Intro to Embryology I Week 1 Embryonic Development PDF

02/11/23 Intro to embryology I: from fertilised egg to bi-laminar disc: Learning objective: de ne and describe the process of fertilisation ( revise gametogenesis ) Learning objective: describe the process of zygote cleavage Learning objective: de ne and describe the process of embryo hatching and i...

02/11/23 Intro to embryology I: from fertilised egg to bi-laminar disc: Learning objective: de ne and describe the process of fertilisation ( revise gametogenesis ) Learning objective: describe the process of zygote cleavage Learning objective: de ne and describe the process of embryo hatching and implantation Learning objective: describe the formation of bi-laminar embryonic disc Learning objective: appreciate spatio-temporal issues First week of embryonic development: Revision of spermatogenesis: Nothing happens in the testis until puberty, this is when testosterone is produced. Each spermatid will carry either the X or the Y chromosome which will determine the sex of the sex of the future baby. Spermatid will differentiate into spermatozoa via spermiogenesis. Revision of oogenesis: 1-2 months before birth, most of the 7 million oogonia die, and the remaining sur viving oogonia enter meiosis I and become primary oocytes. These arrest at prophase I until the start of the menstrual cycle ( LH- luteinising hormone ). The secondary oocytes arrest at metaphase II until fertilisation. If fertilisation doesn’t take place, the secondary oocyte will die. Asymmetrical division; a polar body is formed. I consists of follicular cells Fertilisation: Before the sperm can fertilise the egg it must undergo a process of capacitation. This is a conditioning process that involves epithelial interactions bet ween the sperm and mucosal surface of the uterine tube. A glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa. Phases of fertilisation: 1. Penetration of the corona radiata: Enzyme hyaluronidase which released from the acrosome of the sperm and assisted by tubal mucosal enzymes plays a major part in dispersal of the follicular cells of corona radiata. This process is also aided by movement of the sperm tail. 2. Penetration of the zona Pellucida: The Z.P is composed of glycoproteins and it facilitates and maintains sperm binding. It also induces the acrosome reaction leading to the release of the proteolytic enzyme acrosin as well as esterases and neuraminidase that causes the lysis of the Z.P allowing the sperm to come in contact with the plasma membrane of the ovum. 3. Fusion of the plasma membranes of the ovum and the sperm: The fusion results in cortical or zona reaction, a change in Z.P that prevents other sperm from entering. The head and tail of the sperm enter the cytoplasm of ovum, but the plasma membrane and the mitochondria of the sperm remain behind. 4. Completion of the second meiotic division of the secondary oocyte: Completion of the second meiotic division occurs immediately after entry of the sperm. This produces the second polar body and the mature ovum. The Chromatin material of the mature ovum is arranged into the female pronucleus. 5. Formation of the make pronucleus: Within the cytoplasm of the ovum, the chromatin material of the sperm ( 22X or 22Y ) is organised into the male pronucleus. The sperm tail detaches and degenerates. The combination of 23 chromosomes in each pronucleus results in a zygote with 46 chromosomes. During growth of male and female pronuclei ( both haploid ) each pronucleus replicates its DNA. 6. Metabolic activation of the ovum: Immediately after DNA synthesis, breakdown of the pronuclear membranes occur followed by condensation and mixing of paternal and maternal chromosomes. Chromosomes organise on the spindle in preparation for a normal mitotic division and the rst cleavage division of the zygote occurs. Results of fertilisation: Stimulation of the secondary oocyte to complete meiosis II, producing the second polar body. Restoration of the normal diploid number of chromosomes. Variation of human species; fertilisation is random. Determination of the sex of the baby. Metabolic activation of the oocyte that initiates cleavage of the zygote. Fertilisation abnormalities: Dizygotic t wins ( fraternal, non-identical ) arise from separate fertilisation events involving t wo separate oocyte and spermatozoa. These have separate placentas. Monozygotic t wins ( identical ) are produced from a single fertilisation event and therefore share the same genome. They also share the same placenta. Cleavage of zygote and blastocyst formation: Cleavage: repeated mitotic divisions of the zygote, resulting in a rapid increase in the number of cells that are called blastomeres. After the third cleavage, compaction process takes place. Approximately 3 days after fertilisation a 16-cell morula is formed. Approximately 4 days after fertilisation the morula enters the uterine cavity and the blastocyst cavity or blastocoele is formed. The blastomeres are organised into the trophoblast and the embryo blast ( inner cell mass ) and the embryo is called a blastocyst. Trophoblasts provide a supportive role whereas the embryoblasts will form the embryo. Embryo hatching and implantation: Approximately 5 days after fertilisation the embryo frees itself from the Z.P through a process called hatching. Hatching is a result of a series of expansion-contraction cycles that are also supported by enzymes that dissolve Z.P. Approximately 6 days after fertilisation the hatched blastocyst attaches to the endometrial epithelial ( decidua ). As a result of trophoblast and endometrial interaction, the trophoblast cells begin to rapidly differentiate and proliferate into: cytotrophoblasts and syncytiotrophoblasts. The syncytiotrophoblasts produce proteolytic enzymes that erode the maternal tissue enable to embryo to burrow into the endometrium. By the end of the rst week, the embryo is super cially implanted into the endometrium and is deriving its nourishment from eroded maternal tissues, y Week 1 summary: