Neurulation And Organogenesis
Category : 12th Class
Post gastrulation involves two main process. Neurulation is process of laying the neural plate to form the nervous system. The establishment of the germ layers initiates the final phase of embryonic development, i.e., organogenesis. The latter involves differentiation and specialization of groups of cells in the individual germ layers. The cells of such groups change their form and give rise to morphologically recognizable tissues and organs of the new individual. The groups of differentiated cells separate from their germ layers in an orderly manner and with unique precision. Separation of the differentiated cell groups may occur by folding off from the germ layer or by migration of cells individually and reaggregation at a new place. In this manner, the primordial cells of the germ layers gradually and accurately give rise to the tissues and organs of the offspring.
By four weeks after fertilization, the embryo has a simple heart, limb buds and eye rudiments. It also has a tail and pharyngeal pouches, the vestiges of its early vertebrate ancestors that disappear later in development. After the second month, the embryo is recognizable as a primate. From this stage on, the embryo is often called foetus.
|
Time from fertilization |
Stage/organs |
Event |
|
24 hours |
Cleavage |
Embryo is at two-cell stage |
|
3 days |
Modula |
The module reaches uterus |
|
7 days |
Blastocyst |
Implantation of blastocyst begins |
|
2.5 weeks |
Notochord |
Notochord formed, differentiation of tissues that will give rise to heart, blood cells formed in yolk sac and chorionic. |
|
3.5 weeks |
Organ system |
Neural tube formed, primordial eye and ear vesicle, pharyngeal pouches formed, liver bud differentiates, respiratory system and thyroid gland begin to develop, heart tube bends and begins to beat, blood vessels are formed. |
|
4 weeks |
Limb buds |
Development and appearance of limb buds, brain forms three primary vesicles. |
|
2 months |
Muscles and gonads |
Muscles differentiate, embryo capable of movement, gonads distinguishable as testes or ovaries, ossification of bones begins, cerebral cortex is differentiated, blood vessels assume final position. |
|
3 months |
Sex differentiation |
By external examination sex can be determined, notochord degenerates, lymph glands develop. |
|
4 months |
Face |
Face begins to look human, eye, ear and nose look ‘normal’, cerebral lobes differentiate. |
|
6-9 months (third trimester) |
Lanugo (hairs) body growth |
Lanugo appear but are shed later, tremendous growth of body occurs, neurons become militated |
|
266 days |
Birth (parturition) |
Baby is born, neonate arrives in outer world. |
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