In this cartoon movie of fetal growth, observe the changing relative sizes of the head and body.
Then consider fetal skeletal changes occurring within the head associated with ossification processes and palate development.
Finally consider fetal changes that are occurring in auditory pathway.
These are 2 views of the same 12 week 92 mm CRL human fetus head, double stained to show both cartilage (blue) and newly-formed bone (red). The head undergoes two different forms of ossification (endochondral and intramembranous) in separate regions of the skull.
In this lateral (external) view, note the distribution of new bone formation by intramembranous ossification in the plates of the cranial vault, temporal bone, orbit, upper jaw (maxilla) and lower jaw (mandible) regions. (More? Skull | Face) Bony regions in the lower jaw (mandible) region also show spaces where tooth formation is occuring. (More? Tooth Development)
In this medial (internal) view, note the distribution of cartilage from the nasal region through the base of the skull showing endochondral ossification, also occuring in the atlas/axis (with new bone forming). See also the original Meckel's cartilage within the newly forming bony mandible.
The cartilage template of the mandible and the base of the skull are replaced by early bone development.
Selected midline medial head view showing key features of head musculoskeletal and neurological development.
Note extensive nasal cartilage, nasal conchae, pituitary, secondary palate, oral cavity, tongue, mandible, hyoid, choana, oropharynx.
Also note the developing tongue musculature and its mandibular attachment site.
(More? Human 12 Week Fetus)
Note that the cranial vault, the portion of the skull enclosing the brain, ossifies by a unique bone formation process, intramembranous ossification.
Because the head contains many different structures also review notes on Special Senses (eye, ear, nose), Respiration (pharynx), Integumentary (Teeth), Endocrine (thyroid, parathyroid, pituitary) and Musculoskeletal (tongue, skull).
Week 9 - Mesenchyme surrounding membranous labryinth (otic capsule) chondrifies. (More? Human 10 Week Fetus)
Week 12-16 - Capsule adjacent to membranous labryinth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph.
Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.
3rd Trimester - Vibration acoustically of maternal abdominal wall induces startle response in fetus.
Inner Ear (organ of corti)
26 weeks - human brainstem auditory pathway is anatomically formed.
28 weeks - AABR can be recorded.
less than 34 weeks - latencies of AABR components (I, III, and V) decrease as a function of gestation
Link to next page in this Practical - Postnatal and Abnormalities
The next page will look at the fetal events in face and ear development.
Use the alphabetical list below to find definitions of terms that are new to you or use the Google search window to search UNSW Embryology site.
AABR - acronym for Automated Auditory Brainstem Response
bone - A connective tissue forming the main structural component of the skeleton. There are two processes of bone formation (ossification): endochondrial, replacing a pre-existing cartilage template (most of the skeleton); or intramembranous, ossification directly from a membrane (cranial vault, scapula). Adult bone gross histology is described as either trabecular bone (also called cancellous or spongy bone) or compact bone (does not have any spaces or hollows). (More? Musculoskeletal Development - Bone | Musculoskeletal Development | Blue Histology - Bone)
canalis reuniens – (ductus reuniens, canaliculus reuniens, canalis reuniens, Hensen's canal, Hensen's duct, uniting canal, canalis reuniens of Hensen) short narrow canal connecting the cochlea duct to the saccule. (Victor Hensen, 1835-1924)
cartilage - The connective tissue formed from mesoderm by the process of chondrogenesis. In the embryo cartilage initially forms most of the skeleton and is later replaced by bone (ossification). In the adult, cartilage is found in many skeletal regions including the surface of bone joints. (More? Musculoskeletal Development - Cartilage Development | Musculoskeletal Development)
centre of ossification - (ossification centre) The region where bone formation commences within embryonic connective tissue. In endochondrial ossification, this process occurs within cartilage. In membranous ossification, this process occurs within a membrane. (More? Musculoskeletal Notes)
chondrogenesis - the process of forming cartilage, which is a connective tissue formed from from mesoderm (or neural crest in the head region). In the embryo, cartilage initially forms most of the skeleton and is later replaced by bone (ossification). In the adult, cartilage is found in many skeletal regions including the surface of bone joints. (More? Musculoskeletal Development - Cartilage Development | Musculoskeletal Development)
cribiform plate - Anatomical structure of the head, describing the perforated portion of ethmoid bone of skull allows passage of olefactory axons from nasal epithelium to olefactory bulb. (More? Senses Notes)
endochondral ossification - (Greek, endon = within) The term used to describe the process of replacement of cartilage, which forms the first embryonic body skeleton, with bone. The osteoblasts are the bone-forming cells. (More? Musculoskeletal Development - Bone)
eustacian tube - (auditory tube) A narrow canal connecting the middle ear space to the back of the oral cavity. The direct physical connection has two main functions to allow pressure equalization in, and fluid drainage from, the middle ear. In development, the canal is initially both horizontal and very narrow leading to poor drainage and easy blockage (More? Hearing Notes | Abnormal Hearing Notes)
