Embryology at a glance Watch this Then this

C. Cell Differentiation • By day 4, the embryo forms a morula –a ball containing 16- cells –the cells have now differentitated into • an inner cell mass – gives rise to the morula embryo proper (namely YOU) (means “mulberry”) •an outer cell mass embryonic contribution to the placenta

D. Cavitationand the Blastocyst • Next a fluid-filled outer cell mass cavity “blastocyst” and the “cavitation” forms inner cell mass between the inner cell mass • This is called • This gives rise to the blastocyst “blastocyst”= “germ-bag”

Week #2: BilaminarDisk Formation (1) •The inner cell mass now differentiates into 2 distinct cell layers epiblast hypoblast –the epiblast –the hypoblast • This is called the bilaminar embryonic disk

Week #2: BilaminarDisk Formation (2a) • A cavity forms next to the epiblast –It is called the amniotic cavity –it becomes filled with amniotic cavity amniotic fluid –this will eventually surround and suspend the embryo and provide • a shock-free environment • a constant temperature –and allow free movement

Week #2: BilaminarDisk Formation (2b) • The cells lining the amniotic cavity are shed on occasion • These cells, along with proteins, carbohydrates, fat, enzymes, hormones and embryonic excretions can be extracted and examined amniotic cavity • This procedure is called an amniocentesis

Week #2: BilaminarDisk Formation (3) amniotic cavity yolk sac

Week #3: Gastrulation (1)… • With further cell differentiation, a 3rdcell layer forms between the epiblastand hypoblast • The embryo is now a trilaminardisk • This process is called gastrulation and forms the mesoderm around day 16 Forms lining of GI tract

Week #3: Gastrulation(2)… • The epiblastis now called the ectoderm • The hypoblast is now called the endoderm • The middle layer, composed of cells that migrated from the epiblast, becomes the mesoderm • These 3 cell layers give rise to all the cells of your body!

Week #3: Gastrulation(3a)… •The ectoderm will give rise to –organs that maintain contact with the outside world • epidermis (epithelial lining of the skin) And the nervous system skin

Week #3: Gastrulation(3b)… •The endoderm will give rise to –the epithelial lining of the digestive and respiratory tracts –plus outgrowths of the digestive tract that give rise to the: • liver pancreas

Week #3: Gastrulation(3c)… •The mesoderm will give rise to –connective tissues • connective tissue proper That is, basically everything between the inner and outer linings of your body! bone cartilage blood –muscle

While the external “look” of a human is called bilateral symmetry because if you cut us in half on the median plane, both sides look roughly exact. However, our internal design is called tube within a tube as seen here.

Four Weeks Figure 29–7a, b

Eight Weeks the 8thweek the fingers and toes have • The face Figure 29–7c, d • The face • By the end of formed now begins to look human-like • (The fetus remains at very high risk of malformations through the 8th week)

Twelve Weeks • This is the Figure 29–7c, d • This is the period of functional maturation of all organ systems • (By this age, the fetus is at lower risk of malformations)

HOMEWORK What are the three germ layers and what mature features do they form?