Body Logistics

Question 1

What is embryology?

Answer 1

• study of profession of a single cell to a baby in 9 months (including molecular, cellular, and structural factors)

Question 2

Define:

Embryonic age.
Gestational age.

Germinal stage.

Embryonic period.

Foetal period

Answer 2

• the time since fertilisation
• the time since last menstruation (embryonic age plus 2 weeks)
• the time from fertilisation to the end of the second week
• The time from third week to the end of the eighth week
• the time from the beginning of the ninth week to birth (week 38)

Question 3

Describe the process of ejaculation to fertilisation.

Answer 3

• millions of sperm enter vagina and will flow through or die in acidic environment
• passes through cervix (open for few days during ovulation) or die in mucus or folds of cervix
• tiny cilia push egg towards sperm, some sperm die in cilia
• chemicals in reproductive tract make sperm swim harder and faster
• sperm must push through layer of cells and zona pelucida (release digestive enzymes) on egg
• cell membranes of sperm and egg fuse (no other sperm can get in)
• male genetic material spreads inside egg, male pronucleus and female pronucleus fuse (2 sets)

Question 4

In general, list the key events in the process of early development?

Answer 4

• fertilisation and implantation
• gastrulation (formation of 3 germ layers)
• neurulation (transformation of neural plate into neural tube through folding)
• segmentation ( divisions of cells into different parts/ sections
• folding

Question 5

What occurs in the first week of development after fertilization?

Answer 5

• Zygote (2-cell stage)
• Early cleavage (4-cell stage)
• Morula (bundle of cells , day 3-4 )
• Blastocyst with trophoblast (hollow ball of cells, day 4-5, becomes embryo where cells starts to differentiate)
• Implantation (day 6, in endometrium- lining of uterus)

Question 6

What is the zona pellucida?

Answer 6

• thick transparent membrane that prevents morula/blastocyst from sticking to the oviduct before implantation
• Stops the morula from enlarging

Question 7

What is the trophoblast?

Answer 7

• cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta

Question 8

Where is the normal implantation site?

Answer 8

• blastocyst implants on superior and posterior wall of uterine cavity
  (determines the site of formation of the placenta, implants on day 6)

Question 9

What occurs in the second week of development?

Answer 9

• trophoblast differentiates into 2 laters
  - cytotrophoblast
  - syncytiotrophoblast
• Embryoblast forms 2 layers - the bilaminar disc
  - epiblast
  - hypoblast
• Hypoblast contributes to the formation of 2 cavities
  - the yolk sac
  - chorionic cavity

Question 10

What occurs in the embryonic period?

Answer 10

• weeks 3-8
• all major structures are formed (period of most change)
• greatest risk of major congenital malformation (from environmental exposure/drug therapy)

Question 11

What is the primitive streak?

Answer 11

• forms on the back surface of epiblast towards the caudal end
• leading to migration and invagination of epiblast
• marks the start of gastrulation

Question 12

What occurs in gastrulation and when does gastrulation occur?

Answer 12

• occurs in third week (start of embryonic period)
• from primitive streak, bilaminar disk (epiblast and hypoblast) converted to a trilaminar disk (ectoderm, mesoderm, and endoderm)
• Ensures correct placement of precursor tissues to continue process

Question 13

What is derived from the ectoderm, mesoderm, and endoderm?

Answer 13

• ectoderm - organs and structures that maintain contact with outside world (eg nervous, epidermis)
• Mesoderm - supporting tissues (eg muscle, cartilage, bone, vascular system )
• Endoderm - internal structures (eg epithelial lining of GI tract, respiratory tract, parenchyma of gland )

Question 14

What is the notochord in neurulation and how does the neural tube form?

Answer 14

• Solid rod of cells running in the midline with important signalling role
• directs conversion of overlying ectoderm to neurectoderm
• Signals the overlying ectoderm to thicken
• Slipper-shaped neutral plate forms
• edges elevate out of the plane of the disk and curl towards each other, creating the neural tube

Question 15

What are somites?

Answer 15

• organization of paraxial mesoderm into segments
• gives rise to repeating structures (Vertebrae, Ribs, Intercostal muscle, Spinal cord segments)
• Guides innervation
• Dermatome (skin section) - dermis
• Myotome (muscle section) - muscle
• Sclerotome (hard tissue section) - bones

Question 16


What does folding achieve?

Answer 16

• creates primordium of the gut
• puts the heart and primordium in the right place
• creates a new cavity within the embryo

Question 17

What has occurred by the end of the 4th week?

Answer 17

• The nervous system has started to form
• Segments have appeared, assigning specific tasks to 
specific cells
• Embryo has folded, putting everything in right place.