Embryology
EMBRYOGENESIS
Embryogenesis is part of ontogenesis, it includes time from the moment of fertilization of oocyte by spermatozoon and formation of zygote till the birth.
Embryogenesis is divided into three periods:
1) primary - the first week of development;
2) embryonic - second to eight weeks of development;
3) fetal - continues from ninth week of intranatal development till the birth of child.
Embryogenesis is closely connected with progenesis. Progenesis is period of development and mature of sexual cells - oocytes and spermatozoa. As a result of progenesis in mature sexual cells haploid number of chromosomes appears, structures providing their ability to fertilization and development of new organism are formed.
Spermatozoa.
Human sperm is formed during the whole active life. Among sperms fifty per cent contain X-chromosomes and fifty per cent - Y-chromosomes. Weight of X-chromosome is more then weight of Y-chromosome, that is why sperms containing X-chromosome are less mobile, then containing Y-chromosome.
In human being volume of ejaculation in norm is about three milliliter. In one ejaculate about three hundred and fifty million spermatozoa are present. For providing fertilization general number of spermatozoa must be about one hundred and fifty million, and concentration of spermatozoa in one milliliter - about sixty million.
Thanks to high mobility spermatozoa can reach cavity of uterus after thirty minutes - one hour; and after one and a half to two hours - in distal (ampular) part of uterine tube, where fertilization occurs. They keep their fertile ability during one to two days.
Oocyte.
Oocyte has a diameter about one hundred and thirty micrometers and is surrounded by dense brilliant zone or membrane and corona of follicular cells. Human oocyte is secondary isolecital type.
It contains a little yolk granules located in ooplasm regularly. Reserve of nutritive substances human oocyte lost during twelve to twenty-four hours after ovulation, then it is dead, if fertilization does not take place.
Fertilization
Fertilization is fusion of male and female sexual cells with formation of unicellular embryo - zygote with diploid number of chromosomes. It occurs in ampullar part of uterine tube.
The first two weeks of the human development are called the preembryonic period. This period begins with the fertilization. Fertilization is the beginning of the pregnancy and can be considered as the beginning of a new life.
In order to have successful fertilization several events have to be taken their course (e.g., gametogenesis). Female gamete (oocyte) is produced during the menstrual cycle and expelled during the ovulation. During each ovarian cycle, only one follicle with an oocyte reaches full maturity. At the fourteenth day in an average twenty eight day cycle this follicle bulges on the surface of the ovary. Immediately before the ovulation, oocyte and some surrounding cells (cumulus oophorus) detach from the interior of the follicle. Also shortly before the ovulation the fimbriae of the oviduct start covering the surface of the ovary. During ovulation, follicle bursts and oocyte is expelled into the uterine tube.
The expelled secondary oocyte is surrounded by the zona pellucida and several layers of the follicular cells arranged as the corona radiata. Male gametes are produced during the spermatogenesis and stored in the epididymis. Upon ejaculation into the female genital tract, the spermatozoa are not capable of fertilizing the oocyte. They must undergo a capacitation period that lasts approximately seven hours during which the glycoprotein coat and seminal proteins are removed from the surface of the sperm acrosome by the action of the substance secreted by uterus or uterine tubes. When the capacitated spermatozoa come into contact with the corona radiata surrounding the secondary oocyte, they undergo the acrosomal reaction. This process includes release of the acrosomal vesicles content that helps the sperm digest its way to the oocyte plasma membrane in order to fuse with it.
Fertilization, the process by which the male and female gametes fuse, marks the beginning of the pregnancy. It lasts twenty-four hours and occurs in the ampullary region of the uterine tube. The first event is the scattering of the corona radiata cells by the released contents of the acrosomal vesicle (hyaluronidase), tubal mucosa enzymes and sperm tail movements. Penetration of the zona pellucida is enabled by the action of other enzymes released from the acrosome (acrosin and neuraminidase). When the first sperm passes through the zona pellucida, cortical reaction makes it impermeable to other sperms. The mechanism ensures that each oocyte is fertilized by only one sperm.
When the sperm enters the oocyte, it leaves its plasma membrane behind. After the sperm entry, the secondary oocyte finishes its second meiotic division, forming an ovum and a second polar body. The nucleus of the mature oocyte is known as the female pronucleus. Male pronucleus is formed by the enlarging of the nucleus in the head of the sperm. During the growth of the pronuclei they replicate their DNA. At this stage, male and female pronuclei are indistinguishable. Fertilization ends with the fusion of the female and male pronuclei and formation of zygote.
