Question

Write the fate of three germ layers developed in embryonic development of human beings.

Answer 1

The development of embryonic layers takes place during gastrulation during which the cells of the inner mass of the blastula move to their new and final location. This movement occurs by morphogenetic movements. Three germ layers ectoderm, mesoderm and endoderm develop into different parts of a human being.

Complete answer:
A zygote is formed after fertilization which afterwards undergoes cleavage and mitosis and converts zygote into blastula which is a hollow mass of cells. Blastula further undergoes gastrulation forming a gastrula during which germ layers are formed. Germ layers include:

Endoderm – Initially consists of flattened cells which become columnar afterwards. Endoderm forms the epithelial lining of the digestive tract, lining of glands, lining of auditory tube and tympanic cavity, bronchi, trachea, alveoli of lungs, urinary bladder and parts of urethra
Fate of endoderm –
-Lining of the gut
-Lining of glands – liver, pancreas, gastric and intestinal glands, thymus, parathyroid
-Inner layer of tympanic membrane
-Lining of middle ear
-Lining of respiratory tract
-Lining of excretory organs.

Mesoderm – It is the layer present in between the ectoderm and endoderm by the migration of cells towards inside during gastrulation. Mesoderm leads to the formation of coelom and the organs formed inside the coelom are free to grow, move and develop independent of the body wall and fluid support that protects them from shocks.
Fate of mesoderm –
-Forms connective tissues
-Eye muscles except iris and ciliary muscles
-Dermis layer of the skin
-Gonads
-Kidneys
-Heart, blood and lymph vessels
-Reproductive and urinary ducts
-Most of the skeleton
-Coelomic epithelium
-Pleura and pericardium
-Cortex of adrenal glands
-Gut wall except the lining

Ectoderm – This germ layer forms the outer layer of the embryo and develops from the epiblast of the embryo.
Fate of ectoderm –
-Nervous system
-Enamel of teeth
-Epidermis of skin
-Epidermal derivatives of hair, epidermal glands and nails
-Eyes: cornea, conjunctiva, lens, iris, retina, ciliary muscles
-Nasal and olfactory epithelium
-Internal ear
-Posterior and intermediate lobes of pituitary glands, medulla of adrenal gland and pineal gland
-Nasal and olfactory epithelium
-Epithelium of hindgut and foregut
-Mammary glands, sweat glands, oil glands, lacrimal glands and salivary glands.

Note: Neural crest is considered at the fourth germ layer though it is derived from ectoderm and forms the neurons, schwann cells, arachnoid and pia mater, melanocytes, adrenal medulla, bones of neurocranium, aorticopulmonary septum and pharyngeal arch bones which are mandible, maxilla, incus and malleus.