General Embryology Revision PDF

# GENERAL EMBRYOLOGY ## Spermatogenesis - **Definition**: It is a process by which complete motile spermatozoa are produced from primordial male germ cells called spermatogonia. It includes the different stages by which the spermatogonia are transformed into sperms in the testis. - **Start**: It starts after puberty and continues into old age. - **Duration**: It takes about 60-64 days (including the process of spermio-genesis) - **Site**: in the seminiferous tubules of the testis It passes in 4 stages: - **Stage of proliferation**: - The spematogonium divides by mitosis to form 2 daughter spermatogonia. - Each contains 23 pairs of chromosomes, 2n. - **Stage of Growth**: - Daughter spermatogonia enlarge to form 1ry spermatocytes (2n), the largest germ cells in the seminiferous tubules of the testis. - **Stage of maturation**: - **First meiotic division**: 1ry spermatocyte divides by meiosis to from two 2ry spermatocytes (Each contains 23 chromosomes) - **Second meiotic division**: Each secondry spermatocyte divides to form two spermatids (Each contains 23 chromosomes) - **Stage of transformation**: - **Spermio-genesis**: change in shape with no further division of spermatids change to motile sperms: 1. The nucleus condenses into ovoid chromatin to form the head of sperm. 2. Aggregation of Golgi complex in the supranuclear area forming a big acrosomal vesicle then acrosomal cap which covers the anterior 1½ of the head. Acrosome secretes enzymes which facilitate penetration of the ovum. 3. The 2 centrioles (anterior and posterior), the axial filament starts from the posterior one. 4. Mitochondria: collect in a helical sheath in the middle piece around the axial filament. 5. Cytoplasm: (most of it is shed). ## Structure of sperm - **Head**: acrosome + nucleus. - **Neck**: Follows the head & contains the centrioles. - **Body**: middle piece containing the mitochondria. - **Tail**: end piece containing the axial filament. ## Seminal fluid It is composed of 2 main components: - **Spermatozoa**: produced by the testis - **Fluid secretions**: from the accessory sex glands: seminal vesicles, prostate and bulbo-urethral glands. - It is an alkaline fluid that contains fructose, and vitamin C. - The normal amount of one ejaculate is 3-5 с.с., where each c.c. contains about 100 million sperm. However, men whose semen contains 20 million sperms per milliliter are probably fertile.. - In ejaculation the spermatozoa pass through the duct of epididymis, vas deferens, ejaculatory duct and finally the urethra. - **NB**: Sperms remain viable (able to fertilize ova) for only after ejaculation 48h = 2 days ## Anomalies of spermatogenesis ### Abnormal forms of sperms - Giant, dwarf, bicephalic... - May be present in the semen, they should not exceed 20% of the total count. ### Necrospermia - Dead sperms in the ejaculate. ### Oligospermia - Sperm count is below normal. ### Azospermia - Complete absence of sperm in the semen. ## Oogenesis - **Definition**: Changes that take place in the ovary during which the oogonia cells (primordial female germ cells) differentiate into 2ry oocyte (ready for fertilization). - **Start**: From 3rd month of intrauterine life till birth. then continues after puberty (during each ovarian cycle), and after fertilization & end at menopause. - **Site**: in the ovary ## Steps: During Fetal Life (Pre-natal maturation) 1. **Proliferation** - During early fetal life, oogonia proliferate by mitotic division giving daughter oogonia (each contains diploid (46=2n) number of chromosomes) (44 autosomes + 2X) 2. **Growth** - Each daughter oogonium increases in size to form primary oocyte which contains (44 autosomes + 2X) chromosomes. - A primary oocyte, together with flat epithelial cells surrounding it, is known as a primordial follicle. - When the surrounding flat cells become cuboidal, the follicle is called a primary follicle. - **NB**: All oogonia enlarge to form primary oocytes before birth. Iry oocytes begin the 1st meiotic division before birth, but completion of prophase does not occur until adolescence. ## Oogenesis - **Oogonium** 2n - **Primary oocyte** 2n - **Secondary oocyte** n - **Ovum** n - **3 Polar bodies** n - **Primary polar body** n - **Secondary polar body** n ## Post-natal maturation - **At time of birth**: all primary oocytes have reached 1st meiotic division & remain dormant till puberty under the effect of (oocyte maturation inhibitor) secreted by the follicular cells. - **At puberty**: 1. **First meiotic division** is completed shortly before ovulation - The 1ry oocyte divides by meiosis giving: - 2ry oocyte [haploid (23 = 1n) number of chromosomes, 22 + X] - 1st polar body (also haploid) receives a little amount of cytoplasm and is extruded to the perivitelline space where it degenerates. 