Differentiation of the Germ Layers

Germ Layer	Adult Derivatives
Ectoderm	Surface ectoderm	Epidermis and its derivatives (hairs, nails, sweat and sebaceous glands); Mammary glands Epithelial lining of the oral cavity structures, ameloblasts, salivary glands; Lens of eye; Adenohypophysis; Epithelial lining of external auditory meatus; utricle, semicircular ducts Olfactory placode; Epithelial lining of lower anal canal
	Neuroectoderm		Nerve tube	CNS (all neurons and glial cells); Retina; Pineal gland
			Nerve crest	PNS end many other structures (table)
Mesoderm	Paraxial (SOMITES)		Dermatome	Skin dermis
			Myotome	Skeletal muscles of trunk, limbs, head and neck; extraocular muscles; intrinsic muscles of tongue
			sclerotome	Vertebrae and ribs; cranial bone
	Intermediate (Nephrogonotome)		Kidneys Reproductive system organs
	Lateral (splanchnotome)		Serous membranes of body cavities (mesothelium) Carddiogenic mesoderm (splanchnic mesoderm) → heart Suprarenal cortex cells
Mesenchyme	Blood, lymph, connective tissue, smooth muscle cells, blood vessels, endocardium
Endoderm	Epithelial lining of the gastrointestinal tract, the liver, the pancreas, the gall bladder; epithelial lining of the respiratory system (trachea, bronchi, lungs), the thymus stroma, the thyroid and the parathyroid glands endocrine cells; epithelial lining of the urinary bladder, the female urethra and most of the male urethra

DIAGRAM of CONSECUTIVE STAGES of HUMAN EMBRYOGENESIS

1- uterine mucosa; 2- epithelium; 3- connective tissue; 4- trophoblast;

5- embryoblast; 6- amniotic vesicle (ectoderm material); 7- entoderm material;

8- extraembryonic mesoderm; 9- extraembryonic mesenchyme;

10- differentiation of trophoblast; 11- yolk sac; 12-amniotic stalk;

13- chorion cavity; 14- lacunae; 15- symplastotrophoblast; 16- cytotrophoblast;

17- intraembryonic mesoderm; 18- primitive streak; 19- chorion frondosum;

20- smooth chorion; 21- amnion; 22- body fold; 23- yolk sac; 24-embryo body;

25- amniotic membrane; 26- allantois; 27- chorionic villi

Fig.1 Cleavage in mammals

1 – blastomere;

2 – fertilization membrane

Fig.2 Implantation. Differentiation of trophoblast.

1 – trophoblast; 2 – embryoblast; 3 – epithelium of endometrium;

4 – connective tissue of endometrium; 5 – hypoblast; 6 – epiblast;

7 – roof of amniotic vesicle; 8 – amniotic cavity; 9 – blood vessels of endometrium;

10 – symplastotrophoblast; 11 – cytotrophoblast; 12 – mitosis in cytotrophoblast;

13 – extraembryonic mesoderm; 14 – fibrinoid;

15 – lacunae within symplastotrophoblast; 16 – exocoelomic cavity.

Fig.5 Extraembryonic organs in human

A – the third week;

B – the beginning of the forth week

1 – amnion wall;

2 – cardiac primordium;

3 – blood islets; 4 – oral membrane;

5 – cloacal membrane;

6 – chorionic plate;

7 – tertiary chorionic villi;

8 – chorionic blood vessels;

9 – allantois; 10 – primitive foregut;

11 – primitive midgut;

12 – primitive hindgut;

13 – yolk sac cavity;

14 – primitive umbilicus;

15 – amniotic cavity;

16 – extraembryonic cavity

(chorion’ cavity)

Fig.7. Changes in extraembryonic membranes and the embryo’ correlation in

the process of development

1 – amniotic vesicle; 1a – amniotic cavity;

2 – the embryo body;

3 – yolk sac;

4 – extraembryonic coelom (chorion cavity)

5 – primary chorionic villi;

6 – secondary chorionic villi

7 – connective stalk;

8 – tertiary chorionic villi;

9 – allantois;

10 – umbilical cord

Fig.8 HUMAN PLACENTA

A – General plan of placenta structure (arrows point blood circulation in one of lacunae);

B, C, D – consecutive stages of chorionic villi’ formation;

E – structure of placentar barrier in the first trimester of pregnancy;

F – structure of cotyledon

1 – amniotic epithelium; 2 – chorionic plate; 3 – stem villus; 4 – fibrinoid; 5 – reducing yolk sac;

6 – umbilical cord; 7 – septum between cotyledons; 8 – lacuna; 9 – spiral artery;

10 – decidua basalis; 11 – myometrium; 12 - symplastotrophoblast; 13 – cytotrophoblast;

14 – basal lamina of trophoblastic epithelium; 15 – hemocapillar of chorionic villus;

16 – fibroblast; 17 – mesoderm of secondary chorionic villus;

18 – blood vessel of tertiary chorionic villus.