Embryology Lecture Notes PDF

**OUTLINE** +-----------------------------------------------------------------------+ | I. | | | | |...

**OUTLINE** +-----------------------------------------------------------------------+ | I. | | | | | | | | A. B. C. | | | | | | | | II. | | | | | | | | A. Figures 1 | | | | B. Tables 1 | | | | | | | | III. IV. V. VI. | +-----------------------------------------------------------------------+ +-----------------------------------------------------------------------+ | I. DEVELOPMENT OF THE PLACENTA | +-----------------------------------------------------------------------+ | A. **INTRODUCTION** | +-----------------------------------------------------------------------+ - The placenta and fetal membranes separate the fetus from the endometrium---the inner layer of the uterine wall. - An interchange of substances, such as nutrients and oxygen, occurs between the maternal and fetal bloodstreams through the placenta. - The vessels in the umbilical cord connect the placental circulation with the fetal circulation. - The fetal membranes include the chorion, amnion, umbilical vesicle (yolk sac), and allantois. -------------- **PLACENTA** -------------- - The placenta is the primary site of nutrient and gas exchange between the mother and fetus. - The placenta is a fetomaternal organ that has two components: - **fetal part** that develops from the chorionic sac, the outermost fetal membrane - **maternal part** that is derived from the endometrium, the mucous membrane comprising the inner layer of the uterine wall - The placenta and umbilical cord form a transport system for substances passing between the mother and fetus. - Nutrients and oxygen pass from the maternal blood through the placenta to the fetal blood, and waste materials and carbon dioxide pass from the fetal blood through the placenta to the maternal blood. - The placenta and fetal membranes perform the following functions and activities: - Protection - nutrition - respiration - excretion - hormone production - Shortly after birth, the placenta and fetal membranes are expelled from the uterus as the **afterbirth.** ------------- **DECIDUA** ------------- - The decidua is the endometrium of the uterus in a pregnant woman. - It is the functional layer of the endometrium that separates from the remainder of the uterus after parturition (childbirth). - The three regions of the decidua are named according to their relation to the implantation site (Fig. 7-1): - The decidua basalis is the part of the decidua deep to the conceptus (embryo and membranes) that forms the maternal part of the placenta. It is the most important - The decidua capsularis is the superficial part of the decidua overlying the conceptus. - The decidua parietalis is all the remaining parts of the decidua. - In response to increasing progesterone levels in the maternal blood, the connective tissue cells of the decidua enlarge to form pale-staining decidual cells. - These cells enlarge as glycogen and lipid accumulate in their cytoplasm. - The cellular and vascular changes occurring in the endometrium as the blastocyst implants constitute the decidual reaction. - Many decidual cells degenerate near the chorionic sac in the region of the syncytiotrophoblast and, together with maternal blood and uterine secretions, provide a rich source of nutrition for the embryo. - It has also been suggested that they protect the maternal tissue against uncontrolled invasion by the syncytiotrophoblast and they may be involved in hormone production. - Decidual regions, clearly recognizable during ultrasonography, are important in diagnosing early pregnancy. +-----------------------------------------------------------------------+ | II. FORMATION OF PLACENTA | +-----------------------------------------------------------------------+ | A. **IMPLANTATION** | +-----------------------------------------------------------------------+ Imagen: Stages of embryo implantation **STAGES OF IMPLANATION** 1. On days 5 and 6 of development, the embryo begins to hatch until it detaches from its zona pellucida, the external protein membrane that protects it in its first days after fertilization. As the embryo increases in size, the zona pellucida becomes thinner until it finally breaks down. Finally, the embryo manages to leave it through a series of contractions and begins to interact with the endometrium. 2. During this phase, the embryo seeks its position on the endometrial tissue and remains immobile while it is oriented so that its internal cell mass points towards the endometrium to allow the proper formation of the placenta later on. 3. ADHESION The blastocyst trophoectoderm adheres to the endometrial epithelium and is united by the action of the adhesion molecules: integrins β1, β3, and β4, L-selectins, proteoglucans, fibronectins, etc. This happens about 7 days after fertilization, when the blastocyst already has a diameter of 300-400 µm. 4. INVASION The blastocyst, more specifically the trophoblast or embryonic trophectoderm, proliferates towards the endometrium, displaces the epithelial cells, and finally invades the endometrial stroma, making contact with the maternal blood. This entire invasion mechanism is controlled by cytokines, molecules that act as mediators of implantation and allow dialogue between the embryo and the