Life Cycle of an embroyo - From the start

Early Hours
Fertilization begins when a sperm penetrates an oocyte (an egg) and it ends with the creation of the zygote. The fertilization process takes about 24 hours. A sperm can survive for up to 48 hours. It takes about ten hours to navigate the female productive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the upper part of the vagina, only 1%, 3 million, enter the uterus.

At 6 hours
The sperm must make the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced. Penetration of the zona pellucida takes about twenty minutes.

At 1 - 5 days
Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization. The zygote now begins to cleave, with each division occurring into two cells called blastomeres. The zygote's first cell division begins a series of divisions, with each division occurring approximately every twenty hours. Each blastomere within the zona pellucida becomes smaller and smaller with each subsequent division. When cell division ungenerated about sixteen cells, the zygote becomes a morula (mulberry shaped). It leaves the fallopian tube and enters the uterine cavity three to four days after fertilization.

At 6 days
The blastocyst hatches from the zona pellucida around the sixth day after fertilization, as the blastocyst enters the uterus. The trophoblast cells secretes an enzyme which erodes the epithelial uterine lining and creates an implantation site for the blastocyst.In a cyclical process of hormonal stimulation, the ovary is induced to continue producing progesterone while human chorionic gonadotropin (hCG) is released by the trophoblast cells of the implanting blastocyst. Endometrial glands in the uterus enlarge in response to the blastocyst and the implantation site becomes swollen with new capillaries. Circulation begins - a process needed for the continuation of pregnancy.

At 12 days
Trophoblast cells engulf and destroy cells of the uterine lining creating blood pools, both stimulating new capillaries to grow and foretelling the growth of the placenta. The inner cell mass divides, rapidly forming a two-layered disc. The top layer of cells will become the embryo and amniotic cavity, while the lower cells will become the yolk sac. Ectopic pregnancies can occur at this time and sometimes continue for up to 16 weeks of pregnancy before being noticed. Diagnosed quickly, ectopic pregnancies can be treated pharmacologically without surgery, reducing danger to the mother, and preserving the site of the ectopic pregnancy.

At 15 - 18 days
The formation of blood and blood vessels of the embryo begins. The blood system appears first in the area of the placenta surrounding the embryo, while the yolk sac begins to produce hematopoietic or non-nucleated blood cells. The embryo is attached by a connecting stalk (which will later become part of the umbilical cord), to the developing placenta. A narrow line of cells appears on the surface of the embryonic disc. This primitive streak is the future axis of the embryo and it marks the beginning of gastrulation, a process that gives rise to all three layers of the embryo: ectoderm, mesoderm and endoderm.

At 19 - days
The embryonic area is now shaped like a pear, and the head region is broader than the tail end. The ectoderm has thickened to form the neural plate. The edges of this plate rise and form a concave area known as the neural groove. This groove is the precursor of the embryo's nervous system and it is one of the first organs to develop. The blood cells of the embryo are already developed and they begin to form channels along the epithelial cells which form consecutively with the blood cells

at 21 days
If you could look at the embryo from a top view, it would resemble the sole of a shoe with the head end wider than the tail end, and a slightly narrowed middle. Somites, which are condensations composed of mesoderm, appear on either side of the neural groove. The first pair of somites appear at the tail and progress to the middle. One to three pairs of somites are present. Every ridge, bump and recess now indicates cellular differentiation. A head fold rises on either side of the primitive streak. The primitive streak now runs between one-fourth to one-third of the length of the embryo. Secondary blood vessels now appear in the chorion/placenta. Hematopoietic cells appear on the yolk sac simultaneously with endothelial cells that will form blood vessels for the newly emerging blood cells. Endocardial (muscle) cells begin to fuse and form into the early embryo's two heart tubes.

at 23 - 26 days
Rapid growth and change as the embryo becomes longer and the yolk sac expands. On each side of the neural tube, between four and twelve pairs of somites can exist by the end of this stage. The cells which become the eyes appear as thickened circles just off of the neural folds. The cells of the ears are also present. Neural folds are rising and fusing at several points along the length of the neural tube concomitant with the budding somites which appear to zipper the neural tube closed. Neural crest cells will eventually contribute to the skull and face of the embryo. The two endocardial tubes formed in before fuse here to form one single tube derived from the roof of the nueral tube, which becomes S-shaped and makes the primitive heart asymmetric. As the S-shape forms, cardiac muscle contraction begins.

