CHAPTER 29 - C HAPTER 29 DEVELOPMENT AND I N HERITANCE...

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CHAPTER 29 DEVELOPMENT AND INHERITANCE Fertilization occurs when genetic material from ovum and sperm merge into a single nucleus. Only about 1% of the 300-500 million sperm ejaculated reach the secondary oocyte. Fertilization usually occurs in the Fallopian tube 12-24 hours after ovulation. Sperm can travel up the Fallopian tube and reach the oocyte within minutes, but they are not capable of fertilization at that point. The oocyte is surrounded by several layers of follicular cells, the corona radiate, and a glycoprotein layer, the zona pellucida. After several hours in the female reproductive tract, sperm have undergone capacitation and are capable of fertilization. Release of enzymes of the acrosomes of a number of sperm enable one sperm to penetrate the protective layers of the oocyte – syngamy. syngamy causes the secondary oocyte to complete Meiosis II and form the ovum. The fertilized ovum contains a nucleus that is a combination of chromosomal material from both ovum and sperm and is diploid. Dizygotic (fraternal) twins are produced when 2 ova are released and both are fertilized by separate sperm. They may or may not be the same sex and are no more closely related than other siblings. Monozygotic (identical) twins develop from a single fertilized ovum that splits at an early stage of development. They are always the same sex and contain identical genetic material. After fertilization, rapid mitotic cell division called cleavage takes place. A solid mass of cells called the morula is produced (still in the uterine tube). Although there are greater numbers of cells, the morula is still about the same size as the zygote. The progressively smaller cells it contains are called blastomeres. By the fifth day, the morula has changed into a hollow ball of cells, the blastocyst, and reached the uterine cavity. The blastocyst has these parts: 1. Trophoblast – outer covering that comes in contact with the uterine wall and enables the blastocyst to burrow into the endometrium 2. Inner cell mass 3. Blastocele – fluid-filled cavity The blastocyst remains free in the uterine tube for 1-2 days. About 6-7 days after fertilization the blastocyst attaches to the endometrium – implantation. the blastocyst then proceeds to burrow into the endometrium.
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At this stage, the trophoblast has 2 functions: 1. Secretes enzymes that aloow the blastocyst to burrow into the uterine wall 2. Develops into the chorion (one of the fetal membranes) 3. Secretes hCG, which causes the corpus luteum, to continue to secrete estrogens and progesterones to maintain pregnancy. Secondary oocyte Mature ovum Zygote Morula Blastocyst – this stage implants (burrows in to the wall of the uterus Following implantation, the endometrium is known as the deciduas and consists of: 1. Decidua basalis – portion between embryo and stratum basalis of the uterus – will form the maternal part of the placenta 2. Decidua capsularis – portion between embryo and uterine cavity 3. Decidua parietalis – remain endometrium
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This note was uploaded on 07/26/2011 for the course BSC 1093 taught by Professor Porter during the Fall '11 term at Edison State College.

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CHAPTER 29 - C HAPTER 29 DEVELOPMENT AND I N HERITANCE...

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