endometrium. In response to this dialogue, the blastocyst is activated and initiates differentiation of the trophoblast into cytotrophoblast and syncytiotrophoblast. - Site of implantation - Size of blastocyst - Interstitial implantation completed at 11^th^ day +-----------------------------------------------------------------------+ | B. **TROPHOBLAST DIFFERENTIATION** | +-----------------------------------------------------------------------+ - By the beginning of the second month, the trophoblast is characterized by a great number of secondary and tertiary villi, which give it a radial appearance (Fig. 8.7). Stem (anchoring) villi - Trophoblast will differentiate into - synctiotrophoblast - cytotrophoblast ------------------- **LACUNAR STAGE** ------------------- - Spaces from within ST - ST erodes maternal decidua - Maternal blood( from uterine spiral arteries) enter lacunar network - Erodes maternal residua - Invasion of blood vessels week 2 ----------------------------- **CYTOTROPHOBLASTIC SHELL** ----------------------------- - Formed from cytotrophoblasts from tertiary villi - Anchoring villi - Branching villi - EXTRAEMBRYONIC VASCULAR SYSTEM - Build up of core of villi with capillaries of chrionic plate and connecting stalk - Established at week 2 - Spiral artery - SPIRAL ARTERY REMODELLING +-----------------------------------------------------------------------+ | III. STRUCTURE OF PLACENTA | +-----------------------------------------------------------------------+ | **4^TH^ MONTH** | +-----------------------------------------------------------------------+  - By the beginning of the fourth month, the placenta has two components: - \(1) a fetal portion, formed by the chorion frondosum, and - \(2) a maternal portion, formed by the decidua basalis (Fig. 8.1OB). On the fetal side, the placenta is bordered by the chorionic plate (Fig. 8.13); on its maternal side, it is bordered by the decidua basalis, of which the decidual plate is most intimately incorporated into the placenta. - In the junctional zone, trophoblast and decidual cells intermingle. - This zone, characterized by decidual and syncytial giant cells, is rich in amorphous extracellular material. - By this time, most cytotrophoblast cells have degenerated. - Between the chorionic and decidual plates are the intervillous spaces, which are filled with maternal blood. - They are derived from lacunae in the syncytiotrophoblast and are lined with syncytium of fetal origin. - The villous trees grow into the intervillous blood lakes (Figs. 8.8 and 8.13) ----------------- **5^TH^ MONTH** ----------------- - During the **fourth and fifth months**, the decidua forms a number **of decidual septa**, which project into intervillous spaces but do not reach the chorionic plate (Fig. 8.13). - These septa have a core of maternal tissue, but their surface is covered by a layer of syncytial cells so that at all times, a syncytial layer separates maternal blood in intervillous lakes from fetal tissue of the villi. - As a result of this septum formation, the placenta is divided into a number of compartments, or cotyledons (Fig. 8.14). - Because the decidual septa do not reach the chorionic plate, contact between intervillous spaces in the various cotyledons is maintained. - As a result of the continuous growth of the fetus and expansion of the uterus, the placenta also enlarges. - Its increase in surface area roughly parallels that of the expanding uterus, and throughout pregnancy, it covers approximately 15% to 30% of the internal surface of the uterus. - The increase in thickness of the placenta results from arborization of existing villi and is not caused by further penetration into maternal tissues. ------------------------ **FULL TERM PLACENTA** ------------------------ - At full term, the placenta is discoid with a diameter of 15 to 25 cm, is approximately 3cm thick, and weighs about 500 to 600 g. - At birth, it is torn from the uterine wall and, approximately 30 minutes after birth of the child, is expelled from the uterine cavity as the afterbirth. - When the placenta is viewed from the maternal side, 15 to 20 slightly bulging areas, the cotyledons, covered by a thin layer of decidua basalis, are clearly recognizable (Fig. 8.14B). - Grooves between the cotyledons are formed by decidual septa. Ihe fetal surface of the placenta is covered entirely by the chorionic plate. - A number of large arteries and veins, the chorionic vessels, converge toward the umbilical cord (Fig. 8.14A). The chorion, in turn, is covered by the amnion. - Attachment of the umbilical cord is usually eccentric and occasionally even marginal. Rarely, however, does it insert into the chorionic membranes outside the placenta (velamentous insertion). --------------------------------- **CIRCULATION OF THE PLACENTA** --------------------------------- - Cotyledons receive their blood through 80 to 100 spiral arteries that pierce the decidual plate and enter the intervillous spaces at more or less regular intervals (Fig. 8.13). - Pressure in these arteries forces the blood deep into the intervillous spaces and bathes the numerous small villi of the villous tree in oxygenated blood. As the pressure decreases, blood flows back from the chorionic plate toward the decidua, where it enters the endometrial veins (Fig. 8.13). - Hence, blood from the