Embryology PDF

Embryology Embryology is a branch of science that deals with the study of formation and development of an organism. ֍ Growth is an increase in the number and size of cells. ֍ Differentiation is the specialization of the tissues of different organs to perform special functions. N.B: Development...

Embryology Embryology is a branch of science that deals with the study of formation and development of an organism. ֍ Growth is an increase in the number and size of cells. ֍ Differentiation is the specialization of the tissues of different organs to perform special functions. N.B: Development involves growth and differentiation. periods of human embryology Most of the clinicians divide human prenatal development as first, second and third trimesters, 3-month period for each. Embryologically, prenatal development is divided into 3 periods: 1\. Germinal period : First two weeks after fertilization. 2\. Embryonic period: From 3rd till the end of the 8th week (2 nd month). 3\. Fetal period (organ growth): From 3 rd month till the end of the pregnancy. Embryological Periods 238 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 First Week of Development Events in the first week of development The first week of development Fertilization Definition: fusion of mature sperm with an ovum ( haploid germ cells ) to form a single zygote (diploid cell ). Usual site of fertilization: lateral 1/3 of uterine (fallopian) tube ( = Ampulla of fallopian tube). Preparation of sperms for fertilization 1\. Capacitation of spermatozoa Capacitation is a process of conversion of immature spermatozoa to mature one by removal of glycoprotein coat covering the head of sperm. 239 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 It takes 7 hrs. Site: Female reproductive tract (uterus and uterine tubes). Effect of capacitation: -- Increases sperm motility -- Sperm becomes capable of penetrating the outer layers of the egg N.B: Spermatozoa are artificially capacitated and used in vitro fertilization (IVF). 2.Acrosomal reaction: The acrosomal cap release of its contents (acrosin, hyaluronidase, and trypsin like substance) needed to pass through corona radiata and penetrate zona pellucida). Phases of fertilization: 1\. Passing through corona radiata: out of 300-500 sperms reach the site of fertilization. Only one fertilizes the ovum (the rest only help it to pass through the corona radiata). 2\. Penetration of zona pellucida: by the aid of enzymes released from the acrosomal cap. 3\. Fusion of oocyte-sperm cell membranes: The head of the spermatozoon enter the cytoplasm of the oocyte. Phases of fertilization 240 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 The changes of oocyte in response to the spermatozoon entrance 1.Cortical and zona reactions: Cortical receptors alter their structure. Zona pellucida changes its structure and composition to become impermeable. These prevent polyspermy. 2\. As soon as the sperm penetrate the secondary oocyte , the latter undergoes Second meiotic division and gives rise to an ovum. Results of fertilization: 1\. The head of sperm swells to form the male pronucleus , also the nucleus of the ovum swells to form the female pronucleus. The male and female pronuclei ( each containing 23 chromosomes ) unite together to form one nucleus ( containing 46 chromosomes ) and now the fertilizaed ovum is called zygote. 2\. The number of chromosomes become (46). 3\. Determination of sex. 4\. Initiation of cleavage: Fertilization provides energy for repetitive cell divisions of zygote. Without fertilization Oocyte degenerates. CLEAVAGE -Definition: -Cleavage is a process of repeated mitotic divisions of zygote to give rise to smaller cells called blastomeres. -Duration: Up to 6--7 days (after fertilization till implantation). -Site: begins in the ampulla of fallopian tube, ends in the uterine cavity (where the conceptus reaches the site of implantation). -Fertilized ovum divides to form 2-cell, 4-cell stage and so on. About the 3 rd day, a 16-cell stage is formed (known as morula) that surrounded by zona pellucida. 241 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 Cleavage Blastocyst Formation: -Morula enters the uterine cavity around day 4. -Fluid enters between the blastomeres and form a cavity (blastocele). This stage is called blastocyst. \- The outer cell mass flatten (now called trophoblast) and inner cell mass (now called embryoblast) lie at embryonic pole. -By the end of 4th day, zona pellucida starts degeneration and disappears by the end of 5th day to allow beginning of implantation. IMPLANTATION OF BLASTOCYST Definition: - It is the process of embedding of blastocyst into the uterine endometrium. Time: begins on 6th or 7th day after fertilization and completes by the 12th day. Implantation Site of Implantation: - The normal site is the midline of the upper part of the body of the uterus in the anterior or posterior wall, near the fundus. Normal site of implantation 242 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 Implantation process: Blastocyst fixes its embryonic pole to site of implantation. The trophoblastic cells rapidly proliferate & becomes formed of 2 layers: a-An outer Syncytiotrophoblast formed of cytoplasm with randomly dispersed nuclei and without cell membrane. b-An inner Cytotrophoblast formed of cells with well-defined walls. Syncytiotrophoblast is phagocytic and makes a pit in the endometrium. Blastocyst is gradually sinks into the pit until it sinks completely into the endometrium. Blastocyst completely embeds in the endometrium by the 12th day of development. Site of penetration is closed by fibrin plug at 9th day. Surface epithelium overgrows the fibrin plug at 11th day. Endometrium of pregnancy called decidua. Implantation process 243 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 Abnormal site of Implantation If conceptus does not implant at the usual site, it is called abnormal or ectopic implantation and it results in ectopic pregnancy. Ectopic pregnancy or implantation are classified according to their site of implantation as follows: A. Uterine abnormal implantation Placenta previa: It is an implantation in lower uterine segment. placenta previa lateralis = placenta is attached to the lateral wall of the body of uterus. placenta previa marginals = covers internal os partially. placenta previa centralis = covers internal os completely. Placenta previa B. Tubal implantation: It is an implantation in uterine tube. It is the most common extrauterine implantation C. Abdominal implantation: It is a rare ectopic implantation. It usually occurs in the ovary or mesentery N.B.: - Implantation outside the cavity of the uterus leads to ectopic pregnancy which cannot reach full-term due to inadequate nutrition. 244 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 Abnormal sites of implantation THE DECIDUA Definition: It is the endometrium of the pregnant uterus after implantation. Parts of the decidua: - 3 parts in relation to blastocyst: - 1-Decidua capsularis: - The part between blastocyst and uterine cavity. 2-Decidua basalis: - The part between blastocyst and wall of uterus. 3-Decidua parietalis: - The part that lines the remaining wall of the uterus. 245 ENDOCRINE & REPRODUCTIVE SYSTEM MODULE 2024-2025 The decidua Events occur in the second Week of Development