Shannon’s Normal Labor Assignment: Week 7 ​Fetal Structures Involved In Labor Holistic Midwifery Vol II​, pp. 7-27 1. How many different elements, or “parts”, are there to the fetal skull? The fetal skull is made up of 51 bony elements or parts separated by either cartilage or connective tissue and are only partly ossified at birth. These parts develop from either one of two ossification processes, which are called the intramembranous or endochondral processes. 2. What are sutures? What are fontanels? Sutures are gaps that are spanned by a flexible bridge of fibrous membranous tissue and run along the edges of membranous cranial bones in the fetus. Varying in width according to the bones that they border, they allow the bones in the baby’s skull to shift and mold as the baby makes his way through the birth canal during labor. Fontanels are open areas at the junctions of each suture line that are covered with membranes and skin. The most significant fontanels are the anterior (bregma) and posterior (lambda) ones. Their position and shape help to identify the position of the vertex (the top-back of the baby’s head) of the fetal skull in his mother’s pelvis during labor. Other fontanels also play an important role in that they allow the surrounding bones to move during labor as needed. 3. Explain suture and fontanel locations, shapes, and functions in detail along with their names. ● The sagittal suture divides the cranial vault in half from front to back, running between the parietal bones. It begins at the anterior fontanel and ends at the posterior fontanel and ranges in size from 3 to 17 mm wide when the fetus is full term. ● The two lambdoidal sutures run from the posterior fontanel in an outward sweeping curve along the back of the fetus’ head. They follow the curve down and around the border of the occipital vault on either side and separate the interparietal portion of the occiput from the two parietal bones. When the fetus is full term, they may measure from 1.5 to 11 mm wide. ● The two coronal sutures run transversely and downward from the anterior fontanel to the sphenoid fontanel on either side and separate the parietal bones from the frontal bones. At term, they measure from 1 to 11 mm in width. ● The frontal, or metopic, suture is located between the two frontal bones and is the anterior continuation of the sagittal suture. It extends from the central, anterior point of the anterior fontanel to the glabella (The glabella is located down the center of the forehead to the area between the eyebrows above the bridge of the nose.). Sometimes it is mistaken for the sagittal suture in deflexed presentations during internal exams, but when identified correctly, it can help distinguish a face presentation. ● The two temporal (also called the lateral or squamous) sutures separate the temporal bone from the parietal bone on either side of the head. Each extends from the temporal fontanel in front, forming a gentle arch, dipping down near the uppermost tip of the pinna (the outer ear or auricle) and running almost straight to the posterolateral fontanel in the back. During birth, the temporal suture allows some upward and downward mobility of the parietal bones. It also allows the temporal bones to move upward and forward toward the sphenoidal fontanel. ● The anterior fontanel (This is also called the bregma, which refers to the fontanel as well as the area surrounding it.) is located at the junction of the sagittal, frontal, and coronal sutures. It is a diamond-shaped opening bordered by the two frontal and two parietal bones. It is also the largest fontanel, averaging about 3 cm in width by 2 cm in length when the baby is term. ● The posterior fontanel (also called the lambda fontanel) is located where the sagittal suture meets the two lambdoidal sutures. Bordered by the interparietal occiput posteriorly and the parietal bones anteriorly, it forms an inverted “Y.” The tail of the “Y” becomes the sagittal suture and the arms of the “Y” begin the lambdoidal sutures on either side. The posterior fontanel is triangular-shaped and is much smaller than the anterior fontanel, measuring about 0.8 cm. ● The sagittal fontanel (also called false or Gerdy’s) is a developmental fontanel that usually continues to ossify until the fifth month of gestation when only the tiny parietal foramina (​an opening for the parietal emissary vein, which drains into the superior sagittal sinus) ​in each parietal bone remains. It consists of a small, diamond-shaped space, usually located one-quarter of the distance anterior to the posterior fontanel in the sagittal suture. If found during an internal exam, it is most often mistaken for the anterior fontanel because it is similarly diamond-shaped. While this feature is most often benign, it may be associated