Embryonic Development PDF

Mammalian embryonic development Mammalian embryonic development  The fertilization and embryo development is internal.  The eggs of higher mammals (Euteria) lose their vitelline and become alecitic (oligolecitic).  The stages of embryonic development are common to all multicellular...

Mammalian embryonic development Mammalian embryonic development  The fertilization and embryo development is internal.  The eggs of higher mammals (Euteria) lose their vitelline and become alecitic (oligolecitic).  The stages of embryonic development are common to all multicellular organisms - cleavage, blastula, gastrula, organogenesis. The common characteristics of the embryonic development of all mammals:  mammalian eggs mature only after the penetreation of sperm  fertilization takes place in the initial part of the fallopian tube  development takes place in the mother's uterus  cleavage is totall- equal  already at the blastula (blastocyst) stage, the embryo has the shape of a disk, and further development is partial-discoid.  Cleavage  Cleavage begins a few hours after fertilization.  By cleavage a single-celled zygote develops into a multi-celled embryo.  It is a series of mitotic divisions without cell growth.  During cleavage division, the volume ratio between the nucleus and cytoplasm changes - the size of the nucleus remains almost the same by maintaining the amount of nucleoplasm. Early cleavage stages in human embryos Cleavage morphology  Cleavage is total-equal (complete, i.e. holoblastic), and all blastomeres are of the same size.  The zygote divides into 2, 4, 8, 16, etc. blastomere-embryonic cells. Cleavage morphology  During cleavage, intensive synthesis of genetic material takes place.  With each division, the total amount of embryo DNA increases, along with the increase in the number of embryonic cells.  When the embryio consists of approximately 16 cells, it is called morula (derived forom the Latin word meaning mulberry. Morula Blastula  The morula is a short-lived stage.  Cavitation- 4 days after fertilization. Fluid-filled space is known as the blastocoele (blastocyst cavity)  At this stage, the embryo as a whole is known as a blastocyst.  In mammals, the blastula is a specific blastocyst. A. Morula, showing the beginning of cavitation. B. Blastocyst, showing a well-defined inner cell mass ad blastocoele  Blastocyst  At the blastocyst stage, the embryo, consists of two types of cells:  An outer epithelial layer (the trophoblast) that surrounds a small inner group of cells called the inner cell mass.  These two cell typer reflects major organizational changes that have occurred within the embryo and represents the specialization of the blastomeres into two distinct cell lineages.  Cells of the inner cell mass give rise to the body of the embryo itself, whereas cells of the trophoblast form only extraembryonic structures, including the placenta.  Blastocyst  In further embryogenesis, the trophoblast will form the placenta.  The trophoblast differentiates into an inner part- cytotrophoblast and an outer- syncytiotrophoblast, in which there are no borders between cells.  Embryoblast is a plate formation consisting of: -epiblast-from the dorsal side -hypoblasta- on the ventral side.  In the blastocyst above the two-layered embryonic disc, the amnion is observed. Cell and tissue lineages in the mammalian embryo Gastrulation and the embryonic germ layers  Gastrula is the embryonic stage in which germ layers are formed from the undifferentiated blastoderm.  It is formed by: intussusception, migration, delamination and epiboly.  The gastrula of oligolecitic eggs is invaginated.  Primary germ layers are ectoderm and endoderm (1st gastrulation phase).  Mesoderm (2nd stage of gastrulation) is secondary germ layer. Gastrulation in mammals  Gastrulation begins with the formation of the primitive streak, a linear midline condensation of cells derived from the epiblast in the posterior region of the embryo,  Through the primitive streak, cells migrate under the epiblast and form the mesoblast.  As a result, a depression - a primitive groove - is formed along the entire length of the primitive streak. Gastrulation  Cell migration is the most intense in the end part of the streak- Henzen's node is formed there.  In front of Henzen's node, a head extension will be formed.  All 3 germ layers are now visible on the cross section. Differentiation of three embryonic germ layers Extra embryonic membranes in mammals  Yolk sac  Amnion  Chorion  Allantois  Yolk sac  It is formed by the evagination of the endoderm at the same time as the amnion.  Does not contain vitellus.  Since it has no role in the nutrition of the embryo, it is reduced.  Blood stem cells differentiate in the mesoderm of the yolk sac.  In the endoderm of the yolk sac, the stem cells of the gametes-primary gonocytes differentiate.  Amnion  It is formed early, around 12-14. days of embryogenesis in humans, from the dorsal side of embryo.  Amnion is filled with amniotic fluid.  The role of the amnion is to protect the embryo from mechanical damage and drying.  The amnion wall is composed of ectoderm on the inside and mesoderm on the outside.  Chorion  In the period of gastrulation, when the mesoderm differentiates, the cells lining the cytotrophoblast form the parietal mesoderm (extraembryonic mesoderm).  This mesoderm with the trophoblast forms the chorion.  Chorion  The chorion forms numerous evaginations - chorionic villi, which penetrate to different layers of the uterus.  In evolutionarily older mammals, they lie on the epithelium of the uterus, and in evolutionarily younger ones, they enter the mesoderm or penetrate the blood vessels of the uterus.  In humans, they are submerged in spaces filled with the mother's blood.  The blood of the fetus and the mother never mix because there is a placental membrane.  Chorion  The arrangement of the villi can be different:  -cotyledonary, in the form of cotyledons (cattle)  -diffuse, even (pigs)  -discoidal, disc-shaped (man)  -zonal, in the form of a belt (dog).  Chorionic villi with the wall of the uterus form the placenta.  Placenta  This is fetomaternal organ.  It is made of fetal tissue-pars fetalis (chorionic villi) and the tissues of the mother - pars materna (uterus wall).  The placenta and the umbilical cord are a transport system for substances between the mother and the fetus.  Blood from the placenta goes to the embryo via the umbilical vein, and returns to the placenta via the umbilical arteries. Functions of placenta: -embryo nutrition and gas exchange -removal of harmful and unnecessary substances -protection of the embryo-prevents the passage of infectious agents into the fetal circulation -secretion of hormones: estrogen, progesterone, hCG, etc. important for maintaining pregnancy in humans.  Allantois  It arises as a enlargement of the hindgut.  Endoderm and visceral mesoderm participate in the construction of the allantois.  It has the role of the urinary bladder - in reptiles and birds, and in mammals in supplying the embryo with oxygen and nutrients through the allantoic network of blood vessels.  In humans, it develops early, already around the 21st day of embryogenesis.  The endodermal part of the allantois atrophies, while the mesodermal part expands and extends into chorine villi (allantochorion).