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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

mention two processes of bone formation
Ngopiro Reply
what is the process for the bone formation
Ngopiro
what are the importance of studying anatomy and physiology?
Justine Reply
what are the importance of studying anatomy and physiology?
Justine Reply
bones and structures
Nkechi Reply
good morning
Cb
I wand anatomy notes 1st year
Cb
it's called going to class and taking notes
fyn sir
patrick
lmfao slay thank you for making my day lol
Bianca
heloo
King
I felt sleepy when I go for study immediately, how can I change
Abdi
what are the Hodgkin's disease
Mwasiti Reply
what are the components of cardiovascular system
Mwasiti Reply
Heart Blood Blood Vessel
Md
what are the components of the bone
Ngopiro
what is Hodgkin's disease
Mwasiti Reply
What is anatomy
Munira Reply
anatomy is a study of physical and structural functions of human body parts
Mohd
is the study of structure of the body and physical relationship involve between the body system
Mwasiti
is the study of structure of the body and physical relationship involve between the body system
Mwasiti
why simple columnar support peristalisis
Camilius Reply
why when sex of an individual is determined by y chromosome found in male therefore why there is many women than male
pamfili Reply
I'm not sure if I fully understand your question.
are you sure it is more
samir
i don't remember what I said.
I'm not sure if I fully understand your question.
roughly taking a lead.
samir
Muscle of mastication
JIMOH Reply
what is anatomy
Akanle
Sorry....it's the study of human body and it's functions.
Irene
Anatomy is the study of human organs
Kity
what is serum protein?
Salum
what is anatomy
Akanle
Anatomy is the study of human organs
Kity
Who is a bovin
Kity
Who is a bovin
Kity
Why do you ask tough questions I'm in JHS 3
Kity
pls does broken bones get healed
Desmond
yes
Belinda
how
Desmond
by surgery or medication
Desmond
Medication
Irene
why do a bovin not menstruate unlike women?
patrick
Why do you ask tough questions I'm in JHS 3
Kity
Anatomy is the study of structure of the human body
Belinda
yes
Belinda
medication
Belinda
yes
Belinda
animals within that group include cows as well as others.
ihope that was helpful
aaa,kity.bovin is a terminology used by veteranians to refer a cow.for dogs its carnine,cats it's ferline...etc
patrick
i believe if you look up the classification of Bovin , it tell clarify more for you.
ihope that was helpful
i don't think my first message about taxonomy sent
i don't think my first message about taxonomy sent
difference between seminiferous tubules and ejaculatory duct
Muhammad Reply
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Musibi
wats dis group
Kity
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Agemo Reply
what is azygous vein
Karan Reply
Sir muscle contraction ka topic kis part m milega
sonugora Reply

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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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