ANAT 1502 Key Embryo Terms

ANAT 1502 Key Embryo Terms - Key concepts of gametogenesis...

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Unformatted text preview: Key concepts of gametogenesis Gametogenesis Generic term for the genesis of haploid gametes out of diploid primordial germ cells through meiosis. Gametes Collective designation for male and female germ cells (oocytes = mature egg cells and spermatozoa); they arise through oogenesis and spermatogenesis and, following the meiotic reduction division, contain only a single (= haploid) chromosome set. Through the fertilization the diploid zygote is generated from the male and female gametes. Germline Comprises the non- somatic cells of an individual that are determining for the transmission of the genetic material. Oogenesis comprises the entire development from oogonium to a mature oocyte. Oogonia Descendants of the primordial germ cells in the female gonadal primordium. Primary follicle The outer surface consists of a single-layered, cuboidal epithelium and it contains the oocyte. Primordial follicle The outer surface consists of a single-layered, flat epithelium and it encircle the primary oocyte. Primordial germ cells are generated in the wall of the vitelline sac (yolk sac) towards the end of the 3rd week. They make their way into the gonadal primordium until the end of the 6th week. Primordium The earliest discernible indication of an organ or structure during embryonic development. Secondary follicle The surface epithelium has several layers; it harbors just the oocyte. Spermatogenesis comprises the entire development from spermatogonia up to spermatozoa. It can be subdivided into spermatocytogenesis (from spermatogonia up to spermatides) and spermiogenesis (spermatohistogenesis). Spermatocytogenesis Commentary about meiosis In the metaphase of the first meiosis, the tetrads (i.e., the two homologous chromosomes that have divided into two chromatids [= (2n, 4c], respectively) are visible. In the first meiosis the chromosomes are distributed to the daughter cells (reduction). Thereby each daughter cell receives one of the two homologous chromosomes (1n, 2c), while the two chromatids from the respective chromosomes remain bound together. Note: in the usual mitosis the two chromatids of each chromosome end up in various daughter cells! In the second meiosis (representing a division) the centromeres divide, whereby the two chromatids are separated. Thereby each daughter cell contains a set of 23 chromatids (now termed chromosomes again). The cells are now haploid and are called spermatids. In the prophase of the first meiotic division various stages can be distinguished: leptotene, zygotene, pachytene, diplotene, diakinesis. In the Meiosis of the oocyte an in-between stage is added: the dictyotene, the socalled interphase. In the leptotene (= condensing) stage the DNA forms itself into chromosomes that already consist of 2 daughter chromosomes (= chromatids), due to the doubled DNA content. In the zygotene stage the homologous (i.e., maternal and paternal) chromosomes become paired by a synaptonemal complex as bivalents Every bivalent inserts with both ends on the inner surface of the nuclear membrane. Due to their close proximity they can be mistaken for a haploid set of chromosomes! In the pachytene (= exchange of genetic material) stage, which lasts the longest time, the chromosomes shorten and thicken themselves by substantial knotting of the DNA and the two chromatids of each chromosome draw away from each other along their lengths. They remain connected together only by the centromere, so that the typical scissor or pliers form of the chromosomes becomes visible. One recognizes the 4 chromatids of a bivalent (tetrad formation) only now. In this stage the formation of the so-called chiasmata, the "crossing over", occurs. In the diplotene stage the synaptonemal complexes loosen and partial separation of each pair of sister chromatids from their homologous counterparts occurs. The chromatids are still held together at the centromeres and the sites of crossing over. The dictyotene stage is the resting phase of the oocyte. It lasts from birth up to sometime between the twelfth and fiftieth year of age, depending on the moment of the atresia of the follicle or of the ovulation. The chromosomes are decondensed during this phase. In the diakinesis a further condensation of the chromosomes and, simultaneously, a further separation of the chromatids takes place that, however, still hang together at the chiasmata. Nucleolus and the nuclear membrane dissolve. Key concepts having to do with fertilization Acrosome reaction: The release of various lytic (dissolving) enzymes from the acrosomal cap of the spermatozoon. The enzymes help the sperm cell make its way through the corona radiata and the pellucid zone. Capacitation: Technical term for the changes that must take place to a freshly ejaculated spermatozoon so it gets closer to being capable of fertilizing an oocyte. Corona radiata: When the follicle tears, the oocyte is washed out together with cumulus cells that surround it. These granulosa cells that form the cumulus oophorus in the mature follicle look like a wreath and are thus called the corona radiata. Coitus: The sexual act with the aim of producing conception/pregnancy. Ejaculate: The ejaculate is made up of spermatozoa (10%) and the seminal plasma fluid (90%). Fertilization: Union of the male and female germ cells to become a new organism. The first signs of fertilization (pronucleus) that can be observed in assisted fertilization occur roughly 16-20 hours after the impregnation. Follicle fluid: the watery-yellow fluid (2-6 ml) that pours out of the follicle into fallopian tube at the time of ovulation. Impregnated oocyte: An oocyte in which the spermatozoon has entered (impregnation), but the fusion of the pronucleus (which normally takes between 18 - 26 hours) has not yet taken place. Impregnation (human medicine) / insemination (veterinary medicine): 1. 