2. **Second meiotic division**: occurs after ovulation, only if fertilization occurs: - The 2ry oocyte divides by mitosis into: - Mature ovum (haploid number of chromosomes, 22+ X) - 2nd polar body (also haploid) extruded to the pervitelline space and degenerates ## N.B.: - **The number of 1ry oocytes for each ovary at birth is 700,000-2,000,000** - **Number of Iry oocytes for each ovary at puberty is 40,000, the rest undergo degeneration (atresia). ** - **The long duration of the first meiotic division (up to 45 years) may account in part for the relatively high frequency of meiotic errors, such as non-disjunction (failure of paired chromatids to dissociate), that occur with increasing maternal age.** - **No Plimary oocytes form after birth in females, in contrast to the continuous production of primary spermatocytes in males** ## Differences between spermatogenesis and oogenesis | **Spermatogenesis** | **Oogenesis** | | ---------------------- | ------------------- | | Occurs in the testis | Occurs in the ovary | | Starts after puberty | Starts during intrauterine foetal life | | Continues till old age. | Ends at menopause | | Every daughter spermagonium gives rise to 4 equal spermatids. | Every daughter oogonium gives rise to one ovum and 3 polar bodies. | | Every spermatid transforms to a motile sperm. | No stage of transformation occurs and the ovum is non-motile. | ## Differences between a sperm and an ovum (2ry oocyte): | **Sperm** | **Ovum** | | ------------------ | ----------------------- | | Small (60 microns) total length | Large (120 microns) | | Numerous (100 million/cc) | Rare (400) per cycles | | Highly motile | Immotile | | Not covered by other cells | covered by other cells | | Little cytoplasm | Abundant cytoplasm | | 2 types: 22 + x or 22 + Y | Only 1 type: 22 + X | ## Female Reproductive Cycles - Starting from puberty, females undergo reproductive cycles in the ovaries and uterus every 28 days. These cyclic changes are under the effect of hormones: FSH & LH (secreted by pituitary gland) and estrogen & progesterone (secreted by the ovary in response to pituitary hormones) ### 1) Ovarian cycle - At puberty, under the influence of FSH, a number of 1ry oocytes begin to mature with each ovarian cycle, however, usually, only one primary follicle develops into a mature follicle & ruptures through the surface of the ovary to expel the oocyte. #### Phases: - **Follicular phase (14 days):** controlled by FSH hormone & ends by ovulation - The follicular cells surrounding the primary oocyte proliferate to form several layers of granulosa cells (stratum granulosum), that become separated from the oocyte by a hyaline thick membrane termed zona pellucida. - Stroma cells from the ovarian cortex condense around the stratum granulosum to form a sheath (theca) for the follicle, that divides into 2 layers: - Theca interna: consists mainly of cells and blood capillaries. - Theca externa: consists mainly of fibrous tissue and larger vessels. - Fluid filled spaces appear amidst the follicular cells and coalesce to form a single large cavity called the follicular antrum. This separates the granulosa cells into 2 parts: - An outer part: termed stratum granulosum. - An inner part: termed corona radiata around the oocyte + cumulus ovalis (cumulus oophorus) attaching the oocyte to one side of the follicle. - **The oocyte & the surrounding follicular cells in addition to the surrounding 2 theca sheathes is now called growing follicle or Graafian follicle.** - **NB**: - Theca interna is the main source of estrogen hormone (the main ovarian hormone that regulates reproductive organs). - With each Ovarian Cycle number of follicles begin to develop but only one reachess full maturity while others degenerate and become atretic. - **Luteal phase (post ovulatory phase, 14 days):** - It follows ovulation and leads to the formation of corpus luteum under the effect of luteinizing hormone (LH) - The corpus luteum will secrete progestrone hormone which is responsible for the secretory stage of uterine cycle. ### Oogenesis / Ovulation / Ovarian cycle - **Follicular phase** - **Ovulation phase** - **Luteal Phase** ## Ovarian Cycle Phases: - **View of ovary**: - **Follicular phase** - **Follicle grows** - **Follicle is released, ovulation ** - **View of follicle/ corpus lutem**: - **Corpus luteum forms then degenerates** ## Ovulation - **Definition**: rupture of the Graafian follicle & discharge of 2ry oocyte outside the ovary - **Onset**: usually on the 14th day of menstrual cycle. - **Signs**: - In some women there is some pain (middle pain) - The basal body temperature is raised (37.5°c). - **Causes**: the exact reason is unknown but some factors have a role: - **Mechanical factor**: by the increasing of follicle size & its squeezing effect on the capsule of the ovary at a protruded area (stigma) causing its atrophy to pave the way for ovulation. - **Hormonal factor**: FSH and LH hormones lead to accumulation of fluid inside the mature follicle. - **Endosmotic factor**: due to increase of concentration of salts in the intrafollicular fluid ## Steps : 1. **At ovulation, the mature follicle ruptures at the surface of the ovary then the 2ry oocyte with the corona radiata escape to enter the uterine tube.** 2. **If fertilization occurs, the 2nd meiotic division is completed to give mature ovum & 2nd polar body** 3. **The ovum life span is up to 1 day then it is either fertilized or degenerate.** 4. **After ovulation some changes occur in the follicle under the effect of LH = Luteinization: lipid deposition in the granulosa cells, so appear yellow in color, and is termed corpus luteum Granulosa lutein cells start synthesis of progesterone.** ## Fate of corpus Luteum - **If Fertilization does not occur**: the corpus luteum rapidly degenerates and becomes transformed into a white mass, corpus albicans. - **If fertilization takes place**: the corpus luteum persists (under the effect of chorionic gonadotrophines, secreted by trophoblast surrounding the growing embryo) and continues to secrete progesterone for 3-4 months until the placenta is formed, then degenerates. ## Structure of a mature ovum - **Shape & size**: large oval cell of 120 - 150 micron in diameter - **Covering**: It has 3 coverings: - Inner thin: vitelline membrane. - Outer thick: Zona pellucida (in between there is the previtelline space). - Outer surrounding of 2-3 layers of corona radiata ## 2) Uterine Cycle - Changes occur every 28 days in the endometrium of uterus under the influence of ovarian hormones. During each ovarian cycle, the endometrium of the fundus and body of uterus undergoes changes that end bleeding (menstruation). The mucosa of cervix and uterine tubes are not involved in these cyclic changes. ### 1- Stage of menstruation (bleeding) - **Duration**: about 5 days. - **Amount of blood lost**: about 50-60 с.с. - **Consists of**: blood and pieces of superficial layers of the endometrium (This blood does not clot). ### 2- Stage of repair - **Epithelial cells from the bases of the uterine glands divide & spread to cover the raw endometrium by flat cells** - **Duration**: about 3 days; from the 6th to the 9th day of menstrual cycle. ### 3-Stage of proliferation - **Duration**: 7 days; from the 10th to the 16th day of menstrual cycle. - **The cells become low columnar.** - **The endometrium grows to become 3 mm. thick.** - **The uterine glands become longer & straight, later they become slightly convoluted.** - **N.B**: The stages of repair & proliferation are under the effect of the estrogen hormone secreted by the theca interna cells of the Graaftan follicle. ### 4-Stage of secretion - **Duration**: from the 17th day to the 28th day of menstrual cycle. - **The endometrium become thicker (5-7 mm.) and is formed of 3 layers:** - Superficial compact layer, middle spongy layer & deep basal layer. - The cells become columnar. - The uterine glands become more convoluted and full of secretion. - The spiral arteries become dilated and tortuous. - The endometrium becomes edematous and soft. - **N.B.:** This stage is under the effect of the hormone progesterone secreted by the corpus luteum. ## Possibilities on the 28th day of the menstrual cycle: - **A.** **If fertilization had occured**: - The endometrium becomes thicker & more vascular. - No menstuation occurs - **B.