Department of Histology, cytology and embryology Dr. Makarchuk Iryna

The individual development of living organisms is an ontogenesis. The embryology is a science, which studies lows of formation of an embryos and process of his development. The individual development of living organisms is an ontogenesis. In individual development are two basic stages: Prenatal ontogenesis is development till birth Postnatal ontogenesis is development from birth up to death of an individual So, the embryogenes is a part of ontogenesis.

1. Fertilization is fusion of a female and male gamete. In development of embryos some stages characterized by certain quantitative and qualitative changes are observed: 1. Fertilization is fusion of a female and male gamete. 2. Cleavage is the series of rapid cell divisions of the zygote with the formation of blastula 3. Gastrulation is the formative process by which the three germ embryonic layers are established in embryos (ectoderm, mesoderm and endoderm). 4. Histogenesis-development of tissues 5. Organogenesis development of organs 6. Systemogenesis development of systems

The spermatozoon The spermatozoon consists of 3 main components: -the head -the neck -the tail The tail is subdivided into three segments: -the middle piece -the principal piece -the end piece There is a nucleus in the head. The nucleus involves a very condensed chromatin. The acrosomal cap surrounds the anterior two-thirds of the nucleus. Acrosome contains different hydrolytic and mucolytic enzymes. The enzymes disaggregate the cells of the corona radiata and dissolve the zona pellucida during fertilization. There are two centrioles in the neck. The tail consists of axoneme and a lot of mitochondrias.

Ovum or ovocyte The ovum is the female haploid gamete which fuses with the sperm and develop into an organism after fertilization. As a rule they have a round form, larger than spermatozoons volume of cytoplasm and nucleus, and do not have the ability to move actively. The ovum is characterized by the presence of protein-lipid inclusion of yolk in cytoplasm. The ovocyte is coated by ovolemme or an initial envelope. The human ovocyte is secondary olygolecithal, what means that the amount of a yolk in cytoplasm little, and isolecithal what means that yolk distributed evenly on cytoplasm. The mammalian ovum, except of ovolemme is coated with two more envelopes: jelly-like zona pellucida and a radiate crown corona radiata. Composition of zona pellucida enters glycosaminoglycans (GAG) and glycoproteins. There are some cortical granules in the periphery of cytoplasm.

Fertilization and Cleavage The fertilization represents penetration of a spermatozoon into an ovum as a result of which it is restored number of chromosomes and the single-celled embryo a zygote is formed. The fertilization consists of 3 phases: Distant interaction and coming together of gametes Contact of gametes Infiltration of sperm cell in an ovum and gametic syngamy The zygote undergoes a number of ordinary mitotic divisions that increase the number of cells in the zygote but not its overall size. Each cycle of division takes about 24 hours. The individual cells are known as blastomeres. At the 32-cell stage the embryo is known as a morula (L. “mulberry”), a solid ball consisting of an inner cell mass and an outer cell mass. The inner cell mass will eventually become the embryo and fetus, while the outer cell mass will eventually become part of the placenta.

Here is a pictorial representation of the main landmarks in human prenatal development: Day 0 - Development of the new individual begins when a spermatozoon penetrates the jellylike zona pellucida and contacts the plasma membrane of the secondary oocyte. Note the smaller first polar body alongside the secondary oocyte within the zona pellucida. This was formed during the first meiotic division. Several follicular cells are shown still attached to the outer surface of the zona pellucida. After the arrival of the fertilizing sperm, no further spermatozoa are allowed to enter.

Day 0 In response to the fertilizing sperm, the oocyte completes its second meiotic division. The resulting cell division is unequal - the larger daughter cell becomes an ovum, and the other much smaller daughter cell becomes the second polar body. While the oocyte is dividing, the first polar body may also divide. The head of the spermatozoon enters the cytoplasm of the oocyte and swells, forming the male pronucleus. (The mid-piece and tail of the spermatozoon do not enter the ovum.)

Day 0 After completing its second meiotic division, the nucleus of the larger ovum becomes the female pronucleus. The male pronucleus swells, and the two pronuclei approach each other and merge. This establishes the diploid genome and also the genetic sex of the new individual. The fertilised ovum is called a zygote. The nuclear DNA is replicated and after several hours the zygote begins its first mitotic cell division. This illustration shows metaphase, with the chromosomal pairs arranged at the equator of the mitotic spindle ready to be separated and moved to opposite ends of the spindle.

Day 0 The nuclear DNA is replicated and after several hours the zygote begins its first mitotic cell division. This illustration shows metaphase, with the chromosomal pairs arranged at the equator of the mitotic spindle ready to be separated and moved to opposite ends of the spindle.

Day 1 The two-cell stage. The cytoplasm of the original zygote has been subdivided to form two smaller cells - there is little synthesis of new cytoplasmic material at this time. This also applies to subsequent cell divisions during the first few days, so they are often called ‘cleavage divisions’. The cells remain enclosed by the zona pellucida. The polar bodies are less prominent, and probably degenerate. As cleavage continues, a closely-packed ball of cells is produced - the morula. Each one of these cells is still very high in developmental potential, and could each give rise to a new individual, although usually they will co-operate in the development of just one baby.