2. Addition of spermatozoa to the oocyte during assisted fertilization. Artificial introduction of spermatozoa into the inner female genital tract. Meiosis: A special form of cell division, occurring in sex cells, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species. Oocyte: Synonym for egg cell. Primary oocyte ==> oocyte before the end of the first meiosis. Secondary oocyte ==> oocyte after the end of the first meiosis (driven by the rapid rise in LH [luteinizing hormone] and FSH [follicle stimulating hormone] concentrations) Pellucid zone: A membrane that envelops the oocyte. The cumulus cells are anchored there. Pronucleus (synonym for haploid nucleus): The paternal or maternal nuclear material that is formed in the impregnated oocyte. When both come together, they arrange themselves into a prophase nucleus. Seminal plasma: Collective term for all the glandular secretions (from the ampulla of the deferens duct, seminal gland, prostate, bulbourethral gland, urethral gland) which are mixed together during the ejaculation process. Spermatozoon, spermatozoa / sperm cell, sperm cells are synonyms Zygote: The first cell of the embryo. It only exists for a short time (a few hours). From the very beginning it is "born" with a prophase nucleus and its "job" is to divide as soon as possible. Key concepts of the preimplantation phase Zygote: The first cell of the embryo. It exists from the metaphase of the first mitosis to the twocell stage. It thus exists only for a short time. Cleavage: Mitotic division, beginning with the zygote into ever smaller daughter cells (blastomeres) until the morula stage Morula: (Diminutive of the Latin "morum", mulberry) solid, spherical collections of 16 - 30 blastomeres; result of the cleavage on the 4th day after the fertilization. Develops further to blastocyst. Blastomeres: Cells resulting from the cleavage of the zygote; they divide without growing and by each division they become smaller, the plasma/nucleus-relationship shifts in the direction of more nucleus. Blastocyst: The cell collection that develops from the morula on roughly the 5th day after the fertilization, forming the blastocyst cavity (blastula) and in which an outer cellular layer (trophoblast) and an inner cell mass (embryoblast) can be distinguished. Compaction: Thickening of the cell packing. The prerequisite is the setting up of cell contacts and a reduction of the cellular volume. Trophoblasts: Cellular outer wall of the blastocyst that at the nidation in the endometrium differentiates further into two layers: the syncytiotrophoblast and the cytotrophoblast. Embryoblasts: Consisting of only few cells, the small, inner part of a blastocyst from which the embryonic disk develops following a successful implantation. Abembryonic pole and embryonic pole of the blastocyst: The location within the blastocyst where the small cell collection, consisting of a few cells, is found is termed the embryonic pole. The opposite pole is then called the abembryonic pole. Hatching from the pellucid zone occurs at the abembryonic pole whereby implantation in the endometrium happens at the embryonic pole end. Hatching: Emergence of the blastocyst out of the protective pellucid zone at the end of the 5th day. Pellucid zone: Protective, tough covering of unchangeable size that surrounds the oocyte and the cells of the embryo up till the moment of hatching. The various forms of hormonal contraception • Estrogen and progesterone, combined or sequentially in pill form • Progesterone alone in the form of pills • Progesterone alone in the form of an implant • Progesterone given off by an intra-uterine system How the hormonal method of contraception functions • They lower FSH secretion so that normal follicle maturation does not occur and no LH peak happens. This effect is mainly due to estrogen, but progesterone can also inhibit ovulation when ingested in high dosages. • They transform the uterine endometrium so it becomes pseudoatrophic, thereby making an implantation practically impossible, should an ovulation and fertilization take place. • They modify the cervix mucous so that it becomes less permeable for sperm cells (this effect is mainly due to the progesterone). Most pills are monophasic, i.e., they always contain the same amounts of estrogen and progesterone for the whole cycle. Nevertheless, bi- and triphasic pills also exist that attempt to imitate the cyclic physiological changes. The ideal pill yields the lowest amounts of estrogen and progesterone but, at the same time, is an efficient contraceptive with minimal side effects. The main side effects • Thrombo-embolisms: primarily due to the alteration of coagulation factors that are induced by the estrogen. Risk factors are age (for those older than 35), arterial hypertonia and tobacco abuse • Myocardial infarction and cerebral insult, especially with smokers that are older than 35. The arterial disease is primarily due to the effects of progesterone. This lowers the HDL-2 value (high density lipoprotein) that has a heart protective activity. • Hypertonia: blood volume and angiotensin secretion are elevated. • Some studies have suggested that patients who are older than 25 and take progesterone for a long time have an increased risk of breast cancer. Oral contraceptives taken for an extended period of time are also supposed to increase the risk for cervical carcinomas. Less prominent side effects • • • Estrogen: Bleeding, feeling bad, aching breasts, headaches Estrogen + progesterone: Weight gain, absence of menstruation bleeding Progesterone: Acne, depression, lowering of libido ...
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This note was uploaded on 11/21/2011 for the course ANAT 1502 taught by Professor Johnh.romfh during the Spring '10 term at Life Chiropractic College West.

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