overview of systemic embryology

Question 1

the trilaminar embryonic germ disc -embryonic period or period of organogenesis

Answer 1

occurs from the 3rd/4th to the 8th weeks of development
is the time when each three embryonic primary germ disc layers give rise to specific tissues and organs
by the end the main organ systems have been developed

Question 2

derivatives of the ectodermal germ layer

Answer 2

gives rise to the organs and structures that maintain contact with the outside world
-induced to form neuroectoderm that leads to the formation of the neural tube
-also gives rise to neural crest cells and pituitary gland
eg skin nose ears eyes

Question 3

paraxial mesoderm

Answer 3

forms segmented blocks called somitomeres which in the head region is called neuromeres
the neuromeres contribute to the mesenchyme in the head
from neck downwards somitomeres form blocks of mesoderm called somites
first pair of somites arises in the occipital region of the embryo at about the 20th day of development
age of an embryo can be calculated by counting somites

Question 4

intermediate mesoderm

Answer 4

contribute in the formation of the structures of the urogenital system including the primordial germ cells, gametes and gonads

Question 5

lateral plate mesoderm

Answer 5

splits into the parietal (Somatic) and visceral (splanchnic) layers
parietal layer lines cavity walls
forms the lateral body wall
mesoderm of the visceral layer together with the endoderm form the wall of the gut tube

Question 6

derivatives of the endodermal germ layer

Answer 6

epithelial lining of the gastrointestinal tract including the pharynx
-also forms the lining of the respiratory tract,urinary bladder,urethra,tympanic cavity and auditory tube
the parenchyma of the thyroid,parathyroids ,liver
NOTE: The walls of the GIT-smooth muscles and other connective tissues are derived from the mesoderm

Question 7

morphogenesis

Answer 7

form-shaping process in an embryo,controlled by fundamental cell behaviours that result in differential tissue growth
interfere with differential tissue growth in an embryo , could be occasioned by genetic mutation, teratogen exposure

Question 8

dysmorphogenesis and birth defects

Answer 8

responses due to the influence of genetic variations an environmental factors during development are considered and an abstract norm is defined
individuals who develop within a close range of this abstract norm both in form and function are said to have developed normally-normogenesis

Question 9

teratogen

Answer 9

any factor or agent that CAUSES birth defect or congenital malformation or congenital anomaly

Question 10

what factors determine the capacity of an agent to produce birth defects

Answer 10

• genotype of the conceptus and the maternal genome
• development stage at the time of exposure to teratogens
• dose and duration of exposure to teratogen

Question 11

gastrulation

Answer 11

embryonic development-beginning of 3rd week

Question 12

holoprosencephaly

Answer 12

cells killed of anterior midline of germ disc ,leading to deficiency of midline in craniofacial structures
-lack of nasal tissue, eyes too close together

Question 13

siren omelia

Answer 13

due to mesodermal insufficiency in the caudal most region of the embryo

Question 14

situs inversus

Answer 14

a condition with transposition of the viscera in the thorax and abdomen

Question 15

left-sided bilaterality

Answer 15

polysplenia

Question 16

right sided bilaterality

Answer 16

asplenia or hypoplastic spleen

Question 17

sacrococcygeal teratomas

Answer 17

remnants of the primitive streak persist in the sacrococcygeal region to form tumours