at 28 - 31 days
Thirteen to twenty pairs of somites are present and the embryo is shaped in a modified S curve. The embryo has a bulb-like tail and a connecting stalk to the developing placenta. A primitive S-shaped tubal heart is beating and peristalsis, the rhythmic flow propelling fluids throughout the body, begins. However, this is not true circulation because blood vesel development is still incomplete. At this stage, the neural tube determines the form of the embryo. Although the primary blood vessels along the central nervous system are connecting, the central nervous system appears to be the most developed system. If twenty somites are present in the embryo, the forebrain is completely closed.


at 32 days
The brain differentiates into the three main parts: the forebrain, midbrain and hindbrain. The forebrain consists of lobes that translate input from the senses, and will be responsible for memory formation, thinking, reasoning, problem solving. The midbrain will serve as a relay station, coordinating messages to their final destination. The hindbrain will be responsible for regulating the heart, breathing and muscle movements.Thyroid continues to develop and the lymphatic system, which filters out bacteria, starts to form. Otic placode invaginates and forms the otic vesicle, which will develop into the structures needed for hearing and maintenance of equilibrium. Retinal disc presses outward and touches the surface ectoderm. In response the ectoderm proliferates forming the lens disc. Specific parts of the eye, such as the retina, the future pigment of the retina and the optic stalk are identifiable. Primitive mouth with a tongue is recognizable. Thyroid continues to develop and the lymphatic system, which filters out bacteria, starts to form. Heart chambers are filled with plasma and blood cells making the heart seem distended and prominent. The heart and liver combined are equal in volume to the head by this stage. Blood circulation is well established, though true valves are not yet present.. The villous network is in place to accommodate the exchange of blood between the woman and the embryo. Aortic arches 4 and 6 develop and 5 may appear. Lung buds continue to form. Gall bladder, stomach, intestines, pancreas continue to form and the metanephric bud appears in the chest cavity. The stomach is in the shape of a spindle and the pancreas may be detected at the intestinal tube. The developing liver receives blood from the placenta via the umbilical cord. The amnion encloses the connecting stalk helping to fuse it with the longer and more slender umbilical vesicle (the remnant of the yolk sac). Upper limb buds are visible as ridges and the lower limb buds begin to develop. Folding is complete and the embryo is now three-dimensional and is completely enclosed in the amniotic sac. The somites will be involved in building bones and muscles. The first thin surface layer of skin appears covering the embryo.

at 35 days
The brain and head grow rapidly. The mandibular and hyoid arches are noticeable. Ridges demarcate the three sections of the brain (midbrain, forebrain and hindbrain). The spinal cord wall at this stage contains three zones: the ventricular, the mantle and the marginal. The ventricular zone will form neurons, glial cells and ependymal cells, the intermediate mantle will form neuron clusters and the marginal zone will contain processes of neurons. Adenohypophyseal pouch, which will develop into the anterior pituitary, is defined. Lens vesicle opens to the surface and is nestled within the otptic cup. Otic vesicle increases its size by approximately one-fourth and its endolymphatic appendage is more defined. Nasal plate can be detected by thickened ectoderm. Esophagus, the tube through which food is swallowed, forms from a groove of tissue that separates from the trachea, which is also visible. Semilunar valves begin to form in the heart. Four major subdivisions of the heart (the trabeculated left and right ventricles, the conus cords and the truncus arteriosus) are clearly defined. Two sprouts, a ventral one from the aortic sac and a dorsal one from the aorta, form the pulmonary (sixth aortic) arch. Right and left lung sacs lie on either side of the esophagus. Ureteric bud appear. Metanephros, which will eventually form the permanent kidney, is developing. Upper limbs elongate into cylindrically-shaped buds, tapering at tip to eventually form hand plate. Nerve distribution process, innervation, begins in the upper limbs.

