Anatomy - Outcome 3 Flashcards
What ‘periods’ make up the first trimester of pregnancy?
The preimplantation period and the embryonic period make up the first trimester of the pregnancy
What period makes up the last two trimesters of pregnancy?
the fetal period constitutes the last two trimesters
When is the preimplantation period?
The preimplantation period is the first week.
When is the embryonic period?
The embryonic period extends from the beginning of the second week to the end of the eighth week; at this time the developing life is known as an embryo. The embryonic period is the most critical time in development.
When does the fetal phase/period begin?
The fetal phase begins with the ninth week and lasts until birth.
Embryonic Germ Layers
During the third week of development, the cells of the embryo form the three primary embryonic layers
Ectoderm
Mesoderm
Endoderm
Ectoderm (outer layer)
→ epithelium covering outside the body and lining the oral cavity, nasal cavity, sinuses, tooth enamel, and nervous system
Mesoderm (middle layer)
→ skeletal system, muscles, blood, lymph, kidneys, and other internal organs
Endoderm (inner layer)
→ epithelial lining of pharynx, stomach, intestines, lungs, bladder, urethra, vagina
The face and its associated tissue begin to form during what week of prenatal development?
The face and its associated tissue begin to form during the fourth week of prenatal development, within the embryonic period.
What layers are involved in facial development?
All three embryonic layers are involved in facial development: ectoderm, mesoderm, and endoderm
Stomodeum and oral cavity formation:
The stomodeum (primitive mouth) appears as a shallow depression in the embryonic surface ectoderm at the cephalic end before the fourth week
It is limited in depth by the oropharyngeal membrane.
The membrane separates the stomodeum from the primitive pharynx.
The primitive pharynx is the cranial part of the foregut, the beginning of the future digestive tract.
Mandibular Arch & Lower Face Formation
After the formation of the stomodeum, two bulges of tissue appear inferior to the primitive mouth, the two mandibular processes.
These processes consist of a core of mesenchyme.
These paired mandibular processes fuse at the midline to form the mandibular arch.
After fusion, the mandibular arch then extends as a band of tissue inferior to the stomodeum
The mandibular arch directly gives rise to the lower face, including the lower lip.
Frontonasal process and upper face formation
The frontonasal process forms as a bulge of tissue in the upper facial area.
On the outer surface of the embryo are placodes, which are rounded areas of specialized, thickened ectoderm found at the location of developing special sense organs.
The facial area of the embryo has 2 lens placodes, which are initially located fish-like on each side of the frontonasal process.
The tissue around the nasal placodes on the frontonasal process undergoes growth, starting the development of the nasal region and the nose.
The placodes become submerged, forming a depression in the center of each placode, the nasal pits.
The middle part of the tissue growing around the nasal placodes appears as two crescent‑shaped swellings located between the nasal pits. These are the medial nasal processes.
On the outer part of the nasal pits are two other crescent‑shaped swellings, the lateral nasal processes.
Maxillary process and midface formation
During the fourth week of prenatal development, an adjacent swelling forms from increased growth of the mandibular arch on each side of the stomodeum, the maxillary process.
Each maxillary process will grow superiorly and anteriorly around the stomodeum.
The upper lip is formed when each maxillary process fuses with each medial nasal process on both sides of the stomodeum due to the underlying growth of the mesenchyme
The maxillary processes contribute to the sides of the upper lip, and the two medial nasal processes contribute to the philtrum.