Fragmentation (cleavage).
After the fertilization, the zygote undergoes a series of rapid divisions called cleavage. Human cleavage is:
complete (cleavage is characterized by complete division of the mass of the fertilized egg to form the blastomeres);
unequal (cleavage in which the dark and light blastomeres are unequal in size).
The zygote first divides into two cells known as blastomeres (thirty hours after fertilization). Three days after the fertilization, a rapid increase in the number of the cells results in the formation of a solid ball of twelve to six cells, the morula. These repeated mitotic divisions happen during the zygote’s passage through the utrerine tube towards the uterus. Four days after fertilization, when the morula enters the uterus, fluid-filled spaces between the blastomeres appear and fuse a central cavity called the blastocoele. At this stage of the development, the conceptus is called a blastocyst.
Blastocyst later loses the zona pellucida through the process called blastocyst hatching. The blastocyst consists of two parts, an outer layer of small slightly flattened cells (light blastomeres) the trophoblast, and an inner cell mass consisting of a small group of larger polyhedral cells (dark blastomeres) contained within the trophoblastic vesicle. The cavity of the blastocyst (blastocoele) separates the trophoblast from the inner cell mass except for the small area where they are in contact. Embryoblast gives rise to the embryo and a part of the amnion. The trophoblast cells form most of the extraembryonic membranes, i.e., the bulk of the placenta. During the sixth day after fertilization, the blastocyst attached to the endometrial epithelium with it’s embryonic pole. This triggers the differentiation of the trophoblast into an inner cytotrophoblast and outer syncytiotrophoblast. By the end of the first week, the blastocyst is superficially implanted. At about seven days a flattened layer of cuboidal cells, called hypoblast (primitive endoderm) appears on the surface of the embryoblast.
Gastrulation.
It occurs on the seventh to eighth day. The first phase of gastrulation occurs by delamination. Cells of embryoblast is divided into two layers: the hypoblast layer, consisting of small cuboidal cells, and the epiblast layer, consisting of high columnar cells.
The second phase of gastrulation - immigration. Immigration starts on the fourteenth to fifteenth day. As a result of immigration primary stria is formed. From primary stria cells migrate. Embryo gets three-layered structure. Embryo consists of ectoderm, endoderm and mesoderm.
To the end of the 2nd - beginning of the 3rd week the differentiation of germ layers starts. One part of cells transform to rudiments of tissues and organs of embryo, another - into provisory organs.
Embryonic histogenesis.
Embryonic histogenesis is process of appearance of specialized tissues from slightly differentiated cellular material of embryonic rudiments taking place during embryogenesis. Embryonic rudiments are source of development of tissues and organs during ontogenesis. As a result of histogenetic processes formation of tissues from embryonic rudiments occurs. Histogenesis is accompanied with reproduction and growth of cells, their transfer - migration, differentiation of cells and their derivatives, intercellular and intertissue interactions.
Ectoderm develops into: skin ectoderm, neuroectoderm (nerve tube, nerve comb and ganglious lamina), placoda, prechordal lamina, provisory ectoderm (source of formation of epithelium of amnoin and umbilical cord). From ectoderm germ layers are formed.
Mesoderm develops into: dorsal mesoderm (is divided into myotom, sclerotom and dermatom), ventral mesoderm (splanchnotom) gives the beginning to myocardium, cortex of suprarenal glands, mesothelium, segmented pedunculi (nephrogonotom) forms epithelium of kidneys and gonads, paramesonephral duct (epithelium of female reproductive organs), mesenchyme, provisory mesoderm (connective tissue of yolk sac, amnion, chorion).
Entoderm develops into: entoderm of intestine tube (epithelium of organs of gasrointestinal tract and glands of gastrointestinal tract), provisory entoderm (epithelium of yolk sac and allantois).
Extraembryonic organs.
Umbilical cord. The umbilical cord is vascular cable (~fifty-five centimetres) that connects the embryo to placenta. The umbilical cord of the fetus is covered by the amniotic epithelium and contains two umbilical arteries and one umbilical vein embedded into the Wharton’s jelly. It is first formed during the fifth week of embryonic life and initially contains the yolk sac and body stalk with the enclosed allantois.
Amnion. The thin, transparent, tough membrane lining the fluid-filled cavity which contains the embryo.
