Timeline of embryonic development: Weeks 3 and 4

00:01 Moving into day 15, we have a primitive streak appearing on the opposite side of the embryo from the prechordal plate.

00:08 So if the prechordal plate marks where the head will develop, the primitive streak is developing in the more caudal or tail region of the developing embryo.

00:18 Moving into day 16, the primitive streak is still present and epiblast cells have begun to move through it and migrate in to displace hypoblast cells, and this process will eventually create three layers where there were only two.

00:34 Secondary and tertiary villi are now forming in the cytotrophoblast.

00:39 So not only do we have cytotrophoblast, That little horns sticking out into the uterine lining, but we're getting a core of extraembryonic mesoderm and vessels growing into those villi to set up that circulation that will make the placenta.

00:57 Moving into day 17, we start to see that the bilaminar embryo is going away and been replaced by the three layered trilaminar embryo with its components of: ectoderm, mesoderm, and endoderm.

01:12 Moving into day 18, I'd like to note that we are now able to measure crown-rump length because we now have something that could be distinguished as the head and the eventual tail.

01:26 Crown-rump length at this time is still very small, 0.2 to 0.3 mm in length.

01:32 But development is preceding the pace.

01:35 The lateral plate mesoderm on the opposite side, left to right of the embryo is beginning to split which will eventually form the gut tube and the anterior body wall.

01:46 The intraembryonic coelom is forming as that separation occurs and the somatopleure begins to migrate anteriorly to enclose the body cavity and endocardial tubes are beginning to form in the mesoderm just in front of the eventual face.

02:04 Moving into day 19, Crown-rump length is now about 0.4 mm and the oropharyngeal and cloacal membranes form.

02:13 This marks the eventual occasion of the mouth, from the oropharyngeal membrane and the urogenital sinus and anal membranes from the cloacal membrane.

02:22 Day 20, We're still at about 0.4 mm in length but the neural folds have become distinct and the grooves are visible on the surface of the ectoderm and will soon begin migrating in to form the central nervous system and neural tube.

02:38 Primordial germ cells are migrating to the genital ridges from the yolk sac.

02:43 It's important to note these germ cells were initially part of the epiblast but migrated to the yolk sac very early during development and are now migrating back to become the spermatocytes and oocytes of the embryo.

02:58 Day 21, still roughly 0.4 mm from crown to rump but the neural folds and grooves are definitely present and neuro relation is proceeding.

03:08 The primordial germ cells have migrated to their mature position in the general ridges and will now be a wrapped up in either the developing testes or developing ovary.

03:19 During week 4, most women discover their pregnancy.

03:23 Week 4 to week 6 is the major time this occurs.

03:25 And I want you to think about all the important things that have happened, in neuro relations beginning, we've got the beginning of the heart, and all the germ cell layers are present.

03:36 So a lot has happened to before most people are even aware that a pregnancy is present.

03:41 Moving into week 4, at day 22, the crown-rump length still roughly 0.4 mm.

03:48 The notochord has fully separated from endoderm and has now gonna drive the formation of the neural tube and the other nearby structures.

03:57 The neural tube closes starting in the middle of the embryo and zipping forward and posteriorly.

04:03 The embryo also begins to fold into its mature permutation, so we get a body wall and we also have folding in the sagittal plane bringing the head anteriorly and also tucking the tail segment anteriorly as well.

04:18 The heart begins to beat on day 22.

04:21 Day 23, we are now quite a bit larger, point 1.0 to 1.5 mm in length as the elongation of the developing embryo has occurred relatively rapidly.

04:34 The neural tube is closing between the cranial and caudal neuropores so it's all the way shot except to the very top and the very bottom of the neural tube.

04:44 The otic placodes that will form the inner ear have formed and they are starting to move deeper and deeper into the mesenchyme of the head and the respiratory diverticulum has begun to bud off of the gut tube.

04:56 Day 24. 1.0 to 1.5 mm from crown to rump, the heart is bulging into the pericardial cavity not only is it beating, it's starting to expand outward into the space that surrounds the heart.

05:11 Pharyngeal arches 1 and 2 are forming, and will begin to become visible during this week.

05:16 The aortic arches within those pharyngeal arches are also forming, carrying blood from the beating heart to the dorsal aorta.

05:23 And the upper limb bud begins forming at this time.

05:27 Day 24, another important hallmark of this time is that the Rostral neuropore or the cranial neuropore is closing, and at that point, the neural tube defects are gonna become less pronounced as it closes, and later the caudal neural pore closes.

05:44 Day 25. We're elongating a bit more, 1.5 to 2.5 mm in length.

05:51 We've already got pharyngeal arches one and two, so we add, pharyngeal arch 3 and now we have the mesonephros, the very early embryonic kidney forming along with its duct, the mesonephric duct.

06:04 Day 26, we're moving a little bit further along, 1.5 to 2.5 mm in length, and the gastrointestal glands, Such as the liver, the pancreas, the gall bladder, and a little bit further back, the spleen, are budding off of the endoderm of the gut tube.

06:23 The upper limb bud is not only present but visible at this time.

06:28 So maybe it begun forming on day 24, but it's definitely visible by day 26.

06:33 By day 27, still about 1.5 to 2.5 mm in length, the otic vesicles have moved fully into the mesenchyme of the head and the optic vesicle is starting to extend off the diencephalon to enduce formation of the lens and create the eye.

06:51 By day 28, grown a bit more, 2.0 to 3.5 mm in length, at this time the caudal neuropore closes.

06:59 If the neuropores have closed, we no longer have to worry too much about spina bifida.

07:04 The lower limb bud begins forming at this state.

07:07 roughly four days after the upper limb bud began forming, and the lower limb will always lag, just a bit, behind formation of the upper limb.

07:16 The bronchial buds have now extended off the respiratory diverticulum, so we have our first division of the respiratory system into a finer level of detail.

07:26 Body folding is more or less complete by day 28, and the septum primum, and muscular ventricular septum begin to form.

07:35 So from a single, linear pumping, the heart is now beginning to separate into its four chambers that will create two separate circuits.

07:44 The pulmonary and the systemic.

07:46 By day 28, we also have the ureteric bud growing off of the mesonephric duct and inducing formation of the final adult kidney, the metanephros.