Human Embryo Genesis

We can use the visual embryo site to quickly sketch out the structure of the embryogenesis process for humans. In other words, we can surmise the distinct processes by noting what and when different tissues/organs begin to develop. The initial phases are:

Stage 1: Fertilization: 24 hours after the fusion of the egg & the sperm nuclei the zygote is created. This is the first cell of the new embryo.
Stage 2: Cleavage: within the next three days the zygote undergoes a series of cell divisions. A division occurs approximately every 20 hours. After about 16 cells have been generated the zygote becomes a morula, that is mulberry shaped.
Stage 3: Early Blastocyst: In about another day a cavity (blastocele) forms in the center of the morula. The cells inside the cavity flatten & compact. The resulting structure is called a blastocyst.
Stage 4: Implantation: In about another day the blastocyst travels towards the uterus and implants itself in its wall.
Stage 5: Two weeks post ovulation: Placental circulation system begins: in the next week or so the cell mass divides rapidly forming a 2 layer disc. The top layer will become the embryo and the amniotic cavity. The bottom layer will become the yolk sac.
Stage 6: 14 days post ovulation: Gastrulation: Blood system appears in the placenta area. The yolk sac begins to produce blood cells with no nuclei. A narrow streak of cells appears on the embryonic disc.
Stage 7: Three weeks post ovulation: Neurulation & Notochordal Process: Two kinds of cells are generated from the streak changing the two layered disc into a three layered disc: ectoderm, mesoderm & endoderm. The endoderm will form: linings of lungs, tongue, tonsils, urethra, bladder & digestive tract. The mesoderm will form: muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, genital & excretory systems. The ectoderm will form: the skin, nails, hair, lens of eye, lining of internal & external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands & the entire nervous system.
Stage 8: 23 days post ovulation: embryo pear shaped, head side broader, the ectoderm forms the neural plate and the neural groove which is the precursor of the nervous system. Embryo blood cells begin to form. They in turn, form blood channels.
Stage 9: 25 days post ovulation: Somites appear. Somites, condensation of mesoderm, appear on either side of the neural groove. They appear from the tail side & moving towards the middle. One to three pairs of somites should be present at this point. Every ridge, bump and recess indicates cell differentiation.
Stage 10: 4 weeks post ovulation: Neural folds & heart fold fuse. Between four and twelve pairs of somites appear by this stage. Eye precursor cells appear just off the neural folds, same is true for ear cells. Neural tube is being zipped close. A single endocardial tube forms and becomes S-shaped. Cardiac muscle contraction begins.
Stage 11: 30 days post ovulation: 13-20 pairs of somites, head side neural tube closes, optical vesicles appear, two pharyngeal arches appear. Primitive S-shaped heart is beating, peristalsis propelling fluids through the body begins. Central nervous system appears to be most developed system. If 20 pairs somites are present the forebrain is completely closed.
Stage 12: 32 days post ovulation: 21-29 pairs of somites. Tail side neural tube closes, three to four pharyngeal arches appear, upper limb buds appear. Arches for face & neck becoming evident under the enlarging forebrain, once neural tube is closed eye & ear will begin to form. At this point the brain & spinal cord are the largest & most compact tissue. Valves and divisions may appear in the heart. Digestive layer begins to differentiate into buds for liver, lung, stomach & pancreas. The liver forms before the rest of digestive system.
Stage 13: 34 days post ovulation: Four limb buds, lens disc & optic vesicle. thin skin layer covering embryo. 30-40 somites pairs.
Head & neck region: brain differentiates in fore, mid & hind brain. Forebrain lobes for sensory inputs. Will implement: memory, thinking, problem solving. Midbrain relay station for messages. Hindbrain controls: heart, breathing, muscles. Thyroid continues to develop. Lymphatic system starts to form. Ear placode starts to invaginate to form ear. Retinal disc presses outwards and touches ectoderm which starts forming the lens disc. Retina, its pigment & optic stalk are identifiable. Primitive mouth & tongue can be recognized.
Thorax: Heart chambers are filled with plasma & blood. Heart & liver equal the head volume. Blood circulation is well established. Lung buds continue to form.
Abdomen & pelvic regions: Gall bladder, stomach, intestines, pancreas continue to form.
Limbs: Upper limb buds visible, lower limb buds begin to develop. Embryo is 3-dimensional & completely enclosed in the amniotic sac. The somites will be involved in building bones & muscles.
Stage 14: 5 weeks post ovulation: lens pit and optic cup appear.
Head & neck: brain & head grow rapidly. Spinal wall differentiates into: ventricular, mantle & marginal zones. Ventricular will form neurons, glial & ependymal cells. Mantle will form neuron clusters. Marginal will contain neurons chains (processes). Pouch for the pituitary is defined. Lens vesicle opens to surface & nestles in optic cup. Otic vesicle (ear) grows 25%. Nasal plate detected in thickened ectoderm.
Thorax: Esophagus form from tissue groove that separates from trachea. Four major heart divisions visible. Heart valves begin to form. Lung sacs lie on either side of esophagus.
Abdomen & pelvic regions: ureteric bud appears, interim kidney appears.
Limbs: Upper limbs become cylindrically shaped buds tapering at tip to eventually form hand plate. Innervation of the upper limbs begins.
Stage 15: 38 days post ovulation: Lens vesicle, nasal pit, hand plate, trunk widens, future cerebral hemispheres differentiate.