fetal period - (foetal period) In humans, the development week 9 to 36 is the fetal stage (second and third trimester) and during this time organs formed in the embryonic period continue to develop and the fetus grows in size and weight. The first 8 weeks of development is considered the embryonic period and is divided into 23 Carnegie stages based upon developmental milestones. Note when searching an alternate spelling "foetal". (More? Fetal Notes | Embryo Stages)
hard palate - he bony anterior portion of the palate formed by maxillary and palatine bones. The muscular posterior portion is called the soft palate. (More? Head Note)
intramembranous ossification - The term used to describe the process of replacement of a membranous mesenchyme with bone by osteoblasts (bone-forming cells). Mesenchymal cells (osteoprogenitor cells) differentiate into osteoblasts at the initial sites of ossification (ossification centre). (More? Musculoskeletal Development - Bone)
meatal plate - An ectodermal plug that temporarily blocks the external auditory meatus of the ear. (More? Senses Notes)
Meckel's cartilage - A temporary cartilage located in the first pharyngeal arch (mandibular component) that forms the template for formation of the mandible and middle ear bones. Named after Johann Friedrich Meckel, the Younger a German anatomist (1781 - 1833). (More? Head Notes | Who Named it? Johann Friedrich Meckel | PalaeosThe Gill Arches: Meckel's Cartilage )
medial epithelial seam - (MES) Embryonic structure formed by the fusion of the two palatal shelves, forming a two-layered medial edge epithelial seam, which is then lost with palate development. (More? Face - Abnormalities | Face Notes | | Head Notes | Medline Plus - Cleft Lip and Palate)
mesethmoid cartilage - The ventral component of the nasal capsule. In the chicken embryo, it is induced by sonic hedgehog (Shh) expression from endoderm (endoderm zone I). (More? Face Notes)
oropharynx - The second portion of the pharynx (throat) that is posterior to the oral cavity. The other pharynx regions are the nasopharynx and laryngopharynx (hypopharynx). (More? Respiratory Notes)
otic capsule - The skull skeleton enclosing the inner ear membranous labrynth, developing as cartilage then at ossification centers becomes completely bony and unified at (humans approx week 23-24).
palate - The roof of the mouth (oral cavity) a structure which separates the oral from the nasal cavity. Develops as two lateral palatal shelves which grow and fuse in the midline. Initally a primary palate forms with fusion of the maxillary processes with the nasal processes in early face formation. Later the secondary palate forms the anterior hard palate which will ossify and separate the oral and nasal cavities. The posterior part of the palate is called the soft palate (velum, muscular palate) and contains no bone. Abnormalities palatal shelf fusion can lead to cleft palate. (More? Face - Abnormalities | Face Notes | Head Notes | Medline Plus - Cleft Lip and Palate)
primary palate - The fusion of the maxillary processes of the first pharyngeal arches with the medial nasal prominences to form the upper jaw. In human embryos this occurs between stage 17 and 18, going from an epithelial seam to the mesenchymal bridge. This is a separate and earlier event than secondary palate, palatal shelf fusion, which occurs during the early fetal period.
Reichert's cartilage - Neural crest forming the cartilage band of the second pharyngeal arch. (More? Head Notes)
second trimester - Clinical term used to describe and divide human pregnancy period (9 months) into three equal parts of approximately three calendar months. The first trimester corresponds approximately to embryonic development (week 1 to 8) of organogenesis and early fetal. The second and third trimester correspond to the fetal period of growth in size (second trimester) and weight (third trimester), as well as continued differentiation of existing organs and tissues. (More? Embryo Stages | Human Fetal Period | Development Week by Week)
secondary palate - The palatal shelves develops bilaterally during embryonic week 7, grow, elevate and fuses by week 9. This forms the hard palate separating the nasal and oral cavities. The secondary palate shelves fuse togenter and with the earlier primary palate (stage 17 and 18) and nasal septum.
soft palate - (velum, muscular palate) The muscular posterior portion of the palate forming the roof of the oral cavity. The bony anterior portion of the palate is called the hard palate, formed by maxillary and palatine bones. (More? Head Notes)
third trimester - Clinical term used to describe and divide human pregnancy period (9 months) into three equal parts of approximately three calendar months. The third trimester corresponds to the fetal period of growth in weight, as well as continued differentiation of existing organs and tissues. The respiratory system matures late in teh third trimester. The first trimester corresponds approximately to embryonic development (week 1 to 8) of organogenesis and early fetal period, the second trimester is the fetal period of growth in size. (More? Embryo Stages | Human Fetal Period | Development Week by Week)
vestibular ganglion - (Scarpa's ganglion) primary afferent vestibular neuron ganglion of the vestibular nerve. Located within the internal auditory meatus.
vestibular membrane - (Reissner’s) extends from the spiral lamina to the outer wall and divides the cochlea into an upper scala vestibuli, a lower scala tympani.
Vestibulocochlear Nerve - Cranial Nerve VIII