W1 - Embryology Flashcards
LO1: Define the three periods of the prenatal period
The 38 week period from fertilisation to birth (divided into three shorter periods of pre-embryonic, embryonic, and foetal period)
Pre-embryonic + embryonic = embryogenesis
LO1: Define the three periods of the prenatal period - pre-embryonic
The first two weeks of development – the developing offspring during this period is referred to as a ‘pre-embryo’
LO1: Define the three periods of the prenatal period - embryonic
the third to the eighth weeks of development – the developing offspring during this period is referred to as an ‘embryo’
LO1: Define the three periods of the prenatal period - foetal
the ninth week of development until birth – the developing offspring during this period is referred to as a ‘foetus’ (for the first four weeks after birth, it is referred to as a ‘neonate’, after which it is referred to as an ‘infant’ until around 1 year of age)
LO1: Define the three periods of the prenatal period - embryogenesis
The developmental events that occur during the pre-embryonic and embryonic periods
- Cleavage: the cell division of the zygote, which is the single cell formed once an oocyte (female gamete or sex cell) is fertilised by a sperm (male gamete or sex cell)
- Gastrulation: the formation of the three primary germ layers of the embryo
- Organogenesis: the arrangement of the three primary germ layers to give rise to all organs of the body
LO2: Describe the major developmental events of the pre-embryonic period (first week)
- Fertilisation
An oocyte (female gamete)is fertilised by a sperm. Occurs in the ampulla in the uterine tube - Zygote
Once the process of fertilisation is complete, the resulting single cell is called a zygote. - Cleavage
The zygote moves through the uterine tube towards the uterus and as it does, it undergoes a series of cell divisions, which is a process called cleavage. During this process, the number of cells increases but the overall size of the structure does not. - Morulla
After the third cleavage of the zygote, the cells form a tightly packed ball and the structure is then called a morula. The morula continues to divide and enters the uterus. - Blastocyst
Once the morula enters the uterus, a fluid-filled cavity called the blastocyst cavity develops within it and the structure is then called a blastocyst.
outer ring = trophoblast
The blastocyst has two distinct components, which are the outer ring of cells called the trophoblast and the inner cell mass called the embryoblast.
LO2: Describe the major developmental events of the pre-embryonic period (second week)
The two components of the blastocyst each differentiate into two layers:
Trophoblast: differentiates into an inner layer called the cytotrophoblast and an outer layer called the syncytiotrophoblast
Embryoblast: differentiates into the epiblast and hypoblast, which are collectively referred to as the bilaminar embryonic disc (‘bi’ = two; ‘lamina’ = layer)
LO3: Describe the process of gastrulation (define gastrulation)
The transformation of the bilaminar embryonic disc into trilaminar
this forms the primary germ layers of the embryo (ectoderm, mesoderm, endoderm) now called an embryo
- 3RD WEEK
LO3: Describe the process of gastrulation (describe its stages)
Primitive streak: Thin depression on epiblast surface (starts gastrulation)
Cell migration:
- Epiblast cells move through primitive streak
- Form mesoderm
- Displace hypoblast to form endoderm
- Remaining epiblast cells become ectoderm
Notochord:
- Cylindrical mass of mesoderm
- Forms internal and parallel to primitive streak
- Establishes embryo’s axial orientation
LO3: Describe the process of gastrulation (embryo folding in week 3-4)
Folding: Trilaminar disc folds into a cylindrical shape (late week 3 to week 4)
Germ layer arrangement:
Ectoderm: Most external layer (e.g., skin, nervous system)
Mesoderm: Middle layer (e.g., muscles, bones, cardiovascular system)
Endoderm: Most internal layer (e.g., gut lining, respiratory tract)
Outcome: Establishes foundational body plan; all body tissues derived from these three layers
LO4: Name the three primary germ layers of the embryo and provide examples of major tissues and structures derived from these layers (identify)
ectoderm, mesoderm, endoderm
LO4: Name the three primary germ layers of the embryo and provide examples of major tissues and structures derived from these layers (ectoderm)
The most external primary germ layer split into surface ectoderm and neuroectoderm
surface ectoderm: externally located structures in the body, including the epidermis of the skin and its derivatives (e.g. hair and nails) and the enamel of the teeth.
neuroectoderm: nervous tissue in the body and structures related to the nervous system such as sensory organs (e.g. the retina of the eyes) and glands such as the pineal gland, posterior pituitary gland and part of the adrenal glands.
LO4: Name the three primary germ layers of the embryo and provide examples of major tissues and structures derived from these layers (mesoderm)
The middle primary germ layer
- leads to muscle tissue and connective tissue in the body
- leads to dermis of the skin, which is composed of connective tissue.
Other structures that are derived from the mesoderm include the serous membranes of the body (pleura, pericardium and peritoneum), the heart, the epithelial lining of blood and lymphatic vessels, part of the adrenal glands and most of the urinary and reproductive systems.
LO4: Name the three primary germ layers of the embryo and provide examples of major tissues and structures derived from these layers (endoderm)
The most internal primary germ layer
- leads to internally located structures in the body, including most internal epithelial linings.
- Also most of the digestive system, thymus and glands such as the thyroid gland and parathyroid glands.
LO5: Briefly describe the process of organogenesis and the foetal period (organogenesis)
Organogenesis: Process where germ layers form all body organs
Timing: Starts in week 4, continues through embryonic and foetal periods
End of Embryonic Period: Rudimentary forms of most organ systems developed
First System to Develop: Cardiovascular system (heart develops early)
LO5: Briefly describe the process of organogenesis and the foetal period (foetal period)
Timing: Ninth week of development until birth
Terminology change: Embryo → Foetus
Key Features:
Maturation: Tissues and organs mature
Growth: Rapid body growth
Growth Patterns:
- Months 3-5: Significant increase in foetal length
- Last 2 months: Substantial increase in foetal weight
