Chapter Fifteen Development 311

PREEMBRYONIC STAGE

The process of development begins with the union of the sperm and oocyte. After ovulation, the secondary oocyte moves down the uterine tube and, if fertilization occurs by sperm, it usually happens in the uterine tube. Once fertilization occurs, the oocyte and the sperm unite to become a zygote. The zygote divides during this preembryonic stage and forms a two-celled stage. These cells go through numerous divisions and are called blastomeres. The two blastomeres divide and become four cells and this process continues until a cluster of cells

(16 to 32 of them) is formed called a morula. As division continues this cluster becomes a hollow ball of cells called a blastocyst. The hollow cavity of the blastocyst is called the blastocele and most of the wall of the blastocyst consists of a layer of simple squamous epithelia called the trophoblast. One part of the wall consists of an inner cell mass known as the embryoblast. Some of these cells will develop into the embryo. Label the structures in the preembryonic stage of development. Color in the various stages in different colors and use one color for the trophoblast and another for the embryoblast.

Preembryonic Development

Answer Key: a. Ovulation, b. Fertilization, c. Two-cell stage, d. Morula, e. Blastocyst, f. Zygote, g. Blastomere, h. Trophoblast, i. Embryoblast, j. Blastocele kaplan.. ■ vmh medical 313

EMBRYONIC STAGE

The blastocyst is the stage of development in which implantation in the uterus occurs. Implantation is the imbedding of the blastocyst in the endometrium of the mother. Once this occurs, a hollow space develops in the embryoblast and this is called the amniotic cavity. At this time, the embryoblast is divided into a bilaminar germ disk with two primitive tissues called the epiblast and the hypoblast. The primitive streak forms along the anterior/posterior axis of the embryo and it becomes a region of growth in the early stage of development.

From the epiblast the embryoblast begins to form three primary germ layers. These are the endoderm, ectoderm, and mesoderm. The structure is now referred to as a trilaminar germ disk (meaning a developmental structure with three layers). The development of the notochord begins and this structure will make up the center part (nucleosus pulposus) of the intervertebral disks in the adult. The yolk sac also forms during this period. Once the germ layers are formed, the preembryonic stage ends and the developing tissue is known as an embryo. The embryonic stage begins about day 16 after fertilization and lasts until about the eighth week of pregnancy. During the embryonic stage, the major organs of the body are initiated in a process called organogenesis.

During the first part of the embryonic phase, the ectoderm begins to fold in on itself and becomes a neural groove. This will develop into the nervous system of the body. Other derivatives of the ectoderm are the epidermis and some of the facial bones and muscles. The mesoderm gives rise to most of the bones and muscles of the body, the dermis, and the circulatory system. The endodermis gives rise to the linings of the gastrointestinal tract and respiratory system, and some glands. As development continues, the neural groove folds in on itself and becomes a neural tube and the formation of the gut takes place. Label the structures in the embryonic phase and use blue colors for the ectoderm and derivatives of the ectoderm such as the neural tissue. Use red for the mesoderm and color the endoderm in yellow.

Primitive Groove Bilaminar Trilaminar

Answer Key: a. Epiblast, b. Hypoblast, c. Ectoderm, d. Mesoderm, e. Endoderm, f. Amniotic cavity, g. Bilaminar germ disk, h. Primitive streak, i. Notochord, j. Trilaminar germ disk, k. Neural groove, I. Yolk sac, m. Neural tube, n. Gut

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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