** **If fertilization did not occur**: Menstruation occurs on the 28'h day and the cycle is repeated. ## Events of the 1st Week ### 1- Fertilization - **Definition**: It is the process of union between the male and female gametes to form the zygote - **Site**: the lateral 1/3 of the uterine tube. #### Steps: 1. **Preparatory steps** - After insemination, the sperms pass rapidly from the vagina into the uterus and the uterine tube, (movement of sperm by its tail is helped by contraction of the uterus and uterine tubes). - Out of the 200 - 300 million ejected sperms in female genitalia, only 300 - 500 reach the site of fertilization and only one of these succeed to penetrate the 2ry oocyte. - The power of fertilization of the sperm is lost within 48 hours after ejaculation. 2. The sperms cannot fertilize an ovum before 2 processes happen: - **Capacitation of the sperm (activation)**: final step of sperm maturation after ejaculation, occurs in female genital organs for 7 hours during which the glycoprotein coat is removed from the plasma membrane overlying the acrosomal region. - **Acrosomal reaction**: occurs after binding to the zona pellucida. This leads to the release of enzymes needed to penetrate the zona pellucida. - **The released acrosomal enzymes are**: - Hyaluronidase: for digestion of corona radiata. - Trypsin-like substance & zonalysin: for penetration of the zona pellucida ### Release of acrosomal enzymes allows sperm to penetrate the zona, thereby coming in contact with the plasma membrane of the oocyte. - Permeability of the zona pellucida changes when the head of the sperm comes in contact with the oocyte surface. - This contact results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte. - In turn, these enzymes alter properties of the zona pellucida (zona reaction) to prevent other sperm penetration. ## II- Actual steps of fertilization - **Phase 1-penetration of the corona radiata by**: - The hyaluronidase enzyme released from the acrosome. - Tubal mucosa enzymes. - Movements of the tail of the sperm. - **Phase 2 - penetration of zona pellucida by**: - The trypsin like substance & zonalysin released from the acrosome - **Phase (3): union of 2ry oocyte + sperm cell membranes.** - As soon as the sperm contact oocyte cell membrane, both cell membranes fuse then the oocyte Responds by 3 different ways: - The oocyte (vitelline) membrane becomes impenetrable to other spenns. - The zona pellucida prevents entry of other sperms. - The oocyte finishes its 2nd step of reduction division to give 2nd polar bodies + the female pronucleus of the mature ovum. - **The head of the sperm becomes rounded (swollen) to form male pronucleus that unite with the female pronucleus to finish the process of fertilization. The activating factors are probably carried by the spermatozoon.** - **The male and female pronuclei (each containing 23(n) chromosomes) unite together to form one nucleus (containing 46 chromosomes, 2n) & the fertilized ovum is now called the zygote** ## Results of Fertilization 1. **The oocyte completes its second meiotic division.** 2. **The number of chromosomes become 46 again (restoration of normal chromosomal number, diploid).** 3. **Determination of the sex as follows:** - **Sperm of (22+Y) + ovum of (22+X) --> (44+XY) male offspring.** - **Sperm of (22+X) + ovum of (22+X) --> (44+XX) female offspring.** 4. **Determination of the general characters by the autosomes. It allows variation in the human species through mingling of maternal and paternal chromosomes.** 5. **The zygote begins to divide to form the different organs (Cleavage).** ## In vitro fertilization (IVF) - In cases of sterility especially Fallopian tube occlusion, in vitro fertilization can be used as a treatment. - 2ry oocytes are obtained from the wife by an endoscope & fertilized by sperms of the husband on a glass plate. - Under special circumstances, a dividing zygote is obtained & transferred to the uterus of the wife. ## II - Cleavage Of The Zygote - **Definition**: Repeated division process (series of mitosis) by which a unicellular zygote is transformed into a multicellular mass of embryonic cells (blastomeres). These blastomeres become smaller with each successive cleavage division. - **Zygote** - **1st cleavage**: 2 Cells - **2nd cleavage**: 4 Cells - **3rd cleavage**: 8 Cells ## The functions of zona pellucida are: - **Zonal block prevent penetration of another sperm after fertilization.** - **Keep blastomeres together during cleavage.** - **Prevent adhesion of blastomeres to the uterine tube.** - **Start approximately 30 hours after fertilization.** ## Steps 1. **After fertilization the zygote shows 2 main features:** - **It undergoes repeated mitotic divisions.** - **It moves towards the uterine cavity (by action of cilia of cells lining the uterine tube).