intervillous lakes drains back into the maternal circulation through the endometrial veins. Collectively, the intervillous spaces of a mature placenta contain approximately 150 mL of blood, which is replenished about three or four times per minute. - This blood moves along the chorionic villi, which have a surface area of 4 to 14 m2. - Placental exchange does not take place in all villi, however, only in those that have fetal vessels in intimate contact with the covering syncytial membrane. - In these villi, the syncytium often has a brush border consisting of numerous microvilli, which greatly increases the surface area and, consequently, the exchange rate between maternal and fetal circulations (Fig. 8.8D). The placental membrane, which separates maternal and fetal blood, is initially composed of four layers: (1) the endothelial lining of fetal vessels, (2) the connective tissue in the villus core, (3) the cytotrophoblastic layer, and (4) the syncytium (Fig. 8.8C). - From the fourth month on, the placental membrane thins because the endothelial lining of the vessels comes in intimate contact with the syncytial membrane, greatly increasing the rate of exchange (Fig. 8.8D). - Sometimes called the placental barrier, the placental membrane is not a true barrier, as many substances pass through it freely. - Because the maternal blood in the intervillous spaces is separated from the fetal blood by a chorionic derivative, the human placenta is considered to be of the hemochorial type. - Normally, there is no mixing of maternal and fetal blood. - However, small numbers of fetal blood cells occasionally escape across microscopic defects in the placental membrane. +-----------------------------------------------------------------------+ | IV. FUMCTION OF THE PLACENTA | +-----------------------------------------------------------------------+ | **4^TH^ MONTH** | +-----------------------------------------------------------------------+ I. 1^st^ Main Point: a. Description / Supporting Point b. Supporting Point 1. Detail 2. Detail II. 2^nd^ Main Point a. c. Details d. Details - - Books - Previous transes - Online sources --------------- B. SUBTOPIC Sub-sub topic --------------- - **LIST Style: Listing 1** - Style: Listing 2 - Style: Listing 3 - Style: Listing 4 - Style: Listing 5 - **NUMBER LISTING Style:** I. a. e. Style: Number 3 3. Style: Number 4 - **Tips**: - If inserting a sentence, make sure the alignment is **justified** - Avoid putting in paragraphs - Instead, paraphrase - Make sure the idea is still the same - Take advantage of listing bullets and numbers - Make sure there are **no redundant information** +-----------------------------------------------------------------------+ | II. **TABLES AND FIGURES** | +-----------------------------------------------------------------------+ | **A. FIGURES** | +-----------------------------------------------------------------------+ - For Figures - Positioned at the **CENTER** - Use **in-line with text wrapping** - Center picture - Figure Label - Place under the figure - Font - Century Gothic size: 8 - Centered (alignment) - Bold - "Figure", number, and period only - Ex. Figure 1. - Regular (not bold) - For description or figure title  Figure 2. Make image **wrapped in line with text** - **PLEASE MAKE SURE to PUT TITLE or CAPTION.** **PLEASE MAKE sure that the labels are READABLE.** +-----------------------------------------------------------------------+ | A. | +-----------------------------------------------------------------------+ **Table 1.** Sample table ------------- ------------- **Title 1** **Title 2** ------------- ------------- +-----------------------------------------------------------------------+ | III. **COLOR SCHEME** | +-----------------------------------------------------------------------+ Table 2. Color Scheme ---------------- ----------------------------------------------------------------------------------- Black General notes **Black bold** **Important concepts** Navy blue Clinical correlations or significance Red Must-know concepts Green Concepts from other references (i.e., previous transes, books, online references) ---------------- ----------------------------------------------------------------------------------- +-----------------------------------------------------------------------+ | IV. **READING ASSIGNMENT** | +-----------------------------------------------------------------------+ | - If inserting a long sentence or a composition of sentences, | | please make sure that the alignment is justified. If inserting a | | long sentence or a combination of sentences, please make sure | | that the alignment is justified. | +-----------------------------------------------------------------------+ +-----------------------------------------------------------------------+ | V. **CHECKPOINT** | +-----------------------------------------------------------------------+ | 1. High-yield Question \#1 | | | | 2. Question \#2 | | | | 3. Minimum of 5 questions | | | | 4. This will serve as | | | | 5. Self-assessment | +-----------------------------------------------------------------------+ +-----------------------------------------------------------------------+ | VI. | +-----------------------------------------------------------------------+ - Font: Century Gothic 8 - PLEASE USE APA FORMAT. - Example: