07+Gr+emb+Placentation PDF

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Veterinary embryology...

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General embryology: Placentation

6- Placentation in Mammals: =In all viviparous animals, the development of the young takes place inside the uterus of the mother because the amount of the stored yolk in the egg doesn’t remain sufficient for the development of the embryo. =Therefore, the developing embryo has to depend on the mother for nourishment, oxygen supply and other physiological processes. =Such embryos often get attached with the uterine wall to draw necessary substances from the maternal uterus circulation by means of an organ called “Placenta.” =========================================================================================================================

THE PLACENTA (chorion-decidual membrane): It is developed as following: 1- Development of the chorionic villi. (Described before) 2- Development of the decidua from the endometrium of the uterus. 3- Combination of decidua (maternal part) and chorion (fetal part). Functions of the placenta: 1- Nutrition of the fetus, 2- Barrier,

3- Excretion of fetal waste products, 4- Synthesis of progesterone, estrogen, ------etc.

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Placental membrane "barrier:" 1) What is meant by the placenta/barrier? =It is the membranes that separate the fetal blood which found in the capillaries of the villi from the maternal blood in the intervillus spaces. 2) The placental barrier in early pregnancy: is composed of four layers: (i) The endothelial lining of the capillaries in the chorionic villi, (ii) The connective tissue of the core of these villi. (iii) A layer of cyto-trophoblast, (iv) A layer of syncytio-trophoblast covering the cyto-trophoblast (3) The placental barrier in late" pregnancy: consists of two layers only: Most of the villi begin to lose two of their components the connective tissue core of the villi and the layer of cytotrophoblast. (4)- What are the main functions of the placental barrier? 1- It allows the passage of O2, food, vitamins and minerals from maternal to fetal blood, CO2 and waste products (e.g. urea) from fetal to maternal blood. 2- It allows the passage of some antibodies against diseases from the mother to the fetus. 3- It prevents the passage of “most" (but not all) organisms and harmful substances from the mother to the fetus.

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

General embryology: Placentation

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The placenta has two surfaces: 1- Fetal surface: smooth and covered by amnion and the umbilical cord attached to the center of this surface. 2- Maternal surface: irregular and consists of cotyledons that separated from each other by grooves into which the placental septa are projected. =Maternal blood lies in intervillus spaces between the chorionic villi and the decidua. =Ridges of the decidua projects in the intervillus spaces are called "placental septa."

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The placenta develops from two parts: 1- Fetal placenta: extraembryonic membranes. 2- Maternal placenta: uterine endometrium. =It is noted that the maternal placenta is formed of a single component, the endometrium, while the fetal placenta has four elements, amnion, chorion, yolk sac and allantois. =The amnion makes no direct contribution to the placenta. The chorion, because of its most external position, it is the membrane makes immediate contact with the endometrium. =In chick embryos, the chorion plays its role by way of a vascular supply through the allantois. =In mammals, two possible sources of the chorionic vascularization, vitelline circulation provided by the yolk sac and the allantoic circulation provided by the allantois. =When the fused vascular choriovitelline membranes become attached to the endometrium, the resulting placenta is known as a choriovitelline placenta. This type of placentation is commonly encountered in marsupials ‫الجرابيات مثل الكنجارو‬. When the chorioallantoic membrane becomes attached to the endometrium, the resulting placenta is referred to as a chorioallantoic placenta. = While this is the definitive form of placentation in higher mammals (eutherians), it may be preceded ‫ مسبوق‬by and co-exist with a temporary choriovitelline placenta.

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

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General embryology: Placentation

Then in mammals, there are two main types of placenta; choriovitelline and chorioallantoic placentae. A- Choriovitelline placenta (Yolk sac placenta): =The yolk sac is formed early in development in higher mammals, usually while the blastocyst is still unattached in the uterine cavity. =In most mammals, with the exception of humans, the endoderm of the early yolk sac combines with the trophoblastic layer of the blastocyst, forming the bilaminar yolk sac. =When the vascular mesoderm becomes interposed between the chorion and the endoderm, the bilaminar structure becomes a trilaminar yolk sac which functions as the embryonic component of the choriovitelline placenta. =While the choriovitelline placenta persists as the definitive placenta in most marsupials (Kangaroo), among domestic mammals the choriovitelline placenta exists only as an early temporary structure, losing its exchange function when the extraembryonic coelom extends into the mesoderm of the trilaminar yolk sac, separating the mesoderm into splanchnic and somatic layers. =As these changes take place rapidly in cattle, sheep and pigs, this functional yolk sac placenta is of short duration. In dogs and cats, the choriovitelline placenta functions up to the 21st day of pregnancy, whereas in horses it functions up to the eighth week of pregnancy. =The choriovitelline placenta does not establish an extensive and intimate contact with the endometrium. =========================================================================================================================

