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By the end of this section, you will be able to:
  • Distinguish the stages of embryonic development that occur before implantation
  • Describe the process of implantation
  • List and describe four embryonic membranes
  • Explain gastrulation
  • Describe how the placenta is formed and identify its functions
  • Explain how an embryo transforms from a flat disc of cells into a three-dimensional shape resembling a human
  • Summarize the process of organogenesis

Throughout this chapter, we will express embryonic and fetal ages in terms of weeks from fertilization, commonly called conception. The period of time required for full development of a fetus in utero is referred to as gestation    (gestare = “to carry” or “to bear”). It can be subdivided into distinct gestational periods. The first 2 weeks of prenatal development are referred to as the pre-embryonic stage. A developing human is referred to as an embryo    during weeks 3–8, and a fetus    from the ninth week of gestation until birth. In this section, we’ll cover the pre-embryonic and embryonic stages of development, which are characterized by cell division, migration, and differentiation. 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.

Pre-implantation embryonic development

Following 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. During its journey to the uterus, the zygote undergoes five or six rapid mitotic cell divisions. Although each cleavage    results in more cells, it does not increase the total volume of the conceptus ( [link] ). Each daughter cell produced by cleavage is called a blastomere    (blastos = “germ,” in the sense of a seed or sprout).

Approximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. The cells that had been loosely grouped are now compacted and look more like a solid mass. The name given to this structure is the morula    (morula = “little mulberry”). Once inside the uterus, the conceptus floats freely for several more days. It continues to divide, creating a ball of approximately 100 cells, and consuming nutritive endometrial secretions called uterine milk while the uterine lining thickens. The ball of now tightly bound cells starts to secrete fluid and organize themselves around a fluid-filled cavity, the blastocoel    . At this developmental stage, the conceptus is referred to as a blastocyst    . Within this structure, a group of cells forms into an inner cell mass    , which is fated to become the embryo. The cells that form the outer shell are called trophoblasts (trophe = “to feed” or “to nourish”). 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 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. Totipotency lasts for only a few days before the cells’ fates are set as being the precursors to a specific lineage of cells.

Questions & Answers

they both receive carbon dioxide for purification. then the right lung is quite smaller than the left one because much of the space is occupied by the heart
jaston Reply
left lung is smaller than the one at the right
Manokiruthika
Not that the right lung is bigger than the left one
Bessy
it will stop preventing food to enter into trachea
jaston Reply
the conducting zone consists of organs that does not involve direct in gas exchange while the respiratory zone involves the organs that involves direct in gas exchange
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is the tip of the nose
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is the organ of respiratory system consists of bronchioles only
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epiglostis
Sayed
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in order to have knowledge about our bodies.to know how many bones we have and to know there functions.
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All
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all the 206 bones
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enzyme are organic catalyst
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biological catalysts that speeds up the chemical reaction in our body
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catalyst that speed up the rate of a chemical reaction without it been changed
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Which one of the following is correct about Which one of the following is correct about the frontal plan the frontal plan
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What are the physiologic examples of positive feedback
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ATP
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bleeding through a tear in a blood vessel ( decrease in ECF volume ) activates some plateles to accumulate at site bleeding
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What are the extracellular fluids
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sweat,urine
Fatou
cerebrospinal fluid
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Thax
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Fahad Reply
anatomy is the study of structure of the body and the physical relationship between systems
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what are the biggest bone to our body?
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femur
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starnd of MRI and ?why
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what is haploid
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half of diploid(2n) n no of chromosomes I.e human have 46 no of chromosomes and its haploid is 23
Sayed
Half of diploid
ruha
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Haploid describes as a cell that contains a single set of chromosomes.
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what are oxygen free radical
Fateemah Reply
are very reactive molecules that can react with every cellular component
Irene
is a molecule that contains atleast one unpaired electron
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is the study of structure of the body
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is the study of the structure and function of the human body
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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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