Day 4 The morula drifts along the fallopian tube as cleavage continues. When it reaches the uterus, the zona pellucida dissolves and fragments, releasing the morula. Fluid is drawn into the centre of the morula, creating a hollow blastocyst. Within the hollow, fluid-filled blastocyst there is a special groups of cells called the inner cell mass. The cells forming the outer surface of the blastocyst are called trophoblast cells.

Day 7 The blastocyst makes contact with the lining of the uterus - the endometrium. This has prepared for such a possibility by thickening, becoming more glandular, and developing a rich blood supply. The blastocyst begins to implant, a process that will take several days. Implantation brings the outer cells of the blastocyst into direct contact with the maternal cells of the endometrium. The conceptus is genetically different from the mother since half its chromosomes have come from the father. Usually, genetically different cells would be rejected by the mother’s immune system, but this does not occur during normal pregnancy. During implantation, the trophoblast thickens and forms two layers - an outer syncytiotrophoblast (grey) and an inner cytotrophoblast. Meanwhile, the inner cell mass becomes organised into a two-layered plate of cells called the embryonic disc, with the amniotic cavity above and the yolk sac below. The two layers are called ectoderm and endoderm.

The conceptus implants itself completely within the endometrium The conceptus implants itself completely within the endometrium. The enlarging syncytiotrophoblast develops fluid-filled lacunae within it and comes into contact with the maternal blood vessels. Extra-embryonic mesodermal cells derived from the cytotrophoblast form a layer around the external surfaces of the amnion and yolk sac. The embryonic disc will give rise to the baby itself. The first sign of the midline axis of the baby is the formation of the primitive streak in the ectodermal layer facing the amniotic cavity.

In this diagram, part of the embryonic disc has been removed to show the primitive streak in cross-section. Ectodermal cells migrate towards the primitive streak and then tuck inwards to form a new layer of cells in between the ectoderm and endoderm. The new layer is called the mesoderm. These three layers of cells - ectoderm, mesoderm, and endoderm -then give rise to all parts of the body by a process called morphogenesis.

The primitive streak is situated in the midline towards the tail end of the future embryo. Further towards the future head end, the ectoderm thickens to form the neural plate. This begins to fold, initially forming a groove and then closing over to form the neural tube. The neural tube will give rise to the brain and spinal cord. Closure of the neural tube begins in the future hindbrain region and then continues for several more days, extending both forwards and caudally until closure is complete. Clusters of mesodermal cells produce pairs of somites alongside the developing neural tube. The number of somites increases steadily as neural closure continues.

In the future brain region, the neural folds enlarge as they close and begin to overshadow the part of the embryonic disc that lies further forwards. Within the mesoderm of this part the heart begins to develop. As the heart develops, blood vessels form in the wall the yolk sac, the embryo itself, and in the stalk that connects the embryo to the trophoblast. Blood cells are also formed in the yolk sac wall. On day 23 after fertilisation, the heart begins to beat and the circulation of blood is established.

As the embryo develops, the amniotic sac enlarges and gradually the yolk sac begins to diminish in relative size. The surrounding trophoblastic tissue develops finger-like processes which extend outwards into the endometrium. These extensions are best-developed across the most deeply-implanted part of the conceptus, and will contribute to formation of the placenta. Limb buds begin to form alongside the neural tube and somites. The developing eyes are visible, and the beating heart is now tucked ventrally in the future thoracic region.

The fingers and toes are becoming apparent, and the external ear is visible on the side of the neck region. The connection between the embryo and surrounding trophoblast is becoming the umbilical cord, and passing through this is a narrow duct connecting the yolk sac with the developing digestive tract in the embryo. By the end of the second month, all the different parts of the new individual have formed. Morphogenesis is complete. However, the embryo is only about 3 cms long from the top of its head to its rump. Most of the body organs and systems are only partially functional.

In the third month, the fetal period begins In the third month, the fetal period begins. It is a time of rapid growth in size and weight, the further maturation of function in organs and body systems, and the rehearsal of increasingly complex activities that must be perfected before birth - breathing movements, swallowing, production of urine, and digestion, for example. The uterus and placenta continue to enlarge to accommodate the growing fetus and meet its increasing needs.

Towards the end of pregnancy, space is at a premium, and the placenta is finding it increasingly difficult to meet the needs of the fetus. Approximately 9 months after conception, the process of birth begins. A difficult transition must be achieved with the baby’s systems taking over many of the responsibilities that were met previousy by the placenta and mother.

Gastrulation Gastrulation is the formative process by which the three germ embryonic layers are established in embryos (ectoderm, mesoderm and endoderm). Human gastrulation includes two main processes: Delamination, which establishes bilaminar disk composed of two layers, the epiblast and hypoblast Migration, which establishes three-laminar embryonic composed of ectoderm, mesoderm and endoderm. These three layers give rise to all tissues and organs of the adult. Gastrulation begins in the same time as implantation