B- Chorioallantoic placenta: =The embryonic component of a chorioallantoic placenta is formed by the attachment and fusion of the outer wall of the expanding allantoic sac with the adjacent chorion. =This

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the

definitive form of placentation which occurs in higher mammals and is characterized by an extensive area of contact between the embryonic placental component and the endometrium. =Increased surface of contact is achieved through folding of the chorioallantois and the endometrial surface, formation of chorionic villi and the establishment of chorionic labyrinths ‫متاهة‬. Components of a choriovitelline placenta and of a chorioallantoic Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

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General embryology: Placentation

Development of the placenta A- Development of Chorionic villi: =Formation of the primary, secondary and tertiary chorionic villi B- Decidua: (1) - Decidua basalis C- Chorion: (1) - Chorion laeve

(2) - Decidua parietalis (3) - Decidua capsularis (2) - Chorion frondosum

B- The decidua: 1. First, what is meant by decidua? =The decidua is the modified endometrium of pregnancy after the blastocyst is completely embedded in it and the endometrium becomes so intimately associated with the fetus that it will be "shed away” with the fetus at birth. (Decidua = shed normally). =The uterus is thickened due to the activity of the progesterone hormone (internal secretion of the corpus luteum). =This thick, glandular and vascular endometrium is called "deciduas;" it contains leucocytes and decidual cells. =As a result of the embryo growth, the decidua is divided anatomically into 3 areas: 1- Decidua basalis: the part situated between the embryo and uterine muscles. 2- Decidua capsularis: the part enveloping the growing embryo. 3. Decidua parietalis (D. Vera): the part lining the rest of the uterine cavity.

3. What happens to the different parts of the decidua? *As the embryo grows, the decidua basalis develops to form the maternal part of the placenta while the other two parts of the decidua degenerate.

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

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General embryology: Placentation

4. How does the decidua capsularis and the decidua parietalis degenerate? *As the embryo grows, the decidua capsularis expands with it and projects into the cavity of the uterus. **As the embryo grows more, the decidua capsularis becomes stretched, begins to degenerate and comes nearer to the decidua parietalis which becomes compressed and begins also to degenerate. *Finally, the decidua capsularis and decidua parietalis come in apposition, and fuse with each other; and in this way the cavity of the uterus becomes obliterated. =========================================================================================================================

C- THE CHORION: it is formed of; A- An outer layer: consisting of trophoblast. B- An inner layer: consisting of primary mesoderm. The trophoblast: will form the chorionic villi in the decidua basalis and capsularis. i- Chorionic laeve: (laeve =smooth ‫)أملس‬ =The decidua capsularis gets thickened and compressed the chorionic villi in facing chorion that become atrophied and disappear, this is called "Chorionic laeve." ii- Chorionic Frondosum and early placenta:

(Frond =leaves ‫) ورق الشجر‬

The villi in the region of the decidua basalis increase in size and become more branched and form the beginning of the placenta, this part of the chorion is called "Chorionic Frondosum."

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

General embryology: Placentation

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Classification of Chorio-allantoic Placentation (Types of placenta): 1- According to the degree of contact between fetal tissue and uterine mucosa: which varies and may involve partly the loose apposition ‫ تراكب‬of these two tissues termed apposed ‫ متراكبة‬placentation, or their intimate fusion termed conjoined ‫ متحدة‬placentation. A= in an apposed placenta: =There is no fusion of the maternal and fetal tissue, and separation is easily achieved at parturition without damage to maternal tissue. =This form of placentation is termed apposed and non-deciduate or semiplacenta which is found in horses, ruminants, pigs and some other mammals. B= in conjoined placenta: =An intimate connection is formed between maternal and embryonic tissue, and at birth, some maternal tissue is lost with the fetal tissue. =This type of placentation is termed conjoined and deciduate or placenta Vera which is found in humans, dogs, cats and rodents. C= Contra-deciduate placenta: =A modified type of placentation in moles ‫ خلد الماء ويسمى الجنين الكاذب‬where not only there is a loss of maternal tissue but also fetal tissue are absorbed in situ by maternal leukocytes. =========================================================================================================================

2- Anatomical classification of the placenta (Goss): =Chorioallantoic placentae can be classified according to their shapes and the relationship of the extra-embryonic membranes to the endometrium. =Formation of the chorionic villi, their distribution on the surface of the chorionic sac and their relationship with the endometrium are used to define some placental characteristics. Placental morphology and areas of the chorionic villous attachment can be described as diffuse, cotyledonary, zonary or discoidal. 1- Diffuse placenta; =Which occurs in mares and sows, is characterized by uniform distribution of villi on the outer surface of the chorion. 2- Cotyledonary placenta; =Which occurs in ruminants (cow and ewe), chorionic villi are restricted to definite areas referred to as cotyledons, which are distributed over the surface of the chorionic sac and attached to the caruncles of the endometrium to form the placentomes. 3- Zonary placenta; =Which occurs in domestic carnivores (bitch and queen), is characterized by Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

General embryology: Placentation

chorionic villi are confined to a girdle-like structure around the middle of the chorionic sac. 4- Discoidal placenta; =Which occurs in humans, monkeys and rodents, chorionic villi are restricted to disc-shaped areas on the chorionic sac (mono-discoidal in human and bi-discoidal in rodents).

