Final answer:
Morphogen distribution can be established before fertilization and also result from secretion by neighboring cells during embryogenesis, influencing cell differentiation and specialization into various tissues and organs.
Step-by-step explanation:
The distribution of morphogens inside a cell can vary and is crucial for the processes of differentiation and early embryonic development. Morphogens are substances that help to pattern the tissues of an organism by forming concentration gradients that cells respond to by differentiating into different types. This distribution can be established prior to fertilization of the oocyte, often as a result of the maternal contribution to the egg's cytoplasm, setting up initial conditions for embryogenesis. Moreover, morphogen distribution can arise due to localized secretion by neighboring cells during development. This causes cells to respond and specialize based on their location relative to the source of the morphogen. For example, during human reproduction, following fertilization, as the zygote undergoes cleavage and forms into a blastula, it contains the inner cell mass, which will eventually give rise to the embryonic stem cells. These stem cells will later form the three primary germ layers - the ectoderm, mesoderm, and endoderm - through the process of differentiation, directed by the morphogens and their gradients.
Specialization of cells into tissues and organs is dependent on the differential gene expression triggered by morphogen gradients and by the cells' interaction with the extracellular matrix, involving proteins such as fibronectin and integrins, which play roles in cell adhesion and signaling during tissue differentiation. The intricate coordination of these processes during embryonic development leads to the formation of the various organ systems.