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WEEK 16 HANDOUT FOR 03051, VERTEBRATE ZOOLOGY:

REPRODUCTION & DEVELOPMENT

5/9/00 Text (7th ed): Ch. 14, (Ch. 12, pp. 266-269, 282-287, for hormonal concerns)

Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

possible web site: http://www.med.unc.edu/embryo_images/ (tutorial on mammalian development)

HVCC a/v material: VT 3089,"The Odyssey of Life: The Ultimate Journey" (to be shown in class)

HORMONES: BASIS OF ACTION

In contrast to the rapid communication of the nervous system, vertebrates also have a complementary chemical message system that is a bit slower: the endocrine system. These internal glands produce hormones, or chemicals produced by one cell and that affect the metabolism of another cell in small amounts. Hormones accomplish this in two general ways:

1. a water-soluble peptide (small protein) hormone binds to a receptor on the target cell surface and sets off a secondary messenger to change activities within the cell (rapid onset of action).
2. a fat-soluble steroid hormone passes through the target cell plasma membrane, binds with a receptor in the cytoplasm, and the resulting hormone-receptor complex interacts with the DNA directly (slower onset of action).

Generally, hormones are not secreted constantly, but at specific times. Usually, there is a negative feedback control system that will regulate the amount of hormone produced at one time. A major example of hormonal functioning is in the vertebrate reproductive system.

OVERVIEW OF VERTEBRATE REPRODUCTION

As clearly seen in humans, males and females have different hormones and different cycles in their reproductive systems. In female humans, apes, and old-world monkeys, hormonal control leads to a monthly menstrual cycle. In contrast, most mammals have an estrous cycle that is much less regular. With these animals (e.g., dogs), the female is sexually receptive ("in heat") and ovulating (releasing eggs from the ovary) only a few times a year.¹ Other animals are induced ovulators, e.g., female rabbits ovulate with copulation. In contrast, in human males, hormone levels are more constant, so that sperm production and testosterone influence occur continuously. The few eggs vs. the many sperm produced are an indication of these different hormonal states.

A human pregnancy lasts 38 weeks. In that time, one has gone from one fertilized egg to a bouncing baby boy/girl. The first trimester is when most of the important events in development occur. Fertilization (conception) occurs in the fallopian (uterine) tube, only within a narrow time frame after ovulation.

A fertilized egg is called a zygote. While still in the fallopian tube, the zygote begins to divide (cleavage) into smaller cells or blastomeres that make up a blastocyst. This blastocyst will encamp on the endometrial lining of the uterus for the next nine months. The inside (inner cell mass) of the blastocyst will go on to become the fetus, while the outer layer (trophoblast) will make up the extraembryonic membranes. Gastrulation in the inner cell mass sets up the germ layers of endoderm, mesoderm, and ectoderm, and most organs are set up by 10 weeks. The rest of the 38 weeks is maturational, with some organs, e.g., lungs, maturing later than others.

|main page| |background| |03028: Physiology| |03048: Anatomy|

|03050: Invertebrate Zoology| |03051: Vertebrate Zoology| |03074: Economic Botany|

Please send comments and questions to: cronewil@hvcc.edu

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This web page last updated on April 24, 2000