** 2. **The zygote, by mitotic division, passes through stages: 2- cell stage, 4-cell stage, & so on till morula stage (12-16 cell stage) after 72 hours.** ## 3- The morula: - Will continue division and the inner cells of the morula constitute the inner cell mass (embryo proper), and surrounding cells compose the outer cell mass (trophoblast). - It reaches the uterine cavity on the 4th day after fertilization where it undergoes the following changes: - **The zona pellucida disappears.** - **Fluid absorbed from the uterine cavity and/or secreted by trophoblast, accumulates in the morula leading to the formation of blastocyst.** - **The cavity of the blastocyst is called blastocele.** - **Its cells become arranged into 2 masses:** - An outer cell mass: forms the trophoblast - An inner cell mass: (formative cell mass or embryoblast) will give rise to the embryo. - **The pole of the blastocyst at which the inner cell mass is attached is termed the embryonic pole while the opposite one is the abembryonic pole.** ## Results of cell cleavage: - Increase in number of cells & decrease in size of cells to reach normal size of the species. ## Embryonic stem cells (ES cells) - **Are derived from the inner cell mass of the embryo.** - **Because these cells are pluripotent and can form virtually any cell or tissue type, they have the potential for curing a variety of diseases, including diabetes, Alzheimer's and Parkinson's diseases, anemias, spinal cord injuries, and many others.** ## III - Implantation - **Definition**: it is the process of embedding of blastocyst into the uterine wall. - **Site**: the normal site is the upper part of the body of the uterus mainly the posterior wall, in the midline, near the fundus. ### Steps 1. **At the 6th day after fertilization** - **The trophoblastic cells over embryonic pole adhere to the uterine wall then it erodes the endometrium and the whole blastocyst progressively penetrates it (start of Implantation).** 2. **The trophoblastic cells rapidly proliferate** - **& become formed of 2 layers as soon as it touches the endometrium (Decidua).** - **An outer Syncytial trophoblast formed of a mass of protoplasm with randomly dispersed nuclei.** - **An inner Cellular trophoblast (cytotrophoblast) formed of cells with well-defined walls.** 3. **At about the 9th day** - **The blastocyst becomes more deeply embedded in the endometrium and the penetration defect in the surface epithelium is closed by coagulation plug.** 4. **At the 11th and 12th days** - **The blastocyst becomes completely implanted, and the surface epithelium is restored.** ## N.B.: - **The blastocyst is superficially implanted in the compact layer of the endometrium and is deriving its nourishment from the eroded maternal tissues** ## Abnormal sites of implantation: - **These are sites other than the normal site of implantation (which is the upper part of the posterior wall of the uterus)** ### 1-Intrauterine sites: (placenta previa) - **Implantation inside the endometrium very Close to the cervix of the uterus (i.e. in the lower uterine segment)** - 1 - Placenta previa lateralis: Just above internal os - 2 - Placenta previa marginalis: Covers internal os partially - 3 - Placenta previa centralis: Covers internal os completely ### 2-Extra-uterine sites - **Tubal Pregnancy**: In the uterine tube - **Abdominal Pregnancy**: In the abdominal cavity - **Ovarian Pregnancy**: On the surface of the ovary ## N.B.: - **Implantation outside the cavity of the uterus leads to ectopic pregnancy which cannot reach full-term due to inadequate nutrition.** ## IV - The Decidua - **Definition**: Is the name given to the endometrium of the pregnant uterus, or gravid endometrium. (Decidua: means, structure that separate, because it is shedded completely at the time of child birth) - **Just after fertilization, the endometrium starts a decidual reaction which becomes more marked after embedding of the blastocyst.** ## The decidual reaction: - **Stroma, cells**: become enlarged, vacuolated and filled with glycogen & lipids (called decidua cells). - **Uterine glands**: are distended greatly with secretions. ### Parts of the decidua - **Formed of 3 parts in relation to blastocyst:** - **Decidua capsularis**: the part that lies superficial to the blastocyst (between blastocyst and uterine cavity). - **Decidua basalis**: The part that lies deep to the blastocyst (between blastocyst and muscle wall of uterus). - **Decidua parietalis**: The part that lines the remaining wall of the uterus. ## Events of the 2nd Week ### 1 - Formation Of Bilaminar Embryonic Disc - **Implantation of the blastocyst is completed during the second week (11th day)-12** - **As implantation of the blastocyst progresses, a small space appears in the embryoblast; the primordium of the amniotic cavity.