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1. #Lumen learning boundless anatomy and physiology full
2. #Lumen learning boundless anatomy and physiology free

As a result of this increased production, hCG accumulates in the maternal bloodstream and is excreted in the urine. These functions of hCG are necessary for creating an environment suitable for the developing embryo. The trophoblast secretes human chorionic gonadotropin (hCG), a hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses. In response, the uterine mucosa rebuilds itself and envelops the blastocyst. When implantation succeeds and the blastocyst adheres to the endometrium, the superficial cells of the trophoblast fuse with each other, forming the syncytiotrophoblast, a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall. Ovulation, fertilization, pre-embryonic development, and implantation occur at specific locations within the female reproductive system in a time span of approximately 1 week.

#Lumen learning boundless anatomy and physiology free

In a process called “hatching,” the conceptus breaks free of the zona pellucida in preparation for implantation.įigure 2. Pre-embryonic cleavages make use of the abundant cytoplasm of the conceptus as the cells rapidly divide without changing the total volume.Īs the blastocyst forms, the trophoblast excretes enzymes that begin to degrade the zona pellucida. Totipotency lasts for only a few days before the cells’ fates are set as being the precursors to a specific lineage of cells. The inner mass of embryonic cells is totipotent during this stage, meaning that each cell has the potential to differentiate into any cell type in the human body. These cells will develop into the chorionic sac and the fetal portion of the placenta (the organ of nutrient, waste, and gas exchange between mother and the developing offspring). The cells that form the outer shell are called trophoblasts (trophe = “to feed” or “to nourish”). Within this structure, a group of cells forms into an inner cell mass, which is fated to become the embryo. At this developmental stage, the conceptus is referred to as a blastocyst. The ball of now tightly bound cells starts to secrete fluid and organize themselves around a fluid-filled cavity, the blastocoel. It continues to divide, creating a ball of approximately 100 cells, and consuming nutritive endometrial secretions called uterine milk while the uterine lining thickens. Once inside the uterus, the conceptus floats freely for several more days. The name given to this structure is the morula (morula = “little mulberry”). The cells that had been loosely grouped are now compacted and look more like a solid mass.

Each daughter cell produced by cleavage is called a blastomere (blastos = “germ,” in the sense of a seed or sprout).Īpproximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. Although each cleavage results in more cells, it does not increase the total volume of the conceptus. During its journey to the uterus, the zygote undergoes five or six rapid mitotic cell divisions. By the end of the embryonic period, all of the organ systems are structured in rudimentary form, although the organs themselves are either nonfunctional or only semi-functional.įollowing fertilization, the zygote and its associated membranes, together referred to as the conceptus, continue to be projected toward the uterus by peristalsis and beating cilia. In this section, we’ll cover the pre-embryonic and embryonic stages of development, which are characterized by cell division, migration, and differentiation. A developing human is referred to as an embryo during weeks 3–8, and a fetus from the ninth week of gestation until birth. The first 2 weeks of prenatal development are referred to as the pre-embryonic stage. It can be subdivided into distinct gestational periods.

#Lumen learning boundless anatomy and physiology full

The period of time required for full development of a fetus in utero is referred to as gestation (gestare = “to carry” or “to bear”). Throughout this chapter, we will express embryonic and fetal ages in terms of weeks from fertilization, commonly called conception. Explain how an embryo transforms from a flat disc of cells into a three-dimensional shape resembling a human.Describe how the placenta is formed and identify its functions.List and describe four embryonic membranes.