Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 354 . For convenience, the gestation period is described as consisting of three integrated trimesters, each three months in duration. The first trimester is the period of embryological and early fetal development. The second trimester involves the development of organs and organ systems, a process that nears completion by the end of the sixth month. The third trimester is characterized by rapid fetal growth and deposition of adipose tissue. Many complex developmental events occur during the first trimester, such as cleavage, implantation, placentation, and embryogenesis. Cleavage is a sequence of cell divisions that begins immediately after fertilization when the cells start to form the embryo. Cleavage subdivides the cytoplasm of the zygote in a series of mitotic divisions; the zygote becomes a morula (pre- embryo) and then a blastocyst. Implantation begins with the attachment of the blastocyst to the endometrium of the uterus and continues as the blastocyst invades maternal tissues. When implantation occurs outside the uterus, such as in one of the uterine tubes, it is called an ectopic of 368 Chapter 26: Development and Inheritance The gradual modification of anatomical structures and physiological characteristics during the period from fertilization to maturity is called development . The formation of different types of cells during development is called differentiation . Development begins at fertilization , or conception . The study of embryological development is called embryology . Fetal development begins at the start of the ninth week and continues until birth. Embryological and fetal developments are sometimes referred to collectively as prenatal development . Postnatal development commences at birth and continues to maturity, the state of full development or completed growth. Differences in individual genetic makeup create unique specific characteristics. The term inheritance refers to the transfer of genetically determined characteristics from generation to generation. The study of the mechanisms responsible for inheritance is called genetics . 26.1 Prenatal Fertilization Fertilization involves the fusion of two haploid gametes, each containing 23 chromosomes, producing a zygote that contains 46 chromosomes. The male and female gametes provide very different roles and contributions. The spermatozoon delivers the paternal chromosomes to the site of fertilization. The female gamete must provide all other components of the cellular organelles, including nourishment and genetic programming necessary to support the development of the embryo for nearly a week after conception. The spermatozoa that have been implanted in the vagina during sexual intercourse cannot accomplish fertilization until they have been exposed to conditions in the female reproductive tract. Fertilization typically occurs near the junction between the ampulla and isthmus of the uterine tube, generally within a day after ovulation. Although approximately 200 million spermatozoa are introduced into the vagina in a typical ejaculation, only about 10,000 enter the uterine tube, and fewer than 100 reach the isthmus. Pr natal Development The time spent in prenatal development is known as gestation