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

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General embryology: Placentation

3- Histological classification of placenta: =Based on the number of tissue layers interposed between the fetal and maternal blood stream, four basic types of placentation can be described. =In the simplest form, maternal endothelium, maternal connective tissue, maternal uterine epithelium, fetal (chorionic) epithelium, fetal connective tissue and fetal endothelium separate the maternal blood and fetal blood. =In the most complex form, the maternal layers are successively broken down until the chorionic epithelium (trophoblast) comes in direct contact with the maternal blood supply. =By using the name of the maternal tissue which is related to the chorion, the following types of placentation, based on histological features, can be described: Epitheliochorial, Synepitheliochorial, Endotheliochorial and Haemochorial. 1- Epitheliochorial placenta (6 layers) =The endometrial epithelium remains intact and is opposed to the chorionic epithelium. This type of placentation occurs in horses, donkeys, pigs and whales. 2- Synepitheliochorial (Syndesmochorial) placenta (5 layers) =The term “syndesmochorial," which describes removal of uterine epithelium leaving the chorion in contact with maternal connective tissue, was formerly used to describe the histological form of placentation in ruminants. =Electron microscope studies, however, have demonstrated that an attenuated layer of combined maternal and fetal epithelium persists in ruminant placentae. =Consequently, the term “syndesmochorial” has been replaced by the term “synepitheliochorial.” =The prefix “syn” implies a union of fetal and maternal cells in the cryptal epithelium. =The term “syndesmochorial” is an inappropriate description of ruminant placentation. 3- Endotheliochorial placenta (4 layers) =The uterine epithelium and connective tissue are removed and the chorionic epithelium comes in direct contact with the endometrial capillaries. =Placentae of this type are found in dogs, cats and elephants. 4- Haemochorial placenta (3 layers) =The maternal endothelium is removed and chorionic epithelium comes in direct contact with the maternal blood. =This type of placentation is found in some rodents, rabbits, guinea pig and higher primates.

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

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General embryology: Placentation

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4 3 1, Epitheliochorial. 2, Synepitheliochorial. 3, Endotheliochorial. 4, Haemochorial. Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

General embryology: Placentation

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Development of the umbilical cord: =It is the pathway which connects the placenta with the ventral aspect of the embryo. =It is a soft tortuous cord which has smooth surface as a layer of amnion covers it. It contains; 1- Three umbilical vessels (one vein and two arteries) embedded in mesodermal cells which form the ground substance of the cord and transformed into a gelatinous (jellylike) material called Wharton's jelly. 2- Body stalk 3- Yolk sac stalk and its vitelline vessels 4- Part of allantois. 5- Remnants of the extra-embryonic coelom separating the body stalk from the yolk sac stalk. How does the umbilical cord develop? A- Early stages: Formation of “primitive umbilical ring “and “primitive umbilical cord.” 1- As the embryo grows, the embryonic disc bulges into the amniotic cavity, results in the junction between the embryonic disc and the amnion is carried into the ventral aspect of the embryo and its line of reflection becomes oval shaped and is called “the primitive umbilical ring.” 2- The primitive umbilical ring constricts changing the ring into a tubular sheath of amnion called “primitive umbilical cord.”

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University

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General embryology: Placentation

3- Rapid enlargement of the amniotic cavity occurs and excessive lengthening of the cord B- Later stages of development of the umbilical cord: 1- Extra-embryonic mesoderm of the body stalk, forms Wharton’s jelly. 2-The cavity of the yolk sac stalk becomes obliterated and vitelline vessels disappear. 3-The extra-embryonic coelom disappear gradually. 4-The part of the allantois in the umbilical cord becomes obliterated while the allantoic vessels persist and enlarge to form umbilical vessels. The length of the umbilical cord differs greatly according to the species. =It is about the same length of the body in pig, 1/2 in horse and dog, 1/3 in cat, 1/4 in cow and sheep, and 1/6 in goat. =The umbilical cord of man is the longest as it is about double the length of the body (60cm).

The arrangement of the blood vessels and urachus in the umbilical cord in the mare and in the sow.

Dr. M.G. Tawfiek, MVSC., PhD, Anatomy and Embryology Department, College of Veterinary Medicine, Beni-Suef University...