Reproduction + Development

Prenatal Development

The single-celled zygote forms a multicellular organism with specialized cells through the processes of:  **mitosis and differentiation**

By the end of the 8th week of pregnancy the embryo is called a **fetus** and all of the major structures are present

When the zygote is implanted in the uterine  wall a temporary organ called the **placenta** is formed from maternal and embryonic tissues

The developing embryo is attached to the placenta by the **umbilical cord,** this cord contains blood vessels

The fetus is surrounded by a membrane called the **amnion** which is filled with **amniotic fluid** 

The fluid protects the fetus by giving it a stable environment and absorbing shock

zygote 🡪 multiple identical cells 🡪 differentiated cells (tissues) 🡪 organs 🡪organ systems 🡪 individual

Umbilical Cord

two arteries and a vein connects the fetus to the placenta

* **There is NO exchange of blood between the embryo and the mother!**

Placenta

**site of diffusion**

* **Oxygen & nutrients diffuse from the mother’s blood vessels, across the placenta, and into the baby’s blood vessels**
* **Wastes diffuse from the baby’s blood vessels into the mother’s blood vessels**

Human gestation

* the period between fertilization and birth   
* approximately 38-40 weeks
* When gestation is complete the baby passes through the **cervix** and then through the **vagina**

Teratogens

* **Substances that may harm the developing fetus and result in the formation of birth defects**


* **Include alcohol, certain drugs/medications, infections, and certain chemicals**

Fertilization

the fusion of the sperm cell nucleus with the egg cell nucleus to produce a zygote (fertilized egg)

* IF fertilization takes place, it occurs in the **oviduct**  
* The egg cell is viable for approximately 24 hours after ovulation

Sperm + ovum 🡪 zygote

Implantation

* After approximately a week, the developing embryo is implanted into the **uterus**

**Embryo**:  conception to 8 weeks

Identical twins (monozygotic)

One egg is fertilized by one sperm

Embryo splits into two during the early stages of development

Have identical genes and must be of the same sex

(Incidence:  about 3 in every 1000 births)

**Fraternal twins (dizygotic)**

Two eggs are ovulated and are each fertilized by a different sperm cell.

No more related than any other sibling in the family (can be of the same or different sexes)

Maternal age, genetics, use of assisted reproductive technologies are factors

varies by geography and ranges from 6 to over 20 per 1,000 deliveries

Embryonic Development

A series of **cell divisions** and **differentiation** into various tissues and organ systems 

Embryo

**a multicellular organism in the early stages of development**

The beginning developmental processes are always the same in all animals:

1)  **rapid mitotic cell divisions** (cleavage)

2)  **growth**

3)  **differentiation**

**Cleavage**

* **The cells divide rapidly with little time in between for growth**


* **The cells become smaller and smaller**
* **The cells divide until they become a solid ball called a morula**
* **Cells continue to divide and are pushed to the outside to form a fluid-filled ball of cells called a blastula**

Gastrulation

**The cells further divide until they fold in on each other called invagination**

* **This forms 2 cell layers**


* **The outside layer is called** the **ectoderm** layer


* **The inside layer is the endoderm layer**


* **As cells continue to divide a third cell layer forms in the middle: mesoderm**

Differentiation

* the changing of unspecialized embryonic cells into the specialized cells, tissues and organs of a multicellular animal


* the ectoderm, mesoderm, and endoderm **differentiates** to form tissue and organs systems
*  **Each cell contains the same genes, but different**

genes are __**expressed**__ in different cells (embryonic cells use different portions of their genetic information)
*   **The type of cell that forms through differentiation is controlled by genes, hormones and cell location**

Germ Layers

**Ectoderm: Outer layer**

**Endoderm: Inner Layer**

**Mesoderm: Middle Layer**

Ectoderm

* forms nervous system including brain,

 spinal cord and nerves

*    forms skin, sweat glands, hair, nails

Endoderm

* forms lining of digestive tract and     associated organs (liver, pancreas)


*  forms lining of respiratory tract

Mesoderm

* forms bones and muscles
* forms blood and blood vessels
* forms reproductive and excretory systems

Menstrual Cycle

a series of changes controlled by **hormones** that help prepare the female uterus for a possible pregnancy

The cycle is controlled by hormones of the **pituitary gland** in the brain and **ovaries**

The menstrual cycle occurs in 4 stages

Begins at **puberty**

Usually lasts 28 days, but can vary due to illness & other factors

**Stops** when a women is **pregnant**

**Humans and primates** are the only mammals to have a menstrual cycle

**Stops permanently** during **menopause**

Stage 1 - Follicle stage

Lasts 10-14 days

Ovarian hormones (**estrogen and progesterone**) are in **low** concentrations

Pituitary gland produces **FSH** (follicle stimulating hormone) which **causes a follicle** (egg cell capsule) **to form**

as the follicle grows it makes **increased amounts of estrogen**

* **estrogen** has a **negative feedback** effect on the pituitary that **stops FSH secretion**


* **estrogen** causes the **uterine lining** (endometrium) to **grow thicker**

Stage 2 – Ovulation (middle of cycle)

* increased estrogen from the follicle stimulates the pituitary gland to produce **LH (luteinizing hormone)**


