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