Final answer:
The lesser or posterior fontanelle, also known as the occipital fontanel, is a small, triangular soft spot on a newborn's skull that closes within the first 2-3 months after birth. It allows the skull to be flexible during childbirth and provides space for brain growth postnatally. It is made up by the lambdoid and sagittal sutures and located between the parietal and occipital bones.
Step-by-step explanation:
The lesser or posterior fontanelle, also known as the occipital fontanel, is one of the 'soft spots' on a newborn's skull, which are technically known as fontanelles. This particular fontanel is smaller and has a triangular shape, positioned at the back of the head and is bordered by the lambdoid and sagittal sutures, between the parietal bones and the occipital bone. The posterior fontanelle is significantly smaller than the anterior fontanelle, usually about a couple of centimeters across at birth and it closes relatively quickly, typically within the first two or three months of life as part of the ossification process, where connective tissue ossifies and converts into bone, leading to the fusion of the adjacent skull bones in a process known as synostosis.
The role of the posterior fontanelle, along with other fontanelles, is crucial during childbirth, as they allow the bony plates of the skull to compress and overlap to facilitate passage through the birth canal. After birth, these fontanelles provide the necessary space for the rapid growth of the baby's skull and brain. Over time, as the brain and skull grow, the fontanelles become smaller, the bones thicken, and the connective tissue narrows down to form sutures, which will eventually ossify and fuse in some areas, causing the suture lines to gradually disappear later in life.