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;

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