Head & neck: brain 33% larger than previous stage & still larger than trunk. Head side of neuropore closed now. Four pairs of pharyngeal arches visible. Maxillary & mandibular (jaws forerunners) prominences of 1st arch are clearly visible. The stomoderm, ectoderm depression, forerunner of mouth & oral cavity appears between the forebrain & the fused mandibular prominence. Swelling of external ears forms. Lens pit has closed, retinal pigment may appear in external layer of optic cup. Lens body forms. Two symmetrical nasal pits may appear as depressions in nasal disc.
Thorax: Esophagus lengthens, Blood flow through the heart is divided into left & right streams. Lobar buds appear in bronchial tree.
Abdomen & pelvic regions: Intestines lengthens, ureteric bud lengthens & tip expands beginning to form permanent kidneys.
Limbs: Distinct regions: handplate, forearm, arm & shoulder can be seen in upper limb bud. Lower limb bud begins to round at top, tip of its tapering end will form foot. Innervation of lower limb buds begins.
Spine: The relative width of trunk increases because of growth of spinal ganglia, muscular plate & mesenchymal tissues.
Stage 16: 6 weeks post ovulation:
Head & neck: brain well differentiated in hemispheres. Hindbrain begins to develop. Future lower jaw is visible. Future upper jaw is present but not demarcated. Mesenchymal cells from the primitive streak & the prechordal plate continue to form skull & face. External retina pigment visible, lens pit grown to a D shape. Nasal pits still separate plates but rotate face down as head widens.
Thorax: primary cardiac tube separates into aortic & pulmonary channels. Mammary gland tissue begins to mature.
Abdomen and pelvic regions: mesentery which attaches intestines to rear abdominal wall now clearly defined. Ureter for urine from kidney to bladder continues to lengthen. Gonadal primordium begins.
Limbs: Hand region differentiates into carpal part & digital plate. Lower limb buds differentiate in: thigh, leg & foot areas.
Stage 17: 45 days post ovulation: four chambered heart & sense of smell.
Head and neck: Jaw & facial muscles developing. Nasofrontal groove is distinct & olfactory bulb develops in the brain. Ear hillocks become recognizable. Teeth buds begin to form. Pituitary begins to form. Trachea, larynx & bronchi begin to form.
Thorax: Heart begins to separate into four chambers. The diaphragm forms.
Abdomen: Intestines begin development in umbilical cord will later migrate into embryo.
Pelvis: primitive germs cells arrive in genital area to develop male or female genitals.
Limbs: digital rays appear in foot plate. Finger rays are more distinct.
Spine: Trunk becomes straighter.
Stage 18: 7 weeks post ovulation: Ossification of skeleton starts.
Head and neck: nerve begin to develop in scalp, eyes are pigmented & eyelids begin to develop.
Thorax: pulmonary artery separates from aorta trunk within heart. Nipples appear on chest. Body more cubelike.
Abdomen: kidney begin to produce urine.
Pelvis: Genital tubercle, urogenital & anal membranes appear.
Limbs: arms at proper location and proportion. Hand plate not finished. Wrist clearly visible. Hands show future separation of fingers and thumbs.
Spine and skeleton: ossification of skeleton begins.
Stage 19: 52 post ovulation days: brain waves and muscles.
Head and neck: brain has first detectable brain waves. Head more erect. Semicircular canals form in inner ear for sense of equilibrium.
Thorax: in heart the primum & intermedium septa fuse.
Pelvis: gonads form. Sex will be recognizable in about a week.
Limbs: indentations indicate knee & ankle locations. Legs at proper location & proportion. Toes almost completely notched & toenails begin to appear. Joints more distinct.
Spine, skeleton and muscles: trunk elongates & straightens. Bone cartilage begins to solidify. Muscles get stronger.
Stage 20: 8 weeks post ovulation: Spontaneous involuntary movements.
Head and neck: brain connected to muscles & nerves, spontaneous movements are possible. Scalp plexus is present. Nasal openings & nose's tip fully formed.
Limbs: upper limbs become longer, bend at elbows & extend forward. Foot plate's skin folds down between future toes which are distinguishable.
Pelvis: anal membrane perforated, urogenital membrane differentiates in male/female embryos. Testes & ovaries are distinguishable.
Stage 21: 58 days post ovulation: Intestines begin to recede in body cavity.
Head and neck: eyes well developed but on side of head. External ears are set low on head. Tongue development will complete in next few days.
Abdomen: Intestine begin to migrate within umbilical cord towards body. Liver causes a ventral prominence.
Limbs: fingers lengthens & distinct grooves form between them. Feet approach each other but still fan shaped & toe digits webbed.
Stage 22: 60 days post ovulation: Heart development ends.
Head and neck: fissures characteristic of humans appear. Eyelids and external ear more developed. Upper lip fully formed. Brain can move muscles.
Thorax: critical period of heart development ends.
Pelvis: in females clitoris begins to form. In males penis develops from the same tissue.
Limbs: ossification centers appear in long bones for replacement of cartilage with bone. This begins, usually, in upper limbs. Digits of fingers fully separate. Feet lengthen and become more defined.
Spine, skeleton and muscles: stubby tail still present but much smaller.
Stage 23: 9 weeks post ovulation: Essential external & internal structures complete.
Head: head is erect & rounded. External ear fully developed.Eyes are closed, retina is fully pigmented, eyelids begin to unite & are half closed. Taste buds appear on tongue. Primary teeth are at cap stage. Palate bones begin to fuse. Scalp plexus reach top of head.
Limbs: Upper & lower limbs well formed. Toes no longer webbed. All digits are separate.
Spine, skeleton, muscles and skin: layer of flattened cells, precursor of skin's surface layer, replaces the thin ectoderm. Tail has disappeared

At this point the basic architecture of the embryo is in place, after about nine weeks from fertilization, the embryo is only 23-26 mm long, that is about one inch! For our purpose this is the most interesting portion of the 42 weeks gestation since it covers most of the important cell differentiation processes.