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PinkMonkey Online Study Guide-Biology

(C) Morula                      (D) Blastocyst

Figure 24.10 Early stages in development of an embryo

The union of the sperm nucleus and the nucleus of the ovum to form a zygote is known as fertilization. It normally occurs in outer part of the uterine tube, i.e. in Fallopian tube, usually within 24 hours after ovulation. The spermatozoa swim up the female tract by movements of their flagella (tails), and is also assisted by muscular contractions of the uterus.

The spermatozoa must remain in the female genital tract for 4-6 hours. During this time they are activated by the secretion of the enzyme hyaluronidase to form finger-like projection called an acrosome. The enzyme partially dissolves the membrane covering of the ovum for the entry of the sperm via insertion of its acrome. Normally, only one spermatozoan fertilizes an ovum. After the entry of the sperm the tail is shed, the second polar body is cut off within the perivitelline space, the fertilization membrane is formed around the ovum which prevents the entry of other sperms, chromosomes are rearranged within the female pronucleus, the male pronucleus rotates through 180o angle and gradually approaches the female pronucleus, the union of the male and female pronuclei takes place exactly in the center of the cell to form a single segmentation nucleus or zygote, a process termed fertilization. The zygote contains the diploid number of chromosomes (46).

Cleavage (Figure 24.10). The zygote undergoes rapid cell division as it passes down the oviduct and into the uterus. This early division of the zygote is called cleavage. The cleavage is total or holoblastic, and takes place at regular intervals. The dividing zygote floats freely, absorbing nutrients from the uterine secretions. The cleavage or segmentation continues (mitotic type), and results in the formation a solid mass of cells known as the morula. The morula arrives at the uterus about 5 days after fertilization. The uterine fluid enters the morula and provides nutritive elements at this stage. If the two first-formed cells of one zygote part, they develop into identical twins. If two ova are fertilized simultaneously, they form fraternal twins.

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(E) Blastocyst

Blastocyst embedded in endometrium (F) Implantation

Figure 24. 11 Furthur Development of the Embryo

Blastula - The fluid imbibed by the morula, develops into a fluid filled cavity called the blastocoel. The blastocoel splits the cell mass into an inner cell mass and an outer layer of cells, the trophoblast (strophectoderm). The morula at this stage is called the blastocyst or the blastula (Figure 24.11). The inner cell mass will develop into the embryo, and the trophoblast forms an encompassing sac that becomes part of the placenta.

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Table of Contents

24.0 Introduction
24.1 Human reproduction
24.2 Female reproductive cycle
24.3 Embryonic Development

Chapter 25


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