A zygote (first cell of a future offspring) resulting after fertilization undergoes mitosis, giving rise to two new cells known as blastomeres in turn divides into four cells which also divides into eight cells and so on. The divisions occur rapidly, with little time for growth. This phase of early rapid cell division is known as cleavage. During cleavage (cell division), the tiny mass of cells moves through the uterine tube to the uterine cavity. This movement takes about three days, and by then the structure consist of a solid ball of about sixteen cells. These cells looks like a mul berry called morulla, at this stage the morulla is still surrounded by the zona pellucida. The morulla remains free within the uterine cavity for about three days, during this time the zona pellucida of the original egg cell then degenerate. After this, the morulla forms a blastocyst, which superficially implants in the endometrium. The development of embryo is divided into two stages- the embryonic stage and fetal stage.
- EMBRYONIC STAGE OR DEVELOPMENT
About the time of implantation, certain cells on the inner face of the blastocyst organize into a group called the inner cell mass, which then give rise to the offspring. This offspring is termed an embryo until the end of eight weeks, then from the eight week until birth, the offspring is then called fetus. As the blastocyst develops further, it gives rise to the tissues, organs of the embryo and numbers of structure that support the embryo and help to acquire nutrition. On the sixteenth day of embryonic development, the embryo acquire the form of embryonic disc with two distinct layers- an outer ectoderm and an inner endoderm. A short period of time later the ectoderm and endoderm folds to form the third layer called mesoderm in between. All the organs are formed from the 3 germ layers. The tissues of the body are derived from the three embryonic germ layers.
THE DERIVATIVES OF ECTODERM
The following are derived from ectoderm: skin, hair, nail, the enamel of the teeth, and the lens of the eye (musculature of the eyeris). nervous system including the neurons, neuroglia (except microglia), swan cell, pia arachnoid, the lips, cheeks, parts of the floor of the mouth, the plalet and the nasal cavity, sweat glands, parotic, mammary and lacrimal gland,hypophysis or pituitary and adrenal glands.
DERIVATIVES OF ENDODERM
The epithelia lining of some parts of the mouth, platelet, tongue, esophagus, stomach, small and large intestine, the upper part of the anal canal, the epithelium of the respiratory tracts, endocrine glands like the thyroid gland, parathyroid, thymus, and islet of Langerhans, exocrine glands like the liver, pancreas, and glands in the GIT.
DERIVATIVES OF MESODERM
All the connective tissues of the body including the superficial and deep facial, ligament, tendon, aponeurosis and the dermis of the skin, the dentine of the teeth, all muscles (smooth, cardiac, skeletal muscles except musculature of the iris), the heart and all blood vessels, the lymphatic, urinary bladder and part of the prostate gland.
A connecting stalk attaches the embryonic disc to the developing placenta. The cell surrounding the embryo and cells of the endometrium form the placenta. The placenta attaches the embryo to the uterine wall and exchanges nutrients, gasses and waste between maternal blood and the embryo's blood. Placenta is divided into two portions- the embryonic portion and the maternal portion. The embryonic portion consist of the chorion and its villi, while the maternal portion is the area within the uterine wall where the villi is attached. A thin membrane separates embryonic blood with the capillary of a chorionic villus from maternal blood in a lacuna. Across the membrane, maternal and embryonic blood exchanges substance. Oxygen and nutrients diffuse from the maternal blood into the embryo's blood, while carbon dioxide and other waste diffuse from the embryo's blood into the maternal blood.
During the second week of development, the placenta forms another membrane called amnion, which develops around the embryo, its margin attaches around the edge of the embryonic disc. As the embryo becomes cylindrical, the amnion margins folds enclosing the embryo in the amnion and amniotic fluid. The amnion envelops the tissues on the underside of the embryo by which the embryo attaches to the chorion and the developing placenta, in this manner the umbilical cord forms. The umbilical cord contains three blood vessels namely- two umbilical arteries and one umbilical vein that transport blood between the embryo and the placenta. the umbilical cord suspends the embryo in the , the amniotic fluid filling the amniotic cavity allows the embryo to grow freely without compression from surrounding tissues and also protects the embryo from jarring movements of the woman's body. Other two embryonic membranes are also formed during embryonic development; they are the yolk sac and the illantosis.
* The yolk sac is formed during the second week of development, it attaches to the underside of the embryonic disc. The yolk sac forms blood cells, give rise to cells that later become sex cells.
* The illantosis is formed during the third week as a tube extending from the yolk sac into the connecting stalk of the embryo. It also form blood cells and give rise to umbilical veins and arteries.
As the embryo implants in the uterus, proteolytic enzymes from the trophoblast breakdown endometrial tissues, providing nutrients for the developing embryo. A space appears between the ectoderm and the trophoblast, which forms the amniotic cavity filled with amniotic fluid. The roof of this cavity is formed by amniogenic cells derive from trophoblast, while its floor is formed by the ectoderm. A second layer begins to line the trophoblast together forming a structure called the chorion. soon slender projection grow out from the trophoblast including new cell layer, eroding their way into the surrounding endometrium by continuing to secrete proteolytic enzyme. These projections become increasingly intricate and form the highly branched chorion villi. As the chorionic villi develops at the end of the fourth week, embryonic blood vessels appear within them and are continuous with those passing through the connecting stalk to the body of the embryo. At the same time, a lacuna- between the villi is filled with maternal blood that escaped from eroded endometrial blood vessels.
At the fourth week of development, the head and jaws develop the heartbeats and forces blood through the blood vessels, and tiny buds, which gives rise to the upper and lower limbs also forms. Through the fifth- seventh week, the head grows rapidly and becomes rounded and erect. The face with developing eye, nose, and mouth becomes more humanlike, the upper and lower limbs elongate and the finger and toes also appear. By the end of the seventh week, all the main internal organs are now present.
- FETAL STAGE
The fetal stage begins starting from the end of the eight week until birth. At this stage, growth is rapid, body proportion changes considerably. At the beginning of this stage, the head of the fetus is disproportionately large, and the lower limbs are short. Gradually the proportion becomes more like those of a child. At the third month, body lengthening accelerates but the head grows slowly, the upper limbs reaches the relative length they will maintain throughout development. By the twelfth week, the external reproductive organs are distinguished as either male or female.
In the fourth month, the body grows rapidly and reaches a length of up to 20 centimeters. In this month, the fetus can turn away from a light-flashed on the pregnant woman's belly. At the fifth month growth slows, the lower limbs reaches their final relative proportions. The skeletal muscles contracts and at this time the woman feels the movement of the fetus in her womb. Hair appears on the head of the fetus and a cheesy mixture of dead epidermal cells and sebum from the sebaceous glands covers the skin.
At the sixth month the eyebrow and eyelashes appears, the fetus gains a reddish appearance due to blood vessels in the skin. The fetus also gains weight at this time. In the seventh month, the eyelids, which fused during the third month reopens.in this same month fat is being deposited in the subcutaneous tissues thereby, smoothing the skin. At the eight month, fetal brain cells rapidly form network as organs specializes and grow. A layer of fat is laid down beneath the skin. In male the testes descends from regions near the developing kidney through the inguinal canal and into the scrotum. Digestive and the respiratory system are the last to mature which makes it difficult for premature infants to digest milk and breathe.
At the end of the ninth month (average of 266 days), the fetus is full-termed. This time the finger and toes have well-developed nails, the skull bones are largely ossified, and the fetus is now positioned with its head toward the cervix and ready to be born.