Histology III:Embryology and differentiation
Histology III:Histology III:Embryology and Embryology and differentiationdifferentiation
Dr. Carmen E. RexachAnatomy 35
Mt San Antonio College
We all start as one fertilized oocyte!
Gestation lasts ≈266 days
Trimesters:Clinical Approach
8 weeks5 months
Full term
2nd trimester = 13-24 weeks
3rd trimester = 25 weeks to birth
1st trimester = 1st 12 weeks
Stages of Development
Growth and maturation8-40wksFetal
Differentiation of 1o germ layers16d-8wksEmbryonic
Trophoblast + inner mass4-16dBlastocystBall of 16+ blastomeres3-4dMorulaIdentical blastomeres30-72 hrCleavageA single diploid cell0-30 hrZygote
Pre-embryonic
Pre-embryonic
• Cleavage• Implantation• Embryogenesis
Cleavage• Blastomeres
– Smaller cells formed by division of zygote • Morula
– 16+ cells• Blastocysts
– Hollow sphere >100 cells– Internal cavity forms = blastocoel– Two parts
• Trophoblast• Embryoblast
Implantation
• Attachment of blastocystto endometrium
• Two parts– Syncytiotrophoblast
• Superficial layer in contact with endometrium
– Cytotrophoblast• Deep layer near
embryoblast
Embryogenesis• Arrangement of
blastomeres into 3 primary germ layers
• Formation of amniotic cavity
• Embryoblast becomes embryonic disc with two cell layers– Epiblast(6)– Hypoblast(8)
• Forms yolk sak
(1) Blood vessels of endometrium(2) Endometrium(3) Syncytiotrophoblast(4) Cytotrophoblast(7) Is the amniotic cavity
Embryogenesis• Primitive streak forms establishing bilateral
symmetry• Gastrulation
– Three germ layers are established– Basic body plan is established– Primitive streak = beginnings of primary body axis– Cells are brought into new placement inside cell
• Allows cells to interact directly with each other that could not before
Conceptus is now called an embryo2mm long and 16 days old
Differentiation--how cells become different
Primary germ layers• Endoderm
– Forms inner lining of the digestive tract
• Mesoderm– Differentiates into
mesenchyme– Gives rise to muscle,
bone, blood• Ectoderm
– Gives rise to skin, nervous system, some glands
Embryonic Stage• Age of conceptus
– 16 days-8 wks• Major events
– Development of placenta & accessory structures
– Placenta becomes primary nutrient source
– Differentiation of germ layers into organs and organ systems
7 weeks
Folding and organogenesis
• Embryonic folding forms C shape enclosing primitive gut– Surface covered with ectoderm– Mesoderm splits into two layers forming
coelom– Thoracic and abdominal cavities form
when diaphragm divides coelom– Thoracic cavity divides to form pleural
and pericardial cavities
Folding
Longitudinal section Cross section
Major events of organogenesis
• Development of the neural tube• Development of pharyngeal pouches• Development of somites
Neurulation• Appearance of
neural plate at week 3
• Sinks and becomes neural groove
• Neural folds come together to form neural tube– Brain forms at
cephalic end– Caudal end
becomes spinal cord
Pharyngeal pouches
• Formation of 5 pairs of pockets at 4-5 weeks gestation
• Separated by pharyngeal arches
• Give rise to middle-ear cavity, palatine tonsils, thymus, parathyroid gland, part of thyroid gland
Characteristic of all chordates
Somites• Segmentation• Appear at day 20
resulting in 42-44 pairs by 35d
• Each divides into 3 tissue masses– Sclerotome = surrounds
neural tube, gives rise to vertebral column
– Myotome = trunk muscles– Dermatome = skin and
subcutaneous tissues
Embryonic membranes
• Amnion• Yolk sac• Allantois• Chorion
Amnion• Completely encloses embryo• Penetrated by umbilicus• Filled with amniotic fluid derived from
mother’s plasma– Symmetrical development– Prevents adhesion of surface tissues– Protects against trauma, infection– Mediates temperature fluctuations– Movement for muscle development
Yolk sac• Attached to primitive gut• Vitelline duct• Produces first blood cells and stem
cells– Migrate into embryo– Colonize bone marrow and other tissues
• Blood cells to bone marrow• Stem cells to future gonads
Allantois and chorion• Allantois
– Connected to gut by allantoic duct– Foundation for growth of umbilicus – Becomes urinary bladder
• Chorion– Outermost membrane– Chorionic villi = branch to form fetal portion of
placenta
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