Allantois. One of the extraembryonic membranes, providing respiratory exchange. It is covered by a conjoined vascular layer containing umbilical or allantoic vessels which are the vascular connections with the placenta.
Vitelline sac. An extraembryonic structure, attached to the umbilical cord and lined by endoderm. Vitelline sac is a site of hematopoiesis.
Chorion. The outermost of the fetal membranes of mammals, formed from extraembryonic somatopleuric mesoderm and the overlying trophoblast. The trophoblast divides into an outer syncytiotrophoblast and inner cytotrophoblast. The chorion develops villi, engaged in fetal-maternal exchange.
Placenta.The blastocyst normally attached to the uterine mucosa about six days after fertilization. The outer layer of the blastocyst proliferates to form the trophoblastic cell mass or chorionic shell. Trophoblastic cells derived from this mass penetrate the columnar epithelial cells of the maternal endometrium. The invasion of the endometrium begins on the seventh day after fertilization and is complete by about the twelfth day. The primitive trophoblast rapidly differentiates into two layers. The inner layer is composed of mononuclear cells and is known as the cytotrophoblastic layer, whereas the outer layer consists of multinucleated cells and is known as the syncytiotrophoblast. Intercommunicating fluid-filled spaces (lacunae) appear in the rapidly enlarging trophoblastic mass from the eighth day after fertilization.
The end of the second week is characterized by the first appearance of chorionic villi. The layer of the placenta from which the villi project is called the chorionic plate. Primary villous stems have a central core of cells derived by the proliferation of the cytotrophoblast. The primary villi gradually develop mesenchymal cores, which convert them into the secondary villi. The mesenchymal cells within the villi differentiate into blood capillaries, thus forming the tertiary villi. The vessels from the villi soon become connected with the embryonic heart via vessels that differentiate in the mesenchyme of the chorion and in the connecting stalk.
By the end of the third week, embryonic blood is circulating through the capillaries of the villi. As the growth continues, the villi on the decidua capsularis (abembryonic) pole degenerate to form the chorion laeve, while the villi adjacent to the decidual plate rapidly grow and expand to form the chorion frondosum. The chorion frondosum forms up the placenta together with the decidual plate. By the end of the fourth month of the pregnancy, the placenta has attained its definitive form and undergoes no further anatomical modifications. It has two components: the maternal portion, formed by the decidual plate, and a fetal portion, made by the chorion frondosum. Growth continues by further ramification of the stem villi into the surrounding intervillous spaces. On the maternal side decidual septa extend into the intervillous spaces, dividing placenta into ten to thirty eight cotyledons.
The villous surface area continues to increase until term, although the rate of the increase slows from approximately thirty four to thirty six weeks of gestation. The placenta has a circular shape and measures about fifteen to twenty centimetres in diameter, weighing five hundred to six hundred grammes at full term.
Bilaminar embryonic disc.
Implantation of the blastocyst in the endometrium of the uterus is completed during the second week of the development. The cells of the inner cell mass (embryoblast) differentiate into two layers: the hypoblast layer, consisting of small cuboidal cells, and the epiblast layer, consisting of high columnar cells. The two-layered plate that will differentiate into embryo is called the embryonic disc.
The epiblast forms the floor of the amniotic cavity and is peripherally continuous with a thin epithelial layer of the amnion. Flattened cells probably originating from the hypoblast, form an exocoelomic membrane (Hauser’s membrane). This membrane and the hypoblast form the lining of the exocoelomic cavity (primitive yolk sac).
Cells derived from the yolk sac form the extraembryonic mesoderm and fill the space between the trophoblast externally and the amnion and exocoelomic membrane internally. Large cavities within the extraembryonic mesoderm become confluent and form the extra embryonic coelom. The extraembryonic coelom splits the extraembryonic mesoderm into two layers: the extraembryonic somatic mesoderm, lining the trophoblast and amnion, and the extraembryonic splanchnic mesoderm, covering the yolk sac. The extraembryonic somatic mesoderm and the layers of trophoblast constitute the chorion. The extraembryonic somatic mesoderm and the extraembryonic part of the ectoderm constitute the amnion.
The endodermal germ layer produces additional cells which form a new cavity, known as the secondary or definitive yolk sac. The extraembryonic coelom expands to form a large chorionic cavity, within which the embryo and the attached amniotic and yolk sac are suspended by the connecting stalk.