Sexual Reproduction Embryonic Development OVERVIEW Embryonic Development early

Sexual Reproduction Embryonic Development

OVERVIEW Embryonic Development: early development of an organism � Humans: first 2 months after fertilization Why study embryonic development? � To understand the process compared to other species � To help scientists design new technology to assist in animal reproduction or to cure genetic diseases

STAGES OF EMBRYONIC DEVELOPMENT After fertilization, the zygote undergoes mitosis and a series of rapid cell divisions 2. At the end of the 1 st week the zygote becomes a morula ◦ Morula: a ball of cells about 0. 2 mm in diameter 3. At the end of the 2 nd week a blastula forms ◦ Blastula: a hollow ball of cells about 1. 5 mm in diameter ◦ Cells also known as embryonic stem cells at this point 1.

4. Cells of blastula organize into 3 layers and developing embryo is now called the gastrula ◦ Ectoderm: outside layer of the gastrula �Will eventually form skin and nervous system ◦ Mesoderm: middle layer of gastrula �Will form kidneys, muscles, blood vessels, reproductive organs, and bones ◦ Endoderm: inner layer of gastrula �Will form lungs, liver, and lining of digestive system

Differentiation: process of cell layers eventually forming the organs and tissues of a baby � Continues for 38 weeks � Often divided into 3 periods of time called trimesters ◦ Each trimester = 3 months and involve major development changes in each trimester

First Trimester: Developing Organ Systems � All organs begin to develop/form ◦ 4 weeks: brain and spinal cord developing ◦ 8 weeks: bone cells forming �Embryo now called a fetus ◦ 12 weeks: organ systems have completely formed � On average, fetus is about 28 g in mass and about 9 cm long by end of 1 st trimester

Second Trimester: Growth � Fetus grows rapidly between 12 -16 weeks � Growth slows between 20 -24 weeks � 20 weeks: mother can feel fetus moving � On average, fetus weighs about 650 g and about 35 cm long at end of 2 nd trimester

Third Trimester: Continued Growth � Fetus continues to grow, preparing for birth � Significant growth of brain � 32 weeks (8 th month): fat is deposited under skin for insulation � On average, fetus weighs approximately 3300 g and is 40 -50 cm long

Advantages of Disadvantages of Sexual Reproduction