First 2 weeks of development (pre-embryo).

Embryonic period:
3-8 weeks (embryo = rudimentary forms of most organ systems).

Foetal period:
9-birth (foetus) - (4 weeks after birth = neonate, <1 year = infant).

Question 3

Describe the 3 stages of embryogenesis (developmental events) that occur during the pre-embryonic and embryonic periods?

Answer 3

Cleavage:
The cell division of the zygote (after the oocyte is fertilised by a sperm).

Gastrulation:
Formation of the 3 primary layers of the embryo.

Organogenesis:
Arrangement of the 3 primary germ layers to all organs of the body.

Question 4

Describe the major developmental events (FIRST WEEK) of the pre-embryonic period?

Answer 4

Fertilisation = sperm fertilises an oocyte.

Zygote = complete fertilisation.

Cleavage = the cell division of the zygote.

Morula = enters the uterus after the 3rd cleavage of the zygote.

Blastocyst = blastocyst cavity develops.

Implantation = blastocyst begins to implant itself in the lining of the uterus.

Question 5

Describe the major developmental events (SECOND WEEK) of the pre-embryonic period?

Answer 5

Second Week (2 components of the blastocyst differentiate into 2 layers):

Trophoblast:
Inner layer = cytotrophoblast & outer layer = syncytiotrophoblast.

Embryoblast:
Differentiate into the bilaminar embryonic disc → epiblast & hypoblast).

Question 6

Describe the process of gastrulation?

Answer 6

Formation of primitive streak:
Thin depression on the surface of the epiblast.

Formation of mesoderm:
Cells from the epiblast detach and migrate through the primitive streak.

Formation of endoderm:
Other migrating cells from the epiblast eventually displace the hypoblast.

Formation of ectoderm: Remaining cells in the epiblast form this.

***3-4 weeks of development: trilaminar embryonic disc folds = flat surface to cylindrical structure.

Question 7

Name the three primary germ layers of the embryo and provide examples of major tissues and structures derived from these layers?

Answer 7

Ectoderm:
Epidermis of skin, nervous tissue, sensory organs.

Mesoderm:
Muscle/connective tissue, dermis of skin, heart, serous membranes, urinary/reproductive.

Endoderm:
Most internal epithelial linings, most of the digestive system.

Question 8

Briefly describe the process of organogenesis and the foetal period?

Answer 8

Organogenesis:
4th week of embryonic period to the foetal period (heart, digestive, gonad, nervous).

Foetal period:
Foetal length ↑↑↑ (3-5 months of pregnancy) & foetal weight ↑↑↑ (during the last 2 months).