BGDA Practical 7 - Week 5
Introduction
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Key Events of Human Development during the fifth week (week 5) following fertilization or clinical GA week 7.
By this week there are many different organs and tissues differentiating throughout the embryo, only a few select systems will be discussed.
| Stage 15 - Ventral View
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| Stage 15 | Embryo Slides |
Cardiovascular
- Heart - septation starts, atrial septation and ventricular septation
- Vascular - 3 vascular systems (systemic, placental, vitelline) extensively remodelled
See also Heart Development Timeline - Basic Cardiac Embryology
Respiratory
- left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)
Neural
- Central nervous system - secondary brain vesicles
- Sensory - Hearing cochlear part of otic vesicle elongates (humans 2.5 turns)
Stage 14
Stage 15
Cardiovascular
Heart Development Timeline (see Basic Cardiac Embryology)
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Atrial Septation
Respiratory
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Week 4 - laryngotracheal groove forms on floor foregut.
Week 5 - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity) Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes. Week 7 - enlargement of liver stops descent of heart and lungs. Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant. Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and alveolar sacs. |
The trachea can be see making its initial branching into the lung bud mesenchyme.
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| Bifurcation of the trachea into the main or primary bronchi, the left and right bronchus. | Adult bronchial tree. |
Neural
During week 5 the five secondary brain vesicles form from the original primary vesicles formed in week 4.
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Rapid growth folds the neural tube forming 3 brain flexures:
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Neural tube - brain flexures (stage 13)
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Additional Information
| Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page. |
Timeline
| Week 5 - Human Embryo Stages and Events (GA week 7) | ||
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| Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9 | ||
| Event | ||
| pituitary - Week 5 elongation, contacts infundibulum, diverticulum of diencephalon
heart - Week 5 septation starts, atrial and ventricular respiratory - Week 5 left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity) respiratory - Week 5 to 17 lung histology - pseudoglandular hearing - Week 5 cochlear part of otic vesicle elongates (humans 2.5 turns) | ||
| Stage 14 |  mesoderm - continued segmentation of paraxial mesoderm (somite pairs), heart prominence Head - 1st, 2nd and 3rd pharyngeal arch, forebrain, site of lens placode, site of otic placode, stomodeum Body - heart, liver, umbilical cord, mesonephric ridge visible externally as bulges. Limb - upper and lower limb buds growing neural - first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable[1] liver hepatic gland and its vascular channels enlarge, hematopoietic function appears[2] | |
| Stage 15 |  neural - cranial nerves (except olfactory and optic) are identifiable in more advanced embryos[3] | |
| vision - 35 to 37 days retinal pigment present | ||
| Note - the day timing of stages is only approximate, system names link to first page of that specific system, and events are based upon the literature cited below. | ||
References
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Neural Development
| Embryonic Central Nervous System | |||
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| Stage 13 | Stage 14 | Stage 16 | Stage 21 |
scale bar = 1 mm |
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| Week 4 | Week 5 | Week 6 | Week 8 |
| Neural Tube | Primary Vesicles | Secondary Vesicles | Adult Structures |
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| week 3 | week 4 | week 5 | adult |
| prosencephalon (forebrain) | telencephalon | Rhinencephalon, Amygdala, hippocampus, cerebrum (cortex), hypothalamus, pituitary | Basal Ganglia, lateral ventricles | |
| diencephalon | epithalamus, thalamus, Subthalamus, pineal, posterior commissure, pretectum, third ventricle | ||
| mesencephalon (midbrain) | mesencephalon | tectum, Cerebral peduncle, cerebral aqueduct, pons | |
| rhombencephalon (hindbrain) | metencephalon | cerebellum | |
| myelencephalon | medulla oblongata, isthmus | ||
| spinal cord, pyramidal decussation, central canal | |||
References
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