** - **Soon amnioblasts (amnion-forming cells), separate from the epiblast and form the amnion, which encloses the amniotic cavity.** - **Concurrently, morphologic changes occur in the embryoblast that result in the formation of a flat, almost circular, transparent bilaminar embryonic disc, consisting of two layers:** - **Epiblast (future ectoderm)**: consists of high columnar cells related to the amniotic cavity. - **Hypoblast (future endoderm)**: consists of small Cuboidal cells adjacent to the exo-coelomic ## N.B.: - **Extra-embryonic structures forming during the second week**: - Amniotic Cavity - Amnion - Yolk Sac - Connecting Stalk - Chorion ### II - Formation Of The Extra-Embryonic Mesoderm - **Extra-embryonic (1ry) mesoderm arises from the hypoblast (1ry endoderm).** - **It surrounds the amnion and the yolk sac & separates them from the trophoblast.** - **Small cavities appear in this mesoderm, that coalesce together to form a single cavity called the extra-embryonic coelom, which splits the mesoderm into two layers:** - **An outer layer** - Lining the trophoblast and covering the amnion, is called somatic or parietal layer of extraembry- onic mesoderm. - **An Inner layer** - Covering the yolk sac, is called visceral (splanchnic) layer of extraembryonic mesoderm. - **The trophoblast & the underlying somatic extra-embryonic mesoderm is called chorionic plate, and the blastocyst is now called chorionic vesicle.** - **However, the coelom incompletely surrounds the vesicle, as a part of the mesoderm persists without cavitation, this connects the caudal part of the embryonic disc to the wall of the chorionic vesicle.** - **This area of extraembryonic mesoderm is termed the connecting (body) stalk through which foetal blood vessels pass between the embryo and placenta.** - **Later, the amniotic cavity expands while the yolk sac shrinks & the mesoderm covering the amnion comes in contact with that lining the chorion.** - **As a result the extra-embryonic coelom becomes greatly obliterated.** ## Development of the chorionic villi - **The wall of the chorionic vesicle is termed the chorion.** - **It is composed of 3 layers (from outside to inside):** - **Syncytiotrophoblast** - **Cytotrophoblast** - **Somatic layer of primary mesoderm ** - **The outer surface of the chorionic vesicle shows a large number of proliferations termed chorionic villi** - **Thought to be induced by the underlying extra-embryonic somatic mesoderm, distributed as follows:** - **Chorion Frondosum**: The villi lying opposite the decidua basalis Are well-developed - **Chorion Laeve**: The villi lying in contact with the decidua capsularis Are less developed (atrophic) ## Types of chorionic villi - **There are 3 types of villi according to the complexity of their structure:** - **Primary villi**: formed of the 2 layers of trophoblast separated from each other by lacunae (spaces filled with maternal blood). - **Secondary villi**: are 1ry villi invaded by cores of m mesoderm. - **Tertiary villi**: These are 2ry villi invaded by foetal blood vessels within the core of the Iry mesoderm. ## Development of Placenta - **The placenta is the primary site of nutrient and gas exchange between the mother and fetus.** ### Steps of formation: 1. **Fetal Part**: Chorion Frondosum. 2. **Maternal Part**: Decidua Basalis. - **By the beginning of the 4th month the placenta develops from two components:** ### During 4th - 5th month - **The decidual cells form a number of septa that project into the inter-villous space but don't reach to chorionic plate.** - **These septa divide the placenta into a number of compartments called cotyledons.** ## Gross appearance of full-term placenta: - **Shape**: Flattened disc - **Measurements**: - **Diameter**: 15-25 cm. - **Thickness**: 3 cm. - **Weight**: 500-600 gm. - **In general it covers 25-30% of internal surface of the uterus (its increase in thickness is due to branching of the existing villi).