EMBRYOLOGY

Question 1

Embryology is

Answer 1

The study of embryological development; from fertilization, until the end of the first trimester.
Embryo – a multicellular diploid eukaryote in its early stages of development.

Question 2

Gestation

Answer 2

Gestation period – the time period of carrying an embryo/fetus and its development.
𝐹𝑒𝑟𝑡𝑖𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 → 𝐶𝑙𝑒𝑎𝑣𝑎𝑔𝑒 → 𝐼𝑚𝑝𝑙𝑎𝑛𝑡𝑎𝑡𝑖𝑜𝑛 → 𝐺𝑎𝑠𝑡𝑟𝑢𝑙𝑎𝑡𝑖𝑜𝑛 → 𝑁𝑒𝑢𝑟𝑢𝑙𝑎𝑡𝑖𝑜𝑛 → 𝐹𝑒𝑡𝑢s

Question 3

Pregnancy

Answer 3

Lasts 37-40 weeks, and its divided into 3 trimesters:
1. 1st trimester (0-13 weeks) – major organs begin to develop in the embryo, and by the end it is
referred to as fetus.
2. 2nd trimester (14-27 weeks) – fetus undergoes tremendous amount of growth. Its organs mature
and sex is finally decided by appearance.
3. 3rd trimester (28th week till the end) – rapid growth and further brain development.

** Birth before the 37th week defined as premature delivery.

Question 4

Sperm Cell

Answer 4

Sperm cells are haploid cells that includes head which carries the chromosomes and acrosome hut that will
react with zona pellucida. They also have a neck that is rich with mitochondria from the movement of
flagella.

Question 5

Oocyte

Answer 5

The oocyte is surrounded by supporting cells, forming the zona pellucida. On top of that, we can find
molecular layer, called corona radiata.

Question 6

Fertilization

Answer 6

Fertilization occur in the oviduct duct, as the acrosome of the sperm reacts with zona pellucida using the
enzyme 𝑍𝑃3. After successful penetration of the sperm head, the zona pellucida is becoming impermeable
to another sperm. After this, a zygote is formed with a two set of chromosomes.

Process:
1. Sperm secretes acrosomal enzymes to digest the corona radiata. Then it binds to the zona pellucida
with the help of 𝑍𝑃3 protein.
2. The sperm penetrates the plasma membrane, causing a reaction that makes it impermeable to
other sperm.
3. Fusion of egg and sperm nuclei creates a zygote.

Question 7

Cleavage

Answer 7

The zygote that was formed in fertilization is undergoing cleavage to increase the number of cells.When reaching 16 to 32 cells, the complex is called morula the cleavage is continuing to form blastocyst.
In the blastocyst we can differentiate between the inner cell mass, that will later from the embryo; and to
peripheral cells, called trophoblast, that will later form the fetal placental.
The blastocyst will make implantation.

Question 8

Implantation

Answer 8

Occurs 5 to 8 days after fertilization. The blastocyst is implanting itself into the decidual layer of the
endometrium. The decidual layer is made of decidual cells that are specialize in protecting and nursing the
developing embryo. These cells will from the maternal placental (week 12).

Question 9

Gastrulation

Answer 9

During gastrulation, the inner cell mass is creating three layers:

• Outer ectoderm – will from the skin and nerve tissues.
• Middle mesoderm – will give rise to internal organs. Such as bones, muscles and connective tissues.
• Inner endoderm – will give rise to the internal linings. Such as alveoli and 𝐺𝐼𝑇.

Question 10

Neurulation

Answer 10

This Is the stage where the start of the development of the nervous system from the ectoderm begins.
1. A rod of mesodermal calls (a.k.a. notochord) forms along the axis of the embryo (like a spinal cord).
2. The notochord causes endodermal cells to fold inward, forming neural folds that surround a neural
groove.
3. The neural folds grow toward on another until they fuse into a neutral tube that give rise to the
central nervous system.
4. At the tip of each neural fold are neural crest cells which give rise to the peripheral nervous system.

Question 11

Defects in Neurulation

Answer 11

Spina bifida – birth defect that occurs due to the improper closing of the neural tube.
* Spina bifida occulta – outer part of some of the
vertebrae is not completely closed.
* Meningocele (𝑀𝑀) – protrusion of meninges
through the gaps between the vertebrae.
* Myelomeningocele – unfused portion of the spinal
column allows the spinal cord and
meninges to protrude through an opening,
forming a sec enclosing the spinal elements.

Question 12

Fetal Developments

Answer 12

After the first trimester, the placenta is completed. The placenta is a tissue that allows exchange of gases,
nutrients and waste product. Placenta also have endocrine function to produce 𝐻𝐶𝐺 – a hormone that
helps maintaining the pregnancy.

The placenta has two sides:

• The fetal placenta.
• The maternal placenta.
• *Blood is not mixed between the embryo and the mother

Question 13

Umbilical cord

Answer 13

The umbilical cord is exchanging blood between the fetus and the placenta. Umbilical cord
consists of:
* Two arteries – contain deoxygenated blood and waste product.
* One vain – contains oxygenated blood and nutrients from the mother.

Question 14

Chorion sac

Answer 14

Outer membrane connected with the placenta. This is the site of umbilical cord attachment

Question 15

Amnion

Answer 15

Inner protecting layer that contain amniotic fluid that surround the fetus. It is filled with amniotic
fluid that serves as a shock absorber during pregnancy and labor.

Question 16

During the second trimester…

Answer 16

Organs develop. Including, the reproductive track. Male reproductive track is
developing from the wolffian duct, while the female reproductive track is developing from the mullarian
duct.