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Timeline of embryonic development: Weeks 5 and 6

by Peter Ward, PhD
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    00:01 At the beginning of week 5, day 29, the embryo, not surprisingly, has elongated a bit more to 2.5 to 4.5 mm in length.

    00:10 And now, the dorsal root ganglia coming from the neural crest cells alongside the neural tube are present.

    00:16 By day 30, we've grown a bit more, 3.0 to 5.0 mm in length and facial development begins in earnest as the pharyngeal archers and the frontal nasal prominence begin fusing to form the various structures of the check, the mandible, and the nose.

    00:32 At this time, the lens pits, the optic cups and the nasal pits are all forming.

    00:37 So we are beginning to get the eyes and nose a little more fully invested.

    00:42 The ears have already begun forming but the external ears will not be notable until much later in development.

    00:49 By day 31, Crown-rump length is now 5.0 to 6.0 mm in length and the optic cup is extending to form the lens completely.

    00:59 The lens epithelium has been inducted but now it's wrapping around the lens which is beginning to migrate deeper into the head.

    01:07 The developing eye is still gonna be a bit lateral and will not just swing forward until a bit further, but we can now see distinctive nasal pit and primordial mouth or stomodium.

    01:18 Moving to day 32, We're roughly 4.0 to 6.0 mm in length and the embryo is now completely surrounded by its amniotic sac.

    01:28 As the embryos continue to fold and develop It's pulled the amniotic sac around itself as the interior body wall has fused.

    01:36 By day 33, we're now 5.0 to 7.00 mm in length and the rate of elongation is only gonna get faster as we develop more quickly moving through the last half of the embryonic period towards the beginning of the fetal period.

    01:52 By day 33, the umbilical cord is fully formed, stretching from the umbilicus to the placenta and the handplate is now present on the upper limb bud.

    02:03 Distinctive fingers, or digital race are not there yet but the handplate is notable.

    02:08 By day 34, we are now 5.0 to 7.0 mm in length and induction of the lens has now fully begun and the lens has moved into the head and then surrounded by the optic cup.

    02:21 The cerebral vesicles inside the head, the telencephalon, the diencephalon, mesencephalon, metancephalon, and myelincephalon are all distinct at this time and lagging just a bit behind the upper limb, we now have a footplate present on the lower limb bud.

    02:37 By day 35, we've now stayed roughly 5.0 to 7.0 mm in length, but all pharyngeal arches; one, two, three, four and six are present.

    02:48 The nasal pits have begun to form a full deepening nasal cavity and the limb muscle masses within the limb buds have begun to form into distinctive ventral flexor masses and dorsal extensor masses.

    03:01 Beginning the week 6, day 36, crown-rump length has extended just a bit more to between 7.0 and 9.0 mm in length.

    03:10 The midgut has moved into the umbilical cord As it's elongated to the degree it can no longer fit strictly inside the abdominal cavity.

    03:18 The pleuropericardial and pleuroperitoneal folds have begun forming and sealing the peritoneal cavity from the pericardial cavity and creating two distinct pleural cavities surrounding the heart and its pericardial cavity.

    03:34 The oral and nasal cavities have now grown together.

    03:37 The choanae have formed and ruptured, allowing the nose and oral cavity to be continuous.

    03:44 By day 37, we're still somewhere between 7.0 and 9.0 mm in length and development of the face is continuing as the frontal nasal prominence moves down and the maxillary and the mandibular prominences fuse with variant portions of the medial and the lateral nasal prominences.

    04:02 By day 38, We're now 8.0 to 11.00 mm in length; motor nerves have reached the muscle masses in each limb and are gonna begin innervating those flexor and extensor muscle.

    04:14 The head is quiet notable, has taken up a huge proportion of the developing embryo's size at this point.

    04:21 And the upper lip and nose have now completely formed.

    04:24 They might not fully fuse but they are formed and visible at this time.

    04:30 By day 39, We're still roughly 8.00 to 11.00 mm in length, the stomach and the duodenum are beginning to rotate to their mature positions as the stomach rotates and expands as the greater curvature forms.

    04:42 By day 40, we're between 8.00 and 11.00 mm in length and the internal ear begin forming quite a while ago and now the external ear begins to form as the auricular hillux on pharyngeal archers 1 and 2, become distinct and start migrating to form the auricle.

    05:01 The sternal bars have formed inside the anterior portion of the thoracic wall coming from the somatopore and will begin fusing with each other to completely cover the developing heart.

    05:13 By day 41, we're now 11.0 to 14.0 mm in length, the atrial septum has completely formed with the foramen ovale present, to allow blood to travel from the right to the left atrium from the placenta.

    05:29 The urorectal septum is forming and has grown inferiorly and will shortly separate the cloaca into a rectum and urogenital sinus.

    05:38 The kidneys have begun to ascend so the metanephric buds have formed and the metanephros is beginning to ascent as it's filtering urine and releasing it into the urogenital sinus.

    05:51 By day 42, we are now 11.0 to 14.00 mm in length.

    05:55 The tooth buds present from the dental lamina inside the maxilla and mandible begin to form.

    06:03 And digital rays begin to become visible in the hands and feet.

    06:07 These are not yet distinct fingers or toes but little condensations where those will eventually develop.

    06:13 The yolk sac and umbilical vessels have been completely surrounded by the body wall and are just extending from the umbilicus.

    06:20 The yolk sac and its vitelline duct will regress.

    06:23 The umbilicus will maintain its connection to the placenta up to, and just past the time of delivery.

    06:30 Endocardial cushions are fusing, separating the atrioventricular canals into a right and left atrioventricular canal, and also closing the ostium primum and ventricular septum.


    About the Lecture

    The lecture Timeline of embryonic development: Weeks 5 and 6 by Peter Ward, PhD is from the course Conception, Implantation and Fetal Development. It contains the following chapters:

    • Embryonic Development: Week 5
    • Embryonic Development: Week 6

    Author of lecture Timeline of embryonic development: Weeks 5 and 6

     Peter Ward, PhD

    Peter Ward, PhD


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