** ### Surface - **Foetal surface**: - The inner Smooth surface which is covered by the amnion. - The umbilical cord is attached near its centre. - **Maternal surface**: - The outer rough surface that shows a number of elevated areas called cotyledons (15-20) separated from each other by grooves, representing the sites of the placental septa. ## Microscopic structure of Placenta - **The placenta shows:** - **Tertiary chorionic villi of chorion frondosum** will exhibit further changes to form foetal part of placenta: - **Stem or anchoring villi**: extend from the chorion to get attachment to the decidua basalis. - **Free or absorbing villi**: extend laterally to increase surface area, thus increase the change between fetal & maternal blood. - **Intervillous space** - Lined with syncytial trophoblast. - It is filled with maternal blood from the maternal vessels which open into the spaces. - **Cytotrophoblastic shell** - The cytotrophoblast at the end of the anchoring villus penetrates the syncytiotrophoblast and extends in a lateral direction to fuse with similar cytotrophoblast from other villi thus forming a continuous shell surrounding the embryonic vesicle to prevent further penetration of decidua by syncytiotrophoblast. ## Placental barrier - **It is the membrane separating the maternal blood present in the intervillous space from the foetal blood present in the foetal vessels within the free villi.** - **During the 1st half of pregnancy it is thick & is composed of:** - Syncytiotrophoblast - Cytotrophoblast - 1ry mesoderm. - Endothelium of foetal blood vessels. - **By the 2nd half of pregnancy**: - the cytotrophoblast and primary mesoderm disappear - the other layers of the barrier become very thin and consequently more permeable allowing rapid exchange (i.e. syncytiotrophoblast + endothelium of fetal blood vessels). ## N.B.: - **The maternal & foetal blood never mix together but remain always separated by a barrier except during birth, they could be mixed.** ## Functions of the placenta: 1. **Nutritive function:** - by transporting: (Water, lucose, amino acids, antibodies, antigens, free fatty acids, neutral fat, electrolytes and vitamins) to the foetus. 2. **Respiratory function:** - Exchange of oxygen. & carbon dioxide takes place. 3. **Excretory function:** - It gets rid of the few amounts of urine constituents secreted by the foetal kidney. 4. **Secretory function:** - It secretes: - Chorionic gonadotrophins, Estrogen, Progesterone & Placental lactogen. - Chorionic gonadotrophins start to appear in mother's urine from the 9th day of pregnancy (detected by pregnancy tests). 5. **Barrier:** - To protect against the transmission of infection from mother to fetus. - Antibodies also can pass from mothergiving immunity to the fetus against some infections as diphtheria measels, and small pox. - But some viruses can pass as that of german measles, syphilis, poliomyelitis & AIDS. - Rh factor antibodies & agglutinins can also pass. Also some drugs can pass. ## Placental circulation - **Fetal blood arrives to the placenta by 2 umbilical arteries which branch to form capillaries of the villi where gaseous exchange occurs with the maternal blood in the intervillous space around the villi.** - **Finally, the oxygenated blood returns to the fetus by the left umbilical vein.** ## Anomalies of the placenta ### 1. Abnormal attachment - **If the placenta is attached to the lower uterine segment (placenta previa)** - 1 - Placenta previa lateralis: Just above internal os - 2 - Placenta previa marginalis: Covers internal os partially - 3 - Placenta previa centralis: Covers internal os completely ### 2. Abnormal distribution of chorionic villi - **Diffuse placenta (placenta membranacea)** - The chorion frondosum occupies a more extensive area of the chorion - **Divided placenta** - Bipartite or tripartite: - Separate accessory lobule (placenta succenturiata). ### 3. Abnormal sites of umbilical cord attachment - **Battledore placenta**: The umbilical cord is attached to the margin of the placenta (normally, it is attached near its centre) - **Velamentous insertion of the cord**: The umbilical cord is attached to the adjacent foetal membranes just away from the margin of the placenta ### 4. Chorion epithelioma - **malignant tumor of placenta.** ## Twins ### Uniovular (Monozygotic. One egg) twin: - 1 zygote - 2 embryos. - Identical except finger prints. Same sex. - Usually single placenta & 1 or 2 amnion Sac - Incidence: 30%. ### Conjoined twins: - If the embryoblast or embryonic disc does not divide completely (incomplete separation), some identical twins become fused and various types

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