* **LH surge causes the follicle to rupture and the egg cell is sent to the oviduct=ovulation**

Stage 3 – Corpus Luteum Stage

13-14 days

**ruptured follicle** heals inside the ovary forming the **corpus luteum**

corpus luteum produces **progesterone**

**progesterone** maintains the **added growth of the uterine lining** 

If fertilization takes place:

* An embryo in the uterus will secrete a chemical called **hCG**  (human chorionic gonadotropin)


* **hCG** prevents the corpus luteum from decomposing which maintains **progesterone** levels
* The **uterine lining** is **not shed**
* hCG is the chemical that is detected by pregnancy tests

Stage 4 - Menstruation

* if an embryo __**is not present**__, approximately 2 weeks after ovulation the **corpus luteum decomposes**


* **estrogen & progesterone levels decrease**
* **uterine lining is shed** and moves out through the vagina (menstruation)
* **pituitary increases** the release of **FSH** & a new follicle matures – the cycle continues throughout the woman’s reproductive years

Male reproductive system

**Testes** – the male gonad;  produce sperm and testosterone

* Located outside of the body in structure called the **scrotum**


* Lower temperature is best for sperm production and storage
* Testes are 3°C lower than body temp

Mature sperm are stored in the **epididymis**

* The sperm is transported from the testes (testicles) to the **vas deferens**


* **vas deferens:**  tubes that connect to urethra

Glands that add fluid are the ***Cowper’s gland, seminal vesicles*** and ***prostate***

* Fluid nourishes and transports sperm, protects sperm from acidity of female reproductive tract
* Sperm + seminal fluid = **semen**

Female reproductive system

**Ovaries**

**Oviduct** (fallopian tube)

**Uterus**

**Cervix**

**Vagina**

vas deferens

tubes that connect to urethra

**Ovaries**

the female gonad;  produce egg cells (ova) and hormones estrogen and progesterone

**Oviduct**

a tube that carries the egg cell (ovum) to the uterus;  

******site of fertilization of the egg cell**

* Cilia draws ovulated egg cell into the oviduct

**Uterus**

a thick, muscular, pear shaped organ (aka womb)

* A baby develops here

**Cervix**

narrow neck that is the opening to the uterus

**Vagina**

the birth canal

Meiosis

the process by which **gametes** (egg and sperm) are formed \n Meiosis only occurs in the **gonads** (ovaries and testes)

In meiosis, the **chromosomes replicate once**, and the **cell divides twice…**

**Result = 4 new cells, each with ½ the number of chromosomes as the parent cell**

**The 4 cells are monoploid (aka haploid): have one set of chromosomes**

**The rest of the body cells are diploid: have the full number of chromosomes (2 sets)**

The chromosomes are genetically different from the chromosomes of the parent cell

First Meiotic Division

Interphase I

Prophase I

Metaphase I

Anaphase I

Telophase I

**Interphase I**

**chromosomes and centrioles replicate**

Prophase I

* chromosomes condense


* centrioles move to opposite poles of the cell
* spindle fibers form, attach to centromeres of chromatids
* each pair of chromatids lines up with its __homologous pair__ = __**synapsis**__

Homologous chromosomes

* chromosomes that are __**similar**__ in terms of size, shape and genetic content


* In each pair, one chromosome came from the mother (maternal); one chromosome came from the father (paternal)

Synapsis

pairing of homologous chromosomes

Crossing over

exchange of segments of chromosomes during synapsis;

results in recombination of the genes

Genetic Recombination

Genetic Recombination is the result of crossing over

**Because of crossing over, each gamete (egg or sperm cell) receives a unique set of genes from their parents**

Chiasmata

Points at which crossing over and exchange of genetic material occur between the chromosomal strands

Metaphase 1

tetrads line up in the middle of the cell

Anaphase 1

* homologous chromosomes separate


* \*\*\* This is called disjunction

Telophase 1

* two daughter cells are formed;


* \*each has half the number of chromosomes, but they are still replicated

Second Meiotic Division

* just like mitosis


* duplicated chromosomes separate - but they are actually recombined due to crossing over

Differences between Mitosis and Meiosis

__**Mitosis -**__


1. Daughter cells are genetically identical to parent cells:  same # and types of chromosomes
2. 2 diploid cells are formed
3. Occurs in growth & repair of body cells
4. Chromosomes replicate once; cell divides once
5. Homologous chromosomes do not pair; no recombination of genes

__**Meiosis -**__


1. Daughter cells are not genetically identical to parent cells:  have ½ the number of chromosomes and they have different genetic combinations
2. 4 monoploid cells are formed
3. Forms gametes (egg & sperm cells) in the gonads
4. Chromosomes replicate once; cell divides once
5. Synapsis, crossing over and genetic recombination of genes

Oogenesis

* the production of egg cells (ova) in the ovary of the female


* cytoplasm is divided unequally
* one of the daughter cells receives most of the cytoplasm – the small cells are called polar bodies
* the large egg cell formed contains yolk to nourish the developing embryo before the placenta forms
* the polar bodies disintegrate
* for each cell that undergoes meiotic division, there is only 1 ovum (egg cell) produced

Spermatogenesis

* The production of sperm in the testes
* The division of the cytoplasm is equal
* For each cell, there are 4 sperm produced