Process Of A Developing Embryo And Fetus example essay topic

Development of the Human Zygote November 16, 1995 Hundreds of thousands of times a year a single-celled zygote, smaller than a grain of sand, transforms into an amazingly complex network of cells, a newborn infant. Through cellular differentiation and growth, this process is completed with precision time and time again, but very rarely a mistake in the " blueprint' of growth and development does occur. Following is a description of how the pathways of this intricate web are followed and the mistakes which happen when they are not. The impressive process of differentiation changes a single-cell into a complicated system of cells as distinct as bold and bone. Although embryonic development takes approximately nine months, the greatest amount of cellular differentiation takes place during the first eight weeks of pregnancy. This period is called embryo genesis.

During the first week after fertilization, which takes place in the Fallopian tube, the embryo starts to cleave once every twenty-four hours (Fig. 1). Until the eight or sixteen cell stage, the individual cells, or blastomeres, are thought to have the potential to form any part of the fetus (Lease, Conaghan, Martin, and Hardy, April 1993). As the blastomeres continue to divide, a solid ball of cells develops to form the morula (Fig. 1). The accumulation of fluid inside the morula, transforms it into a hollow sphere called a blastula, which implants itself into the inner lining of the uterus, the endometrium (Fig. 1). The inner mass of the blastula will produce the embryo, while the outer layer of cells will form the trophoblast, which eventually will provide nourishment to the ovum (Pritchard, MacDonald, and Gant, 1985). Figure 1: Implantation process and development (Pritchard, MacDonald and Gant, 1985) During the second week of development, gastrulation, the process by which the germ layers are formed, begins to occur.

The inner cell mass, now called the embryonic disc, differentiates into a thick plate of ectoderm and an underlying layer of endoderm. This cellular multiplication in the embryonic disc marks the beginning of a thickening in the midline that is called the primitive streak. Cells spread out laterally from the primitive streak between the ectoderm and the endoderm to form the mesoderm. These three germ layers, which are the origins of many structures as shown in Table 1, begin to develop. Table 1: Normal Germ Layer Origin of Structures in Some or all Vertebrates (Harrison, 1969) Normal Germ Layer Origin of Structures in Some or All Vertebrates Ectoderm Mesoderm Endoderm Skin epidermis Hair Feathers Scales Beaks Nails Claws Sebaceous, sweat, and mammary glands Oral and anal lining tooth enamel Nasal epithelium Lens of the eye Inner earBrainSpinal cord Retina and other eye parts Nerve cells andgangliaPigment cells Canal of external ear medulla of the adrenal glandPituitarygland Dermis of the skin Connective tissueMusclesSkeletal componentsOutercoverings of the eye Cardiovascular system Heart Blood cells BloodvesselsKidneys and excretory ducts Gonads and reproductive ducts Cortex of the adrenal glandSpleenLining of coelo mic cavities Mesenteries LiverGallbladderPancreasThyroid gland Thymus gland Parathyroid glands Palatine tonsilsMiddleearEustachian tube Urinary bladder Primordial germ cells Lining of all organs of digestive tract and respiratory tract During the third week of development, the cephalic (head) and caudal (tail) end of the embryo become distinguishable.

Most of the substance of the early embryo will enter into the formation of the head. Blood vessels begin to develop in the mesoderm and a primitive heart may also be observed (Harrison, 1969). Cells rapidly spread away from the primitive streak to eventually form the neural groove, which will form a tube to the gut. When the neural folds develop on either side of the groove, the underlying mesoderm forms segmentally arranged blocks of mesoderm called somite. These give rise to the dermis of the skin, most skeletal muscles, and precursors of vertebral bodies. the otocyst, which later becomes the inner ear, and the lens pla codes, which later form the lenses of the adult eyes, are derived from the ectoderm. The strand of cardiovascular functioning is apparent during the fourth week.

The heart shows early signs of different chambers and begins to pump blood through the embryo which simultaneously has well developed its kidneys, thyroid gland, stomach, pancreas, lungs, esophagus, gall bladder, larynx, nd trachea (Carlson, 1981). Several new structures are observed, organs continue developing, and some previously formed structures reorganize during the fifth week. The cranial and spinal nerves begin to form and the cerebral hemispheres and the cerebellum are visible. The spleen, parathyroid glands, thymus gland, retina, and gonads, all new structures, also begin to form. Thegastrointestimer tract undergoes considerable development as the middle part of the primitive intestine becomes a loop larger than the abdominal cavity.

Next, it must then project into the umbilical cord until there is room for the entire bowel. Finally, the heart develops walls or atrial and ventricular septa cushion. These cushions thicken the junction of the atrium and ventricle. the atrial and ventricular septa meanwhile divide their respective chambers into right and left halves (Harrison, 1969). The sixth week is characterized by the completion of most organ formation. The embryo has a more identifiable human face with basic structure of the eyes and ears now developed.

Hard and soft palates appear, the salivary glands begin to form, and there is an early differentiation of the cells that later develop into the teeth. Division of the heart is essentially completed and the valves begin to form. The primitive intestinal tract is divided intothe anterior and posterior chambers that will later develop into the urinary bladder and the rectum, respectively. At the end of the week, the gonads are histologically recognizable as either testes or ovaries (Pritchard, MacDonald, and Gant, 1985). The embryo looks similar to miniature human when it enters the seventh week of embryo genesis. During this last week, the pituitary gland takes a definitive structure, the eyelids become visible, the last group of muscles begin to form, and bone marrow appears for the first time. the main concerns of this period are the different developments taking place in the male and female.

This is first shown as the M" ducts degenerate in males, but continues to develop in females, where they will later differentiate to become the Fallopian tubes, the uterus and the inner part of the vagina. The Wolff ian ducts degenerate in female embryos, but continue to develop into the duct us deferens in the male. Although the external genitalia continue to grow and develop, the yare still unable to be visibly identified as male or female. By the end of this week the placenta begins to take on definite characteristics, and for the first time blood from the maternal circulation enters the placental circulation (Carlson, 1981). After this period of embryo genesis the embryo is given the name fetus. The remainder of pregnancy is primarily concerned with growth and cellular differentiation, but during this period of growth, mistakes which can cause birth defects are still highly effective, as they were in the first seven weeks of development.

What are some of these defects which begin during the first trimester of pregnancy and how are they caused? Obviously the process of a developing embryo and fetus is very complicated and although most of the babies born each year are free from any abnormalities, up to five percent of all newborn infants have congenital anomalies, birth defects (Cunningham, MacDonald, and Gant, July / August 1989). Seventy percent of birth defects are unknown spontaneous errors of development. Of the thirty percent which are known, twenty-five percent are associated with genetic factors that include major chromosomal defect and point mutations, three percent with venereal diseases such as syphilis and rubella, and two percent with teratogens, medications and drugs (Cunningham, MacDonald, an.