with certain fetal anomalies. ● The two anterolateral fontanels (also called the sphenoidal fontanels) are formed at the junction of the parietal, frontal, sphenoid, and temporal bones on either side of the skull. They allow the temporal bone to move upward and forward. They cannot be palpated during a normal labor and therefore do not serve as important directional landmarks. ● The two posterior, or mastoid, fontanels are roughly rectangular membranous openings located laterally and posteriorly (well behind the ears) at the junction of the parietal, temporal, and occipital bones. While they allow the temporal and the occipital bones to move during labor, they cannot normally be palpated and are therefore not relevant directional landmarks. 4. What is the difference in a “membranous bone” and an “endochondral bone”? Why does this matter in regards to labor? Membranous bones form by hardening or ossifying outward from a central region called the growth center. Numerous thin spikes of bone fan out from this region, denser at their growth center and more fragile and flexible near their edges. In a newborn, the edges of membranous bones are smooth and slightly separated by sutures. Endochondral bones ossify differently than membranous bones do. A cartilaginous template in the shape of the eventual bone forms first. This cartilage is gradually reabsorbed and replaced by hard, calcium-filled bone as the baby matures. During ossification, which occurs uniformly (contrast with membranous bones which ossify more densely at their growth centers, becoming more fragile the farther away they get) throughout the cartilaginous template, regions of uniform thickness become consistently firm throughout, making them much less flexible than membranous bones at the same stage of development. Endochondral bones are not separated by sutures. Instead, they are separated by hyaline cartilage or fibrocartilage and meet at synchondrodial joints. At birth, these joints are much less flexible than sutures, but do allow for some movement of the surrounding more ossified regions of bone. After birth and as the baby grows, the synchondrodial joints become progressively less mobile. This varies according to the individual bone, but some eventually fuse together completely, joining ossified regions into one solid bone. The most significant synchondrodial joints related to birth join the occipital and sphenoid bones together. The difference between these two types of bones matter in regards to labor because it is necessary for the majority of the fetus’ skull bones to be very flexible during labor so that it can mold and pass through the birth canal properly. It is of great significance therefore, that most of the bones in the fetal skull are membranous bones since these are more flexible and moldable than endochondral bones are. 5. How many regions of the fetal skull are there? Name them and their relative locations. The fetal skull is divided into three main regions: the face, the vault, and the base. ● The face extends from the bridge of the nose to the junction of the chin (mentum) and neck. It includes features such as the mentum or chin, the mouth, and the nasion or bridge of the nose. ● The vault of a fetus’ skull is also called the cranium. In a newborn, the sides of the vault are made up of the two parietal bones, the two temporal bones, the two frontal bones, the occiput, and the pterygoid processes of the temporal bones, and the lateral extremities of the sphenoid. ● The base or floor of the skull is the part of the fetus’ skull that joins the spinal column to the back of the head (occiput). It is formed by portions of several bones, including the central portion of the sphenoid bone (Which is behind the eyes.), the thick inferior petrous portion of the temporal bones, the basiocciput and condylar parts of the occiput, the ear ossicles, the lowermost border of the frontal and ethmoid bones, and the inferior parts of the maxillae, palatine, inferior nasal conchae and vomer bones of the face. 6. What are “parietal” bones? What is their importance in labor and birth? Parietal bones are the two large membranous bones that extend from the center of the top of the fetus’ head down to the ears on either side of the skull to form most of the sides of the skull vault. Each parietal bone has four blunt corners and is shaped like a shallow bowl. Each is bordered by the occipital parietal bone superiorly, the frontal bones anteriorly, the occipital bone posteriorly, and the temporal and sphenoid bones inferiorly. They are thin and can become “dented” during a difficult birth involving excessive pressure. These bones are important because they cover the most area of the fetal skull and contain three landmarks that are clinically important during labor and birth.