Embryology

Question 1

Prenatal period?

Answer 1

• 38 weeks from conception to birth (average) “fetal” age

- gynecologic timing has been from LMP so refers to 40 weeks gestational age (LMP is on avg 2 weeks b/f ovulation)

Question 2

Embryonic period?

Answer 2

• first 8 weeks

- all major organs are formed

Question 3

Fetal period?

Answer 3

• remaining 30 weeks

- organs grow large and become more complex

Question 4

Process of ovulation and fertilization?

Answer 4

• ovulation: egg released into peritoneal cavity
• fertilization: occurs as egg travels down fallopian tube
• zygote: the cell that forms when the ova and the sperm fuse
• cell division: begins immediately as zygote travels down fallopian tube and into uterus

Question 5

Zygote cleavage - leads to blastomeres?

Answer 5

• zygote divides repeatedly moving down tube toward uterus (cleavage)
• daughter cells are called blastomeres
• morula: solid cluster of 12-16 blastomeres at about 72 hrs
• day 4: 60 cell morula enters uterus, taking up fluid becoming a blastocyst

Question 6

2 distinct types of cells of blastocyst? floats around for how long?

Answer 6

• inner cell mass: forms embryo
• trophoblast: layer of cells surrounding the cavity which helps form the placenta
• floats for about 3 days

Question 7

When does implantation occur?

- how are monozygotic twins formed?

Answer 7

• day 6 post conception
• trophoblast erodes into uterine wall
• takes 1 week to complete
• if inner cell mass of single blastocyst divides: monozygotic twins

Question 8

What occurs during week 2 post conception?

Answer 8

• inner cell mass divides: epiblast and hypoblast
• 2 fluid filled sacs: amniotic sac from epiblast and yolk sac from hypoblast
• bilaminar embryonic disc - gives rise to whole body

Question 9

formation of 3 primary germ layers?

Answer 9

• bilaminar disc - forms trilaminar disc in week 3
• ectoderm
• endoderm
• mesoderm

Question 10

description of 3 germ tissues?

Answer 10

early specialization of cells:

• ectoderm and endoderm are epithelial tissue (form sheets of tissue)
• mesoderm is a mesenchyme tissue
• mesenchyme cells are star shaped and don’t attach to one another, they migrate freely

Question 11

Formation of 3 germ layers?

Answer 11

• primitive streak - on dorsal surface of epiblast
• grastulation: invagination of epiblast cells
• day 14-15: they replace hypoblast becoming endoderm
• day 16: mesoderm (new third layer)
• epiblast cells remaining on surface are ectoderm

Question 12

Notochord formation?

Answer 12

• primitive node epiblast cells invaginate
• migrate anteriorly with some endoderm cells
• rod defining the body axis is formed
• future site of the vertebral column

Question 13

Process of neuralation?

Answer 13

• notochord signals overlying ectoderm
• formulation begins of spinal cord and brain
• neural plate to neural groove to neural tube: pinched off into body

Question 14

Mesoderm differentiation in week 3?

Answer 14

• lateral to notochord, week 3
• extends cranially and caudally (from head to tail or crown to rump)
• division of mesoderm into 3 regions: somites - 40 pairs of body segments (repeating units) by end of week 4
• intermediate mesoderm: just lateral to somites
  lateral plate: spilts to form coelom

Question 15

When does neural tube close? What is impt for this step?

- formation of neural crest?

Answer 15

• begins at end of week 3, complete by end of week 4
• folic acid impt for this step
• extends cranially (eventually brain) and caudally (spinal cord)
• neural crest: lateral ectodermal cells, pulled along and form sensory nerve cells and other structures

Question 16

Divisions of the mesodermal lateral plate?

Answer 16

• somatic mesoderm: apposed to ectoderm
• splanchnic mesoderm: apposed to endoderm
• coelom in b/t will become the serous cavities of the ventral body cavity:
  peritoneal
  pericardial
  pleural

Question 17

When does embryo take rudimentary shape of adult?

Answer 17

• week 4

- look at cross sections - basically similar

Question 18

Major derivatives of ectoderm?

Answer 18

• epidermis, hair, nails, glands of skin
• brain and spinal cord
• neural crest: sensory nerve cells and some nervouse structures; pigment cells; portions of skeleton, blood vessels in head and neck

Question 19

Major derivatives of mesoderm?

Answer 19

• somite: sclerotome: vertebrae and ribs
• dermatome: dermis
• myotome: trunk and limb musculature
• intermediate: kidneys, and gonads
• lateral plate mesoderm:
  somatic - CT of limbs
  splanchnic: wall of digestive and respiratory tracts, visceral serosa, heart, blood vessels

Question 20

Major derivatives of endoderm?

Answer 20

• epithelial lining of glands of digestive and respiratory tract

Question 21

When does heart start pumping?

Answer 21

• 29 days

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Question 22

Importance of early abstinence of drugs in pregnancy?

Answer 22

• before 2 month mark

- by 8 weeks all major organs are at least a rudimentary form, many drugs taken during this time can cause birth defects

Question 23

Weeks 3-8 organogenesis significance?

Answer 23

• stem cell populations est for each organ primordia
• this is period when most gross structural birth defects are induced
• 3rd and 4th weeks are particularly vulnerable and mother may not even know she is pregnant (somites which produce majority of head mesenchyme starts in cervical region at day 20 and spreads caudally at rate of 3 pairs per day
• SIG is that mother MUST avoid environmental, dietary and drug exposure during this period!