at 42 days
s the brain has increased in size by one-third since the last stage, it is still larger than the trunk. Rostral neuropore is closed and four pairs of pharyngeal arches are visible now, though the fourth one is still quite small. The maxillary and mandibular prominence of the first arch are clearly delineated. The stomodeum, the depression in the ectoderm which will develop into the mouth and oral cavity, appears between the prominent forebrain and the fused mandibular prominence.Swellings of the external ear begin to appear on both sides of the head, formed by the mandibular arch. Lens pit has closed, retinal pigment may appear in the external layer of the optic cup and lens fibers form the lens body. Two symmetrical and separate nasal pits may appear as depressions in the nasal disc. Esophagus lengthens. Blood flow through the atrioventricular canal is divided into left and right streams, which continue through the outflow tract and aortic sac. The left ventricle is larger than the right and has a thicker wall. Lobar buds appear in the bronchial tree. The intestine lengthens. Ureteric bud lengthens and its tip expands, thus beginning the formation of the final and permanent set of kidneys. Distinct regions of the handplate, forearm, arm and shoulder may be discerned in the upper limb bud. Lower limb bud begins to round at top and tip of its tapering end will eventually form the foot. Innervation, the distribution of nerves, begins in the lower limb buds. The relative width of the trunk increases from the growth of the spinal ganglia, the muscular plate and the corresponding mesenchymal tissues.

at 44 days
Brain is well marked by its cerebral hemispheres. The hindbrain, which is responsible for heart regulation, breathing and muscle movements, begins to develop. Future lower jaw, the first part of face to be established, is now visible while future upper jaw is present, but not demarcated. Mesenchymal cells originating in the primitive streak, the neural crest and the prechordal plate, continue to form the skull and the face. External retina pigment is visible and the lens pit has grown into a D shape. Nasal pits are still two separate plates, but they rotate to face ventrally as head widens. Primary cardiac tube separates into aortic and pulmonary channels and the ventricular pouches deepen and enlarge, forming a common wall with their myocardial shells. Mammary gland tissue begins to mature. The mesentery, which attaches the intestines to the rear abdominal wall, holds them in position and supplies them with blood, nerves and lymphatics, is now clearly defined. Ureter, the tube that will convey urine from the kidney to the bladder, continues to lengthen. Proliferation of the coelomic epithelium indicates the gonadal primordium. Hand region of upper limb bud differentiates further to form a central carpal part and a digital plate. The thigh (rostrolateral part), leg (the caudomedial part) and foot areas can be distinguished in the lower limb buds.

at 46 days
Jaw and facial muscles are now developing. The nasofrontal groove becomes distinct and an olfactory bulb (sense of smell) forms in the brain. Auricular (ear) hillocks become recognizable. The dental laminae or teeth buds begin to form. The pituitary, which is the master gland responsible for growth of hormones that regulate other glands, such as the thyroid, adrenal glands, gonad) begins to form.Trachea, the larynx and the bronchi begin to form. The heart begins to separate into four chambers. The diaphragm, the tissue that separates the chest cavity from the abdomen, forms. Intestines begin to develop within the umbilical cord and will later migrate into the abdomen when the embryo's body is large enough to accommodate them. Primitive germ cells arrive at the genital area and will respond to genetic instructions to develop into either female or male genitals. Digital rays in appear in the foot plates and finger rays are more distinct. Trunk becomes straighter.

at 51 days
Nerve plexuses begin to develop in the region of the scalp. Eyes are pigmented and eyelids begin to develop and may fold. Within the heart, the trunk of the pulmonary artery separates from the trunk of the aorta. Nipples appear on the chest. Body appears more like a cube. Kidneys begin to produce urine for the first time. Genital tubercle, urogenital membrane and anal membrane appear. The critical period of arm development ends, and the arms are at their proper location, roughly proportional to the embryo. However, the hand plates are not finished, but develop further in the next two days. The wrist is clearly visible and the hands already have ridges or notches indicating the future separation of the fingers and the thumbs. Ossification of the skeleton begins.