- Human physiological development
- Egg, sperm, and fertilization
- Early embryogenesis - Cleavage, blastulation, gastrulation, and neurulation
- Germ layer derivatives
- Major motor milestones
- Motor development
- Neonatal reflexes
- Physical development in adolescence
- Brain changes during adolescence
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- [Instructor] When we talk about adolescence, we're talking about a transition period that exists between childhood and adulthood. But what does that actually mean? Honestly, it means different things for different cultures. So, cultures where children leave school after a few years, and then go and find a job and are self-sufficient by the time that they're teenagers, they don't really have this adolescent period. Even within Western cultures, the idea of adolescence is sort of a new one. Historically, children, and definitely teenagers, were basically treated as kind of mini adults, or just adults themselves. I think that you could definitely have a long debate, and I could definitely make a whole video, about whether or not adolescence is a social construct. But for today, in the U.S. and most of the Western world, we would say that adolescence starts at the beginning of sexual maturity and ends with independent adult status. So that would mean living apart from your family and having your own interests, your own job, et cetera. Mostly, it's about being self-supported. And when we talk about adolescence, we typically say that it begins with puberty. It's about a two year long period of sexual maturation, during which time a person becomes capable of reproduction. It starts at different times in males and females. For males, it typically starts around age 13. For females, it starts around age 11. And again, that's just when it generally starts. It could start earlier or later, depending on the person. And although puberty is actually a process, we usually associate it with two very specific incidents that occur during this time, two landmarks. For males, that event is their first ejaculation, which usually occurs as a nocturnal emission. For females, it's their first menstrual cycle, or their first period. Just like puberty starts at different times for males and females, these milestones are also reached at different times, usually around age 14 for boys, and somewhere between 12 and 13 for females. And this, as well as the earlier start time in general, is why girls tend to be taller than boys between the ages of 11 and 14. But then, once boys catch up in terms of puberty, they have a growth spurt and typically wind up being taller. Aside from these milestones, puberty is also associated with many physical changes. These include two main things. The first is primary sex characteristics. This basically describes the development of our reproductive organs, or the parts of our body that are actually used for reproduction. So this would include the testes in males and the ovaries in females, and just overall genital development in both groups. And we say that they're primary sex characteristics because these are the things that make sexual reproduction possible, which is, after all, what puberty is all about. Puberty also brings about the development of secondary sex characteristics. These are nonreproductive sex characteristics. Meaning that even though they are related to sexual development, they are not required for sexual reproduction. And for males, this would include things like a change in voice, and a growth of body hair, and also a growth spurt. For females, this includes the development of breasts and hips. For both genders, puberty tends to bring about the development of both underarm hair and pubic hair. There are a couple of things that we need to keep in mind. First, is that the sequence of physical changes is typically the same in all individuals. However, the timing is not. Some females enter puberty as early as age nine or as late as age 16. It depends on a ton of different things: their height, their weight, their nutrition. And usually, an earlier period or a late period isn't actually a sign that anything is wrong. It's just a sign of individual differences. However, there are different social effects that are associated with early puberty. Boys that go through puberty early, who mature faster, tend to be stronger and taller, and therefore more athletic, and with that comes a number of social things. They tend to be popular and self-assured and independent. And, while I'm sure that we will all agree that it's nice to be popular, it's nice to be liked, but there are also some negative things that are associated with early puberty in males, things like increased tendency for delinquency, and increased alcohol use. For females, the story isn't really so clear. In fact, there aren't really any positives for going into puberty early. These girls aren't typically more popular. In fact, they're typically the focus of a lot of teasing and sometimes even sexual harassment. They tend to be out of sync with their friends in terms of their interests. And while a lot of it depends on the attention that they get and the interactions that they have, and the responses that they get from both adults and their peers, early physical maturation in females doesn't really go along with any positive social factors.